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Concrete segment tunnel lining sealant performance under earthquake loading
Tunnelling and Underground Space Technology 31 (2012) 51–60
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Tunnelling and Underground Space Technology journal homepage: www.elsevier.com/locate/tust
Concrete segment tunnel lining sealant performance under earthquake loading Faisal I. Shalabi a,⇑, Edward J. Cording b, Stanley L. Paul b a b
Department of Civil Engineering, King Faisal University, Al-Ahssa, Saudi Arabia Department of Civil Engineering, University of Illinois at Urbana-Champaign, Urbana, IL, USA
a r t i c l e
i n f o
Article history: Received 21 October 2011 Received in revised form 30 March 2012 Accepted 2 April 2012 Available online 1 May 2012 Keywords: Tunnel Lining Concrete segments Gasket sealant Earthquake
a b s t r a c t This work was conducted to provide an understanding of the leakage behavior of gasketed segmental tunnel linings subjected to static ground loads and earthquake shaking of the recommended design of the Los Angeles (LA) Metro. The proposed lining was made of bolted and double gasketed precast concrete segment lining with convex to convex longitudinal joint surfaces. Lining evaluation included the sealant performance of different gasket materials under water pressure less than 90 psi (Neoprene and EPDM gaskets with open base. Testing program was designed to evaluate the longitudinal joint and T-joint sealant behavior under static and dynamic loading using large scale concrete segments. The results showed that concrete around the gasket groove, cracked zones, gasket–gasket groove interface, and bolting pockets are the places of leakage in concrete segments. Besides that, damage to the side of the gasket groove reduced the gasket confinement and led to leakage. Also, the results showed that longitudinal joint sealant capacity was improved by cycling as a result of the increase in bonding between the gasket and gasket groove. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction The use of bolted precast concrete segments as initial and final tunnel lining (single-pass system), mostly eliminates the need for using two pass lining system, which consists of initial lining and final lining. Initial lining for a two-pass lining system is usually shotcrete and rock bolts in rock ground, while in soil it can be steel ribs and lagging or expanded segmental lining. The final lining is usually cast-in-place plain or reinforced concrete. One advantage of using bolted concrete segments (single-pass system) rather than an expanded segmental lining as initial support for the two-pass system is that the ground loss and movement that might occur behind the shield tail can be minimized. This can be done by grouting between the bolted segments and the ground while the shield is moving forward and before the gap start to be filled with soil. In the case of expanded segment lining, expansion cannot be reached until the lining is cleared from the tail of the shield at the end of the shove which leads to considerable ground loss and settlement as the soil rapidly invades the gaps behind the lining (Cording et al., 1998). This issue becomes very critical for tunneling through granular soils in urban areas. Another advantage of using the single-pass bolted concrete segments lining rather than the expanded lining as initial support for two-pass system, is the construction schedule, which is less for the former lining system.
⇑ Corresponding author. E-mail addresses: [email protected] (F.I. Shalabi), [email protected] (E.J. Cording), [email protected] (S.L. Paul). 0886-7798/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tust.2012.04.006
For transportation tunnels in sandy materials below water table, control of leakage is one of the most important problems to be considered in lining design. Presence of gases, such as hydrogen sulfide or methane may cause serious safety problems if the gas and water leak into the tunnel. One of the solutions to this problem is to use a membrane between the initial and final linings to serve as a barrier for gas and water flow. For a single-pass lining system of bolted precast concrete segments, most of the tunnel leakage will take place at the joints. Leakage through the joint system can be minimized by properly designed and installed gaskets extending around the four edges of each segment. Ground shaking and ground static loads tend to deform the tunnel lining into an oval shape. However it might be possible to consider the deformation due to shaking as temporary. For a concrete segment lining, part of the distortion will take place at the longitudinal joints. This will lead to changes in joint opening near the edges where the gaskets are located and may significantly affect the sealant potential of the lining. The amount of deformation of the lining segments and the rotation of the joints is highly dependent on lining and soil stiffness. Tunnel lining stiffness is affected by many factors such as (a) number of segments in each ring, (b) shape of the contact surfaces along the longitudinal joint, (c) bolting between the segments, and (d) interaction and relative stiffness between segments of the adjacent rings. Other factors may cause joint opening. Segment misalignment, incomplete bolting between the segments and the rings, and muck accumulation between segments can cause larger gap opening than that caused by earthquake and static ground loads. These factors also need to be considered in the design of a gasketed lining.
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Gasketed precast concrete segment tunnel liner 1) Non uniform ground loads 2) Earthquake shaking Installation problems: 1) Manufacturing tolerances 2) Segment misalignment
Deformed liner Depends on lining stiffness & amount of distortion
3) Muck accumulation between the segments 4) Incomplete bolting
1) Change in the lining joint gaps 2) Change in gasket contact loads 3) Rings offset at the T-joint
Tunnel integrity and structural problems. e.g 1) Gasket groove damage 2) Gasket distortion
Loss in gasket confinement & reduction in gasket contact loads
Leakage problems ?
Fig. 1. Problems associated with the erection of gasketed tunnel lining.
Integrity of the tunnel lining is also an important issue that needs to be considered in tunnel design, especially in areas of earthquake. For precast concrete segments, the tunnel joints (especially the longitudinal joints) are the places where most of the tunnel permanent and temporary deformations take place. Using a convex geometry for the longitudinal joint is expected to minimize segment edge damage and spalling that might take place as a result of lining distortion and joint rotation. Any damage to the segment edges due to severe joint rotation will reduce the gasket confinement and may lead to leakage into the tunnel. But the use of convex to convex bearing surfaces for the longitudinal joints of the concrete segments tends to reduce the structural capacity due to splitting, which is the expected mode of failure for this kind of joint contact. Gaskets have been used as a sealant material for gas and water in bolted precast concrete segment linings for about 40 years. Although tests are conducted on gasket materials and sections of gaskets, there have been few studies to investigate the deformation and leakage characteristics of the concrete segment-gasket system, especially the effect of lining distortion and joint rotation that might take place as the lining is subjected to permanent deformation due to non-uniform ground loads, installation difficulties or temporary deformation due to earthquake shaking. Fig. 1 summarizes the problems associated with the erection of gasketed concrete tunnel lining. To obtain reasonable limits for the testing program over the full range of joint movement, joint rotation and opening for the tunnel lining were evaluated based on the conservative assumption that the lining segments were rigid, which means that all the lining distortion and rotation would take place only at the joints. Results of finite element analysis by Shalabi and Cording (2005) showed that the change in the joint gap of 5-segment rigid segments lining is
Long. joint
Circ. joint
T-joint
(a) Staggered concrete segment tunnel lining
Ring B
Ring A
Ring A
Ring B
Gasket Δ Gap Δ
(b) Lining deformation and rings offset Fig. 2. Tunnel lining offset and deformation with staggered concrete segment.
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F.I. Shalabi et al. / Tunnelling and Underground Space Technology 31 (2012) 51–60 Table 1 Engineering properties of elastomeric compounds (Construction Polymers, 2000). Common name Chemical name
Polyisoprene
EPDM Ethylene propylene
Neoprene Polychloroprene
Tensile strength (psi) Hardness range (shore A durometer) Specific gravity Elongation (Max% at room temp.) Service temp. Min Max Tear resistance Abrasion resistance Compression set Impermeability to gases Resistance to: Swelling in lubricating oil Oil and gasoline Water absorption Oxidation Ozone Flame Microbiological deterioration
>3000 40–90 1.15 750 60 212 Very good Excellent Excellent Good
>2000 40–90 1.15 600 60 212 Good Excellent Good Good
>2000 40–90 1.35 600 40 250 Good Very good Good Good
Poor Poor Very Very Very Poor Very
Poor Poor Very good Excellent Excellent Poor Good
Good Good Very good Very good Very good Very good Very good
good good good good
joint (segment–segment interaction) and the circumferential and longitudinal joints at the T-joint (ring and segment interaction) using open-base (finger-base) gaskets. The study focuses on gasket performance and sealant behavior under the effect of no joint rotation, effect of joint rotation (offset), shaking (cycling), timing, and segment damage.
