Large Caverns Research Articles

Numerical prediction of unloading in underground caverns excavation based on damage mechanics

Unloading is a quite common phenomenon in the excavation process of large underground caverns, but the formed unloading region in the surrounding rock will bring an extremely adverse effect on the system stability. At present, correlation studies mainly focus on the distribution of the stress field and the displacement field after the excavation completed. However, studies that specifically test the fracturing process with the rock properties weakening during the excavation process are rare. Considering the above reasons and based on the damage mechanics theory, this paper established a numerical model used to analyse the damage-fracturing process of surrounding rock mass, with the nonuniformity properties of rock materials explicitly considered. Taking the underground caverns of Jinping Hydropower Station as the study object, the evolution of the unloading region was simulated dynamically during the excavation process, and good simulating results were presented. In sum, the study findings point out that the developed numerical model is not only applicable to the study of the fracturing mechanism of surrounding rock mass, but also can be used as a new method to analyse problems related to underground caverns excavation.

열에너지 저장용 암반 공동의 최적 종횡비 결정을 위한 역학적 안정성 해석

It is generally well known that the stratification of thermal energy in heat stores can be improved by increasing the aspect ratio (the height-to-width ratio) of the stores. Accordingly, it will be desirable to apply a high aspect ratio so as to demonstrate the good thermal performance of heat stores. However, as the aspect ratio of a store increases, the height of the store become larger compared to its width, which may be unfavorable for the structural stability of the store. Therefore, to determine an optimum aspect ratio of heat stores, a quantitative mechanical stability assessment should be performed in addition to thermal performance evaluations. In the present study, we numerically investigated the mechanical stability of silo-shaped rock caverns for underground thermal energy storage at different aspect ratios. The applied aspect ratios ranged from 1 to 6 and the mechanical stability was examined based on factor of safety using a shear strength reduction method. The results from the present study showed that the factor of safety of rock caverns tended to decrease with the increase in aspect ratio and the stress ratio of the surrounding rock mass was influential to the stability of the caverns. In addition, the numerical results demonstrated that under the same conditions of rock mass properties and aspect ratio, mechanical stability could be improved by the reduction in cavern size (storage volume), which indicates that one can design high-aspect-ratio rock caverns by dividing a single large cavern into multiple small caverns.

Analysis of drill stem testing technology of the Malm carbonate rocks of the Carpathian Foothills, in the aspect of geological research and exploration decisions

1. InTRoDUcTIonThe credibility of drill stem tests results performed with DST tools depends largely on the technology used. The Polish Oil and Gas Company most commonly use double-cycle drill stem tests, which, in the case of inflow to DST tool oil or reservoir water, enable among others the evaluation of [1, 2]:‒ changes of permeability of reservoir rocks in closer and more distant parts of the near-wellbore area test with DST,‒ change of yield index in the first and in the second sampling cycle,‒ relieving the given horizon of reservoir rocks from the repression of the column of the drilling mud.In the case of natural gas inflow to DST and mud spacer self-acting outflow from DST column, the technology of drill stem test changes from double-cycle to multi-cycle with clas-sical open flow operation. Thanks to open the flow operation, it is possible to remove the mud from bottom hole and declogging the near-wellbore area rocks.The Baker Inflatable type tool allows us to perform selective reservoir test, mostly in open hole sections of boreholes but also in the case of large caverns in borehole walls [1]. Then the inflow of reservoir fluid is from a predetermined interval of wellbore. This allows to shutoff the inflow of bottom water to the DST tool and reduces forming the water cone.

