Reliability Based Design of Pile Raft Foundation
Engineering and Technology Journal,
2010, Volume 28, Issue 12, Pages 2361-2378
AbstractThe Combined Piled Raft Foundation (CPRF) is a modern concept in
which the total load coming from the superstructure is partly shared by the raft
through contact with soil and the remaining load is shared by piles through skin
friction and/or base capacities. A CPRF system is economical compared to the
traditional “piled foundation” design where the pile cap is assumed to be sustained
by piles only.
A “case study” (Basrah elevated water tank project) is studied thoroughly
in this work. The 1365 m3 elevated water tank located at 3 nearby sites was
originally designed as a piled foundation with 25 bored piles for each site (0.7m
diameter and 24m length). Theoretical analysis reveals that the piles have an
allowable capacity of 2245 kN. On the other hand static pile tests were preformed
on 17 piles out of 75 piles and it appeared that the allowable capacity
demonstrated erratic values below the expected pile capacity.
A re-analysis of the pile raft is performed establishing the CPRF concepts.
The case study was modeled by STAAD Pro computer package to determine the
loads on both piles and soil with the corresponding settlement values.
The reliability aspects of behavior of both “piled foundation” and CPRF
are investigated. In this approach the influence of autocorrelation for the stiffness
modulus (of both piles and soil) and raft thickness are considered.
The safety of both systems is obtained in terms of traditional factor of
safety (FS) and reliability index (b). The results showed that the “piled
foundation” system is “unsafe” for 3 criteria for both FS and b. On the other hand,
the CPRF is “safe” for the 4 criteria for FS concept while it is “unsafe” for 3
criteria for b.
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