Optimized Use of Settlement Reducing Piles for Raft Foundation on Clay Soil
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Date
2020-10-11
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Addis Ababa University
Abstract
In this study, a concept of using piles for settlement control has been examined through
a Finite Element modeling technique. A raft alone may be adequate in terms of bearing
capacity but predicted settlement may exceed tolerable values. Hence, piles may be
provided under the raft foundation to make the system sound. The study begins with an
analysis of an unpiled raft foundation to decide the requirement of piles followed by an
extensive parametric study to optimize the pile usage through three-dimensional analysis.
The results indicated that the di erential settlement requirement can be meet with an
increase in raft thickness but the vertical settlement increases in the same order. Vertical
and di erential settlement decreases with an increase in length and number of piles. For
the considered loading and subsoil condition a central pile group area ratio of 26% reduced
the di erential settlement signi cantly and the corresponding load shared by the piles
reach more than 40%. The pile spacing is an important parameter and the study shows
a vertical settlement decreases with an increase in pile spacing owing to a reduction of
interaction among piles while di erential settlement increases with increasing pile spacing
due to the sti ness reduction of the central portion of the piled raft in which maximum
settlement is expected . The load sharing ratio increases with an increase in pile spacing.
Strategic use of non-uniform pile length con gurations, shorter piles at the center, and
longer at edges, enhances the settlement performance of piles raft foundations for the
same total pile length due to relatively longer piles at edges supports the cantilevered
raft portion. Pile diameter variation shows no signi cant change in settlement values
as the settlement increase associated with increase in pile diameter is compensated by
the frictional resistance increase due to pile circumference frictional area increase. The
e ect of consolidation in the load sharing ratio reaches up to 14% along with its e ect
in increasing the settlement and hence the usual practice of pre-proportioning the super-
structure load to the foundation units may not be the best solution as the long term
behavior of these units will be altered due to consolidation.
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Keywords
Piled raft foundation, Finite Element modeling, Clay soil