An optimization model for managing compressors in salt cavern gas storage

In light of the upcoming gas market liberalization, the storage in underground gas stor‐ age should be treated as a commercial activity, providing profit to investors. Gas fuel con‐ sumed for the operation of underground gas storage facility, mainly by work of compressors injecting gas into storage, will have to be carefully accounted and bought at market prices. In a competitive gas market, technical and economic optimization of the underground gas storage operation takes on particular significance. Liberalized market requires that the under‐ ground gas storage operator manages the operation in the most efficient way, affecting the profitability of service storage provision. Optimization of operation of underground gas storage in salt cavern (CUGS) is an im‐ portant operational problem in CUGS management. Despite many works on this subject [6, 16, 17, 20], it is not totally resolved. The reduction of gas storage capacity in a bed of rock salt, called convergence is inevitable, but its size also depends on the cavern depth and the operating storage [5, 19]. Operation practice suggests that the first step is to inject a large cavern convergence, then the lower and finally the shallowest cavern lying. In the withdraw process, the situation is reversed. However, the optimization that takes into account only the speed of storage caverns convergence is not ideal. You may find that maintaining a regime of operation that minimizes the effect of reducing the volume of the storage spaces can increase the cost of the gas installation (surface infrastructure). It is because Injection of the gas to the deepest caverns requires more power compressors. This occurs in CUGS Mogilno where the system can transport up to 400,000 standard cubic meter per hour [SCm/hr] of natural gas which about 2% is used by the compressor sta‐ tions to inject gas into storage caverns. It is estimated [13] that the global optimization of the

Construction Schedule Real-Time Control Coupling Construction Quality in Large Scale Underground Powerhouse

Underground powerhouse of hydraulic and hydroelectric engineering is a large cavern group structure. Construction process of underground powerhouse is a complex system, whose construction schedule is associated with construction quality. It is necessary to arrange construction schedule adopting construction simulation method and combining with tunnel stability analysis, which can advance the level of construction organization design. By using real-time control theory and methodology, the real-time control model of construction schedule and quality was established, considering construction procedure, geological conditions and resource limitation. The real-time control method of construction schedule and quality was presented, and the practical control procedure was studied. Good effect is obtained through the application of the real-time control method for construction schedule and quality in one real underground powerhouse.

State of the art for fabricating and emplacing concrete containers into large horizontal disposal caverns in the French geological repository

The research and development work presented in this paper was initialized by Andra in 2007. The work necessary for manufacturing and testing a full scale demonstrator is presently implemented. The case story is twofold.The first part is related to the initial development and fabrication of high performance concrete storage containers (containing Intermediate Level and Long Lived Waste primary canisters) to be stacked and emplaced into 400-m long concrete lined horizontal disposal vaults (also called caverns), excavated in the Callovo-Oxfordian clay host formation at a 550 to 600-m depth, with an inside diameter of approximately 8-m. The fabrication of the concrete boxes is illustrated.The second part presents the outcome at the end of the detailed design phase, for an industrial prototype system which is now being manufactured (for further test and assembly), for the emplacement of the concrete containers inside the vault. The application was engineered for remote emplacing a pile of 2 concrete containers (the containers being preliminarily stacked in a pile of 2, inside a hot cell, thanks to a ground travelling gantry crane). The emplacement process is justified and the related transfer and emplacement synoptic is illustrated. The test campaign is scheduled in 2011-2012.The successful completion of the technical trials is mandatory to confirm the mechanical feasibility of remotely emplacing concrete containers into large horizontal disposal caverns over long distances.The later display of the machinery at work in Andra’s showroom will be instrumental for the necessary confidence building process involving the various stakeholders concerned by the public debate period (mid-2013) preceding the deep geological repository license application (2015).

RandWPSO-LSSVM optimization feedback method for large underground cavern and its engineering applications

According to the characteristics of large underground caverns, by using the safety factor of surrounding rock mass point as the control standard of cavern stability, RandWPSO-LSSVM optimization feedback method and flow process of large underground cavern anchor parameters were established. By applying the optimization feedback method to actual project, the best anchor parameters of large surge shaft five-tunnel area underground cavern of the Nuozhadu hydropower station were obtained through optimization. The results show that the predicted effect of LSSVM prediction model obtained through RandWPSO optimization is good, reasonable and reliable. Combination of the best anchor parameters obtained is 114131312, that is, the locked anchor bar spacing is 1 m × 1 m, pre-stress is 100 kN, elevation 580.45–586.50 m section anchor bar diameter is 36.00 mm, length is 4.50 m, spacing is 1.5 m × 2.5 m; anchor bar diameter at the five-tunnel area side wall is 25.00 mm, length is 7.50 m, spacing is 1 m × 1.5 m, and the shotcrete thickness is 0.15 m. The feedback analyses show that the optimization feedback method of large underground cavern anchor parameters is reasonable and reliable, which has important guiding significance for ensuring the stability of large underground caverns and for saving project investment.