almost twice as much as that of flexible segments lining. Fig. 2 shows the expected offset and deformation of two staggered rings of segmental concrete tunnel lining. Experience with flexible linings shows that, under the effect of installation and ground static loads, the initial tunnel lining is typically subjected to a change of 0.5% of its diameter (Peck, 1969) (20-ft-diameter tunnel will have 1.2 in. change in diameter). For LA Metro, under earthquake conditions, the free-field shear displacements for a 20-ft-diameter tunnel are predicted to be 0.52 in. and 1.2 in. for ODE and MDE respectively (giving a change in tunnel diameter of 0.26 and 0.60 in. for ODE and MDE respectively). These values are based on shear wave velocity of 640 ft/s, maximum horizontal ground accelerations of 0.3 g and 0.6 g, and peak particle velocities of 1.4 ft/s and 3.2 ft/s for ODE and MDE, respectively (Hendron and Fernandez, 1983). In this work, gasket performance and sealant behavior under gasket-segment and gasket-segment-ring interaction is studied. The study considers the leakage performance of the longitudinal
2. Testing setup and tests description The testing program focused on the gasket sealant potential to prevent gas and water from leaking into the tunnel through the joint system. Leakage tests were performed with gasket in groove concrete segments. Tests were performed at different joint gaps. During the tests, water pressure and total loads were measured. Leakage tests were performed before and after cyclic tests in order to investigate the effect of ground shaking on gasket sealant potential. Also, tests were performed using unaligned concrete segments
36.0''
Box steel beam
C-beams 1.5'' 2.0'' 1.0'' 2.0''
Load cell
54.5''
Steel columns
Servo ram
15.0''
68''
Point of rotation Gasket
Specimen
2.0'' 1'' 2''
Specimen
Load cell
Axial load ram
26'' 10''
Seg. support
Longitudinal joint
~15'' Test floor
Ram support
Fig. 3. Setup of the longitudinal joint cyclic tests (segment to segment).
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the ends of the two segments to allow this kind of rotation. The tests were performed under working load conditions with axial load of 125 kips (35 ft soil cover). The tests were performed by rotating the longitudinal joint an amount corresponding to joint ovalling due to ground static loads, then cycling the joint upward and downward around the offset an amount corresponding to the MDE rotation. Joint rotation due to ground static loads and MDE are 0.01 and
in order to investigate the sealant potential under the effect of permanent joint rotation. Table 1 summarizes the engineering properties of the used gaskets material. For the longitudinal joint performance, a test setup was designed with a servo-controlled ram to allow the longitudinal joint to be cycled upward and downward. The dimensions of the segments were 36 36 10 in. for each. Hinged supports were placed at
Reaction box beam attached to 4 H-columns
Load cells hyd. jac k
89''
6''
24'' 16''
36'' 56''
21x6x2'' plate
6'' 8.0'' 0.5 4''
18''
2
10.0''
Top beam
0.5
0.5 2
2
T- Joint
9''
9''
4' '
8''
Rotating rod
29'' 4 threaded bolts to apply Load cells normal loads on circ. joint
2''
Circ. joint Bracket for Ram
41''
Concrete block
Bolt pockets Long. joint
concrete Block
36''
Circ. joint T- Joint
Bottom beam
12''
6''
36''
36''
120'' Box beam 10 in. X14 in. Located at mid length of the C-beams 14 in.
Load cell Normal loads
C-beam 15 in. x 48 in.
Steel plates 4x4x0.5 in
Load cell
Hyd. Jack
15 in.
29 in.
Servo controlled ram for cycling
Gaskets
Steel column
Top of the concretebeam
41'' Concrete Block
36'' Bolts throught test floor
36''
60'' 66'' Bolt throught test floor
Fig. 4. T-joint cyclic test and water leakage test setup.
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0.0055 rad respectively. These values are based on the assumption that all the lining distortion and rotation would take place only at the joint. For 36-in. long concrete segments, the cycling amplitudes that cause this amount of joint rotations are 0.18 in. for ground static loads and 0.1 in. for MDE. Tests were performed at amplitudes a little more than the values mentioned above. Fig. 3 shows the design
setup for the longitudinal joint performance (segment–segment interaction). For T-Joint cyclic tests, a new test setup was designed to contain both longitudinal and circumferential tunnel joints and allowed testing of the tunnel lining segments under cyclic conditions, especially at the T-joint (interaction between longitudinal and circum-
1) Set the joint for the required gap opening by tightening the nuts
2) Measure the load cell to calculate the gasket initial contact pressure
3) Fill the area between the gaskets by water through the inlet. Evacuate all the air through the outlet
4) Get the water under pressure (Nitrogen gas was used for the tests). The pressure was applied in 5 psi increments
5) For each pressure increment, visually monitor any leakage around the gasket. Keep water under the current pressure for 1-hour
No
le a
k
Le ak
Increase the water pressure by 5 psi, and go to step 5
Keep the test under the same pressure for 1-hour.
e ag ak nue e L nt i co
Leakage stop
The current pressure is the leakage pressure for the joint gap. Record the load cells load to evaluate the gasket contact pressure at leakage and re-check the joint opening
increase the pressure by 5 psi and go to step 5
Fig. 5. Flow chart of water leakage test.
90
Water pressure at leakage (psi)
80
Ex1026 gasket (open-base gasket) Leakage took place between gasket and gasket groove
A : Before any cyclic test B : Before any cyclic test, and after reparring side groove C : Before any cyclic test and after fixing end rods for cyclic tests
70
D : After the first cyclic test
60 50
After cyclic tests
40
E
E : After the third cyclic test
D
F : Joint rotated by raising it upward 0.27 in. over 36'' segment length (0.17'' due to ground loads & 0.1'' due to MDE)
C
Before cyclic tests
30
B F
A
20 10 0 0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
0.18
0.20
Joint gap at leakage (in.) Fig. 6. Water pressure vs. joint gap at leakage condition for longitudinal joint (segment–segment) test.
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90
Water pressure at leakage (psi)
80
Ex1026 gasket (open-base gasket) Longitudinal joint gap = 0.0717 in
70
Leakage occurred between gasket and gasket groove of the longitudinal joint (Circ. joint gap = 0.063 '')
60
After the Ist cyclic test (after replacing the damaged segment), amp. = +-0.05 in.
Before cycling
D Time effect (T=2months)
A
50
G
40
E B
30 20
F
After cycling Leakage occurred through the top corner of the damaged segment
After the 2nd cyclic test (100 cyc., amp. = +-0.1 in.)
C
Before cycling line Before cycling (After replacing the damaged segment)
10 0 0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
0.18
0.20
Circumferential joint gap at leakage (in.) Fig. 7. Water pressure vs. joint gap at leakage condition for the circumferential for the T-Joint (before and after cycling) test.
Gasket
(a) Effect of cycling on the longitudinal joint gasket (Pure compression)
Packing
1/2 cyclic amplitude Destroyed contact and gasket separation
(b) Effect of cycling on the circumferential joint gasket (Rolling and shearing)
grooves to match the gaskets at the top and bottom of the two segments. The whole gasket system along the two sets of joints was forming a double I-shaped closed volume. Axial load, representing ground static load, was applied horizontally to one end of the two segments by using two 200-kip hydraulic rams with load cells. Four threaded bolts running through the two beams were used to control the circumferential joint opening. Load cells were attached to the threaded bolts in order to measure the normal loads along the circumferential joints. Cycling was performed by using a displacement-controlled ram with 25-kip capacity. The ram was attached to the two segments and displaced them laterally relative to the top and bottom beams. This relative movement is similar to the movement that would occur at the T-joint when the lining ovals as a result of ground shaking. Cycling led the segments to rotate at the convex–convex bearing surface with points of rotation located at segments far edges. The bottom beam was supported laterally by the testing floor using angles, while the top beam was supported by H-beams connected to the steel columns. This support system was expected to minimize the beams rotation and translation during cycling. Instrumentation was designed to measure the change in longitudinal joint opening. LVDTs were mounted at the top and bottom of the longitudinal joint on both sides of the segments. Other LVDTs were fixed at the ends of the segments to monitor segments translation and rotation. Fig. 4 shows the setup for cyclic and leakage tests of the T-joint. Water leakage tests were performed before and after cyclic tests. The tests were performed by applying incremental water pressures. Water proofing material was used to coat the inside concrete surfaces to prevent water from flowing into pores or small concrete channels beneath the surface. Fig. 5 summarizes the water leakage test procedures.
