南阳膨胀土裂隙面强度试验研究

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

裂隙性是膨胀土的基本特性之一,膨胀土中原生裂隙面的存在往往导致膨胀土边坡的失稳。选取南水北调中线工程南阳段膨胀土进行裂隙面强度特性试验,提出了裂隙面强度三轴试验新方法。首次将计算机X射线断层扫描技术引入裂隙面的强度试验,通过测量裂隙面真实产状,准确分析裂隙面上的破坏应力,提出了裂隙面强度参数的整理方法。研究成果表明,裂隙面的峰值强度不仅远小于两侧土的峰值强度,而且也小于两侧土的残余强度;裂隙面强度较低的原因主要是其具有较高的含水率、蒙脱石含量以及颗粒排列定向度。地下水淋滤作用对于膨胀土裂隙面存在3个方面的影响:微观上使得膨胀土中铁的氧化物由三价转变为二价,形成与两侧土颜色完全不同的灰白色裂隙黏土;细观上改变了母体土的矿物成分和颗粒排列结构,使得裂隙填充物具有较高的蒙脱石含量和颗粒定向度;宏观上形成了裂隙面与两侧土体力学强度的显著差异。

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.