Stability Of Caverns Research Articles

Experimental study on the mechanical and permeability behaviors of limestone under hydro-mechanical-coupled conditions

In situ stress and underground water are known to significantly influence the deformation and stability of deep underground caverns. These factors should be considered in the design of excavation and supporting schemes. However, the rock failure mechanism and permeability characteristics under the coupling condition remain largely unexplained. Based on the Xianglushan diversion tunnel in Central-Yunnan Water Diversion Project, we conducted triaxial compression experiments on both intact rock specimens and cracked rock specimens with three confining pressures (11 MPa, 18 MPa, and 25 MPa) and three osmotic pressures (2 MPa, 6 MPa, and 10 MPa). The experimental results show that rock strength can be expressed as a function of confining pressure and osmotic pressure. Osmotic pressure can reduce rock strength, while the effect of confining pressure is opposite. With the increase of osmotic pressure, the initial permeability and the peak permeability increase. Compared with the stress-strain curve, the variation of permeability-strain curve is “hysteretic.” For cracked specimens, the stress-strain curve after failure presents a “step down” trend, and the permeability-strain curve appears a “bimodal shape.” The failure pattern changes from shear failure to tensile failure as the osmotic pressure increases. With the increase of osmotic pressure, the crack increases gradually and becomes tortuous and rough. The generation and propagation of cracks gradually develop towards vertical load.

Tightness and stability evaluation of salt cavern underground storage with a new fluid–solid coupling seepage model

The effects of operating pressure and ground stress on the seepage parameters (permeability and porosity) of surrounding rock and stability and tightness of salt cavern have been investigated, and a method is proposed for safe operating pressure classification. In previous works, the analysis of stability and tightness of salt caverns were mostly carried out independently, and few studies have considered the interaction between them. However, when used as a storage space for natural gas, petroleum and brine, there are many fluid–solid coupling problems in salt cavern underground storages. To address these problems, we establish a fluid–solid coupling seepage model based on the relationships between pressure, rock deformation and seepage parameters, and apply it to actual engineering. The coupling analysis results show that the relationship between shrinkage displacement of salt cavern and operating pressure is an exponential function with negative correlation, while the relationship between seepage range of a salt cavern and the operating pressure is a power function with positive correlation. Therefore, the effects of operating pressure on the stability and tightness of salt caverns are diametrically opposite. The seepage range of gas in surrounding formations gradually increases with time, and eventually tends to become stable. Under an operating pressure of 12 MPa and after operating for 30 years, the maximum seepage range and shrinkage displacement of the studied salt cavern are 99.72 m and 1.86 m, respectively. Hence the stability and tightness of this salt cavern meet the requirements of the regulations in China, and a gas leakage accident will not occur in this storage during operation. The seepage parameters of the surrounding formation increase with operating time of gas storage, and the increase in permeability is larger than that of porosity. The research results can provide reference and guidance for safe operation and parameter design of salt cavern underground storage.

Long-term mechanical and acoustic emission characteristics of creep in deeply buried jinping marble considering excavation disturbance

To obtain a complete understanding of the stability of deep caverns after excavation disturbance, a laboratory simulation method for testing surrounding rock under excavation disturbance was systematically proposed. The rock creep mechanical behavior considering excavation disturbance was compared with that of rock without excavation disturbance, and the results show that the long-term rock strength considering excavation disturbance is significantly lower than that without excavation disturbance at a 2400 m depth. Based on this method, a step-loading triaxial creep and associated acoustic emission (AE) test was carried out with a single-stage loading time of 5 days and a total loading time of approximately 50 days. The results show that with an increase in stress level, the transient strain of the deep rock increases nearly linearly, while the axial creep strain rate increases exponentially. The AE monitoring results demonstrate that when the rock creep transforms from the initial creep stage to the steady-state creep stage, and from the steady-state creep stage to the accelerating stage, the AE amplitude will clearly exhibit a band-shaped distribution, and the AE count rate and AE energy rate will show a single peak or multiple peaks. Multistage AE data can be used to determine the long-term strength of rock. In addition, with an increase in the stress level, the microcracks of the deep rock under excavation disturbance transform from closed primary microcracks to initiating, propagating and coalescence secondary microcracks, which ultimately leads to macroscopic shear failure. The above research results can provide a reference for deep resource exploitation and deep cavern stability evaluation.

