Parameters Mapping Of The Rock Mass Heterogeneity Around The Underground Water-sealed Cavern And Its Application

The underground water-sealed caverns are often constructed in the hard granite rocks with ample groundwater and random fractures,where heterogeneity or spatial variability of parameters(permeability and deformability and shear strength)are inevitably significant.Under the high groundwater pressure maintained by the water curtain system,the heterogeneity of parameters may lead to catastrophic deformation and failure of the surrounding rock mass,i.e.,existing a high probability of failure zone around the cavern.However,the limited understanding of the heterogeneity in practice severely restricts the identification of the high probability of failure zone around the cavern,which ulteriorly limits the reliability of support design.In view of this,this thesis takes the heterogeneous surrounding rock mass of the underground water-sealed caverns as the research object,and inverses the rock mass parameters,i.e.,permeability(K_s),effective Young's modulus(E?),effective cohesion(c?),and effective internal friction angle(??).Subsequently,we accurately identify the high probability of failure zone around the cavern,and then propose a targeted support scheme.Taking the Ningbo Baidinian underground water-sealed caverns project as an example,the size and anisotropy effect of the representative element volume(REV)of each parameter of rock mass are studied firstly,and the necessity of considering the heterogeneity of rock mass is clarified.The field data are then used to determine the spatial geostatistics of these four parameters to portrait the spatial variability of the rock mass properties over the entire field site.The spatial geostatistics of these four parameters include the mean,the variance,and the correlation scale,which are also regarded as the priori information required for parameter inversion.After that,four parameter random fields satisfying the prior information are generated as the reference field,and the forward calculation is performed on the reference field to collect the hydraulic head and the displacement information.Subsequently,based on the spatial cross-correlation analysis of these four parameters and displacement,the hydraulic tomography and the displacement back analysis are established by successive linear estimator,which are used to inverse the spatial distribution and its associated conditional uncertainty of the permeability(K_s)and the mechanical parameters(E?,c?,and??)respectively through fusion of the hydraulic head and the displacement information.Afterward,a feasible probability of failure assessment method yielded from the probability theory is used to quantitatively identify the probability of failure for different parts around the cavern,so as to feedbackly indicate the imperative strengthen location of the cavern.Finally,the construction procedure of underground water-sealed caverns considering the rock heterogeneity is proposed.The main conclusions of this thesis are as follows:(1)The size of the permeability REV and the mechanical parameter REV have small discrepancy with the cavern's geometry so that the equivalent continuous homogenization model cannot be used.Therefore,it is necessary to consider the heterogeneity or spatial variability of rock mass parameters.The spatial geostatistics of the parameters demonstrate that the degree of spatial variability of the permeability is significantly greater than that of the mechanical parameters.Based on the fracture network model,numerical experiments are carried out to find that the correlation scale of the permeability is almost equal to the average maximum trace length of fractures,and the ascendant occurrence of fractures controls the statistical anisotropy of the random field.Conversely,if fractures do not have ascendant occurrence,the random field parameters are statistically isotropic.(2)The displacement at a point in the surrounding rock is negatively correlated with these four parameters near the point,and is only positively correlated with permeability away from the point.The displacement is affected by the permeability and the effective Young's modulus over the entire model,but just affected by strength parameters(c?and??)nearby.In addition,in view of the cross-correlation magnitude,the displacement have the largest cross-correlation forK_s and the smallest cross-correlation for E?,while the cross-correlation for c?and??are medium.(3)The forward calculation results based on the reference field indicate that considering the heterogeneity of mechanical parameters only does not capture the true deformation and failure locations at the carven.Since the water curtain system maintain high groundwater pressure and heterogeneity of K_s can alter the distribution seepage forces,the spatial variability ofK_s plays a significant role on the cavern stability assessment.For this reason,it is of practical significance to take the heterogeneity of both hydrological and mechanical properties into account during the safety analysis of underground cavern excavation.(4)Although the hydraulic gradient field based on the inversion K_s field cannot reflect every details of the real hydraulic gradient field,it grasps the overall characteristics,especially the large hydraulic gradient zone around the cavern.The hydraulic gradient field based on the inversion K_s field is enough to estimate the seepage force.Based on the estimated seepage force and displacement back analysis,the estimated E?field in the displacement sampling locations closely resembles the reference parameter field,while the estimated E?field in other locations is smoother than the reference field,due to lack of observations at this region.In comparison with the estimated E?field,the resolutions of the estimated c?and??fields are low except near the cavern periphery.In addition,the conditional variance(or uncertainty)maps illustrate the uncertainty of the estimated E?,c?,and??near the observation positions are small.More importantly,the uncertainty around the cavern is the smallest,indicative of high confidence in the estimated values around the cavern,which are important for assessing the probability of failure of rock masses around the cavern.(5)If the prior information is used only in the assessment of probability of failure,we will underestimate the probability of failure.In contrast,using the inversion results leads to a realistic assessment of the probability of failure.In addition,the targeted anchoring scheme,based on the identified high probability of failure zone,have a better anchoring effect than that of based on the large magnitude of displacement.