Fig. 8. Effect of cycling on gasket movement inside the groove.
3. Results and discussion ferential joints) for both damage and leakage performance. The dimensions of the segments were 36 36 10 in. Two concrete beams were placed at the bottom (89 21 12 in.) and top (89 21 18 in.) of the two segments to form two staggered-ring configurations. Each segment had two gaskets placed at the top, bottom, and vertical sides. The gaskets formed a closed double Ushape. The contact between the concrete segments and the two beams represented the circumferential joints. Two picture frame gaskets were placed at the inside face of the two beams in gasket
3.1. Open-base gasket performance under no joint rotation conditions 3.1.1. Longitudinal joint performance (before cycling) Results of water leakage test on the longitudinal joint (segment–segments interaction) using Ex1026 neoprene gasket showed that leakage started to occur at water pressure of 30 psi at joint gap of 0.0717 in. in which there was concrete to concrete contact at the center line of the bearing surfaces, as shown in Fig. 6 (point A). In this test, leakage took place between the gasket
57
Water pressure and gasket contact pressure at leakage (psi)
F.I. Shalabi et al. / Tunnelling and Underground Space Technology 31 (2012) 51–60
220 Ex1026 gasket (open-base gasket)
200
Leakage took place between gasket and gasket groove
180 160 140 120 100 80
Picture frame steel device (Shalabi 2001)
60 Leakage continues
40 T-joint concrete segment setup
20 0 0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
Joint gap (in.) Fig. 9. Water pressure at leakage for T-joint setup and picture frame steel device.
and the gasket groove at one of the segment corners. Leakage occurred at the corner because of the lack of confinement at the ends of the gasket groove (the steel strips that were used to cast the gasket grooves were extended to the segment ends). Filling the corners of the segments with mortar to provide confinement for the gasket at the corners increased the water pressure at leakage to 35 psi, before the leak started to occur again between the gasket and gasket groove at the corners. (Fig. 6, point B). 3.1.2. T-joint leakage tests (gasket-ring interaction) Results of water leakage test on Ex1026 showed that leakage started to occur at the longitudinal joint at a joint gap of 0.0717 in. and water pressure of 50 psi, as shown in Fig. 7 (point A). Leakage occurred between the gasket and gasket groove at the top and bottom of the longitudinal joint near the T-joints (not at the T-joint). The circumferential joint gaps at leakage were 0.063 in. The gasket-ring interaction at the T-joint held more water pressure than the segment–segment interaction at the same joint gap. For the longitudinal joint, leakage occurred between the gasket and gasket groove at the corner at a water pressure of 35 psi. This can be explained due to the effect of the amount of the gasket volume at these joints. T-joint corners usually had more gasket material than the corners of the longitudinal joint, which means more contact stresses at the T-joint. Paul (1978), based on water leakage tests on neoprene gasket using steel frame device, found that leakage is most likely to occur at the straight portion of the joint rather than at the T-joint. 3.2. Open-base gasket performance under joint rotation conditions Results of water leakage tests on the longitudinal joint of the concrete segments (segment–segment) showed that joint rotation caused a reduction in the gasket sealant capacity. Fig. 6 shows that at 0.27 in. longitudinal joint displacement from aligned position (0.17 in. due to ground static loads and 0.1 in. due to MDE), which represents a joint rotation of 0.0155 rad, water pressure at leakage was 31 psi (point F). Before joint rotation, water pressure at leakage was 50 psi (point E). The reduction in water pressure from 50 psi to 31 psi at leakage was mainly due to the increase in the longitudinal joint gap from 0.0717 in. where the segments were aligned to 0.12 in. where the segments were rotated 0.0155 rad, which in turn resulted in a reduction in gasket contact pressure. The leakage test on the rotated segments (point F, Fig. 6) was performed after a series of cyclic and water leakage tests on joints that were not rotated.
Top beam
C-Joint Leakage zones at 45-50 psi water pressure
L-Joint
C-Joint
Bottom beam
Fig. 10. Leakage near the T-joint corner before any cycling.
3.3. Effect of ground shaking on open-base gasket sealant behavior Results of water leakage tests on the longitudinal joint (segment–segment) and T-joint showed that there was an effect of cycling on gasket performance. As can be seen in Fig. 6, cycling of the longitudinal joint increased the gasket sealant capacity from a leakage pressure of 35 psi before cycling to 53 psi after cycling (50% increase in gasket performance). On the other hand, for the T-joint tests, the leakage test results showed that cycling does not always improve the gasket performance after each cyclic test. As can be seen in Fig. 7, before cycling, leakage occurred between the gasket and gasket groove at 30 psi water pressure (point C). After the first cyclic test, the gasket started to leak at the gasket groove interface at 55 psi water pressure (point D) while after the second cyclic test, the water pressure at leakage dropped to 35 psi (point E). The difference in gasket performance in longitudinal joint and T-joint was mainly due to the difference in the gasket response to cycling. For the longitudinal joint tests, cycling subjected the gasket to cycles of pure compression–uncompression inside the gasket groove, as can be seen in Fig. 8a. This type of gasket deformation tends to increase the bonding between the bottom of the gasket and the gasket groove (or at least not cause damage to this bond) and led to an increase in gasket sealant potential. For T-
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the cause of water pressure drop at leakage from point D to point E (Fig. 7).
joint tests, the increase in gasket performance after the first cyclic test (Fig. 7, point D) is expected to be due to the following reasons: (1) Redistribution of the normal loads along the circumferential joint. Before cycling, the normal load due to gasket compression may not be uniform. This may cause leakage in the areas where the local normal loads are less than the average normal loads. (2) Zones along the circumferential joint where the section of the gasket is not uniformly compressed inside the gasket groove. After the first cycling, the gasket tended to move laterally to fit the groove in a more uniform way.
3.4. Effect of time on gasket performance Water leakage tests on Ex1026 gasket showed that there was a time effect on gasket sealant behavior. As shown in Fig. 7, leaving the T-joint system undisturbed for two months increased the water pressure at leakage from 35 psi (point F) to more than 40 psi (point G). (Uncontrolled leakage started to occur through porous concrete at the outside of the gasket groove at 40 psi. Tests were stopped to seal these pores). The increase in gasket performance with time is expected to be due to two reasons. The first one is the healing process that takes place at the gasket–gasket groove interface. During the waiting time period between the two water leakage tests, bonds between the gasket and gasket groove might be re-built. The second reason is the buildup in gasket contact pressure with time (gasket recovery). Water leakage test of point F was performed shortly after a series of cyclic tests under circumferential joint unloading (opening the joint gap after each cyclic test). During the 2-month period, the gasket at point G
After series of cyclic tests (100 cycles with ±0.1 in. amplitude), the movement of the gasket inside the gasket groove started to have a destructive effect on gasket performance. Intensive lateral movement of the gasket inside the gasket groove tends to destroy the adhesive bond between the gasket and the gasket groove. Fig. 8b shows an illustrative drawing for the lateral movement of the circumferential joint gasket during cycling and the effect of this movement on gasket–groove contact. This effect is expected to be
Top beam 8''
Leakage occurred through the damaged part at 35 psi water pressure
9''
C-Joint
L-Joint
Damaged segment
C-Joint
West
Bottom beam
(a) Front
Top beam
5''
13''
Bottom beam
(b) Back Fig. 11. Leakage through the damaged corner after the first T-joint cyclic test.
West
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Damaged due to cycling. Epoxy and hydrocal were used for repairing
Top beam
Top beam Repaired zone ~27'' C-Joint
C-Joint Leakage through the repaired zone of the gasket groove at 35 psi
Leakage between gasket & gasket groove at 21 psi
L-Joint
C-Joint
L-Joint
C-Joint Bottom beam
Bottom beam
Leakage through porous concrete beneath gasket groove
Leakage between gasket and gasket groove at 55 psi
Fig. 14. Leakage between gasket and gasket groove and through porous concrete beneath gasket groove after the first cyclic test; EPDM gasket.
Fig. 12. Leakage through the T-joint corner and the repaired zone after cyclic test, EX1026 gasket.
