Rock Caverns Research Articles

Instability risk analysis and risk assessment system establishment of underground storage caverns in bedded salt rock

Stability is an important part of geotechnical engineering research. The operating experiences of underground storage caverns in salt rock all around the world show that the stability of the caverns is the key problem of safe operation. Currently, the combination of theoretical analysis and numerical simulation are the mainly adopts method of reserve stability analysis. This paper introduces the concept of risk into the stability analysis of underground geotechnical structure, and studies the instability of underground storage cavern in salt rock from the perspective of risk analysis. Firstly, the definition and classification of cavern instability risk is proposed, and the damage mechanism is analyzed from the mechanical angle. Then the main stability evaluating indicators of cavern instability risk are proposed, and an evaluation method of cavern instability risk is put forward. Finally, the established cavern instability risk assessment system is applied to the analysis and prediction of cavern instability risk after 30 years of operation in a proposed storage cavern group in the Huai’an salt mine. This research can provide a useful theoretical base for the safe operation and management of underground storage caverns in salt rock.

Estimation of the REV Size and Equivalent Permeability Coefficient of Fractured Rock Masses with an Emphasis on Comparing the Radial and Unidirectional Flow Configurations

A method to estimate the representative elementary volume (REV) size for the permeability and equivalent permeability coefficient of rock mass with a radial flow configuration was developed. The estimations of the REV size and equivalent permeability for the rock mass around an underground oil storage facility using a radial flow configuration were compared with those using a unidirectional flow configuration. The REV sizes estimated using the unidirectional flow configuration are much higher than those estimated using the radial flow configuration. The equivalent permeability coefficient estimated using the radial flow configuration is unique, while those estimated using the unidirectional flow configuration depend on the boundary conditions and flow directions. The influences of the fracture trace length, spacing and gap on the REV size and equivalent permeability coefficient were investigated. The REV size for the permeability of fractured rock mass increases with increasing the mean trace length and fracture spacing. The influence of the fracture gap length on the REV size is insignificant. The equivalent permeability coefficient decreases with the fracture spacing, while the influences of the fracture trace length and gap length are not determinate. The applicability of the proposed method to the prediction of groundwater inflow into rock caverns was verified using the measured groundwater inflow into the facility. The permeability coefficient estimated using the radial flow configuration is more similar to the representative equivalent permeability coefficient than those estimated with different boundary conditions using the unidirectional flow configuration.

Direct Tensile Test on Brittle Rocks with the Newly Developed Centering Apparatus

Failure of brittle rock normally initiates from tensile fracture due to its extraordinary lower resistance to tension rather than to compression. Intensive study on the deformability and accurate acquirement of the mechanical properties of rock under tensile stress is crucial for the stability analysis of rock slope and rock caverns. A new set of centering apparatuses, which consist of the specimen bonding part and the axial pulling part, were developed in this study. The misalignments when bonding the specimen and conducting the uniaxial tensile test can be effectively eliminated. Direct tensile tests were conducted on various brittle rocks cored from several large-scale hydropower station sites in China. The tensile strength, tensile elastic modulus, maximum tensile strain, and complete stress-strain curves were successfully obtained. The direct tensile test results indicated that the fracture surface formed in the central part of the specimen, which was under uniform tensile stress. The fracture surfaces were mostly perpendicular to the specimen axis. Fresh fractures without accumulation of rock fragments can be observed and characterized as tensile brittle failure. The microscopic failure modes of brittle rocks under the direct tensile test were not unique but consisted of the intragranular and intergranular fractures. The direct tensile strength (DTS) was generally lower than the Brazilian tensile strength (BTS) (DTS/BTS = 0.60~0.71). The direct tensile test was therefore recommended for the measurement of the tensile mechanical properties in order to ensure the safety of rock engineering.

