Closure of "Full-Scale Lateral Load Tests of Pile Groups"

Analytical model for laterally loaded pile groups in layered sloping soil

Numerical predictions of the immediate moment carrying capacity of a short pier foundation in saturated clay are presented. Three-dimensional finite element analyses are carried out using linear and non-linear programs and using a linear axi-symmetric program.



Preliminary investigations are made to determine suitable boundary distances for analysis at full-scale and size of loading increment for non-linear analyses.



Predictions of the behaviour of prototype pier and of conventional and centrifuge models of this pier are then made. It is shown that the axi-symmetric program yields significantly higher rotations per unit moment than the linear three-dimensional program and that, using both of these programs, elastic analyses of the conventional and centrifuge models and of the prototype yield very similar results. It is also shown that non-linear analyses of the conventional and centrifuge models yield significantly different moment/rotation relationship in accordance with the behaviour actually observed in the model tests. The relationship obtained for the centrifuge model is shown to differ only slightly from that obtained for the prototype, due to the boundary restrictions in the model, and to be of the same order as the centrifuge test result at working condition but not at ultimate capacity.

Numerical Studies of the Moment Carrying Capacity of a Short Pier Foundation in a Clay Soil

The response of piles and two-pile groups to lateral loading has been studied by field tests and computationally. Due to the lack of field test data and because of uncertainty concerning the pile/soil system it has been suggested that further experimental studies of pile groups under lateral loading should be undertaken. The research was conducted through a series of tests on vertical single piles and two-pile groups at various spacing and pile cap overhang heights, to identify the lateral stiffness, bending moment and axial force distribution. Attempts were also made to measure the in-situ total lateral soil pressure on the pile walls. Piles were designed to behave as "long" pile since most piles used in the U.K. are long and flexible. Piles were instrumented with strain gauges for measurement of bending moments and axial forces. Field tests were conducted in a sand trench using 4.0m long piles. A stiff steel pile cap was used to connect head of the two piles firmly together. Linear elastic back analyses of single pile tests were carried out to estimate the soil modulus profile with depth. Thereafter comparisons were made between the field test results on two-pile groups, published analyses and also a three dimensional finite element analysis. Tests results showed that the lateral stiffness of a two-pile groups tends towards a limit as spacing increases. A similar result was found from predictive and finite element analyses. The ratio between the maximum pile shaft bending moment and horizontal force varied between dry and wet season, being greater in the latter. The ratio between maximum reverse bending moment and horizontal load increased as the pile spacing and the overhang increased. Similar results results were found by finite element analysis. One of the main achievements in this research was the measurement of the axial forces in the vertical piles due to lateral loading. It was found that as the pile spacing increased and pile cap overhang height decreasd the peak axial forces per unit load decreased. Similar results were obtained by three dimensional finite element analysis.

The behaviour of laterally loaded two-pile groups

Offshore wind energy offers a huge potential for expansion of renewable energy in the world. However, placement of wind turbines in harsh offshore environments is an engineering challenge, which requires development of suitable foundation to support turbines. Among 4 typical types of offshore foundations, monopile foundation is selected and designed for 2.3 MW offshore wind turbine (OWT) in Weihai, China, considering water depth, soil conditions, foundation cost and other factors. In a three dimensional (3D) finite element model of monopile, the layered seabed soil where the pile is embedded is considered in a soil-structure interaction formulation. Maximum stress and deformation of the monopile are computed under 4 load cases in different operating phases. The natural frequencies of the monopile foundation are given through the modal analysis. Results show that the designed monopile can effectively preclude resonance with the wind turbine. Moreover, the dynamic response of monopile foundation under wave and seismic loads are carried out by transient analysis method, the time histories of the deformation and stress are presented, and dynamic amplification factors (DAF) of maximum stress and deformation are discussed. It is suggested that the dynamic effect must be attached importance in the design.

M.R. Arta

Papers

Three-dimensional finite element analysis of pile groups under lateral loading