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Model tests on installation techniques of suction caissons in a soft clay seabed
Applied Ocean Research 34 (2012) 116–125
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Model tests on installation techniques of suction caissons in a soft clay seabed Zhen Guo, Lizhong Wang ∗ , Feng Yuan, Lingling Li College of Civil Engineering and Architecture, Zhejiang University, Hangzhou, China
a r t i c l e
i n f o
Article history: Received 4 December 2010 Received in revised form 22 July 2011 Accepted 19 September 2011 Available online 24 October 2011 Keywords: Suction caisson Soft clay Dead weight Suction pressure Soil heave Intermittent pumping
a b s t r a c t A series of model tests have been carried out in order to investigate the behavior of suction caissons during installation in a soft clay seabed and explore more effective installation techniques. The test results indicate that when the suction pressure instead of dead weight is adopted to help penetrating the caisson, a greater resistance may be encountered at a large depth. During the process of suctionassisted penetration (SP), all the soil displaced by the caisson wall flows inwards, and even more volume of soil would enter into the caisson before the reverse bearing failure at the caisson tip occurs. This phenomenon would result in a larger soil heave, which makes the final insertion depth of the suction caisson less than the target depth. Although plug failure occurs during the SP process it is possible to install the caisson further, and the inverse bearing capacity failure could be progressive. The test result illustrates that different initial penetration depths by dead weight have little influence on final heights of soil heave inside the caisson. It is beneficial to suppress the development of soil heave by releasing the suction inside the caisson at intervals during the SP process. Adopting a new method of intermittent pumping can effectively reduce the soil heave but not increase the time for caisson installation in a soft clay seabed. Crown Copyright © 2011 Published by Elsevier Ltd. All rights reserved.
1. Introduction In recent years, suction caissons have been increasingly used as an effective solution for anchoring in deep water. A suction caisson is a large cylinder structure, which is usually made of steel, openended at the bottom and closed at the top. The suction caisson can be installed by first allowing it to penetrate under its dead weight (self weight and ballast), and then pumping out the entrapped water to cause further penetration of caisson to the desired depth. These two ordinal phases can be generally named as “dead-weight penetration” (DP) and “suction-assisted penetration” (SP), respectively. Most previous works have stressed on the SP process, during which sufficient pressure differential on the top cap should be guaranteed to overcome the seabed resistance, and too large soil heave inside the caisson should be avoided in order to achieve the desired penetration depth. The seabed resistance is not only decreased by the remolding of surrounding soil around the caisson wall, but also influenced by the underpressure (suction pressure) inside the caisson. In a sandy seabed, the suction pressure could induce seepage flow through the soil, which will result in a reduction in the effective stress at the caisson tip and along the inner wall, and thus reduce the seabed resistance. This characteristic of sandy seabed can evidently
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promote the installation of suction caisson [1–4]. In clayey soil, both API RP 2SK [5] and DNV RP-E303 [6] have presented the similar formulae for calculating the required suction pressure. Recommended formulae just take the suction pressure acting on the top cap as an equivalent vertical force, but its influence on the surrounding soil is ignored. Houlsby and Byrne [7] suggested that the inner underpressure would reduce the end bearing on the annular rim of the caisson. EI-Sherbiny [8] carried out a series of suction caisson installation tests in soft kaolin clay, and concluded that the resistance was in close agreement for insertions by suction and for insertions by dead weight, but the side friction acting on the caisson walls during insertion by dead weight was larger than that during insertion by suction. This was thought to be mainly due to the lower internal side friction during suction insertion, while the side friction on the outer tube was practically unaffected by the applied suction. Cao et al. [9] and Rauch et al. [10] have performed some model tests on the installation of suction caisson in soft clay, and all their test results suggested that the resistance for insertions by suction would be obviously larger than that for insertions by dead weight. The seabed resistance encountered during the SP process is critical to determine how to apply suction pressure to achieve the caisson installation, but the above mentioned references show some controversies over the magnitude of soil resistance with different installation methods. Another focus of previous works is the heave of the interior mudline during the SP process, which is generally caused by the inward flow of soil near the caisson tip. Large applied suction
0141-1187/$ – see front matter. Crown Copyright © 2011 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.apor.2011.09.004
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Fig. 1. (a) Photograph and (b) sketch of the model caisson.
pressure will result in an excessive soil heave inside the caisson, and make the caisson top cap contact the interior mudline so quickly that the desired penetration depth cannot be achieved. It was the earliest experience of unanticipated soil heaves during the SP process in the Gorm Field in 1980 [11,12]. In that case, the suction caisson was installed in a sand layer followed by soft clay on top of stiff clay, and the large soil heave was mainly due to the liquefaction of the interior sand under a great suction pressure. For a soft clay seabed, the excessive soil heave may also appear as a result of the instability of soil near the caisson tip. API RP 2SK [5], DNV RP-E303 [6] and Houlsby and Byrne [7] gave the calculation equations for the allowable suction pressure, which is the critical underpressure causing a general reverse bearing failure at the caisson tip. According to API RP 2SK [5] and DNV RP-E303 [6], the volume of soil heave during the SP process is just equal to the volume of soil displaced by the caisson wall. Rauch et al. [10] and Whittle et al. [13] suggested that almost 100% of soil displaced by the wall moves inwards during the SP process. Andersen et al. [14] performed a centrifuge study on suction caisson installation and found that at a penetration depth of nearly half the maximum penetration depth, the volume of the soil heave inside the caisson actually increased more than the volume of the displaced soil. On the basis of model tests in normally consolidated kaolin clay, House [15] reported that all the soil displaced by the caisson wall would flow into the caisson during the early SP process (before the depth of about 4–5 diameters), and afterwards the failure of inner soil plug would occur and the final height of soil heave can be as large as 30% of the caisson length. In contrast, Newlin [16] stated that the amount of soil drawn into the caisson during suction installation is much less than 100% of that displaced by the caisson wall, and Chen et al. [17] also indicated that only about half of the caisson wall is accommodated by inward flow of the soil. So, the process and volume of soil upheaval need further clarification with more lab tests. Traditionally, the suction caisson is installed by continuous pumping after having been inserted by dead weight. Either the seabed resistance or the interior soil heave can be affected by the operations in the DP and SP processes, such as different initial DP depth and different methods of water pumping. Allersma et al. [18,19] have presented a new method, called “percussion technique”, to install suction piles by creating short pressure pulses on
the top cap. The pulse time is ranged between tens and hundreds of milliseconds. This percussion technique has proved to be effective for reducing the soil heave, especially in coarse sand and layered soil (clay overlying sand). But, it also suggested that there was little difference for caisson installation in clay between by continuous pumping and using percussion technique. Using a specially designed model experimental system, several small scale tests are carried out in over-consolidated clay. This paper presents test data of caisson installation by different operation techniques as well as the interpretations. By systematically studying the effects of different operations, this paper aims to clarify the soil resistance, the soil heave process and volume during the SP process, and then it is anticipated to explore more effective installation techniques in a soft clay seabed. 2. Model caisson and installation equipment 2.1. Model caisson As shown in Fig. 1, the steel model caisson is 88 mm in diameter, 2 mm in wall thickness and 400 mm in length. The surface of the caisson wall is smooth, and the sealed top cap extends 20 mm inside the tube. The total mass of the model caisson is 3.20 kg. Ignoring the soil heave inside the caisson, the expected embedment-todiameter ratio is about 4.3. The installation test can be conducted by jacking the model caisson to a specified depth in the soft clay, and then pumping entrapped water out of the interior to accomplish the final insertion. The top cap has one threaded attachment used for connecting the insertion/pullout rod to the caisson. In addition, the top cap contains several ports for pore pressure instrumentation, venting and vacuum application. A pressure sensing line, which is connected directly underneath the top cap (point U in Fig. 1), is used to measure the underpressure acting on the caisson during the SP process. 2.2. Test bed soil As shown in Fig. 2, a 3.0 m long, 1.2 m wide and 1.5 m high tank was used to accommodate the test bed soil. The tank was made of stainless steel plates to protect against possible corrosion and
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Fig. 2. Construction of the test tank.
