ANALYSIS OF LATERALLY LOADED PILES NEAR CREST OF SLOPING GROUND

Abstract

Pile foundations are generally provided for resist vertical loads. However, it has to withstand the lateral loads generated by wind, wave action, and earthquakes. In many practical situations, structures subject to lateral loading are located near excavated slopes or embankments. Development in hilly regions often involves construction of buildings and bridges on slopes which are to be supported by piles. Different analytical and numerical methods to analyze lateral response of pile include elastic continuum method, finite element method and Winkler spring. Out of the available analytical and numerical approaches of pile analysis in level ground, only finite element analysis can be employed for piles near slope. Elastic continuum method is not applicable to sloping ground conditions and Winkler springs must account for reduced stiffness. From the literature available, it is evident that limited studies had considered the effect of ground slope and edge distance on the lateral load capacity of piles and pile group in clay. Effect of pile-soil separation is barely considered So far the pile response was obtained for a specific horizontal load at top. But in the practice, pile is designed for limited allowable displacement. Hence in the present study, it was aimed to investigate the lateral response of single pile and pile group near clayey slope through 3-D FEM using PLAXIS 3D. Effects of slope geometry and soil properties are critically investigated. A detailed parametric study was planned on single pile and different pile group configurations to investigate the effect of slope and pile parameters. The effect of slope angle, undrained shear strength, restraint, pile diameter and pile-soil interface adhesion on the safe pile capacity with and without pile cap embedded at the crest of clayey slope were investigated. A constant Length to diameter ratio (L/D = 25), Slope angles  form 0° to 30°, undrained cohesion cu = 50 to 100 kPa, pile diameter D of 0.4, 0.6, 0.8, 1.0 m and pile-soil interface adhesion coefficient, α of 0.25, 0.5, 0.75, 1 are considered for the study. Almost a linear variation in safe pile capacity with undrained cohesion, pile diameter and slope angle were observed. However, the change was not linear with variation in pile-soil adhesion. From the obtained results, an expression is given for the safe lateral pile capacity, in terms of cu and D. Values of Reduction factors to account slope are also given.A constant pile diameter (D = 0.6 m), length to diameter ratio of pile (L/D = 25) and pile-soil interface adhesion coefficient (α = 1.0) are considered for the study the response of single pile, pile group in series and parallel arrangement and square pile group near clayey slope. Slope angles of 0°, 10°, 20°, 30°, cu = 50 to 100 kPa, S/D of 0,1,3,5,7 are considered. Pile capacity is considered as load corresponding to top displacement equal to 20% of pile diameter, to expose the nonlinear behavior of the soil and the effect of slope angle, undrained cohesion, and edge distance from crest on the ultimate lateral load capacity of single pile embedded in clayey slope. Increment in the undrained cohesion of soil results improvement in the lateral pile capacity. Similar effect of undrained cohesion is also observed in sloping ground. Increase in the ground slope reduces the soil available on the sloping side for passive resistance. The effect of slope on lateral pile capacity is relatively more for higher values of undrained cohesion. The ultimate loads are increasing with increment in the edge distance. A correlation between the effect of undrained cohesion, slope angle and edge distance from crest on pile capacity is established. Hyperbolic p-y relationships are established for different edge distances along depth for a slope angle of 30° for cu = 50 kPa and 100 kPa. Initial slope Kmax and ultimate soil resistance pu, reduced with increment in the ground slope. The effect of slope angle, undrained shear strength, edge distance of the pile cap near front pile from crest on the ultimate lateral load capacity of pile group in series and parallel arrangements and 2 × 2 pile group embedded near clayey slope were examined. Load sharing ratio of piles and the effect of aforesaid parameters on the same for these configurations are also studied. Increment in the cu results an increase in the lateral capacity for level and sloping ground. Increment in the ground slope causes reduction in the in lateral capacity. The ultimate loads are increasing with increase in the edge distance. A correlation between the effect of undrained cohesion, slope angle and edge distance from crest on Lateral load capacity of the 2 Piles in Series, 2 Piles in Parallel and 2 × 2 pile group are established. It is to be noted that these constants are different for both the cases and are applicable only for cu ranging from 50 kPa to 100 kPa. In series arrangement, for the case of horizontal ground, the load taken by Pile 1 is approximately 10% higher than Pile 2 due to shadowing effect. Whereas for the case of pile configuration at crest, the ratio of load sharing ratio of the piles (P1/P2) reduced due to lesser soil passive resistance available for Pile 1. It also observed that with increase in the edge distance from the crest, the load sharing ratio increased. In level ground case front pile share more load as compared to rear piles. This is attributed to shadowing effect of stress zone on rear piles. But in the case of sloping ground, front pile share lesser load as compared to rear pile because of proximity of slope. For the case of Parallel arrangement, both in horizontal and sloping ground conditions, the load sharing ratio of the piles (P1/P2) is approximately equal to one, as shadowing effect and effect of passive resistance from soil is equally experienced by both the piles. From the load sharing of the individual piles in 2 × 2 pile group, it is observed that the response of Pile 1 and Pile 2 (Piles near slope) is similar and Pile 3 and Pile 4 (Piles far from slope) is similar. For the case of horizontal ground, the load taken by front piles (P1 and P2) is approximately 20% higher than rear piles due to shadowing effect. For the case of pile configuration at crest, the ratio of load sharing capacity reduced due to lesser soil passive resistance available for front piles. It can also be observed that with increment in the edge distance, the load sharing ratio increased.