Study On Conceptual Design Of Variation Rigidity Composite Foundation Based On The Controlling Settlement Principle

In recent years, the concept of design methods of composite foundation based on the controlling settlement principle attracts more and more attention of the civil engineering professions. With regard to the research about composite foundation according to the controlling settlement, foundation's settlement is a critical point. And the design methods of composite foundation according to the controlling settlement should consider various factors sufficiently. This paper summarizes various factors on the settlement of the raft foundation, and focus on the impact of the supporting structure. Based on the principle of reducing the raft foundation differential settlement, this paper also studies the conceptual design method of variable stiffness composite foundation.The Q-S curve will be used to calculate the settlement of composite foundation. Using the numerical simulation method and considering of the supporting structure and the depth of foundation, a three-dimensional finite element model of composite foundation is established. The loads distribution of frame-shearwall structure is large in the center and small on the edge of raft. Under the circumstances, this paper analyses the settlement of variable rigidity composite foundation with long-short-pile of the same pile spacing, with long piles in the center and short piles on the edge of raft, with a small pile spacing in the center and a large pile spacing on the edge of raft, with reducing the equal rigidity composite foundation edge piles. The main conclusions are as followed:(1) The presence of supporting structure affects the settlement of raft. However, the influence on the foundation that has reinforced body(piles) is not distinct. But the presence of supporting structure affects the settlement of natural foundation significantly.(2) To increase the length of pile can reduce the settlement of the raft. But that can not solve the problem of differential settlement of raft in the equal rigidity (3) The loads distribution of frame-shearwall structure is large in the center and small on the edge of raft. Under the circumstances, to reduce the differential settlement, and basing on the principle that high bearing stiffness in the center regional and low bearing stiffness on the edge regional, the area of high bearing stiffness must be larger than the core area of upper structure. According to the analysis, the variable rigidity composite foundation with long piles in the center and short piles on the edge of raft is the best method. And the settlement control is very good when center area of long piles occupies about 130% of the core-tube load distribution area. In this case, different positions of the pile share load evenly. But this range is concerned with the ratio of the bearing stiffness.(4) The purpose of reducing the bearing stiffness on the edge of raft is mainly to increase the settlement on the edge of raft, while the settlement in the central part varies slightly, so that the settlement curve is smooth and the differential settlement decreases markedly. However, if the bearing stiffness on the edge regional is reduced severely, the settlement value will be very small in the center but large on the edge of raft. That fails to control the settlement.(5) The key point of reducing the differential settlement is to choose the right ratio of the center and the edge regional bearing stiffness and the corresponding arrangement region of bearing stiffness.