Reliability Based Design of Pile Raft Foundation

Abstract

The Combined Piled Raft Foundation (CPRF) is a modern concept inwhich the total load coming from the superstructure is partly shared by the raftthrough contact with soil and the remaining load is shared by piles through skinfriction and/or base capacities. A CPRF system is economical compared to thetraditional “piled foundation” design where the pile cap is assumed to be sustainedby piles only.A “case study” (Basrah elevated water tank project) is studied thoroughlyin this work. The 1365 m3 elevated water tank located at 3 nearby sites wasoriginally designed as a piled foundation with 25 bored piles for each site (0.7mdiameter and 24m length). Theoretical analysis reveals that the piles have anallowable capacity of 2245 kN. On the other hand static pile tests were preformedon 17 piles out of 75 piles and it appeared that the allowable capacitydemonstrated 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 theloads on both piles and soil with the corresponding settlement values.The reliability aspects of behavior of both “piled foundation” and CPRFare investigated. In this approach the influence of autocorrelation for the stiffnessmodulus (of both piles and soil) and raft thickness are considered.The safety of both systems is obtained in terms of traditional factor ofsafety (FS) and reliability index (b). The results showed that the “piledfoundation” 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 3criteria for b.