Development of Hybrid Foundation consisting of Bearing Piles and a Mat Foundation

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There are several types of foundations for buildings, such as pile foundations, shallow foundations, and floating foundations. Although a shallow foundation and a floating foundation with a mat slab are usually more economical than a pile foundation, it is occasionally difficult for these foundations to secure a sufficient safety factor against a failure of soft subsoil. On the other hand, the settlement of a building supported by piles is small even if the soft subsoil is very thick. However, the pile-heads beneath the mat often slip out of the soil, and are sometimes damaged by strong seismic motions when the subsoil subsides due to consolidation.In order to avoid these problems and reduce costs, this project develops a new hybrid foundation consisting of a small number of bearing piles and a mat foundation. Its primary functions are to simultaneously utilize the bearing capacities of both the soft subsoil beneath the mat slab and the rigid layer where the bottom tips of pile are pen … More etrated. In order to secure the subsoil reaction force against the mat foundation, the building itself must subside together with the soil. This is made possible by a "reaction control device" at the head of the pile between the pile and the mat slab.For the first step of this project, members developed two kinds of "reaction control device". The first one is composed of three steel plates called fixed plates connected with the mat slab and two steel plates called movable plates connected with the pile head. A pre-stressed concrete (PC) bolts bind all the plates. The axial force of the bolt is, next, applied to create a large friction force between the fixed plates and the movable plates prior to on-site installation. When the load applied to a pile exceeds the maximum limit of the friction force, the pile starts to slip but keeps supporting a load equivalent to the maximum limit. The mat slab, thus, maintains contact with the soil and a constant subsoil reaction force even when the subsoil settles due to consolidation. The second one is composed of double steel piles at the connection between the mat and a pile.One of the main issues of this foundation is the stress concentration with depth due to the negative friction from the soil to a pile. A series of small scale stress concentration tests and a large scale test were carried out in this project. The soil model of the small scale test is φ =25cm and h=33cm, and the large test scale is φ =50cm and h=100cm. The diameters of piles used are 19.6mm and 46mm, respectively. As the test and numerical analysis results, fully mobilized negative friction force acts on the piles. It was, therefore, made clear from the present study that the maximum negative friction should be taken into consideration in the design of this hybrid foundation system. Less