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

建筑物的基础有几种类型，如桩基础、浅基础和浮动基础。虽然浅基础和垫板浮动基础通常比桩基础更经济，但这些基础有时难以确保足够的安全系数，以防止软土地基的破坏。另一方面，即使软土地基很厚，桩支撑建筑的沉降也很小。然而，垫层下面的桩头经常会滑出土壤，有时在地基因固结而下沉时被强烈的地震运动破坏。为了避免这些问题并降低成本，本项目开发了一种由少量承桩和垫层基础组成的新型混合基础。它的主要功能是同时利用垫板下的软基和桩底端处的刚性层的承载力。为了确保底土反作用力对垫层基础的作用，建筑物本身必须与土壤一起下沉。这是通过在桩头和垫板之间的“反作用力控制装置”实现的。在项目的第一步，成员们开发了两种“反应控制装置”。第一种是由与垫板连接的三块固定钢板和与桩头连接的两块活动钢板组成。用预应力混凝土（PC）螺栓将所有钢板固定。然后，在现场安装之前，施加螺栓的轴向力以在固定板和活动板之间产生较大的摩擦力。当桩所受的荷载超过最大摩阻力极限时，桩开始发生滑移，但仍继续支撑着相当于最大极限的荷载。因此，垫板保持与土壤的接触，并保持恒定的底土反作用力，即使底土因固结而沉降。第二种是在垫板与桩的连接处由双钢桩组成。该基础的主要问题之一是由于土与桩之间的负摩擦引起的应力随深度的集中。本项目进行了一系列小型应力集中试验和大型试验。小比例尺试验土模型φ =25cm, h=33cm，大比例尺φ =50cm, h=100cm。桩径分别为19.6mm和46mm。试验和数值分析结果表明，桩上的负摩擦力充分调动。因此，本研究明确了在设计混合基础体系时应考虑最大负摩阻力。少

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