Field monitoring and numerical analysis on piled-raft foundation : case study

Abstract

This thesis presents the result of detailed back-analysis, using three-dimensional finite-element analysis, of the instrumented piled-raft foundation in monitoring site. The piled-raft foundation is a composite foundation structure that consisting of piles, raft and surrounding soils acting as a whole system.

To check the reliability of soil taking load under the raft and obtain a reasonable value of load proportion taken by piles for the soil conditions in Hong Kong, a piled-raft foundation was partially instrumented in the monitoring site. The pile head loading, raft-soil contact pressure of specified area and settlement at raft top for selected locations were being monitored. Comparisons of overall settlement, differential settlements and the load carried by the piles show reasonably good agreement.

Followed by a 3D finite element modeling of the entire piled-raft foundation of the monitored site, the analysis includes a pile-soil slip interface model. The numerical analysis is performed to give insights to (1) load transfer behavior of the piled-raft foundation (2) effects of pile reduction on pile load ratio.

Combined the observation from site monitoring and analysis results from the numerical analysis, the proportion of load shared between piles and raft is revealed as 7:3. The lower limit of pile ratio is proposed as 0.67 for the site after the parametric study by removing piles strategically. In spite of the settlement-reducing purpose of the piles, the design of piled-raft foundation still concentrate on providing adequate axial capacity, with settlement requirement treated as a secondary issue. The significance of the study is that it provides factual evidence of soil taking the load under the raft, and the economical benefits of piled-raft foundation as a reduction of piles will save more than 2 million of the construction budgets.