Experimental Research On The Mechanism Of Strength Deterioration Of Silty Mudstone Of Badong Formation Under The Combined Action Of Wet-dry Cycles And Seepage

Since the impoundment of the Three Gorges Reservoir,the water level of the reservoir has fluctuated periodically in the range of 145 m to 175 m every year,forming a 30 m hydro-fluctuation belt.The effect of reservoir water on the rock mass in the hydrofluctuation belt is mainly summarized in two aspects: on the one hand,the fluctuation of reservoir water level causes the rock mass in hydro-fluctuation belt to be in a periodic "saturated-dry" wet-dry cycle state for a long time;On the other hand,the seepage resulted from reservoir water causes the rock mass to simultaneously undergo the action of seepage pressure.The wet-dry cycle and seepage are the key factors causing the deterioration of rock mass in the hydro-fluctuation belt.Badong Formation is a typical "sliding-prone strata" in the Three Gorges Reservoir area,which is characterized by soft and hard interbedded rock mass structure,especially the silty mudstone.The silty mudstone of the Badong Formation has high argillaceous content,low mechanical strength and poor engineering geological properties,which is a typical "weakening rock" in the Badong Formation.The strength of the silty mudstone of the Badong Formation subjected to wet-dry cycles and seepage caused by reservoir water will gradually deteriorate,leading to bank slope instability,causing significant casualties and economic losses.Therefore,it is necessary to study the deterioration mechanism of typical "weakening rock" silty mudstone of Badong Formation in the reservoir hydro-fluctuation belt under the combined action of wet-dry cycles and seepage.In this thesis,silty mudstone of Badong Formation was taken as the research object to conduct thin section identification test,saturated water absorption tests,nuclear magnetic resonance tests,and stress-seepage coupling triaxial compression tests under different wet-dry cycle.The microstructural evolution and macroscopic mechanical property deterioration laws of the silty mudstone are analyzed,the strength deterioration mechanism of silty mudstone under the combined action of wet-dry cycles and seepage is revealed,and the shear strength parameter deterioration model is established considering the number of wet-dry cycle and seepage pressure.The following research results are obtained in this thesis:(1)The law of microstructural evolution of silty mudstone under the combined action of wet-dry cycles and seepage is analyzed.Both wet-dry cycles and seepage pressure will cause the deterioration of the micro parameters of silty mudstone.With the increase of wet-dry cycles,the saturated water absorption and porosity of silty mudstone gradually increase,especially in the first six wet-dry cycles;The mineral particles change from uniform and dense cementation state to loose and porous structure,from clear subangular particle boundary to smooth mineral boundary,and there are intragranular and intergranular cracks under the wet-dry cycles;In the process of seepage,the seepage fluid first enters the main fracture channel,and gradually enters the small pores of the rock sample under the continuous seepage pressure.The wet-dry cycles and seepage pressure will cause more fracture seepage channels,inducing the "compression phenomenon" of the rock blocks on both sides of the fracture.The contact area between the fluid and the rock sample increases,which is the main reason that inducing the increase of the permeability coefficient;(2)The deterioration law of macroscopic mechanical properties of silty mudstone under the combined action of wet-dry cycles and seepage are analyzed.The combination of wet-dry cycles and seepage pressure leads to the deterioration of the strength deformation of silty mudstone.The greater the number of wet-dry cycles,the more obvious the deterioration effect of seepage pressure on the rock.The triaxial compression strength,Young’s modulus,internal friction angle and cohesion of silty mudstone are negatively correlated with the number of wet-dry cycles and seepage pressure.And the deterioration degree of the strength and deformation parameters of silty mudstone is different under the combined action of wet-dry cycles and seepage.The order of deterioration degree from large to small is: Triaxial compression strength > cohesion >internal friction angle;Silty mudstone exhibits shear failure and axial splitting failure characteristics,with the increase of wet-dry cycles and seepage pressure,the shear failure characteristics weaken and the fracture angle decreases;(3)The strength deterioration mechanism of silty mudstone under the combined action of wet-dry cycles and seepage is revealed.Based on the results of laboratory tests,the relationship between the macro and micro deterioration parameters of silty mudstone is established,and the strength deterioration mechanism of silty mudstone is analyzed,which is mainly summarized as three aspects: clay mineral expansion,pore increase and crack extend.These three mechanisms are interconnected and occur at the same time.Clay mineral expansion is the fundamental cause of pore increase and crack extend of silty mudstone.The pore increase and crack extend are the direct result of clay mineral expansion behavior.All three factors play an important role in the strength deterioration of silty mudstone,and jointly promote the loss and evolution of silty mudstone,The strength deterioration model of the silty mudstone in Badong Formation considering the wet-dry cycle and seepage pressure is established,which provides a theoretical basis for scientifically determining the strength parameters and reasonably evaluating the stability of the reservoir bank rock mass.