The Anisotropy Of Wudang Group Schist And Its Application In Tunnel Support

Affected by the inherent weak nature of the schistose planes,the schist often shows a remarkable anisotropy in the mechanical properties,which usually brings about a geotechnical challenge during construction projects.Especially in the process of tunnel construction,after tunnel excavation,the schist anisotropy easily leads to some engineering problems such as asymmetric deformation damage,longitudinal crack and lining cutting for tunnel.The Shiyan-Fangxian highway through the steep mountainous areas in northwest Hubei,and exposed rocks mostly are Wudang metamorphic schist.As typical schist,Wudang schist exhibited a significant anisotropy due to the weak schistose plane development,which results in great trouble to the tunnel construction along the highway.So,a comprehensive understanding of the anisotropy properties of schist plays a critical role to design a safe and reliable engineering project.The present study has systematically studied the micro-and macro-anisotropy properties of Wudang group schist and its application in tunnel engineering by using multi-methods such as engineering geological survey,polarizing microscope,scanning electron microscope,indoor physical test,acoustic velocity measurement,uniaxial and triaxial compression tests,numerical simulation.The main findings of the present study are as follows:(1)This paper summarizes geologic environmental conditions of Wudang group schist and analyzes the causes of such schist in this condition.It is concluded that a sandstone-siltstone-silty mudstone clastic sedimentary rock group experienced orogenic metamorphism and came into being the Wudang group schist.As the original rock lithology and growth environment change with the geographical location are slightly different,the mineral composition,content and structure of Wudang group schist have a certain spatial difference.(2)The microscopic characteristics of the lithofacies and structure of the Wudang group were systematically analyzed by polarizing microscope and scanning electron microscope respectively.The qualitative analysis results show that the Wudang group has two kinds of structural elements with granular and flaky form,and the two units show a certain degree of uniaxial elongation.The flaky minerals are sandwiched between granular minerals,The relative content,boundary morphology and distribution of mineral and micro-pore fissures of Wudang group were measured by area percentage,fractal dimension and degree of orientation.It was found that the microstructure of Wushang group schist with different characteristics showed different law.The correlation between mineral content and pore fissure content,mineral orientation and fracture orientation,mineral fractal dimension and fracture fractal dimension are revealed,and the spatial relationship between mineral and pore fissure in microcosmic system is revealed.(3)The current study investigated the macroscopic anisotropy properties of Wudang group schist under the influence of water and confining pressure by using the wave velocity test,uniaxial compression test and triaxial compression test.Furthermore,we examined the anisotropy of Wudang group schist by comprehensive the microstructure and macrostructure of schist.The longitudinal wave velocity,compressive strength and elastic modulus of Wudang group show obvious anisotropy,and the longitudinal wave velocity increases with the angle ? of the longitudinal axis of the specimen and the direction of the sheet distribution.The compressive strength and The elastic modulus decreases first and then decreases with the decrease of ?,and the minimum value is ? = 30 °.Under the influence of mineral composition and structure,water,confining pressure and other factors,the longitudinal wave velocity,compressive strength,elastic modulus and anisotropy of these index show obvious laws.And we examined the effect of water,mineral composition and structure(including mica mineral and fissure content,mineral and fracture morphology,distribution of minerals and fissures)on the compressive strength,elastic modulus and anisotropy of schist under uniaxial compression test.Furthermore,we investigated the effect of the mica content,mineral orientation,and mineral fractal dimension on the anisotropy of schist.The results revealed that mineral orientation has the greatest influence on the strength anisotropy of schist,followed by the mineral fractal dimension,the effect of mica content is relatively small.The failure mode of schist is studied by means of laboratory test and numerical test.The results show that there are three kinds of failure modes in the rock with no confining pressure: shear failure oblique the foliation plane,stretch splitting failure and shear failure along foliation plane.Under the confining pressure condition of 5-20 MPa,it shows two kinds of failure modes,such as shear failure oblique the foliation plane and shear failure along foliation plane.The failure mode are related to confining pressure and the angle between loading direction and schistosity plane.The fractal dimension and decreasing function of mineral are introduced to improve the Jaeger-Donath formula.The improved formula is used to predict the uniaxial compressive strength of Wudang group schist with different water states and different structures.Then,the compressive strength of Wushang group schist under confining pressure is predicted by Ramamurthy criterion and improved Hoek-Brown anisotropic failure criterion.(4)We constructed the method of rock mass grading for the schist area,and identified the method of the mechanical parameters for the schist mass.For example,based on the BQ formula,we established the modified formula of rock mass gradingwhich including anisotropy S,uniaxial saturated compressive strength,rock mass integrity coefficient by regression analysis.Moreover,the fit rate of results gained by the modified formula of rock mass grading and the situation of engineering practice is up to 84.8%.The grade of correction factors influencing on the basic quality of the rock mass,namely the occurrence of schistosity plane and the structural plane as well as the expansion of the rock were studied.We identified the correction coefficient range and the correction coefficient weight of different grading.An improved BQ?formula obtaining the correction factor was established.The example shows that surrounding rock grading obtained by the formula has a higher fit to the actuality of the surrounding rock.Finally,we put forward the method of obtaining the maximum and minimum rock mechanics parameters of anisotropic schist,based on the suggestion of rock mechanics parameters in the national standard and the empirical evaluation of rock parameters by using Hoek-Brown criterion.(5)We investigated the response law between the stability and the anisotropy of surrounding rock under different schistose plane angle after different buried depth of tunnel excavation.We used the Udec to simulate the tunnel model under different buried depth and the three schistose plane angles(?=0°,?=45°and ?=90°).The results show that: a)The distribution of plastic zone of surrounding rock is closely related to the stress concentration and the combination of the principal stress and schistose plane angles.For example,the distribution of plastic zone shows a symmetry elliptic and hat-shape along cRvKthe longitudinal axis of tunnel when ? = 0° and ? = 90°,and a spiral fan along the long axis and schistose plane angle when ? = 45°.b)With the decrease of the ratio of the strength and stress,the size and characteristic index of plastic zone under different schistose plane angles show a power function or near-power functional growth.c)There are significant differences in the displacement direction and the maximum displacement value of surrounding rock under different schistose plane angles.d)With the increase of buried depth,the degree of anisotropy of rock decreases.So the influence of schistosity plane on the deformation of surrounding rock tends to weaken.On the contrary,the influence of schistosity plane on the deformation of surrounding rock increases suffering from water.And we further put forward the optimal supporting method to control the deformation of surrounding rock by numerical simulation and orthogonal test method.Based on the stress characteristics of the anchor bolt in the anisotropic surrounding rock,this paper puts forward the measures of large scale weakening and local strengthening of the anchor bolt on the basis of the best supporting scheme,so as to achieve the ideal supporting effect.