Propagation Characteristics of Blasting Stress Waves in Layered and Jointed Rock Caverns

The existence of joint fissures makes explosive actions between rock masses more complex. Therefore, it is of great significance to carry out experiments studying blasting stress waves propagating in jointed rock masses. Based on the Froude Similarity principle, the geological mechanical models of intact rock masses and jointed rocks have been proposed. A blasting vibration experiment was carried out and demonstrated that the propagation of the blasting stress waves and changing structures have an important relationship. A numerical simulation of the blasting stress wave propagation law in a layered jointed rock mass was carried out. This study found that with an increase in the joint angle, the peak velocity of blasting stress wave, transmission coefficient and reflection coefficient all gradually increased, while the attenuation coefficient gradually decreased. With an increase in joint spacing, the attenuation rate of the blasting stress waves increased.

Numerical simulation for the coupled thermo-mechanical performance of a lined rock cavern for underground compressed air energy storage

Compressed air energy storage (CAES) is a technology that uses compressed air to store surplus electricity generated from low power consumption time for use at peak times. This paper presents a thermo-mechanical modeling for the thermodynamic and mechanical responses of a lined rock cavern used for CAES. The simulation was accomplished in COMSOL Multiphysics and comparisons of the numerical simulation and some analytical solutions validated the thermo-mechanical modeling. Air pressure and temperatures in the sealing layer and concrete lining exhibited a similar trend of 'up–down–down–up' in one cycle. Significant temperature fluctuation occurred only in the concrete lining and sealing layer, and no strong fluctuation was observed in the host rock. In the case of steel sealing, principal stresses in the sealing layer were larger than those in the concrete and host rock. The maximum compressive stresses of the three layers and the displacement on the cavern surface increased with the increase of cycle number. However, the maximum tensile stresses exhibited the opposite trend. Polymer sealing achieved a relatively larger air temperature and pressure compared with steel and air-tight concrete sealing. For concrete layer thicknesses of 0 and 0.1 m and an initial air pressure of 4.5 MPa, the maximum rock temperature could reach 135 °C and 123 °C respectively in a 30 day simulation.

Use of alkali-silica reactive sedimentary rock powder as a resource to produce high strength geopolymer binder

This paper reports on an innovative way to utilize alkali-silica reactive (ASR) rocks as a natural resource to produce high strength geopolymer binder. Excavation of the Jurong rock caverns in Singapore has produced large quantities of sedimentary rocks. These rocks, however, cannot be used for ordinary Portland cement concrete production due to their ASR reactivity. An alternative way to beneficially utilize these rocks is to produce geopolymer binder. The excavated rocks were classified based on petrography into four types and then converted to powder form in a sequence of steps. The rock powders were used to synthesize geopolymers by replacing metakaolin in different replacement ratios. Results showed that geopolymer binder with 67wt% rock powder and only 33wt% metakaolin can achieve a high compressive strength of 80MPa. Incorporation of sedimentary rock powder enhanced the compressive strengths by 15–30% as compared to pure metakaolin geopolymers. Microstructure analysis revealed that the enhancement in compressive strengths were primarily due to the densification of binder by filling the voids in matrix with rock powders.

A method to estimate the pressure arch formation above underground excavation in rock mass

Arching effect has been attributed as a factor of roof stability of underground rock excavations, which affects excavation geometry and rock support. In this paper, a series of numerical investigations were performed using the 2D finite element method (FEM) to study the formation and features of pressure arches. Based on horizontal and vertical stress distributions, three characteristic lines of the arching area were defined, i.e., the outer boundary line, the inner boundary line and the centroid line. It is found that the height of the inner boundary of the arching area is an indicator of the roof stability of underground excavations. Furthermore, detailed parametric studies including the Geological Strength Index (GSI from 20 to 80), the overburden depth (H from 40 m to 420 m), in-situ stress ratio (k0 from 0.8 to 3) and excavation roof rise-to-span ratio (h/B ratio from 0.1 to 0.5) were conducted to discuss their influences on the arching area. The results show that the initial stress state (H and k0) has more significant effects on the roof stability of underground rock caverns than the GSI and cavern geometry (h/B). A semi-circular roof is optimal design scheme for underground excavation in surrounding mass if only considering the cavern roof stability.

