Experimental Study On Redesign Of Soil And Foundation For Increasing Storey On Existing Buildings

Before existing buildings are rehabilitated to increase their storey, the first problem is to evaluate the characteristics of subsoil correctly. This paper studies on the properties of subsoil, which is completely simulated under four different period loads by three large scale indoor model tests. The four different loads are respectively the construction period load, the service period load, and the storey increasing rehabilitation by enlarging the foundation area or not period loads. After the measured P-S curve, deep layer settlement value, subgrade reaction force and pre-and-post indoor subsoil test data are analyzed, this paper gets some significant conclusions:(1) Under long-term sustained load, the characteristic of the soil has changed greatly, the soil has occurred compaction effects and the P-S curve becomes gentle when additional load is added.(2) The compaction effects of the subsoil mainly occur at the range of 1.25B (B is the shorter width of the foundation) under the foundation. The soil out of the foundation changes a little.(3) Long-term sustained load test is applied to ascertain the bearing capacity of subsoil for existing building rehabilitation. According to the measured P-S curve, the bearing capacity of the subsoil can be achieved by the foundation code. Among methods of measuring the bearing capacity of subsoil, the long-term sustained load site test method is the most simple and reliable one.(4) For storey increasing rehabilitation directly design, if the settlement of the existing building is stable and the foundation structure can meet the need of normal use, the bearing capacity of subsoil of the built building can be obtained according to the local construction experience. According to the analysis of this paper, if the soil is uniform and the superstructure rigidity is adaptable, the bearing capacity of the building can increase by 20-30 percents.(5) The slope of linear branch of the measured P-S curve which is obtained by static-load test can be directly applied to calculate the additional settlement for existing building rehabilitation.(6) Under long-term sustained load, if the foundation area is enlarged to be two times of the original one, the P-S curve is still similar to that of the natural subsoil. In other words, when the foundation of the built building is enlarged, the original building's investigation data can be directly applied to measure the bearing capacity and settlement of the subsoil.(7) The compaction of soil is the key factor of enhancing the subsoil bearing capacity.