employed to measure the soil movements. Pore pressures, ground settlement profiles behind the excavation and bending moment profiles along the pile were ...
Time-dependent Pile Behavior Due to Excavation-Induced Soil Movement in Clay Comportamiento en el Tiempo de Pilotes Afectados por Movimientos del Suelo Inducidos por Excavaciones en Arcillas Ong, D. E. L., Leung, C. F. and Chow, Y. K. Centre for Soft Ground Engineering, Department of Civil Engineering, National University of Singapore, Singapore 117576
Abstract Centrifuge model tests have been conducted to study the time-dependent behavior of a single pile subjected to lateral soil movements in an unstrutted excavation in kaolin clay. An enhanced image processing system was employed to measure the soil movements. Pore pressures, ground settlement profiles behind the excavation and bending moment profiles along the pile were monitored during and after the excavation. The long-term behavior of a single pile located behind a stable and a collapsed retaining wall is presented in detail.
Resumen Ensayos de modelación centrífuga se llevaron a cabo para estudiar el comportamiento en el tiempo de un pilote sometido a movimientos laterales del suelo generados en una excavación sin entibar, hecha en arcilla caolinítica. Un sistema mejorado de proceso de imágenes fue empleado para medir los movimientos del suelo. Las presiones de poros, los perfiles de asentamientos detrás de la excavación y los perfiles de momentos flexionantes a lo largo del pilote fueron inspeccionados durante y después de la excavación. El comportamiento a largo plazo de un solo pilote situado detrás de un establo y de un muro de contención derrumbado se presenta detalladamente.
1 INTRODUCTION Excavation involves drawdown of water table and dissipation of excess pore water pressure over time. In practice, piles are not instrumented to study its long-term behavior. Besides that, it is not feasible to build a real-life structure and then test it to failure. Therefore, a centrifuge model is deemed attractive to study the effect of excavation-induced soil movement on piles located behind a stable and collapsed retaining wall. Leung et al. (2001) presented centrifuge model test results of pile behavior due to excavationinduced soil movement in clay. The study is extended in the present paper to examine the longterm pile behavior caused by a nearby excavation in clay. In-flight excavation is simulated by the draining of zinc chloride solution, whose unit weight is similar to that of the clay. Similar work
in sand has been done by Leung et al. (2000). The pile behavior during and after excavation is discussed with particular reference to wall movements, ground deformations behind the wall as well as excess pore water pressure behavior in the clay. 2 MODEL SETUP AND PROCEDURES Figure 1 shows the experimental set-up of the present study. All tests were conducted at 50g on the National University of Singapore geotechnical centrifuge. 2.1 Model pile and retaining wall The model pile was fabricated from a hollow square aluminium tube with an outer dimension of 9.53 mm and a wall thickness of 3.18 mm. Ten pairs of strain gauges were glued at opposite faces of the model pile at vertical intervals of 25 mm. A thin layer of epoxy was applied to the entire pile
length to ensure that the strain gauges and all connections were waterproof. The final width of the pile is 12.6 mm (630 mm in prototype scale). The total length of the pile is 350 mm (17.5 m) with an embedment depth of 250 mm (12.5 m). The prototype bending rigidity, EI, of the model pile is approximately 2.2 x 105 kNm2. This is equivalent to a 610-mm diameter steel pipe pile with 12.7 mm wall thickness.
