TIME DEPENDENT BEHAVIOUR OF SINGLE PILES UNDER LATERAL LOADS IN COHESIVE SOIL

Abstract

The time-load-deformation behaviour of single vertical and batter piles under lateral loads in cohesive soils have been studied on the basis of both theoretical and experi mental investigation. Also, the flexure behaviour of vertical and batter piles has been studied. In addition to this a few tests were also performed on pile bents. Tests were carried out in soft clay deposit. Only a few tests were performed in stiff clay. A review of the literature pertaining to analytical studies on single vertical and batter piles is presented. The experimental investigations have also been discussed. The literature review indicates that among the different approaches„ the Winkler's hypothesis is most popular. Elastic solutions have also been developed to predict the behaviour of laterally loaded piles. None of these solutions, however, account for the time-dependent behaviour of single piles under lateral loads, which is an important aspect. This timeload- deformation characteristics of laterally loaded single piles embedded in soft cohesive deposits have been studied. It is shown that time-dependent deformations of piles under lateral load can be predicted by the proposed rheological model representing soil-pile system by a strain softening spring in combination with a time thickening dashpot. The K stiffness of spring,^ = -~§ and viscous resistance of -XVItime thickening dashpot t] = n0 t°. Thus, the relationship developed to predict the timeload- deformation behaviour of the laterally loaded pile is of the form: (P-Ko) N *t = B.0o N It consists of three rheological constants, i.e. K . o' t)0 and N. It is observed that these constants depend on the soil-pile interactional behaviour and for a particular system they have a unique value. Knowing these constants, the pile head displacement can be predicted under a given load for the entire range of time. Experimental procedure has been developed to determine the proposed rheological constants. The laboratory studies indicate that these rheological constants may be used effectively to describe the time-load-deformation behaviour of a field pile. The analysis is also extended to predict the timedependent behaviour of batter piles. The deformation could be given by the equation yt = Ae± t*~ The experimental investigations were carried out on 25 mm and 2? mm outer diameter long instrumented aluminium -XV11- model piles. The deflections at ground level were measured independently using dial gauges. The strain gauge observations were taken after each load increment or after suitable inter val of time in case of sustained lateral loads. The data obtained from the tests were reduced on a IBM-1130 digital computer. From the observed bending strains moments were computed. Fitted moments, rotation, deflection, shear, soil reaction and soil modulus were obtained for the entire length of the pile. These results have been inter preted and analysed to evaluate the flexural and timedependent behaviour of soil-pile system. On the basis of the investigation, the following conclu sions are drawn: 1. Time-load-deformation behaviour for a single vertical and batter piles can be predicted knowing the rheo logical constants of the proposed model. An experi mental technique has been developed to determine these constants. Thus, for the entire range of time, the pile head displacements can be predicted for the given load level. 2. In case of soft cohesive deposits, the modulus of subgrade reaction appears to increase linearly with depth. 3. As the soft cohesive soils exhibit a non-linear character, susceptable to creep and having strain -XV111- softening characteristics, the values of modulus of subgrade reaction are affected by the load level and also duration of its application. Experimental evidences show that the values of modulus of subgrade reaction decreased in both the cases. 4. In stiff cohesive deposits, the modulus of subgrade reaction remains constant with depth. 5. The mechanism of deformation in cohesive soils is quite different as compared to piles embedded in sands. The experimental evidences show that in case of soft cohesive deposits with piles under small magnitude of load a negative batter pile takes less load as compared to a vertical pile- The positive batter piles can sustain more lateral loads as compared to a vertical pile for e^ual lateral deformations.