全风化花岗岩(CDG)和花岗岩残积土(GRS)是华南地区易发生工程事故和地质灾害的土类。 这2种土风化程度不同,其工程性质也有显著差异。 现有规范主要根据野外特征和标准贯入试验实测击数进行划分,但在实际地质勘探中仍存在识别上的困难。 对现有规范中关于确定CDG和GRS的相关条款进行了评述和讨论。 鉴于CDG和GRS的级配特征存在一定的差异,前者一般为黏土质砂,后者一般为含砂黏土,建议利用某些级配特征作为区分这两种土的辅助指标。 从某基坑工程中取土样进行了物理性质试验和三轴剪切试验。物理性质试验表明:残积土孔隙比较大,缺乏中等粒径,属于不良级配; 三轴剪切试验中,原状CDG和GRS均有较强的初始结构性,主要表现出以下特征:在剪切变形过程中,土样出现应变局部化的剪切带,并以鼓状-剪切带模式破坏,而重塑土样则呈现标准鼓状破坏模式; 原状CDG排水剪切摩擦角略高于GRS,黏聚力明显高于残积土,其原因为前者结构性更强; 重塑的CDG与GRS均破坏了原始结构性,两者的凝聚力接近; 在低围压条件下,原状土的结构强度对总抗剪强度的贡献较大,可达60%~70%,高围压的压缩作用可使土体的初始结构完全破坏,结构强度将消失。

Completely decomposed granite (CDG) and granite residual soil (GRS) are prone to trigger engineering accidents and geological disasters in South China. Despite that the weathering degree and engineering properties of CDG and GRS both vary remarkably, it is still a challenging task to distinguish CDG and GRS in practical geological investigation, in which the field geological appearance and the measured hit number of standard penetration test according to existing specification are taken as the major indicators for identification. In this paper, we discussed the relevant clauses about identifying CDG and GRS in some current specifications. In view of the difference in gradation between of CDG and GRS, i.e., the former is usually clayey sand, while the latter sand clay, we suggest to take grading characteristic as the subsidiary index of identifying CDG and GRS.According to physical properties test and triaxial shear test of soil samples from a foundation pit, we found that GRS sample had a larger pore ratio with a poor gradation lack of some intermediate particle sizes. Both the undisturbed CDG and GRS present initial structural features reflected by shear zone with strain localization in shear deformation, displaying drum-shear zone failure mode; the remolded samples present a standard drum-like failure mode. The friction angle of drainage shear of undisturbed CDG is slightly higher than that of GRS, and the cohesion obviously higher as CDG boasts a stronger structure; the cohesion of remolded CDG and remolded GRS are close due to the lack of original structure. Under low confining pressure, the structural strength of undisturbed soils contribute up to 60%-70% of the total shear strength, and the compression action of high confining pressure can completely destroy the initial structure of soils, in which case the structural strength will be eliminated.