Experimental Study On Tensile Strength Of Unsaturated Cohesive Soil

The study of unsaturated soil has always been a hot and difficult topic,especially the theoretical study of the strength of unsaturated soil.The actual engineering of buildings,roads and bridges,earth and rock dams,tunnels and other projects are closely related to the strength of unsaturated soil.From the material point of view,unsaturated soil is a special engineering material,its strength can be divided into shear strength,compressive strength,tensile strength,because of the poor extension of the soil,mostly brittle damage,people study more on the shear and compressive strength of unsaturated soil,while the tensile strength is relatively less.It is well known that the tensile strength of unsaturated sandy soil is very small,generally below a few kilopascals,but according to the available research,the tensile strength of clay soil can reach hundreds of kilopascals or even higher,which is worthy of attention,so the research on the tensile strength of unsaturated clay soil needs to be paid attention to.To this end,based on the National Natural Science Foundation of China(No.51978249),this paper takes Hunan red clay as the research object,based on the existing test apparatus WDW-10E microcomputer-controlled electronic universal testing machine,and develops the tensile specimen fixing mold according to the tensile apparatus,and analyzes the influence on the tensile strength of unsaturated Hunan red clay from three aspects of dry density,water content and tensile rate respectively,and also adopts high magnification digital microscope to take pictures of the specimen section to observe Its microscopic damage morphology under different conditions,and finally the SWCCs in the full suction range measured by the pressure plate apparatus method,filter paper method and saturated salt solution method are used to establish the connection between matrix suction and tensile strength by combining the relationship between water content and tensile strength.From the above experimental study,the main conclusions are as follows:(1)Four dry densities(1.4g/cm~3,1.5g/cm~3,1.6g/cm~3,1.7g/cm~3),five moisture contents(16.18%,17.77%,20.2%,21.57%,25.48%),four tensile rates(0.1mm/min,0.2mm/min,0.5mm/min,1mm/min)to analyze and study the tensile strength of Hunan red clay,and the results showed that the tensile strength generally increased linearly with the increase of dry density,which is well fitted by linear fitting of the one-dimensional equation;With the growth of mass moisture content,the tensile strength first increases to the peak and reaches the maximum value near the optimal moisture content,and then gradually decreases,which is fitted by the cubic polynomial,and the fitting effect is good;with the acceleration of the tensile rate,the tensile strength first increases rapidly and then increases slowly.(2)Using a high magnification digital microscope to observe the section of the specimen after tensile damage,it is obvious from the microscopic photographs taken that the pore size of the internal pores of the soil gradually decreases with the increase of dry density at the same mass water content,and the large soil particles are closer to the small soil particles with a higher degree of compactness;the internal pore structure of the soil at a water content of 20.2%is the most uniformly distributed and thus the most stable.Therefore,the qualitative analysis by microscopic observation reinforces the above conclusion(1).(3)The combination of pressure plate apparatus method,filter paper method and saturated salt solution method was used to determine the soil-water characteristic curves of Hunan clayey soil under low,medium and high suction respectively,and the connection between matrix suction and tensile strength at different dry densities as well as tensile rates was obtained comprehensively by the relationship between mass water content and tensile strength in conclusion(1),and the study showed that with the increase of matrix suction,the tensile strength showed a first increase and then decrease The study shows that with the increase of matrix suction,the tensile strength shows a trend of increasing and then decreasing,and the difference between the two is getting larger and larger,which also plays a pioneering role for the study of Hunan red clay fracture.