Fig. 4. Settlement of slurry during consolidation of test bed soil.
to reduce the friction between the soil and the wall surface. The test bed soil was prepared by mixing several batches of slurry in a smaller steel barrel and then carefully pouring it into the test tank. The initial water content of as-prepared slurry was about 90–100%. In order to accelerate the consolidation of clay slurry, a drainage layer with a ball valve connected to atmosphere was deployed on the bottom of the tank (Fig. 3(a)), and a layer of bricks with an underlay geotextile (about 1.60 kPa overloaded) were placed on the clay surface, as shown in Fig. 3(b). Under the effect of the upper ballast and downward drainage, the settlement of slurry with time was measured at four different points (P-1, P2, P-3 and P-4 in Fig. 2), which lay about 30 cm off the tank wall. The fit curve of average soil surface settlement is also shown in Fig. 4. After about six months consolidation, the final thickness of soft clay was about 0.8 m, and the upper bricks and geotextile were removed. Both the continuous downward drainage and the removal of the ballast could result in an over-consolidation condition of test bed soil. Before the model tests, the valve at the bottom was closed and the water line was set to be 0.1 m above the mudline. The water content and total unit weight of the soft clay were determined from soil samples acquired with a piston sampler (Fig. 5(a) and (b)). The liquid limit of the clay ranged between 43% and 49% and the plasticity index between 24% and 28%. The consolidation coefficient of test bed soil was also obtained by the consolidation test, which was locating in the range from 0.03 to 0.05 mm2 /s. As shown in Fig. 6(a), the undrained strength of test bed soil was measured in situ using two vane shear tests and two T-bar penetration tests (just for comparison with the vane shear test). The T-bar was made of steel and had a diameter of 40 mm and a length of 250 mm. The undrained shear strength can be calculated with a bearing factor of 10.5 [20]. The thickness of test soil is too small to perform cyclic penetration of T-bar test, so the vane shear test is adopted to obtain the remolded strength of soil. The profile of soil sensitivity is established from the data of vane shear tests, as shown in Fig. 6(b).
Fig. 5. (a) Water content and (b) total unit weight in the test bed soil.
2.3. Installation equipment The suction caisson will be inserted into the sea bed using dead weight and suction pressure in sequence. Accordingly, the model experiment system designed for suction caisson installation contains two main parts: dead-weight loading rig and suction penetration apparatus. During the DP process, the dead-weight loading rig is used to jack the model caisson into the test bed soil at a constant rate, and simultaneously the vertical resistance and displacement are measured by the load cell and the linear variable differential transformer (LVDT), as shown in Fig. 7. Air and water entrapped in the caisson are allowed to vent through reserved ports on the top cap.
Fig. 3. (a) A layer of gravel and (b) the ballast (a layer of bricks).
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Fig. 6. (a) Undrained shear strength and (b) soil sensitivity.
The rate of dead-weight insertion is set to be 0.5 mm/s in order to avoid large interior overpressure that would break the seal formed between the caisson wall and soil [21]. After achieving enough initial depth by dead weight, the model caisson is then penetrated by the suction penetration apparatus, as shown in Fig. 8. The water pumping system can supply suction pressure by pumping water from the interior of the model caisson and adjusting the pressure by changing the opening degree of ball valve 1. The water supply with ball valve 2 is used to control the seal condition of the interior of model caisson. The vertical displacement, underpressure in the caisson and water weight pumped out of the interior are measured through the whole SP process. It is generally accepted that suction pressure should be gradually increased to keep the insertion rate as steady as possible. Suction penetration of the model caisson is stopped just when the soil inside the caisson is observed flowing out the top cap vent, indicating that the top cap had contacted the interior mudline. It should be noted that a filter screen is arranged near the top cap vent in order to prevent the soil flowing into the suction tube line.
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The model caisson is made of steel, so direct observations of the internal mudline are not possible. According to the previous works of House et al. [22] and Houlsby and Byrne [7], we can investigate the variation of internal mudline during the SP process through measuring the volume of water withdrawn from the caisson. After the DP process of the model caisson, the seal between the caisson wall and clay is formed, and the interior of the model caisson is fulfilled with water. Because of the low permeability of the clay, the interior void of the caisson, suction tube and vessel compose a closed system, in which the total volume of water keeps constant during the SP process. As shown in Fig. 9, when the suction pressure is applied, the volume of water E(t) pumping into the vessel equals the volume within the caisson displaced by the clay, which is the consequence of downward insertion of caisson d(t) and upheave of interior mudline h(t). The pumping water volume E(t) and the vertical penetration volume d(t) can be calculated from water weight change in vessel and vertical displacement of model caisson, which are measured by the load cell and LVDT respectively (Fig. 8). If more water is evacuated than that accounted for by the caisson wall insertion, this indicates that soil heave has occurred inside the caisson. Therefore, the volume h(t) due to soil heave is equal to the difference of E(t) and d(t), and the average height of soil heave can be obtained by h(t)/Ain , where Ain is the interior cross-sectional area of the model caisson. 3. Caisson installation tests Eight caisson installation tests have been carried out in different locations within the test bed, as shown in Fig. 10. The tests can be divided into two sets: one set, which consists of tests JP, CP-1, CP-2 and CP-3, aims to study the effect of different initial penetration depths on the caisson installation behavior; the other set containing tests IP-1, IP-2, IP-3 and IP-4 is performed in order to explore the feasibility of the intermittent pumping method for caisson installation in soft clay. Table 1 gives the summary of all model tests, including installation types, penetration distances and durations of different phases. 4. Test results and interpretation 4.1. Tests of different initial penetration depths Four installation results of tests JP, CP-1, CP-2 and CP-3 are given as recorded time histories in Fig. 11, which illustrates the whole processes from the touch down of the caisson tip to complete penetration of the model caisson by suction. In test JP, the caisson was jacked into the clay until its top cap reached the exterior mudline of the test bed soil, and no water pumping was applied. For tests CP1, CP-2 and CP-3, the caisson was pushed to 74 mm, 140 mm and 200 mm depths below mudline respectively, and the penetration rate of the model caisson under jacking force was about 0.5 mm/s. Afterwards, the jacking force was removed, and the dead weight during the SP process is only the self weight of the model caisson. During the SP process, the opening degree of ball valve (valve 1 in Fig. 8) was adjusted in order to keep a steady insertion rate of about 0.5 mm/s. The durations of the SP process for tests CP-1, CP-2 and CP-3 are 376 s, 358 s and 486 s, respectively.
Fig. 7. Dead-weight loading rig.
4.1.1. Penetration resistance During the DP process, the penetration resistance was measured by the load cell (Fig. 7), while the resistance during the SP process could be obtained indirectly by measuring suction pressure inside the caisson (Fig. 8). As shown in Fig. 12, the penetration resistance encountered in these four installation tests increased with the insertion of the model caisson. When the caisson was inserted
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Fig. 8. Suction penetration apparatus.
Fig. 9. Water flow during suction pressure insertion.
into the soil by jacking force, good repeatability was observed in the resistance in three tests JP, CP-1 and CP-2. While for test CP3, because of some degree of caisson tilt during the DP process, a small crack appeared on the soil surface near one side of the caisson, which thereby imported some water and reduces the friction between the external caisson wall and its surrounding clay. After changing from the DP to the SP process, the penetration of the model caisson was triggered at a much lower resistance compared with that encountered at the same depth by dead weight under the almost same penetration rate; with the increase of penetration depth, the resistance to the caisson grew gradually and finally exceeded the resistance at the same depth by dead-weight. It can be noted that when the caisson is deeply installed in soft clay by suction pressure, greater insertion forces may be required to overcome the resistance to the caisson. This is quite different from the
case of sandy seabed, in which a reduced resistance always occurs due to continuous upward seepage flow through the sand. This phenomenon of elevated resistance [10] in deeper penetration depth is bound up with the flow pattern of clay around the model caisson. Therefore, the clay flow pattern and the development of soil heave are investigated in the following section. 4.1.2. Soil heave inside the caisson In this paper, the initial position of the mudline before the DP process is set to be the zero position. The downward motion of the interior mudline is denoted by a negative sign and the upward motion is denoted by a positive sign. It can be seen from Table 2 that the interior mudline always decreased to some extent after the DP process. This lowering of the mudline could be calculated by comparing the total pumping water volume with the interior
Table 1 Summary of conditions in the caisson installation tests. Test ID
JP CP-1 CP-2 CP-3 IP-1 IP-2 IP-3 IP-4
Installation type
No pumping By dead weight and continuous pumping By dead weight and intermittent pumping
Penetration by dead weight
Penetration by suction pressure
Distance (mm)
Duration (s)
Distance (mm)
Duration (s)
405 74 140 200 140 140 142 140
831 154 285 408 286 285 291 287
0 213 158 118 138 125 138 181
0 376 358 486 189 267 357 340
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Fig. 10. Locations of installation tests within the test bed.
Fig. 13. Development curves of soil heave inside the caisson.