Study on Construction Blasting Vibration Control and Effect of Space Coupling of the Large Cavern under High Crustal Stress

Taken extremely complex and specific underground chamber excavation of TBM assembly in Jinping Ⅱ Hydropower Station as the background, the research was done on construction blasting vibration effects and control technology in the large cavern under high crustal stress. Combined theoretical analysis with field trials and other methods, chamber controlled blasting program and its parameters of better control of blasting vibrations were set in regard with the geological conditions of TBM assembly chamber, cross-section shape and size, chamber body structure and sequence of construction methods. The blasting vibration test program under complex conditions and measuring points were arranged appropriately. The result is that the blasting vibration attenuation coefficient values and was 1.24 and 0.56. Practice shows that the impact of blasting vibration on rock and underground structures is lesser, therefore the chamber driving speed and construction safety can be ensured by the chamber controlled blasting technology for construction. The blasting vibration attenuation that is deduced from the practice can better reflect the effect of blasting vibration chamber: according to the correlation between dose and level distance of blasting, safe construction blasting can be guided. In addition, there is a greater relevance between blasting vibration control method, materials and strength of underground structures. As blasting dose is the same, with increased age and strength of concrete, its ability to withstand the intensity of blasting vibration is also increased dramatically.

Analysis and Design Approach for Large Storage Caverns

AbstractThis paper presents a case study of an underground crude oil storage project. This storage site is researched with detailed field and laboratory investigations. The layout, cross section, and elevation of the caverns are analyzed and designed based on geological and geotechnical information obtained during these investigations. In this study, two-dimensional (2D) elastoplastic analysis is performed using the finite-element code Phase 2D to study the rock mass behavior caused by staged excavation of the cavern (20 m wide×30 m high). The excavation of the cavern is simulated with different stages, i.e., excavation of a pilot tunnel and subsequent side slashing followed by excavation for a pilot bench and subsequent side slashing with a total of three benches. Displacement values obtained analytically are compared with the observed displacement as the excavation progresses. Because the 2D FEM code is based on the plain strain concept, it could not take into account the effect of excavation in other d...

First investigations of an ice core from Eisriesenwelt cave (Austria)

Abstract. Investigations into the genesis and dynamical properties of cave ice are essential for assessing the climate significance of these underground glaciers. We drilled an ice core through a 7.1 m-thick ice body filling a large cavern of the dynamic ice cave Eisenriesenwelt (Austria). In addition to visual core inspections, quasi-continuous measurements at 2 cm resolution comprised particulate matter, stable water isotope (δ18O, δD) and electrolytic conductivity profiles supplemented by specifically selected samples analyzed for tritium and radiocarbon. We found that recent ablation led to an almost complete loss of bomb-derived tritium removing any ice accumulated since, at least, the early fifties leaving the actual ice surface even below the natural tritium level. The small particulate organic masses rendered radiocarbon dating inconclusive, though a crude estimate gave a basal ice age in the order of several thousand years. The visual stratigraphy and all investigated parameters showed a clear dichotomy between the upper 2 m and the bottom 3 m of the core, which points to a substantial change in the ice formation process. Main features of the core comprise the changing appearance and composition of distinct cryocalcite layers, extremely low total ion content and a surprisingly high variability of the isotope signature. Co-isotope evaluation (δD versus δ18O) of the core in comparison with data from precipitation and karst spring water clearly indicate that ice formation is governed by (slow) freezing of dripping water.

