Deep underground energy storage refers to the storage of energy resources such as petroleum，natural gas，hydrogen，compressed air and CO2，and strategic scarce materials such as helium in deep formations. Rock salt formation is an excellent geological host body for deep underground energy storage. Using rock salt formation for energy storage is an important development direction for large-scale energy storage in China. At present，Jintan salt cavern gas storage，Jintan salt cavern compressed air energy storage power station，and Jianghan salt cavern gas storage，etc.，have been constructed. Rock salt formation in China has some unfavorable geological disadvantages for the energy storage，such as thin salt layers，high impurity content and many interlayers，which take a series of theoretical and technical challenges for large scale energy storage. To overcome these theoretical and technical challenges，the Institute of Rock and Soil Mechanics CAS has carried out continuous researches for more than 20 years. Many breakthroughs were obtained，such as experimental device research and development，mechanical properties of bedded rock salt，construction technology of salt caverns，first debrining technology，and operating pressure optimization technology，etc.，which have been used for guiding the construction of several salt cavern storages in China. Large-scale storage of natural gas，compressed air，petroleum and hydrogen by deep salt caverns is one of the key development directions of deep underground energy storage in China. Deep underground energy storage involves complex situations such as multi-field multi-phase coupling and multi-scale. It is urgent to carry out researches on multi-scale migration of energy media，performance evolution of geological host body，intelligent construction of deep energy storages and smart operation of deep energy storages，so as to solve the key scientific problems restricting the technical bottlenecks of deep underground energy storage in China. Deep underground energy storage is the breakthrough of deep cross fusion of geotechnical engineering，engineering geology and energy storage，and is expected to form a new professional discipline.

The model test and digital-image correlation technology were employed to investigate the initiation and propagation characteristics of rock mass cracks during loading of the tunnel-type anchorage installed in soft rock strata. The results indicate that the bearing process of the tunnel-type anchorage installed in soft rock goes through four states successively：approximate linear elastic deformation，nonlinear deformation，plastic deformation and failure state. The failure of the tunnel-type anchorage installed in soft rock strata gradually evolved from a single rock mass failure in the early stage to a double failure of the rock mass failure and debonding of the interface between the plug body and rock mass. The progressive failure process of the tunnel-type anchorage can be generalized as follows：In the plastic deformation state，the tunnel-type anchorage first enters failure stage I，which is mainly characterized by the shear cracks initiation in the surrounding rock near the back end of the plug body crown and invert. Subsequently，the tunnel-type anchorage enters the double failure stage II，which is mainly characterized by the compression-shear and tension-shear failure of rock mass and sliding failure of the interface. In the complete failure state，the tunnel-type anchorage enters the failure stage III in which the crack extends to the ground surface and the interface is debonded. In the process of propagation to the ground surface，the cracks in the surrounding rock of the upper part of the plug body go through the evolution process of shear failure，tension-shear composite failure，and tensile failure. The surrounding rock of the lower part of the plug body is dominated by shear cracks approximately parallel to the axis of the plug body. The ground surface cracks propagate in a direction approximately perpendicular to the vertical projection of the plug body on the ground surface. The ground surface cracks experience the evolution from tensile cracks to shear or tensile-shear composite cracks，to tensile cracks，and then to tensile-shear composite cracks during propagating. The block shape cut by the crack inside the rock mass gradually transits from strip-shaped to block-shaped from the back of the plug body to its front end. The vertical dislocation value between adjacent blocks decreases from backward to forward. The cracks shape in the deep part of the rock mass gradually changed from a convex trumpet shape to a concave arc shape with the increase of the peeling depth，and the damaged area gradually shrinks to the shallow surface and the front end of the plug body crown.

