Research On Hydraulic Pressure Tunnel Under High Geotemperature Conditions By Structual Model Test And Its Bearing Characteristics Of Surrounding Rock

Geothermal disaster induced by the high geothermal problem is becoming increasingly universal in the water conservancy project,especially the project with deep-buried long hydraulic tunnels,which is a special geological problem to be solved urgently.It is a complicated thermal-hydraulic-mechanical(THM)coupled problem in hydraulic pressure tunnel under high geothermal conditon.Aiming at the problem of the unclear bearing mechanism of the surrounding rock of the hydraulic pressure tunnel with high-geotemperature,this paper conducts a large-scale model test study on hydraulic pressure tunnel under high geotemperature and a numerical simulation is carried out by using the proposed thermal-hydraulic-mechanical-damage(THMD)coupling model considering the degradation process of damage of rock mass,which succefully revealed the THM coupled effect and the cracking chracteristics of the surrounding rock of the hyraulic tunnel under high geoteperature,and obtained the rules influenced by temperature difference,internal water pressure,in-situ stress and linear thermal expansion coefficient on the bearing characteristics of surrounding rock.Finally,combined with the THMD coupled model,a reliability method based on the dynamic surrogate model is propsed which has been successfully applied to the engineering practice of hydraulic pressure tunnels with high geotemperature,therefore providing a scientific basis for the construction of the structural design theory system of hydraulic tunnels in high-geotemperature areas and the formulation of related specifications.The main research work and conclusions are as follows:(1)In view of the THM coupling characteristics in hydraulic presure tunnel with high geotemperature,a similar material which is suitable for structural model tests of hydraulic pressure tunnel under high geothermal conditon has been developed to lay the foundation for structural model tests of hydraulic pressure tunnel with high geotemperature.The obtained results indicate that the thermal expansion coefficient,uniaxial compressive stress,uniaxial tensile stress,and the brittleness of the developed similar material are theoretically similar with the class I or class II hard surrounding rock,which indicates that the developed material is applicable to be used for simulating the mechanical behavior of the surrounding rock of the high temperature and high pressure hydraulic tunnel.(2)A structural model testing system with large proportional scale and ability of applying real water pressure is established for modelling the mechanical behavior of the surrounding rocks of the high temperature and high pressure hydraulic tunnel is proposed.Using this system,the cracking process of the surrounding rocks caused by multi-field(temperature,seepage,and mechanics)coupling under different ground temperature gradient and varied internal water pressure can be reasonably reproduced and monitored.(3)The model test reveal several important findings.1)The critical internal water pressure when hydraulic fracturing occurring in the hydraulic tunnel with high geotemperature is obvious lower than that in the common hydraulic tunnel without high geotemperature.2)During the model test,the mechanical behavior of the surrounding rocks is dominated by the tensile failure,the ratio of the tensile type cracks is up to 98.43%.3)Once the damage of the surrounding rocks occur,the THM coupling effect appears obviously,the cracks propagation and new cracks generatation,exhibit a alternant appearance of “burst period” and “quiet period” and a discontinuous step-growth type,;4)Evolution of the temperature and seepage field is consistent with that of the timing-spatial of AE events.5)Cracking model of the surrounding rocks of high hydraulic pressure tunne with high geotemperature differs from that of the hydraulic tunnel without high temperature: for the former,many macro cracks with secondary cracks among them can be observed and the maximum damage area ranges from the boundary of the tunnel to the place with a distance about 5 time of the tunnel diameter;for the latter,only several macro cracks can be found.(4)THMD coupling model for the numerical calculation of the THM behavior of the hydraulic tunnel is proposed,in which post-peak strength degradation of brittle material and damage evolution is considered.The studying results indicate that the proposed method is feasible.In addition,this method exhibits excellent application in predicting the mechanical behavior and assessing the bearing capacity of the surrounding rock of hydraulic pressure tunnel.with high geotemperature.(5)The influences of temperature difference,internal water pressure,thermal expansion coefficient,in-situ stress on the bearing characteristic of the surrounding rocks are revealed by the numerical deduction using THMD coupling model.1)Under high temperature and high internal water pressure conditions,the deformation of the surrounding exhibits obvious THM coupling effect: before hydraulic fracturing,the TM coupling effect is obvious;after hydraulic fracture,the THM coupling effect dominates accompanied by the successive initiation and propagation of the new cracks.2)Cracking of the surrounding rocks is dominated by tensile failure caused by the high tensile stress as a result of the superposition of the temperature stress and internal water pressure,the damage of the surrounding linearly increases with the rising of the temperature difference,internal water pressure,and thermal expansion coefficient,indicating that the high temperature difference,internal water pressure,and thermal expansion coefficient is adverse to the stability of the surrounding rock.3)the geotectonic stress significantly affect the bearing characteristics of the high temperature hydraulic tunnel: when the later pressure coefficient ?=1,the even-distributed geotectonic stress will suppress the initiation of the hydraulic fracture and propagation of the previous cracks in the surrounding rocks,which is in favor of the stability of the surrounding rocks;when ??1 predominate crack caused by hydraulic fracturing is parallel to the maximum geotectonic stress.(6)A method based on the dynamic surrogate model of least squares support vector machine(LSSVM)is proposed for the structural reliability calculation of the surrounding rock of hydraulic presure tunnel with high geotemperature.The results indicate that the proposed method is not only feasible but also effective and high-precision.It can overcome the limitation of the traditional response surface method in solving the structural stability problem.Furthermore,this method can be easily integrated with the calculation procedure of THMD coupling model and is applicable to the stability analysis of the surrounding rock of the high-pressure,high-temperature hydraulic tunnel in which the high-precision and high computational cost are needed.