Top beam Top beam
C-Joint
C-Joint L-Joint
Leakage through concrete at 35 psi
L-Joint Repaired corner with epoxy and steel anchors
C-Joint
C-Joint
Bottom beam
Bottom beam
(a) Front Leakage between gasket & gasket groove at the repaired corner at 20.7 psi
Leakage through porous concrete beneath gasket groove
Top beam Fig. 15. Leakage between gasket and gasket groove and through porous concrete after repairing the damaged segment, EPDM gasket.
C-Joint Leakage between gasket and gasket groove at 35 psi
3.5. Difference in open-base gasket performance in steel forms and concrete segments
L-Joint C-Joint
Bottom beam
(b) Back Fig. 13. Leakage through the concrete around the groove and between gasket and groove after first cyclic test (after repairing the top beam).
gained more contact pressure than that at point F due to gasket recovery. Unfortunately, there were no load measurements for the water leakage tests.
Results of water leakage tests showed that there was a difference between the gasket performance in steel forms and in concrete segments. As can be seen in Fig. 9, concrete segment tests showed that Ex1026 gasket could hold a water pressure in the range of 30–50 psi for a joint gap in the range of 0.135 in. and 0.080 in., respectively. On the other hand, the same gasket did not show any leakage when it was tested in picture frame steel device at a joint gap of 0.16 in. and a water pressure of 100 psi (Shalabi, 2001 and Shalabi et al., 2009). The difference in gasket performance in steel forms and concrete segments is considered primarily due to the effect of concrete roughness between the gasket and gasket groove and variation in gasket groove tolerances. Also, due to the porous nature of the concrete, open water channels
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underneath the gasket can form under water pressure and cause leakage at or just beneath the interface between gasket and gasket groove. Water channels can also cause leakage through the sides of the gasket groove and around the bolt pockets where the porosity of the concrete is high. High concrete porosity around the gasket groove and bolt pockets was mainly due to inefficient concrete compaction around the groove and the pockets.
continued to occur through concrete channels and between gasket and gasket groove of the repaired corner at water pressure of 20 psi. The reduction in water pressure at leakage was due to the increase in the size of the open channels in the concrete that were located around the gasket groove, as leakage continued.
3.6. Locations of leakage in the concrete segments and the effect of segments damage on leakage
For water leakage tests, pairs of concrete segments were tested using open-base (finger-base) Ex1026 gasket. Tests were performed for aligned segments, 0.27 in. longitudinal joint lateral movement (0.17 in. due to ground static loads and 0.1 in. due to Maximum Design Earthquake), and after cycling the joint to the MDE. The results showed the following:
Results of water leakage tests using T-joint and longitudinal joint test setups showed that leakage of the segmental concrete lining could take place through the following areas: (1) Leakage near the T-joint between the gasket and gasket groove. (2) Leakage at the interface between the gasket and gasket groove. (3) Leakage through porous concrete at the gasket groove sides. (4) Leakage through the damaged zones of the gasket grooves. (5) Leakage through the bolt pockets. (6) Leakage through micro-cracks and porous concrete around the bolt pockets. Longitudinal joint water leakage tests on Ex-1026 gasket showed that leakage initially occurred between the gasket and gasket groove interface at water pressure of 35 psi before cycling. After couple of leakage tests, leakage started to occur through porous concrete at the sides of the bolt pockets and at the segment surface near the corners of the pockets. Leakage through the porous concrete started at nearly 30–35 psi water pressure and later on at 10–15 psi when the size of the pores got larger as leakage continued. Filling the pockets and covering the segment face near the pockets with epoxy stopped leakage through the concrete. Before starting cyclic tests, water leakage tests on the Ex1026 gasket showed that leakage took place near the T-joints at water pressure of 45–50 psi, as shown in Fig. 10. During the first cyclic test, damage to the top corners of the west side concrete segment took place, as shown in Fig. 11. Leakage occurred through the damaged corner at about 35 psi water pressure. After replacing the damaged segment with a new one and repairing the top beam damaged groove with epoxy and hydrocal, leakage tests after the T-1–3 cyclic test showed that leakage started to occur at the bottom T-joint between the gasket and gasket groove and through the repaired zone of the top beam gasket groove at 55 and 35 psi water pressure respectively. Fig. 12 shows the locations of leakage of this test. After repairing the top beam gasket groove with just epoxy and steel bars, water leakage tests showed a shift in the location of leakage. After the T-1–6 cyclic test, water started to leak through porous concrete around the gasket grooves of the circumferential joint and through the interface between the gasket and gasket groove at 35 psi water pressure. The locations of leakage are shown in Fig. 13. Leakage through the porous concrete around the gasket groove of the circumferential joint continued even after repairing the outside groove by a 1-in-thick strip of epoxy material. Tests on the EPDM gasket before cyclic showed that leakage through the porous concrete and between the gasket and gasket groove started to occur at a water pressure of 21 psi. Leakage locations of this test are in Fig. 14. During the first cyclic test, damage to the bottom corner of the east segment took place as shown in Fig. 15. The damaged corner was removed and a cast epoxy corner was molded. Steel plate and steel anchors were used to provide lateral support for the gasket at the corner. Water leakage tests showed that leakage
4. Conclusions
(1) Leakage of concrete segments with open base gaskets occurred at the gasket/gasket–groove interface. (2) Longitudinal joint lateral movement reduced the gasket sealant capacity as the joint gap opened. (3) Segment to segment cycling improved the longitudinal joint sealant capacity. The increase in water pressure at leakage after cycling was mainly due to the increase in bonding between the gasket and gasket groove as the gasket was subjected to pure compression and uncompression during cycling, and due to better seating of the gasket inside the groove after cycling. (4) Gasket sealant capacity had been improved after the first cyclic test (12 cycles at ±0.05 in. amplitude), while after severe cycling (100 cycles and ±0.1 in. amplitude), gasket sealant capacity was reduced. (5) Damage to the side of the gasket groove reduced the gasket confinement, which in turn reduced the gasket sealant capacity. (6) Gasket sealant capacity improved slightly with time. The increase was mainly due to the healing process that took place at the gasket/gasket–groove interface and due to the buildup in gasket contact pressure with time (gasket recovery). (7) Concrete around the gasket groove, cracked zones, gasket– gasket groove interface, and bolting pockets were the places of leakage in concrete segments. (8) Gasket sealant capacity of picture frame steel device was higher than that of the concrete segment at the same joint gap.
References Construction Polymers Company, 2000. Tunnel liner gasket systems, USA. Cording, E.J., Paul, S., Rood, M., Shalabi, F., Lee, S., 1998. Testing program for selection of the joint system for concrete segments for the LA Metro Red Line extension. Unpublished report, University of Illinois at Urbana-Champaign. Hendron, J., Fernandez G., 1983. Dynamic and static design consideration for underground champers. In: Proc., Seismic Design of Embankments and Caverns, ASCE National Convention, Philadelphia, Pennsylvania, May 16–20. Paul, S.L., 1978. Sealability tests of gaskets between precast concrete tunnel lining segments. Unpublished report, Dept. of Civil Engineering, Univ. of Illinois at Urbana-Champaign. Peck, R.B., 1969. Deep excavation and tunneling in soft ground. In: Proc., Seventh International Conference on Soil Mechanics and Foundation Engineering, Mexico City, state-of-the-art volume. Shalabi, F.I., 2001. Behavior of gasketed segmental concrete tunnel lining. Ph.D. Thesis, University of Illinois at Urbana-Champaign, Urbana, IL, USA. Shalabi, F., Cording, E., 2005. 3D-finite element analysis of segmental concrete tunnel lining deformation and moments under the effect of static and earthquake loading. In: 11th International Conference on Computer Methods and Advances in Geomechanics (IACMAG), Torino, Italy, June 19–24. Shalabi, F.I., Cording, E.J., Paul, S.L., 2009. Sealant behavior of gasketed segmental tunnel lining. In: Proceedings of the Workshop of the International Tunneling and Underground Space Association, Underground Structures in Hot Climate Conditions, Riyadh, KSA, December 8–9.
