Collapse Mechanism And Stability Analysis On Bank Slope In Repeated Strata Of Sandstone And Mudstone In The Three Gorges Reservoir Area

The bank slope of the interbedded structure of sandstone and mudstone is one of the common types of bank slopes in the Three Gorges reservoir area.The periodic variation of the water level of the Three Gorges Reservoir between 145 and 175 m elevation makes the rocky soil of the bank slope subject to periodic dry and wet cycles.Because the watersoil properties of sandstone and mudstone are different,and the anti-weathering ability of mudstone is obviously lower than that of sandstone,mudstone is prone to softening or even disintegrating under the action of reservoir water,which may cause collapse and destruction of sand-shale interbedded bank slope.This paper relies on the National Science and Technology Support Program topic “Study on the Catastrophic Mechanism and Process Control of Typical High Slope in Southwest Mountain Area”(No.2015BAK09B01-1),and studies the unilateral immersion,dry and wet cycles and overlying by theoretical analysis and laboratory experiments.The disintegration characteristics of mudstone under loading conditions are analyzed,and the collapse mechanism and stability of typical sand-shale interbed bank slopes in the Three Gorges reservoir area are analyzed.The stability analysis method is used to test the stability of the engineering examples.The specific research content and main The research results are as follows:(1)Based on the combination relationship of rock stratum structure planes,the structural types of sand-shale interbedded bank slopes are divided,and the failure modes of sand-mudstone interbed bank slopes with different structure types are summarized.The study of typical sand-shale interbedded slopes in the Three Gorges reservoir area is studied.Collapse mechanism.The types of sand-shale interbedded bank slopes are divided into five categories: smooth layered structure,anti-dip layered structure,oblique layered structure,steeply inclined layered structure and near horizontal layered structure.The near-horizontal layered structure is a study of the typical sand-shale interbedded bank slope in the Three Gorges reservoir area.The collapse mechanism is: the mudstone cavity generated by the softening and disintegration of the mudstone will make the upper sandstone vacant,and the suspended sandstone and overlying strata will be broken in the mechanical equilibrium.After that,the cantilever pull crack will cause the bank slope to collapse as a whole;(2)Based on the hydraulic characteristics and unilateral disintegration test of mudstone,the disintegration characteristics of mudstone are studied and the formula for calculating the disintegration depth of mudstone is proposed based on the test results.The mudstone rock properties were studied to study the material composition,softening,disintegration and water absorption of the mudstone.The one-sided disintegration test consisted of one-side immersion test and one-side dry-wet cycle test.The one-side immersion test was based on the upper loading.The situation is divided into single side immersion upper part without loading,single side immersion upper part loading 0.4MPa,single side immersion upper part loading 0.8MPa;single side dry and wet cycle test is divided into unilateral dry and wet circulation upper part without loading,unilateral dry according to upper loading condition The upper part of the wet cycle is loaded with 0.4 MPa,and the upper part of the dry and wet cycle is loaded with 0.8 MPa.According to the test results,the relationship between mudstone disintegration depth and water immersion time,dry and wet cycle number and overburden load is obtained,and the calculation formula of mudstone disintegration depth under unilateral immersion and unilateral dry-wet cycle conditions is derived.(3)Based on the collapse mechanism of the typical sand-shale interbed bank slope in the Three Gorges reservoir area,the stability analysis of the sand-shale interbed bank slope is carried out.According to the stress distribution law of the cantilever beam bending and the ratio of the maximum tensile stress and tensile strength on the bending moment section,the calculation formula of the critical suspension length and the crack resistance coefficient of the sandstone under the upper load is proposed,and combined with the mudstone disintegration depth.The calculation formula analyzes the stability of the typical sand-shale interbed bank slope in the Three Gorges reservoir area;(4)According to the engineering geological conditions of Tianbai Reservoir in Kaixian County,the stability analysis of sand-shale interbedded bank slopes is carried out.