In situ damage testing of rock mass in large underground cavern

To understand the spatial damage and deterioration mechanism of rock materials during excavation, an in situ test is carried out in an underground cavern 28·5 m in width, 21 m in height and 325 m in length. In this experimental cavern, the spatial distribution and intensity of rock damage are exposed by non‐destructive ultrasonic testing after excavation. The distribution function of spatial damage has been calculated based on the measured data. To protect wall rock, reparative measures of damaged rocks are discussed based on the measured results and the conception of damage reparation, which gives inspiration to the underground engineering design.

Groundwater flow and transport and potential sources of groundwater nitrates in the Ryukyu Limestone as a mixed flow aquifer in Okinawa Island, Japan

We investigated the groundwater flow and the transport and potential source of groundwater nitrates in the typical karst setting of the Ryukyu Limestone aquifer in the southern part of Okinawa Island, Japan. Analysis of groundwater hydrographs indicated that this is a “mixed flow” aquifer with the coexistence of slow diffuse flow in the matrices and rapid conduit flow in the caves and caverns. This relationship is indicated by the travel time of groundwater flow: 70 days in the matrices of the aquifer and 6 days through the caves and caverns. The conduit flow system was also confirmed by the distribution of relatively low concentrations of 222Rn near caverns. The sampling sites were categorized into upland field (UF) type and residential area (RA) type according to the land-use ratio on the upstream side with a 600-m influential radius, and cave and cavern (CC) type according to the hydrogeologic setting near two large caverns, even though the CC type should be categorized as the UF type from the viewpoint of land use. Cross plots of NO3-N versus SO4 2− showed that the predominant source of UF groundwater nitrates was chemical fertilizer. A difference was observed in average δ15N values between UF (8.9‰) and RA (10.0‰). On the other hand, the average δ15N value for CC (10.5‰) was similar to that for RA, indicating that CC nitrates were not related to the surrounding land use. This phenomenon is considered as evidence that CC groundwater nitrates were carried by rapid groundwater flow through caves and caverns from residential areas located higher upstream compared to the influential areas. According to previous studies, animal and human waste was considered the predominant sources of RA and CC groundwater nitrogen. The contribution ratio of chemical fertilizer (R CF) was calculated using mass balance equations under assumed predictability. There was a relatively high correlation between the rate of upland areas and of residential areas and R CF. Average R CF for UF, RA and CC was 41, 27, and 25%, respectively.

Large-scale geomechanical model testing of an underground cavern group in a true three-dimensional (3-D) stress state

The stability of a large cavern group at great depth is discussed on the basis of large-scale three-dimensional (3-D) geomechanical model tests and numerical simulations. The model tests are described in detail. Improvements in the tests were made in terms of experimental techniques and advanced measurement methods. The model tests utilized active loading on six sides of a rock mass in a true 3-D stress state. During the model construction, precast blocks were fabricated and monitoring holes were defined prior to test initiation. Newly developed combination ball-sliding walls were installed on each of the major loading surfaces to reduce the friction induced by model deformation. A unique grouting and installation technique employing prestressed cables was adopted in the tests. A digital photogrammetric technique, displacement sensing bars using fiber Bragg grating (FBG) technology, and mini multipoint extensometers were developed for measuring deformation. Overloading tests were then conducted for different overburden depths, and 3-D numerical analyses were performed to simulate the testing procedures. Conclusions regarding the stability of the cavern group were developed based on a comparison between the experimental and numerical results.

Performance of Four Axial Flow Impellers for Agitation of Pulp Suspensions in a Laboratory-Scale Cylindrical Stock Chest

