桂东南花岗岩风化土与残余节理的微观结构及演化

花岗岩残余节理是指花岗岩高度风化土体内的节理,常常被铁锰氧化物充填,呈褐黑色或黑色薄夹层,对边坡稳定性有重要影响。选取广西东南部容县陈村风化花岗岩滑坡内残余节理土及其两侧土体(全风化土)和上覆残积土为研究对象,分别研究这3种土样的矿物成分、化学成分、微观结构、孔隙分布特征,并探讨残余节理的演化过程及其对边坡稳定性的影响。结果表明:组成花岗岩全风化土和残积土的主要黏土矿物为高岭石、石英及少量伊利石。其中,全风化土内高岭石单晶尺寸在数微米至数十微米,呈书本状构造;残积土内高岭石矿物渐碎片化,单晶尺寸小于全风化土,呈杂乱无章分布。残余节理土的主要矿物类型为赤铁矿和水钠锰矿。初始风化阶段铁锰氧化物胶体呈球状、粒状分布,颗粒孔径主要为数微米至数十微米,表面呈多孔蜂窝状,具有较大的比表面积和胶体活性;随着风化作用持续,铁锰氧化物胶体颗粒被拉长呈椭球状,沿节理走向呈片状定向挤密排列;最终形成薄板状定向分层排列,且胶体颗粒的蜂窝状结构逐渐消失,比表面积逐渐降低,圆度降低。残余节理土的饱和渗透系数比两侧全风化土约低两个数量级,节理由风化早期的渗流优先通道逐渐转变为风化后期的隔水夹层,从而影响风化土体内地下水渗流过程,对其边坡稳定性产生不利影响。

Micro-structure and Evolution of Relict Joints and Weathered Granite Soils in the Southeastern Guangxi,China

The original granite joints are retained in the highly weathered stage and called as relict joints. It is often filled with Fe-Mn oxides, showing a thin interlayer of brown to black, which is significantly different from the surrounding matrix as soil interlayers and has an important impact on slope stability in tropical and humid regions. Soil mineral composition, chemical composition, micro-structure and pore size distribution of granite residual, completely weathered and relict joint soils from a weathered granite slope of Chencun in Rongxian of the southeastern Guangxi were respectively studied by X-ray diffraction(XRD), scanning electron microscopy(SEM), energy spectrum and mercury intrusion analyses;meanwhile, the evolution process of relict joints and the influence on slope stability were discussed. The results show that the main clay mineral compositions of granite residual and completely weathered soils are kaolinite and quartz, and a small amount of illite. The kaolinite in completely weathered soils has a book-shaped structure with a single crystal size of about several microns to tens of microns;the kaolinite in residual soils is fragmented gradually, the single crystal size is smaller and micro-structure is disordered. The main mineral types of relict joint soils are hematite and birnessite. In initial weathering stage, the colloids of Fe-Mn oxides are spherical and granular in shape, the particle pore size is mainly in the range of several microns to tens of microns, and the surface is porous honeycombed with a large specific surface area and colloidal activity;as weathering continues, the colloid particles of Fe-Mn oxides show the ellipsoidal compact arrangement pattern to the patchy and directional arrangement pattern along the joint direction;the honeycomb structure of colloidal particles disappears gradually, and the specific surface of particles decreases gradually, and the roundness also decreases. The saturated permeability coefficient of residual joint soils is about two orders of magnitude lower than that of completely weathered soils on both sides, the joints are preferential flow paths in the early stage of weathering, and gradually transform into low permeability interlayers in highly weathered granite, which affects the percolation process of groundwater in weathered soils, and has adverse effect on slope stability.