Research On Influence To Earth Pressure By Existing Composite Foundation

With the accelerated urbanization process in China, the high-rise buildings develop rapidly and the underground engineering especially excavation engineering obtain unprecedented opportunities for development in quantity and scale. The increasingly serious shortage of land makes buildings more and more crowded, and the surroundings around proposed foundation are more complex. The influence of existing buildings should be taken into account in design and construction of pit excavation. In especial, the adjacent buildings affect the calculation mode of earth pressure, which is the primary problem in designing excavation engineerings.At present, the additional stress in natural foundation has been considered in determining earth pressure. When existing buildings adopt composite foundation, earth pressure will be special and should not be calculated by existing theory because of the complexity of material composition and the load transfer mechanism in the composite foundations. So it is important to study the distribution characteristics and calculation method for the earth pressure when composite foundation is close to pit.Based on previous theories and research results, the influence of composite foundation on earth pressure is thoroughly researched and a tentative calculation method aiming at this situation is proposed. The specific work and achieved research results are as follows:(1) The calculating formula for earth pressure of the finite soil between the basement and retaining structure is derived. The laws of slip surface inclination and earth pressure coefficient with soil parameters’change are studied. Through the comparative analysis of the finite earth pressure and Coulomb’s earth pressure, It is concluded that the finite earth pressure increases to Coulomb’s with rise of the ratio of finite soil’s width and height.(2) Based on the conception of equivalent entity and the assumption that the friction force and lateral pressure of finite soil between retaining structure and composite foundation are equally distributed, a new computational model of earth pressure is built. The formulas of earth pressure, slip surface inclination and earth pressure coefficient are builded. In addition, a few of examples with concrete values are given and the laws of slip surface inclination and the earth pressure coefficient with the change in forces and the aspect ratio of finite soil are summarized.(3) When existing building near foundation pit is constructed on natural foundation or composite foundation, the deformations of retaining structure and distributions of earth pressure are discussed by numerical simulation analysis. By comparing these results with that without considering existing building, the influence on excavation of adjacent buildings with composite foundation is obtained.(4) On the basis of the current earth pressure theory considering construction loads, an equivalent handling to calculate the earth pressure due to excavation is put forward. When composite foundation is adopted, the load could be divided into two loads on the natural foundation according to two partition coefficients. One of the loads is acting on the top of basement and the other one is acting on the pile tip in natural foundation. Some relevant parameters of composite foundation are varied to prove the new method’s applicability.(5) The process of pit excavation is simulated when composite foundation of existing building adopts cement-soil piles. The cement-soil piles and soil are regarded as a composite material with strength indexes and deformation index, and the influence laws on retaining structure displacement and earth pressure with the variation of indexes are obtained. It is concluded that the strength indexes are major factors. The higher strength indexes are, the smaller are earth pressure and lateral displacement.