There are a large number of discontinuous interfaces such as natural fractures，faults and bedding planes in deep reservoirs. These interfaces dominate the generation of the fracture network and efficiency of the stimulation treatment. This paper presents a brief review on the study of hydraulic fracturing in reservoirs with interfaces in three aspects：experiments，theory and numerical simulation. Firstly，this paper investigates the current state of laboratory hydraulic fracturing experiments including loading mode，sample preparation and fracture monitoring. Secondly，based on the results of hydraulic fracturing experiments，the theory of the interaction between hydraulic fracture and single interface is analyzed. In addition，the key factors affecting the generation of the fracture network are concluded. Then，the difficulties of numerical simulation of the interaction between hydraulic fractures and the interface are presented. The treatment of the fracture intersection，and the numerical research progress of the interaction between hydraulic fractures and interfaces and the generation of fracture network are reviewed. Finally，a few issues and trends of hydraulic fracturing research in reservoirs with interfaces are proposed.

In view of the heterogeneous of rock and soil during the gestation and evolution of landslides，the strength test，bottom friction model test and scanning electron microscope test of similar materials of landslides were carried out，and the mechanical mechanism of landslide evolution and its heterogeneity characteristics were deeply studied from macro and micro perspectives. In addition，the strain softening model and Burgers model are connected in series to form a constitutive model considering the heterogeneous rheological characteristics，which is applied to the numerical example analysis and early warning study of landslide time evolution. The results show that there are time and spatial heterogeneity. In terms of time，the faster the degradation rate of rock and soil parameters of the landslide，the shorter the time from creep deformation to the final instability failure. There is heterogeneity in the mechanism and microstructure. The constitutive model considering the heterogeneous rheological characteristics can better reflect the three-stage evolution characteristics of landslide deformation. The accuracy of the heterogeneous rheological model is verified through the combination of actual engineering and numerical analysis，which is the analysis of landslide instability and failure，and early warning to provide useful reference.

To explore the mesoscopic fracture mechanism of granite under high temperature and confining pressure，GBM in PFC was adopted to simulate the triaxial tests of granite specimens after thermal treatment，and then the strain-strain curve，strength，failure modes and the fracture process were investigated. Based on the above analysis，the following conclusions can be obtained：GBM can reflect the interlock between grains and the nonlinear variation characteristic of the peak strength with confining pressure，and it also can overcome the problem that the circular particles is insufficient locking force. The peak strength of granite under different confining pressures is almost constant first and then decreases rapidly with increasing temperature，and 450 ℃ is the threshold temperature. The internal friction angle and cohesion first increase and then decrease with temperature，and the variation of the mechanical parameters is closely related to the mechanical structure of granite. After quartz underwent ?-? phase transition(573 ℃)，a large number of intra-granular cracks and inter-granular cracks occur in granite specimen. Under uniaxial compression，the fracture process of the specimen is controlled by thermal cracks，and the specimen exhibits ductile failure after the peak strength. Under high confining pressure，the shear band passes through the grain，loading to the brittle failure after the peak strength.

To study the damage characteristics of the pore structure of coal samples under the action of liquid nitrogen (LN2) freeze-thaw，image analysis，nuclear magnetic resonance(NMR) technique and mercury intrusion were used to analyze the evolution law of fracture expansion，porosity，bound fluid volume，free fluid volume，pore volume，specific surface area and pore size distribution of coal samples under LN2 freeze-thaw. The experimental results show that：(1) The primary fracture gradually expands and connects to form secondary fracture with increasing the number of LN2 freeze-thaw cycles，and the isolated fractures are connected to form fracture network with primary and secondary fractures after the number of freeze-thaw cycles reaches a certain level. (2) LN2 freeze-thaw can promote the development of pores，and the micropores and minipores of coal samples gradually expand，develop and connect to form mesopores and macropores after LN2 freeze-thaw，resulting in enhanced pore connectivity of coal samples，a decrease in the proportion of the bound fluid and increase in the proportion of the free fluid，and the increase of the total porosity，the residual porosity and the effective porosity. (3) The number of pores and the pore size increases after LN2 freeze-thaw，and the pore density in some areas continues to increase and expand to form microfractures. (4) The total pore volume and the specific surface area of coal samples increases after LN2 freeze-thaw，and the change of the pore volume is mainly concentrated in the mespores and macropores. The micropres of coal sample gradually transform to the mesopores and macropores，which leads to the increase of proportion of mesopores and macropores.