Analysis of Initial Geostress Field of a Hydropower Project without Measured In-situ Stress

Initial geostress field is an important factor affecting the stress distribution, displacement and failure mode of surrounding rock of underground engineering, and is also one of the important indexes of design for underground works, especially large hydropower projects. At present, the method to obtain the geostress field is mainly based on measured in-situ stress, however some projects are lack of in-situ stress measurement data. To solve this problem, we inversed regional tectonic stress field firstly, and then extracted the boundary load of the project area from the fitted regional tectonic stress field and embedded the underground cavern to be studied in the calculation model to carry out the numerical calculation of the geostress field of the project area. The results show that the direction of regional tectonic principal stress is NNE∼NE, and the average maximum horizontal tectonic stress is about 5.0MPa. The direction of the maximum principal stress in the project area is generally the same as that of the regional tectonic stress field, which indicates that the valley stress field is determined by the regional tectonic stress field. However, due to the influence of river incision, the geostress field of the project area redistributes, and has the basic characteristics of principal stress direction deflection and stress differentiation. The research results provide a basis for the subsequent study of the stability of underground caverns, and have a certain reference value for the method of obtaining initial geostress field of a project without measured in-situ stress.

Fatigue tensile strength of rock salt

Prismatic salt specimens are subjected to four-point bending tests under three different loading conditions: monotonic loading, tension cyclic loading (TC), and compression-tension cyclic loading (CTC). CTC loading specimens exhibit tensile strength about half of those under TC loading. Fatigue strength curves from both loading conditions can be well represented by logarithmic equations. Decreasing loading frequencies by one order of magnitude reduces the fatigue strengths by only 5–10%. Tensile fractures induced by CTC loading are dominated by inter-crystalline separation, and hence result in lower tensile strength, as compared with those under TC loading where cleavage fracturing with higher bonding strength is induced. Strain-time curves from cyclic loading show instantaneous, transient, steady state, and tertiary creep phases. Salt at the crack initiation point can accumulate strain energy density more quickly when it is subjected to CTC loading than under TC loading. Test results under tension to fully reverse compression (CTC) may be more suitable to assess long-term stability of storage caverns, as its loading path would be similar to those that occur at the cavern boundary.

Creep Properties and Constitutive Model of Salt Rock

Due to the advantages of low porosity, low permeability, high ductility, and excellent capacities for creep and damage self‐healing, salt rock is internationally considered as the ideal medium for underground storage of energy and disposal of radioactive waste. As one of the most important mechanical properties of salt rock, creep properties are closely related to the long‐term operation stability and safety of salt rock underground storage cavern. A comprehensive review on the creep properties and constitutive model of salt rock is put forward in this paper. The opinions and suggestions on the research priority and direction of salt rock′s mechanical properties in the future are put forward: (1) permeability variation of salt rock under the coupling effect of temperature and stress; (2) damage mechanism and evolution process under the effect of creep‐fatigue interaction and low frequency cyclic loading; (3) microdeformation mechanisms of salt rock and the relationship between microstructure variations and macrocreep behavior during creep process; (4) the establishment of the creep damage constitutive model with simple form, less parameters, easy application, and considering the damage self‐healing ability of salt rock simultaneously.

Failure analysis for gas storage salt cavern by thermo-mechanical modelling considering rock salt creep

Salt cavern is ideal vessel for underground gas storage due to the high deliverability. During cyclic gas operations of injection-and-withdrawal, the temperature change in salt cavern imposes thermal stress on cavern wall. The temperature change, together with the pressure change in salt cavern, causes the effective stress on cavern wall to alter, leading to variation of creep rate. In this study, a coupled thermo-mechanical model is proposed for failure analysis of salt cavern in rock salt prone to creep deformation. Six cases that consider rock salt with different creep tendency have been conceived for the coupled thermo-mechanical model to investigate the stability of the salt cavern in terms of cavern convergence and failure indices. The results indicate that the affected region by cyclic pressure and temperature is up to 10 m inside the rock salt from the cavern wall. It is also revealed from the results that largest displacement occurs on the top of the cavern, indicating that the cavern top is most liable to deformation damage. Although convergence of cavern fluctuates over the injection and withdrawal sessions, long-term convergence of cavern depends on the creep tendency of rock salt rather than the cyclic loading; rock salt with stronger creep tendency leads to larger cavern convergence. The results also demonstrate that there is stress concentration on the top and bottom of the cavern; the cavern wall is likely subject to shear failure according to Mohr‒Coulomb failure criteria, however, the cavern is unlikely to fail due to expansion according to Drucker‒Prager failure criteria. Both Mohr‒Coulomb and Drucker‒Prager failure indices are smaller in the cases where rock salt has stronger creep tendency. The proposed thermo-mechanical model provides an approach for evaluation of long-term stability of underground salt caverns.

