Study On Fissure Characteristics And Its Impact On Slope Stability Of Expansive Soil

Expansive soil is one of the most complex global engineering geological problem.Multi-fissures are typical characteristics of expansive soils. Fissures constitute thefractured structure, resulting in a complex physical and mechanical effects on expansivesoil. Fissures damage integrity of expansive soil,and the distribution of fissures haveheterogeneity and tendency, which cause intensity anisotropy of strength and make theslope instability mechanism of expansive complexity. Therefore, studying fissurecharacteristics of expansive soil and its influence on side slope stability has importantacademic value and very high practical value.This thesis focuses on the Mid-route of South-to-North Water Transfer Project ofNanYang section. Through the field investigation of slope and landslide, we discuss therelation between the spatial distribution of fissures and the landslide of expansive soil. Weobtain the density of undisturbed fissure sample and physicochemical properties andmechanical properties of the fillings in fissure through lots of laboratory test. Systemresearch the effect of fissure angle and thickness to the strength of expansive soil throughartificial simulation fracture. Quantitative analyses the effect of fissure length、fissureangle and fissure depth to the slope stability of expansive soil, which based on the limitequilibrium method considering crack factors of Geo-slope, and applicated in the middletypical slope stability analysis of Mid-route of South-to-North Water Transfer Project.Main content and results of this thesis are as follows:（1） Scene fissure investigation reveals the distribution of fissure in expansive soil ofthe Nanyang test segment：the main development of fissures below the surface3⁷m. Lotsof fissures extending for a length of0.5².0m grow in the range of3⁷m, Lots offissures extending than2.0m grow in the range of8¹2m. The majority of the fissureobliquity are low-angle and middle-angle, and few dip fissures. With the increase offissure buried depth, the number of low-angle fissure is gradually increased and thenumber of middle-angle and dip fissures is decreased. The fissure dip is relevant to itsextension scale. With the increase of fissure length, the ratio of low-angle fissure is higher.There are two general species of fissure: smooth unfilling fissure and filling fissure. Thefissure fillings generally of gray and its water content is significantly greater than the sidesof expansive soil while dry density is significantly smaller than the sides of expansive soil. （2） Through the laboratory test of fissure fillings, we can know that the percentage ofclay mineral in fissure fillings is54%while the the expansive soil of both sides is51%.The mainly chemical composition of filling is SiO2, its percentage is59.1%, which lowerthan the both sides. The filling’s content of Fe2O3is lower than expansive soil of bothsides, and the ration is2:3, that is why the fissure fillings are gray. Study the mechanicalproperties of the fissure fillings and the sides at the same dry density through consolidateddrained triaxial test. The results show that the strain-stress curve of both fissure fillingsand the sides are strain-hardening, and the cohesion is bigger than the sides of expansivesoil while internal friction angle is smaller.（3） Through triaxial test of undisturbed sample, we exploration a new test method offissure strength: ensuring the fissure obliquity in45+φ/2±10and fissure face inaccord with the shear plane, then, the strength of the test is the fissure face strength. Thistest method have high success rate and the mechanics meaning is clear, which can avoidexperimental errors by direct shear test thoroughly. The studies show that the fillingfissure strength of undisturbed sample is very low, the cohesion is10.0kPa and internalfriction angle is17.2°. Through artificial simulation fissure and using the data processingmethod in this paper, study the influence of fissure angle and thickness on the expansivesoil strength. The studies show that fissure angle and thickness have significant influenceon the shear strength index of expansive soil, which decreases with the increases of fissureangle and thickness. Using the data processing method in this paper,（σ₁-σ₃）f、c and φ allincreased in different extent,which is is safer from engineering perspective, but it is morerealistic.（4） Based on the Geo-slope and limit equilibrium analysis method of considering thespatial distribution of fissure, we study the influence of fissure length、fissure angle andfissure depth on the slope stability of expansive soil. The results show that the safetyfactor of slope decreases with the increases of fissure length and angle, and the safetyfactor of slope decreases and then increases with the decreases of fissure depth. Theresults demonstrate that this approach can reflect the actual accurately.