Fig. 11. Caisson installation processes for different initial penetration depths.
void of caisson, which is between the interior mudline at the start of the DP process and the inner surface of the top cap. As illustrated in Section 2.3, the development of soil heave during the SP process can be obtained indirectly by measurement of the volume of water pumping into the vessel. Thus, Fig. 13 presents the data of soil heave in tests CP-1, CP-2 and CP-3 and the corresponding fitting curves. In this figure, four straight lines represent the incremental soil heave inside the caisson obtained by assuming that all of the clay displaced by the caisson wall moves inward [5,6]. Pumping of water into the vessel may result in some oscillation of water in the vessel, which will produce some fluctuations of data, but the general trend of accelerating soil heave during the SP process is shown. As shown in Table 2, for test CP-1, the insertion depth by jacking force during the DP process was 74 mm. The interior mudline only decreased a little (about 4 mm) in the DP process, and the final position of the mudline after the SP process was +93 mm above the mudline. During the early stage of the SP process, the slope of soil heave increase was nearly the same as the straight line that indicated all clay displaced by the caisson wall moved inside the caisson. However, at a penetration depth about 100 mm, the soil heave increased faster than that caused by displaced clay, and when the caisson tip reached point “a” at the depth of 153.6 mm, the increase of soil heave started to accelerate. For tests CP-2, the initial jacking depth was 140 mm and the lowering of the interior mudline in the DP process was about 25 mm. When the SP process got started, the soil displaced by the caisson wall did not flow into the interior, and even the inner soil surface decreased a little from point “1” to point “2”. After point
Fig. 12. Soil resistance profiles for different initial penetration depths.
Table 2 Soil heaves for caisson installation by different methods. Test ID
Interior mudline (mm) Position after DP
JP CP-1 CP-2 CP-3 IP-1 IP-2 IP-3 IP-4
−55 −4 −23 −33 −28 −30 −27 −25
Position after SP −55 +93 +82 +62 +102 +115 +100 +59
Displacement of interior mudline after SP process (mm) 0 97 105 95 130 145 127 84 Fig. 14. Variations of suction pressure for different initial depths.
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Fig. 15. Variations of suction pressure during intermittent pumping process.
“2”, the interior mudline began to rise, and the increased velocity was greater than that induced by all displaced soil flowing into the interior. The increase curve agreed well with the straight line from their intersection point at about 200 mm to the depth of about 220 mm. The point of accelerated soil heave, which is noted as point “b” in Fig. 13, existed at the depth of 228.8 mm, an the finial position of soil heave was +82 mm. In test CP-3, due to its greater initial jacking depth of 200 mm, the interior mudline decreased to 33 mm below the mudline. During the SP process, the curve of incremental soil heave first coincided well with the straight lines until reaching a depth about 230 mm. The point of accelerated soil heave was marked as point “c” and its insertion depth was equal to 276.3 mm. After completion of caisson installation, the finial position of soil heave was +62 mm. From the test results of different initial depths, it can be seen that the early increase of soil heave agrees well with the assumption in API RP 2SK [5] and DNV RP-E303 [6], both of which suggest the volume of soil heave should be equal to the volume of soil displaced by the caisson wall. However, with the increase of suction pressure during the SP process, the soil heave may become greater than the volume of soil displaced by the caisson wall. Another notable characteristic is the increase of soil heave after the acceleration point: the curve of soil heave in test CP-1 was relatively flat during the whole SP process; the soil heave in test CP-2 increased more faster than that in test CP-1; the sharp increase of soil heave in test CP-3 occurred mainly in the last stage of the SP process. After the completion of caisson installation, the final positions of soil heave in these three installation tests are quite different. However, if taking
the initial mudline decrease after the DP process into consideration, the heights of soil heave in CP-1, CP-2 and CP-3 are 97 mm, 105 mm and 95 mm, respectively. It seems that there is not much influence of different initial penetration depths on the final height of soil heave, although the initial penetration has an obvious influence on the rate of later heave increase and the depth from which a large heave occurs. In addition, it can be noted from Fig. 12 that only after reaching some depths (about 132 mm for CP-1, 218 mm for CP-2, 262 mm for CP-3), the suction pressure would lead to a greater seabed resistance. At these depths, more than 100% displaced clay entered into the caisson. Especially, at the end of the SP process, the clay near the caisson tip largely flowed inwards, and the final resistances in tests CP-1, CP-2 and CP-3 were much greater than that in the jacking test. In our opinion, for the same penetration depth, the higher resistance during the SP process may mainly ascribe to the change of interior friction to the model caisson. On one hand, this change is due to the rise of interior mudline, which increases the contact area between the interior wall and soft clay; on the other hand, the suction pressure is great enough to induce large inflowing of surrounding clay, which would evidently enhance the soil friction on the lower portion of interior wall.
4.1.3. Comparisons with the API method When the suction caisson is inserted into clay, bearing capacity failure occurs at the caisson tip, leaving a zone of remolded clay along the caisson wall. It should be noted that if the steel surface of caisson is painted or treated in other ways, a correction factor
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Fig. 16. Curves of suction pressure versus penetration depth.
may need to be applied to the calculation of interface friction between clay and caisson wall [5,6,23,24]. The equations for required and allowable suction pressure from the API method [6] are given in Appendix A as well as the values of the relevant coefficients. Detailed properties of the test bed soil have been given in Section 2.2. The curves of required and allowable suction pressures obtained from the equations in Appendix A are plotted versus penetration depths. The DNV method [6] is almost the same as the API method [5].
In order to investigate the relationship between soil heave and suction pressure, Fig. 14 shows three curves of suction pressure during the SP processes for tests CP-1, CP-2 and CP-3. It can be seen that the suction caisson starts to penetrate once measured suction pressure reaches the required suction pressure calculated by the API method. Thus, the calculated suction pressure agrees quite well with the suction pressure required to activate the caisson insertion. However, with the continuance of caisson insertion, the measured pressure gradually deviates from the line of required
Fig. 17. (a) SP processes by intermittent pumping method and (b) local enlarged view.
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suction pressure, although the penetration velocity of the model caisson is nearly constant. During this phase, the volume of soil heave gradually increases and becomes larger than the volume of soil displaced by the caisson wall, as shown in Fig. 13. It can also be found that the suction pressure reaches the allowable suction pressure at points “d”, “e” and “f”, which are at the depths of 158.6 mm, 225.6 mm and 275.5 mm respectively. Compared with the data in Fig. 13, the points of accelerated soil heave (“a”, “b” and “c”) almost coincide with the intersection points on the line of allowable suction pressure. Therefore, these points correspond to the initiation of reverse bearing failure at the caisson tip. This partly proves that the API method is applicable in the calculation of allowable suction pressure. However, it can also be found from Figs. 11, 13 and 14 that although the reverse failure occurs, the caisson penetration still continues until all entrapped water has been withdrawn from the interior. This phenomenon is supported by the viewpoint of Houlsby and Byrne [7] that although plug failure occurred, it was still possible to install the caisson further, and the consequence of plug failure is only that the full design depth of the caisson cannot be reached. Both the inverse bearing capacity failure and the soil flow into the interior could be gradual during the SP process, and they are controlled by the pumping flux of water. 4.2. Caisson installation tests by intermittent pumping It is generally accepted that excessive soil heave is mainly caused by the suction pressure inside the caisson. During the SP process of a suction caisson, the traditional method of suction application is through continuously pumping water out of the interior. In order to avoid excessive soil heave, a small hole about 1 mm diameter was made in the top cap of the model caisson to dissipate continuous acting suction pressure. As shown in Fig. 8, the flow of water supply can be controlled by opening and closing the ball valve 2. On one hand, the flow through the hole in the top cap should be low enough to not affect the application of suction on the top cap when the ball valve 1 is open; on the other hand, the flow should be large enough to release the suction pressure when the ball valve 1 is closed. It should be noted that due to the inability to measure sudden change of water weight in the vessel, the previous method of measuring soil heave is only suitable for the steady insertion process of the model caisson, such as in tests CP-1, CP-2 and CP-3, but not suitable for installation tests by intermittent pumping method. Although the development curve of soil heave is not available, the final height of soil heave can be obtained from the penetration depth of model caisson, which is measured by the LVDT. The intermittent pumping method has been applied in four caisson installation tests IP-1, IP-2, IP-3 and IP-4, as shown in Table 1. In these tests, all the initial penetration depths of the model caisson by dead weight were about 140 mm, and then intermittent pumping was used to install the caisson in the test bed soil. Fig. 15 shows the durations of suction pulses in these four installation tests. The suction pulse durations for tests IP-1, IP-2, IP-3 and IP-4 were about 189 s, 267 s, 357 s and 340 s, while the corresponding numbers of pulses were 113, 71, 146 and 90, respectively. In each test, the pulse duration is controlled to be as the same as possible during the SP process. The average duration of one pulse (including the interval time between two adjacent suction pulses) for tests IP-1, IP-2, IP-3 and IP-4 was equal to 1.67 s, 3.76 s, 2.45 s and 3.78 s respectively. Fig. 16(a) shows the curve of penetration depth versus suction pressure for test IP-1. It can be seen that the interval time between two pulses was so small that the suction pressure could not dissipate completely before the next suction pulse, so there was always some degree of suction pressure acting on the top cap and the interior mud surface. During the SP process in test IP-1, the maximal applied suction was close to or even exceeds slightly the allowable suction pressure. To reduce the duration of suction application but
