| In the foundation pit engineering, because of the need of the building functions andlayout, a second or multiple grade excavation in the external pit is necessary, which lead to thedepth difference of the foundation, this appeared to multi-level pit, so called “Pit-in-pit”.Current research of the multiple excavation is not enough and the current design specificationsdo not involve such engineering phenomenon as pit-in-pit. The existence of the internal pitleads to the discontinuous basement of the external pit, and the area between the external andinternal pits appears. Research on the soil-structure interaction in the area between the externaland internal pits is the key to ascertain the influence of the pit-in-pit.Soil arching phenomenon was confirmed widespread in the foundation pit and othergeotechnical engineering, in the pit-in-pit region, which between the external and internal pits,soil arching effect also exist. Soil arching effect has a greater impact of the distribution ofearth pressure. Analysis of the mechanism of soil arching between the pit-in-pit region andstudy on the soil arching effect on the soil pressure focus on the shear friction characteristicsof soil and structure in this paper. Firstly, base on appropriate assumptions, improved thepit-in-pit analysis model. The range of the soil arching effects is analysed according to theassumption of the Coulomb’s theory. Secondly, the soil arching effects of sandy and cohesivesoil in the region of pit-in-pit is analysed by formula derivation and example analysis using theparallel walls theory. Then, by using the method of level layer element, the distribution ofpassive earth pressure and non-limit state earth pressure in the pit-in-pit area is analysed.Thirdly, the factors which impact the earth pressure in the pit-in-pit region is explored througha analysis of examples. The main research harvests are as follows:The coefficient of earth pressure is impacted by the smooth degree of the surface ofsupporting structure. When the supporting structure with smooth surfaces, the soil archingdoesn’t form in the pit-in-pit region and the the coefficient of earth pressure is equal to theresults of Coulomb’s theory. When the supporting structure surface is rough, soil archingeffect is obvious, the coefficient of earth pressure between Rankine’s theory and Coulomb’stheory.The coefficient of earth pressure in sand has nothing to do with the depth. Cohesive soil’scoefficient of earth pressure related to the depth, and become large when depth is shallow,tends to a constant value when depth is enough.Soil arching effect leads to thepassive earth pressure which load on the supporting structure distribution in a curve. The distribution of passive earth pressure become small on the top and large on the bottombecause of the soil of shallow layer forming an arch and then unloading to the surface.According to the results of the earth pressure distribution fitting curve, earth pressure is zero ina depth range, which unfavorable for the embedded stability and the deformation control ofthe supporting structure.Putting forward the mechanism of soil arching effect progressive development, whichbelieve that soil arching effect is progressive develops with the amount of displacement of thesupporting. The earth pressure change from static earth pressure to passive earth pressure withthe development of the soil arching effect, and the distribution of earth pressure change fromlinear to non-linear gradually.The better the shear strength of the soil is, the greater the resistance moment on theexternal pit supporting structure. Through the method of foundation reinforcement andincreasing the embedded depth in the pit-in-pit region, the horizontal resistance force of on thesupporting structure improves effectively. When the pit-in-pit region is narrow, the earthpressure which loading on the internal pit structure become large. In this case, strongerhorizontal supporting structure should be set to ensure the stability. |
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