The freezing-thawing damage nature of rock mass is the expansion of existing cracks driven by the frost heaving force in the process of water-ice transformation. Therefore，the study of the generation and evolution of the frost heaving force in cracks is the core problem in the study of rock mass freezing-thawing damage and the basic premise for revealing the freezing-thawing damage mechanism of rock mass. In this paper，the crack internal temperature，the formation process of crack ice，the evolution of the freezing-heaving force inside the crack and the freezing-heaving deformation at the crack end are monitored in the limestone with single crack during freezing-thawing，and effects of different variables (freezing rate，crack water content (water ratio of the crack volume)，crack depth) on frost heave characteristics of cracks. The results show that the temperature change in the crack can be divided into six stages during freezing-thawing. In the rapid freezing stage，there are obvious phenomena of undercooling and thermal relaxation，and the crack water freezes from the outside to the inside，forming an ice shell to restrict the unfrozen water. Fracture internal frost heaving force and frost heave deformation evolution process can be divided into five stages. In the stage 2，the frost heave force grows to peak and then rapid decline，while the frost heave deformation evolution can be divided into two modes：rapid increase first and then rapid decrease，and rapid increase first and then slow increase. The influence of different variables on the degree of undercooling，the duration of thermal relaxation and the maximum frost heaving force in the water freezing process is significant. When the crack depth is large，the crack will appear at the crack end. Based on the above experimental results，it can be concluded that the generation and evolution of the frost heaving force in the freezing-thawing process are controlled by the unfrozen water seal condition in the crack，that the formation of the sealing condition is accompanied by a process of “unfrozen water freezing-ice shell fracture-unfrozen water extrusion-ice shell closure”，and that whether the crack expands after the sealing condition is determined by the freezing rate，the initial water content and the crack depth.

The combinations and occurrence forms of coal seams and roof and floor strata are diverse. In laboratory study of the failure and deformation behavior of coal and rock，the stiffness differences of the roadway surrounding rock structures should be taken into account，which may provide a new method to reveal coal bump mechanism. This paper introduced a self-developed rock testing system with changeable stiffness. The main testing machine of this system is a combined structure of inner and outer frames. The change in the loading stiffness is achieved by using a stiffness servo control system to control the energy accumulation in the inner frame. The tests of the sandstone specimens under three different loading stiffnesses in the testing system show that the loading stiffness does not significantly affect either the uniaxial compressive strength or the Young?s modulus of the rock. However，the post-peak stress-strain curve of the rock becomes smoother and steeper when the loading stiffness decreases，and the stress drop rate increases. It is shown that the stress drop rate has a power function correlation with the loading stiffness. After the peak load，the inner frame of the testing system rebounds several times in high speeds. The magnitude of the instantaneous rebound speed and the rebound duration increase with the decrease of the loading stiffness. Both the mean rebound velocity and the total rebound deformation have power function correlations with the loading stiffness. The loading stiffness has little effect on the post-peak dissipated energy of the specimen，and has a power function relationship with the released energy of the testing machine. The rock testing system with changeable stiffness provides a new kind of testing equipment and technology for rock mechanical behavior testing under different loading stiffness combinations.