Dynamic Stability Evaluation of Underground Powerhouse Cavern Using Microseismic Monitoring

Construction of any excavation adds to the re-distribution of stresses around abutment and crown. This is stabilised by providing extra reinforcement. When a large underground cavern is excavated, stress re-distribution around abutment is obvious. Underground powerhouse cavern of Tala Hydropower Plant with adverse geological setting is one such cavern which encountered failure of a number of rock-bolts and series of other types of rockmass failure during its construction. It puts a question mark on the long-term stability of this powerhouse. To evaluate the dynamic stability of the rockmass (against induced stresses) around this powerhouse, microseismic monitoring network was installed. Microseismic data has been analysed in terms of static and dynamic parameters to assess the overall cavern stability. Spatio-temporal analysis of microseismic events could identify the potential risk zones which has significant cluster of events. To assess the overall stability due to these induced events, the ratio of transverse wave and longitudinal wave energy of these events was used as an index. It is found that cluster of events had shear failure. These events are basically of small non-damaging type and aid in releasing the built-up induced stresses. The slope of log–log plot of energy versus moment has a very high value. This indicates a stiffer rockmass which adds to its capability to absorb the induced stresses and aid to the overall stability of the region. Analysis reveals that no major seismic events are expected around this rockmass cavern and there is no dynamically unstable region against induced stresses.

Seismic stability of an underground cavern in jointed rock mass based on the discrete fracture network

In the construction of underground structures in high seismic intensity areas, it is inevitable that a fault fracture zone will pass through the construction site. Therefore, it is very important to evaluate and analyze the stability of the unstable block, which is cut by the fault and the dominant joint group. In this study, the discrete element method was used to simulate the seismic response of an underground powerhouse of a hydropower station near a high seismic zone. The discrete fracture network (DFN) model was introduced to simulate the joint group. Moreover, to optimize the number of joints and improve computational efficiency under dynamic conditions, the joints in the DFN with little influence on the stability of the cavity, were deleted by the algorithm, and a nonlinear time-history response analysis method was used for dynamic calculation. Based on the simulation results obtained, it was possible to classify the instability of the unstable block, and corresponding suggestions for construction were provided. Additionally, these results can be used as a reference for the seismic design of similar underground structures.

Study on the Long-Term Reliability of the Tangshuiya Reservoir Dam

Du, P. and Wang, Y.Y., 2020. Study on the long-term reliability of the tangshuiya reservoir dam. In: Bai, X. and Zhou, H. (eds.), Advances in Water Resources, Environmental Protection, and Sustainable Development. Journal of Coastal Research, Special Issue No. 115, pp. 556-560. Coconut Creek (Florida), ISSN 0749-0208.Weifang section of the Yellow River Diversion Project is a comprehensive large-scale water conservancy project, which mainly solves the water supply of Weifang, and also uses farmland irrigation, flood control and hydropower generation. The tangshuiya reservoir dam is a gravity earth rock dam. The research on the rheological properties of rock mass in the project area is the basis of analyzing the long-term stability of slope and cavern. The creep pattern under different levels of tri-axial stress has been experimentally studied. A creep damage model is established. By comparing the fitting result with the experimental data, the model can accurately predict the creep characteristics of shale at different stress levels. Based on the research results of this paper, it can provide guidance for flood control and disaster reduction, improve the creep resistance performance of the reservoir dam and river embankment, and avoid major disasters due to creep.

The mechanical behavior of rock salt under different confining pressure unloading rates during compressed air energy storage (CAES)

Compressed air energy storage (CAES) is a promising energy conversion storage technology and underground salt caverns are recognized as the appropriate storage places for it. The reduction of air during the withdrawal periods leads to the surrounding rock being unloaded, which threatens the stability of the salt cavern. To investigate the mechanical behavior of surrounding rock under different unloading rates, the confining pressure unloading tests in six unloading rates have been conducted on rock salt. Results show the rock salt undergoes volume expansion at the beginning of unloading due to the reduction of elastic compression strain. The volume strain expands rapidly beyond a threshold, i.e. accelerated strain point. The stresses at failure point and accelerated strain point show that a low unloading rate is beneficial to enhance rock strength. Nevertheless, larger inelastic strain appears in a lower unloading rate. The effect of the unloading rate on rock salt failure mode is that rapid unloading will promote crack growth. Moreover, the relationship between the unloading rate of surrounding rock and the air withdrawal rate is established. The outcome derived from the research can provide some instruction and inspiration for air withdrawal rate design in CAES.