Contents lists available at SciVerse ScienceDirect
Tunnelling and Underground Space Technology journal homepage: www.elsevier.com/locate/tust
Concrete segment tunnel lining sealant performance under earthquake loading Faisal I. Shalabi a,⇑, Edward J. Cording b, Stanley L. Paul b a b
Department of Civil Engineering, King Faisal University, Al-Ahssa, Saudi Arabia Department of Civil Engineering, University of Illinois at Urbana-Champaign, Urbana, IL, USA
a r t i c l e
i n f o
Article history: Received 21 October 2011 Received in revised form 30 March 2012 Accepted 2 April 2012 Available online 1 May 2012 Keywords: Tunnel Lining Concrete segments Gasket sealant Earthquake
a b s t r a c t This work was conducted to provide an understanding of the leakage behavior of gasketed segmental tunnel linings subjected to static ground loads and earthquake shaking of the recommended design of the Los Angeles (LA) Metro. The proposed lining was made of bolted and double gasketed precast concrete segment lining with convex to convex longitudinal joint surfaces. Lining evaluation included the sealant performance of different gasket materials under water pressure less than 90 psi (Neoprene and EPDM gaskets with open base. Testing program was designed to evaluate the longitudinal joint and T-joint sealant behavior under static and dynamic loading using large scale concrete segments. The results showed that concrete around the gasket groove, cracked zones, gasket–gasket groove interface, and bolting pockets are the places of leakage in concrete segments. Besides that, damage to the side of the gasket groove reduced the gasket confinement and led to leakage. Also, the results showed that longitudinal joint sealant capacity was improved by cycling as a result of the increase in bonding between the gasket and gasket groove. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction The use of bolted precast concrete segments as initial and final tunnel lining (single-pass system), mostly eliminates the need for using two pass lining system, which consists of initial lining and final lining. Initial lining for a two-pass lining system is usually shotcrete and rock bolts in rock ground, while in soil it can be steel ribs and lagging or expanded segmental lining. The final lining is usually cast-in-place plain or reinforced concrete. One advantage of using bolted concrete segments (single-pass system) rather than an expanded segmental lining as initial support for the two-pass system is that the ground loss and movement that might occur behind the shield tail can be minimized. This can be done by grouting between the bolted segments and the ground while the shield is moving forward and before the gap start to be filled with soil. In the case of expanded segment lining, expansion cannot be reached until the lining is cleared from the tail of the shield at the end of the shove which leads to considerable ground loss and settlement as the soil rapidly invades the gaps behind the lining (Cording et al., 1998). This issue becomes very critical for tunneling through granular soils in urban areas. Another advantage of using the single-pass bolted concrete segments lining rather than the expanded lining as initial support for two-pass system, is the construction schedule, which is less for the former lining system.
⇑ Corresponding author. E-mail addresses: [email protected] (F.I. Shalabi), [email protected] (E.J. Cording), [email protected] (S.L. Paul). 0886-7798/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tust.2012.04.006
For transportation tunnels in sandy materials below water table, control of leakage is one of the most important problems to be considered in lining design. Presence of gases, such as hydrogen sulfide or methane may cause serious safety problems if the gas and water leak into the tunnel. One of the solutions to this problem is to use a membrane between the initial and final linings to serve as a barrier for gas and water flow. For a single-pass lining system of bolted precast concrete segments, most of the tunnel leakage will take place at the joints. Leakage through the joint system can be minimized by properly designed and installed gaskets extending around the four edges of each segment. Ground shaking and ground static loads tend to deform the tunnel lining into an oval shape. However it might be possible to consider the deformation due to shaking as temporary. For a concrete segment lining, part of the distortion will take place at the longitudinal joints. This will lead to changes in joint opening near the edges where the gaskets are located and may significantly affect the sealant potential of the lining. The amount of deformation of the lining segments and the rotation of the joints is highly dependent on lining and soil stiffness. Tunnel lining stiffness is affected by many factors such as (a) number of segments in each ring, (b) shape of the contact surfaces along the longitudinal joint, (c) bolting between the segments, and (d) interaction and relative stiffness between segments of the adjacent rings. Other factors may cause joint opening. Segment misalignment, incomplete bolting between the segments and the rings, and muck accumulation between segments can cause larger gap opening than that caused by earthquake and static ground loads. These factors also need to be considered in the design of a gasketed lining.
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Gasketed precast concrete segment tunnel liner 1) Non uniform ground loads 2) Earthquake shaking Installation problems: 1) Manufacturing tolerances 2) Segment misalignment
Deformed liner Depends on lining stiffness & amount of distortion
3) Muck accumulation between the segments 4) Incomplete bolting
1) Change in the lining joint gaps 2) Change in gasket contact loads 3) Rings offset at the T-joint
Tunnel integrity and structural problems. e.g 1) Gasket groove damage 2) Gasket distortion
Loss in gasket confinement & reduction in gasket contact loads
Leakage problems ?
Fig. 1. Problems associated with the erection of gasketed tunnel lining.
Integrity of the tunnel lining is also an important issue that needs to be considered in tunnel design, especially in areas of earthquake. For precast concrete segments, the tunnel joints (especially the longitudinal joints) are the places where most of the tunnel permanent and temporary deformations take place. Using a convex geometry for the longitudinal joint is expected to minimize segment edge damage and spalling that might take place as a result of lining distortion and joint rotation. Any damage to the segment edges due to severe joint rotation will reduce the gasket confinement and may lead to leakage into the tunnel. But the use of convex to convex bearing surfaces for the longitudinal joints of the concrete segments tends to reduce the structural capacity due to splitting, which is the expected mode of failure for this kind of joint contact. Gaskets have been used as a sealant material for gas and water in bolted precast concrete segment linings for about 40 years. Although tests are conducted on gasket materials and sections of gaskets, there have been few studies to investigate the deformation and leakage characteristics of the concrete segment-gasket system, especially the effect of lining distortion and joint rotation that might take place as the lining is subjected to permanent deformation due to non-uniform ground loads, installation difficulties or temporary deformation due to earthquake shaking. Fig. 1 summarizes the problems associated with the erection of gasketed concrete tunnel lining. To obtain reasonable limits for the testing program over the full range of joint movement, joint rotation and opening for the tunnel lining were evaluated based on the conservative assumption that the lining segments were rigid, which means that all the lining distortion and rotation would take place only at the joints. Results of finite element analysis by Shalabi and Cording (2005) showed that the change in the joint gap of 5-segment rigid segments lining is
Long. joint
Circ. joint
T-joint
(a) Staggered concrete segment tunnel lining
Ring B
Ring A
Ring A
Ring B
Gasket Δ Gap Δ
(b) Lining deformation and rings offset Fig. 2. Tunnel lining offset and deformation with staggered concrete segment.
53
F.I. Shalabi et al. / Tunnelling and Underground Space Technology 31 (2012) 51–60 Table 1 Engineering properties of elastomeric compounds (Construction Polymers, 2000). Common name Chemical name
Polyisoprene
EPDM Ethylene propylene
Neoprene Polychloroprene
Tensile strength (psi) Hardness range (shore A durometer) Specific gravity Elongation (Max% at room temp.) Service temp. Min Max Tear resistance Abrasion resistance Compression set Impermeability to gases Resistance to: Swelling in lubricating oil Oil and gasoline Water absorption Oxidation Ozone Flame Microbiological deterioration
>3000 40–90 1.15 750 60 212 Very good Excellent Excellent Good
>2000 40–90 1.15 600 60 212 Good Excellent Good Good
>2000 40–90 1.35 600 40 250 Good Very good Good Good
Poor Poor Very Very Very Poor Very
Poor Poor Very good Excellent Excellent Poor Good
Good Good Very good Very good Very good Very good Very good
good good good good
joint (segment–segment interaction) and the circumferential and longitudinal joints at the T-joint (ring and segment interaction) using open-base (finger-base) gaskets. The study focuses on gasket performance and sealant behavior under the effect of no joint rotation, effect of joint rotation (offset), shaking (cycling), timing, and segment damage.
