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Filtration of cement-based grouts measured using a long slot
Tunnelling and Underground Space Technology 43 (2014) 101–112
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Tunnelling and Underground Space Technology journal homepage: www.elsevier.com/locate/tust
Filtration of cement-based grouts measured using a long slot Almir Draganovic´ ⇑, Håkan Stille Division of Soil and Rock Mechanics, Department of Civil and Architectural Engineering, KTH, S-100 44 Stockholm, Sweden
a r t i c l e
i n f o
Article history: Received 13 February 2012 Received in revised form 11 February 2014 Accepted 28 April 2014
Keywords: Penetrability Filtration Long slot Cement-based grouts Grouting
a b s t r a c t Penetrability of cement-based grout is an important issue when sealing the rock around tunnels and measurement of this property of the grout is needed for designing the grouting process and the development of grout. This paper investigates plug-building or the filtration process in a long slot where a slot constriction is placed relatively far from both the ‘‘borehole’’ and the end of the slot. In this slot, a certain shear stress develops before and after a constriction, which may influence plug-building at the constriction. This method is also compared with short slot and penetrability meter. The smallest groutable fracture for all three measured grouts is reasonably close to 75 lm. Measurements using the long slot showed better penetrability results compared to the short slot and the penetrability meter. The short slot is more practical and gives reasonably good results. The penetrability meter underestimates the penetrability of the grouts. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction 1.1. Background Grouting is a method of rock sealing and cement-based grout is the most common agent used in this process. To create a sufficiently watertight zone, a grout must penetrate a certain distance into rock fractures. During penetration through the fractures, a grout could be stopped at a fracture constriction due to cement grains building a stable arch over the constriction. This process is known as plug-building or filtration of grout and has been studied by many researchers e.g. Widmann (1996) and Schwarz (1997). The results are not unambiguous and no consensus exists of how penetrability of the grout should be measured and how it should be related to rock fractures. 1.2. Existing equipment for measuring penetrability of cement-based grouts Widmann (1996) used a pressure chamber with a filter of known permeability to determine the filtration stability of cement-based grout. The filter may consist of fleece, fine sand, soft rock, standard porous stone, or similar. Schwarz (1997) used a sand column to study the permeability and filtration of cement-based grout. A pressure chamber with a filter of known permeability or ⇑ Corresponding author. Tel.: +46 87908026; fax: +46 87907928. E-mail addresses: [email protected] (A. Draganovic´), [email protected] (H. Stille). http://dx.doi.org/10.1016/j.tust.2014.04.010 0886-7798/Ó 2014 Elsevier Ltd. All rights reserved.
a sand column may be appropriate methods to study penetration in sand or soil but there are doubts about using these methods to study the penetration of cement-based grouts in fractured rock. Other devices have also been developed to study the process, including the filter pump developed by Hansson (1995) and the penetrability meter developed by Eriksson and Stille (2003). Both these devices use a mesh of thin woven steel wires in a square pattern to represent a fracture aperture. Even though the two devices use the same mesh to measure filtration, they showed different results. Measurements with the filter pump showed that a higher water to cement ratio (w/c) improves penetrability (Hansson, 1995) while measurements with the penetrability meter showed that the w/c has no significant influence on penetrability (Eriksson et al., 2004). The reason might be the way in which the grout is pushed through the mesh. The filter pump sucks grout through the mesh while the penetrability meter presses the grout from a pressure chamber. The magnitude of the pressure which presses or sucks grout through the mesh is also different. Eriksson and Stille (2003) defined two parameters called bmin and bcritical. Parameter bmin is the minimum aperture in which grout can penetrate at all and parameter bcritical is the minimum aperture through grout can penetrate without filtration (critical fracture aperture). The grout is filtered in apertures between bmin and bcritical. Eklund and Stille (2008) studied how well a mesh simulates a constriction in a fracture. They tested the penetrability of cement-based grouts using both slot and mesh to simulate an aperture and showed that penetrability varies with different geometries. Slot geometry in this measuring method refers to a 62 mm
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long and 75–125 lm wide opening in an 0.5 mm thick metal plate. The diameter of the plate is 95 mm. Sandberg (1997) developed another type of measuring device where a fracture aperture is represented by two parallel metal discs without any constriction between them. This method is known as the NES-method. The method was further developed by Nobuto et al. (2008) where a slot between the discs is grouted with real grouting and mixing equipment where mixing, grouting and agitation are continuous. An important advantage of this method compared to the filter pump and penetrability meter is the possibility to use a relatively high pressure during grouting since research has shown that a higher grouting pressure improves penetrability (Eriksson et al. (1999), Hjertström (2001), Nobuto et al. (2008), and Draganovic´ and Stille (2011)). Axelsson and Gustafson (2010) developed a measurement device called the PenetraCone to determine the penetrability of cementbased grouts in the field. In this device, a slot also represents a fracture aperture as in the NES device. The difference is that the slot walls in this device are spherical instead of flat. The PenetraCone consists of two conical cylinders; an outer cylinder and an inner cylinder. By turning the inner cylinder, the gap between the inner and outer cylinders can be adjusted between 20 and 200 lm. Axelsson and Gustafson (2010) performed some comparison measurements with the PenetraCone and the penetrability meter but the results do not show any clear relation between the measurements. Axelsson and Gustafson (2010) were of the opinion that the slot formed by these two cylinders represents a fracture in a better way than a mesh. Draganovic´ and Stille (2011) developed a measuring device called the short slot. The short slot is similar to the NES device where grout is pressed through a slot between two parallel metal discs, but there is a significant difference between the two devices. In the short slot, the slot aperture is reduced. This aperture reduction represents a constriction in a fracture. There is no constriction between the discs used in the NES-method and in the authors’ opinion the slot geometry in the NES-method measures plugbuilding at an intersection between a fracture and a borehole rather than at a constriction in a fracture. 1.3. Objective of the paper A great many different measuring devices exist, indicating the difficulties to construct a device, which can correctly represent the filtration process in a rock fracture. Further, measurements with these devices showed different results regarding how factors such as w/c ratio, pressure, the cement’s grain curve, mixing, hydration and flocculation, and additives influence the penetrability of grouts, which is also an indication that all or some of them do not represent the filtration process satisfactorily. Measuring equipment with a pressure chamber with sand or steel mesh as a filter could not measure the influence of grout flow through a fracture before and after constriction when a plug forms. Nor could these methods follow the plug-building process over time. NES and short slot were able to follow the plug-building process over time but could not measure the influence of grout penetration before and after a constriction. The aim of this paper is to present results from measurements of the plug-building or the filtration process in a long slot where a constriction is placed a relatively long distance from both the ‘‘borehole‘‘ and the end of the slot. The results are compared with measurement with penetrability meter and short slot. 1.4. Bingham flow and plug-building at a fracture constriction The flow of cement-based grout through a fracture can be described as a flow of a Bingham fluid characterized by viscosity
and yield stress e.g. Lombardi (1985) and Håkansson et al.(1992). The shear stress s developed during penetration can be described by Eq. (1),
s ¼ s0 þ lc
ð1Þ
where s0 is the yield strength of the grout, l the plastic viscosity, and c the shear rate. Fig. 1 illustrates shear stress, yield strength, and velocity profile of a Bingham fluid flowing through a fracture with a constriction. A solid core is formed in the middle of the fracture where shear stress developed in the grout is less than yield strength. Closer to fracture walls the shear stress exceeds the yield strength and the grout is sheared. The shear stress is largest in contact with the fracture wall. The velocity of the grout is larger after fracture constriction, V2(z) compared to before V1(z) since the flow is constant. This causes a higher shear stress in the grout and the thickness of the solid core is reduced. Therefore, during the flow from larger to smaller fracture, the thicker solid core must be destroyed, which causes an extra pressure loss. Fig. 1 also shows that the gradient or rate of pressure loss, (Dp/Dl) vary before, at, and after a constriction. The rate of pressure loss in zone 2 is much higher than in zone 1 or 3 due to constriction. The grout in zone 2 has a higher velocity, i.e. higher shear, than in zone 1. Extra energy is also needed to destroy the relatively thick solid core. The pressure loss in zone 3 is higher than in zone 1 since the shear rate is higher. The rate of pressure loss (Dp/Dl) at the constriction will also increase over time in case of filtration or plug-building. When grout passes a constriction in a fracture there is a risk of plug-building, which might cause the flow to stop. Particles in the grout can initiate plug-building and build an arch of particles at the constriction as illustrated in Fig. 2. Plug-building is a complex process and mainly a function of the cement grain-size curve, w/c ratio, tendency to flocculate and pressure (Draganovic´ and Stille, 2011; Eklund and Stille, 2008). Draganovic´ and Stille (2011) showed that the relationship between critical fracture aperture (bcritical) and d95 varies between 2 for coarser and relatively finegrained cements and 21 for very fine-grained cements due to a larger degree of hydration and flocculation. A similar relationship was also shown by Eklund and Stille (2008). The cement with d95 around 30 lm showed the best penetrability. A higher w/c-ration also gives a better penetrability since grain concentration is lower. Grouting pressure also influences penetrability. Higher pressure reduces the risk of arching and increased erosion of the plug at the constriction as shown in Draganovic´ and Stille (2011). Plug-building decreases flow velocity, i.e. decreases shear rate in the grout. During plug-building, the pressure in front of the constriction will increase and decrease after the constriction. The pressure loss in zones 1 and 3 will be reduced and the main pressure loss will occur in zone 2, i.e. at the constriction. This will be described in more detail in sub-chapter 2.1. 1.5. Influence of pressure change on plug-building at a constriction During grouting, grout penetrates from a borehole into a fracture and continues to penetrate through it. Through this penetration, grout can meet a fracture constriction at any distance from the borehole and plug-building might cause the flow to stop. When the grout front reaches the first constriction, substantial pressure can be mobilized to erode a plug in case of plug-building since the accumulated shear stress along the fracture wall at this time is then relatively small. Pressure distribution in a fracture at this moment is illustrated by curve t1 in Fig. 3. When the grout front passes the constriction, illustrated by curve t2, a pressure loss at the constriction will occur. The rate of pressure loss (Dp/Dl) will be higher at the constriction than in the fracture before constriction. From this moment, a pressure difference (DP = P2 – P3) will exist at the constriction. This pressure
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Fig. 1. Flow of a Bingham fluid in a fracture with a constriction.
