Cyclic Lateral Response of Model Pile Groups in Clay

Transcription

1 The 2 th International Conference of International Association for Computer Methods and Advances in Geomechanics (IACMAG) -6 October, 28 Goa, India Cyclic Lateral Response of Model Pile Groups in Clay S. S. Chandrasekaran, A. Boominathan 2 and G. R. Dodagoudar 3 Research Scholar, 2 Professor, 3 Assistant Professor Department of Civil Engineering, Indian Institute of Technology Madras, Chennai, India. Keywords: cyclic lateral load, critical spacing, bending moment profile, soft clay ABSTRACT: This paper presents the results of two-way cyclic lateral load tests conducted in the laboratory on the model pile groups embedded in soft clay. The purpose of this research work is to investigate the effects of spacing, number of cycles of loading and cyclic load level on the pile group behaviour in clay. A pneumatic system is used to simulate cyclic loading typical of wave loading. Similitude laws are adhered to in selecting the material and size of the model piles. Piles are instrumented so that bending moments developed along the piles can be calculated. The results emphasized highly nonlinear nature of load-deflection behaviour. Group interaction effect under cyclic lateral loading is predominant for groups with spacing to diameter ratio less than 7. It is found that the cyclic load levels exceeding.5 times of static ultimate capacity, produce large deflections of the pile group due to gaps developed at the pile-soil interface, remoulding of clay and subsequent reduction in the stiffness. The bending moments in the piles are increased with the number of cycles and the location of maximum bending moment shifted downwards along the length of the pile. Numerical analysis using software GROUP is also carried out for closely spaced pile groups subjected to static lateral loads and the results are compared with the experimental ones. Introduction Pile groups supporting coastal and offshore structures are subjected to lateral loads due to wave action. These lateral loads are cyclic in nature with different amplitudes and duration. Mechanics of the behaviour of group of laterally loaded piles is more complex than those of the axially loaded pile group. Piles in the group subjected to lateral loading are influenced by the existence of similarly loaded nearby piles due to pile-soil-pile interaction, leading to reduction in lateral load capacity of the pile group. Pile group behaviour under cyclic lateral loading is nonlinear and involves complicated group interaction. Most of the pile foundations in marine area are constructed in clay deposits. Cyclic loading in clay under undrained condition leads to degradation of stiffness and reduction in shear strength. The problems associated with cyclic lateral loading in clay such as formation of gap at pile-soil interface, buildup of excess pore pressure and remoulding of clay lead to higher deflections and higher bending moments than static loading. Brown et al. (987) investigated the behaviour of large scale pile group in stiff clay under two-way cyclic lateral loading. It was shown that maximum soil resistance for the piles in group is greatly reduced as compared to single piles due to cyclic loading. Moss et al. (998) studied the behaviour of closely spaced model linear pile group embedded in clay, subjected to two-way cyclic lateral loading. It was observed that magnitude of maximum bending moment increases and location of maximum moment moving down the pile length with increasing numbers of cycles of loading. Ramakrishna and Rao (999) conducted one-way cyclic lateral load tests on model pile groups embedded in clay consisting of two piles arranged in series and parallel configuration. It was shown that capacities of piles reduce at cyclic load levels exceeding 6% of the static capacities. Rollins et al. (26a, b) performed series of full scale cyclic lateral load tests on pile groups with various spacing to study the effect of pile spacing on the behaviour of pile group. It was concluded that the group interaction effect decreases considerably with increase of spacing from 3.3 to 5.65 times the diameter of pile. Peng et al. (26) summarized various devices used for applying cyclic lateral load to model piles. As noticed from the literature, the experimental data on the behaviour of pile groups in clay under cyclic