almost twice as much as that of flexible segments lining. Fig. 2 shows the expected offset and deformation of two staggered rings of segmental concrete tunnel lining. Experience with flexible linings shows that, under the effect of installation and ground static loads, the initial tunnel lining is typically subjected to a change of 0.5% of its diameter (Peck, 1969) (20-ft-diameter tunnel will have 1.2 in. change in diameter). For LA Metro, under earthquake conditions, the free-field shear displacements for a 20-ft-diameter tunnel are predicted to be 0.52 in. and 1.2 in. for ODE and MDE respectively (giving a change in tunnel diameter of 0.26 and 0.60 in. for ODE and MDE respectively). These values are based on shear wave velocity of 640 ft/s, maximum horizontal ground accelerations of 0.3 g and 0.6 g, and peak particle velocities of 1.4 ft/s and 3.2 ft/s for ODE and MDE, respectively (Hendron and Fernandez, 1983). In this work, gasket performance and sealant behavior under gasket-segment and gasket-segment-ring interaction is studied. The study considers the leakage performance of the longitudinal
2. Testing setup and tests description The testing program focused on the gasket sealant potential to prevent gas and water from leaking into the tunnel through the joint system. Leakage tests were performed with gasket in groove concrete segments. Tests were performed at different joint gaps. During the tests, water pressure and total loads were measured. Leakage tests were performed before and after cyclic tests in order to investigate the effect of ground shaking on gasket sealant potential. Also, tests were performed using unaligned concrete segments
36.0''
Box steel beam
C-beams 1.5'' 2.0'' 1.0'' 2.0''
Load cell
54.5''
Steel columns
Servo ram
15.0''
68''
Point of rotation Gasket
Specimen
2.0'' 1'' 2''
Specimen
Load cell
Axial load ram
26'' 10''
Seg. support
Longitudinal joint
~15'' Test floor
Ram support
Fig. 3. Setup of the longitudinal joint cyclic tests (segment to segment).
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the ends of the two segments to allow this kind of rotation. The tests were performed under working load conditions with axial load of 125 kips (35 ft soil cover). The tests were performed by rotating the longitudinal joint an amount corresponding to joint ovalling due to ground static loads, then cycling the joint upward and downward around the offset an amount corresponding to the MDE rotation. Joint rotation due to ground static loads and MDE are 0.01 and
in order to investigate the sealant potential under the effect of permanent joint rotation. Table 1 summarizes the engineering properties of the used gaskets material. For the longitudinal joint performance, a test setup was designed with a servo-controlled ram to allow the longitudinal joint to be cycled upward and downward. The dimensions of the segments were 36 36 10 in. for each. Hinged supports were placed at
Reaction box beam attached to 4 H-columns
Load cells hyd. jac k
89''
6''
24'' 16''
36'' 56''
21x6x2'' plate
6'' 8.0'' 0.5 4''
18''
2
10.0''
Top beam
0.5
0.5 2
2
T- Joint
9''
9''
4' '
8''
Rotating rod
29'' 4 threaded bolts to apply Load cells normal loads on circ. joint
2''
Circ. joint Bracket for Ram
41''
Concrete block
Bolt pockets Long. joint
concrete Block
36''
Circ. joint T- Joint
Bottom beam
12''
6''
36''
36''
120'' Box beam 10 in. X14 in. Located at mid length of the C-beams 14 in.
Load cell Normal loads
C-beam 15 in. x 48 in.
Steel plates 4x4x0.5 in
Load cell
Hyd. Jack
15 in.
29 in.
Servo controlled ram for cycling
Gaskets
Steel column
Top of the concretebeam
41'' Concrete Block
36'' Bolts throught test floor
36''
60'' 66'' Bolt throught test floor
Fig. 4. T-joint cyclic test and water leakage test setup.
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F.I. Shalabi et al. / Tunnelling and Underground Space Technology 31 (2012) 51–60
0.0055 rad respectively. These values are based on the assumption that all the lining distortion and rotation would take place only at the joint. For 36-in. long concrete segments, the cycling amplitudes that cause this amount of joint rotations are 0.18 in. for ground static loads and 0.1 in. for MDE. Tests were performed at amplitudes a little more than the values mentioned above. Fig. 3 shows the design
setup for the longitudinal joint performance (segment–segment interaction). For T-Joint cyclic tests, a new test setup was designed to contain both longitudinal and circumferential tunnel joints and allowed testing of the tunnel lining segments under cyclic conditions, especially at the T-joint (interaction between longitudinal and circum-
1) Set the joint for the required gap opening by tightening the nuts
2) Measure the load cell to calculate the gasket initial contact pressure
3) Fill the area between the gaskets by water through the inlet. Evacuate all the air through the outlet
4) Get the water under pressure (Nitrogen gas was used for the tests). The pressure was applied in 5 psi increments
5) For each pressure increment, visually monitor any leakage around the gasket. Keep water under the current pressure for 1-hour
No
le a
k
Le ak
Increase the water pressure by 5 psi, and go to step 5
Keep the test under the same pressure for 1-hour.
e ag ak nue e L nt i co
Leakage stop
The current pressure is the leakage pressure for the joint gap. Record the load cells load to evaluate the gasket contact pressure at leakage and re-check the joint opening
increase the pressure by 5 psi and go to step 5
Fig. 5. Flow chart of water leakage test.
90
Water pressure at leakage (psi)
80
Ex1026 gasket (open-base gasket) Leakage took place between gasket and gasket groove
A : Before any cyclic test B : Before any cyclic test, and after reparring side groove C : Before any cyclic test and after fixing end rods for cyclic tests
70
D : After the first cyclic test
60 50
After cyclic tests
40
E
E : After the third cyclic test
D
F : Joint rotated by raising it upward 0.27 in. over 36'' segment length (0.17'' due to ground loads & 0.1'' due to MDE)
C
Before cyclic tests
30
B F
A
20 10 0 0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
0.18
0.20
Joint gap at leakage (in.) Fig. 6. Water pressure vs. joint gap at leakage condition for longitudinal joint (segment–segment) test.
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F.I. Shalabi et al. / Tunnelling and Underground Space Technology 31 (2012) 51–60
90
Water pressure at leakage (psi)
80
Ex1026 gasket (open-base gasket) Longitudinal joint gap = 0.0717 in
70
Leakage occurred between gasket and gasket groove of the longitudinal joint (Circ. joint gap = 0.063 '')
60
After the Ist cyclic test (after replacing the damaged segment), amp. = +-0.05 in.
Before cycling
D Time effect (T=2months)
A
50
G
40
E B
30 20
F
After cycling Leakage occurred through the top corner of the damaged segment
After the 2nd cyclic test (100 cyc., amp. = +-0.1 in.)
C
Before cycling line Before cycling (After replacing the damaged segment)
10 0 0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
0.18
0.20
Circumferential joint gap at leakage (in.) Fig. 7. Water pressure vs. joint gap at leakage condition for the circumferential for the T-Joint (before and after cycling) test.
Gasket
(a) Effect of cycling on the longitudinal joint gasket (Pure compression)
Packing
1/2 cyclic amplitude Destroyed contact and gasket separation
(b) Effect of cycling on the circumferential joint gasket (Rolling and shearing)
grooves to match the gaskets at the top and bottom of the two segments. The whole gasket system along the two sets of joints was forming a double I-shaped closed volume. Axial load, representing ground static load, was applied horizontally to one end of the two segments by using two 200-kip hydraulic rams with load cells. Four threaded bolts running through the two beams were used to control the circumferential joint opening. Load cells were attached to the threaded bolts in order to measure the normal loads along the circumferential joints. Cycling was performed by using a displacement-controlled ram with 25-kip capacity. The ram was attached to the two segments and displaced them laterally relative to the top and bottom beams. This relative movement is similar to the movement that would occur at the T-joint when the lining ovals as a result of ground shaking. Cycling led the segments to rotate at the convex–convex bearing surface with points of rotation located at segments far edges. The bottom beam was supported laterally by the testing floor using angles, while the top beam was supported by H-beams connected to the steel columns. This support system was expected to minimize the beams rotation and translation during cycling. Instrumentation was designed to measure the change in longitudinal joint opening. LVDTs were mounted at the top and bottom of the longitudinal joint on both sides of the segments. Other LVDTs were fixed at the ends of the segments to monitor segments translation and rotation. Fig. 4 shows the setup for cyclic and leakage tests of the T-joint. Water leakage tests were performed before and after cyclic tests. The tests were performed by applying incremental water pressures. Water proofing material was used to coat the inside concrete surfaces to prevent water from flowing into pores or small concrete channels beneath the surface. Fig. 5 summarizes the water leakage test procedures.