Fig. 2. Plug-building in a fracture with a constriction.
difference over time without filtration at the constriction is illustrated by DPt2, DPt3, and DPt4 in Fig. 3. With further penetration, accumulated shear stress increases and flow velocity decreases. A lower flow velocity gives a lower shear rate, which causes a decrease in the pressure difference at the constriction. This means that further penetration might increase the risk of plug-building at the constriction. The pressure difference at the constriction will change, i.e. increase over time in case of filtration or plug-building. This might also influence plug-building and erosion. The distance between constriction and borehole could also influence the risk of plug-building at a constriction. A longer distance will give a lower pressure for erosion of a partially formed plug and might increase the risk of plug-building. 2. Material and methods 2.1. Description of the long slot and definition of filtration when measuring using long slot The long slot was 4 m long and 100 mm wide. A longitudinal section of the slot is illustrated in Fig. 4a. The aperture of the first
two meters of the slot is 0.5 mm, which then changes to 1 mm along a length of 30 mm. The aperture is then reduced to 75 lm. The reason for having a length of 30 mm before the reduction of the aperture of 1 mm is to have a similar geometry to the short slot. Fig. 4b illustrates plug-building at a constriction during grouting. When grouting begins, grout flows without accumulation of grains at the constriction (t = t1). After a while, some grains stick at the constriction, which initiates accumulation of the grains. The accumulation continues and the plug grows and at the end a complete plug has formed, stopping the flow (t = t4). Plug-building can start anywhere at the constriction but often starts at the edges of the slot, where velocity is lowest. An uncompleted plug could be eroded during grouting, as illustrated in Fig. 2. This can be observed from the pressure measurements. A sudden decrease in pressure in front of the plug and a sudden increase in pressure behind the plug indicate erosion of the plug. Four characteristic places in the slot are chosen to measure pressures during penetration, as shown in Fig. 4a. The measured pressures are: P1, the pressure in the grout container; P2, the pressure measured 7.5 mm before the constriction; P3, the pressure measured 22.5 mm after the constriction; and P4, the pressure measured 52.5 mm from the end of the slot. The diameter of the holes where pressure-gauges P2, P3, and P4 are located is 3 mm. The graphs in Fig. 4c and d illustrate the development of the pressures P1, P2, P3, and P4 during grouting in cases without plug-building and with plug-building when the grout is filtered. In the case without plug-building, all pressures remain constant during the entire grouting time after steady state. In the case with plug-building, the pressures change when plug-building occurs and grout is filtrated until the full flow stop occurs. The beginning of filtration of the grout is detected by the increase in pressure P2 and the decrease in pressures P3 and P4. A grooving filter cake reduces the flow area at the slot constriction and causes a decrease in flow. Due to the lower flow, the pressure loss is reduced in the slot before the constriction and this causes an increase in P2 and a decrease in P3 and P4. The filter cake or plug may or may not reach the P2 pressure gauge before the flow stops. In cases when the flow stops before the filter cake reaches the P2 pressure gauge, pressure P2 will reach pressure P1 and pressures P3 and P4 will fall to zero. In cases
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Fig. 3. Pressure distribution in a fracture during grouting with a cement-based grout. Curve t4 illustrates pressure distribution along the fracture just before pressure equilibrium, i.e. flow stop.
when the filter cake reaches the P2 pressure gauge before the flow stops completely, pressure P2 will begin to decrease from the moment when the filter cake reaches the P2 pressure gauge. Penetrability measurements with short slot and penetrability meter were also carried out in this study. Detailed descriptions of these two methods can be found in Draganovic´ and Stille (2011) and Eriksson and Stille (2003) respectively. The main differences of the used methods are listed in Table 1. 2.2. Performed tests, cements used, and mixing A clear and long filtration process in the long slot during penetration is a desired result since the possibility to measure the filtration is one of the aims of this study. An aperture of 75 lm in the long slot is therefore chosen on the basis of measurements performed using a short slot (Draganovic´ and Stille, 2011). 75 lm is judged to be close to the critical aperture for the grouts based on micro-cement used in this study. Tests using the long slot are also carried out to study the pressure losses in the slot before, at and after the constriction. The cements used in the study were Injektering 30 (INJ30), Microfine 20 (MF20), and Ultrafin 12 (UF12). INJ30 is a relatively fine-grained cement with a d95 of 32 lm and a surface area of 1300 m2/kg. MF20 is a very fine-grained cement with a d95 of 20 lm and a surface area of 2560 m2/kg and UF12 is also a very fine-grained cement with a d95 of 12 lm and a surface area of 2200 m2/kg. Surface area is measured using the BET method, i.e. the nitrogen absorption method. INJ30 and UF12 are based on the same clinker minerals as ordinary Portland cement. The raw material for MF20 cement is Byggcement, which is a Portland cement of type CEM II. In addition to the cement clinker, Byggcement also contains approximately 13% limestone. The tests performed with the chosen grouts are listed in Table 2. The tests with the long slot were repeated twice to investigate the repeatability. The grouts were chosen on the basis of measurements performed using short slot as presented in Draganovic´ and Stille (2011). The w/c ration was chosen to fulfill normal requirements to get a stable grout and with the best penetrability. All grouts are mixed at high speed for 3 min using a high-shear field mixer of colloidal type. The grouting pressure of 15 bar (1.5 MPa) was chosen because it is in the range of often used grouting pressures in the field
(Holmberg et al. (2012), Hernqvist et al. (2009)). Mixing of grout and size of grouting pressure is as in real grouting. The grout from grout container is pressed with compressed nitrogen. See Fig. 4a. The grouting pressure during grouting time is constant i.e. is not pulsing as in real grouting. Pressure gouges used to measure pressures P1, P2, P3 and P4 are SML 10.0/0–25 bar/4–20 mA from Thermokon-Danelko. They are calibrated against pressure gauge at pressure regulator mounted at gas container (Fig. 4a). The same type of pressure gauges are used in test set up in Draganovic´ and Stille (2011) and worked properly. Rheological properties and density of tested grouts are presented in Table 3. Since possibilities to measure rheological properties at the same time during tests with long slot are limited, these properties and density were later measured in the laboratory. Yield strength and plastic viscosity were measured with a Brookfield LV-II+ Programmable rheometer. The measurements were performed with small sample adapter SC4-13R and spindle SC-34 for INJ30 w/c = 0.6 grout and with spindle SC-31 for MF20 w/c = 0.9 and UF12 w/c = 1.2 grouts. Densities were measured with mud balance model 140, Fann Instrument Company. 3. Tests and results 3.1. Results of the tests performed using long slot, short slot, and penetrability meter 3.1.1. Penetrability test using INJ30 w/c=0.6 grout The results of a penetrability test with INJ30 w/c = 0.6 grout performed with the long slot are shown in Fig. 5. The four curves in the graph show pressures P1, P2, P3, and P4, as illustrated in Fig. 4d. By opening valve V1 (Fig. 4a), the pressure in the grout container (P1) jumps to a grouting pressure of approximately 15 bar. The grouting of the slot is begun by opening valve V4. Pressure P2 (the pressure before the constriction) then rises to approximately 13.5 bar, P3 (the pressure after the constriction) to approximately 13 bar, and P4 (the pressure close to the outlet of the slot) to approximately 4 bar. The grouting continues for approximately 80 s to time = 105 s, when the gas from the gas container reaches the outlet of the slot. This is indicated by a sudden increase in pressure P4 and the measuring is terminated by closing valve V4, and P4 then falls to zero.
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Fig. 4. Test of the grout filtration in the long slot by measuring some characteristic pressures. Sketch (a) illustrates a longitudinal section of the long slot with four measured pressures. The sketches in (b) illustrate plug-building in time at the constriction in the middle of the slot during grouting. The graphs (c) and (d) illustrate the pressures during grouting in cases without plug-building and with plug-building.
Table 1 Significant properties of the used methods. Method
Type of opening
Aperture of the constriction
Total area of the constriction
Average gradient over the equipment
Max volume
Long slot Short slot Penetrability meter
Slot Slot Wire mesh
75 lm 75 lm 75 lm
7.5 mm2 9.0 mm2 166 mm2
1.5 Mpa/4 m = 0.375 MPa/m 1.5 MPa/0.15 m = 10 MPa/m 0.1 MPa/0.15 m = 0.667 MPa/m
1.7 l 1.7 l 1.0 l
Table 2 Tests performed using long slot, short slot and penetrability meter. Grout
INJ30, w/c = 0.6 MF20, w/c = 0.9 UF12, w/c = 1.2 a
Measuring equipment
Table 3 Measured rheological properties and density of tested grouts. Yield strength (Pa) Plastic viscosity (Pas) Density (g/cm3)
Long slot
Short slot
Penetrability meter
Grout
x, x x, x x, x
x x x
x x xa
INJ30, w/c = 0.6 11.00 MF20, w/c = 0.9 5.02 UF12, w/c = 1.2 3.64
0.3750 0.0596 0.0378
1.72 1.40 1.37
30 min old grout.
When the grout front reaches the constriction, pressures P2 and P3 jump to approximately 13.5 and 13 bar, respectively. They then begin to increase slowly over the penetration time until the front of
the grout reaches the end of the slot, which is when P4 jumps to 4 bar. Pressures P2 and P3 continue to increase for a short period of approximately 10 s.
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Fig. 5. Penetrability test performed using the long slot and INJ30 w/c = 0.6 grout.
After this period a first filtration process of the grout is observed and marked in Fig. 5. Pressure P2 continues to slowly increase while P3 and P4 begin to slowly decrease. This filtration process continued for approximately 15 s to around 50 s in the graph. This filtration process follow period of pressure changes as predicted in Fig. 4d before the filter cake reaches pressure sensor P2. Later, pressures P2, P3, and P4 remain constant for a new period of approximately 10 s. This period of constant pressures indicates that the filtration of the grout is currently stopped and the flow during this time is constant. Then, at t 64 s, a small fall in pressure P2 and a small jump in pressures P3 and P4 occur. This could be a result of the sudden erosion of a partially built plug. Following the sudden erosion, a new period of filtration begins and continues until t reaches 80 s. This filtration period is also indicated by a slow increase in P2 and a slow decrease in P3 and P4. From the time of 80 s, all the pressures were in principle constant, which might mean that flow was steady without any further filtration until the gas reached the outlet. When gas reaches the end of slot in a test, the test is classified as 100% of the grout having passed the slot even if some grout remains in the slot. The grout container is empty. This test is classified as 100% of the grout having passed the slot. In a repeated test using the same grout, i.e. a grout with the same recipe, all the grout once again passed the slot. In this test, the filtration did not begin early as in the previous test but approximately 45 s after the commencement of the grouting. The filtration period marked in the graph in Fig. 6 is also indicated by a slow increase in P2 and a decrease in pressures P3 and P4. The sudden increase in pressures P3 and P4 at the end of the filtration period might be due to erosion, but pressure P2 should decrease while here it remains constant. Shortly afterwards, the gas reached the end of the slot, indicated by the P4 pressure jump to 4 bar. After approximately 5 s the grouting was stopped and pressure P4 fell to close to 0 bar. The partially formed plug in this test can be seen in Fig. 7. The holes in the upper plate are also marked where pressures P2 and P3 are measured. The rate of the pressure loss in the slot from these two tests is estimated at the beginning and end of the filtration processes. The results are shown in Table 4. The pressure loss in the slot after constriction, i.e. between pressure gauges P3 and P4, is approximately ten times larger than the pressure loss in the slot before constriction, i.e. between pressure gauges P1 and P2, due to higher flow
Fig. 6. Penetrability test performed using the long slot and INJ30 w/c = 0.6 grout.
velocity. The measured pressure loss at the constriction, i.e. between pressure gauges P2 and P3, is much larger than in the other parts of the slot. This is a reasonable result since that slot aperture is reduced 6.6 times. At the end of these filtration processes, the pressure loss at the constriction further increases due to plug-building, i.e. a further reduction in the flow area. The rate of pressure loss over time (Dp/Dt) is 2.11 and 1.46 bar/s in the respective test. The penetrability of the same grout was also tested using the short slot with a 75 lm aperture. The result of this test is shown in Fig. 8. Only 800 g, or 25%, of the grout passed the slot. This is a somewhat poorer result compared to the weight passed in tests with the long slot. The penetrability of this grout was further tested using a penetrability meter. This measurement also showed relatively poorer penetrability than tests with the long slot. The amount of grout passed through a 75 lm filter was only 60 ml and estimated bmin and bcritical were 68 lm and 172 lm, respectively. 3.1.2. Penetrability test using MF20 w/c = 0.9 grout The results of the first measurement performed with the long slot with MF20 w/c = 0.9 grout are shown in Fig. 9. All the grout passed the slot in approximately 18 s and for most of this time, the grout was to some extent filtered. The filtration began early but the pressure managed to keep the slot open for the flow. Some erosion occurred near the end of the grouting time and shortly afterwards the gas reached the outlet and pressure P4 jumped to 8 bar and the test was stopped.