Figure 1 Centrifuge model set-up (all dimensions in mm) The model retaining wall is simulated using a 3 mm (150 mm in prototype scale) thick aluminum plate. The prototype bending rigidity, EI, is 24 x 103 kNm2/m, which is equivalent to a FSP II A sheet pile. The embedment depth of the wall is 160 mm (8 m). 2.2 Soil Malaysian kaolin clay and sand with physical properties shown in Tables 1 and 2, respectively, were used in the model tests. Table
1
Physical properties kaolin clay Specific gravity, Gs Liquid limit, LL Plastic limit, PL Compression index, Cc Swelling index, Cs Void ratio at 115 kPa at NC line Permeability at 115 kPa at NC line
of
Malaysian 2.6 80 % 40 % 0.65 0.14 1.67
1.3x10-8 m/s
Table 2 Physical properties of sand Mean grain size 0.16 mm Uniformity coefficient 1.3 Specific gravity, Gs 2.65 Friction angle (50-100 kPa) 43o
2.3 Test procedures The kaolin powder was mixed with water at a water content of 120% in a de-airing mixer. Simultaneous mixing and de-airing was done for about 4 to 5 hours. The four walls of the container were greased to reduce the soil-wall friction during testing. Subsequently, the model container was filled with water before the sand was rained down from a height of 600 mm. The kaolin slurry was then placed under water. Two miniature pore pressure transducers (Druck PDCR81), which had been de-aired, were embedded in the kaolin slurry. Subsequently, a 17-kg plate was placed on top of the slurry to stiffen it. Then, the sample was placed on a loading frame for 1-D consolidation under a load of 20 kPa for 3 days. After that, self-weight consolidation of clay was carried out under 50g for about 6 to 7 hours. The ground surface settlements were monitored by the LDVTs. After about 90% consolidation had been reached, the centrifuge was spun down. The back face of the model container was then removed so that additional PPTs could be embedded at positions shown in Figure 1. A polyethylene tube, which would be used to drain off the zinc chloride (ZnCl2) solution from the latex bag, was fastened through the valve at the bottom corner of the back face, with one end of the tube connected to the bottom of the latex bag and the other end to a solenoid valve. When power was supplied during centrifuge flight, the valve would open to release the ZnCl2 solution. Subsequently, the model wall and pile were jacked vertically into place using a guide at 1g. Excavation was then carried out. The excavated clay was replaced by ZnCl2 solution contained in a latex bag. The density (16.5 kN/m3) and height of the ZnCl2 solution were made to be identical to those of the clay that had been excavated. After that, the front perspex face of the container was removed so that soil movement markers could be placed on the clay at 20 mm square grids. LVDTs were installed to measure the ground settlements behind the excavation. The pile head deflection, during and after excavation, was monitored by two non-contact laser transducers (NAIS LM10, model ANR1250). Each laser transducer has a maximum linearity error of ±0.5% on aluminium. This translates to a linear error of 0.25 mm at prototype scale. A CVM1 2/3" CCD progressive scan high resolution image processing camera was mounted in front of the perspex window of the model container. For a high resolution image, a pixel-to-pixel spacing of less than 0.1 mm can be achieved.
Finally, the model package was put together and spun up to 50g for reconsolidation. When both pore water pressures and ground settlements behind the wall showed negligible changes, the ZnCl2 solution was then released to depict excavation at 50g. At prototype scale, the simulated excavation rate was about 0.56 m per day. All data acquired and still images captured would be stored in the computer and could be retrieved after the tests. The ground water level for the tests was kept slightly higher than the ground surface. The height of the ground water level could be monitored by placing 2 PPTs (not shown in Figure 1 for clarity) at different locations on the ground surface so that corrections could be made when analyzing the experimental data. The stress history of the clay sample was intended to simulate that of a normally consolidated clay deposit with a 2.5 m overconsolidated crust. Two tests were conducted in the present study. The configuration of the tests are shown in Table 3.
excavation depth just reaches 1.2 m, a bending moment of 88.5 kNm is recorded. However, the bending moment increases to 92.0 kNm 22 days later. After 240 days, it reduces to 87.0 kNm as shown in Figure 2. The time-dependent responses of pile are thus evident.