lateral loads are limited. The effects of spacing and cyclic load level on pile-soil-pile interaction of pile group under cyclic loading have not previously been studied extensively. Moreover only a few tests have been carried out using two-way cyclic lateral loading on piles. Available methods for analysis of pile groups subjected to cyclic loading use group interaction factors derived from static tests. Hence in the present study, a comprehensive experimental investigation was carried out on model pile groups embedded in soft clay under two-way cyclic lateral load to study the effects of spacing, cyclic load ratio and number of cycles on load deflection and bending behaviour of pile group. Based on the results, the critical spacing and critical load level under cyclic lateral loading are arrived. The important features of 36

2 the bending moment profiles of single pile and piles in the group are discussed. The results of numerical analysis of pile groups subjected to static lateral loads are compared with the experimental results. 2 Materials used 2. Soil The clay soil collected from the Siruseri area of the Chennai city is used in the present study. The properties of the clay are: liquid limit = 67%, plastic limit= 28%, plasticity index= 39% and undrained shear strength at consistency index of.38 = 9 kpa. The soil is classified as high plastic clay (CH) as per the Indian standard soil classification system. 2.2 Pile Similitude laws are adhered to in selecting model pile material and dimensions. The prototype pile is 55 mm diameter solid section made of reinforced cement concrete with M 25 grade concrete. The model pile dimensions are selected to suit the prototype requirements. The following scaling law proposed by Wood et al. (22) is used: Em Im = 5 Ep Ip n () where E m = Modulus of elasticity of model pile, E p = Modulus of elasticity of prototype pile, I m = Moment of inertia of model pile, I p = Moment of inertia of prototype pile, and /n = Scale factor for length. Aluminium tube of outer diameter 25.6 mm and inner diameter 8.6 mm is selected as model pile with length scaling factor of /. The other scaling factors used in the study are presented in Table. Aluminium plate of 2 mm thick is used as pile cap. Pile cap is attached at the top of the piles leaving 5 mm above the ground surface as a free standing length for the group. A free head condition is obtained by screwing piles in the threads provided in the cap. TABLE. Scaling factors adopted in the study. Variable Scaling factor Length Density Stiffness Stress Strain Pore fluid density Flexural rigidity of pile 5 Modelling of models procedure is used to verify scaling factors, in which the prototype pile is modelled using different scaling factors and the predicted prototype static load-deflection behaviour matches very well. The scaling law used in this study is further verified by modelling a full scale steel pile driven in clay and subjected to static lateral load reported by Rollins et al. (998). The predicted load-deflection curve matches reasonably well with the measured curve. 3 Experimental programme 3. Experimental setup and instrumentation Experimental setup used for cyclic lateral load test on pile group is shown in Figure. Cyclic lateral load tests are conducted on model pile groups embedded in clay in steel test tank of size.5 m diameter and.3 m height which was sufficiently large enough to avoid boundary effects. Two-way cyclic lateral loading is applied on model piles using two pneumatic power cylinders attached on the loading frame on both sides of the pile cap and connected to pile cap with wire rope. Filtered compressed air is regulated through the precision pressure regulator which is used to control the cyclic load level. Electronic timer is used to control the period of loading. Two solenoid valves are used to supply the compressed air to one cylinder during half of the period while cutting the supply to other cylinder and vice versa which in turn move the pistons back and forth and enabling two-way 37