Fig. 7. Photo showing the partially formed plug at the upper plate at the constriction in the test presented in Fig. 6 and holes where pressures P2 and P3 are measured.
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Table 4 Measured rate of pressure loss (Dp/Dl) (bar/m) in tests with INJ30 w/c = 0.6 grout. Test
Time
Between P1 and P2
Between P2 and P3 at constriction
Between P3 and P4
Fig. 5
At the start of the filtration, t = 36 s At the end of the filtration, t = 50 s At the start of the filtration, t = 55 s At the start of the filtration, t = 108 s
0.51
21.29
4.82
0.43
50.85
5.00
0.56
4.85
4.88
0.45
82.27
4.58
Fig. 6
Fig. 10. Penetrability test performed using the long slot and MF20 w/c = 0.9 grout.
Fig. 8. Penetrability test performed using the short slot with an aperture of 75 lm and INJ30 w/c = 0.6 grout.
ing time. This is the reason why the flow was much lower than in the previous test. At the end, after approximately 6 min, this channel was also plugged, which stopped the flow. The amount of grout passed was approximately 0.5 l or approximately 30% of the initial amount of grout in the container. During this test it was observed that the grouting pressure (P1) was 3 bar lower than the normal grouting pressure of around 15 bar. This could be the reason for the poorer penetrability. However, when grouting had proceeded for 200 s the pressure was adjusted to 15 bar but this probably did not subsequently influence penetration. On the other hand, the sudden drop in pressures P2 and P3 during grouting is difficult to explain. An electrical problem with the measuring equipment cannot be ruled out. The rate of pressure loss in the slot from the test presented in Fig. 9 is estimated and presented in Table 5. As in the tests with INJ30 w/c = 0.6 grout, the largest pressure loss is at the constriction. The pressure loss in the slot in front of and behind the constriction is somewhat lower in this test compared with INJ30 w/ c = 0.6 grout. This is a reasonable result since the grout has lower yield strength and plastic viscosity. The rate of pressure loss over time (Dp/Dt) is 3.10 bar/s. This indicates a faster filtration process compared to tests with INJ30 w/c = 0.6 grout. The rate of pressure loss in the test presented in Fig. 10 is not estimated since it is difficult to define a clear filtration process. Most of the flow takes place through the small channel as described above. This grout showed good penetrability when tested with the short slot. The grout passed the slot in 22 s, which can be seen in Fig. 12. This is the only grout where all the grout penetrates through the short slot. The measurement using the penetrability meter also showed that this grout has the best penetration ability of the three grouts. The volume passed with a 75 lm filter was 530 ml, which is almost nine times as much as the INJ30 w/c = 0.6 or UF12 w/c = 1.2 grouts. The estimated bmin is 47 and bcritical is 105 lm.
Fig. 9. Penetrability test performed using the long slot and MF20 w/c = 0.9 grout.
The results of the second measurement with the same grout are shown in Fig. 10. The filtration of the grout began immediately, which caused an increase in pressure P2 and a decrease in pressure P3. In this test, the plug was completely formed and erosion did not occur during grouting as in the previous test. The test was terminated after approximately 370 s (approximately 6 min). Photo 1 in Fig. 11 shows the lower plates at the slot constriction with a built plug of around 120 mm in length. A small channel can be observed in the middle of the plug. It seems that the flow of the grout took place through this small channel for most of the grout-
3.1.3. Penetrability test with UF12 w/c = 1.2 grout The last series of measurements using the long slot, the short slot, and the penetrability meter were performed with UF12 w/ c = 1.2 grout. The test results measured by means of the long slot are shown in Figs. 13 and 15. In the first measurement, the whole amount of grout passed through the slot in approximately 80 s. The measurement showed that plug-building or filtration began immediately, indicated by the increase in P2 and decrease in P3 and P4. The filtration proceeded for between 18 and 38 s and at the end of this period sudden erosion occurred.
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Fig. 11. Photo showing the built plug at the constriction at the lower plate in the test presented in Fig. 10. It can be seen that the plug was approximately 120 mm long and a channel in the middle of the plug is also visible.
Table 5 Measured rate of pressure loss (Dp/Dl) (bar/m) in test with MF20 w/c = 0.9 grout. Test
Time
Between P1 and P2
Between P2 and P3 At constriction
Between P3 and P4
Fig. 9
At the start of the filtration, t = 15 s At the start of the filtration, t = 26 s
0.45 0.30
32.23 66.41
3.48 3.16
Fig. 12. Penetrability test performed using the short slot with an aperture of 75 lm and MF20, w/c = 0.9 grout.
Fig. 13. Penetrability test performed using the long slot and UF12 w/c = 1.2 grout.
After the sudden erosion there was another short period of filtration. The second filtration is followed by a period when pressure P2 in principle remained constant while P3 was mainly decreasing the rest of the time with a large variation. This period ended when gas reached the end of the slot, indicated by a sudden small jump in P4. The measurement was terminated and pressures fell. This large variation in pressure P3 may have been caused by frequent changes between plug-building and the erosion of the partially built plug. This conclusion is based on the observed pulsation of the outflow at the outlet, which was filmed. The plug from this test is shown in photos 1a (lower plate) and 1b (upper plate) in Fig. 14. In the middle of the plug, a small channel can be seen. This channel was probably open for the entire grouting time and gas passed through it at the end of the measurement. When injection was stopped by closing valve V4
at time 100 s, pressure P2 did not fall to zero but decreased to 6 bar and pressure P3 to 1 bar over a period of approximately 15 s. During this time, the small channel in the middle of the cake was completely plugged. A channel in the middle of the slot along its entire length can be seen in photo 2 in Fig. 14. This channel is the result of gas erosion at the end of the test. The test is classified as 100% of grout having passed the slot. It should be noted that pressure P2 should be somewhat higher than P3 when the grout front passes the slot constriction. The measurements at the beginning showed the opposite. This could mean that some of the P2 or P3 pressure gauges showed a higher or lower pressure but the important thing is the trend of the measured pressures. This is also the reason why the pressure loss in the slot from this test is not estimated.
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Fig. 14. Photos showing the long slot after the test presented in Fig. 13.
Fig. 15. Penetrability test performed using the long slot and a UF12 w/c = 1.2 grout.
The results of the repeated test with the same grout are shown in Fig. 15. In this measurement, the filtration of the grout also began immediately as in the previous test. The difference was that in this test the filtration continued for the entire grouting time. No erosion of the plug occurred and the plug was growing during the entire grouting. A certain outflow at the outlet was observed over the whole grouting time. According to pressure P2, the plug reached pressure gauge P2 at t = 36 s, which is detected by the commencement of a decrease in pressure. The filter cake continued to grow. At time t = 155 s, the measurement was terminated even though full flow stop was not achieved and only approximately 25% of the grout had passed the slot. The filter cake from this test is shown in photos 1, 2, and 3 in Fig. 16. Photos 1 and 2 show the filter cake at the upper plate and photo 3 shows part of the filter cake at the lower plate. Photo 2 also shows the places where pressures P2 and P3 were measured. It can be seen that the filter cake passed the P2 gauge by approximately 50 mm, confirming the previous explanation that pressure P2 begins to decrease when the filter cake passes pressure gauge P2. Photo 4 shows the lower plates after the test. It can be seen that even during plug-building, the slot after the constriction could be partially grouted by this grout.
Fig. 16. Photos showing the long slot after the test presented in Fig. 15. Photos 1 and 2 show the plug at the upper plate and photo 3 shows part of the filter cake at the lower plate. Photo 2 also shows the distances in mm where pressures P2 and P3 are measured. Photo 4 shows the long slot after the test.
The result of the penetrability test measured using the short slot is shown in Fig. 17. 2250 g of the grout passed the slot in approximately 35 s with a constantly decreasing rate of outflow, which indicates filtration. There are similarities between this test and tests performed with the long slot where grout was also filtered from the beginning of grouting. The measurement with the penetrability meter showed poor penetrability. The volume passed measured with a 75 lm filter was 78 ml and is approximately the same result as in the test with the INJ30 w/c = 0.6 grout. The estimated bmin is 49 and bcritical is 419 lm.
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Fig. 17. Penetrability test performed using the short slot with an aperture of 75 lm and UF12 w/c = 1.2 grout.
4. Discussion of the test results measured by the long slot, the short slot, and the penetrability meter 4.1. Comparison of the results measured with different measuring equipment Table 6 shows the summarized results of the measurements performed using the long slot and the short slot with a 75 lm aperture and the penetrability meter with a mesh with 75 lm openings. The test results estimated with the three different measuring methods are presented as percentages of grout passed. Comparison of these results between the methods is complex due to different flow geometries and different initial grout volumes. The measurements showed some different results between measurements using the long slot and the short slot. The INJ30 w/c = 0.6 grout showed the best penetrability measured by means of the long slot while the MF20 w/c = 0.9 grout showed the best penetrability when measured by means of the short slot and the penetrability meter. In almost all tests performed with the long and short slots, the grouts were evidently filtered for a longer time. In spite of filtration, a significant amount of the grout had passed through the slot in almost all the tests. Therefore it has been estimated that the narrowest fracture aperture to be grouted is close to 75 lm as initially was judged based on previous tests. This is also supported by the fact that different results were achieved when the tests were repeated with the long slot with the MF20 w/c = 0.9 and UF12 w/ c = 1.2 grouts. The only exception is the measurement with short slot with MF20 w/c = 0.9 grout where all grout passed the short slot without filtration. More testing with the long slot with varying slot aperture is needed to be able to better estimate the narrowest fracture aperture to be sealed.