The development of techno-economic models for large-scale energy storage systems

The development of a cost structure for energy storage systems (ESS) has received limited attention. In this study, we developed data-intensive techno-economic models to assess the economic feasibility of ESS. The ESS here includes pump hydro storage (PHS) and compressed air energy storage (CAES). The costs were developed using data-intensive bottom-up models. Scale factors were developed for each component of the storage systems. The life cycle costs of energy storage were estimated for capacity ranges of 98–491 MW, 81–404 MW, and 60–298 MW for PHS, conventional CAES (C-CAES), and adiabatic CAES (A-CAES), respectively, to ensure a market-driven price can be achieved. For CAES systems, costs were developed for storage in salt caverns hard rock caverns, and porous formations. The results show that the annual life cycle storage cost is $220–400 for PHS, $215–265 for C-CAES, and $375–480 per kW-year for A-CAES. The levelised cost of electricity is $69–121 for PHS, $58–70 for C-CAES, and $96–121 per MWh for A-CAES. C-CAES is economically attractive at all capacities, PHS is economically attractive at higher capacities, and A-CAES is not attractive at all. The developed information is helpful in making investment decision related to large energy storage systems.

Water Curtain System Pre-design for Crude Oil Storage URCs: A Numerical Modeling and Genetic Programming Approach

In this paper the main criteria of the water curtain system for unlined rock caverns (URCs) is described. By the application of numerical modeling and genetic programming (GP), a method for water curtain system pre-design for Iranian crude oil storage URCs (common dimension worldwide) is presented. A comprehensive set of numerical simulations is performed using the finite element based commercial software (COMSOL 5.1) where the results are used as database for genetic programming. Describing equations of water inflow to the filled and empty caverns and water production rate of water curtain boreholes are generated using GP. By equating the proposed equations to each other, water curtain system can be pre-designed. Relative error of the generated GP equations shows their ability and accuracy. Applying a standard regression coefficient method, sensitivity analysis of parameters related to water curtain performance and water inflow to the caverns is performed as well. The results help the design of the water curtain system for crude oil storage caverns worldwide.

22.03: Acoustic ceiling designed for trains passing at 250km/h: Holmestrand – The first station in the world inside a mountain entrance hall

ABSTRACTThe Holmestrand station will be located inside a rock cavern, with two separate tracks for platforms and two tracks for through trains in the same hall. Hence the entrance hall will be designed to allow high speed trains to pass at 250 km/hour, creating challenges concerning pressure, fatigue, wind speed and noise levels within the station hall.The steel structure comprises of a suspended ceiling inside the station of more than 12,000 m2. The ceiling shall prevent drip from the rock and act as an acoustic silencer. In addition, great emphasis has been placed on the architectural configuration. The primary steel is designed for a life time of 100 year.The ceiling is divided into 3 parts, one above each platform and one above the tracks.Main beams and cross beams form a grid that is suspended from the mountain by individual hangers of adjustable lengths. On top of this grid a corrugated aluminium plate prevents drip from the rock, and below this grid acoustic panel is fitted to reduce noise problems. All steel between the rock surface and the corrugated plate on top of the ceiling consists of duplex steel. Below the corrugated steel plate the main and cross beams are hot dip galvanized (HDG) to minimum 200 μm and the acoustic panels are sandwich elements with perforated steel surface. All joint connections are bolted and most of them with preloaded stainless steel bolts. There are many friction grip connections that lead to testing prior to assembly of the preloaded stainless steel bolts and the friction surfaces for the different material combinations.In addition to duplex steel, HDG steel and aluminium the ceiling above the platforms is also supported by weathering steel for architectural reasons. Thus the detailing required special attention in order to secure the required service life without accelerated corrosion from either galvanic effects as combination of different materials are used or the detailing of e.g. weathering steel.The new 14 km double track through the town of Holmestrand will greatly improve the rail services on the Vestfold line. Rambøll is responsible for designing 7 km of this line. Total Cost: NOK 6.6 billion.The work for Rambøll started in 2010 and the station opened on November 27, 2016

