| Pingtou Hydropower Station is located on Meigu River, which is a first level tributary of Jinsha River, in Liangshan Sichuan province. It is the last cascade hydropower station of "One-reservoir and five-cascade" development plan in the downstream. The underground powerhouse of Pingtou Station perches in the backwater area of Xiluodu Hydropower Station on the Jinsha River. Pingtou Station is a kind of dam-diversion power station, with the height of sluice gate38.5m and a storage capacity of620900m3. Its left diversion tunnel is12.7km long with an available water head298m, combined with three sets of machines underground, the installed capacity reaches to180MW. The station site23km far away from the dam is built on the left bank of Meigu River, which reaches the Ergu Valley upwards and Longtou Valley downwards. The underground site located at0.5～1.0km up the Longtou Valley, consists of lower horizontal section of pressure pipes, underground powerhouse, transformation room and tailrace tunnel. Hydraulic structures lie at the depth of10-465m laterally and5-325m vertically in the rock blocks, with Dengying Formation of Sinian dolomite as wall rock which causes great development of karstification and fractures. According to the original design, the base surface heights of pressure pipes, underground powerhouse, transformation room and tailrace tunnel are573.8m,569.3m,587.7m and570.8m, respectively. Except the transformation room, all of the structures are lower than the water level of local groundwater and Meigu River,583-587m and580.24m, respectively. However, as the Xiluodu Hydropower Station is completed, the level of the reservoir would stay at600m a.s.l. to generate electricity; the water level of Meigu River in the site region, influenced by the backwater of the reservoir, will rise to600m too, higher than the base surface levels of structures. Thus, it will cause drain water of groundwater and Meigu River back up into the chambers during the excavation and construction of structures, obviously. To undertake the construction and operation of the hydraulic structures in the underground site area, the anti-seepage preventions of the chambers underground should be the key and treated seriously.Because the site situates at the drainage area of karst system formed by the Dengying dolomite, it is a challenge to construct the hydrology conceptual model and numerical model with high degree of simulation, which could better predict the characteristics of the groundwater seepage system and the amount of chamber gushing under different operating conditions.To instruct the application of hydraulic structures properly and design the project of conservancy construction (anti-seepage prevention), hydrological investigation, geological exploration, related experiments and quantitative and qualitative analyze were carried on. The following works were finished to ensure the project under security construction and functioning well:(1) Hydrological conditions and the groundwater system of the research area were analyzed;(2) The regularity of karstification and fractures development were evaluated and the Representative Elementary Volume was defined;(3) A3-D heterogeneous anisotropy numerical simulation model was constructed;(4) The optical application of project and anti-seepage prevention schemes were proposed through simulating the characteristics of seepage field and the amount of water gushing under different operating conditions and courses. It concludes as follows:Firstly, Pingtou Hydropower Station is located in the main discharge area of the karst groundwater system originating from Longtou Valley, with a water catchment of46Km2area and an aquifer system of12Km2. Naturally, the groundwater receive recharge through infiltration of precipitation, then flows North to South and sinks at the structures sites, discharges to the Meigu River as fountain groups finally. There were severe problems of water gushing in the hydraulic structures, which would block the discharge routine of the Longtou Valley groundwater system and induce backwater of Meigu River during daily construction, according to original design. Thus, the amounts of water gushing were measured through experiments and a tendency was acquired for further prediction:the chamber water flux could reach to0.38m3/s at normal water level, and then increase to1m3/s and5.9m3/s during flood season and the period when the level of Xiluodu reservoir stayed at600m, respectively.Secondly, the investigation of hydrological conditions and the regularity of fracture development in the site area reveal that:(1) Fracture network was formed by the finely connected fractures, and caused corrosion effect to an extent in the site area. It could be solution-enlarged fracture-fracture media.(2) The media of aquifer controlled by the lithology and tectonism behaves as heterogeneous anisotropy medium. Heterogeneity of medium is influenced by the lithology:with better extensibility and expansibility, the permeability of thicker fine dolomite is larger than that of the thin microcrystal dolomite. The anisotropy is controlled by the fractures direction, especially affected by the strata fracture with better extensibility and expansibility:the permeability becomes weaker from northeast to northwest differing from1.5times.(3) The permeability of the aquifer decreases with the depth increasing vertically. The lithology of the borehole and results of water-pressure test in the site show as follows:Above480m a.s.l., fractures and karstification of the medium are well-developed with permeability larger than3Lu (according to Lv Rong); below450m a.s.l., the medium could be aquiclude with fracture of medium merely-developed; between450m and480m a.s.l., the permeability ranges from1to3Lu with fractures developed but no corrosion effect on the medium.Thirdly, through the systematically measurements and statistical calculations of the fractures on the top drainage gallery (610m a.s.l. high) of the underground powerhouse, this study illustrates the existence of scale effect of the fracture aquifer medium. For the same point, the fracture rate shows the characteristics of the spatial variation when the scale is small, while the fracture rate tends to be stable when the scale increases to a certain extent. Moreover, the certain stable scale of different measurement points is basically at a range from2.5m to3.5m. This work provides the possibility for using continuous model depicting the characteristics of seepage field in