| As a kind of rock mass with poor engineering characteristics,cataclastic rock mass has appeared frequently in many projects such as water conservancy and hydropower engineering,underground geotechnical engineering,road slope and roadbed in recent years.Therefore,the structural characteristics and mechanical characteristics of fractured rock mass determine the safety of the project.Many scholars have conducted in-depth research on the cataclastic rock mass in various projects,and have made great progress in how to improve the overall performance of cataclastic rock mass and the stability of cataclastic slope in engineering construction.The formation reasons of fragmented rock mass have certain commonness,but also have obvious regional characteristics.Therefore,the study of cataclastic rock mass can be comprehensively analyzed from the perspectives of tectonic environment,geological history and fragmentation characteristics.In this paper,the fractured surrounding rock of Dacun Tunnel under construction on Miyu Expressway is taken as the research object.The macroscopic fragmentation causes and formation mechanism of surrounding rock fragmentation are discussed by consulting and collecting geological data and combining with field investigation.The macroscopic geological background of fragmentation is explained by analyzing the geological background of the study area,the tectonic movement experienced,the uplift and erosion process of the Sinian strata and the three-order unloading experienced by the Sinian system.The video and image information collected on site are analyzed in detail,and the phenomenon of surrounding rock fragmentation is analyzed and evaluated from the perspectives of excavation method and efficiency,tunnel face characteristics and slag fragmentation characteristics,and the formation mechanism of surrounding rock fragmentation is identified.The main conclusions are as follows :(1)The Qilu Lake Basin where the Dacun Tunnel is located is located in the Kunming platform fold beam of the secondary structural unit of the eastern Yunnan platform margin fold belt.The Sinian system in the study area and its periphery was previously continuously covered by the thick Cambrian-Cretaceous stratigraphic caprock.The denudation of the caprock caused the present Sinian system to be exposed to the surface,indicating that the tectonic movement and denudation it experienced were strong,and the rock mass quality should be very poor.(2)The study area and its periphery have been disturbed by five tectonic movements since Paleozoic,including Caledonian movement,Hercynian movement,Indosinian movement,Yanshan movement and Himalayan movement.Coupled with factors such as adjacent tectonic units,the study area and its periphery Sinian outcrop area show a mosaic structure pattern composed of multi-scale fault blocks.At the same time,the formation of joints and fracture systems in different periods may cut the rock mass in the study area into mosaics composed of blocks as small as centimeters.(3)The strike-slip movement of the Red River fault zone and the eastward extension of the Xiaojiang fault zone driven by its left-lateral slip provide source power and derived driving force for the continuous uplift of the study area and its periphery.The Sinian System in the study area and its periphery was widely exposed at the end of the Early Tertiary or the beginning of the Late Tertiary,and has been exposed for more than 23 million years.The evolution process of near surface will lead to the decrease of the integrity and continuity of Sinian rock mass,resulting in rock fragmentation.(4)The Sinian system that the Dacun tunnel passes through has undergone three long-term and slow unloading periods : the Sinian unloading in the study area and its periphery,the unloading in the Qilu Lake Basin,and the unloading in the Dacun slope.The joint and fissure system generated under the original deep burial conditions can appear,and the stress release after unloading further promotes the development of the joint and fissure system and promotes the occurrence of fragmentation.(5)The construction of Dacun tunnel adopts the construction method of long time static excavation of excavator and high construction efficiency,which reflects that the surrounding rock of tunnel is easy to excavate and highly fragmented.The fragmentation degree of dolomite exposed on the tunnel face is generally higher than that of sandstone,and the maximum equivalent diameters of the structure are 20 cm and 40 cm,respectively.The maximum size of the slag debris exposed by excavation is generally below 20 cm,and some pile numbers are below 10 cm.(6)The fracture surface of the columnar and tabular bodies of the Dacun Tunnel should be an X-shaped joint system formed in a compressive tectonic stress environment.X-type shear joints cut rock mass on the horizontal plane,which is the basis of rock fragmentation.Bedding shear is divided into bedding joints and bedding faults,both of which can further cut the block cut by X-type joint system.The combination of X-type shear joints and bedding shear can cut the surrounding rock into fragments of different sizes.(7)The macroscopic mechanism of the development of bedding faults in the surrounding rock of the tunnel is that the thin mudstone is transformed into a slip zone,and the upper and lower parts of the rock mass slide along the mudstone.The microscopic mechanism is the directional arrangement of nanoscale clastic clay minerals near the slip surface.The bedding fault not only causes the fragmentation of mudstone,dolomite and sandstone in the upper and lower plates,but also causes the increase of joint opening in the X-type shear joint system in the upper and lower plates,which improves the degree of fragmentation of surrounding rock and reduces the excavation difficulty of surrounding rock.(8)Brittle failure is one of the important auxiliary mechanisms for the fragmentation of dolomite surrounding rock in Dacun Tunnel.The main contribution should be the dolomite debris below the confining equivalent diameter of 20 cm,especially a few centimeters. |
PDF Full Text Request