폐밀봉선원 처분시스템 예비 폐쇄후 안전성평가

An optimum disposal plan of disused sealed radioactive sources (DSRSs) should be established to ensure long-term disposal safety at the low- and intermediate-level radioactive waste (LILW) disposal facility in Gyeongju. In this study, an optimum disposal system was suggested and preliminary post-closure safety assessment was performed. The DSRSs disposal system was composed of a rock cavern and near surface disposal facilities at the Gyeongju LILW disposal facility. The assessment was conducted using GoldSim program, and probabilistic assessment and sensitivity analysis were implemented to evaluate the uncertainties in the input parameters of natural barriers. Deterministic and probabilistic calculations indicated that the maximum dose was below the regulatory limits (<TEX>$0.1mSvyr^{-1}$</TEX> for the normal scenario, <TEX>$1mSvyr^{-1}$</TEX> for the well scenario). It was concluded that the DSRSs disposal system would maintain environmental safety over a long-time. Moreover, the partition coefficient of Np in host rock, Darcy velocity in host rock, and density of the host rock were the most sensitive parameters in predicting exposure dose in the safety assessment.

Low cost colorimetry for assessment of fire damage in rock

Tunnels and caverns excavated for the purpose of mining tend to be designed for short term use and would usually be unlined. Fire hazard within a mining cavern or tunnel poses concerns on the structural integrity of the surrounding rock. Hence, a fast, non-destructive method for assessing the strength of a rock mass after a fire would be useful in deciding whether to salvage a mining operation or to abandon it. Colorimetry as a technique to estimate the heating temperature of concrete has proven useful. However, the use of colorimetry imposes a need to balance ease of use, such as low cost digital cameras and computer scanners, with accuracy achievable with expensive spectrometers and spectrophotometers. This technical note shall investigate the use of a Digital Single Lens Reflex (DSLR) camera and MATLAB as a means of performing non-destructive testing on rocks exposed to temperatures of between 200°C to 500°C to estimate the degradation of the rocks' tensile strength. The CIE L*a*b* color space is used. The effects of soot and smoke on the rocks are also simulated using high temperature grease to mimic real world conditions. The Brazilian tensile strength and P-wave velocity are two other properties investigated in this technical note. The results suggest that the a* and b* color values have an almost linear relationship with the heating temperature of the rock, whereas the tensile strength of the rock has a quadratic relationship. Additional tests indicate the robustness of the colorimetry process as the simulated smoke and soot damage did not significantly affect the relationship between heating temperature, a* and b* color values, provided the surface of the rock was cleaned after the fire event. The mechanism of the strength degradation may be attributed to generation of microcracks due to thermal loading, while the color change is attributed to either chemical changes within the rock, or scattering of light due to the generated microcracks. A key finding of this study is that the colorimetry process outlined is not only relatively cheap to use, as compared to expensive spectrometers and spectrophotometers, but also fast, as the photo taking process can be completed within 10min. This makes it easier for multiple teams to be deployed after a fire incident in an underground rock tunnel or cavern, minimising the amount of time that workers need to spend in potentially hazardous environment.

Design and construction of shaft for rock caverns in Singapore

Access shaft is of critical importance to the construction and operation of underground rock caverns. It usually has a relatively large cross-section and penetrates through fill materials, soil layers, and weathered rocks before reaching the caverns excavated in solid bedrock. In this paper, the design and construction of vertical shafts are reviewed in terms of diameter, depth, geological conditions, and support structure. Three shaft alternatives, namely alternative I: vertical shaft with spiral roads, alternative II: upper shaft with spiral roads & lower tunnels, alternative III: plain shaft, are proposed based on a simplified geological profile of the Jurong formation, Singapore. The advantages and limitations of the three types of shafts are discussed. The key issues relating to shaft design and construction, such as the shaft sinking, water control, support structure, are also discussed with a series of solutions provided, such as the sequential excavation, pre-grouting and diaphragm walls.