3 cyclic lateral load application to the pile cap. 6 e L Loading Frame 9 Solenoid Valves 2 Steel Tank Pneumatic Power Cylinder 3 Clay Load Cell 4 Pile 2 LVDT 5 Pile Cap 3 Strain Gauges 6 Air Compressor 4 Pore Pressure Transducer 7 Pressure Regulator 5 Amplifier (MGC+, Spider8) 8 Electronic Timer 6 DAS Figure. Schematic diagram of cyclic lateral load test setup. Piles are instrumented with electrical resistance strain gauges along the length to study the bending behaviour. Load carried by pile cap is measured using load cell. Pile head and ground line deflections are measured using Linear Variable Displacement Transducers (LVDTs). Miniature pore pressure transducers embedded at various depths into the soil bed and are used to record the excess pore pressure generation. A 4 channel data acquisition system comprising HBM make MGC Plus carrier frequency amplifier system and SPIDER8 with Catman Professional software is used to monitor and store the data automatically. 3.2 Test procedure 3.2. Clay bed preparation In the present study, the clay was mixed in a separate mixing tank with required amount of water to get the soft consistency (consistency index, I c =.38) and cured for a period of two days. After fixing the piles in the centre of the test tank, uniformly mixed clay was placed and hand packed in the test tank in several layers each of 5 mm thickness and tamped with template in order to remove entrapped air and to ensure homogenous packing. Water content, density and undrained shear strength tests carried out at various depths of the soil bed confirmed the homogeneity of the prepared clay bed Cyclic lateral load test Two-way cyclic lateral loading representing wave loading is applied to the model single piles and pile groups at different cyclic load ratios. The Cyclic Load Ratio (CLR) is defined as the ratio of magnitude of cyclic lateral load to static ultimate lateral capacity of the pile (Poulos, 982). The different magnitudes of cyclic lateral load corresponding to CLR of.35,.5,.65 and.8 representing wave loading (Brown et al., 987; Peng et al., 26) are applied to the pile group. The period of cyclic loading is kept as s. The cyclic loading has been applied to 38

4 the piles and the loading is stopped around 3 cycles. 3.3 Test programme The arrangement of piles used in 2 2 and 4 pile groups is shown in Figure 2. The spacing to diameter ratio (S/D) is taken as 3, 5, 7 and 9. The transverse spacing is kept constant as three times the diameter of pile for the 2 2 pile group. Loading is applied in line with all the piles in the case of 4 pile group. Piles with the embedded length (L) to diameter (D) ratio (L/D) of 5 is used which is classified as rigid pile as per the relative stiffness factor (Poulos and Davis, 98). S T = 3D D S S S S S = Spacing of piles in the direction of loading S = Spacing of piles in the direction of loading S T = Transverse spacing D = Diameter of pile (a) 2 2 pile group (b) 4 pile group Figure 2. Pile groups used in this study. 4 Load deflection behaviour of 2 2 pile group 4. Effect of cyclic load ratio (CLR) on pile head deflection Figure 3 shows the pile head deflection for a closely spaced 2 2 pile group under different magnitudes of cyclic loading i.e., for different values of CLR. It can be seen from the figure that at low magnitude of loading (CLR <.5), the pile head deflection increases gradually with the number of cycles but nonlinearly up to a certain number of cycles and then practically becomes constant irrespective of the increase in the number of cycles. But, at high magnitude of cyclic loading (CLR >.5), as shown in Figure 3, there is a steep rise in deflection within a few number of cycles and it gradually increases with the increase in the number of cycles. The sudden steep rise of deflection is mainly due to the formation of gap around the piles up to a depth of five times the pile diameter from the surface which leads to reduction of passive resistance of the soil. Poulos (982) defined critical cyclic load level as the cyclic load at which a dramatic increase in deflection occurs. In the present study, the critical cyclic load level is corresponding to CLR of Effect of number of cycles of loading on load deflection behaviour The load-deflection behaviour of closely spaced 2 2 pile group (S/D = 3) at different number of cycles of loading is shown in Figure 4. The nature of load-deflection curves at different cycles of loading indicates nonlinear behaviour of pile group. It can be easily noticed from Figure 4 that the degree of nonlinearity increases with an increase in the number of cycles of loading. At low number of cycles, the nonlinear behaviour is related to degradation of stiffness of the soil due to