Study On The Test Of Tensile Strength Of Loess In The South Of Jinghe

As a common material of soil mass,loess has been studied thoroughly by the scholars from its shear strength and compression strength long before.But the tensile strength of loess is not caused much attention and studies by the scholars,until the geological disasters and engineering problems related to the tensile strength of loess have frequently emerged since this year.The tensile strength of loess is a kind of ability to resist external axial tensile load.When the tensile stress can right produce the loess fracture,then the loess body has reached its ultimate tensile strength.The tensile strength of loess is rather small and not easy to be measured accurately,thus it is often ignored by people in the experimental research and practical engineering.The studies on tensile strength of loess can be generally divided into two kinds of test methods: the direct test method and the indirect.Direct tests consist of uniaxial tensile test and triaxial tensile test;while the indirect tests consist of bending test of curved beam,cylindrical specimens of axial or radial fracturing test method.These two kinds of test methods have their own advantages and disadvantages,in which the tensile stress by indirect test is not as accurate as by direct test and so is the the accuracy of stress-strain curve.But the indirect test method is simple and easy to operate,the determination of the tensile strength is relatively close to the actual situation.This thesis has experimented on the undisturbed landslide loess from Zhaitou village in south Jinghe River via uniaxial tensile test method.By virtue of large number of rigorous tests to measure the size of the uniaxial tensile strength of loess and explore its influencing factors,this thesis has achieved the following results:（1）Though the uniaxial tensile strength of loess is small,only from thousands of PA to tens of thousands of PA,it does exist,which size alters closely related with the water content in soil mass.That is,with the increase of water content,the tensile strength decreases rapidly.From the beginning to the end of the test,the axial tensile strain of loess specimen becomes very small,almost invisible in the test process.After multiple groups of tensile test,we can find that after reaching the ultimate tensile strain,the specimens’ sudden rupture process is similar,which belongs to the brittle fracture;（2）the uniaxial tensile strength of loess can be affected by the specimens’ height-diameter ratio,but the effect is not obvious.With the same diameter and different heights,the ratio of height to diameter of the loess specimen are 2.04（height,80mm）,2.30（height,90mm）,2.56（height,100mm）.Under the same experimental conditions,the average value of the uniaxial tensile strength is between 14.29 KPa and 14.65 KPa,whose ultimate displacement’s average value is between 4.13 * 10^-2mm to 4.71 * 10^-2mm,whose ultimate tensile strain is between 0.047% to 0.051%,which changes in a small rang.It indicates that under the same experimental conditions,the uniaxial tensile strength of loess does not change greatly with the change of its height-diameter ratio.In order to get the strength formula which can judge both the tensile fracture and the shear fracture of loess,this thesis,based on Mohr--Coulomb strength theory and inspired by the empirical formula of joint strength theory,intends to work an elliptic curve fitting on the straight line of the envelope line from Mohr--Coulomb strength theory in a stretch zone,and validate it by the real test data through the empirical formula derived.The final joint strength formula of elliptic curve based on Mohr--Coulomb strength theory can basically judge the undisturbed loess when it is subjected to tensile and shear fracture at the same time.Meanwhile by the employment of this empirical formula,under the condition of knowing the compression strength and shear strength in the undisturbed loess,can calculated reversely the tensile strength and produce a rather reasonable result.The instability and deformation and fracture of loess slope is not a single shear fracture,but a relatively complicated shear-tensile fracture.Before the collapse and the landslide of loess,a tensile crack often emerges on the trailing edge of the slope.With the double effects of both rainfall erosion and irrigation,the slope water content has increased,while the mechanical properties of the loess including tensile strength and shear strength has decreased sharply,resulting in the instability and deformation and fracture of loess slope.Therefore,according to the ideal model of loess landslide,from the soil-plasticity limited equilibrium point to conduct the approximate calculation formula that can predict the back tension fracture position of loess slope,and conform the trailing edge crack position,will contribute a positive significance for both the prevention and monitoring of loess landslide.