Thermodynamics and economic performance comparison of three high-temperature hot rock cavern based energy storage concepts

Employing a thermal energy storage (TES) as a medium for storing power in an energy storage system was recently proposed and analyzed in two different configurations. The first proposal is employing the TES as the boiler of a Rankin cycle based (RCB) energy storage plant. In this configuration, heat production along with power production may or may not be an objective (RCB1 and RCB2). The other proposal is employing the TES as the combustion chamber of an Erickson cycle for energy storage applications (ECB). In this work, a detailed energy, exergy and economic performance comparison between the three systems is accomplished, and the positive and negative features of each of them are addressed. All of the three systems are designed for a 100 MWp wind power in Denmark as the case study. Although it is demonstrated that the application of the ECB system is limited to locations with high heating demand, it outperforms both of the RCB systems due to its very fast response and also the high efficiency that it offers. The overall energy efficiencies of the RCB1, RCB2 and ECB systems are 85%, 32% and 80% while their exergy efficiencies are 47%, 58% and 58%, respectively.

Principles of rockbolting design

This article introduces the principles of underground rockbolting design. The items discussed include underground loading conditions, natural pressure zone around an underground opening, design methodologies, selection of rockbolt types, determination of bolt length and spacing, factor of safety, and compatibility between support elements. Different types of rockbolting used in engineering practise are also presented. The traditional principle of selecting strong rockbolts is valid only in conditions of low in situ stresses in the rock mass. Energy-absorbing rockbolts are preferred in the case of high in situ stresses. A natural pressure arch is formed in the rock at a certain distance behind the tunnel wall. Rockbolts should be long enough to reach the natural pressure arch when the failure zone is small. The bolt length should be at least 1 m beyond the failure zone. In the case of a vast failure zone, tightly spaced short rockbolts are installed to establish an artificial pressure arch within the failure zone and long cables are anchored on the natural pressure arch. In this case, the rockbolts are usually less than 3 m long in mine drifts, but can be up to 7 m in large-scale rock caverns. Bolt spacing is more important than bolt length in the case of establishing an artificial pressure arch. In addition to the factor of safety, the maximum allowable displacement in the tunnel and the ultimate displacement capacity of rockbolts must be also taken into account in the design. Finally, rockbolts should be compatible with other support elements in the same support system in terms of displacement and energy absorption capacities.

A numerical procedure for modeling the seepage field of water-sealed underground oil and gas storage caverns

Underground rock caverns have been considered as an effective technique for oil storage, commonly with water curtains for maintaining a desired and stable groundwater level and preventing the products from escaping. This study presents a three-dimensional analysis of groundwater flow in the surrounding rocks of water-sealed underground oil storage caverns. The water curtain consisting of numerous boreholes of small diameter, long extension and dense spacing is accurately modeled with a substructure technique. The influence of the oil vapor or mixed gas pressure in the caverns on the groundwater flow is considered by integrating it into the water head and complementary boundary conditions. The proposed method is applied to assess the performance of the water curtain system in the Huangdao oil storage caverns, the first-built underground oil storage project in China. The numerical results show that the gas pressure in the caverns leads to development of gas invasion zones around the cavern roofs, which decreases the thickness of the groundwater barrier and increases the risk of oil and oil vapor leakage. Based on the numerical observations, the design criteria for the water curtain system are discussed and modified.

Geotechnical Investigations and Support Design of Underground Pump House Cavern: A Case Study from Lift Irrigation Project

For better rock mass classification and support design of supporting system, geotechnical investigations were carried out for the proposed 196.80 m long, 50.30 m high and 25.24 m wide underground pump house cavern of a lift irrigation scheme. Geotechnical site investigations, rock support design, excavation and related works and modification of supports design according to the observational construction method are the principal activities for the construction of underground rock cavern. The investigation includes engineering geological mapping, geological logging of drill holes, in situ permeability test and laboratory testing on core samples. Detailed geotechnical investigations carried out supporting the feasibility of underground excavations of cavern within the hillock. Support system recommended is the rock bolt and steel fibre reinforced shotcrete based on empirical approaches and their capacity is determined. Capacity of support system was conducted to evaluate the efficacy of the proposed support system.

