Unloading depth of rock masses and its relations with river downcutting in deep valleys in Southwest China

Abstract

Intense and deep unloading is a common manifestation of rock masses in deep valley in Southwest China. In this work, we investigated the evolution of four main rivers and the characteristics of the unloading cracks in rock masses in the river valleys, and we studied the relations of the tensile unloading fractures of rock masses with the downcutting of rivers based on the investigation results on the unloading crack features of 33 hydropower stations in the study area. The Lancang River, Jinsha River, Dadu River and Yalong River in the study area had downcut of 250–660 m, 120–430 m, 400–670 m and 350–460 m, respectively, since the Quaternary. The unloading and the redistributed stress of valley caused by the river downcutting had important effects on the unloading fracture of rock masses. The maximum strong and weak tensile unloading fracture depths of the rock masses exceeded 160 m and 230 m, respectively, in these valleys. The unloading depths of the rock masses increased with increasing river level elevations and the heights of the river valleys and varied within a certain range. In a certain valley, the unloading depth increased with the elevation of the rock masses. An empirical method was proposed in this work for roughly estimating the strong and weak tensile unloading depths of a rock masses. • The maximum strong unloading depths of the rock masses exceeded 160 m in deep valley Southwest China. • The unloading depths of the rock mass increased with river level elevation, valley height and elevation of rock mass. • An empirical method was proposed in this work to roughly estimate the strong and weak unloading depths.