Influence Of Layer Thickness Ratio On Mechanical Properties Of Alternatively Distributed Sandstone And Mudstone Rock Mass

The alternatively distributed sandstone and mudstone rock mass are widely distributed in southwestern China.The existing standard specifies the basic mechanical parameters of the alternatively distributed sandstone and mudstone rock mass.It indicates that the mudstone value is used when the layer thickness is less than 5 meters,and the relationship between different engineering scenes and different layer thickness ratios is not further distinguished.In this paper,the ratio of mudstone thickness to sandstone thickness is called the layer thickness ratio.From the factors affecting the mechanical parameters of alternatively distributed sandstone and mudstone rock mass,FLAC3 D is used to carry out triaxial numerical test and uniaxial numerical test.Through the test results,the changes of shear strength parameters,compressive strength and elastic modulus of alternatively distributed sandstone and mudstone rock mass under different layer thickness ratio,layer thickness,rock dip angle and model size are analyzed.This provides a reference for the value of the mechanical parameters of alternatively distributed sandstone and mudstone rock mass in actual engineering.Obtained the following main results:(1)If the other conditions are the same,the following rules are obtained by analyzing the layer thickness ratio: When the layer thickness ratio is different,the mechanical properties of the alternatively distributed sandstone and mudstone rock mass are also different.As the layer thickness ratio increases,its mechanical parameters tend to decrease.When the layer thickness ratio of alternatively distributed sandstone and mudstone rock mass is in the range of 0.1~0.75,the shear strength parameters,compressive strength and elastic modulus decrease with the increase of the layer thickness ratio,and the variation range is large.The influence of the layer thickness ratio should be considered,and the value should be taken according to the field test results.When the layer thickness ratio is in the range of 0.75~2,the above mechanical parameters vary little with the layer thickness ratio.They are basically consistent with the mechanical parameters of mudstone,so it is objective and feasible to take the values of mudstone parameters in actual engineering.(2)The influence of layer thickness on the mechanical properties of alternatively distributed sandstone and mudstone rock mass needs to be determined by distinguishing the layer thickness ratio.The cohesion,internal friction angle and compressive strength of the alternatively distributed sandstone and mudstone rock mass increase with the decrease of the layer thickness.When the layer thickness ratio is in the range of 0.1~0.75,the smaller the layer thickness,the greater the cohesion,internal friction angle and compressive strength,and much larger than the mudstone parameters,which means that the actual engineering should not only be based on the mudstone cohesion,internal friction angle and compressive strength.Specific value recommendations are determined based on actual conditions or field trials.When the layer thickness ratio is in the range of 0.75~2,the cohesion,internal friction angle and compressive strength change little with the layer thickness,and are basically consistent with the mudstone parameters,which can be taken according to the mudstone parameters.The influence of layer thickness on the elastic modulus of rock mass is reduced,and the elastic modulus of alternatively distributed sandstone and mudstone rock mass decreases.When the layer thickness ratio is between 0.1 and 0.5,the elastic modulus of the alternatively distributed sandstone and mudstone rock mass changes greatly with the decrease of the layer thickness,and the elastic modulus is larger.In the interval of 0.5~2,the elastic modulus changes more smoothly,but it is still larger than the elastic modulus of mudstone.(3)The influence of rock dip angle on the cohesion,internal friction angle,compressive strength and elastic modulus of alternatively distributed sandstone and mudstone rock mass should also be determined by different layer thickness ratios.When the inclination angle of the rock is less than 50° and the layer thickness ratio is between 0.1 and 0.75,the cohesion and compressive strength are basically consistent with the parameters of the mudstone.In actual engineering,the cohesive force and compressive strength of the mudstone can be taken.When the inclination angle of the rock layer is greater than 50° and the layer thickness ratio is 0.75~2,the cohesion and compressive strength increase greatly.In actual engineering,the value should be taken according to the actual situation.The influence of the dip angle of the rock layer on the internal friction angle and the elastic modulus is generally "U" type change.(4)The size of the model has little effect on the mechanical properties of the alternatively distributed sandstone and mudstone rock mass,the model size increases,and the mechanical parameters of the rock mass decrease.The shear strength parameters,compressive strength and elastic modulus layer thickness ratio change greatly in the mechanical parameters of alternatively distributed sandstone and mudstone rock mass in the range of 0.1~0.75.In actual engineering,it should be determined according to the actual test conditions.They are basically unchanged in the interval of 0.75~2,and they are similar to the parameters of mudstone.In actual engineering,the values of mudstone can be taken.(5)Taking the north anchor foundation pit of Changshou Second Bridge as an example,by comparing the deformation data of the foundation pit with the actual monitored deformation data,it can be seen that in actual engineering,factors such as layer thickness ratio,rock thickness and rock dip angle should be considered.It is more practical to take the value according to the equivalent parameters.