Effect of ground subsidence on piled raft foundation

Abstract

Extensive researches for piled rafts have been performed but the evaluation of their behavior in ground subsidence condition is still a challenge. In this study, centrifugal tests and finite element (FE) analyses were performed in order to investigate the effect of ground subsidence on load distribution between piles and raft, foundation settlement as well as moment and shear in raft. Six cases of centrifugal tests consisted of group 1 (soil condition 1) with case 1: raft alone, case 2: piled raft (s = 2d), case 3: piled raft (s = 4d) and group 2 (soil condition 2) with case 4: raft alone, case 5: friction piled raft (s = 4d), case 6: end-bearing piled raft (s = 4d) were conducted under centrifugal force field of 50g. The centrifugal test results can be concluded as followed; 1) foundations settled almost linearly with ground subsidence, 2) as piled spacing increases, settlement of foundation increases and axial load of piles increases, 3) negative skin friction of end-bearing piled raft was much larger than that of friction piled raft in ground subsidence condition. After centrifugal tests had been conducted, parametric studies were made for centrifugal piled raft models and three cases of piled rafts on Bangkok soil (case 1, case 3 and case 3 with levels of raft at -15 m, -7.5 m and + 0.0 m, respectively) under ground subsidence condition by a commercial finite element analysis program (Plaxis 3D). The calculations were done by drained, plastic analysis with effective parameters. Results showed that load-settlement curves, load distributions between piles and raft, axial force of piles, average and differential settlements, raft moment and shear were significantly affected by number of piles, Young’s modulus of soil, pattern of loads, raft thickness, piled spacing and ground water levels.