Analysis of the results is difficult due to the randomness of the filtration processes. Since the flow geometry is similar, the comparison between the results estimated using the long and the short slots is easier than the comparison between the results estimated using these two methods and the penetrability meter. One way to compare these results is to show the amount of grout passed per flow area at the outlet (ml/mm2). The long slot has a flow area at the outlet of 7.5 mm2 and the short slot 9 mm2. The penetrability meter has a much larger outlet. The diameter of the outlet is 24 mm and the diameter of the wires used to manufacture a mesh of 75 lm is 50 lm. This gives an effective flow area at the outlet of 166 mm2 for a 75 lm mesh. The effective flow area at the outlet means that the area of the wires is excluded. The test results in Table 7 are presented as volume of grout passed divided by the area of the outlet. The results where the whole amount of the grout passed the slot are also marked as 100%, which means that even more grout would have passed through the slot if the grouting container had been larger. In all tests with the penetrability meter the flow was terminated due to complete filtration. This occurred after very small volumes of grout had passed the constriction compared to the passed volumes at the other tests. This might mean that the penetrability meter significantly underestimates the penetrability of the grouts. The measurements with INJ30 w/c = 0.6 performed using the long slot showed much better penetrability than measurements performed with the short slot. The comparison between the measurements performed using the long and the short slots for the MF20 w/c = 0.9 and UF12 w/c = 1.2 grouts is somewhat more complicated since the results measured with the long slot varied. However, the mean values of the results from the long slot are only slightly better than the results measured with the short slot. The flow regime for all three methods was roughly estimated. Reynolds number in the long slot after constriction for all three grouts was estimated to be between 1 and 35. This indicates laminar flow. The slot aperture before constriction is larger and the flow must be also laminar. The flow regime at the constriction is difficult to estimate. Especially in case of plug building or erosion the flow could be turbulent. Reynolds number in short slot and penetrability meter was somewhat higher but still indicates lami-
Table 7 Results shown in Table 6 shown as the volume of grout passed divided by the area of the outlet [ml/mm2] measured with 75 lm slot aperture and 75 lm mesh width. Grout
Long slot (ml/mm2)
INJ30, w/c = 0.6
226 (100%) 226 (100%) 226 (100%) 66 226 (100%) 56
MF20, w/c = 0.9 UF12, w/c = 1.2
Short slot (ml/mm2)
Penetrability meter (ml/mm2)
51
0.36
171 (100%)
3.19
173
0.47
Table 6 Amount of grout passed measured with the long and short slots with a 75 lm aperture and the penetrability meter mixed with a field mixer. 100% in the long and short slots corresponds to 1.7 liters while in the penetrability meter it corresponds to 1 liter. Grout
Long slot V = 1.7 l
Short slot V = 1.7 l
Penetrability meter Amount passed for 75 lm mesh width (ml,%), bmin–bcritical (lm–lm)
INJ30, w/c = 0.6
100% passed with filt. 100% passed with filt. 100% passed with filt. 30% passed with filt. 100% passed with filt. 25% passed with filt.
25% passed (stop after 5 s)
60 ml or 6% Filt. 68–172
100% passed without filt.
530 ml or 53% Filt. 47–105
95% passed (filt. during whole penetration time of 40 s)
78 ml or 8% Filt. 49–419
MF20, w/c = 0.9 UF12, w/c = 1.2
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nar flow. In real grouting pressure is pulsing. It is difficult to evaluate which flow give higher risk for plug building. The measurements using the long slot (Figs. 5, 9 and 13) indicated erosion with a sudden decrease in pressure in front of the constriction and a sudden increase in pressure after the constriction. The erosion could also occur in the short slot, which was indicated by the measurements with varying pressures presented in Draganovic´ and Stille (2011). These observations suggest that erosion could also be a process which takes place during penetration of the grout through the fractures and higher pressure may increase the erosion. It was expected that the pressure loss in the long slot before and after the constriction might reduce the penetrability of the grouts compared to flow through the short slot. This reduction in penetrability was not experimentally shown by comparing the results presented in Table 7. The results rather showed the opposite. The long slot could not be used for daily measurement of the penetrability of the grouts but the measurements using this method are probably closest to the truth, i.e. based on engineering judgment, the long slot simulates penetration through a fracture better than measurements with the short slot or the penetrability meter. The variation in the results measured with the long slot made this evaluation difficult and uncertain. The differences between the long and short slots are not significant. However, it can be concluded that the penetrability meter underestimates penetrability. More measurements with all three methods need to be carried out in order to obtain a better prediction. 4.2. In-laboratory measured penetrability related to fracture aperture The ultimate objective of testing penetrability in laboratory is to establish if a given grout will penetrate the required lowest fracture aperture related to design requirements. The complexity of the fracture geometry and variations implies that testing in laboratory always will be indirect and a simplification of reality. It means that the results based on testing in laboratory have to be validated against observations of penetrability of rock fractures in the field. The main objective of this investigation is to study the penetrability in laboratory with some different methods. Validation against field data is outside the scope of the work. However, the complexity of such investigation is shortly discussed below. Penetrability of grout is related to the size of the grains in comparison with the fracture aperture. It therefore seems reasonable to assume that the physical apertures of the fracture will be representative for evaluating the penetrability. The physical aperture can be characterized by the arithmetic mean value. The geometry of fractures is described by the aperture, roughness and contact area and theirs spatial correlations. The focus of many investigations has been both related to mechanical properties and water bearing capacities. One main issue is the relationship between the geometry of the apertures and the hydraulic aperture measured by water loss measurements or other types of flow measurements. Many researchers have contributed like e.g. Hakami (1989), Zimmermann and Bodvarsson (1996), Barton and Quadros (1997). They found that the measured hydraulic aperture is less than the arithmetic mean aperture. This conclusion has been confirmed in later research e.g. Tsuji et al. (2013) showing that the aperture defining the grout take always is greater than the hydraulic aperture. Penetrability of cement based grout into sand column, filter stone or mesh seems to be far from penetrability into a rock fracture. The geometry of the voids or openings has no similarity with a fracture. Comparison, like those in this study, indicates that it is easer for the grout to penetrate a slot instead of a mesh. Measurements also indicate that the penetrability can occur even if filtration
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of the grout is ongoing. The two defined slot apertures bmin and bcritical describing the filtration process can therefore be regarded as upper and lower bounds of the penetrability. Important issues to be further investigated is how (i) the hydraulic aperture is related to the arithmetic mean of a fracture and how well (ii) the arithmetic mean represents the true penetration and (iii) the evaluated penetrability in the laboratory testing will reflect the penetrability of a real fracture. 5. Conclusions Tests using the long slot showed that the filtration process could be observed by pressure measurements along the slot. The measurements also showed that erosion of the partially formed plug would occur during plug-building. The observed behavior indicates that the narrowest aperture to be grouted without filtrations for all three measured grouts is close to 75 lm. The poor repeatability of the long slot tests indicates that the plug-building process is randomn in its nature. It was further observed that the grouts more easily penetrated in the long slot compared to what was measured with the short slot and penetrability meter with a similar aperture. The results measured by means of the long slot are probably closest to the truth. Based on engineering judgment, the long slot simulates penetration into a fracture better than the other two methods. The short slot gives somewhat conservative results. The conclusion is that measurements of penetrability with short slots are more practical and give reasonable good results. The long slot could not be used for daily measurements. The penetrability meter, which maybe is the easiest method to use, will underestimate the penetrability of the grouts. The number of tests was limited and more measurements with the long slot method are needed to obtain reference values for a deeper discussion of the suitability of measurements with the short slot and the penetrability meter. References Axelsson, M., Gustafson, G., 2010. The PenetraCone, a new robust field measurement device for determining the penetrability of cementitious grouts. Tunn. Undergr. Space Technol. 25, 1–8. Draganovic´, A., Stille, H., 2011. Filtration and penetrability of cement-based grout: study performed with a short slot. Tunn. Undergr. Space Technol. 26, 548–559. Barton, N., Quadros, E.f., 1997. Joint aperture and roughness in the prediction of flow and groutability of rock masses. Proc. of NY Rocks´97. Linking Science to Rock Engineering. In: K. Kim (Ed.), Int. J. Rock Mach. and Min. Sci., 34(3–4), pp 907–916. Eklund, D., Stille, H., 2008. Penetrability due to filtration tendency of cement-based grouts. Tunn. Undergr. Space Technol. 23, 389–398. Eriksson, M., Friedrich, M., Vorschulze, C., 2004. Variation in the rheology and penetrability of cement-based grouts – an experimental study. Cem. Concr. Res. 34, 1111–1119. Eriksson, M., Stille, H., 2003. A method for measuring and evaluating the penetrability of grouts. ASCE Publ., 1326–1337. Eriksson, M., Dalmalm, T., Brantberger, M., Stille, H. 1999. Separations- och filtreringsstabilitet hos cementbaserade injekteringsmedel. In: Department of Soil and Rock Mechanics, KTH, Stockholm. KTH Rapport 3065 in Swedish. Hakami, E. 1989. Water flow in single rock joints. Licentiate thesis. Luleå University of Technology, Sweden. Hansson, P. 1995. Filtration stability of cement grouts for injection of concrete structures. In: Proc. IABSE Symposium, San Francisco, pp. 1199–1204. Hernqvist, L., Fransson, Å., Gustafson, G., Emmelin, A., Eriksson, M., Stille, H., 2009. Analyses of the grouting results for a section of the APSE tunnel at Äspö hard rock laboratory. Int. J. Rock Mech. Min. Sci. 46, 439–449. Hjertström, S. 2001. Microcement-pentration versus particle size and time control. In: 4th Nordic rock grouting symposium, Stockholm: SveBeFo Rapport 55, pp. 61–71. Holmberg, M., Tsuji, M., Stille, B., Stille, H., 2012. Evaluation of Pre-Grouting for the City Line Project Using the RTGC Method. Eurock, ISRM International Symposium, Stockholm. Håkansson, U., Hässler, L., Stille, H., 1992. Rheological properties of microfine cement grouts. Tunn. Undergr. Space Technol. 7 (4), 453–458. Lombardi, G., 1985. The Role of Cohesion in Cement Grouting of Rock. Quinzième Congres des Grandes Barrage, Lausanne.
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Nobuto, J., Nishigaki, M., Mikake, S., Kobayashi, S., Sato, T., 2008. Prevention of clogging phenomenon with high-grouting pressure. Doboku Gakkai Ronbunshuu C 64 (4), 813–832 (in Japanese with English abstract). Sandberg, P. 1997. NES-metod för mätning av injekteringsbruk inträngningsförmåga, Svensk Bergs- & Brukstidning. Schwarz, L. 1997. Roles of Rheology and Chemical Filtration on Injectability of Microfine Cement Grouts. Dissertation Thesis, Northwestern University, Evanstone, Illinois.
Tsuji, M., Holmberg, M., Stille, B., Rafi, J., Stille, H. 2013. Optimization of the grouting procedure with RTGC method. Data from trial grouting at City Line project in Stockholm. R-12-16, SKB Svensk Kärnbränslehantering AB, Stockholm. Widmann, R., 1996. International society for rock mechanics, commission on rock grouting. Int. J. Rock Mech. Min. Sci. Geomech. Abstr. 33 (8), 803–847. Zimmermann, R.W., Bodvarsson, G.S., 1996. Hydraulic conductivity of rock fractures. Transp. Porous Media 23 (1), 1–30.