not decrease the efficiency of caisson installation, tests IP-2, IP-3 and IP-4 were carried out, as illustrated in Fig. 16(b)–(d). During these three tests, the interval time between two suction pulses was large enough to allow the suction pressure to dissipate. The main difference is the magnitude of suction pulse during the SP process: the suction pulses in test IP-2 were obviously larger than the allowable suction pressure from API method; the suction maximum in test IP-3 was quite close to the value of allowable suction pressure; and the suction pulses of test IP-4 were larger than the required suction pressure, but smaller than the allowable suction pressures. Fig. 17 illustrates the four SP processes for different intermittent pumpings. For comparison, the SP curve of test CP-2 with the same initial penetration depth is also shown. In test IP-1, the pulse duration was short, and undissipated suction pressure made the SP curve a relatively smooth line. Due to longer pulse duration in test IP-3, the suction pressure could dissipate fully between two adjacent pulses and the SP process showed a little stepwise penetration characteristic. For tests IP-2 and IP-4, the pulse duration was much longer and the characteristic of stepwise penetration during the SP process became more obvious. That means that the caisson penetrates into the clay to some distance under the effect of one pulse, then the suction pressure inside the caisson vanishes and the caisson penetration ceases until the next pulse. It should be noted that the SP curves for tests CP-2, IP-2 and IP-4 were close before the depth of 250 mm. In test CP-2, when the penetration depth was greater than 250 mm, the penetration velocity became slow because of large soil heave inside the caisson, and the final penetration depth was about 298 mm. The duration of one pulse in test IP-2 was similar to that in test IP-4, but the final penetration depth was only 265 mm due to larger magnitude of the pulse, which exceeded the allowable suction pressure calculated by the API method; in test IP-4, the total time of the SP process was a little shorter than that in test CP-2, but the final penetration depth reached 321 mm, which was the deepest in all of these caisson installation tests (Table 2). It can be seen from Figs. 15 and 16 that both the magnitude of suction pulses and their interval time are critical. Therefore, if an appropriate mode of suction pulse is adopted, the intermittent pumping method can show remarkable advantages in reducing the soil heave but not increasing the installation time.
5. Conclusions A series of caisson installation tests have been carried out using the specially designed model experimental system in order to investigate the influence of initial penetration depths on the caisson behavior during the suction-assisted penetration process, and to study the feasibility of a new caisson installation technique called “intermittent pumping”. The test results show that during suction-assisted penetration process, the penetration of the model caisson can be triggered at a much lower resistance compared with that encountered at the same depth by dead weight at almost the same penetration rate; with the increase of penetration depth, the resistance to the caisson grows gradually and even exceeds the resistance at the same depth by dead weight. Therefore, for caisson inserted to some large depth by suction pressure, greater insertion forces may be required to install the caisson. During the suction-assisted penetration process, the volume of clay entering the caisson is equal to, or even greater than, the volume displaced by the caisson wall, prior to the occurrence of inverse bearing failure. It can also be noted that although inverse bearing capacity failure occurred, it was still possible to install the caisson further, and the consequence of inverse bearing capacity failure is only that the full design depth cannot be reached. The inverse bearing capacity failure could be progressive
Z. Guo et al. / Applied Ocean Research 34 (2012) 116–125
during suction-assisted penetration process because of continuous water pumping. On the basis of the API method, the start suction of caisson insertion in soft clay agrees well with the required suction pressure, while the accelerated increase of soil heave inside the caisson coincides with the allowable suction pressure. It seems that different initial depths by dead weight have little influence on the final heights of soil heave inside the caisson. The intermittent pumping method has expended the scope of the installation technique of suction caisson in a soft clay seabed. Although the installation techniques of suction caisson in the field are relatively mature, the soil heave inside the caisson is still a critical problem now. The occurrence of soil heave inside the caisson is a progressive process. It is beneficial to suppress the development of soil heave by releasing the suction inside the caisson at intervals during the SP process. Adopting intermittent pumping method could be effective with no increasing the time of caisson installation. However, it is also found that for the caisson installation tests by intermittent pumping method, different suction pulses can result in different results. Both the magnitude of suction pulses and their interval time are critical, so it is necessary to explore the appropriate mode of suction pulse in the future research. Acknowledgements The authors would like to acknowledge the support of the Grant No. 51079128 from the National Natural Science Foundation of China and Grant No. R1100093 from Excellent Youth Foundation of Zhejiang Scientific Committee. Appendix A. According to the API method [6], the required suction pressure, srequire, and the allowable suction pressure, sallow, can be calculated by Eqs. (1a) and (1b): srequire =
Awall · · (˛ins Su )AVE + (Nc · Su,tip + · z) · Atip − W
sallow = Nc∗ · Su,tip +
Ain Ainside · · (˛ins Su )AVE Ain
(1a) (1b)
where Awall is the sum of inside and outside wall area embedded into the soil; is the correction factor for interface friction between clay and caisson wall, and is equal to 0.5 [25]; ˛ins is the shear strength factor and equals 1/St , where St is the sensitivity of soil; Su is the undrained shear strength of soil at one penetration depth, which can be obtained by vane shear tests; (˛ins Su )AVE is the average side friction from mudline to depth z; Nc is the bearing capacity factor in plane strain conditions, and is set to be 7.5 [6]; Su,tip is the undrained shear strength of soil at caisson tip level; is the effective unit weight of soil; Atip is the cross sectional area of the caisson tip; W is the submerged weight of the caisson during installation; Ain is the plan view inside area where suction pressure is applied; Ainside is the inside lateral area of caisson wall; Nc∗ is the reverse bearing capacity factor varying from 6.2 to 9.0 depending on depth/diameter ratio during penetration [26].
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References [1] Hogervorst JR. Field trails with large diameter suction piles. In: Proceedings of 12th annual offshore technology conference. OTC 3817. 1980. [2] Tjelta TI. Geotechnical experience from the installation of the Europipe Jacket with bucket foundations. In: Proceedings of 27th annual offshore technology conference. OTC 7795. 1995. p. 897–908. [3] Erbrich CT, Tjelta TI. Installation of bucket foundations and suction caissons in sand: geotechnical performance. In: Proceedings of 31st annual offshore technology conference. OTC 10990. 1999. [4] Tran MN, Randolph MF. Variation of suction pressure during caisson installation in sand. Géotechnique 2008;58(1):1–11. [5] API RP 2SK. Recommended practice for design and analysis of stationkeeping systems for floating structures, third ed., Washington, DC; 2005. [6] Det Norske Veritas. Geotechnical design and installation of suction anchors in clay. DNV Recommended Practice RP-E303, Høvik; 2005. [7] Houlsby GT, Byrne BW. Design procedures for installation of suction caissons in clay and other materials. Geotechnical Engineering 2005;158(2):75–82. [8] EI-Sherbiny RM. Performance of suction caisson anchors in normally consolidated clay. Ph.D. Dissertation, The University of Texas at Austin; 2005. [9] Cao J, Phillips R, Popescu R, Al-Khafaji Z, Audibert JME. Penetration resistance of suction caissons in clay. In: Proceedings of 12th international offshore and polar engineering conference. 2002. p. 800–6. [10] Rauch AF, Olson RE, Luke AM, Mecham EC. Measured response during laboratory installation of suction caissons. In: Proceedings of 13th international offshore and polar engineering conference. 2003. p. 780–7. [11] Senpere D, Auvergne GA. Suction anchor piles – a proven alternative to driving or drilling. In: Proceedings of 14th annual offshore technology conference. 1982. p. 483–93. [12] Tjelta TI. Suction piles: their position and applications today. In: Proceedings of 11th international offshore and polar engineering conference. 2001. p. 1–6. [13] Whittle AJ, Germaine JT, Cauble DF. Behavior of miniature suction caissons in clay. London: Offshore Site Investigation and Foundation Behavior ‘98; 1998. p. 279–300. [14] Andersen KH, Jeanjean P, Luger D, Jostad HP. Centrifuge tests on installation of suction anchors in soft clay. Ocean Engineering 2005;32:845–63. [15] House AR. Suction anchor foundations for buoyant offshore facilities. Ph.D. Dissertation, The University of Western Australia; 2002. [16] Newlin JA. Suction anchor piles for the Na kika FDS mooring system. Part 2: installation performance. In: Proceedings of international symposium on deepwater mooring systems: deepwater mooring system: concepts, design, analysis, and materials. 2003. [17] Chen W, Zhou H, Randolph MF. Effect of installation method on external shaft friction of caissons in soft clay. Journal of Geotechnical and Geoenvironmental Engineering 2009;135(5):605–15. [18] Allersma HGB, Hogervorst JR, Dufour C. Orientation tests on suction pile installation by the percussion method. In: Proceedings of 19th international conference on offshore mechanics and arctic engineering. OMAE 4036. 2000. [19] Allersma HGB, Hogervorst JR, Pimoulle M. Centrifuge modeling of suction pile installation in layered soil by percussion method. In: Proceedings of 20th international conference on offshore mechanics and arctic engineering. OMAE 1036. 2001. [20] Stewart DP, Randolph MF. T-bar penetration testing in soft clay. Journal of Geotechnical Engineering 1994;120(12):2230–5. [21] Huang J, Cao JC, Jean ME. Geotechnical design of suction caisson in clay. In: Proceedings of 13th international offshore and polar engineering conference. 2003. p. 770–9. [22] House AR, Randolph MF, Borbas ME. Limiting aspect ratio for suction installation in clay. In: Proceedings of 9th international offshore and polar engineering conference. 1999. p. 676–83. [23] Dendani H. Suction anchors: some critical aspects for their design and installation in clayey soils. In: Proceedings of 35th annual offshore technology conference. 2003. p. 1–7. [24] Andersen KH, Murff JD, Randolph MF. Deepwater anchor design practice – phase II report to API/Deepstar, vol. II. Suction caisson anchors. Report submitted to API/Deepstar; 2004. [25] Potyondy JG. Skin friction between various soils and construction materials. Géotechnique 1961;11(4):339–53. [26] Skempton AW. The bearing capacity of clays. In: Proceedings of building research congress, division I, London, Part III. 1951. p. 180–9.
