南阳膨胀土的工程地质特征和填筑适宜性

施工阶段的工程地质调查发现，宁（南京）西（西安）铁路内乡-镇平段南阳膨胀土成因类型上不仅有第四系冲积洪积粘土，而且发育上第三系湖相沉积粘土。不同成因的膨胀土具有不同的空间分布特征和工程性质，X射线衍射测试表明，南阳膨胀土的主要膨胀性粘土矿物是伊利石/蒙脱石混层矿物。物质组成和物理化学性质差异导致击实土在含水量变化条件下膨胀力和膨胀变形有明显的差别；对同一种膨胀土材料，其击实含水量越小，膨胀性越强，甚至含水量降低1 %～2 %，膨胀力就可以增加到200 %以上；击实膨胀土的直剪强度受到钙质结核含量的影响，对于膨胀势为高或强的膨胀土，结核含量小于15 % 时，经过干湿循环后直剪强度衰减量为40 %～65 %，结核含量为25 %～30 % 的强膨胀土直剪强度衰减量则低于30 %；对膨胀土的地质特征、工程性质、击实土的膨胀性和直剪试验结果进行分析，探讨其用于填筑铁路路基的适宜性，进行了填筑适宜性分级。依据研究成果制定的路基施工工艺取得了比较理想的工程效果

Engineering geological characteristics and suitability for embankment of Nanyang expansive soils

The geological investigations during construction of Neixiang-Zhenping section of Nanjing-Xi’an railway revealed that Nanyang expansive soils origins not only from Quaternary alluvial, diluvial clays but also Neozoic lacustrine clays. Expansive clays with different origins differ in terms of space distribution and engineering characteristics. X-ray diffraction tests indicated main expansile clay mineral of Nanyang Expansive soils is mixed-layer illite/montmorillonite. Change of water content will result in significant difference in swelling force and dilatancy for the compacted soil sample because of the variation in material composition and physicochemical properties. To compacted samples prepared by same expansive soil material, the smaller the prepared moisture content the higher the expansive force. Even if the moisture content only reduced 1 %-2 %, the corresponding expansive force would upper to 200 %. The direct shear strength is affected by calcareous cement degree and nodes content. As high or very high expansive clays, if nodes content were less than 15 %, the reduced value of direct shear strength after dried and saturated cycle would be 40 %- 60 %; Whereas direct shear strength would reduced no more than 40 % if nodes content were 25 %-30 %. The geological properties, engineering characteristics, expansibility of compacted samples and direct shear test date of swelling soils are analysed, suitability for embankment is discussed, and suitability grades is presented in this paper. Applications of the subgrade-construction techniques developed based on the research work have achieved good effect.