Permeability Of Granitic Gneiss Rock Mas Surrounding Rock Stability Of The Undergrou Storage Caverns Under Water Sealed Condi

Permeability characteristics of the fractured rock mass are the important scientific problems and engineering applications in the rock mass hydraulic. That is also one of the focus and difficult problems in the rock mechanic. And rock mass permeability has become a critical theoretical foundation for the large projects construction like hydropower, underground oil and radioactive nuclear waste storage, and so on.To avoid the risk of economic crisis induced by oil shortage, the strategic crude oil storage construction is ongoing in China at this stage, which is the underground rock cavern by water sealed. The underground energy storage cavern is the man-made rock cavern to store Oil and Liquefied Gas, which is located well below the surrounding ground water level and prevented leakage by water sealed. It has many advantages over aboveground storage in terms of safety, environment and economy, and is the main mode to strategic oil storage in the world.The most critical issue of the underground storage cavern is the gas or oil containment and surrounding rock stability. The building rock mass permeability is the one fundamental scientific question for the containment. And it is the key to evaluating the effective and efficiency of the water curtain system. Because of the caverns below the water level and in the long-term groundwater seepage, it is the crucial problem to caverns safety evaluation that the research of the surrounding rock stability with the effect of water curtain system.Although, several underground oil or Liquefied Petroleum Gas (LPG) storage caverns have been constructed in China, the technologies were whole or part from the foreign companies. We have many problems should be solved and much core technology to be mastered. Now, four strategic underground crude oil storage caverns and some business ones are ongoing in China. It is necessary to study the interrelated theory and technology to break the monopoly of foreign companies. That has important theoretical and strategic significance and valuable to engineering applications.Combining the characteristics of water sealed underground storage cavern, some work was done with the guide of the theory of Engineering Geology, Hydrogeology, Rock Mechanics and Hydraulic. In the work, we used the methodology of geological survey, in situ and laboratory test, calculation and numerical simulation. The main work is as followed.1. The characteristics of discontinuous in granitic gneiss rock mass. (1) Based on the image of Borehole Television (BHTV), the distinguished methods of discontinuous information were proposed.The information of discontinuous is about the Dip, Aperture, Roughness and Type. According to the geometric relationship of the discontinuous and inclined drilling, it could be determined occuiTence information of the discontinuous at any angle in the borehole images by using depth and orientation of3points from it. The real aperture was also figured out by the depth and orientation of two points. It was adopted a sine curve ruler to measure the fluctuating quantity of the joints in images, and transformed the oval roughness of borehole wall to straight line. Then the Joint Roughness Coefficient (JRC) was calculated by the fractal dimension. The types of the joints were identified by the shape, aperture and roughness in the BHTV images. The type of the granitic gneiss is metamorphic differentiation and tectonic joint.(2) Optimal arrangement principle of the survey line direction.The regularities of the joints were different in the outcrop and the deep rock mass, which were the main errors caused by the measuring direction. The result shows that the included angle between measuring direction and joint was much bigger, distribution probability is bigger. If the angle is above69.4°, relative error could be controlled within10%, while the angle is above79.3°, error is only less than5%. For elimination of influence from the measuring direction, each group of joint could be revised with factor of X=1/sinθ.(3) The inspection of the joint characteristic in access tunnels.The methods of joint distinguished and measuring direction correction were inspected by the990joints in the two access tunnels. And that proved the methods were reasonable and effective. There were4groups of preferred joint in the granitic gneiss, such as134°～144°∠63°～80°,52°～63°∠70°～79°,740°∠71°and160°∠40°. Then the4groups of joint were demonstrated seepage joints according to the relationship of groundwater level monitoring data in borehole and tunnels excavated information, which the water monitoring was last about20months in21borehole.2. The permeability characteristics of granitic gneiss rock mass.