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Applied Ocean Research journal homepage: www.elsevier.com/locate/apor
Model tests on installation techniques of suction caissons in a soft clay seabed Zhen Guo, Lizhong Wang ∗ , Feng Yuan, Lingling Li College of Civil Engineering and Architecture, Zhejiang University, Hangzhou, China
a r t i c l e
i n f o
Article history: Received 4 December 2010 Received in revised form 22 July 2011 Accepted 19 September 2011 Available online 24 October 2011 Keywords: Suction caisson Soft clay Dead weight Suction pressure Soil heave Intermittent pumping
a b s t r a c t A series of model tests have been carried out in order to investigate the behavior of suction caissons during installation in a soft clay seabed and explore more effective installation techniques. The test results indicate that when the suction pressure instead of dead weight is adopted to help penetrating the caisson, a greater resistance may be encountered at a large depth. During the process of suctionassisted penetration (SP), all the soil displaced by the caisson wall flows inwards, and even more volume of soil would enter into the caisson before the reverse bearing failure at the caisson tip occurs. This phenomenon would result in a larger soil heave, which makes the final insertion depth of the suction caisson less than the target depth. Although plug failure occurs during the SP process it is possible to install the caisson further, and the inverse bearing capacity failure could be progressive. The test result illustrates that different initial penetration depths by dead weight have little influence on final heights of soil heave inside the caisson. It is beneficial to suppress the development of soil heave by releasing the suction inside the caisson at intervals during the SP process. Adopting a new method of intermittent pumping can effectively reduce the soil heave but not increase the time for caisson installation in a soft clay seabed. Crown Copyright © 2011 Published by Elsevier Ltd. All rights reserved.
1. Introduction In recent years, suction caissons have been increasingly used as an effective solution for anchoring in deep water. A suction caisson is a large cylinder structure, which is usually made of steel, openended at the bottom and closed at the top. The suction caisson can be installed by first allowing it to penetrate under its dead weight (self weight and ballast), and then pumping out the entrapped water to cause further penetration of caisson to the desired depth. These two ordinal phases can be generally named as “dead-weight penetration” (DP) and “suction-assisted penetration” (SP), respectively. Most previous works have stressed on the SP process, during which sufficient pressure differential on the top cap should be guaranteed to overcome the seabed resistance, and too large soil heave inside the caisson should be avoided in order to achieve the desired penetration depth. The seabed resistance is not only decreased by the remolding of surrounding soil around the caisson wall, but also influenced by the underpressure (suction pressure) inside the caisson. In a sandy seabed, the suction pressure could induce seepage flow through the soil, which will result in a reduction in the effective stress at the caisson tip and along the inner wall, and thus reduce the seabed resistance. This characteristic of sandy seabed can evidently
∗ Corresponding author. Tel.: +86 571 88208678; fax: +86 571 88206240. E-mail address: [email protected] (L. Wang).
promote the installation of suction caisson [1–4]. In clayey soil, both API RP 2SK [5] and DNV RP-E303 [6] have presented the similar formulae for calculating the required suction pressure. Recommended formulae just take the suction pressure acting on the top cap as an equivalent vertical force, but its influence on the surrounding soil is ignored. Houlsby and Byrne [7] suggested that the inner underpressure would reduce the end bearing on the annular rim of the caisson. EI-Sherbiny [8] carried out a series of suction caisson installation tests in soft kaolin clay, and concluded that the resistance was in close agreement for insertions by suction and for insertions by dead weight, but the side friction acting on the caisson walls during insertion by dead weight was larger than that during insertion by suction. This was thought to be mainly due to the lower internal side friction during suction insertion, while the side friction on the outer tube was practically unaffected by the applied suction. Cao et al. [9] and Rauch et al. [10] have performed some model tests on the installation of suction caisson in soft clay, and all their test results suggested that the resistance for insertions by suction would be obviously larger than that for insertions by dead weight. The seabed resistance encountered during the SP process is critical to determine how to apply suction pressure to achieve the caisson installation, but the above mentioned references show some controversies over the magnitude of soil resistance with different installation methods. Another focus of previous works is the heave of the interior mudline during the SP process, which is generally caused by the inward flow of soil near the caisson tip. Large applied suction
0141-1187/$ – see front matter. Crown Copyright © 2011 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.apor.2011.09.004
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Fig. 1. (a) Photograph and (b) sketch of the model caisson.
pressure will result in an excessive soil heave inside the caisson, and make the caisson top cap contact the interior mudline so quickly that the desired penetration depth cannot be achieved. It was the earliest experience of unanticipated soil heaves during the SP process in the Gorm Field in 1980 [11,12]. In that case, the suction caisson was installed in a sand layer followed by soft clay on top of stiff clay, and the large soil heave was mainly due to the liquefaction of the interior sand under a great suction pressure. For a soft clay seabed, the excessive soil heave may also appear as a result of the instability of soil near the caisson tip. API RP 2SK [5], DNV RP-E303 [6] and Houlsby and Byrne [7] gave the calculation equations for the allowable suction pressure, which is the critical underpressure causing a general reverse bearing failure at the caisson tip. According to API RP 2SK [5] and DNV RP-E303 [6], the volume of soil heave during the SP process is just equal to the volume of soil displaced by the caisson wall. Rauch et al. [10] and Whittle et al. [13] suggested that almost 100% of soil displaced by the wall moves inwards during the SP process. Andersen et al. [14] performed a centrifuge study on suction caisson installation and found that at a penetration depth of nearly half the maximum penetration depth, the volume of the soil heave inside the caisson actually increased more than the volume of the displaced soil. On the basis of model tests in normally consolidated kaolin clay, House [15] reported that all the soil displaced by the caisson wall would flow into the caisson during the early SP process (before the depth of about 4–5 diameters), and afterwards the failure of inner soil plug would occur and the final height of soil heave can be as large as 30% of the caisson length. In contrast, Newlin [16] stated that the amount of soil drawn into the caisson during suction installation is much less than 100% of that displaced by the caisson wall, and Chen et al. [17] also indicated that only about half of the caisson wall is accommodated by inward flow of the soil. So, the process and volume of soil upheaval need further clarification with more lab tests. Traditionally, the suction caisson is installed by continuous pumping after having been inserted by dead weight. Either the seabed resistance or the interior soil heave can be affected by the operations in the DP and SP processes, such as different initial DP depth and different methods of water pumping. Allersma et al. [18,19] have presented a new method, called “percussion technique”, to install suction piles by creating short pressure pulses on
the top cap. The pulse time is ranged between tens and hundreds of milliseconds. This percussion technique has proved to be effective for reducing the soil heave, especially in coarse sand and layered soil (clay overlying sand). But, it also suggested that there was little difference for caisson installation in clay between by continuous pumping and using percussion technique. Using a specially designed model experimental system, several small scale tests are carried out in over-consolidated clay. This paper presents test data of caisson installation by different operation techniques as well as the interpretations. By systematically studying the effects of different operations, this paper aims to clarify the soil resistance, the soil heave process and volume during the SP process, and then it is anticipated to explore more effective installation techniques in a soft clay seabed. 2. Model caisson and installation equipment 2.1. Model caisson As shown in Fig. 1, the steel model caisson is 88 mm in diameter, 2 mm in wall thickness and 400 mm in length. The surface of the caisson wall is smooth, and the sealed top cap extends 20 mm inside the tube. The total mass of the model caisson is 3.20 kg. Ignoring the soil heave inside the caisson, the expected embedment-todiameter ratio is about 4.3. The installation test can be conducted by jacking the model caisson to a specified depth in the soft clay, and then pumping entrapped water out of the interior to accomplish the final insertion. The top cap has one threaded attachment used for connecting the insertion/pullout rod to the caisson. In addition, the top cap contains several ports for pore pressure instrumentation, venting and vacuum application. A pressure sensing line, which is connected directly underneath the top cap (point U in Fig. 1), is used to measure the underpressure acting on the caisson during the SP process. 2.2. Test bed soil As shown in Fig. 2, a 3.0 m long, 1.2 m wide and 1.5 m high tank was used to accommodate the test bed soil. The tank was made of stainless steel plates to protect against possible corrosion and
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Fig. 2. Construction of the test tank.
