Research On Mechanical Characteristics And Constitutive Model Of Completely Weathered Granite And Its Engineering Application

Being different from general cohesive soil and sandy soil,completely weathered granite（CDG）is a complex natural geological product.With loose structure and weak mechanical strength,it shows strong disintegration and dilatability when exposed to water,which seriously affects the construction and operation safety of underground tunnel engineering through this kind of stratum.In consideration of the complex engineering properties and occurrence environment in the CDG stratum,the special mechanical behavior of CDG is not well understood,and the effective stability control technology is also relatively rare,resulting in the frequent occurrence of major engineering accidents such as collapses,water and mud inrushes in the surrounding rock.Therefore,from the perspective of actual engineering application,the research on the mechanical properties and constitutive model of CDG under hydraulic environment can be carried out to reveal the deformation and failure mechanism,and then the efficient and rapid excavation and support optimization methods can be proposed,which is of important theoretical significance and engineering application value for promoting the construction of tunnel engineering in the CDG stratum.In this paper,taking the CDG in Wuyue pumped storage power station in Henan Province of China as research object,the mechanical strength and wetting-deformation characteristics of the CDG under different hydraulic environment are systematically studied by employing field investigation,laboratory test,theoretical analysis,and industrial test.Afterwards,the corresponding constitutive models are established.Finally,the support control technique and the support opportunity when wetting deformation occurs are put forward.The main research contents and achievements of this paper are listed as follows:（1）By unconfined compression test,the characteristics of rapid shear mechanical behavior of CDG are experimental studied.When the dry density keeps the same,the UCS value of soil samples increases firstly and then decreases with the increase of water content.When the moisture content is a constant,there is a good positive proportional relationship between UCS and dry density.In each case,there is an optimal moisture content,making the UCS of soil samples reach the maximum value,among which it is 10.5%corresponding to the smaller dry density,and 8.0%corresponding to the large dry density.Meanwhile,there is also a critical moisture content,which is 13.0%,and the strength will decrease sharply once moisture content exceeds this critical value.The deformation and fracture of CDG is mainly shear failure.The water content of the sample dominates the final failure mode,and the dry density of the sample mainly affects the degree of crack propagation.（2）Based on the damage mechanics,a statistical damage constitutive model of CDG under uniaxial stress is developed by the undetermined coefficient method.Compared with other similar models,the superiority and reliability of the model are verified.The model can not only qualitatively reflect the significant characteristics of fully weathered granite soil samples,such as initial compaction,post-peak deformation and failure,but also quantitatively elaborate the key mechanical indexes,i.e.,initial compaction threshold,elastic modulus,peak strength,and peak strain.（3）On the basis of the consolidated drained tests with different environmental variables,the slow shear mechanical behavior of CDG applied with three-dimensional stress is analyzed,and the relationship between its shear strength index and dry density as well as water content is revealed.Under the same moisture content,the strength of soil sample is directly proportional to dry density and confining pressure.When the moisture content increases,the soil strength first decreases significantly and then increases slightly,and the minimum value appears when it reaches16.0%.The deformation and fracture mode of soil sample show a combination of shear failure and dilatancy deformation.The shear failure characteristics are obvious at low water content and low confining pressure condition,and the dilatancy deformation is prominent at high water content.The fracture mode of soil sample is mainly dominated by water content.Furthermore,compared with the dry density,the cohesion and internal friction angle of soil samples are mainly affected by water content,and the change of water content will directly determine the shear strength of soil samples.（4）According to the classical elastic-plastic theory and the diversified type curves obtained from triaxial mechanical test,a new mechanical constitutive model is obtained through dimensional transformation and coefficient revision of Duncan Chang nonlinear model.The model can characterize both softening and hardening characteristics of CDG.By comparing the results of other similar models,this model has a few parameters with simple form,which can more accurately reflect the key characteristics of the test curve under three-dimensional stress state,such as nonlinear elastic behavior,peak value and residual strength.（5）With the variation of the stress level and confining pressure,the mechanical characteristics of CDG soil samples with natural moisture content after soaking,were studied using a large-scale triaxial shear tester,and then the mathematical model of wetting strain increment was obtained.With natural water content,the humidification effect not only significantly weakens the mechanical strength,but also sharply increases the three indexes of axial strain,volumetric strain,and shear strain.Compared with confining pressure,the influence of stress levels on soil behavior is more sensitive.In particular,when the stress level increases by one-time step by step,these three indexes increase by 2.59,1.41,and 3.17 times respectively.（6）Based on the time-strain relationship of wetting deformation,the time-varying effect model on stress level and confining pressure is constructed by using the burgers creep model,and the judgment index of wetting stability is determined.The constitutive model can not only describe the nonlinear and linear viscoelastic deformation behavior of CDG in the stages of attenuation creep and stable creep stage,but also accurately reflect its instantaneous elastic deformation behavior.Also,the stability time of wetting deformation is about 1.36 h,and the corresponding strain rate is 0.03%/h.（7）With the secondary development program in FLAC^3D numerical software,the modified Duncan Chang constitutive model is embedded to further simulate and analyze the support effect of the high-voltage cable adit through CDG stratum under the original design support scheme.After the optimization design,a combined support technique with"long/short bolt or anchor cable"cooperative hierarchical support as a core,and"I-steel arch+local or full section grouting"as an auxiliary reinforcement,was proposed aimed to tackle with broken surrounding rock mass.Compared with the original design support scheme,the overall convergence of surrounding rock under the optimized scheme is reduced by 45.82%,the volume of plastic zone is reduced by 24.74%,and the stress concentration area is obviously reduced.Additionally,the roof subsidence and bottom heave reduce to 55.21 mm and 66.10 mm respectively.（8）The wetting effect of high voltage cable adit during excavation is simulated and analyzed by using the numerical simulation method.After different cyclic excavation steps,the wetting deformation and the corresponding stability time of surrounding rock are mainly related to the stress level and the support method rather than the number of excavation steps.The wetting deformation of surrounding rock tends to be stable after1.37 h（or 80 min）approximately,and the later deformation is generally small.Therefore,the effective measures to prevent and control the wetting collapse of surrounding rock should be to delay the lag of support construction practice by 1.37h and reasonably strengthen the support face.（9）Taking the high-voltage cable tunnel in a CDG stratum as an example,the field industrial test is carried out on the proposed optimized support scheme,and the support effect is evaluated combined with the real-time monitoring data.During the construction support period,the roof of the tunnel did not collapse,and the surrounding rock was well controlled.According to the deformation convergence curve of the surrounding rock in the field,the main deformation of the surrounding rock has been completed in the first 7 days after the initial support.After more than 15days,the deformation value basically tends to be stable.After 60 days,the roof subsidence,bottom heave and the average extrusion of the two sides reach 42.15 mm,61.00 mm and 58.78 m respectively,which could effectively solve the support problem of surrounding rock in tunnel through a CDG stratum.The present research results will have important guiding significance for the construction of tunnels crossing soft and broken strata such as completely or strongly weathered granite in Henan Wuyue pumped storage power station,and also provide important references for other underground tunnels with similar geological conditions.This paper contains 108 figures,31 tables,173 references.