Study On Soil Pressure Of Deep Shaft Under Loess-Paleosol Interbedded Condition

With the continuous development of subway construction in China,the subway shaft continues to develop in a deeper and larger direction.Under the condition of loess-paleosol interbedded,the deepest shaft under construction in Xi ’an area is about 50 m for the first time.In the past,when designing the shaft structure,the earth pressure was often calculated by static earth pressure or Rankine active earth pressure.The calculated lateral earth pressure was very large at a deeper position.However,this is not the case,resulting in the waste of supporting materials.Therefore,the distribution of shaft earth pressure has become a key problem to be solved urgently.Therefore,this paper focuses on the engineering practice and takes the lateral earth pressure of deep shaft as the main research object.Through the study of the physical and mechanical properties of loess-paleosol strata,the field monitoring analysis of shaft,the study of static earth pressure coefficient and the numerical simulation method,the earth pressure of deep shaft under the condition of loess-paleosol interbed is studied.The following conclusions are obtained:(1)Under the same sedimentary age,the void ratio,water content,liquid index and compression coefficient of paleosol are smaller than those of loess,and the saturation and plasticity index are larger than those of loess.Under the condition of loess-paleosol interbedded,water content and void ratio,liquid index and void ratio,saturation and water content,liquid index and plasticity index have good correlation in the depth direction.The soluble salt content test results show that the soluble salt content of the ancient soil layer is higher than that of the loess,and the cation is mainly Ca2+.The static cone penetration test shows that the bottom boundary of the paleosol layer has large resistance value and friction characteristics.The lateral compression test results show that the initial pressure P0,plastic pressure Pf and ultimate pressure PL of paleosol are larger than those of loess.(2)During the excavation process,the convergence deformation of the shaft structure is small at the bottom of the ancient soil.The horizontal displacement results of the deep soil of the inclined pipe show that the horizontal deformation diffusion range is concentrated in the range of 5m at the edge of the shaft.The monitoring results of soil pressure show that the soil pressure fluctuates with the depth direction,and the soil pressure has a minimum value at the bottom of the ancient soil.The measured earth pressure coefficient calculated by soil column theory is in the range of 0.12～0.35.(3)The static earth pressure coefficient of loess-paleosol strata is positively correlated with compression coefficient,void ratio,water content and liquid index,and negatively correlated with dry density.Based on this,a formula for quantitatively calculating the static earth pressure coefficient K0 by physical property index is proposed,and its applicability is preliminarily verified.The relationship between the measured earth pressure coefficient and the calculated static earth pressure coefficient is obtained.The earth pressure is greatly affected by the soil properties.(4)The soil pressure calculation formula of circular shaft with a diameter of 10 m is given under the condition of loess-paleosol interbedded.The verification of the earth pressure monitoring data of another shaft shows that the overall law of the calculated value is in good agreement with the measured value,which preliminarily proves the rationality of the calculation method in this paper.(5)By establishing a three-dimensional numerical model of circular shafts with different diameters,it is found that the soil pressure strength at a specific depth is affected by the excavation unloading of the soil in the range of 10～15m above and below the depth,and the size effect coefficient Kd is proposed.With the increase of shaft excavation diameter D,the influence range of surface settlement is 0.6～0.8D;combined with the size effect coefficient Kd,an earth pressure calculation formula considering the properties of loess-paleosol strata and the diameter of shaft excavation is proposed.