pore pressure buildup. But at higher number of cycles, the occurrence of strong nonlinear behaviour of the pile group is related to both the formation of gap around the piles up to a certain depth and degradation of stiffness of the soil due to pore pressure. The deflection corresponding to 3 cycles of loading with CLR of.8 is about 4 times more than that of the first cycle of loading. The first cycles of loading are very much critical for deflection of the piles accounting for more than 2 times increase over that at first cycle. The ultimate lateral capacity of pile group corresponding to a deflection of 2% of pile diameter (5 mm) under 5 cycles of loading is 2 N which is about 3% less than the corresponding static capacity. 39

5 Deflection (mm) No. of Cycles CLR =.8 CLR =.65 CLR =.5 CLR =.35 Figure 3. Deflection vs. number of cycles for the pile group (S/D = 3). Load (N) S/D = 3 No.of Cycle = No.of Cycles = 5 No.of Cycles = No.of Cycles = 5 No.of Cycles = No.of Cycles = 2 No.of Cycles = Pile Head Deflecton of Group (mm) Figure 4. Load-deflection behaviour of pile group at different number of cycles. 4.3 Effect of pile spacing on load deflection behaviour The load-deflection curves obtained for 2 2 pile group with different pile spacing at 5 numbers of cycles of loading are shown in Figure 5. For a given load, the deflection at 5 cycles for pile groups with S/D ratios of 9 and 7 are less, whereas large deflections occur for groups with S/D ratios of 5 and 3. This is attributed to group interaction effect (i.e., shadowing effect) due to overlapping of stress zones. For the closely spaced pile groups (S/D = 3 and 5), when the piles are subjected to two-way cyclic loading, due to stress overlap, the entire soil column enclosed by the pile group moves as a single block in one direction during first half-cycle of the loading and in other direction during second half-cycle of the loading. Occurrence of block failure mode, under cyclic lateral loading for closely spaced pile groups remoulds and softens the soil column within the group thereby offering less resistance for subsequent cycles of loading, leading to very large deflections. As the spacing of piles in the group increased, the overlap of stress zones reduces. For the 2 2 pile group considered in the present study, the critical spacing under cyclic lateral loading corresponds to S/D ratio of 7. 5 Bending moment profile The effect of cyclic loading on bending moments of single pile (L/D = 5) is shown in Figure 6 for number of cycles N =, 5 and 5 with a load of 39 N corresponding to CLR of.65. The bending moment profiles for both compressive and tensile halves of two-way cyclic loading show similar pattern. The important features of these curves are that the magnitude of maximum bending moment increases with the number of cycles and depth of occurrence of maximum bending moment travelling down the length of pile as the number of cycles increases. It is noted from Figure 6 that the maximum bending moment at N = 5 is about 4% higher than that at first cycle. The depth of maximum bending moment increases from.65 m (.7L) at N = to.9 m (.24L) at N = 5. These effects are attributed to the gapping developed at the pile-soil interface which increases in width and depth with the number of cycles. The gap width at the clay surface is 5 mm (.6D) and propagating to a depth of 2 mm (5D) at 5 cycles of loading. Similar trend is observed in the bending moment profile of piles in x 4 group. The bending behaviour of intermediate piles of the x 4 pile group subjected to an average load of 39 N is compared with the single pile subjected to same load at 5 cycles (Figure 7). It is observed from Figure 7 that the magnitude of maximum bending moment for a pile in the group is % more than that of the single pile under same average load and occurred at much (.4L) deeper depth along the pile than that for single pile which clearly shows that the effect of cyclic loading is more predominant for piles in the group than for single pile. This is attributed to group interaction effect which reduces soil resistance to the piles in the group. Figure 8 shows bending moment profiles of different rows of x 4 pile group subjected to 5 cycles of load of 56 N (CLR =.8). It is observed that the leading row pile carries largest bending moment indicating greater load on this pile. The second row pile has lesser moment compared to leading row but the maximum bending moment occured at a greater depth than the leading row pile. The third and rear row piles carry lesser moments compared to leading and second row piles. These effects are attributed to reduction in soil resistance due to stress over lap and the occurrence of load transfer to the soil at a greater depth for the trailing row piles. 32