Table 3
Figure 2 Development of pile bending moment profile over time (Test 1 on stable wall)
Note: Quantities in prototype scale
3 TESTS RESULTS AND DISCUSSIONS By using limit equilibrium analysis (Bolton and Powrie, 1987), the factors of safety for Tests 1 and 2 are determined to be about 2.5 and 1.3, respectively. From the centrifuge scaling law, the model time scale is 1/N2 (N is 50) of the prototype scale. The results of Tests 1 and 2 are discussed with particular emphasis on the long-term responses of the pile, wall movements, soil deformations and pore water pressures. 3.1 Pile response The development of the pile bending moment profiles over time for Tests 1 and 2 are shown in Figures 2 and 3, respectively. For Test 1, the maximum pile bending moment is located at a depth of 7.5 m below the ground level. When the
20
Bending moment (kNm) 40 60 80
100
Symbol Excavation depth (m)
Depth (m)
2.5
Days after completion of excavation
0.8 1.0 1.2 1.2 1.2 0
0
0
22 240
5 7.5 10
12.5
0
0
50
Bending moment (kNm) 100 150 200
250
Symbol Excavation depth (m)
2.5 Depth (m)
Configuration of Tests 1 and 2 Stable Collapsed wall wall Item Test 1 Test 2 Distance of pile 3m 3m from wall Excavation depth 1.2 m 1.8 m Clay thickness 6.5 m 10.5 m Sand thickness 6.0 m 2.0 m Wall toe 1.5 m 0 m; Floating in embedment in sand clay
0
0
Days after completion of excavation
1.0 1.2 1.4 1.6 1.8 1.8 1.8 0 0 0 0 0 22 240
5 7.5 10
12.5
Figure 3 Development of pile bending moment profile over time (Test 2 on collapsed wall) For Test 2, a different pile behaviour is noted. The maximum pile bending moment is located at 8.75 m below the ground level. A maximum value of 235.7 kNm is recorded at an excavation depth of 1.2 m. Thereafter, as excavation continues to its final depth of 1.8 m, the bending moment reduces to 185.8 kNm. The bending moment is found to decrease further over time as shown in Figure 4.
60 100 BM Deflection
50 0
40
Test 1 Test 2
20
0 0.1 1 10 100 1000 Time from start of excavation (days)
Figure 4 Development of maximum pile bending moment and head deflection over time 3.2 Wall and soil deformations Figure 5 shows the wall head movements and ground settlements over time. It is intuitively correct that the deformations in Test 2 are larger than those of Test 1. In Test 2, due to the instability created after the collapse of the wall, the time taken to achieve equilibrium is thus longer. Figures 6(a) and 6(b) show the soil movements for Tests 1 and 2, respectively. The size of considerable soil movement zone increases as excavation progresses, but the shape does not alter. This observation is consistent with that observed by Bolton and Powrie (1987). For Test 1, it is evident that relatively large soil movements are induced between excavation depths of 1.0 m and 1.2 m. This observation is consistent to the relatively large increase in the pile bending moments after excavation depth exceeds 1.0 m, as shown in Figure 2. This could be due to the non-linear soil behaviour. Figure 6(b) shows a distinct increase in soil movements between excavation depths of 1.2 m and 1.4 m, indicating a total collapse of the wall between these depths. For both Tests 1 and 2, when excavation depth exceeds about 0.6 m, the image processor shows that the clay surface starts to “flow” past the pile head. The observed slip lines, which shows the extent of the deformed zone around the pile, is similar to the characteristics meshes defined by Randolph et al. (1984) for a plastically deforming cohesive soil. Nevertheless, the development of the soil “flow” depth is restricted by the excavation depth. When excavation depth exceeds about 0.8 m, it is observed from the image processor that some
1.4 W all head lateral m ovem ent (m )
80 150
fissures with depths varying from 150 to 250 mm (prototype scale) occur at the ground surface in Tests 1 and 2. Water can flow freely into the increasing number of fissures as excavation progresses. The development of these fissures further reduces the stability of the wall, thus causing the wall to tilt and the clay to be sheared progressively over time. Therefore, the overall clay stiffness is reduced. This explains the increase in pile bending moments and head deflection in both Tests 1 and 2 during the earlier stages of excavation.