Axial flow impellers are commonly used for pulp suspension agitation. Pulp fiber suspensions are non-Newtonian and exhibit a yield stress. In mixing operations, a ‘cavern’ (region of active motion) is created around the impeller, with the size of the cavern affecting the quality of mixing attained. In this work, the cavern size produced by four different axial flow impellers in a Cm = 3% (mass concentration) hardwood pulp suspension was measured using electrical resistance tomography (ERT) and by analysis of dynamic mixing tests. Cavern size is shown to depend on impeller performance as characterized by power number, NP, and axial force number, Nf. At an equal power consumption of 0.53 kW/m3 the largest cavern was produced by the impeller having the largest values of NP and Nf. The measured cavern volumes compared well with predictions of the axial force model developed by Hui et al. [Hui, L. K.; Bennington, C. P. J.; Dumont, G. A. Cavern formation in pulp suspensions using side-entering axial-flow impellers. Chem. Eng. Sci. 2009, 64, 509], which accounted for interaction between the cavern and the vessel walls. When the cavern just filled the vessel volume, the time constants determined using the dynamic mixing test data reached 90% of their theoretical values (with the estimated standard deviation of ±10%), indicating that the chest approached an ideal dynamic response (complete mixing) with the onset of complete motion in the chest.

Influence of cavern spacing on the stability of large cavern groups in a hydraulic power station

Many factors influence the static and dynamic stabilities of the rocks surrounding large cavern groups, such as in a hydropower station. In order to study the influence of cavern spacing on the stability of adjacent caverns, two kinds of dynamic-history simulation models for large cavern groups were set up by considering three different strengths of rock masses (soft rock, medium hard rock, and hard rock). One model included two caverns and the other included three caverns. The numerical simulations of these models were conducted under the loading of gravity and the combined loadings of gravity and earthquake motion, separately. To fully consider the dynamic features of rock masses, a damaged plasticity model with a non-associated potential flow was adopted for the surrounding rocks. Due to the stress redistribution and the scattering effect, simulated results indicate that, taking the caverns located in soft rocks, for example, if the spacing is less than one cavern characteristic length (taken to the biggest width of a cavern in the group), the static and dynamic responses of adjacent caverns are significantly affected by their spacing. The damage and the distribution of tensile stress surrounding the caverns are extremely extensive. Once the spacing approaches or exceeds twice the cavern characteristic length, the damage and the distribution of tensile stress of caverns keep unchanged, and the effect of nearby caverns disappears. In some situations, as the rock strength decreases, the damage becomes more severe and the area of tensile stress becomes more extensive. The critical distance of cavern spacing decreases as the strength of the surrounding rocks increases. The conclusions of this work could be used as a primary guidance to the anti-earthquake design for practical engineering.

Design and Construction of the Venaus Powerhouse Cavern in Calcschists

AbstractThis paper examines the design and observed performance during construction, associated with staged excavation and support requirements of the Venaus powerhouse cavern, a component of the Pont Ventoux‐Susa hydroelectric scheme (in Northern Italy, near the city of Torino), which has been recently put into operation. This cavern, with a maximum height of 49 m, a length of 50 m, and a span of some 23 m, has been excavated in calcschists at a depth of 250 m below surface. Rock reinforcement consisted essentially of 9 to 15 m long primary rock anchors, for long term support, and 6 m long secondary rock dowels, together with a 25 cm thick reinforced shotcrete lining. Following comprehensive geological and geomechanical investigations with laboratory and in situ testing, design analyses have been performed by numerical modelling with both continuum and discontinuum methods. On the basis of the observed performance during early excavation of the upper cross section of the cavern, a discontinuum model, calibrated by back analysis procedures, was used for predicting the overall excavation stability. It is noteworthy that the design concepts adopted allowed this large cavern to be excavated and stabilized without the use of a cast‐in‐place concrete arch for roof support.

Measurement and Analysis of Pore Water Pressure in Large Caverns and Shafts in a Hydro-electric Project

With an objective to understand the behaviour of pore water pressure around large underground openings, piezometers were installed in the rock around the desanding chambers, surge shaft, its drainage galleries, and the transformer cavern of a large hydro-electric project. The monitored data indicated that the pore water pressure was insignificant in the rock surrounding the desanding chambers prior to lining. On completion of the concrete lining of the chambers, the maximum pore water pressure was about 50 kPa as the flow of water was restricted. When one of the desanding chambers was charged with water, the piezometers around it indicated an excessive increase in the pore water pressure due to seepage of water from the chamber. Thus the piezometer readings were useful in taking appropriate remedial measures. In the surge shaft, higher values of pore water pressure were recorded on the hill side of the shaft as compared to the valley side. In one of the drainage galleries, the pore water pressure gradually increased to 450 kPa, which was released by drilling relief holes in the area. In the other drainage gallery, no significant change in the pore water was recorded. In the transformer cavern, the recorded pore water pressures were within 20 kPa.