6 Load (N) Cycles of Loading Pile Head deflection of Group (mm) S/D= 9 S/D= 7 S/D= 5 S/D= 3 Figure 5. Effect of pile spacing on load-deflection behaviour of 2 2 group (5 cycles). Bending Moment (N - m) Tension direction Compression direction Depth (m) N = N = 5 N = 5 Figure 6. Bending moment distribution in single pile under cyclic loading (load = 39 N) 6 Numerical analysis The numerical analysis for static lateral load on 2 x 2 and x 4 groups with spacing to diameter (S/D) ratio of 3, 5, 7 and 9 is carried out using software GROUP. Analysis is carried out by finite difference approach in which the total length of the pile is divided into hundred increments. The model pile is considered as an elastic beam with pile stiffness obtained from moment of inertia and modulus of elasticity. The p-y method based on beam on Winkler foundation model with the soil reaction (p) pile deflection (y) curves attached at nodal points along the length of the pile is used to model the nonlinear lateral resistance of soil. The p-y curve model for soft clay proposed by Matlock (97) is used in this analysis. Pinned head with lateral load and zero moment condition is selected to represent the free head condition. The pile cap is attached at a height of.5m above ground surface and it's weight is considered as a vertical force. The group interaction effect is taken into account by inbuilt p- multipliers which are used to reduce the value of p for the p-y curve of the single pile to obtain p-y curve for the piles in the group. The values of inbuilt p-multipliers were derived for piles in the group by considering line-by-line, side-by-side and skewed reduction due to interaction with surrounding piles. These values depend on location of the pile in the group and spacing between piles in the parallel and perpendicular direction to the direction of loading. The inbuilt reduction factors were obtained from the results of previous experimental studies. The model pile properties external diameter (.256m), cross-sectional area (2.43 x 4 m 2 ), modulus of elasticity (7 GPa), moment of inertia (.52 x 8 m 4 ) are given as inputs in the analysis. The method of pile installation is considered as bored pile. The single layer of soft clay with undrained shear strength (c u = 9 kpa), obtained from vane shear test, is used. The soil modulus parameter k (8 kn/m 3 ) and soil strain parameter ε 5 (.2) are selected based on shear strength and correlations given by Reese and Wang (997). The values of ultimate unit side friction (.7c u ) and ultimate unit tip resistance (9c u ) are also given as inputs. The load-deflection curves obtained from numerical analysis are shown in Figure 9, which indicate strong nonlinear behaviour. The lateral capacities of pile groups are estimated as load corresponding to a deflection equal to 2% of the pile diameter (Broms, 964). They are: 96 N, 225 N, 245 N and 245 N for plies with S/D ratio of 3, 5, 7 and 9 respectively. It is noted that the lateral capacity of pile group at closer spacing is low and improves as the spacing between the piles increases. The low lateral capacities for S/D ratios of 3 and 5 are attributed to the overlap of stress bulbs of individual piles causing loss of passive resistance. The numerical loaddeflection curves are compared with the measured curves in Figure 9. There is excellent agreement between the numerical and experimental curves for S/D ratio of 5 and 7. However, GROUP over estimates the ultimate lateral capacity of the pile group by about 2 % for S/D ratio of 3. Figure shows the comparison of the bending moment curves obtained by the numerical analysis with that of the measured curves for the leading row pile of x 4 group with S/D ratio of 3 when subjected to average load of 3 N. Though, the nature of variation of the bending moment matches very well, the numerical analysis over estimates the maximum bending moment by %. 32