Rock slabbing is a serious threat to the construction safety of deep buried tunnels. However，it is still unclear whether rock slabbing is related to the gradient stress in surrounding rocks. In this paper，the changes of the tangential stress gradient with the buried depth increase were theoretically analyzed first. An improved method of rock gradient stress construction was proposed. Then，by the method，strain observation tests were conducted on cuboid granite samples with the d0 (the length difference of upper and lower surfaces of specimen) between 0.1 and 0.8 mm under gradient stress to confirm whether rock spalling can occur under gradient stress and whether the thickness of formed rock slabs are uniform. The results show that the stress changes linearly along the thickness direction of the specimens，with a rate up to more than 103 MPa/m，which means that the improved method can be used to create gradient stress environment in rocks. It is found that the tensile stress in the low-stress region at the end of the specimen，whose direction is consistent with the loading，occurs in the range of 0.5 to 1.5 MPa under gradient stress. The tensile stress presents a “rise-fall-rise-fall” stage change in the process of loading，and it can be restrained by reducing d0. It is confirmed that rock slabbing occurs under gradient stress，and the formed rock slabs have uniform thickness. Moreover，the formed cracks belong to tension cracks，which is consistent with the previous research results. It is also confirmed that the gradient stress is an important factor that cannot be ignored in the propagation process of deep surrounding rock slabbing. This study lays an experimental foundation for further research on the occurrence conditions of rock slabbing and the influencing factors of the thickness of rock slabs.

Columnar jointed basalt shows highly anisotropic characteristics in strength and deformation modulus，and hence，understanding its physical and mechanical properties is of great significance to engineering safety. According to the natural structural characteristics of columnar jointed rock mass，a three-dimensional polycrystalline discrete element model including double joint networks was established. Using this model，the mechanical properties and failure modes of columnar jointed rock mass under uniaxial conditions were explored. Then，the compression failure simulation of rock mass with specific dip angle under three-dimensional stress conditions was carried out. It is shown that the numerical simulation results are in good agreement with the experimental results，and the samples show three typical failure modes. Under the condition of three-phase stress，the stress-strain curve shows obvious multistage and anisotropy，and the volumetric strain ?V shows dilatancy. After unloading in the ?3 direction，the peak strength and corresponding elastic strain of columnar jointed rock mass decrease significantly，while the brittleness index of the specimen increases significantly. The failure mode presents brittle failure extending from the unloading plane to the center，and the proportion of the dissipated energy increases in energy evolution. The evolution process of microcracks reflects that the degradation process of columnar jointed rock mass presents progressive failure characteristics，and the intermediate principal stress has a promoting effect on the slip failure of the inter column joint plane.

Radon is harmful in the development and utilization of underground space，and it has an important influence on the deep dark matter experiment. Studying the regularity of radon release during underground excavation is the basis of solving radon pollution problems. To study the influence of marble fracture mode on radon release，triaxial compression and radon release tests of marble specimens under different confining pressures were carried out by the self-designed rock triaxial compression radon release test system，and the relationship between Failure Approach Index and accumulative radon concentration was discussed. The results show that significant differences exist in the characteristics of radon release from marble samples at different loading stages under triaxial compression，showing a trend of increasing firstly，then decreasing slightly，next increasing sharply to the peak release，and finally dropping to a lower level. The initial radon emission gradually increases with the increase of the confining pressure，while the peak radon emission gradually decreases with the increase of the confining pressure which is related to the failure mode of rock under different confining pressures. It is found that there is a correlation between Failure Approach Index and accumulative radon concentration by fitting the experimental data，and the change trend of radon emission can be predicted by the failure approach index curve. It is of great significance to reveal the law of radon release in rock for the design of radon discharge/radon isolation.

In order to explore efficient and green grouting materials，the industrial-grade graphene oxide(GO) with a mass ratio of 0.03% of the cement-based slurry combined with fly ash with a mass ratio of 12.5% were mixed into cement-based grouting materials to strengthen the crushed stone. The uniaxial compression tests，acoustic emission(AE) monitoring and scanning electron microscopy(SEM) characterization，were then carried out to study the strength and deformation performance of the cemented specimens，and the macro-and micro-characteristics after the failure of the cemented crushed stone were investigated. The optimization effects and application prospects of GO on cement-based materials are discussed. The results show that the modified slurry can effectively increase the compressive strength of the cemented crushed stone by about 12.6%–22.5%. GO can promote the hydration reaction of cement and guide hydration products to grow on the surface of graphene nanosheets，thereby optimizing the pore structure in the grouting materials and reducing the micro-cracks between the slurry and the crushed stone interface. AE monitoring results，SEM characterization，and fractal analysis further revealed that the incorporation of GO could effectively reduce the degree of micro-damage of the sample during the failure process，ensure the integrity of the sample during the destabilization failure process after cemented，enhance the material toughness and reinforce its ability to resist the compressive load.

