Study On Tests Of Deformation Characteristics Of Deep Clayey Soil In The Typical Area Of Beijing Tianjin And Hebei

The Beijing-Tianjin-Hebei region has a wide area of land subsidence and a high subsidence rate,and the area of serious subsidence is more than 80%of China.In response to the national requirements for comprehensive prevention and control of land subsidence in Beijing-Tianjin-Hebei region,the paper relies on the project undertaken by China Geological Environment Monitoring Institute(DD20160235)‘Land subsidence and ground fissure investigation and geological environment monitoring in Beijing-Tianjin-Hebei region’and the National Natural Science Foundation of China(41877294)‘Soil Deformation Characteristics and Mechanism Driven by Groundwater Level Change in Complex Aquifer System’.Based on the geotechnical test data of 15borehole soil samples in the settlement areas of Lang Fang,Da Xing,Shun Yi and Tian Jin,the variation rules of basic physical and mechanical parameters of clay soil from 0 to400m are analyzed.On the basis of a large number of data statistics,the formula for estimating compression modulus of deep cohesive soil is summarized by normalization method.The applicability of the classical consolidation model in deep cohesive soil is verified,and the equation for estimating the degree of consolidation-time factor for different depths of cohesive soil is summarized.The deformation law of repeated compression rebound of clay soil with different depths is analyzed,and the differences between in-situ stress and consolidation and repeated loading and unloading tests under conventional conditions are summarized.The main conclusions are as follows:(1)The density and compression modulus of clay increase with depth.The liquid index decreases with the increase of depth.Plastic index and limit water content fluctuate with depth in a certain range.In terms of consistency,the deep clay in this area is mostly hard plastic and hard.(2)The compression modulus of clay is very important for estimating land subsidence.The compression modulus is obtained by using various experimental data processing methods,and there is some error between the calculated value and the actual deformation when it is used to calculate the settlement amount.By normalizing a large number of consolidation test data,the fitting curve and envelope of compression modulus can be obtained,which can estimate the maximum,minimum and intermediate values of compression modulus,and provide a scientific basis for the settlement calculation of deep clay in the region.(3)Conventional compression models are well fitted to shallow viscous soil compression deformation,such as e-logp model,log(e+e_c)-logp model and dual curve model,however,for deep clay,the experimental data are poorly fitted by the above models.In the analysis of consolidation process,the Terzaghi consolidation equation is closer to the experimental data than the hyperbolic consolidation equation.However,for deep clay,the Terzaghi consolidation equation deviates from the experimental data greatly,so it is not suitable for the analysis of consolidation process.Based on the analysis of a large number of experimental data,an empirical formula can be obtained to reflect the consolidation process of cohesive soil.The consolidation parameters can be obtained by high pressure consolidation test.The formula can provide a reference for estimating the consolidation degree of soil at different times and calculating the land subsidence.(4)Through repeated loading and unloading consolidation test analysis,for the same depth of cohesive soil,in the process of repeated loading and unloading,the deformation of cohesive soil gradually accumulates and increases,which is greater than the deformation of high pressure consolidation under the same load.In the process of loading and unloading,the deformation of clay shows a certain lag.In the unloading process of soil,the percentage of rebound increases with the increase of load ratio,and the rebound modulus decreases with the increase of load ratio.(5)In consolidation and repeated loading and unloading tests for in-situ stress recovery,the deformation of cohesive soil with the same depth under the same load is greater than that of the conventional consolidation test,indicating that the ground settlement calculated by the conventional consolidation test may be less than the actual ground settlement.