Investigating the Load Sharing Behaviour Of Piled Raft with Pile Tips on Hard Strata Using Finite Element Method: Case of Commercial Bank of Ethiopia Headquarters Building
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Date
2019-01
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Addis Ababa University
Abstract
Piled raft foundation is a geotechnical composite construction consisting of three elements: piles, raft and soil. In piled raft foundation, the superstructure load is shared between the piles and the raft. Therefore, the piled raft foundation allows an increase in the load capacity and reduction of settlements in a very economical way as compared with traditional foundation concepts.
In this study, the contributions of raft on bearing behaviour of piled raft foundation and on load sharing mechanisms between raft and piles in case of pile tips resting on hard stratum has been conducted. The full interaction between the components of piled raft foundation was accounted by a three-dimensional finite-element method. Commercial Bank of Ethiopia (CBE) head quarter building has been taken as case study. This foundation is primarily designed as conventional pile foundation. The raft; pile and soil have been discretized as eight-noded brick elements. The raft and piles were modelled with a linear elastic constitutive model whereas; the soil continuum was modelled with modified Drucker – Prager cap plasticity constitutive model.
The foundation settlements and the load sharing between raft and pile have been investigated to identify the contribution of raft to the total capacity of piled raft foundations. The results of the analyses show that although the foundation is primarily designed by conventional piled foundation, the raft alone is adequate to carry the total superstructure load with acceptable settlement. Based on the result adding 46 piles below the raft has no contribution for settlement reduction. However, almost the entire superstructure load is carried by the piles: the contribution of the raft became negligible only 7% of the total load is carried by the raft. When this result is compared with simplified method; the hand calculations give marginally higher value. It gives 14.6% of the total load is transfer directly to soil by the raft. A parametric study has been conducted to see the effect of weak layer located under hard stratum below pile tips on piled raft behaviour. Based on the results, it can be concluded that the effect of this weak layer is negligible for relative stiffness between the weak and hard strata is greater than 0.5.In addition; the piled raft behaviour is not affected if the weak layer is located below 9 m for all stiffness ratios (0.05 to 0.8) considered in this study.
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pile, raft, raft - soil interaction, finite element modelling and analysis;, weak layer, hard strata, pile tips