Study On Alteration Characteristics Of Granite And Its Influence On Surrounding Rocks Of Cavern

As a special rock mass with poor engineering properties,altered rocks seriously restrict the design and construction of underground engineering.Therefore,based on the Longyangxia large-scale energy storage plant project,this paper takes the tectonic altered granites in the underground powerhouse zone as the research object.Based on the basic principles of mineralogy,petrology and rock mass mechanics,the basic characteristics and spatial distribution rules of tectonic altered granites are studied by field investigation,indoor and outdoor testing and experiments,and numerical calculation methods,and the engineering classification standard of alteration degree is established.The influence of altered rocks on the surrounding rocks of the cavern is analyzed,which provides a basis for the rational design of underground powerhouse.The main research results are as follows:(1)Based on field investigation and fine measurement,the rock mass structure characteristics and spatial distribution patterns of tectonic altered granites in the study area are analyzed,and the genetic types of altered rocks in the study area are determined.The spatial distribution of tectonic altered granites is strictly controlled by the tectonics,and the structural planes within the rock mass are well developed,so it is determined that the tectonic altered granites in the study area are tectonically dynamic altered rocks.The strike of the granite alteration zone is mainly NNW and NE directions,with a larger dip angle,smaller development width,poor extensibility,and approximately equal spacing within the group.The spatial distribution patterns of altered rocks can be classified into two types: fault-controlled and fracture-controlled,depending on their spatial relationship with structural planes.Based on the type of rock mass structure,the interior of the alteration zone is divided into cataclastic zone,block fracture zone,sub block zone,and block zone.Each zone is often symmetrically distributed around the core of the alteration zone.(2)Based on X-ray diffraction test,X-ray fluorescence analysis,scanning electron microscopy,physical and mechanical tests and other methods,comprehensive material composition,microstructure,physical and mechanical properties of altered granites,established the engineering classification standard of alteration degree.The mineral compositions of weakly altered granite and fresh granite in the study area are basically the same.The moderately or strongly altered granite contains high hydrophilic clay minerals such as montmorillonite,chlorite and vermiculite.Compared with fresh granite,it has more potassium feldspar and less plagioclase.Alteration can alter the chemical composition of granite,reduce the content of chemically active oxides,and thereby increase the content of inert oxides.Alteration can also alter the microstructure of granite.The microstructure of weakly altered or fresh granite is scaly,while the microstructure of moderately altered granite is columnar or flaky.The microstructure of strongly altered granite is flocculent.The physical and mechanical properties of granite can also be degraded by alteration,mainly manifested as a decrease in rock density,strength,enhanced water absorption,and increased porosity.For engineering applications,based on characteristics such as material,microstructure,and physical and mechanical properties,the degree of alteration is divided into levels I-IV,which are unaltered,weakly altered,moderately altered,and strongly altered.(3)Based on the basic characteristics of the rock mass in the study area and the spatial distribution and alteration characteristics of altered granites,combined with the preliminary design scheme,numerical model experiments were conducted to study the response characteristics of the surrounding rock of the cavern under the influence of altered rocks.The influence of altered rock on the surrounding rock of the cavern is significant.Compared to areas without altered rock,areas with altered rock mainly exhibit high stress reduction,large displacement,and a larger plastic zone area.The influence of interval distributed alteration zones on the surrounding rock is weaker than that of concentrated distributed alteration zones,and the influence range of each interval alteration zone is independent of each other.