Fig. 8. Effect of cycling on gasket movement inside the groove.
3. Results and discussion ferential joints) for both damage and leakage performance. The dimensions of the segments were 36 36 10 in. Two concrete beams were placed at the bottom (89 21 12 in.) and top (89 21 18 in.) of the two segments to form two staggered-ring configurations. Each segment had two gaskets placed at the top, bottom, and vertical sides. The gaskets formed a closed double Ushape. The contact between the concrete segments and the two beams represented the circumferential joints. Two picture frame gaskets were placed at the inside face of the two beams in gasket
3.1. Open-base gasket performance under no joint rotation conditions 3.1.1. Longitudinal joint performance (before cycling) Results of water leakage test on the longitudinal joint (segment–segments interaction) using Ex1026 neoprene gasket showed that leakage started to occur at water pressure of 30 psi at joint gap of 0.0717 in. in which there was concrete to concrete contact at the center line of the bearing surfaces, as shown in Fig. 6 (point A). In this test, leakage took place between the gasket
57
Water pressure and gasket contact pressure at leakage (psi)
F.I. Shalabi et al. / Tunnelling and Underground Space Technology 31 (2012) 51–60
220 Ex1026 gasket (open-base gasket)
200
Leakage took place between gasket and gasket groove
180 160 140 120 100 80
Picture frame steel device (Shalabi 2001)
60 Leakage continues
40 T-joint concrete segment setup
20 0 0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
Joint gap (in.) Fig. 9. Water pressure at leakage for T-joint setup and picture frame steel device.
and the gasket groove at one of the segment corners. Leakage occurred at the corner because of the lack of confinement at the ends of the gasket groove (the steel strips that were used to cast the gasket grooves were extended to the segment ends). Filling the corners of the segments with mortar to provide confinement for the gasket at the corners increased the water pressure at leakage to 35 psi, before the leak started to occur again between the gasket and gasket groove at the corners. (Fig. 6, point B). 3.1.2. T-joint leakage tests (gasket-ring interaction) Results of water leakage test on Ex1026 showed that leakage started to occur at the longitudinal joint at a joint gap of 0.0717 in. and water pressure of 50 psi, as shown in Fig. 7 (point A). Leakage occurred between the gasket and gasket groove at the top and bottom of the longitudinal joint near the T-joints (not at the T-joint). The circumferential joint gaps at leakage were 0.063 in. The gasket-ring interaction at the T-joint held more water pressure than the segment–segment interaction at the same joint gap. For the longitudinal joint, leakage occurred between the gasket and gasket groove at the corner at a water pressure of 35 psi. This can be explained due to the effect of the amount of the gasket volume at these joints. T-joint corners usually had more gasket material than the corners of the longitudinal joint, which means more contact stresses at the T-joint. Paul (1978), based on water leakage tests on neoprene gasket using steel frame device, found that leakage is most likely to occur at the straight portion of the joint rather than at the T-joint. 3.2. Open-base gasket performance under joint rotation conditions Results of water leakage tests on the longitudinal joint of the concrete segments (segment–segment) showed that joint rotation caused a reduction in the gasket sealant capacity. Fig. 6 shows that at 0.27 in. longitudinal joint displacement from aligned position (0.17 in. due to ground static loads and 0.1 in. due to MDE), which represents a joint rotation of 0.0155 rad, water pressure at leakage was 31 psi (point F). Before joint rotation, water pressure at leakage was 50 psi (point E). The reduction in water pressure from 50 psi to 31 psi at leakage was mainly due to the increase in the longitudinal joint gap from 0.0717 in. where the segments were aligned to 0.12 in. where the segments were rotated 0.0155 rad, which in turn resulted in a reduction in gasket contact pressure. The leakage test on the rotated segments (point F, Fig. 6) was performed after a series of cyclic and water leakage tests on joints that were not rotated.
Top beam
C-Joint Leakage zones at 45-50 psi water pressure
L-Joint
C-Joint
Bottom beam
Fig. 10. Leakage near the T-joint corner before any cycling.
3.3. Effect of ground shaking on open-base gasket sealant behavior Results of water leakage tests on the longitudinal joint (segment–segment) and T-joint showed that there was an effect of cycling on gasket performance. As can be seen in Fig. 6, cycling of the longitudinal joint increased the gasket sealant capacity from a leakage pressure of 35 psi before cycling to 53 psi after cycling (50% increase in gasket performance). On the other hand, for the T-joint tests, the leakage test results showed that cycling does not always improve the gasket performance after each cyclic test. As can be seen in Fig. 7, before cycling, leakage occurred between the gasket and gasket groove at 30 psi water pressure (point C). After the first cyclic test, the gasket started to leak at the gasket groove interface at 55 psi water pressure (point D) while after the second cyclic test, the water pressure at leakage dropped to 35 psi (point E). The difference in gasket performance in longitudinal joint and T-joint was mainly due to the difference in the gasket response to cycling. For the longitudinal joint tests, cycling subjected the gasket to cycles of pure compression–uncompression inside the gasket groove, as can be seen in Fig. 8a. This type of gasket deformation tends to increase the bonding between the bottom of the gasket and the gasket groove (or at least not cause damage to this bond) and led to an increase in gasket sealant potential. For T-
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F.I. Shalabi et al. / Tunnelling and Underground Space Technology 31 (2012) 51–60
the cause of water pressure drop at leakage from point D to point E (Fig. 7).
joint tests, the increase in gasket performance after the first cyclic test (Fig. 7, point D) is expected to be due to the following reasons: (1) Redistribution of the normal loads along the circumferential joint. Before cycling, the normal load due to gasket compression may not be uniform. This may cause leakage in the areas where the local normal loads are less than the average normal loads. (2) Zones along the circumferential joint where the section of the gasket is not uniformly compressed inside the gasket groove. After the first cycling, the gasket tended to move laterally to fit the groove in a more uniform way.
3.4. Effect of time on gasket performance Water leakage tests on Ex1026 gasket showed that there was a time effect on gasket sealant behavior. As shown in Fig. 7, leaving the T-joint system undisturbed for two months increased the water pressure at leakage from 35 psi (point F) to more than 40 psi (point G). (Uncontrolled leakage started to occur through porous concrete at the outside of the gasket groove at 40 psi. Tests were stopped to seal these pores). The increase in gasket performance with time is expected to be due to two reasons. The first one is the healing process that takes place at the gasket–gasket groove interface. During the waiting time period between the two water leakage tests, bonds between the gasket and gasket groove might be re-built. The second reason is the buildup in gasket contact pressure with time (gasket recovery). Water leakage test of point F was performed shortly after a series of cyclic tests under circumferential joint unloading (opening the joint gap after each cyclic test). During the 2-month period, the gasket at point G
After series of cyclic tests (100 cycles with ±0.1 in. amplitude), the movement of the gasket inside the gasket groove started to have a destructive effect on gasket performance. Intensive lateral movement of the gasket inside the gasket groove tends to destroy the adhesive bond between the gasket and the gasket groove. Fig. 8b shows an illustrative drawing for the lateral movement of the circumferential joint gasket during cycling and the effect of this movement on gasket–groove contact. This effect is expected to be
Top beam 8''
Leakage occurred through the damaged part at 35 psi water pressure
9''
C-Joint
L-Joint
Damaged segment
C-Joint
West
Bottom beam
(a) Front
Top beam
5''
13''
Bottom beam
(b) Back Fig. 11. Leakage through the damaged corner after the first T-joint cyclic test.
West
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Damaged due to cycling. Epoxy and hydrocal were used for repairing
Top beam
Top beam Repaired zone ~27'' C-Joint
C-Joint Leakage through the repaired zone of the gasket groove at 35 psi
Leakage between gasket & gasket groove at 21 psi
L-Joint
C-Joint
L-Joint
C-Joint Bottom beam
Bottom beam
Leakage through porous concrete beneath gasket groove
Leakage between gasket and gasket groove at 55 psi
Fig. 14. Leakage between gasket and gasket groove and through porous concrete beneath gasket groove after the first cyclic test; EPDM gasket.
