DISCOVERY AND EXPLANATION OF ONE MUDDY GIRDLE UNDER SHARP TURN OF ONE RIDGE—A CASE STUDY AT THE CSNS SITE OF GUANGDONG PROVINCE

Stress-release zone always connects with alpine valleys or a certain scale of underground engineering, which is generally thought to be caused by stream trenching or large scale engineering excavation. However there are few research in stress-release under the ridge's sharp turn. During the engineering geological investigation in the CSNS site which located in southern Guangdong Province, leakage occurred in drilling process of four adjacent boreholes and found locally broken argillization zone in the intact rock. To study and explain this phenomenon, we first arranged some geophysical lines across the engineering site to probe the underground. And the electrical geophysical profile indicates there is a local low resistivity zone exists under the ridge sharp turn. Then, during the geologic mapping work, we found that the argillization zone just located in the contacting zone between northern granite belt and southern granulite belt. Third, by analysis the engineering geological investigation report, we circled the muddy zone in borehole profiles and contour maps, which tallies with geophysical results. After all these outdoor and indoor works, we used computer building an authentic surface model to simulate the influence of weathering and unloading on the mountain from earliest times. The numerical results show that stress unloading happened in a certain depth under the ridge's sharp turn because of the erosion unloading. It also made partial rock quality worse which approached to the contacting zone of dike and granulite, and formed crushing zone and control the local enrichment of groundwater and the generate of fountain. We analyzed the geological prospecting and geophysical data to conjecture what caused muddy zone in borehole ZK94 and neighboring areas' low RQD, and verified it by numerical simulation. We found that the muddy zone was caused by multiple factors. First and the most important factor, the metamorphic sandstone developed in the contacting zone of granite and granulite was harder and brittler than the other two rocks, which become the internal cause of muddy zone. Second, by analyzing substantial drilling information, there is a low RQD zone at a certain depth, which located in contact zones between granite and granulite. The contact zone was affected by the later southern granite intrusion, and presented as saddle and gully on surface, even made the rock broken near the contact zone. At last, the numerical simulation indicates borehole ZK94 which located at the ridge's sharp turn was a low stress zone, and the original compressional stress changed in this area. With the surface weathering and tectonic uplift, the exposed rock masses unloaded and produced a weak area under the ridge, so that make rock fracture developed further and the rock masses became more broken. Besides, the sudden water inflow happened in borehole ZK99 and ZK101 indicates that headwaters exist nearby. Thus the broken rock masses in the lithologic boundary formed water conducting fracture, and the groundwater via the fracture soften the rock in return. In this circle, it finally formed muddy zone near ZK94. This study reveals the influence of a ridge's sudden change on in-situ stress and rock structure, and it is expected to provide reference to similar geological anomaly analyses.