Behavior of Piled Rafts on Multi-Layered Soil (A Case Study on Orda Project Of Addis Ababa)
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Date
2020-06
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Addis Ababa University
Abstract
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.
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Keywords
Multi-Layered Soil, Orda Project Of Addis Ababa, Piled Rafts