| With the great development of the west regions in China, the economy developed fast in Lanzhou, Gansu province. Meanwhile, so many people swarmed into the city that the existing ground transportation cannot keep up with the needs of people’s travel. However, constrained by the location of that Lanzhou is narrow and both sides surrounded by mountains, which severely restricts the improvement of ground traffic conditions, therefore, it is significant to develop the underground rail traffic in Lanzhou city. Lanzhou subway line 1 officially started in 2012. Due to the stratum structure is relatively single, mainly including miscellaneous fill, loess, and sandy pebble. To some extent, miscellaneous fill and loess are belonged to clayey soil, sandy pebble can represent sandy soil. This kind of geological structure is suitable for the construction of cast-in-situ bored piles so that bored piles have been widely applied to the envelope enclosure of the underground station in Lanzhou city. Nevertheless, if pile spacing between two filling piles are too narrow, which will cost too much and slow down the construction process; too wide may cause flow-disruption of the pile of soil and eventually will cause serious engineering accident.This paper based on soil arching theory to research the reasonable pile spacing, using the finite difference program to establish the mechanism of plane strain model, which can theoretically analysis the mechanism, formation and manifestation of the soil arching effects for three typical soils in Lanzhou city. Finally it is concluded that: soil arching effects are connected with the pile of soil’s asymmetrical displacement, horizontal stress concentration and changes in model calculations of the plastic zone; In order to form cohesive soil arching, horizontal earth pressure must be increased to a certain extent, but sandy soil can form soil arching effect in a small horizontal earth pressure conditions. After a comparative analysis of various computation, we can not only concluded that horizontal stress nephogram and the variation value of horizontal stress on two piles’ vertical wires are good reaction on soil arching’s existence and strength, but also get that under the same level, there is a larger variation value and plastic zone of sandy soil, in a word, sandy soil is easier to form soil arching effect than cohesive soil.The deep excavation is analyzed in three-dimensional simulation, combined with the results of two-dimensional model, the author takes special section of three-dimensional model for analysis of soil arching effect, according to the relative displacement between pile and soil in special section, the soil arching effect of three-dimensional model also affected by the relative displacement of pile and soil and surface overload.Based on the mechanism of soil arching effects, the limit equations of reasonable pile spacing has been established by using Mohr-coulomb yielding criteria. And then considering the impact of pile-soil relative displacement on soil arching effect, the author corrected computational formula of reasonable pile spacing in two-dimensional model. Then, using the Lanzhou area of physical and mechanical parameters of soil and pile spacing l=800mm.Final calculated Lanzhou area controlled by the biggest pile spacing l = 1967 mm. And compared to the results of other scholars recommended formula can be found the results of this paper are conservative and beneficial application engineering.The physical and mechanical parameters of the pile of soil are analyzed with finite difference program: the production and strength of the soil arching effects is mainly affected by the cohesive force of the pile of soil, frictional angle and the bulk modulus. The efforts of dilatancy angle and contact surface properties is not very remarkable; the analysis of simulated result between different pile spacing and the pile diameter ratio verified the correctness of the above formulas derived in this article. |
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