Test study of shear strength of fissure-plane in Nanyang expansive soil

Fissures which almost exist in all of expansive soils always result in failure of expansive soil slope. Shear strength characteristics of fissure-plane are studied; and a new triaxial test method is recommended with typical expansive soil taken from Nanyang section of middle route project for South-to-North Water Transfer. The computerized tomography (CT) technology is firstly introduced to shear strength test of fissure-plane. The real failure stress of fissure-plane is analyzed accurately through measurement of the fissure-plane's real occurrence by CT scanning; and shear strength parameters determined method is provided for triaxial test of fissure-plane. The test results show that the peak strength of fissure-plane is much lower than that of bilateral expansive soil without fissures and even smaller than residual strength of bilateral expansive soil without fissures. The reasons of these phenomena are that gray clay in fissures has higher water content, smectite content and particles orientation degree. Leaching behavior of groundwater generates three aspect alternations to fissures. In microscopic view, leaching changes ferric iron oxide in expansive soil to ferrous oxide and that results gray clay in fissures whose color is different from expansive soil besides fissures. In mesoscopic view, mineral composition and particles arrangement of expansive soil have been modified by leaching and filling material in fissures have higher smectite content and particles orientation degree. In macroscopic view, factors mentioned above result in significant difference between fissure-plane and bilateral expansive soil.