groundwater, and provides the basis for determining unit subdivision size by numerical simulation of continuous medium model.Fourthly, the3-D heterogeneous anisotropic continuum of fracture medium conceptual model and the numerical model of water-flow are suitable for this research area. By comparing the advantages and disadvantages of various fissure medium models, given the investigations of the hydrogeological conditions and the fracture site media development features, It is considered that the fractured medium3-D conceptual model is more suitable for the study area. Through the fractures on site measurement, the groundwater flow field analysis, groundwater tracer test, drilling pressure water test method, we carried out in-depth and meticulous research to identify the boundary condition, hydrogeological parameters partition, hydraulic conductivity determination of initial values etc., which ensure the rationality of the model. Considering the water level of base rock groundwater, and the difficulty in carrying out the groundwater pumping test, we make full use of the engineering condition to take groundwater level recovery test, and acquire the recovery curve of groundwater level for this site. Then as the objective function, the model is reversed to identify the groundwater numerical model, which greatly improves the model simulation, and lays the reliable geological basis for the prediction of subsequent groundwater numerical model.Fifthly, different numerical models for anti-seepage prevention scheme and the proposed adjustment scheme of the underground hydraulic structures are constructed, which could simulate the groundwater flow field and water outflow under45kinds of construction conditions. Three boundary conditions, such as the Meigu River at normal and flood water level period, respectively, and future operation period of the Xiluodu Reservoir Impoundment, are taken into consideration; different seepage conditions during construction, such as along the Meigu River local and whole anti-seepage prevention projects, five kinds of anti-seepage curtains depth underground powerhouse during operation period can be applied; if the whole bottom elevation of structures are raised by15m, various situations from hydrogeological points are put forward with, which represent the groundwater flow field under50kinds of working conditions. Groundwater seepage under various conditions could also be calculated, which can depict the characteristics of groundwater flow field and simulate each chamber water outflow.Sixthly, based on the prediction of groundwater flow field and chamber water outflow, probability and quantity of anti-seepage curtain engineering under various conditions are evaluated and optimized owing to the hydrogeological conditions, and conclude as follows:it is more safe and reliable to carry on the scheme when the overall elevation of building structures rises to15m. The feasibility and rationality of this solution has been fully proved during the engineering construction and operation period.During both construction and operation period, there is a big problem of water gushing in the original design for Pingtou hydropower station. In construction period, the flood flux can be as large as76882m3/d, which may cause a serious problem of seepage stability. Thus, enclosed anti-seepage curtains along the Meigu River and around the plant area (both in plane and section) are needed. However, the engineering quantities and construction difficulties are enormous. Full enclosed curtain depth will reach to160m, while the region does not satisfy the conditions to set multi-layer Grouting Gallery. What is worse, the dolomite fractured aquifer medium has a feature of "Sandy" along the fissures. It is difficult to carry out the engineering and the quality is hard to be guaranteed. There are more serious security risks during construction and operation period.A design that the overall elevation of plant structures should be raised by15m is proposed. According to the comparison of numerical simulation results of seepage field, after raising the elevation, most structures are built above the normal water level. Though there are still water gushing problems in some chambers under some conditions, the volumes greatly decrease (a14456m3/d flood outflow during construction period) and construction condition has been greatly improved. Even if water gushing problems exists due to the storage of Xiluodu Reservoir during operation time, the safety risks during construction period will be lowered with the completed station floor and strengthening standards of structures impermeability,In conclusion, this study is to solve the actual problem of engineering. A series of effective research, exploration and test work were carried out, and combining the groundwater system theory and numerical simulation technology with engineering practice commendably was adopted for analyzes. Its aim is not only to solve the practical problems of this project, but also the conclusions may have important instructive value for similar projects. In addition, Pingtou hydropower station has adopted the conclusions and suggestions of this study and made optical design for structures supporting measures and anti-seepage prevention engineering. Currently, the station has been generating electricity normally for more than two years and running under good conditions.The main characteristics and innovations of this paper are as follows:(1) Through the measurement of the fracture network and the study of groundwater flow field, we find that fracture aquifer system in this area has characteristics of continuous medium. The REV of the fractured rock size is determined as3.5m, which provides theoretical basis for achieving groundwater numerical simulation and ensuring the mesh scale.(2) Drainage projects of the tail water to develop groundwater level recovery tests were developed, which provided technical support for obtaining hydrogeological parameters and model identification. It also provides a new idea which would acquire hydrogeological parameters and identify models for the similar steep and precipitous mountain area, where pumping test couldn’t be carried out in underground chambers.(3) To optimize the design of underground engineering and its seepage control scheme, the groundwater numerical simulations of fifty different scenarios were used to solve the problems encountered in engineering practice under complex conditions. |
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