(1) The estimate model of rock mass permeability based on borehole core, BHTV and sonic test data.Three in situ permeability tests, water lifting, water injection and single packer Lugeon test were done to test the granitic gneiss rock mass permeability. But the quantity was small and the precision was poor. Therefore, we used the double packer Lugeon test data to establish an estimate model which was test in granite of the Wanhua underground LPG storage caverns in Yantai. Five geological parameters including Rock Quality Designation (RQD), rock mass Integrity Designation (RID), Aperture Designation (AD), Lithology Permeability Index (LPI) and Argillaceous Content Designation (ACD) were adopted for establishing the permeability coefficient estimate model, RMP model (RMP=(1-RQD)(1-RID){LPI)(AD)(1-ACD), K=10.855×RMP0.8666). Then the model was inspected, and the results was approximate linear to the test data (R2=0.8601).(2) The application of the RMP model.The peemeability of granitic gneiss rock mass was studied by the PMP model. Based on the data of borehole, BHTV, and sonic test in borehole in Huangdao underground oil storage caverns, the permeability coefficient of granitic gneiss rock mass was estimated by the RMP model. The permeability was varied mainly from10-4m/d to10-'m/d, and didn't show an obvious law in both vertical and horizontal distribution. On the plane of10m (EL.),0m (EL.),20m (EL.),30m (EL.) and40m (EL.), the max permeability coefficient was mainly distributed at the southern part in North to South direction, and center in East to West direction. On the plane of-50m (EL.), the maximum was at the southern part, and the northeast corner was bigger than the rest parts. The geometric average permeability coefficient is larger at the southern (in N-S direction) and center (in E-W direction).(3) The permeability tensor of granitic gneiss rock mass.The rock mass permeability tensor was calculated by the modified hydraulic aperture with the joint roughness. And the tensor was modified by the estimation permeability coefficient of different segments. Through the statistics of permeability rose diagram, the major principal seepage directions are N32°E, N88°E and N21°W. And the predominant directions are N34°E, N15°W, N54°E and N73°W. The maximum of major principal permeability coefficient, K1ranges from4.58to6.79×10-2m/d. The predominant is1.42to8.31×10-3m/d. the anisotropic ratio, c1, is1.06in average and c2is3.22.3. Groundwater pressure distribution characteristics of underground storage caverns.(1) Hydrogeological conceptual model of water sealed underground storage caverns.The aquifer type of water sealed underground storage caverns is fractured unconfined aquifer, which is anisotropic and single-layer. The boundary of surrounding is the water head (H), and the caverns and water curtain system is pressure (P). The permeability coefficient of the rock mass is little and most is10-4-10-3m/d. and the rock is anisotropic obviously.(2) The influence of anisotropic permeability of granitic gneiss on the water pressure distribution in the surrounding rock.The numerical experiments are designed for4kinds of different permeability coefficients (ks) and6kinds of anisotropic ratio (kv/kh) which is among1to5. By the23experiments, it shows that water pressure between two caverns was linear increasing with the kv/kh increased, but pressure below the cavern floor was declined. And it between the roof and water curtain system was slow decline firstly then sharply. The groundwater flow velocity was declared sharply and then slowly which is belong the logarithm decrease.(3) The influence of water curtain system on water pressure distribution in the surrounding rock.5kinds of seepage numerical experiments with or without water curtain system were done. The results indicated that the water curtain system was significant effect to the water pressure in the rock between two caverns and the roof, and little effect on the vertical water pressure of the bottom. The groundwater flow velocity was enhanced by the curtain system in the left wall, roof and right wall. And that changed the distribution characteristic of the velocity around the cavern. the velocity below the bottom was affected by the water curtain system and groundwater water level.4. Surrounding rock stability of the underground storage caverns under the water sealed condition.