Study On Engineering Geological Characteristics And Crossing Scheme Of Saturated Soft Loess Around Xingqing Lake,Xi'an

Based on the construction of the subway tunnel in the Dongguan Street Xingqing Road,Xi'an metro line six,the engineering geological characteristics of the saturated soft loess and its influence on the subway tunnel are studied by field test and laboratory test and numerical simulation.Saturated soft loess in depth of 2?7m,the thickness is between 3.2m and 11.3m,showing the characteristics of loess ridge is thin,loess depression is thick.The characteristics of the large pore,weak cementation and vertical joint development of the loess are the material foundation of the formation of saturated soft loess.The topography and ground cracks have significant influence on the distribution of saturated soft loess.Hydrogeology provides the environment conditions for the formation of saturated soft loess,and the human engineering activity(the excavation and storage of Xingqing Lake)makes the saturated soft soil soft.The distribution of loess is increasing.The natural moisture content of saturated soft loess is 26%?30%and has a positive correlation with depth.The liquid limit is 29.6?32.8,the average value is 30.41,the plastic limit is 17.8-19.6,the average value is 18.28,the natural severe is 14 7kN/m3?2.05kN/m3 and the depth is positively correlated with the depth.The calculated original pore ratio is 1.11?1.20 and has a negative correlation with the depth.The compression coefficient is 0.301?0.586MPa-1,the mean value of the deformation modulus E0 is 3.9MPa,the mean value of the bearing capacity is 70.5kPa,the mean value of the horizontal stress index is 2.5,the mean value of the static side pressure coefficient K0 is 0.48,the ED mean of the lateral bulge modulus is 6,8MPa and the lateral expansion modulus increases with the increase of the depth.The sensitivity is 2.4,the cohesive force c value is 19kPa and the mean internal friction angle is 8.1 degrees when unconsolidated and undrained.When different traversing schemes are adopted,the settlement of the vault caused by shield method is the largest,followed by the step method,and the CRD is the smallest.The maximum settlement of the tunnel occurs at the top of the tunnel section.The maximum settlement of the ground is above the tunnel vault.The main impact of the settlement is about± 12m(about 2 times the hole diameter).When the thickness of the saturated soft loess increases from 3m to 8m,the settlement of the vault of the two methods increases by about 13%than that of the 3m,when the construction speed of the shield is increased from 0.5m/d.When added to 4m/d,the settlement of the earth surface increased by about 1/3,and the settlement of the vault increased by about 1/5,so the CRD method was recommended to pass through interval.