Failure Models And Instability Of Slopes With Soft And Hard Rock Interbedded Laminated Structure In Southwest China

Slopes with soft and hard rock interbedded are widely outcropped in the Southwest of China. These kinds of slopes developed especially in the red layer and metamorphic rock region. With different lithologies, differential weathering occurs obviously between the soft and the hard rocks. Coupled with the complex geological conditions, the soft rock is prone to form the weak discontinuities or weak base of the slope, which is harmful to the stability of slopes. The unique structure makes the slope failure modes complex and difficult to evaluate the slope stability, and the stability of cut slope is turning to be a serious engineering geology issue. The slope engineering properties, the characteristics of both the rock and the slope structures are studied in this article. Taking the spillway slope at Liyutang Reservoir in Chongqing as an example, a systematic summary is made on aspects including deformation-failure model, slope stability evaluation, and support measures of soft and hard rock interbedded slope. A set of failure models analysis, stability assessment and control systems act on slope with hard and soft rock interbedded is established. The research achievements are listed as below:(1) The empirical formula of softening coefficient of soft rock is put forward based on collapse mode and slake-durability. The microscopic analysis of the soft mineral composition and rock structure is carried out with the sandstone and mudstone, which is taken from the Liyutang slope. Due to the poor rock mass is too easy disintegration to carry out the uniaxial compression test, the softening coefficient is difficult to make out. Considered the result of the slake durability test and data from other projects, the empirical method for selecting softening coefficient exactly is put forward .The collapse mode and property of slake durability is taken into account also.(2) The structure features of slopes with soft and hard rock interbedded in Southwest China are systematically summarized. With the consideration of the laminated structure, strength of the rock mass, bedding plane spacing, the integrity of rock mass, the soft and hard interbedded rock mass is classified into very thickly bedded blocky structure, thickly blocky bedded structure, interbedded structure and thinly bedded structure. The engineering geological characteristics of each structure are studied thoroughly.(3) A comprehensive classification system of slopes structure with soft and hard interbedded rock is established. According to the obliquity of the rock, the hard and soft rock interbedded slope is divided into two categories: one is nearly horizontal-dip with a small angle and the other one is inclined slopes. Then the two categories are further classified into three subcategories according to the combinations of the dipping direction, the slope incline and the rock strength etc. The former is divided into three subcategories: mainly thick layer of hard rock with slight soft rock (or with thick soft rock layer underlying), slope with soft and hard rocks interbedded equally and slope with soft rocks containing slight hard rocks. The later is also divided into three subcategories: slope with laminated along-layer structure, countertendency laminated slope and slope with laminated oblique layer.(4) The failure mode of slope with soft and hard rock interbedded is summarized comprehensively, which fits for different types of slope structure.The type of nearly horizontal-dip with a small angle among the slope with soft and hard rock interbedded: the slope with very thickly bedded structure containing slight soft rock is prone to take in form of the sliding– tension– shearing failure, large scale of plastic flowing-cracking, and also, maybe toppling unstable collapse. The slopes with soft and hard rock interbedded mainly develops horizontally pushing landslide, small angle dipping bedded landslide and tension collision; the slopes mainly make up of soft rocks mostly come into being weathering and spalling or sliding with a circle surface.The type of inclined slopes: a large scale landslide with a slipping - tension or slipping– bending mode develops in the consequent slopes; the countertendency laminated slope may have bend - dump and compress– dump failure, even resulting in serious dumping deformation, on which a large scale landslide depends; slope with laminated oblique layer, depends on different attitude, can be divided into step-like bedding slump, wedge sliding and other mesoscale rock landslides, as well as small-scale collision and peeling.(5) The method and principle of slope geological modeling are proposed for stability evaluation, which based on the deformation theory. Questions like"How to select rock mechanics parameters accurately?","How to simplify the modeling of slope with soft rock and soft intercalated?"and"What are the meshing principles?"are answered in this study. All these studies lay a foundation to the numerical simulation of slope.(6) The principles of choosing design proposals and ways of design for the slope supporting on different failure modes are put forward. An example of Liyutang high slope at the spillway is presented to study the stability of the slope with soft and hard rock interbedded. The finite difference method based on deformation theory and block theory based on strength theory, the limit equilibrium theory are used, and then specific supporting proposals are put forward, which are proved to be effective for stabilizing the slope.