南京秦淮东河黄马青组(T_2h)砂岩水致劣化特征分析

WATER INDUCED DETERIORATION CHARACTERISTICS IN SANDSTONE OF TERTIARY FORMATION AT QINHUAI EAST RIVER IN NANJING CITY

南京秦淮东河新开河道在铜家山地段穿越黄马青组(T2h)砂岩,形成高度最大达70 m的高边坡。根据现场调查和钻探结果,在近地表处,岩石中裂隙发育,岩体完整性差,而在深部,根据钻孔取芯结果,岩体中结构面胶结强度高,岩体完整性好。钻孔取岩芯样进行干湿循环试验发现,样品在干燥-饱和循环条件下,一般循环5次以上就易发生崩解,而在自然吸水-干燥循环条件下,循环20次以上仍可以保持完整。分析认为,岩石中小开裂隙发育。根据扫描电镜(SEM)对微观结构分析结果发现,饱和条件下,岩石中小开裂隙进水后导致结构面胶结物软化,并可能造成岩石开裂、吸水膨胀,即水对岩石的劣化作用是物质软化和岩石结构破坏的双重机制;而在自然吸水条件下,小开裂隙中未进水,水对结构面的软化作用较小。矿物分析结果表明,岩石中含有较多的黏土矿物,是导致岩石浸水软化的重要原因。经过干湿循环后,岩石波速下降、致密程度降低,塑性特性增强。多次循环后岩石的单轴抗压强度和弹性模量均呈明显下降趋势,岩石破坏时的变形量随循环次数增大而下降,岩石的孔隙压密和弹性变形过程逐渐消失,加载后迅速进入塑性变形阶段,且破坏速度加快。

The Qinhuai East River,in Nanjing city,is designed to pass through the section of Tongjiashan hill,where the Huangmaqing group of Tertiary(T 2 h)will be excavated for the new river.The excavated stratum is mainly composed of sandstone,cemented by mud and iron.The maximum height of the future slope will arrive at 70 m.Results of the field investigation and drilling holes show that the rock mass near the ground surface has intense cracks and low integrity,but the rock mass at the deep location has good integrity and few cracks.The cracks are closed with the favorable cementation strength.The wet and dry cycle tests are carried out for the rock cores.When the samples are tested for the five times of wet and dry cycles,a majority of the samples are cracked and disintegrated.But when the samples are immersed under the natural conditions,after twenty cycles,the samples are continuative and not cracked.This phenomenon can be explained that the small cracks in the rocks,where the water is not easy to enter under natural condition but enter easily in the vacuum,are very developed.When the water enters the small cracks,the cements are softened and the small cracks open,water swelling.The deterioration of the rocks due to water are caused by the double mechanisms.The mineral composition analysis is taken by the XRD apparatus.The results indicate that the more clay minerals in the rocks are the important causation due to softening of the rock,which result in the weak expansion.After the cycles of dry and wet tests,the wave velocity,unconfined compression strength and elastic modulus decrease obviously.The physical and mechanical characteristics are sensitive to the water.With the cycle numbers increase,under the compressive stresses condition,the pore compaction and elastic deformation process of the rock gradually disappear.After excavation of the high slope for the Qinhuai East River at the section of Tongjiashan hill,the effect of water on the stability should be considered adequately.