A regional-scale engineering geological study for selecting suitable rock masses for constructing unlined oil storage caverns in Southern Zagros, Iran

The southern section of the Zagros zone in southwestern Iran, a well-known sedimentary basin, was investigated to address two key questions: (1) whether it would be feasible to construct large unlined rock caverns (URCs) in moderate-quality sedimentary rocks of this area for the storage of crude oil, and (2) which parts of this wide area were most suitable for constructing such huge underground structures. To answer these questions, first, the geological, hydrogeological and engineering geological properties of typical rock units in the area were investigated, and preliminary analyses were performed to check the stability, permeability and water circulation conditions of typical rock masses at depth. Next, geographical information system (GIS) techniques were applied to select the best regions for constructing the URCs. Parameters including geological characteristics, governmental restrictions, and proximity to facilities were selected as important factors for area zonation and for preparing a suitability map for constructing URCs for storing crude oil. The results of the study indicated that limestone units of the Asmari and Sarvak Formations at a moderate depth (200–300 m) were the most appropriate rock units for construction of URCs with a cross-section area of around 500 m2. In addition, the suitability map showed that two small regions in the central and northern parts of the study area were best suited for URC construction. The key advantages of these regions include a wide distribution of surface exposures of good-quality stratigraphic units (Asmari and Sarvak Formations), limited distribution of active tectonic structures such as salt domes and faults, and proximity to crude oil facilities.

Dynamic energy, exergy and market modeling of a High Temperature Heat and Power Storage System

A novel energy storage system that produces both electricity and heat at high efficiencies and takes advantage of a high temperature hot rock cavern thermal energy storage was recently introduced and designed. This study aims at evaluating the performance of the system in terms of energy and exergy efficiencies under realistic operational conditions where the storage supports a number of wind turbines over a long period. The potential value creation of the energy storage system in the local electricity and heat markets is also assessed. The Western part of Denmark with its high number of wind turbine plants and flexible electricity and heat markets have been chosen for the case study of this work. Having both forecasted and realized wind power generation as well as energy prices for the recent years, the system is designed with rigor and a smart bid strategy for the power plant equipped with the energy storage unit for day-ahead and intra-day markets is determined. The results show that the system is able to compensate the fluctuations of wind power plants, and present high annual overall energy and electricity efficiencies of 80.2% and 31.4% and exergy efficiency of 56.1%.

Cavern Design Philosophy under High Stress Rock Condition a Case Study in Sri Lanka

For stabilizing of rock cavern stress condition and orientation, rock properties and cavern geometry have determinant effects. Numerical and analytical study and knowledge about self-support arching can assist to designer in appropriate understanding of cavern and ground interaction. The comparative studies by E. Hoek, showed that the mushroom shaped cavern was not an acceptable design because of high value stress concentration. The elliptical cavern provide best stress distribution. In this research 5 basic concept in a case study cavern design under high stress condition has been studied. Orientation of cavern and cross section shape have a dominant influence in stress redistribution and forming self-support arch, respectively. Very small tangential stress may create radial crack in periphery of the opening. If the rock mass is hard and brittle, rock burst problem may arises in the area of high compressive stress when the rock mass strength is lesser than the imposed compressive stress. If the strength of rock is low compare to the stresses on it. Instability problems may arise after few years of excavation. Uma Oya underground hydro power project has chosen as a case study. High value of horizontal stress (k>1) might cause buckling effect in walls. Result indicates that high value of horizontal stress might cause local tensile spalling. Plastic zones extend to the pillar between to caverns more than 30 to 1.5 m in the roof. Even after rock bolting, there is considerable tensile zone. About 5 m deep tensile zones are calculated in the walls and a tensile zone of 2 m deep in the roof. The thickness of arching are calculated 4.8 to 5.5 for power house and 1.6 to 1.7 m for transformer cavern, via FEM analysis.