Fig. 4. Settlement of slurry during consolidation of test bed soil.
to reduce the friction between the soil and the wall surface. The test bed soil was prepared by mixing several batches of slurry in a smaller steel barrel and then carefully pouring it into the test tank. The initial water content of as-prepared slurry was about 90–100%. In order to accelerate the consolidation of clay slurry, a drainage layer with a ball valve connected to atmosphere was deployed on the bottom of the tank (Fig. 3(a)), and a layer of bricks with an underlay geotextile (about 1.60 kPa overloaded) were placed on the clay surface, as shown in Fig. 3(b). Under the effect of the upper ballast and downward drainage, the settlement of slurry with time was measured at four different points (P-1, P2, P-3 and P-4 in Fig. 2), which lay about 30 cm off the tank wall. The fit curve of average soil surface settlement is also shown in Fig. 4. After about six months consolidation, the final thickness of soft clay was about 0.8 m, and the upper bricks and geotextile were removed. Both the continuous downward drainage and the removal of the ballast could result in an over-consolidation condition of test bed soil. Before the model tests, the valve at the bottom was closed and the water line was set to be 0.1 m above the mudline. The water content and total unit weight of the soft clay were determined from soil samples acquired with a piston sampler (Fig. 5(a) and (b)). The liquid limit of the clay ranged between 43% and 49% and the plasticity index between 24% and 28%. The consolidation coefficient of test bed soil was also obtained by the consolidation test, which was locating in the range from 0.03 to 0.05 mm2 /s. As shown in Fig. 6(a), the undrained strength of test bed soil was measured in situ using two vane shear tests and two T-bar penetration tests (just for comparison with the vane shear test). The T-bar was made of steel and had a diameter of 40 mm and a length of 250 mm. The undrained shear strength can be calculated with a bearing factor of 10.5 [20]. The thickness of test soil is too small to perform cyclic penetration of T-bar test, so the vane shear test is adopted to obtain the remolded strength of soil. The profile of soil sensitivity is established from the data of vane shear tests, as shown in Fig. 6(b).
Fig. 5. (a) Water content and (b) total unit weight in the test bed soil.
2.3. Installation equipment The suction caisson will be inserted into the sea bed using dead weight and suction pressure in sequence. Accordingly, the model experiment system designed for suction caisson installation contains two main parts: dead-weight loading rig and suction penetration apparatus. During the DP process, the dead-weight loading rig is used to jack the model caisson into the test bed soil at a constant rate, and simultaneously the vertical resistance and displacement are measured by the load cell and the linear variable differential transformer (LVDT), as shown in Fig. 7. Air and water entrapped in the caisson are allowed to vent through reserved ports on the top cap.
Fig. 3. (a) A layer of gravel and (b) the ballast (a layer of bricks).
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Fig. 6. (a) Undrained shear strength and (b) soil sensitivity.
The rate of dead-weight insertion is set to be 0.5 mm/s in order to avoid large interior overpressure that would break the seal formed between the caisson wall and soil [21]. After achieving enough initial depth by dead weight, the model caisson is then penetrated by the suction penetration apparatus, as shown in Fig. 8. The water pumping system can supply suction pressure by pumping water from the interior of the model caisson and adjusting the pressure by changing the opening degree of ball valve 1. The water supply with ball valve 2 is used to control the seal condition of the interior of model caisson. The vertical displacement, underpressure in the caisson and water weight pumped out of the interior are measured through the whole SP process. It is generally accepted that suction pressure should be gradually increased to keep the insertion rate as steady as possible. Suction penetration of the model caisson is stopped just when the soil inside the caisson is observed flowing out the top cap vent, indicating that the top cap had contacted the interior mudline. It should be noted that a filter screen is arranged near the top cap vent in order to prevent the soil flowing into the suction tube line.
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The model caisson is made of steel, so direct observations of the internal mudline are not possible. According to the previous works of House et al. [22] and Houlsby and Byrne [7], we can investigate the variation of internal mudline during the SP process through measuring the volume of water withdrawn from the caisson. After the DP process of the model caisson, the seal between the caisson wall and clay is formed, and the interior of the model caisson is fulfilled with water. Because of the low permeability of the clay, the interior void of the caisson, suction tube and vessel compose a closed system, in which the total volume of water keeps constant during the SP process. As shown in Fig. 9, when the suction pressure is applied, the volume of water E(t) pumping into the vessel equals the volume within the caisson displaced by the clay, which is the consequence of downward insertion of caisson d(t) and upheave of interior mudline h(t). The pumping water volume E(t) and the vertical penetration volume d(t) can be calculated from water weight change in vessel and vertical displacement of model caisson, which are measured by the load cell and LVDT respectively (Fig. 8). If more water is evacuated than that accounted for by the caisson wall insertion, this indicates that soil heave has occurred inside the caisson. Therefore, the volume h(t) due to soil heave is equal to the difference of E(t) and d(t), and the average height of soil heave can be obtained by h(t)/Ain , where Ain is the interior cross-sectional area of the model caisson. 3. Caisson installation tests Eight caisson installation tests have been carried out in different locations within the test bed, as shown in Fig. 10. The tests can be divided into two sets: one set, which consists of tests JP, CP-1, CP-2 and CP-3, aims to study the effect of different initial penetration depths on the caisson installation behavior; the other set containing tests IP-1, IP-2, IP-3 and IP-4 is performed in order to explore the feasibility of the intermittent pumping method for caisson installation in soft clay. Table 1 gives the summary of all model tests, including installation types, penetration distances and durations of different phases. 4. Test results and interpretation 4.1. Tests of different initial penetration depths Four installation results of tests JP, CP-1, CP-2 and CP-3 are given as recorded time histories in Fig. 11, which illustrates the whole processes from the touch down of the caisson tip to complete penetration of the model caisson by suction. In test JP, the caisson was jacked into the clay until its top cap reached the exterior mudline of the test bed soil, and no water pumping was applied. For tests CP1, CP-2 and CP-3, the caisson was pushed to 74 mm, 140 mm and 200 mm depths below mudline respectively, and the penetration rate of the model caisson under jacking force was about 0.5 mm/s. Afterwards, the jacking force was removed, and the dead weight during the SP process is only the self weight of the model caisson. During the SP process, the opening degree of ball valve (valve 1 in Fig. 8) was adjusted in order to keep a steady insertion rate of about 0.5 mm/s. The durations of the SP process for tests CP-1, CP-2 and CP-3 are 376 s, 358 s and 486 s, respectively.
Fig. 7. Dead-weight loading rig.
4.1.1. Penetration resistance During the DP process, the penetration resistance was measured by the load cell (Fig. 7), while the resistance during the SP process could be obtained indirectly by measuring suction pressure inside the caisson (Fig. 8). As shown in Fig. 12, the penetration resistance encountered in these four installation tests increased with the insertion of the model caisson. When the caisson was inserted
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Fig. 8. Suction penetration apparatus.