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Filtration of cement-based grouts measured using a long slot Almir Draganovic´ ⇑, Håkan Stille Division of Soil and Rock Mechanics, Department of Civil and Architectural Engineering, KTH, S-100 44 Stockholm, Sweden
a r t i c l e
i n f o
Article history: Received 13 February 2012 Received in revised form 11 February 2014 Accepted 28 April 2014
Keywords: Penetrability Filtration Long slot Cement-based grouts Grouting
a b s t r a c t Penetrability of cement-based grout is an important issue when sealing the rock around tunnels and measurement of this property of the grout is needed for designing the grouting process and the development of grout. This paper investigates plug-building or the filtration process in a long slot where a slot constriction is placed relatively far from both the ‘‘borehole’’ and the end of the slot. In this slot, a certain shear stress develops before and after a constriction, which may influence plug-building at the constriction. This method is also compared with short slot and penetrability meter. The smallest groutable fracture for all three measured grouts is reasonably close to 75 lm. Measurements using the long slot showed better penetrability results compared to the short slot and the penetrability meter. The short slot is more practical and gives reasonably good results. The penetrability meter underestimates the penetrability of the grouts. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction 1.1. Background Grouting is a method of rock sealing and cement-based grout is the most common agent used in this process. To create a sufficiently watertight zone, a grout must penetrate a certain distance into rock fractures. During penetration through the fractures, a grout could be stopped at a fracture constriction due to cement grains building a stable arch over the constriction. This process is known as plug-building or filtration of grout and has been studied by many researchers e.g. Widmann (1996) and Schwarz (1997). The results are not unambiguous and no consensus exists of how penetrability of the grout should be measured and how it should be related to rock fractures. 1.2. Existing equipment for measuring penetrability of cement-based grouts Widmann (1996) used a pressure chamber with a filter of known permeability to determine the filtration stability of cement-based grout. The filter may consist of fleece, fine sand, soft rock, standard porous stone, or similar. Schwarz (1997) used a sand column to study the permeability and filtration of cement-based grout. A pressure chamber with a filter of known permeability or ⇑ Corresponding author. Tel.: +46 87908026; fax: +46 87907928. E-mail addresses: [email protected] (A. Draganovic´), [email protected] (H. Stille). http://dx.doi.org/10.1016/j.tust.2014.04.010 0886-7798/Ó 2014 Elsevier Ltd. All rights reserved.
a sand column may be appropriate methods to study penetration in sand or soil but there are doubts about using these methods to study the penetration of cement-based grouts in fractured rock. Other devices have also been developed to study the process, including the filter pump developed by Hansson (1995) and the penetrability meter developed by Eriksson and Stille (2003). Both these devices use a mesh of thin woven steel wires in a square pattern to represent a fracture aperture. Even though the two devices use the same mesh to measure filtration, they showed different results. Measurements with the filter pump showed that a higher water to cement ratio (w/c) improves penetrability (Hansson, 1995) while measurements with the penetrability meter showed that the w/c has no significant influence on penetrability (Eriksson et al., 2004). The reason might be the way in which the grout is pushed through the mesh. The filter pump sucks grout through the mesh while the penetrability meter presses the grout from a pressure chamber. The magnitude of the pressure which presses or sucks grout through the mesh is also different. Eriksson and Stille (2003) defined two parameters called bmin and bcritical. Parameter bmin is the minimum aperture in which grout can penetrate at all and parameter bcritical is the minimum aperture through grout can penetrate without filtration (critical fracture aperture). The grout is filtered in apertures between bmin and bcritical. Eklund and Stille (2008) studied how well a mesh simulates a constriction in a fracture. They tested the penetrability of cement-based grouts using both slot and mesh to simulate an aperture and showed that penetrability varies with different geometries. Slot geometry in this measuring method refers to a 62 mm
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long and 75–125 lm wide opening in an 0.5 mm thick metal plate. The diameter of the plate is 95 mm. Sandberg (1997) developed another type of measuring device where a fracture aperture is represented by two parallel metal discs without any constriction between them. This method is known as the NES-method. The method was further developed by Nobuto et al. (2008) where a slot between the discs is grouted with real grouting and mixing equipment where mixing, grouting and agitation are continuous. An important advantage of this method compared to the filter pump and penetrability meter is the possibility to use a relatively high pressure during grouting since research has shown that a higher grouting pressure improves penetrability (Eriksson et al. (1999), Hjertström (2001), Nobuto et al. (2008), and Draganovic´ and Stille (2011)). Axelsson and Gustafson (2010) developed a measurement device called the PenetraCone to determine the penetrability of cementbased grouts in the field. In this device, a slot also represents a fracture aperture as in the NES device. The difference is that the slot walls in this device are spherical instead of flat. The PenetraCone consists of two conical cylinders; an outer cylinder and an inner cylinder. By turning the inner cylinder, the gap between the inner and outer cylinders can be adjusted between 20 and 200 lm. Axelsson and Gustafson (2010) performed some comparison measurements with the PenetraCone and the penetrability meter but the results do not show any clear relation between the measurements. Axelsson and Gustafson (2010) were of the opinion that the slot formed by these two cylinders represents a fracture in a better way than a mesh. Draganovic´ and Stille (2011) developed a measuring device called the short slot. The short slot is similar to the NES device where grout is pressed through a slot between two parallel metal discs, but there is a significant difference between the two devices. In the short slot, the slot aperture is reduced. This aperture reduction represents a constriction in a fracture. There is no constriction between the discs used in the NES-method and in the authors’ opinion the slot geometry in the NES-method measures plugbuilding at an intersection between a fracture and a borehole rather than at a constriction in a fracture. 1.3. Objective of the paper A great many different measuring devices exist, indicating the difficulties to construct a device, which can correctly represent the filtration process in a rock fracture. Further, measurements with these devices showed different results regarding how factors such as w/c ratio, pressure, the cement’s grain curve, mixing, hydration and flocculation, and additives influence the penetrability of grouts, which is also an indication that all or some of them do not represent the filtration process satisfactorily. Measuring equipment with a pressure chamber with sand or steel mesh as a filter could not measure the influence of grout flow through a fracture before and after constriction when a plug forms. Nor could these methods follow the plug-building process over time. NES and short slot were able to follow the plug-building process over time but could not measure the influence of grout penetration before and after a constriction. The aim of this paper is to present results from measurements of the plug-building or the filtration process in a long slot where a constriction is placed a relatively long distance from both the ‘‘borehole‘‘ and the end of the slot. The results are compared with measurement with penetrability meter and short slot. 1.4. Bingham flow and plug-building at a fracture constriction The flow of cement-based grout through a fracture can be described as a flow of a Bingham fluid characterized by viscosity
and yield stress e.g. Lombardi (1985) and Håkansson et al.(1992). The shear stress s developed during penetration can be described by Eq. (1),
s ¼ s0 þ lc
ð1Þ
where s0 is the yield strength of the grout, l the plastic viscosity, and c the shear rate. Fig. 1 illustrates shear stress, yield strength, and velocity profile of a Bingham fluid flowing through a fracture with a constriction. A solid core is formed in the middle of the fracture where shear stress developed in the grout is less than yield strength. Closer to fracture walls the shear stress exceeds the yield strength and the grout is sheared. The shear stress is largest in contact with the fracture wall. The velocity of the grout is larger after fracture constriction, V2(z) compared to before V1(z) since the flow is constant. This causes a higher shear stress in the grout and the thickness of the solid core is reduced. Therefore, during the flow from larger to smaller fracture, the thicker solid core must be destroyed, which causes an extra pressure loss. Fig. 1 also shows that the gradient or rate of pressure loss, (Dp/Dl) vary before, at, and after a constriction. The rate of pressure loss in zone 2 is much higher than in zone 1 or 3 due to constriction. The grout in zone 2 has a higher velocity, i.e. higher shear, than in zone 1. Extra energy is also needed to destroy the relatively thick solid core. The pressure loss in zone 3 is higher than in zone 1 since the shear rate is higher. The rate of pressure loss (Dp/Dl) at the constriction will also increase over time in case of filtration or plug-building. When grout passes a constriction in a fracture there is a risk of plug-building, which might cause the flow to stop. Particles in the grout can initiate plug-building and build an arch of particles at the constriction as illustrated in Fig. 2. Plug-building is a complex process and mainly a function of the cement grain-size curve, w/c ratio, tendency to flocculate and pressure (Draganovic´ and Stille, 2011; Eklund and Stille, 2008). Draganovic´ and Stille (2011) showed that the relationship between critical fracture aperture (bcritical) and d95 varies between 2 for coarser and relatively finegrained cements and 21 for very fine-grained cements due to a larger degree of hydration and flocculation. A similar relationship was also shown by Eklund and Stille (2008). The cement with d95 around 30 lm showed the best penetrability. A higher w/c-ration also gives a better penetrability since grain concentration is lower. Grouting pressure also influences penetrability. Higher pressure reduces the risk of arching and increased erosion of the plug at the constriction as shown in Draganovic´ and Stille (2011). Plug-building decreases flow velocity, i.e. decreases shear rate in the grout. During plug-building, the pressure in front of the constriction will increase and decrease after the constriction. The pressure loss in zones 1 and 3 will be reduced and the main pressure loss will occur in zone 2, i.e. at the constriction. This will be described in more detail in sub-chapter 2.1. 1.5. Influence of pressure change on plug-building at a constriction During grouting, grout penetrates from a borehole into a fracture and continues to penetrate through it. Through this penetration, grout can meet a fracture constriction at any distance from the borehole and plug-building might cause the flow to stop. When the grout front reaches the first constriction, substantial pressure can be mobilized to erode a plug in case of plug-building since the accumulated shear stress along the fracture wall at this time is then relatively small. Pressure distribution in a fracture at this moment is illustrated by curve t1 in Fig. 3. When the grout front passes the constriction, illustrated by curve t2, a pressure loss at the constriction will occur. The rate of pressure loss (Dp/Dl) will be higher at the constriction than in the fracture before constriction. From this moment, a pressure difference (DP = P2 – P3) will exist at the constriction. This pressure
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Fig. 1. Flow of a Bingham fluid in a fracture with a constriction.
