Study On Deterioration Due To Water Intrusion Mechanism And Strength Characteristics Of Interlayer Staggered Zone In Baihetan Hydropower Station

Baihetan Hydropower Station is one of the large-scale hydraulic structures currently under construction in China.The underground caverns of the left and right bank buildings are dozens of large and small groups.The interlayer fault zone and the intralayer fault zone are cut,which adversely affects the stability of the surrounding rock of the cavern.Before the construction of the project,in order to prevent the water penetration from adversely affecting the stability of the project,a number of water cut curtains were set up,but the dam crest elevation of Baihetan Hydropower Station was834m,the maximum dam height was 289m,and the total water storage capacity of the reservoir was 20.51 billion m3.The flooding range is wide.Under the action of large hydrostatic pressure for a long time,the water intercepting curtain may lose its effect,and the water will enter the staggered zone along the soil/rock structure surface,resulting in the reduction of the mechanical strength of the staggered zone,especially the left bank hole.The room group revealed the C2 staggered zone,and the phenomenon of water seepage occurred during the construction period,which adversely affected the safety and stability of the whole project.Therefore,based on the interlayer staggered zone exposed during the excavation process of the underground caverns on the left and right banks of Baihetan Hydropower Station,this paper studies the mechanism of water degradation in the staggered zone and the variation of its mechanical properties under different water content.The content includes several aspects:(1)X-ray diffraction analysis of the sample of the staggered zone revealed that it contains a large amount of clay minerals such as montmorillonite,illite and chlorite,combined with the engineering geological and hydrogeological conditions,and the basalt fissure water is rich.The K~+and alkaline environment facilitates the conversion of montmorillonite to illite,revealing the microscopic mechanism of deterioration due to water intrusion in the interlayer staggered zone.(2)Laboratory test adopts the remodeling sample,and the sample is made into a saturated sample according to the fixed particle size,and then air-dried to the corresponding water content as needed,avoiding the direct addition of distilled water to the corresponding moisture content during the process of moisture to the wrong zone.The influence of the particle composition and the uneven distribution of mineral components on the test results is also avoided.(3)Triaxial shear test with five gradients of moisture content of 1%,3%,5%,7%and 9%on the remodeling of the staggered zone.The test results show that the cohesion and internal friction angles Both of them have a certain deterioration trend with the increase of moisture content,and the change of moisture content has a great influence on the cohesion.The stress-strain curve is strain-hardening type,and the failure mode is a round drum shape.With the increase of moisture content,it has the characteristic of plastic flow failure.Electron microscopy of the samples with different moisture content after the test showed that the higher the moisture content,the smoother the fracture surface of the dislocation zone,and the less new fine particles,the increased water content,and the softening of the cement structure.The activity is enhanced and a water film is formed on the surface of the particles,which reduces the cohesive force and friction between particles,resulting in a decrease in the strength of the dislocation band.(4)The long-term deformation of the staggered zone under different stress levels has an important control effect on the stability of the surrounding rock of the cavern.Therefore,the creep test under different moisture conditions is carried out on the staggered zone,and the Singh-Mitchell creep is established.The model compares the theoretical value calculated by the creep model with the experimental values and the engineering field detection values to verify the rationality of the creep model.In addition,it can be seen from the creep stress-strain curve that under the condition that the long-term strength of the staggered zone is not reached,the deformation of the dislocation belt is basically instantaneously completed under each load,and the subsequent deformation increases with time and the deformation is small.After the stress level reaches or exceeds the long-term strength of the staggered zone,the deformation accelerates with time until the sample breaks.