(1) After the strength and deformation parameters determined of5classifications granitic gneiss rock mass, the stability of the surrounding rock of the caverns mass was calculated without groundwater.In order to determine the strength and deformation parameters of the rock mass of1to V classification, several kinds of methods were adopted, such as Laboratory Test of18samples uniaxial and7groups triaxial compression tests, estimation of Generalized Hoek-Brown criterion and Hoek E(2006), displacement back analysis with the orthogonal numerical test and so on. According to the numerical simulation, the side walls and the bottom deformation of the caverns was large, while the roof deformation was small. The deterioration of the rock mass quality had significance effect on the total displacement of left side wall and horizontal displacements, and the bottom displacement. And the ring stress field was dominant in the distribution of the secondary stress fields, and there were different developing trends in various places of the caverns when the rock mass quality deteriorated. The plastic zone area appeared in surrounding rock mass of Ⅲ Ⅳ and Ⅴ classification. That in the side walls was the maximum and in the corner of the bottom was the minimum. With the deterioration of the rock mass quality, the factor of safety of the caverns was reduced exponentially. The stability of caverns with I, II and III rock mass could meet the security requirements, but the IV and V couldn't.(2) The stability of the surrounding rock mass with anisotropic permeablity was studied without the water curtain system.The influences of the anisotropic permeability of the surrounding rock mass to the stability were studied by changing the ratio kv/kh in two orthogonal directions. The anisotropic permeablity had no influence on the deformations of the caverns, and little effect on the stress. With the value of kv/kh increasing, the major principal stress remained constant for I rock, and the major principal stresses fluctuation increased for Ⅱ and Ⅲ rock mass. However, it increased at the beginning then decreased with fluctuation for IV rock mass. The increments of the plastic zone total area increased with fluctuation and small magnitude for IⅢ rock mass. While it increased with large magnitude for Ⅳ rock mass. The factor of safety of the caverns was generally declined with the increasing of kv/kh, but was the "W" variation for I rock mass.(3) The variation of surrounding rock stability was studied when the water curtain system running.The displacement of surrounding rock was almost same under the water curtain system in standard and extreme running conditions. Most parts were large under standard condition, and the maximum difference was1.5mm. To I and Ⅱ rock mass, the major principal stress was same in anywhere of the cavern. And to classification Ⅲ, the standard was bigger. However, the extreme was bigger to classification IV. The plastic zone area was smaller under the extreme condition to standard, and the maximum change was0.47%of the caverns cross-section area. The factor of safety was improved under the extreme condition. When the rock was classification I and V, the influence of the stability was bigger with the condition changed. However, no affect to III and IV surrounding rock mass.(4) The variation of surrounding rock stability was studied in the construction and operation phase.The surrounding rock deformation of operation phase is little than that of construction, and the maximum relative change rate is8.2%. The change of major principal stress is more prominence on operation phase to construction with the rock mass quality poorer. To Ⅰ and Ⅱ rock mass, the midpoint of the left wall changed large, and the junction of top arch and right wall was larger to Ⅲ and Ⅳ rock mass. The plastic zone area was smaller on operation than construction. And the change was bigger to rock mass Ⅲ. The factor of safety of operation was declined to the construction, and the largest decrease was28.8%. The stability of caverns with I and Ⅱ rock mass was good and met the security requirements both the construction and operation stage. But the Ⅲ rock mass couldn't when the caverns operating. It cannot meet the security requirement to IV and V on both stages.The innovative and distinctive places of the work are as followed.(1) According to the image of Borehole Television (BHTV), the distinguished methods of joints roughness were proposed, and the JRC was estimated by Fractal Theory.(2) Based on the double packer Lugeon test, five geological parameters including Rock Quality Designation (RQD), rock mass Integrity Designation (RID), Aperture Designation (AD), Lithology Permeability Index (LPI) and Argillaceous Content Designation (ACD) were adopted for establishing the permeability coefficient estimate model, RMP model.(3) The water pressure and stability of surrounding rock were researched in the water sealed underground storage caverns under the conditions which the granitic gneiss rock mass with anisotropic permeability. The water pressure between two caverns was linear increasing with the kv/kh increased, but pressure below the cavern floor was declined, and between the roof and water curtain system was slow decline firstly then sharply. The anisotropic permeablity had no influence on the deformations of the caverns, little effect on the stress, and the factor of safety was generally declined with the increasing of kv/kh.