Fig. 9. Water flow during suction pressure insertion.
into the soil by jacking force, good repeatability was observed in the resistance in three tests JP, CP-1 and CP-2. While for test CP3, because of some degree of caisson tilt during the DP process, a small crack appeared on the soil surface near one side of the caisson, which thereby imported some water and reduces the friction between the external caisson wall and its surrounding clay. After changing from the DP to the SP process, the penetration of the model caisson was triggered at a much lower resistance compared with that encountered at the same depth by dead weight under the almost same penetration rate; with the increase of penetration depth, the resistance to the caisson grew gradually and finally exceeded the resistance at the same depth by dead-weight. It can be noted that when the caisson is deeply installed in soft clay by suction pressure, greater insertion forces may be required to overcome the resistance to the caisson. This is quite different from the
case of sandy seabed, in which a reduced resistance always occurs due to continuous upward seepage flow through the sand. This phenomenon of elevated resistance [10] in deeper penetration depth is bound up with the flow pattern of clay around the model caisson. Therefore, the clay flow pattern and the development of soil heave are investigated in the following section. 4.1.2. Soil heave inside the caisson In this paper, the initial position of the mudline before the DP process is set to be the zero position. The downward motion of the interior mudline is denoted by a negative sign and the upward motion is denoted by a positive sign. It can be seen from Table 2 that the interior mudline always decreased to some extent after the DP process. This lowering of the mudline could be calculated by comparing the total pumping water volume with the interior
Table 1 Summary of conditions in the caisson installation tests. Test ID
JP CP-1 CP-2 CP-3 IP-1 IP-2 IP-3 IP-4
Installation type
No pumping By dead weight and continuous pumping By dead weight and intermittent pumping
Penetration by dead weight
Penetration by suction pressure
Distance (mm)
Duration (s)
Distance (mm)
Duration (s)
405 74 140 200 140 140 142 140
831 154 285 408 286 285 291 287
0 213 158 118 138 125 138 181
0 376 358 486 189 267 357 340
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Fig. 10. Locations of installation tests within the test bed.
Fig. 13. Development curves of soil heave inside the caisson.
Fig. 11. Caisson installation processes for different initial penetration depths.
void of caisson, which is between the interior mudline at the start of the DP process and the inner surface of the top cap. As illustrated in Section 2.3, the development of soil heave during the SP process can be obtained indirectly by measurement of the volume of water pumping into the vessel. Thus, Fig. 13 presents the data of soil heave in tests CP-1, CP-2 and CP-3 and the corresponding fitting curves. In this figure, four straight lines represent the incremental soil heave inside the caisson obtained by assuming that all of the clay displaced by the caisson wall moves inward [5,6]. Pumping of water into the vessel may result in some oscillation of water in the vessel, which will produce some fluctuations of data, but the general trend of accelerating soil heave during the SP process is shown. As shown in Table 2, for test CP-1, the insertion depth by jacking force during the DP process was 74 mm. The interior mudline only decreased a little (about 4 mm) in the DP process, and the final position of the mudline after the SP process was +93 mm above the mudline. During the early stage of the SP process, the slope of soil heave increase was nearly the same as the straight line that indicated all clay displaced by the caisson wall moved inside the caisson. However, at a penetration depth about 100 mm, the soil heave increased faster than that caused by displaced clay, and when the caisson tip reached point “a” at the depth of 153.6 mm, the increase of soil heave started to accelerate. For tests CP-2, the initial jacking depth was 140 mm and the lowering of the interior mudline in the DP process was about 25 mm. When the SP process got started, the soil displaced by the caisson wall did not flow into the interior, and even the inner soil surface decreased a little from point “1” to point “2”. After point
Fig. 12. Soil resistance profiles for different initial penetration depths.
Table 2 Soil heaves for caisson installation by different methods. Test ID
Interior mudline (mm) Position after DP
JP CP-1 CP-2 CP-3 IP-1 IP-2 IP-3 IP-4
−55 −4 −23 −33 −28 −30 −27 −25
Position after SP −55 +93 +82 +62 +102 +115 +100 +59
Displacement of interior mudline after SP process (mm) 0 97 105 95 130 145 127 84 Fig. 14. Variations of suction pressure for different initial depths.
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Fig. 15. Variations of suction pressure during intermittent pumping process.
“2”, the interior mudline began to rise, and the increased velocity was greater than that induced by all displaced soil flowing into the interior. The increase curve agreed well with the straight line from their intersection point at about 200 mm to the depth of about 220 mm. The point of accelerated soil heave, which is noted as point “b” in Fig. 13, existed at the depth of 228.8 mm, an the finial position of soil heave was +82 mm. In test CP-3, due to its greater initial jacking depth of 200 mm, the interior mudline decreased to 33 mm below the mudline. During the SP process, the curve of incremental soil heave first coincided well with the straight lines until reaching a depth about 230 mm. The point of accelerated soil heave was marked as point “c” and its insertion depth was equal to 276.3 mm. After completion of caisson installation, the finial position of soil heave was +62 mm. From the test results of different initial depths, it can be seen that the early increase of soil heave agrees well with the assumption in API RP 2SK [5] and DNV RP-E303 [6], both of which suggest the volume of soil heave should be equal to the volume of soil displaced by the caisson wall. However, with the increase of suction pressure during the SP process, the soil heave may become greater than the volume of soil displaced by the caisson wall. Another notable characteristic is the increase of soil heave after the acceleration point: the curve of soil heave in test CP-1 was relatively flat during the whole SP process; the soil heave in test CP-2 increased more faster than that in test CP-1; the sharp increase of soil heave in test CP-3 occurred mainly in the last stage of the SP process. After the completion of caisson installation, the final positions of soil heave in these three installation tests are quite different. However, if taking
the initial mudline decrease after the DP process into consideration, the heights of soil heave in CP-1, CP-2 and CP-3 are 97 mm, 105 mm and 95 mm, respectively. It seems that there is not much influence of different initial penetration depths on the final height of soil heave, although the initial penetration has an obvious influence on the rate of later heave increase and the depth from which a large heave occurs. In addition, it can be noted from Fig. 12 that only after reaching some depths (about 132 mm for CP-1, 218 mm for CP-2, 262 mm for CP-3), the suction pressure would lead to a greater seabed resistance. At these depths, more than 100% displaced clay entered into the caisson. Especially, at the end of the SP process, the clay near the caisson tip largely flowed inwards, and the final resistances in tests CP-1, CP-2 and CP-3 were much greater than that in the jacking test. In our opinion, for the same penetration depth, the higher resistance during the SP process may mainly ascribe to the change of interior friction to the model caisson. On one hand, this change is due to the rise of interior mudline, which increases the contact area between the interior wall and soft clay; on the other hand, the suction pressure is great enough to induce large inflowing of surrounding clay, which would evidently enhance the soil friction on the lower portion of interior wall.
4.1.3. Comparisons with the API method When the suction caisson is inserted into clay, bearing capacity failure occurs at the caisson tip, leaving a zone of remolded clay along the caisson wall. It should be noted that if the steel surface of caisson is painted or treated in other ways, a correction factor
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Fig. 16. Curves of suction pressure versus penetration depth.
may need to be applied to the calculation of interface friction between clay and caisson wall [5,6,23,24]. The equations for required and allowable suction pressure from the API method [6] are given in Appendix A as well as the values of the relevant coefficients. Detailed properties of the test bed soil have been given in Section 2.2. The curves of required and allowable suction pressures obtained from the equations in Appendix A are plotted versus penetration depths. The DNV method [6] is almost the same as the API method [5].
In order to investigate the relationship between soil heave and suction pressure, Fig. 14 shows three curves of suction pressure during the SP processes for tests CP-1, CP-2 and CP-3. It can be seen that the suction caisson starts to penetrate once measured suction pressure reaches the required suction pressure calculated by the API method. Thus, the calculated suction pressure agrees quite well with the suction pressure required to activate the caisson insertion. However, with the continuance of caisson insertion, the measured pressure gradually deviates from the line of required
Fig. 17. (a) SP processes by intermittent pumping method and (b) local enlarged view.