1.2 1
Settlement Settlement Wall head 1.5 m 4.5 m movement behind wall behind wall
0.8 0.6
Test 1
0.4
Test 2
0.2 0 Ground settlem ent (m )
100
200
Pile head deflection (mm)
Maximum pile bending moment (kNm)
120
250
0.2 Test 1 - Excavation 1.2 m takes 2.2 days Test 2 - Excavation 1.8 m takes 3.3 days
0.4 0.6 0.8 1 0.1
1 10 100 1000 Time from start of excavation (days)
Figure 5 Soil and wall deformations over time For Test 1, the effects of the relatively small fissures and the shallow soil “flow” depth on the pile behavior are relatively minor. Owing to progressive shearing of the clay, the maximum pile bending moment and head deflection increase slightly until about 60 days after completion of excavation. Thereafter, it stabilizes and subsequently reduces over time as the soil effective stress gradually increases due to reconsolidation of soil. However, in Test 2, deeper and wider tension cracks (1.0 m deep and 150 mm wide in prototype scale) are noted as shown in Figure 7. Besides that, the depth of soil “flow” is also much greater than that of Test 1. Therefore, a different pile behavior is expected. The occurrence and development of tension cracks in front of the pile during excavation separates the clay mass into two
distinct regions, namely soil in front of the pile (nearer to excavation) and behind the pile. Distance behind wall (m)
Soil movement (mm) 1000
-4
-6
Excavation 1.0 m, 1.8 days
Soil movement (mm) -2 120 100 80 60 40 20
700 600 500 400 300 200 100
-2 -4 -6
Soil movement -2 (mm)
800 700 600 500 400 300 200 100
-6
Soil movement (mm)
900 800 700 600 500 400 300 200 100
-4
-6
Excavation 1.2 m, 240 days
Pile
Tension cracks
-4 -6
Figure 7 Development of tension cracks in front of pile (Test 2)
Excavation 1.8 m, 22 days
Soil movement (mm) -2
Approximate water levels after excavation
Excavation 1.4 m, 2.5 days
-4
Excavation 1.2 m, 22 days
(a)
Soil movement (mm)
-6
Soil movement -2 (mm)
180 160 140 120 100 80 60 40 20
Excavation 1.2 m, 2.1 days
-4
Excavation 1.2 m, 2.2 days
180 160 140 120 100 80 60 40 20
Soil movement -2 (mm) 400 350 300 -4 250 200 150 -6 100 50
-2 -4 -6
Depth (m)
80 70 60 50 40 30 20 10
6
Depth (m)
Soil movement -2 (mm)
4
Depth (m)
2
Depth (m)
Distance behind wall (m) 2 4 6
pile due to tension cracks exposes it and thus creates voids around the pile, resulting in a minimal soil-pile contact. When tension cracks occur, the surrounding lateral soil pressures are believed to have been prevented from transmitting their full pressures onto the pile. Plastic soil “flow” past the pile is also believed not to have exerted full pressure on the pile. This reason might explain the reduction in pile bending moments in Test 2 (see Figure 4). Nevertheless, the pile head deflection remains fairly constant (see Figure 4) as the pile is continuously being pushed by the separated soil mass behind the pile.