For considering the effect of the vertical friction on the lateral bearing capacity of rigid single piles more accurately，the relationship between the maximum compressive soil pressure around the passive side and the total soil resistance was established by presuming that the compressive soil pressure around passive side presents a cosine function distribution. The modified vertical friction coefficient of the pile-soil interface was obtained by inverse derivation of the simulations of compressive soil pressure and vertical friction resistance. On this basis，the analytical expression of the shaft resisting moment was derived. Assuming that both the ultimate soil resistance and the modulus of horizontal subgrade reaction increase linearly with depth and the modulus of the horizontal subgrade reaction decreases nonlinearly with the displacement at ground surface，the equilibrium equations of the horizontal force and the moment were derived under three stages：soil resistance without yielding，soil resistance with yielding only in a region above the rotation point，and soil resistance with yielding in regions both above and below the rotation point. For avoiding the complexity of the numerical iteration program，the analytical solution of the lateral bearing capacity was obtained by regarding the horizontal displacement(y0) at the ground surface as a known quantity and the depth of the rotation point(a) as a variable with consideration of displacement loading method. Compared with the results of finite element analysis，model test and field test，the correctness of the proposed method was verified. Based on the verified analytical solution，a parametric analysis was carried out to explore the influential factors(e.g. pile diameter，internal friction angle and height of lateral force) of the lateral bearing capacity，maximum bending moment and rotation point depth of rigid single pile. The results show that the cosine function can satisfactorily characterize the distribution of compressive soil pressure around the passive side，and the calculated shaft resisting moment from the modified vertical friction coefficient of the pile-soil interface is more realistic. Ignoring the effect of the shaft resisting moment will significantly underestimate the lateral bearing capacity of rigid single pile，and the degree of underestimation increases with the increase of pile diameter，internal friction angle and the height of lateral force.

The traditional log-spiral mechanism cannot be used for three-dimensional (3D) slope kinematic analysis with spatially varying friction angle. To address this problem，a 3D mechanism based on sliding surface discretization is proposed. This mechanism，with a sliding surface consisting of several triangular elements，uses the point-to-point technique in generation process and strictly satisfies the associated flow rule. The boundary conditions and construction process of this mechanism are described，and the equations of the gravity work power and the internal energy dissipation are derived. Further，the safety factor calculation process based on the dichotomous strategy is proposed，and the sensitivity analysis of the model parameters is carried out. The reliability of this mechanism under various conditions is verified by comparative analysis. Finally，the safety factor of a 3D slope with spatial variation of soil strength is calculated using the present mechanism. The results show that the variation coefficient of soil strength has a large influence on the mean value of the safety factor.

The stress direction dependence of soil refers to the property that the mechanical properties of soil change with the change of the stress direction. The inner reason for the special property is that the soil formed by particle accumulation is anisotropic on the micro scale. To quantitatively describe the difference of pore characteristics of anisotropic granular materials along different directions，the pore area ratio is proposed，that is，the ratio of the pore area to the particle area on a certain section of granular material. Then，the quantitative correlation between the pore area ratio and the fabric tensor is established. On this basis，the anisotropic state parameter is defined as the difference between the pore area ratio in the current direction and the average value of the pore area ratio(or the value of the pore ratio)，so as to quantitatively describe the relationship between the state in different directions and the average state of the material. The equivalent stress method is used to “transform” the unified hardening model(UH model)，and the equivalent unified hardening model(EUH model) is established. Only two new parameters are added in the new model which can be directly obtained by the triaxial compression tests. The stress direction dependence of the stress path，pore pressure，stress-strain relationship can be accurate predicted. Moreover，the accurate prediction of the stress direction dependence of each strain component in the main space and physical space is also realized. The result further verifies the scientificity of the equivalent stress method，and lays a solid foundation for the engineering application of the equivalent stress method.