Fig. 2. Plug-building in a fracture with a constriction.
difference over time without filtration at the constriction is illustrated by DPt2, DPt3, and DPt4 in Fig. 3. With further penetration, accumulated shear stress increases and flow velocity decreases. A lower flow velocity gives a lower shear rate, which causes a decrease in the pressure difference at the constriction. This means that further penetration might increase the risk of plug-building at the constriction. The pressure difference at the constriction will change, i.e. increase over time in case of filtration or plug-building. This might also influence plug-building and erosion. The distance between constriction and borehole could also influence the risk of plug-building at a constriction. A longer distance will give a lower pressure for erosion of a partially formed plug and might increase the risk of plug-building. 2. Material and methods 2.1. Description of the long slot and definition of filtration when measuring using long slot The long slot was 4 m long and 100 mm wide. A longitudinal section of the slot is illustrated in Fig. 4a. The aperture of the first
two meters of the slot is 0.5 mm, which then changes to 1 mm along a length of 30 mm. The aperture is then reduced to 75 lm. The reason for having a length of 30 mm before the reduction of the aperture of 1 mm is to have a similar geometry to the short slot. Fig. 4b illustrates plug-building at a constriction during grouting. When grouting begins, grout flows without accumulation of grains at the constriction (t = t1). After a while, some grains stick at the constriction, which initiates accumulation of the grains. The accumulation continues and the plug grows and at the end a complete plug has formed, stopping the flow (t = t4). Plug-building can start anywhere at the constriction but often starts at the edges of the slot, where velocity is lowest. An uncompleted plug could be eroded during grouting, as illustrated in Fig. 2. This can be observed from the pressure measurements. A sudden decrease in pressure in front of the plug and a sudden increase in pressure behind the plug indicate erosion of the plug. Four characteristic places in the slot are chosen to measure pressures during penetration, as shown in Fig. 4a. The measured pressures are: P1, the pressure in the grout container; P2, the pressure measured 7.5 mm before the constriction; P3, the pressure measured 22.5 mm after the constriction; and P4, the pressure measured 52.5 mm from the end of the slot. The diameter of the holes where pressure-gauges P2, P3, and P4 are located is 3 mm. The graphs in Fig. 4c and d illustrate the development of the pressures P1, P2, P3, and P4 during grouting in cases without plug-building and with plug-building when the grout is filtered. In the case without plug-building, all pressures remain constant during the entire grouting time after steady state. In the case with plug-building, the pressures change when plug-building occurs and grout is filtrated until the full flow stop occurs. The beginning of filtration of the grout is detected by the increase in pressure P2 and the decrease in pressures P3 and P4. A grooving filter cake reduces the flow area at the slot constriction and causes a decrease in flow. Due to the lower flow, the pressure loss is reduced in the slot before the constriction and this causes an increase in P2 and a decrease in P3 and P4. The filter cake or plug may or may not reach the P2 pressure gauge before the flow stops. In cases when the flow stops before the filter cake reaches the P2 pressure gauge, pressure P2 will reach pressure P1 and pressures P3 and P4 will fall to zero. In cases
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Fig. 3. Pressure distribution in a fracture during grouting with a cement-based grout. Curve t4 illustrates pressure distribution along the fracture just before pressure equilibrium, i.e. flow stop.
when the filter cake reaches the P2 pressure gauge before the flow stops completely, pressure P2 will begin to decrease from the moment when the filter cake reaches the P2 pressure gauge. Penetrability measurements with short slot and penetrability meter were also carried out in this study. Detailed descriptions of these two methods can be found in Draganovic´ and Stille (2011) and Eriksson and Stille (2003) respectively. The main differences of the used methods are listed in Table 1. 2.2. Performed tests, cements used, and mixing A clear and long filtration process in the long slot during penetration is a desired result since the possibility to measure the filtration is one of the aims of this study. An aperture of 75 lm in the long slot is therefore chosen on the basis of measurements performed using a short slot (Draganovic´ and Stille, 2011). 75 lm is judged to be close to the critical aperture for the grouts based on micro-cement used in this study. Tests using the long slot are also carried out to study the pressure losses in the slot before, at and after the constriction. The cements used in the study were Injektering 30 (INJ30), Microfine 20 (MF20), and Ultrafin 12 (UF12). INJ30 is a relatively fine-grained cement with a d95 of 32 lm and a surface area of 1300 m2/kg. MF20 is a very fine-grained cement with a d95 of 20 lm and a surface area of 2560 m2/kg and UF12 is also a very fine-grained cement with a d95 of 12 lm and a surface area of 2200 m2/kg. Surface area is measured using the BET method, i.e. the nitrogen absorption method. INJ30 and UF12 are based on the same clinker minerals as ordinary Portland cement. The raw material for MF20 cement is Byggcement, which is a Portland cement of type CEM II. In addition to the cement clinker, Byggcement also contains approximately 13% limestone. The tests performed with the chosen grouts are listed in Table 2. The tests with the long slot were repeated twice to investigate the repeatability. The grouts were chosen on the basis of measurements performed using short slot as presented in Draganovic´ and Stille (2011). The w/c ration was chosen to fulfill normal requirements to get a stable grout and with the best penetrability. All grouts are mixed at high speed for 3 min using a high-shear field mixer of colloidal type. The grouting pressure of 15 bar (1.5 MPa) was chosen because it is in the range of often used grouting pressures in the field
(Holmberg et al. (2012), Hernqvist et al. (2009)). Mixing of grout and size of grouting pressure is as in real grouting. The grout from grout container is pressed with compressed nitrogen. See Fig. 4a. The grouting pressure during grouting time is constant i.e. is not pulsing as in real grouting. Pressure gouges used to measure pressures P1, P2, P3 and P4 are SML 10.0/0–25 bar/4–20 mA from Thermokon-Danelko. They are calibrated against pressure gauge at pressure regulator mounted at gas container (Fig. 4a). The same type of pressure gauges are used in test set up in Draganovic´ and Stille (2011) and worked properly. Rheological properties and density of tested grouts are presented in Table 3. Since possibilities to measure rheological properties at the same time during tests with long slot are limited, these properties and density were later measured in the laboratory. Yield strength and plastic viscosity were measured with a Brookfield LV-II+ Programmable rheometer. The measurements were performed with small sample adapter SC4-13R and spindle SC-34 for INJ30 w/c = 0.6 grout and with spindle SC-31 for MF20 w/c = 0.9 and UF12 w/c = 1.2 grouts. Densities were measured with mud balance model 140, Fann Instrument Company. 3. Tests and results 3.1. Results of the tests performed using long slot, short slot, and penetrability meter 3.1.1. Penetrability test using INJ30 w/c=0.6 grout The results of a penetrability test with INJ30 w/c = 0.6 grout performed with the long slot are shown in Fig. 5. The four curves in the graph show pressures P1, P2, P3, and P4, as illustrated in Fig. 4d. By opening valve V1 (Fig. 4a), the pressure in the grout container (P1) jumps to a grouting pressure of approximately 15 bar. The grouting of the slot is begun by opening valve V4. Pressure P2 (the pressure before the constriction) then rises to approximately 13.5 bar, P3 (the pressure after the constriction) to approximately 13 bar, and P4 (the pressure close to the outlet of the slot) to approximately 4 bar. The grouting continues for approximately 80 s to time = 105 s, when the gas from the gas container reaches the outlet of the slot. This is indicated by a sudden increase in pressure P4 and the measuring is terminated by closing valve V4, and P4 then falls to zero.
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Fig. 4. Test of the grout filtration in the long slot by measuring some characteristic pressures. Sketch (a) illustrates a longitudinal section of the long slot with four measured pressures. The sketches in (b) illustrate plug-building in time at the constriction in the middle of the slot during grouting. The graphs (c) and (d) illustrate the pressures during grouting in cases without plug-building and with plug-building.
Table 1 Significant properties of the used methods. Method
Type of opening
Aperture of the constriction
Total area of the constriction
Average gradient over the equipment
Max volume
Long slot Short slot Penetrability meter
Slot Slot Wire mesh
75 lm 75 lm 75 lm
7.5 mm2 9.0 mm2 166 mm2
1.5 Mpa/4 m = 0.375 MPa/m 1.5 MPa/0.15 m = 10 MPa/m 0.1 MPa/0.15 m = 0.667 MPa/m
1.7 l 1.7 l 1.0 l
Table 2 Tests performed using long slot, short slot and penetrability meter. Grout
INJ30, w/c = 0.6 MF20, w/c = 0.9 UF12, w/c = 1.2 a
Measuring equipment
Table 3 Measured rheological properties and density of tested grouts. Yield strength (Pa) Plastic viscosity (Pas) Density (g/cm3)
Long slot
Short slot
Penetrability meter
Grout
x, x x, x x, x
x x x
x x xa
INJ30, w/c = 0.6 11.00 MF20, w/c = 0.9 5.02 UF12, w/c = 1.2 3.64
0.3750 0.0596 0.0378
1.72 1.40 1.37
30 min old grout.
When the grout front reaches the constriction, pressures P2 and P3 jump to approximately 13.5 and 13 bar, respectively. They then begin to increase slowly over the penetration time until the front of
the grout reaches the end of the slot, which is when P4 jumps to 4 bar. Pressures P2 and P3 continue to increase for a short period of approximately 10 s.
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Fig. 5. Penetrability test performed using the long slot and INJ30 w/c = 0.6 grout.
After this period a first filtration process of the grout is observed and marked in Fig. 5. Pressure P2 continues to slowly increase while P3 and P4 begin to slowly decrease. This filtration process continued for approximately 15 s to around 50 s in the graph. This filtration process follow period of pressure changes as predicted in Fig. 4d before the filter cake reaches pressure sensor P2. Later, pressures P2, P3, and P4 remain constant for a new period of approximately 10 s. This period of constant pressures indicates that the filtration of the grout is currently stopped and the flow during this time is constant. Then, at t 64 s, a small fall in pressure P2 and a small jump in pressures P3 and P4 occur. This could be a result of the sudden erosion of a partially built plug. Following the sudden erosion, a new period of filtration begins and continues until t reaches 80 s. This filtration period is also indicated by a slow increase in P2 and a slow decrease in P3 and P4. From the time of 80 s, all the pressures were in principle constant, which might mean that flow was steady without any further filtration until the gas reached the outlet. When gas reaches the end of slot in a test, the test is classified as 100% of the grout having passed the slot even if some grout remains in the slot. The grout container is empty. This test is classified as 100% of the grout having passed the slot. In a repeated test using the same grout, i.e. a grout with the same recipe, all the grout once again passed the slot. In this test, the filtration did not begin early as in the previous test but approximately 45 s after the commencement of the grouting. The filtration period marked in the graph in Fig. 6 is also indicated by a slow increase in P2 and a decrease in pressures P3 and P4. The sudden increase in pressures P3 and P4 at the end of the filtration period might be due to erosion, but pressure P2 should decrease while here it remains constant. Shortly afterwards, the gas reached the end of the slot, indicated by the P4 pressure jump to 4 bar. After approximately 5 s the grouting was stopped and pressure P4 fell to close to 0 bar. The partially formed plug in this test can be seen in Fig. 7. The holes in the upper plate are also marked where pressures P2 and P3 are measured. The rate of the pressure loss in the slot from these two tests is estimated at the beginning and end of the filtration processes. The results are shown in Table 4. The pressure loss in the slot after constriction, i.e. between pressure gauges P3 and P4, is approximately ten times larger than the pressure loss in the slot before constriction, i.e. between pressure gauges P1 and P2, due to higher flow
Fig. 6. Penetrability test performed using the long slot and INJ30 w/c = 0.6 grout.
velocity. The measured pressure loss at the constriction, i.e. between pressure gauges P2 and P3, is much larger than in the other parts of the slot. This is a reasonable result since that slot aperture is reduced 6.6 times. At the end of these filtration processes, the pressure loss at the constriction further increases due to plug-building, i.e. a further reduction in the flow area. The rate of pressure loss over time (Dp/Dt) is 2.11 and 1.46 bar/s in the respective test. The penetrability of the same grout was also tested using the short slot with a 75 lm aperture. The result of this test is shown in Fig. 8. Only 800 g, or 25%, of the grout passed the slot. This is a somewhat poorer result compared to the weight passed in tests with the long slot. The penetrability of this grout was further tested using a penetrability meter. This measurement also showed relatively poorer penetrability than tests with the long slot. The amount of grout passed through a 75 lm filter was only 60 ml and estimated bmin and bcritical were 68 lm and 172 lm, respectively. 3.1.2. Penetrability test using MF20 w/c = 0.9 grout The results of the first measurement performed with the long slot with MF20 w/c = 0.9 grout are shown in Fig. 9. All the grout passed the slot in approximately 18 s and for most of this time, the grout was to some extent filtered. The filtration began early but the pressure managed to keep the slot open for the flow. Some erosion occurred near the end of the grouting time and shortly afterwards the gas reached the outlet and pressure P4 jumped to 8 bar and the test was stopped.