Excavation 1.8 m, 240 days
(b)
Figure 6 Selected soil movement vectorial plots for (a) Test 1 and (b) Test 2 The soil in front of the pile moves more towards the excavation face, thus exposing the front face of the pile, while the soil region behind the pile remains relatively stable with relatively less lateral soil movements. This is reflected in Figure 6(b) which shows distinct difference between the length of vectors in front and behind the pile. The loss of clay support in front of the
3.3 Pore water pressure responses The excess pore water pressure variations are shown in Figures 8 and 9 for Tests 1 and 2, respectively. All PPTs register excess pore water pressures immediately after excavation. Nevertheless, PPT 1 in both tests registers ‘positive’ excess pore water pressures shortly after the completion of excavation. This is due to the water level after excavation being higher than its hydrostatic level caused by ground surface settlements. Owing to relatively greater soil deformations near the wall, PPT 2 registers a greater dissipation of excess negative pore water pressures as opposed to PPT 3, which is located further away from the wall. It is the difference in these excess pore water pressures that creates a hydraulic gradient that leads to pore water pressure redistribution and hence, the reconsolidation of the clay over time. This is somewhat analogous to that observed by Stewart (1992). After excavation, the ground water level at the excavated side will drop. Therefore, this creates a
Excess pore water pressure (kPa)
8 PPT 4
6 4 2
PPT 1
0
4 CONCLUSIONS
PPT 2
-2
PPT 3
-4 -6 0
50 100 150 200 250 300 350
Time since start of excavation (days)
Excess pore water pressure (kPa)
Figure 8 Excess pore water pressure variations over time (Test 1) 5
PPT 1
0 PPT 2
-5
after excavation, as suggested by PPT 4 in Test 2 shown in Figure 9. The excess negative pore water pressures in the retained side for Tests 1 and 2 are relatively small as they may be partly cancelled out by the positive pore water pressures generated by the undrained shearing of the clay. Similar observation is also noted by Kimura et al. (1994).
PPT 3
-10
The preliminary conclusions drawn from this study are as follows: (1) The time-dependent pile behavior may be attributed to the occurence of fissures on the retained clay, progressive wall and soil deformations as well as pore water pressure redistribution that leads to soil reconsolidation. (2) When soil movements are excessive as in the case of a collapsed wall, the occurrence of tension cracks is believed to have prevented the transmission of full soil pressures onto the pile. Hence, the reduction in pile bending moments over time. The time-dependent pile behaviour can be quite different in cases of a stable and collapsed wall. Currently, more tests are being carried out to further verify these findings. REFERENCES
-15 -20
PPT 4
-25 0 50 100 150 200 250 300 350 Time after start of excavation (days)
Figure 9 Excess pore water pressure variations over time (Test 2) seepage regime between the retained and excavated sides. PPT 4 in Test 1 registers dissipation of excess negative pore water pressures until about 125 days later when pore water pressure in excess of hydrostatic pressure is recorded. This is due to the ground water seeping from the retained side to the excavated side of the wall through the bottom sand layer where the wall is embedded in. However, this is not observed in Test 2 as there is no wall embedment in sand and hence, seepage cannot occur easily from the retained side to the excavated side. Since the wall in Test 2 is ‘floating’ in the clay layer, the reconsolidation of soil caused by pore water pressure redistribution becomes more dominant than the seepage caused by the hydraulic gradient
Bolton, M. D. and Powrie, W., (1987). "The collapse of diaphragm wall retaining clay”. Geotechnique Vol. 37, No. 3: 335-353. Kimura, T., Takemura, J., Hiro-oka, A., Okamura, M and Park J., (1994). “Excavation in soft clay using an in-flight excavator”. Centrifuge 94, Leung, Lee and Tan (eds): 649-654. Leung, C. F., Chow, Y. K. and Shen, R. F., (2000). “Behaviour of pile subject to excavation-induced soil movement”. Journal of Geotechnical and Geoenvironmental Engr., Vol. 126, No. 11: 947-954. Leung, C. F., Lim J.K. and Chow, Y. K. (2001). “Behaviour of pile due to excavation-induced soil movement in clay”. 15th Int. Conf. on Soil Mech. And Geotechnical Engineering, Istanbul, Vol. 2: 951-954. Randolph, M. F. and Houlsby, G. T., (1984). "The limiting pressure on a circular pile loaded laterally in cohesive soil”. Geotechnique Vol. 34, No. 4: 613623. Stewart, D. P. (1992). “Lateral loading of piled bridge abutments due to embankment construction”. PhD Thesis, University of Western Australia.
