Behavior of Piled Rafts on Multi-Layered Soil (A Case Study on Orda Project Of Addis Ababa)

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Addis Ababa University


It is becoming abundant in Addis Ababa, to conservatively place high-rise buildings on deeper hard stratum through very long piles. However, the use of piled raft foundation seems to be more advantageous than the conventional pile foundation since both the raft and piles contribute to the resisting capacity of the system. In an attempt to investigate the extent of this principle, a case study was undertaken in this paper. The case under consideration, ORDA project of Addis Ababa, incorporates three blocks of different height, accessed as one building. The highest block of the structure rested on long piles to transfer the load to a basalt layer found at a great depth. In the study, the as-built conventional pile foundation scheme was analyzed and compared with that of a proposed piled raft foundation system. In doing so, a finite element analysis software, ABAQUS, was utilized to investigate the behavior of the foundation systems under study. The Effect of one block on the other as well as differential settlement due to varying loads of the blocks is considered in the analysis. The analysis result revealed that the proposed piled raft foundation was able to safely support the vertical load coming from the super structure by utilizing a shorter and much smaller number of piles than used in the as-built foundation system. It was possible to reduce the number of piles from 162 to 50 and the length of piles from 25.4m to 20.4m, with the adequate margin of safety. The enormous economic benefit of using piled rafts is also shown through direct comparison of expenses for the as-built and proposed foundations. A potential cost reduction of 75% was observed in the analysis.



Multi-Layered Soil, Orda Project Of Addis Ababa, Piled Rafts