A study on in situ measurements of carbonate rock mass degradation in the water-level fluctuation zone of the Three Gorges Reservoir, China

The degradation of rock masses making up carbonate slopes within water-level fluctuation zones after the impoundment of reservoirs is in some cases very intense. This has attracted widespread concern because such degradation can result in evolving geological hazards. Quantitative characterization of carbonate rock degradation is the key to the study of this phenomenon and its subsequent impacts. In this article, a meter-scale representative observation plane (body) is selected as the research object. The continuous medium mode is used to describe a carbonate rock mass with evenly distributed degradation quantitatively by way of a rock mass degradation variable (De), which is the relative area ratio or volume ratio of new macroscopic defects (bedding planes, joints, cracks, pores, holes, etc.) that form in a representative rock mass within a period of time. For carbonate rock with nonuniform degradation, a further rock mass degradation variable, De_A, is developed, which represents the relative area ratio or volume ratio of the new defects within a discontinuous surface or body within a period of time. Direct and indirect methods and formulae for acquiring these values are presented, dependent upon the degradation process involved, such as mechanical erosion, dissolution, and fissure development or expansion. The De value is established based on the measurement of area, volume, trace length, aperture, eroded depth, and acoustic wave velocity. The resulting formulae are used to analyze the degradation of typical rock masses in the Wu Gorge in the Three Gorges Reservoir area quantitatively. From 2010 to 2013, the De value increased continuously from 0.18 to 0.42 in Maocaopo and showed an accelerating trend. For the Qingshi 6# slope, for the period from 2017 to 2019, the De value showed an overall decreasing trend with increasing depth based on acoustic wave velocity data. The De value of the rock mass underground is greater than 0.2 for eight sections and reaches a maximum of 0.69. The degradation of the rock mass presents structural and zonal characteristics in the depth direction. In Jiandaofeng, the De value is calculated to be 0.0041 for the period from 2012 to 2017, while, at the Sunjiawan cliff, the De_A value of a large structural plane reaches to 1.03 after 10 years of impoundment. This research forms a foundation for the measurement of the degradation of rock masses in the Three Gorges Reservoir area and provides basic technical support for the subsequent study of rock degradation and hazard prevention in the area. Furthermore, it also provides a reference for assessing the degradation of rock masses within the water-level fluctuation zone for other areas around the world.