7 Bending Moment (N - m) Tension direction Depth (m) Compression direction.5 Single Pile Intermediate Row Pile in x 4 Group Figure 7. Comparison of bending behaviour of pile in the group with single pile (Average load = 39 N for 5 cycles). Depth (m) Bending Moment (N-m) Leading Row Third row Second Row Rear row Figure 8. Bending moment distribution for each pile in x 4 group (Average load = 39 N for 5 cycles). Load (N) S/D = 3 Expt. S/D = 5 Expt. S/D = 7 Expt. S/D = 9 Expt. S/D = 3 GROUP S/D = 5 GROUP S/D = 7 GROUP S/D = 9 GROUP Deflection of Pile Group (mm) Depth (m) Bending moment (N-m) Measured GROUP Figure 9. Comparison of measured load-deflection curves for 2 x 2 pile group with curves from numerical analysis. Figure. Comparison of measured bending moment curves for x 4 pile group with curves from numerical analysis. 7 Conclusions. Deflection of the pile group increases with an increase in CLR. When CLR is higher than.5, sharp increase in deflection occurred due to large gap developed at the pile-soil interface within a first few cycles of loading. 2. Deflection of the pile group increases with number of cycles of loading due to increased width and depth of the gap and buildup of excess pore pressure, which leads to reduction of shear strength and degradation of the stiffness of clay. Cyclic loading reduce the ultimate lateral capacity of 2 2 pile group by about 3% after 5 cycles of loading. 3. The group interaction effect under cyclic lateral loading is predominant for groups with S/D ratio less than 7 (critical value). For closely spaced pile groups, block mode of failure has occurred in addition to the gap at the pile soil interface. 4. The magnitude of maximum bending moment for a pile in the x 4 group is about % more than that of the single pile under same average load and occurred at much deeper depth along the length of the pile than that of the single pile indicating that the effect of cyclic loading is more predominant for pile group than for single pile. 5. Among different rows of x 4 pile group subjected to cyclic lateral loading, the leading row pile carry largest bending moment indicating greater load on this pile. 6. The load-deflection curves obtained from the numerical analysis of static lateral loading match well with the measured curves for S/D ratio of 5 and 7 for 2 2 pile group. However, for the closely spaced (S/D = 322

8 3) pile group, GROUP over estimates the ultimate lateral capacity of the pile group by about 2 %. 8 References Brown D.A., Reese L.C., O'Neill M.W Cyclic lateral loading of a large-scale pile group, Journal of Geotechnical Engineering, ASCE, 3, Georgiadis M., Anagnostopoulos C, Saflekou S Cyclic lateral loading of piles in soft clay, Geotechnical Engineering, 23, Matlock H. 97. Correlations for design of laterally loaded piles in soft clay. Proc. OTC, Houston (Texas), Paper No. 24, Moss R.E.S., Caliendo, J.A., Anderson L.R Investigation of a cyclic laterally loaded model pile group, Soil Dynamics and Earthquake Engineering, 7, Peng J.R., Clarke, B.G., Rouainia M. 26. A device to cyclic lateral loaded model piles, Geotechnical Testing Journal, ASTM, 29, -7. Poulos H.G Single pile response to cyclic lateral load, Journal of Geotechnical Engineering, ASCE, 8, Poulos H.G.,Davis, E.H. 98. Pile Foundation Analysis and Design, John Wiley and Sons, New York (USA). Ramakrishna V.G.S.T., Rao S.N Critical cyclic load levels for laterally loaded piles in soft clays, Proc. Int. Conf. Offshore and Nearshore Geotech. Eng., Reese L. C., Isenhower W.M., Wang S.T. 26. Analysis and Design of Shallow and Deep Foundations, John Wiley and Sons, New York (USA). Reese L.C., Wang S.T LPILE Plus 3. for Windows Technical Manual, Ensoft. Inc., Austin (Texas). Rollins K.M., Peterson, K.T., Weaver T.J Lateral load behaviour of full-scale pile group in clay, Journal of Geotechnical and Geoenvironmental Engineering, ASCE, 24, Rollins K.M., Olsen R.J., Egbert J.J., Jensen D.H., Olsen K.G., Garrett B.H. 26a. Pile spacing effects on lateral pile group behavior: load tests, Journal of Geotechnical and Geoenvironmental Engineering, ASCE, 32, Rollins K.M., Olsen K.G., Jensen D.H., Garrett B.H., Olsen R.J., Egbert J.J. 26b. Pile spacing effects on lateral pile group behavior: analysis, Journal of Geotechnical and Geoenvironmental Engineering, ASCE, 32, Wood D.M., Crewe A., Taylor C. 22. Shaking table testing of geotechnical models, International Journal of Physical Modelling in Geotechnics,,