Fig. 7. Photo showing the partially formed plug at the upper plate at the constriction in the test presented in Fig. 6 and holes where pressures P2 and P3 are measured.
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Table 4 Measured rate of pressure loss (Dp/Dl) (bar/m) in tests with INJ30 w/c = 0.6 grout. Test
Time
Between P1 and P2
Between P2 and P3 at constriction
Between P3 and P4
Fig. 5
At the start of the filtration, t = 36 s At the end of the filtration, t = 50 s At the start of the filtration, t = 55 s At the start of the filtration, t = 108 s
0.51
21.29
4.82
0.43
50.85
5.00
0.56
4.85
4.88
0.45
82.27
4.58
Fig. 6
Fig. 10. Penetrability test performed using the long slot and MF20 w/c = 0.9 grout.
Fig. 8. Penetrability test performed using the short slot with an aperture of 75 lm and INJ30 w/c = 0.6 grout.
ing time. This is the reason why the flow was much lower than in the previous test. At the end, after approximately 6 min, this channel was also plugged, which stopped the flow. The amount of grout passed was approximately 0.5 l or approximately 30% of the initial amount of grout in the container. During this test it was observed that the grouting pressure (P1) was 3 bar lower than the normal grouting pressure of around 15 bar. This could be the reason for the poorer penetrability. However, when grouting had proceeded for 200 s the pressure was adjusted to 15 bar but this probably did not subsequently influence penetration. On the other hand, the sudden drop in pressures P2 and P3 during grouting is difficult to explain. An electrical problem with the measuring equipment cannot be ruled out. The rate of pressure loss in the slot from the test presented in Fig. 9 is estimated and presented in Table 5. As in the tests with INJ30 w/c = 0.6 grout, the largest pressure loss is at the constriction. The pressure loss in the slot in front of and behind the constriction is somewhat lower in this test compared with INJ30 w/ c = 0.6 grout. This is a reasonable result since the grout has lower yield strength and plastic viscosity. The rate of pressure loss over time (Dp/Dt) is 3.10 bar/s. This indicates a faster filtration process compared to tests with INJ30 w/c = 0.6 grout. The rate of pressure loss in the test presented in Fig. 10 is not estimated since it is difficult to define a clear filtration process. Most of the flow takes place through the small channel as described above. This grout showed good penetrability when tested with the short slot. The grout passed the slot in 22 s, which can be seen in Fig. 12. This is the only grout where all the grout penetrates through the short slot. The measurement using the penetrability meter also showed that this grout has the best penetration ability of the three grouts. The volume passed with a 75 lm filter was 530 ml, which is almost nine times as much as the INJ30 w/c = 0.6 or UF12 w/c = 1.2 grouts. The estimated bmin is 47 and bcritical is 105 lm.
Fig. 9. Penetrability test performed using the long slot and MF20 w/c = 0.9 grout.
The results of the second measurement with the same grout are shown in Fig. 10. The filtration of the grout began immediately, which caused an increase in pressure P2 and a decrease in pressure P3. In this test, the plug was completely formed and erosion did not occur during grouting as in the previous test. The test was terminated after approximately 370 s (approximately 6 min). Photo 1 in Fig. 11 shows the lower plates at the slot constriction with a built plug of around 120 mm in length. A small channel can be observed in the middle of the plug. It seems that the flow of the grout took place through this small channel for most of the grout-
3.1.3. Penetrability test with UF12 w/c = 1.2 grout The last series of measurements using the long slot, the short slot, and the penetrability meter were performed with UF12 w/ c = 1.2 grout. The test results measured by means of the long slot are shown in Figs. 13 and 15. In the first measurement, the whole amount of grout passed through the slot in approximately 80 s. The measurement showed that plug-building or filtration began immediately, indicated by the increase in P2 and decrease in P3 and P4. The filtration proceeded for between 18 and 38 s and at the end of this period sudden erosion occurred.
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Fig. 11. Photo showing the built plug at the constriction at the lower plate in the test presented in Fig. 10. It can be seen that the plug was approximately 120 mm long and a channel in the middle of the plug is also visible.
Table 5 Measured rate of pressure loss (Dp/Dl) (bar/m) in test with MF20 w/c = 0.9 grout. Test
Time
Between P1 and P2
Between P2 and P3 At constriction
Between P3 and P4
Fig. 9
At the start of the filtration, t = 15 s At the start of the filtration, t = 26 s
0.45 0.30
32.23 66.41
3.48 3.16
Fig. 12. Penetrability test performed using the short slot with an aperture of 75 lm and MF20, w/c = 0.9 grout.
Fig. 13. Penetrability test performed using the long slot and UF12 w/c = 1.2 grout.
After the sudden erosion there was another short period of filtration. The second filtration is followed by a period when pressure P2 in principle remained constant while P3 was mainly decreasing the rest of the time with a large variation. This period ended when gas reached the end of the slot, indicated by a sudden small jump in P4. The measurement was terminated and pressures fell. This large variation in pressure P3 may have been caused by frequent changes between plug-building and the erosion of the partially built plug. This conclusion is based on the observed pulsation of the outflow at the outlet, which was filmed. The plug from this test is shown in photos 1a (lower plate) and 1b (upper plate) in Fig. 14. In the middle of the plug, a small channel can be seen. This channel was probably open for the entire grouting time and gas passed through it at the end of the measurement. When injection was stopped by closing valve V4
at time 100 s, pressure P2 did not fall to zero but decreased to 6 bar and pressure P3 to 1 bar over a period of approximately 15 s. During this time, the small channel in the middle of the cake was completely plugged. A channel in the middle of the slot along its entire length can be seen in photo 2 in Fig. 14. This channel is the result of gas erosion at the end of the test. The test is classified as 100% of grout having passed the slot. It should be noted that pressure P2 should be somewhat higher than P3 when the grout front passes the slot constriction. The measurements at the beginning showed the opposite. This could mean that some of the P2 or P3 pressure gauges showed a higher or lower pressure but the important thing is the trend of the measured pressures. This is also the reason why the pressure loss in the slot from this test is not estimated.
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Fig. 14. Photos showing the long slot after the test presented in Fig. 13.
Fig. 15. Penetrability test performed using the long slot and a UF12 w/c = 1.2 grout.
The results of the repeated test with the same grout are shown in Fig. 15. In this measurement, the filtration of the grout also began immediately as in the previous test. The difference was that in this test the filtration continued for the entire grouting time. No erosion of the plug occurred and the plug was growing during the entire grouting. A certain outflow at the outlet was observed over the whole grouting time. According to pressure P2, the plug reached pressure gauge P2 at t = 36 s, which is detected by the commencement of a decrease in pressure. The filter cake continued to grow. At time t = 155 s, the measurement was terminated even though full flow stop was not achieved and only approximately 25% of the grout had passed the slot. The filter cake from this test is shown in photos 1, 2, and 3 in Fig. 16. Photos 1 and 2 show the filter cake at the upper plate and photo 3 shows part of the filter cake at the lower plate. Photo 2 also shows the places where pressures P2 and P3 were measured. It can be seen that the filter cake passed the P2 gauge by approximately 50 mm, confirming the previous explanation that pressure P2 begins to decrease when the filter cake passes pressure gauge P2. Photo 4 shows the lower plates after the test. It can be seen that even during plug-building, the slot after the constriction could be partially grouted by this grout.
Fig. 16. Photos showing the long slot after the test presented in Fig. 15. Photos 1 and 2 show the plug at the upper plate and photo 3 shows part of the filter cake at the lower plate. Photo 2 also shows the distances in mm where pressures P2 and P3 are measured. Photo 4 shows the long slot after the test.
The result of the penetrability test measured using the short slot is shown in Fig. 17. 2250 g of the grout passed the slot in approximately 35 s with a constantly decreasing rate of outflow, which indicates filtration. There are similarities between this test and tests performed with the long slot where grout was also filtered from the beginning of grouting. The measurement with the penetrability meter showed poor penetrability. The volume passed measured with a 75 lm filter was 78 ml and is approximately the same result as in the test with the INJ30 w/c = 0.6 grout. The estimated bmin is 49 and bcritical is 419 lm.
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Fig. 17. Penetrability test performed using the short slot with an aperture of 75 lm and UF12 w/c = 1.2 grout.
4. Discussion of the test results measured by the long slot, the short slot, and the penetrability meter 4.1. Comparison of the results measured with different measuring equipment Table 6 shows the summarized results of the measurements performed using the long slot and the short slot with a 75 lm aperture and the penetrability meter with a mesh with 75 lm openings. The test results estimated with the three different measuring methods are presented as percentages of grout passed. Comparison of these results between the methods is complex due to different flow geometries and different initial grout volumes. The measurements showed some different results between measurements using the long slot and the short slot. The INJ30 w/c = 0.6 grout showed the best penetrability measured by means of the long slot while the MF20 w/c = 0.9 grout showed the best penetrability when measured by means of the short slot and the penetrability meter. In almost all tests performed with the long and short slots, the grouts were evidently filtered for a longer time. In spite of filtration, a significant amount of the grout had passed through the slot in almost all the tests. Therefore it has been estimated that the narrowest fracture aperture to be grouted is close to 75 lm as initially was judged based on previous tests. This is also supported by the fact that different results were achieved when the tests were repeated with the long slot with the MF20 w/c = 0.9 and UF12 w/ c = 1.2 grouts. The only exception is the measurement with short slot with MF20 w/c = 0.9 grout where all grout passed the short slot without filtration. More testing with the long slot with varying slot aperture is needed to be able to better estimate the narrowest fracture aperture to be sealed.
