The Mechanical Properties Of Compacted Loess Soils In The Loess Terraces And Hilly Areas In Lanzhou And Its Engineering Application

Lanzhou is a typical valley-type strip city.The land resources are very scarce and there is insufficient space available.The only way to increase land use in lanzhou is to cut mountains and fill trenches and artificial land on both sides of the city.The Gaoping District and the low hill gentle slope area are the strongest areas of Lanzhou slashing and trench filling activities.In addition to cutting the square slope,a large number of filling sites and filling slopes have also been formed.Loess is the main source of landfill in Lanzhou.The mechanical properties of compacted loess directly control the foundation stability and slope stability in the fill area.Therefore,studying the mechanical properties of compacted loess in Lanzhou area has important engineering significance and application value.Based on the typical takatsubo area and low hills gentle slope area of lanzhou loess as the research object,this paper adopts indoor test methods such as triaxial shear test,direct shear test,variable head permeation test,collapsibility test and direct shear creep test,etc.The variation laws of shear,collapse,infiltration and creep properties of compacted loess with compaction degree and different water content were studied,and the creep parameters obtained from the test were used to evaluate the long-term stability of the filling slope of Pacific site in low hilly area.Get the following main results:1.The C values of cohesion in the Pacific,Fulongping and mixed compacted loess all increase with the increase of the moisture content,and reach the maximum at the optimum moisture content;when the moisture content is greater than the optimum moisture content,C The value decreases rapidly with increasing water content.The internal friction angle ? of compacted loess decreases with increasing water content.When the water content is fixed,the cohesive force C and the internal friction angle ? of the compacted loess increase with the increase of the compaction degree.In different regions,the shear strength of compacted loess is slightly different.When the moisture content and the compaction degree are the same,the cohesive force of the compacted loess in the Pacific is the largest,followed by the mixed loess in the Gaoping District of Taoshuping,Wenchang Pavilion,Xujiashan,Xiguoyuan and Hualinping(abbreviated as mixed loess).The cohesion force of Fulongping compacted loess is the smallest;the inner friction angle of Fulongping loess is the largest,the mixed soil is slightly smaller,and the Pacific loess is the smallest.This isclosely related to the slightly different composition of minerals,granularity,and salt content of the loess.The triaxial shear test shows that the shear strength index is generally larger than that obtained by the direct shear test.This is due to the different instruments used in the test.2.The collapsibility coefficient of compacted loess in both the three regions was reduced with the increase of moisture content.When the moisture content reached 14%,the collapsibility disappeared,and decreased with the increase of compaction;When the pressure increases,and the collapsibility reaches its maximum,the loess termination pressure is greater and the loess' collapsibility is smaller.The collapsibility coefficient of Fulongping compacted loess is greater than that of other compacted loess in Gaoping District and compacted loess in Pacific Ocean.The difference in the collapsibility coefficient of the three types of compacted loess is also closely related to the slight difference between the mineral composition,grain size composition,and salt content of the loess.3.The permeability coefficients of the compacted loess in the three places are all decreased with the increase of compaction degree.At lower dry densities,the measured permeabilities ranged from 10-5,while at larger dry densities,the permeability coefficients decreased significantly,with magnitudes of 10-6-10-7.Fulongping loess has the largest permeability coefficient,the mixed soil is slightly smaller,and the Pacific Loess is the smallest.This is closely related to the slight differences between the mineral composition,grain size composition,and salt content of three loess soils.4.The direct shear creep test of compacted loess in Pacific field revealed that when the normal stress is 100,200,300,400 k Pa and the shear stress is lower than 30,60,90,120 k Pa,respectively,the creep is always in the decay stage,and the final strain tends to a fixed value.When the shear stress under all levels of normal stress is greater than the above shear stress,the shear creep will change from the decay creep state to the stable creep state.When the shear stress under the four levels of normal stress increases to 40,70,100,and 130 k Pa,respectively,the specimen quickly enters the acceleration phase until it is destroyed.The long-term strength of compacted loess increases with the increase of normal stress.The long-term strength cohesion is 10 k Pa and the internal friction angle is 17°.5.Based on FLAC3 D numerical simulation,under the natural conditions,the V section of the filling slope of the Pacific site,after creep of 255800 steps,the wholedeformation of the slope enters the constant-speed creep state,and the maximum displacement of the slope occurs at the shoulder of the slope.The value is about 80-126 cm,the maximum creep rate is 0.39mm/d;the tensile plastic zone appears at the top of the slope,and the rest is shear plastic zone;the slope creep has the characteristic of decreasing velocity from the top of the slope to the slope.Based on the simulation results,it is estimated that after the next eight years,part of the slow-slip may occur in the section of the V-filled section of the Pacific filling slope.