Stability analysis of Cavern No. 5 with the largest span in Heidong large ancient underground quarry

The Heidong Cavern Group of China includes 21 large-scale caverns that have remained stable for 1400 years. Cavern No. 5 of the Heidong Cavern Group is characterized by an unsupported span of up to 92 m, which is considered to be the largest span in a single underground artificial rock cavern. Cavern No. 5 may have gone through a period during which it was the only cavern and a period during which it had adjacent caverns; recent investigations have proved that Cavern No. 5 is suffering from considerably serious partial damage. The factors affecting the stability of Cavern No. 5, including waste rock accumulations, rock mass structures, surrounding rock quality, and cavern structures, were analyzed to prevent further damage and analyze the influence of the existence of adjacent caverns on the stability of Cavern No. 5. The numerical analysis software MIDAS GTS (Geotechnical and tunnel analysis system) was used to simulate and compare the stability of Cavern No. 5 under the single-cavern and adjacent-cavern cases. Research proves that when this cavern is independently considered and the remaining caverns are excluded, the arched roof structure is considerably favorable to ensure the cavern’s stability and the horizontal compressive stress of the roof becomes approximately equivalent to the vertical tensile stress. The horizontal forces generated by the cavern’s inclined roof contribute to the cavern’s stability. The horizontal compressive stress area of the roof and the effect of the arched structure decrease when adjacent caverns are present, causing the roof to become prone to tensile failure. These results provide a reference for modern underground engineering construction and a theoretical basis for long-term protection of the large and ancient Heidong Caverns.

Mechanical and Microstructural Properties of Alkali Wastes as Filling Materials for Abandoned Salt Caverns

Alkali solid waste from soda industries has previously been stacked on surface and contaminated soil and groundwater. In China, an engineering method of filling alkali wastes into abandoned salt caverns is adopted. This method can solve the problem of waste disposal and the potential geological disasters associated with salt caverns. In such filling projects, the engineering properties of alkali wastes are important for the safety and stability of salt caverns. In this study, the microstructural and mechanical properties of alkali wastes were investigated. X-ray diffraction (XRD) and scanning electron microscope (SEM) were used to analyze the microstructure and mineralogy of such wastes. Direct shear tests were conducted to evaluate the mechanical performance and to obtain the strength parameters of alkali wastes. The basic physical characteristics, microstructure, mineral compositions, and mechanical properties of alkali wastes vary significantly at different burial depths. Moreover, the strength properties of alkali wastes are close to those of the two other common filling materials, sandstone–mudstone mixtures and tailings. The research results can be used as a reference for similar filling projects and can also be beneficial for the cyclic utilization of alkali wastes. An engineering method that utilizes alkali solid wastes to fill abandoned salt caverns is introduced, and the engineering properties of alkali wastes are studied.

Stability analysis of underground water-sealed oil storage caverns in China：A case study

The stability of underground water-sealed oil storage caverns is of great importance for safe excavation and operation. To analyze the scope of the failure zone and underground cavern stability accurately, a new method was developed that integrates the rock tunneling quality index Q-system and stability graph method with 3D laser scanning and numerical simulation. The point cloud data were obtained by 3D laser scanning, and the cavern model was built by using DIMINE software, which directly shows the 3D shape of the cavern. The rock mass physical and mechanical parameters and the corresponding stability coefficients were obtained based on Q-system and stability graph method. The plastic zone distribution and deformation characteristics of surrounding rock were analyzed through numerical simulation. Then, the corresponding relationship between caving zone and plastic zone was determined by comparing the numerical simulation results with the 3D laser scanning contour. The new method provides a reliable way to analyze the stability of the underground water-sealed oil storage cavern and also will helpful to design or optimize the subsequent support.

Research on the Stability and Treatments of Natural Gas Storage Caverns With Different Shapes in Bedded Salt Rocks

Because of complex geo-conditions, many caverns by solution mining in bedded salt rocks have different irregular shapes. To verify the feasibility of using irregular-shaped caverns for underground gas storage (UGS), four typical cavern-shapes are selected, and the stability of each type is evaluated and compared by using the numerical simulation methods. The simulation results show that the UGS salt cavern with irregular wall shape has the lowest volume shrinkage and displacement of the wall rock, but larger plastic zones appear in their overhanging and concave parts. Ellipsoid-shape cavern has the best stability. Cylinder-shape cavern and cuboid-shape cavern have the poorest stability. In these two types of caverns, large deformations occur in the roof and sidewall, which pose a great potential of inducing collapse in the wall rock. By comparison of the stability characteristics in different positions of the wall rock, we found that the roof shape has a much greater influence than the sidewall on the stability of a cavern. The roof must be designed as an arch to improve cavern stability. Treatments to improve the stability of irregularly shaped caverns by changing operational pressure and utilization way or by modifying the caverns' shape are also discussed. So, this study not only determined the stability state of different shaped caverns for gas storage in bedded salt rocks, but also provides the ways to modify the irregular-shaped caverns for storage applications.

