DYNAMIC BEHAVIOUR OF BATTER PILES AND PILE GROUPS

Abstract

Batter piles (also known as inclined or raker piles) are often used for supporting bridges, offshore structures, transmission line towers, etc., where the horizontal load per pile may exceed the capacity of vertical pile. During the past seismic events, batter piles have shown both beneficial and detrimental effects. Several researchers have discussed and debated on the virtues and drawbacks of batter piles which further helped in understanding their response in a better manner. A number of documents are available on the seismic performance of batter pile groups based on controlled tests on scaled laboratory models. The primary objective of the present study is to examine the dynamic behaviour of bored cast in-situ batter piles and pile groups in the field conditions. In this thesis, extensive field investigations along with finite element (FE) models are discussed in detail to understand the dynamic response of batter piles and pile groups. This research comprises four major components: (a) dynamic pile load tests on single vertical and batter piles; (b) dynamic pile load tests on vertical and batter pile groups; (c) 3D finite element analysis of single piles and pile groups and (d) measurement of dynamic pile load test induced vibration in adjacent building and its finite element analysis. In the initial phase, the behaviour of bored cast in-situ reinforced concrete (RC) six single piles: (a) single vertical pile (B0); (b) vertical pile with an under-ream bulb (B0U1); (c) 10° batter pile (B10); (d) 10° batter pile with an under-ream bulb (B10U1); (e) 20° batter pile (B20); and (f) 20° batter pile with an under-ream bulb (B20U1), subjected to lateral and vertical dynamic loading is examined experimentally. All these six piles are subjected to dynamic load generated by an oscillator motor assembly firmly mounted on top of the pile cap. In each direction, the piles are subjected to six different intensity of force levels (varied in the form of eccentricity of the oscillating mass). In addition, four different RC pile groups consisting of: (a) three vertical piles (3VG); (b) three 20° batter piles (3BG); (c) four vertical piles (4VG); and (d) four 20° batter piles (4BG), are also constructed by bored cast in-situ method and tested for five different intensity of force levels. Further, the effect of lateral loading direction on the dynamic responses of these piles and pile groups is also explored. The dynamic response of piles and pile groups is recorded in real time using piezoelectric accelerometers placed at appropriated location on the pile cap. From the recorded dynamic response, the variation in resonant frequency, peak displacement, induced strain etc., is obtained and discussed in detail. Numerical investigations are also performed to evaluate the behaviour of batter piles and pile groups subjected to dynamic loading using 3D FE models. Appropriate loading and dynamic boundary conditions, element sizes and lateral extents of the model are used to develop 3D FE iv models. The material properties of both soil and pile are considered similar to the experimental investigation test site. The effect of loading direction, intensity of exciting force level, material nonlinearity etc., are explored in detail. In addition, an attempt is also made to understand the bending moment profiles of batter piles through a hybrid modelling approach. The developed 3D FE models reasonably replicate the experimental findings. Thus, the model developed could also be used for other parametric studies which are not experimentally feasible. The effect of pile-soil modulus ratios (Ep/Es) on the dynamic response of batter piles is also examined using 3D FE model. When dynamic tests are conducted on pile groups, the vibrations propagated through the soil mass to the adjacent structure. These vibrations are measured at the source and at different floor levels (including ground floor and roof) of the adjacent building, in the form of acceleration and velocity time histories. The response obtained through the experiment is also compared with 2D FE analyses of the integrated building-pile group-soil system. The estimated vibration responses in terms of peak ground acceleration (PGA), peak ground velocity (PGV), pseudospectral acceleration (PSA) and predominant frequencies (f) are compared with the permissible limits recommended in the relevant standards and codes of practice.