Research For Strength Reduction Method Of Ultimate Bearing Capacity Of Strip Shallow Foundations Based On Slip-Line Theory

The stability of foundations is an important factor, being considered in the foundation design. The strength reduction method is applied to evaluate the stability of foundations and calculate the bearing capacity of footings. This research can be summarized as the following:(1) The ultimate bearing capacity is calculated without superposition. The numerical computation of ultimate bearing capacity on rough and smooth shallow strip footings can be obtained using ABC (Analysis of Bearing Capacity) software proposed by Martin. The numerical results of bearing capacity factor Ny are calculated based on the formula of Terzaghi.(2) The suggested expressions to bearing capacity coefficient Nγ are proposed without superposition. According to the numerical results, Nγ is not only related to internal friction angle φ but also the dimensionless ratio F(F=γB/(q+ccotφ)). The Nγ is a constant with the determined φ and F. The function relationship of Nγ、φ and F is analysed based on the numerical results. The suggested expressions to approximate Nγ of rough and smooth shallow strip footings are proposed. The errors between approximated values and numerical results are clarified. The calculation results show that the errors can be controlled within 4%, which are of high fitting exactness. The effect of dimensionless factor F has been considered for the results of Nγ calculated by the suggested expressions in this paper, which has been improved compared with the traditional superposed methods.(3) Introducing the strength reduction of slope stability analysis from the theoretical basis and process, the method for stability analysis of foundations based on strength reduction theory has been put forward in this paper. Based on the theoretical formula and numerical calculation program of the ultimate bearing capacity of the smooth and rough shallow strip footings, the shear strength parameters c and tanφ are divided by the same strength safety coefficient Fs, then the reduced bearing capacity and the expression of stability safety factor K and strength safety coefficient Fs are obtained. The errors of results between the fitting formula and numerical calculation can be controlled within 4%. To sum up, strength reduction method is a new way to determine the allowable bearing capacity and the stability safety factor of footings, and the results can be used as reference for safety analysis of foundations.(4) The influence of design parameters including c、B、D and φ on the stability safety factor K and strength safety coefficient Fs is analysed in this paper.The applications to evaluation of foundation stability based on strength reduction theory are explained by typical examples. The foundation safety coefficient K and strength safety coefficient Fs have been analysed with the change of internal friction angle φ、 cohesive strength c、 foundation width B and embedded depth of foundation D. The results show that the foundation safety coefficient K has a wide fluctuation with the variation of these parameters, while strength safety coefficient Fs is more stable, which is more suitable to evaluate the stability of foundations. Strength safety coefficient Fs being a constant value, allowable bearing capacity is basically consistent with the characteristic value of bearing capacity from Code for Soil Foundation of Building(GB50007-2011), which means that the characteristic values of bearing capacity from the code are reasonable.