Research on the Layered Rock Mass Excavation Relaxation Stability of Underground Caverns

In order to compare the influence of rock mass parameters weakening on the deformation and stability of excavation caverns in layered rock mass, based on power generation system caverns of a hydropower station, the stability and deformation of the caverns is analyzed. The results show that the mechanical characteristics of the structure surface play a major role in controlling the stability of caverns. And the displacement and yield zone value of plan 3, which adopt elastic-plastic softening model, are significantly larger than other two. The method which consider the residual strength of structure surface is more suitable for the excavation calculation of layered rock mass cavern.

Flowchart of DEM Modeling Stability Analysis of Large Underground Powerhouse Caverns

The power of the Distinct Element Method (DEM) in solving the stability of underground powerhouse caverns (UPC) is discovered, but when does the DEM need to be adopted? What problems can be solved? The flowchart is provided and applied to analyze the stability of UPC in this paper. With the guide of the flowchart, the damage index (Di) is used as a failure type (gravity‐controlled or stress‐induced) judgment indicator. Through the calculation of three typical engineering, the problems of random blocks stability, dynamic calculation, and support system evaluation are studied, respectively, with the help of the DEM code 3DEC. The method and results of this paper can give reference to engineering projects of its category.

基于FLAC 3D的大型地下洞室群分步开挖稳定性数值模拟研究

基于FLAC 3D的大型地下洞室群分步开挖稳定性数值模拟研究

Stability Analysis of Salt Cavern Gas Storage Using 2D Thermo-Hydro-Mechanical Finite-Element Software

Ensuring the stability and integrity of underground gas storage salt caverns is a very complicated subject due to the non-linear and time-dependent behavior of rock salts under complicated thermal and mechanical loading conditions. For this reason, pressure and temperature fluctuations in the caverns and their surrounding strata must be integrated into the analysis and the numerical tools that are used for this purpose. LOCAS, a 2D axisymmetric finite-element code, dedicated to the stability analysis of underground salt spaces, was applied to assess the effects of various operating and geometrical parameters on the cavern behavior. In this paper, we aimed to give an overall assessment of the behavior of the salt caverns used for natural gas storage. In this work, some specific loading scenarios were considered first, followed by thorough parametric and sensitivity analyses to reveal the impacts of the geometrical parameters and operational parameters involved on the behavior of salt caverns using the modern stability criteria. The findings showed that the onset of dilation was more likely to happen within the first cavern life cycle when pressure dropped to the minimum level. As for the potential of tension occurrence in the surrounding rock, this is more likely to happen by increasing the number of operation cycles, especially in the upper one-third of the cavern wall. Finally, it was seen that the cavern depth and minimum cavern internal pressure had even more important influences than the others on the salt cavern behavior.

Stability study and optimization design of small-spacing two-well (SSTW) salt caverns for natural gas storages

Abstract A small-spacing two-well salt (SSTW) cavern is a relatively new type of cavern. As this type of cavern is less frequently used in underground natural gas storages, study on its stability is quite limited. To promote the application of this type of cavern and accelerate the construction of underground salt cavern gas storages, stability analysis and optimization design of this type of cavern are indispensable to the security of underground natural gas storages. In this paper, numerical calculation and parameters analysis were performed to analyze the effects of internal gas pressure and ratio of long axis to short axis (RLS) on the stability of SSTW caverns. The results show that internal gas pressure has an influence on the stability of SSTW caverns and low pressure should be limited in terms of duration and magnitude. And the ratio of long axis to short axis (RLS) has limited influence on the stability of a SSTW cavern by ways of volume loss rate and roof settlement. Overall, the SSTW cavern behaves similar stability state with that of a SW cavern. In addition, a flatter roof shape appears under greater Ratio value (RLS) and thus a thicker roof salt rock thickness is suggested for engineering design. The most different between them are the waist displacement along short axis, therefore the pillar width close to the adjacent cavern should be a certain lengthened at this direction.