Analyzing the Strength and Stability of Large Underground Cavern Group with Different Generator Socket Construction Schemes Based on Three Dimensional Numerical Simulation

Shuibuya hydroelectric project is the most upstream power station in the Qingjiang cascade development of China. The power plant is designed as underground powerhouse, from seventh to eighth construction step, how to ensure the stability of surrounding rock mass of generator socket and controlling the displacement of lower side wall where the soft stratum located in is a key problem. To solve the problem, the three dimensional numerical model of the underground powerhouse was established based on the results of geological investigation. Then, the detailed construction processes, including the replacement of soft rock, excavation as well as support, were numerically simulated. In order to improve the simulation accuracy, the rock mechanics parameters were back analyzed based on in-situ monitor data before sixth construction step, then using Mohr-Coulomb criterion, the paper simulated and analyzed the damage zones of surrounding rock and displacement of lower side wall corresponding to different schemes. The best scheme was recommended by synthetically considering the stability indexes of each scheme. The study had a scientific meaning to guide Shuibuya project construction. Introduction When the underground cavern is excavated, the initial ground stress is released

ERRATUM

In the article by Kerry Heckenberg entitled “Thomas Mitchell and the Wellington Caves: The Relationship among Science, Religion, and Aesthetics in Early-Nineteenth-Century Australia” in Volume 33, Number 1 (2005), 203–18, the following phrase in the caption to Figure 20, p. 209, “from T. L. Mitchell, Three Expeditions into the Interior of Eastern Australia. 2nd ed. London: T. & W. Boone, 1839,” should be deleted. The caption should read as follows: Figure 20. Thomas L. Mitchell, “Large Cavern at Wellington Valley, NSW.” Sepia sketch, c1830/1838. Courtesy of Ballarat Fine Art Gallery, Gift of Victor Fox, 1979.

Speleology and magnetobiostratigraphic chronology of the GD 2 locality of the Gondolin hominin-bearing paleocave deposits, North West Province, South Africa

Speleological, paleomagnetic, mineral magnetic, and biochronological analyses have been undertaken at the Gondolin hominin-bearing paleocave, North West Province, South Africa. Two fossiliferous but stratigraphically separate sequences, GD2 and GD1/3, which were once part of a large cavern system, have been identified. Although some comparative paleomagnetic samples were taken from the GD 1, 3, and 4 localities that are currently under investigation, the research presented here focuses on the fossil-rich, in situ deposits at locality GD 2, excavated by E.S. Vrba in 1979. The GD 2 deposits are dominated by normal-polarity calcified clastic deposits that are sandwiched between clastic-free flowstone speleothems. The lower flowstone has a sharp contact with the red siltstone deposits and is of reversed polarity. The capping flowstone shows a change from normal to reversed polarity, thereby preserving a polarity reversal. While the paleomagnetic work indicates that the GD 2 fossil material was deposited during a normal-polarity period, the shortness of the sequence made matching of the magnetostratigraphy to the geomagnetic polarity time scale (GPTS) impossible without the aid of biochronology. While lacking multiple time-sensitive taxa, the recovery of specimens attributable to Stage III Metridiochoerus andrewsi is consistent with a deposition date between 1.9 and 1.5 Ma. A comparison of the magnetostratigraphy with the GPTS therefore suggests that the fauna-bearing siltstone of GD 2 date to the Olduvai normal-polarity event, which occurred between 1.95 and 1.78 Ma, and that the reversal from normal to reversed polarity identified in the capping flowstone dates to 1.78 Ma. The main faunal layers therefore date to slightly older than 1.78 Ma. Deposits from the GD 1 locality are dominated by reversed directions of magnetization, which show that this deposit is not of the same age as the faunal layers from the GD 2 locality.