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suction pressure, although the penetration velocity of the model caisson is nearly constant. During this phase, the volume of soil heave gradually increases and becomes larger than the volume of soil displaced by the caisson wall, as shown in Fig. 13. It can also be found that the suction pressure reaches the allowable suction pressure at points “d”, “e” and “f”, which are at the depths of 158.6 mm, 225.6 mm and 275.5 mm respectively. Compared with the data in Fig. 13, the points of accelerated soil heave (“a”, “b” and “c”) almost coincide with the intersection points on the line of allowable suction pressure. Therefore, these points correspond to the initiation of reverse bearing failure at the caisson tip. This partly proves that the API method is applicable in the calculation of allowable suction pressure. However, it can also be found from Figs. 11, 13 and 14 that although the reverse failure occurs, the caisson penetration still continues until all entrapped water has been withdrawn from the interior. This phenomenon is supported by the viewpoint of Houlsby and Byrne [7] that although plug failure occurred, it was still possible to install the caisson further, and the consequence of plug failure is only that the full design depth of the caisson cannot be reached. Both the inverse bearing capacity failure and the soil flow into the interior could be gradual during the SP process, and they are controlled by the pumping flux of water. 4.2. Caisson installation tests by intermittent pumping It is generally accepted that excessive soil heave is mainly caused by the suction pressure inside the caisson. During the SP process of a suction caisson, the traditional method of suction application is through continuously pumping water out of the interior. In order to avoid excessive soil heave, a small hole about 1 mm diameter was made in the top cap of the model caisson to dissipate continuous acting suction pressure. As shown in Fig. 8, the flow of water supply can be controlled by opening and closing the ball valve 2. On one hand, the flow through the hole in the top cap should be low enough to not affect the application of suction on the top cap when the ball valve 1 is open; on the other hand, the flow should be large enough to release the suction pressure when the ball valve 1 is closed. It should be noted that due to the inability to measure sudden change of water weight in the vessel, the previous method of measuring soil heave is only suitable for the steady insertion process of the model caisson, such as in tests CP-1, CP-2 and CP-3, but not suitable for installation tests by intermittent pumping method. Although the development curve of soil heave is not available, the final height of soil heave can be obtained from the penetration depth of model caisson, which is measured by the LVDT. The intermittent pumping method has been applied in four caisson installation tests IP-1, IP-2, IP-3 and IP-4, as shown in Table 1. In these tests, all the initial penetration depths of the model caisson by dead weight were about 140 mm, and then intermittent pumping was used to install the caisson in the test bed soil. Fig. 15 shows the durations of suction pulses in these four installation tests. The suction pulse durations for tests IP-1, IP-2, IP-3 and IP-4 were about 189 s, 267 s, 357 s and 340 s, while the corresponding numbers of pulses were 113, 71, 146 and 90, respectively. In each test, the pulse duration is controlled to be as the same as possible during the SP process. The average duration of one pulse (including the interval time between two adjacent suction pulses) for tests IP-1, IP-2, IP-3 and IP-4 was equal to 1.67 s, 3.76 s, 2.45 s and 3.78 s respectively. Fig. 16(a) shows the curve of penetration depth versus suction pressure for test IP-1. It can be seen that the interval time between two pulses was so small that the suction pressure could not dissipate completely before the next suction pulse, so there was always some degree of suction pressure acting on the top cap and the interior mud surface. During the SP process in test IP-1, the maximal applied suction was close to or even exceeds slightly the allowable suction pressure. To reduce the duration of suction application but
not decrease the efficiency of caisson installation, tests IP-2, IP-3 and IP-4 were carried out, as illustrated in Fig. 16(b)–(d). During these three tests, the interval time between two suction pulses was large enough to allow the suction pressure to dissipate. The main difference is the magnitude of suction pulse during the SP process: the suction pulses in test IP-2 were obviously larger than the allowable suction pressure from API method; the suction maximum in test IP-3 was quite close to the value of allowable suction pressure; and the suction pulses of test IP-4 were larger than the required suction pressure, but smaller than the allowable suction pressures. Fig. 17 illustrates the four SP processes for different intermittent pumpings. For comparison, the SP curve of test CP-2 with the same initial penetration depth is also shown. In test IP-1, the pulse duration was short, and undissipated suction pressure made the SP curve a relatively smooth line. Due to longer pulse duration in test IP-3, the suction pressure could dissipate fully between two adjacent pulses and the SP process showed a little stepwise penetration characteristic. For tests IP-2 and IP-4, the pulse duration was much longer and the characteristic of stepwise penetration during the SP process became more obvious. That means that the caisson penetrates into the clay to some distance under the effect of one pulse, then the suction pressure inside the caisson vanishes and the caisson penetration ceases until the next pulse. It should be noted that the SP curves for tests CP-2, IP-2 and IP-4 were close before the depth of 250 mm. In test CP-2, when the penetration depth was greater than 250 mm, the penetration velocity became slow because of large soil heave inside the caisson, and the final penetration depth was about 298 mm. The duration of one pulse in test IP-2 was similar to that in test IP-4, but the final penetration depth was only 265 mm due to larger magnitude of the pulse, which exceeded the allowable suction pressure calculated by the API method; in test IP-4, the total time of the SP process was a little shorter than that in test CP-2, but the final penetration depth reached 321 mm, which was the deepest in all of these caisson installation tests (Table 2). It can be seen from Figs. 15 and 16 that both the magnitude of suction pulses and their interval time are critical. Therefore, if an appropriate mode of suction pulse is adopted, the intermittent pumping method can show remarkable advantages in reducing the soil heave but not increasing the installation time.
5. Conclusions A series of caisson installation tests have been carried out using the specially designed model experimental system in order to investigate the influence of initial penetration depths on the caisson behavior during the suction-assisted penetration process, and to study the feasibility of a new caisson installation technique called “intermittent pumping”. The test results show that during suction-assisted penetration process, the penetration of the model caisson can be triggered at a much lower resistance compared with that encountered at the same depth by dead weight at almost the same penetration rate; with the increase of penetration depth, the resistance to the caisson grows gradually and even exceeds the resistance at the same depth by dead weight. Therefore, for caisson inserted to some large depth by suction pressure, greater insertion forces may be required to install the caisson. During the suction-assisted penetration process, the volume of clay entering the caisson is equal to, or even greater than, the volume displaced by the caisson wall, prior to the occurrence of inverse bearing failure. It can also be noted that although inverse bearing capacity failure occurred, it was still possible to install the caisson further, and the consequence of inverse bearing capacity failure is only that the full design depth cannot be reached. The inverse bearing capacity failure could be progressive
Z. Guo et al. / Applied Ocean Research 34 (2012) 116–125
during suction-assisted penetration process because of continuous water pumping. On the basis of the API method, the start suction of caisson insertion in soft clay agrees well with the required suction pressure, while the accelerated increase of soil heave inside the caisson coincides with the allowable suction pressure. It seems that different initial depths by dead weight have little influence on the final heights of soil heave inside the caisson. The intermittent pumping method has expended the scope of the installation technique of suction caisson in a soft clay seabed. Although the installation techniques of suction caisson in the field are relatively mature, the soil heave inside the caisson is still a critical problem now. The occurrence of soil heave inside the caisson is a progressive process. It is beneficial to suppress the development of soil heave by releasing the suction inside the caisson at intervals during the SP process. Adopting intermittent pumping method could be effective with no increasing the time of caisson installation. However, it is also found that for the caisson installation tests by intermittent pumping method, different suction pulses can result in different results. Both the magnitude of suction pulses and their interval time are critical, so it is necessary to explore the appropriate mode of suction pulse in the future research. Acknowledgements The authors would like to acknowledge the support of the Grant No. 51079128 from the National Natural Science Foundation of China and Grant No. R1100093 from Excellent Youth Foundation of Zhejiang Scientific Committee. Appendix A. According to the API method [6], the required suction pressure, srequire, and the allowable suction pressure, sallow, can be calculated by Eqs. (1a) and (1b): srequire =
Awall · · (˛ins Su )AVE + (Nc · Su,tip + · z) · Atip − W
sallow = Nc∗ · Su,tip +
Ain Ainside · · (˛ins Su )AVE Ain
(1a) (1b)
where Awall is the sum of inside and outside wall area embedded into the soil; is the correction factor for interface friction between clay and caisson wall, and is equal to 0.5 [25]; ˛ins is the shear strength factor and equals 1/St , where St is the sensitivity of soil; Su is the undrained shear strength of soil at one penetration depth, which can be obtained by vane shear tests; (˛ins Su )AVE is the average side friction from mudline to depth z; Nc is the bearing capacity factor in plane strain conditions, and is set to be 7.5 [6]; Su,tip is the undrained shear strength of soil at caisson tip level; is the effective unit weight of soil; Atip is the cross sectional area of the caisson tip; W is the submerged weight of the caisson during installation; Ain is the plan view inside area where suction pressure is applied; Ainside is the inside lateral area of caisson wall; Nc∗ is the reverse bearing capacity factor varying from 6.2 to 9.0 depending on depth/diameter ratio during penetration [26].
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