Fig. 12. Leakage through the T-joint corner and the repaired zone after cyclic test, EX1026 gasket.
Top beam Top beam
C-Joint
C-Joint L-Joint
Leakage through concrete at 35 psi
L-Joint Repaired corner with epoxy and steel anchors
C-Joint
C-Joint
Bottom beam
Bottom beam
(a) Front Leakage between gasket & gasket groove at the repaired corner at 20.7 psi
Leakage through porous concrete beneath gasket groove
Top beam Fig. 15. Leakage between gasket and gasket groove and through porous concrete after repairing the damaged segment, EPDM gasket.
C-Joint Leakage between gasket and gasket groove at 35 psi
3.5. Difference in open-base gasket performance in steel forms and concrete segments
L-Joint C-Joint
Bottom beam
(b) Back Fig. 13. Leakage through the concrete around the groove and between gasket and groove after first cyclic test (after repairing the top beam).
gained more contact pressure than that at point F due to gasket recovery. Unfortunately, there were no load measurements for the water leakage tests.
Results of water leakage tests showed that there was a difference between the gasket performance in steel forms and in concrete segments. As can be seen in Fig. 9, concrete segment tests showed that Ex1026 gasket could hold a water pressure in the range of 30–50 psi for a joint gap in the range of 0.135 in. and 0.080 in., respectively. On the other hand, the same gasket did not show any leakage when it was tested in picture frame steel device at a joint gap of 0.16 in. and a water pressure of 100 psi (Shalabi, 2001 and Shalabi et al., 2009). The difference in gasket performance in steel forms and concrete segments is considered primarily due to the effect of concrete roughness between the gasket and gasket groove and variation in gasket groove tolerances. Also, due to the porous nature of the concrete, open water channels
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underneath the gasket can form under water pressure and cause leakage at or just beneath the interface between gasket and gasket groove. Water channels can also cause leakage through the sides of the gasket groove and around the bolt pockets where the porosity of the concrete is high. High concrete porosity around the gasket groove and bolt pockets was mainly due to inefficient concrete compaction around the groove and the pockets.
continued to occur through concrete channels and between gasket and gasket groove of the repaired corner at water pressure of 20 psi. The reduction in water pressure at leakage was due to the increase in the size of the open channels in the concrete that were located around the gasket groove, as leakage continued.
3.6. Locations of leakage in the concrete segments and the effect of segments damage on leakage
For water leakage tests, pairs of concrete segments were tested using open-base (finger-base) Ex1026 gasket. Tests were performed for aligned segments, 0.27 in. longitudinal joint lateral movement (0.17 in. due to ground static loads and 0.1 in. due to Maximum Design Earthquake), and after cycling the joint to the MDE. The results showed the following:
Results of water leakage tests using T-joint and longitudinal joint test setups showed that leakage of the segmental concrete lining could take place through the following areas: (1) Leakage near the T-joint between the gasket and gasket groove. (2) Leakage at the interface between the gasket and gasket groove. (3) Leakage through porous concrete at the gasket groove sides. (4) Leakage through the damaged zones of the gasket grooves. (5) Leakage through the bolt pockets. (6) Leakage through micro-cracks and porous concrete around the bolt pockets. Longitudinal joint water leakage tests on Ex-1026 gasket showed that leakage initially occurred between the gasket and gasket groove interface at water pressure of 35 psi before cycling. After couple of leakage tests, leakage started to occur through porous concrete at the sides of the bolt pockets and at the segment surface near the corners of the pockets. Leakage through the porous concrete started at nearly 30–35 psi water pressure and later on at 10–15 psi when the size of the pores got larger as leakage continued. Filling the pockets and covering the segment face near the pockets with epoxy stopped leakage through the concrete. Before starting cyclic tests, water leakage tests on the Ex1026 gasket showed that leakage took place near the T-joints at water pressure of 45–50 psi, as shown in Fig. 10. During the first cyclic test, damage to the top corners of the west side concrete segment took place, as shown in Fig. 11. Leakage occurred through the damaged corner at about 35 psi water pressure. After replacing the damaged segment with a new one and repairing the top beam damaged groove with epoxy and hydrocal, leakage tests after the T-1–3 cyclic test showed that leakage started to occur at the bottom T-joint between the gasket and gasket groove and through the repaired zone of the top beam gasket groove at 55 and 35 psi water pressure respectively. Fig. 12 shows the locations of leakage of this test. After repairing the top beam gasket groove with just epoxy and steel bars, water leakage tests showed a shift in the location of leakage. After the T-1–6 cyclic test, water started to leak through porous concrete around the gasket grooves of the circumferential joint and through the interface between the gasket and gasket groove at 35 psi water pressure. The locations of leakage are shown in Fig. 13. Leakage through the porous concrete around the gasket groove of the circumferential joint continued even after repairing the outside groove by a 1-in-thick strip of epoxy material. Tests on the EPDM gasket before cyclic showed that leakage through the porous concrete and between the gasket and gasket groove started to occur at a water pressure of 21 psi. Leakage locations of this test are in Fig. 14. During the first cyclic test, damage to the bottom corner of the east segment took place as shown in Fig. 15. The damaged corner was removed and a cast epoxy corner was molded. Steel plate and steel anchors were used to provide lateral support for the gasket at the corner. Water leakage tests showed that leakage
4. Conclusions
(1) Leakage of concrete segments with open base gaskets occurred at the gasket/gasket–groove interface. (2) Longitudinal joint lateral movement reduced the gasket sealant capacity as the joint gap opened. (3) Segment to segment cycling improved the longitudinal joint sealant capacity. The increase in water pressure at leakage after cycling was mainly due to the increase in bonding between the gasket and gasket groove as the gasket was subjected to pure compression and uncompression during cycling, and due to better seating of the gasket inside the groove after cycling. (4) Gasket sealant capacity had been improved after the first cyclic test (12 cycles at ±0.05 in. amplitude), while after severe cycling (100 cycles and ±0.1 in. amplitude), gasket sealant capacity was reduced. (5) Damage to the side of the gasket groove reduced the gasket confinement, which in turn reduced the gasket sealant capacity. (6) Gasket sealant capacity improved slightly with time. The increase was mainly due to the healing process that took place at the gasket/gasket–groove interface and due to the buildup in gasket contact pressure with time (gasket recovery). (7) Concrete around the gasket groove, cracked zones, gasket– gasket groove interface, and bolting pockets were the places of leakage in concrete segments. (8) Gasket sealant capacity of picture frame steel device was higher than that of the concrete segment at the same joint gap.
References Construction Polymers Company, 2000. Tunnel liner gasket systems, USA. Cording, E.J., Paul, S., Rood, M., Shalabi, F., Lee, S., 1998. Testing program for selection of the joint system for concrete segments for the LA Metro Red Line extension. Unpublished report, University of Illinois at Urbana-Champaign. Hendron, J., Fernandez G., 1983. Dynamic and static design consideration for underground champers. In: Proc., Seismic Design of Embankments and Caverns, ASCE National Convention, Philadelphia, Pennsylvania, May 16–20. Paul, S.L., 1978. Sealability tests of gaskets between precast concrete tunnel lining segments. Unpublished report, Dept. of Civil Engineering, Univ. of Illinois at Urbana-Champaign. Peck, R.B., 1969. Deep excavation and tunneling in soft ground. In: Proc., Seventh International Conference on Soil Mechanics and Foundation Engineering, Mexico City, state-of-the-art volume. Shalabi, F.I., 2001. Behavior of gasketed segmental concrete tunnel lining. Ph.D. Thesis, University of Illinois at Urbana-Champaign, Urbana, IL, USA. Shalabi, F., Cording, E., 2005. 3D-finite element analysis of segmental concrete tunnel lining deformation and moments under the effect of static and earthquake loading. In: 11th International Conference on Computer Methods and Advances in Geomechanics (IACMAG), Torino, Italy, June 19–24. Shalabi, F.I., Cording, E.J., Paul, S.L., 2009. Sealant behavior of gasketed segmental tunnel lining. In: Proceedings of the Workshop of the International Tunneling and Underground Space Association, Underground Structures in Hot Climate Conditions, Riyadh, KSA, December 8–9.