Analysis of the results is difficult due to the randomness of the filtration processes. Since the flow geometry is similar, the comparison between the results estimated using the long and the short slots is easier than the comparison between the results estimated using these two methods and the penetrability meter. One way to compare these results is to show the amount of grout passed per flow area at the outlet (ml/mm2). The long slot has a flow area at the outlet of 7.5 mm2 and the short slot 9 mm2. The penetrability meter has a much larger outlet. The diameter of the outlet is 24 mm and the diameter of the wires used to manufacture a mesh of 75 lm is 50 lm. This gives an effective flow area at the outlet of 166 mm2 for a 75 lm mesh. The effective flow area at the outlet means that the area of the wires is excluded. The test results in Table 7 are presented as volume of grout passed divided by the area of the outlet. The results where the whole amount of the grout passed the slot are also marked as 100%, which means that even more grout would have passed through the slot if the grouting container had been larger. In all tests with the penetrability meter the flow was terminated due to complete filtration. This occurred after very small volumes of grout had passed the constriction compared to the passed volumes at the other tests. This might mean that the penetrability meter significantly underestimates the penetrability of the grouts. The measurements with INJ30 w/c = 0.6 performed using the long slot showed much better penetrability than measurements performed with the short slot. The comparison between the measurements performed using the long and the short slots for the MF20 w/c = 0.9 and UF12 w/c = 1.2 grouts is somewhat more complicated since the results measured with the long slot varied. However, the mean values of the results from the long slot are only slightly better than the results measured with the short slot. The flow regime for all three methods was roughly estimated. Reynolds number in the long slot after constriction for all three grouts was estimated to be between 1 and 35. This indicates laminar flow. The slot aperture before constriction is larger and the flow must be also laminar. The flow regime at the constriction is difficult to estimate. Especially in case of plug building or erosion the flow could be turbulent. Reynolds number in short slot and penetrability meter was somewhat higher but still indicates lami-
Table 7 Results shown in Table 6 shown as the volume of grout passed divided by the area of the outlet [ml/mm2] measured with 75 lm slot aperture and 75 lm mesh width. Grout
Long slot (ml/mm2)
INJ30, w/c = 0.6
226 (100%) 226 (100%) 226 (100%) 66 226 (100%) 56
MF20, w/c = 0.9 UF12, w/c = 1.2
Short slot (ml/mm2)
Penetrability meter (ml/mm2)
51
0.36
171 (100%)
3.19
173
0.47
Table 6 Amount of grout passed measured with the long and short slots with a 75 lm aperture and the penetrability meter mixed with a field mixer. 100% in the long and short slots corresponds to 1.7 liters while in the penetrability meter it corresponds to 1 liter. Grout
Long slot V = 1.7 l
Short slot V = 1.7 l
Penetrability meter Amount passed for 75 lm mesh width (ml,%), bmin–bcritical (lm–lm)
INJ30, w/c = 0.6
100% passed with filt. 100% passed with filt. 100% passed with filt. 30% passed with filt. 100% passed with filt. 25% passed with filt.
25% passed (stop after 5 s)
60 ml or 6% Filt. 68–172
100% passed without filt.
530 ml or 53% Filt. 47–105
95% passed (filt. during whole penetration time of 40 s)
78 ml or 8% Filt. 49–419
MF20, w/c = 0.9 UF12, w/c = 1.2
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nar flow. In real grouting pressure is pulsing. It is difficult to evaluate which flow give higher risk for plug building. The measurements using the long slot (Figs. 5, 9 and 13) indicated erosion with a sudden decrease in pressure in front of the constriction and a sudden increase in pressure after the constriction. The erosion could also occur in the short slot, which was indicated by the measurements with varying pressures presented in Draganovic´ and Stille (2011). These observations suggest that erosion could also be a process which takes place during penetration of the grout through the fractures and higher pressure may increase the erosion. It was expected that the pressure loss in the long slot before and after the constriction might reduce the penetrability of the grouts compared to flow through the short slot. This reduction in penetrability was not experimentally shown by comparing the results presented in Table 7. The results rather showed the opposite. The long slot could not be used for daily measurement of the penetrability of the grouts but the measurements using this method are probably closest to the truth, i.e. based on engineering judgment, the long slot simulates penetration through a fracture better than measurements with the short slot or the penetrability meter. The variation in the results measured with the long slot made this evaluation difficult and uncertain. The differences between the long and short slots are not significant. However, it can be concluded that the penetrability meter underestimates penetrability. More measurements with all three methods need to be carried out in order to obtain a better prediction. 4.2. In-laboratory measured penetrability related to fracture aperture The ultimate objective of testing penetrability in laboratory is to establish if a given grout will penetrate the required lowest fracture aperture related to design requirements. The complexity of the fracture geometry and variations implies that testing in laboratory always will be indirect and a simplification of reality. It means that the results based on testing in laboratory have to be validated against observations of penetrability of rock fractures in the field. The main objective of this investigation is to study the penetrability in laboratory with some different methods. Validation against field data is outside the scope of the work. However, the complexity of such investigation is shortly discussed below. Penetrability of grout is related to the size of the grains in comparison with the fracture aperture. It therefore seems reasonable to assume that the physical apertures of the fracture will be representative for evaluating the penetrability. The physical aperture can be characterized by the arithmetic mean value. The geometry of fractures is described by the aperture, roughness and contact area and theirs spatial correlations. The focus of many investigations has been both related to mechanical properties and water bearing capacities. One main issue is the relationship between the geometry of the apertures and the hydraulic aperture measured by water loss measurements or other types of flow measurements. Many researchers have contributed like e.g. Hakami (1989), Zimmermann and Bodvarsson (1996), Barton and Quadros (1997). They found that the measured hydraulic aperture is less than the arithmetic mean aperture. This conclusion has been confirmed in later research e.g. Tsuji et al. (2013) showing that the aperture defining the grout take always is greater than the hydraulic aperture. Penetrability of cement based grout into sand column, filter stone or mesh seems to be far from penetrability into a rock fracture. The geometry of the voids or openings has no similarity with a fracture. Comparison, like those in this study, indicates that it is easer for the grout to penetrate a slot instead of a mesh. Measurements also indicate that the penetrability can occur even if filtration
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of the grout is ongoing. The two defined slot apertures bmin and bcritical describing the filtration process can therefore be regarded as upper and lower bounds of the penetrability. Important issues to be further investigated is how (i) the hydraulic aperture is related to the arithmetic mean of a fracture and how well (ii) the arithmetic mean represents the true penetration and (iii) the evaluated penetrability in the laboratory testing will reflect the penetrability of a real fracture. 5. Conclusions Tests using the long slot showed that the filtration process could be observed by pressure measurements along the slot. The measurements also showed that erosion of the partially formed plug would occur during plug-building. The observed behavior indicates that the narrowest aperture to be grouted without filtrations for all three measured grouts is close to 75 lm. The poor repeatability of the long slot tests indicates that the plug-building process is randomn in its nature. It was further observed that the grouts more easily penetrated in the long slot compared to what was measured with the short slot and penetrability meter with a similar aperture. The results measured by means of the long slot are probably closest to the truth. Based on engineering judgment, the long slot simulates penetration into a fracture better than the other two methods. The short slot gives somewhat conservative results. The conclusion is that measurements of penetrability with short slots are more practical and give reasonable good results. The long slot could not be used for daily measurements. The penetrability meter, which maybe is the easiest method to use, will underestimate the penetrability of the grouts. The number of tests was limited and more measurements with the long slot method are needed to obtain reference values for a deeper discussion of the suitability of measurements with the short slot and the penetrability meter. References Axelsson, M., Gustafson, G., 2010. The PenetraCone, a new robust field measurement device for determining the penetrability of cementitious grouts. Tunn. Undergr. Space Technol. 25, 1–8. Draganovic´, A., Stille, H., 2011. Filtration and penetrability of cement-based grout: study performed with a short slot. Tunn. Undergr. Space Technol. 26, 548–559. Barton, N., Quadros, E.f., 1997. Joint aperture and roughness in the prediction of flow and groutability of rock masses. Proc. of NY Rocks´97. Linking Science to Rock Engineering. In: K. Kim (Ed.), Int. J. Rock Mach. and Min. Sci., 34(3–4), pp 907–916. Eklund, D., Stille, H., 2008. Penetrability due to filtration tendency of cement-based grouts. Tunn. Undergr. Space Technol. 23, 389–398. Eriksson, M., Friedrich, M., Vorschulze, C., 2004. Variation in the rheology and penetrability of cement-based grouts – an experimental study. Cem. Concr. Res. 34, 1111–1119. Eriksson, M., Stille, H., 2003. A method for measuring and evaluating the penetrability of grouts. ASCE Publ., 1326–1337. Eriksson, M., Dalmalm, T., Brantberger, M., Stille, H. 1999. Separations- och filtreringsstabilitet hos cementbaserade injekteringsmedel. In: Department of Soil and Rock Mechanics, KTH, Stockholm. KTH Rapport 3065 in Swedish. Hakami, E. 1989. Water flow in single rock joints. Licentiate thesis. Luleå University of Technology, Sweden. Hansson, P. 1995. Filtration stability of cement grouts for injection of concrete structures. In: Proc. IABSE Symposium, San Francisco, pp. 1199–1204. Hernqvist, L., Fransson, Å., Gustafson, G., Emmelin, A., Eriksson, M., Stille, H., 2009. Analyses of the grouting results for a section of the APSE tunnel at Äspö hard rock laboratory. Int. J. Rock Mech. Min. Sci. 46, 439–449. Hjertström, S. 2001. Microcement-pentration versus particle size and time control. In: 4th Nordic rock grouting symposium, Stockholm: SveBeFo Rapport 55, pp. 61–71. Holmberg, M., Tsuji, M., Stille, B., Stille, H., 2012. Evaluation of Pre-Grouting for the City Line Project Using the RTGC Method. Eurock, ISRM International Symposium, Stockholm. Håkansson, U., Hässler, L., Stille, H., 1992. Rheological properties of microfine cement grouts. Tunn. Undergr. Space Technol. 7 (4), 453–458. Lombardi, G., 1985. The Role of Cohesion in Cement Grouting of Rock. Quinzième Congres des Grandes Barrage, Lausanne.
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Nobuto, J., Nishigaki, M., Mikake, S., Kobayashi, S., Sato, T., 2008. Prevention of clogging phenomenon with high-grouting pressure. Doboku Gakkai Ronbunshuu C 64 (4), 813–832 (in Japanese with English abstract). Sandberg, P. 1997. NES-metod för mätning av injekteringsbruk inträngningsförmåga, Svensk Bergs- & Brukstidning. Schwarz, L. 1997. Roles of Rheology and Chemical Filtration on Injectability of Microfine Cement Grouts. Dissertation Thesis, Northwestern University, Evanstone, Illinois.
Tsuji, M., Holmberg, M., Stille, B., Rafi, J., Stille, H. 2013. Optimization of the grouting procedure with RTGC method. Data from trial grouting at City Line project in Stockholm. R-12-16, SKB Svensk Kärnbränslehantering AB, Stockholm. Widmann, R., 1996. International society for rock mechanics, commission on rock grouting. Int. J. Rock Mech. Min. Sci. Geomech. Abstr. 33 (8), 803–847. Zimmermann, R.W., Bodvarsson, G.S., 1996. Hydraulic conductivity of rock fractures. Transp. Porous Media 23 (1), 1–30.