Numerical Analysis Study On The Soil-nailed Supporting System Of Foundation Pit Excavation In The Sand-cobble Deposit

With the rapid development of the high-rise buildings all over the country, the problem of how to efficiently design the even deeper foundation pits attracts the due attention of the engineering community. Aside the conventional supporting systems, many new ones are introduced in a variety of geological sittings, among which the soil-nailed supporting system has been successfully applied in the sand-cobble deposits mainly in Chengdu, Sichuan Province. The paper presents a detailed numerical analysis to elucidate the failure pattern of the pit wall, the mechanism of the soil-nailed supporting system and its spatial behavior.Firstly, a typical two-layer sand-cobble deposit was chosen for a detailed 2D-FEM study. It was shown that due to the distinct behaviors of the upper soft sand layer against the much stiffer lower cobble layer, a bi-linear plane failure was recommended to be used in the limit equilibrium analysis. Secondly, another 2D-FEM analysis is made on a practical engineering of foundation pit excavation supported by soil-nailed system. Then the different distributions of tensile stress, shear stress, plastic failure zone and horizontal displacement obtained by analysis for non-supporting and soil-nailed supporting step-by-step excavation are compared with each other. Some conclusions about mechanism of soil-nailed supporting system are reached from above comparisons. At last, a 3D-FEM model is used for a soil-nailed supporting system in a classical two-layer distribution. The spatial distribution of horizontal displacement is reached by the analysis. Some other analyses are made for the different ratio of length of foundation pit and the effect of the ratio on the largest horizontal displacement of the slope is analyzed.