FLEXURAL BEHAVIOUR OF PARTIALLY EMBEDDED PILE FOUNDATIONS

Abstract

,Piles used in water front structures usually have large unsupported length and are subjected to lateral loads in addition to vertical loads, Most of the available rigorous analyti cal^studies on laterally loaded piles pertain to fully embedded piles. The most popular method of analysis for partially embedded piles, the 'equivalent cantilever method' has certain shortcoming e.g. the depth of fixity cannot be estimated correctly taking into account all the factors. Also, the method does not take into account effect of vertical load on the flexural behaviour of piles. In view of the above, a rigorous analytical solution for partially embedded long flexible pile in cohesive/cohesionless soils subjected to vertical load P, lateral load Q and moment, M has been presented. The analysis is based on modulus of subgrade reaction approach. The governing differential equations for pile deflection are solved and the results are presented in non-dimensional form. Typical deflection and moment curves are presented and the effect of vertical load, end conditions and unsupported length on the flexural behaviour of partially embedded piles is studied. The results of the analysis are compared uith those of the equivalent cantilever method. To enable the ready use of the analysis, charts are presented for the computation of top deflection and maximum moment. ii The modulus of subgrade reaction approach results in a linear relationship between the lateral load and top deflection of the pile whereas the actual relationship is non-linear. This non-linear relationship can be predicted assuming elasto-plasti c soil behaviour. Therefore analysis of partially embedded long flexible pile in cohesive/cohesionless soil is presented assuming the soil behaviour to be elastoplasti c. The results are presented in non-dimensional form to study the influence of (i) vertical load, (ii) end conditions, and (iii) magnitude of plastic resistance of soil on the flexural behaviour of piles. To enable the ready use of the analysis, charts are presented for the computation of lateral deflection and maximum moment for practical ranges of vertical load, plastic resistance parameter and unsupported length. A numerical example is also solved to illustrate the use of the charts. A two dimensional analysis for group of partially embedded piles is presented making use of the solutions obtained for single piles based on modulus of subgrade reaction approach. Pile groups consisting of vertical and batter piles, embedded in cohesi ve/cohesionless soils are considered. Using the analysis, some numerical problems of pile groups consisting of different combinations of vertical and batter piles subjected to lateral load with and without axial load are solved and the results are discussed bringing out the influence of configuration of pile group, unsupported length, batter angle and soil modulus ill on the lateral resistance of pile groups. The analytical studies suggest that vefctical load increases the lateral deflection whereas the results of tests on actual piles and model piles indicate that the vertical load decreases the lateral deflection of a laterally loaded pile. The probable causes of the above contradiction are ^ hypothesised on the basis of a detailed review of the results of investigations reported in the literature. An experimental investigation on model single piles and group of piles (1x2 and 2x2) has been carried out. Tests have been carried out on fully and partially embedded piles. A special loading arrangement Was designed and fabricated which facilitated the Vertical load to remain vertical and at the centre of the pile head/pile cap throughout the lateral load test. The results ' of the tests are analysed and the effect of vertical load on partially embedded pile is brought out. Based on the studies carried out the following signific ant conclusions are drawn *. 1. The vertical load increases lateral deflection and maximum moment significantly in the case of a partially embedded pile unlike the case of a fully embedded pile. For a given vertical load, the percentage increase depends on the degree of fixity at the pile head and unsupported length of pile. 2. The non-linear relationship between the lateral load and top deflection of partially embedded piles can be predicted iv using the analysis based on elasto-plasti c behaviour of the soil. 3. In case of partially embedded pile groups the influence cf soil modulus diminishes as unsupported length increases. In the case of pile groups with large unsupported portion, an error in the estimation of soil modulus is therefore likely to have negligible influence on the predicted values of deflection and maximum moment. 4. Pile groups consisting of batter piles offer more lateral resistance than the groups consisting of all vertical piles, when subjected to only lateral loads. When the pile group is subjected to both vertical and lateral loads the lateral resistance is significantly affected by the direction of the pile batter. 5. The results of experimental investigation, unlike the results of similar investigations reported by other investigators, show that the vertical load magnifies deflection. The observed reduction in lateral deflection due to vertical load, reported in the literature, may possibly due to the defect in loading mechani sm.