Study On Pore Structure And Deformation Strength Characteristics Of Unsaturated Compacted Loess

Compacted loess is a typical unsaturated soil.Because of its special mechanical properties,it is difficult to reasonably explain the engineering geological problems of unsaturated soil with classical soil mechanics theory.Yan'an New Area is located in the hinterland of the Loess Plateau in northwest China,which is dry and rainless,and most of the loess strata are in unsaturated state.The Land Creation Project has greatly changed the original topography and landform of Yan'an New Area,and the large volume of loess filling and compaction has led to geological disasters such as land subsidence and landslide.Therefore,this paper selected the loess in Yan'an New Area as the research object,analyzed the geological situation of the study area,obtains the specific gravity,liquid and plastic limit,optimal water content and other basic parameters of the loess through the physical property test,and obtained the soil-water characteristic curve through the filter paper method.The test scheme was developed based on soil-water characteristic curve,and the deformation strength of compacted loess was studied by GDS unsaturated soil triaxial instrument.After the test,the triaxial sample was cut into mercury injection sample,and the pore structure was analyzed by mercury intrusion test.The main work and research results are as follows:(1)Combined with the existing data,self-test exploration and discussion with relevant technical personnel,a set of test operation and research methods of GDS unsaturated soil triaxial apparatus and Auto Pore IV 9500 mercury porosimeter are summarized.The sample preparation,experimental principle,operation process,precautions and parameter setting were described in detail.(2)Based on the GDS triaxial test system,the test process,stress-strain relationship and strength parameters of unsaturated soil under different water content and different net confining pressure were studied.It is found that the time for suction balance and consolidation of soil samples near the optimal water content is shorter,and the consolidation time under high confining pressure is shorter.All the soil samples showed strain hardening and strong hardening under high confining pressure,and the strength increases with the increase of matrix suction.Based on Fredlund's theory of unsaturated soil strength with double stress variables,the strength parameters of unsaturated soil are calculated by two methods.It is found that with the increase of matric suction,the effective internal friction angle does not change much,and the total cohesive force increases linearly.The suction friction angle can reflect the contribution of matric suction to soil strength.(3)Based on mercury intrusion test,the pore size distribution,pore structure parameters and their relationship with deformation strength of compacted loess were analyzed.The pore distribution of unsaturated soil after triaxial consolidation and drainage shear is obviously concentrated,mainly small pores,which are distributed between 2?8?m.The compaction loess can obtain relatively uniform pore distribution under the optimal water content,and also shows more uniform pore distribution under high confining pressure.The porosity and apparent density obtained by mercury intrusion method are slightly lower than the actual value,and the bulk density is close to the real value.The porosity measured by mercury intrusion is the lowest at the optimal water content,and the actual porosity of soil samples decreases with the increase of initial water content.