Experimental study of the effect of cushion thickness and the distance of the piles on the behavior of the non-connected pile raft foundation

Because of the existence of concentrated forces and high bending moments at the junction of pile and raft in structure with high load using the non-connected pile raft foundation can be an appropriate option for the foundation of the mentioned structures. In this system, the piles and the raft are not connected and a cushion is used between the piles and the raft to redistribute the forces.in this state load transmit from raft to pile by arching mechanism that forming in cushion layer. The behavior of pile raft because of the interactions such as: pile-pile, pile-soil, pile-raft, raft-soil is very complicated. The pile use in pile raft for two purpose: 1- reduce the settlement and 2- increase the bearing capacity. In this study, a series of experimental tests were conducted on a non-connected pile raft located on a sandy soil in order to investigate the effect parameter such as cushion height (H) and pile spacing (S) in forms of non-dimensional (H/B & S/D) on load-settlement behavior, share of piles and raft from total load and axial stress and frictional stress in center and corner piles. For this purpose, states H/B= 0.17, 0.34, 0.5 and S/D=2, 4, 6 were investigated. The axial stress in piles measured according to strain gauges that installed in perimeter of piles. The results shows that in three state S/D=2, 4, 6 optimum state occur in H/B=0.34. With increase the H/B the forces applied in pile is decrease and then the share of piles from total load decrease. The maximum share of piles from total load occur in H/B=0.17 and the minimum occur in H/B=0.5. In low level of load major of load protected by raft that with increase the load level, share of piles from total load increase and stable in a specified value. Due to the fact that part of the load is transmitted to piles by the soil around the pile, the maximum force applied to the piles does not occur at the tip of the pile and a negative friction is formed in the upper part of the pile. In the upper part of the pile, the displacement of the soil is more than the displacement of the pile, resulting in the formation of a negative friction and the formation of a positive friction in the bottom of the pile. With increase the H/B the value of negative friction is increase and neutral axes (a location of piles that displacement of soil and pile are equal) move down. In various pile spacing (S/D=2, 4, 6) and in three state (H/B= 0.17, 0.34, 0.5) maximum bearing pressure observed in S/D= 4. In S/D=2 maximum interaction effect (pile-pile) and minimum Enclosed of sand and in S/D=6 minimum interaction effect (pile-pile) existed and enclosed of sand has few effect because of high distance of piles. In three state H/B= 0.17, 0.34, 0.5 minimum share of piles from total load in S/D=6 and maximum share of piles in S/D=4 observed. Maximum negative friction in S/D=6 and minimum negative friction in S/D=4 observed. In all state in corner piles negative friction is more than center piles.