| The proposed Rumei Hydropower Station is located on the lower reaches of the Lancang River in Mangkang County,Changdu,Tibet.It is the fifth cascade power station planned for the lower reaches of Changdu.The water-retaining buildings are planned to use core-wall rockfill dams with a maximum dam height.315 m,the normal storage level of the reservoir is 2895 m,the control watershed area of the hydropower station is 79,400 km2,the average annual flow rate is 648m3/s,the corresponding storage capacity below the normal storage level is 3.943 billion m3,and the installed capacity is 2100 MW.The preliminary field survey showed that,for example,the Meige Hydropower Station has a complex geological structure background and various geological structures in the hub area.The weathering and unloading characteristics of the slopes on both sides of the bank are significantly different.There are multiple faults and compression zones.Intensive development.In particular,the development of rock bodies,long cracks and intrusive veins in the unloading zone at the shallow surface of the slope is complicated,and the fissure network structure formed with the surrounding wall rock fissures is complex,which constitutes a direct channel for groundwater migration.Bring some difficulties.Based on the engineering geological environment conditions in the dam site area,this paper systematically investigated the development degree and scale of the cracks on the dam foundation rock mass on both sides of the bank,conducted an in-depth analysis of the rock mass structure and rock mass permeability structure,and obtained through the study of the dam foundation rock mass permeability characteristics The permeability coefficients of different rock types are analyzed.Finally,the Visual Modflow software is used to analyze and compare the seepage field before and after water storage in the middle dam site.The effect of the depth of the anti-seepage curtain on the amount of leakage is discussed,and the treatment of the anti-seepage curtain is discussed.An analogy analysis was carried out.The main results achieved are as follows:(1)Summarized and analyzed the development characteristics of the left and right bank dam foundation rock mass structural planes,statistically analyzed the occurrence types,development scales and filling characteristics of different types of structural planes,and found that there were 20 Class III faults exposed on the left bank.The fault development of N5?25°E/NW(SE)?75?88° has an absolute advantage,extending up to 100?400m,which mainly summarizes the spatial development characteristics of fault L72;most of the right bank III-level faults are steeply inclined The width of the broken zone is 10?40cm.?-level faults are mostly steep dip angles on the left and right banks,and developed in groups;?-level structural planes are mainly bedrock fissures,and the fissure planes are mostly closed,and the degree of fissure development is related to the unloading of the rock mass.Different-scale fissures are in space.The distribution and combination of the two constitute the basic geological model of seepage flow in the dam foundation rock mass.The type of rock mass structure of the dam foundation on both sides of the bank changes with the unloading zoning,and the integrity of the rock mass from the surface and the inside of the slope increases as the unloading degree decreases.(2)Summarize and analyze the five types of permeable structures and their composite types commonly found in most engineering rock masses,and divide the permeable structures of different unloading zone rock masses in the Rumei dam site area.It is a band-shaped infiltration structure and a fissure network-shaped infiltration structure.The belt-shaped permeable structure is mainly composed of rock mass with strong unloading zone,large-scale faults,rock veins and dense fissure zone around it,which is the main channel for seepage.The fissure network-like permeable structure is mainly composed of fissure cuts in weak and unloaded rock masses,and it is a secondary channel for seepage.(3)Through the experimental analysis of pressure water achievements and the calculation of the permeability tensor of the fractured rock mass,it is concluded that the permeability of the rock mass of the dam foundation generally decreases gradually with the increase of the vertical burial depth and horizontal depth,and the permeability of the rock mass mainly depends on the weathering and unloading The zoning changes,and the permeability coefficients of rock bodies with different openings are often different.In order to verify the rationality of the calculation parameters,the permeability coefficients and test data of rock masses in multiple unloading sub-zones of hydropower projects were collected,the relationship between rock mass unloading degree and permeability was discussed,and the dam site area was selected in combination with parameter analogy Permeability coefficient of rock mass in each unloading zone.(4)Using Visual Modflow three-dimensional groundwater finite difference software to simulate the groundwater seepage field under different working conditions in the middle dam site,the results show that in the natural state,the shallow groundwater in the middle dam site is discharged from the two banks to the Lancang River,and the deep rock groundwater is Runoff from the right bank to the left bank.When the reservoir is impounded normally,due to the difference in head between the front and back of the dam,the water head contour changes to the back of the dam,and the upstream flow of the reservoir bypasses the rock masses on both banks and leaks downstream,forming a seepage around the dam shoulder.Among them,the strong unloading and weak unloading rock mass of the dam foundation have formed a certain range of seepage around the dam,and with the reduction of the degree of unloading,the scope of the seepage has expanded.After water storage,the groundwater level of the observation holes on both banks increased significantly,the water level on the right bank gradually increased,and the water level on the left bank increased first and then gradually decreased.(5)The leakage problem of dam foundation and abutment caused by water storage is outstanding.The water balance module in the simulation software predicts the leakage of the dam foundation and abutment,which shows that the leakage of the dam foundation and abutment after storage is 10307.968m3/d;The total leakage after setting 120 m anti-seepage curtain is 7495.363m3/d;the total leakage after setting 150 m anti-seepage curtain is 6384.9199m3/d;and the total leakage after setting 200 m anti-seepage curtain is 5690.7113m3/d.The anti-seepage curtain has a good effect on the leakage of the dam foundation,and the anti-seepage effect is good when the curtain depth is 150?200m.(6)Based on the above-mentioned crack development characteristics,rock structure and permeability structure characteristics,dam foundation permeability characteristics and seepage field analysis of the dam site,referring to the anti-seepage design specifications and treatment experience of large earth-rock dam projects at home and abroad,the anti-seepage standards of the Rumei dam site are carried out After a systematic analysis,the extension of the curtain on the dam foundation of the river bed and the abutments on both banks was prepared.Among them,the bed section of the riverbed dam uses q?1Lu as a relatively impermeable layer.It is recommended that the depth of the curtain of the dam foundation(minimum distance from the foundation surface)be 200 m.The middle and upper elevation dam foundations on the left and right banks use q?3Lu as a relatively impermeable layer,and a layer of grouting flats is set according to the height difference of about 50 m.The left and right bank dam foundations are respectively provided with 5 layers of grouting flats for anti-seepage curtain grouting and related hydrology test.(7)It is recommended to remove all the permeable structures such as rock bodies and porphyry veins in the shallow super-unloading zone of the dam site,and to implement anti-seepage treatment in combination with replacement and reinforcement measures;while the deep faults and long cracks that play a leading role,It should be ensured that the direction of curtain grouting is orthogonal to the direction of the dominant fissure,and the leakage channel is blocked to the greatest extent,thereby reducing the leakage of the rock mass of the dam foundation and ensuring the stability of the dam foundation permeability. |
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