| There are many large rivers in southwest China with abundant hydropower resources.In addition,the topography of the river basin changes greatly,which is conducive to the development of hydropower resources.Therefore,in the main river basin in the southwest region,the construction of water conservancy and hydropower projects has been the main theme for a long time.However,rivers in this area generally develop deep overburdens,and dam construction and foundation excavation of water conservancy and hydropower projects will inevitably encounter deep overburdens in riverbeds.Because the overburden is a kind of soil with good permeability and poor engineering properties,the groundwater flowing through it will cause a series of engineering hydrogeological problems in the upper foundation pit or dam.Current research shows that these engineering problems involving groundwater occur more frequently.In order to effectively solve the above problems,it is very necessary to clarify the hydrogeological conditions of the deep overburden as an aquifer system,investigate its hydrogeological structure,and determine the movement law of groundwater in the overburden.In this paper,the existing deep-covering dam base survey data is analyzed,and the basic distribution and development rules and layer group characteristics of the overburden layer in the southwest China are summarized.On this basis,according to the existing hydrogeological structure system theory,three major hydrogeological structures are divided,and a conceptual model is established to determine the natural conditions of the deep overburden of the hydrogeological structure of the dam body and the seepage field after excavation of the foundation pit.At last,taking the Changheba Hydropower Station as an example,the evolution characteristics of the overburden seepage field during the excavation of the foundation pit are analyzed by numerical simulation.The final results are as follows:(1)Through a careful analysis of a large number of survey data of hydropower stations in the main river basins of the southwest region,such as the Jinsha River,Dadu River,Minjiang River and Yalong River,which collected from the field survey and collected at a later stage,the existing research results of the overall distribution characteristics of the overburden in the southwest region and the distribution and development of the deep overburden in each basin are summarized.According to the survival law of the typical covering layer in each basin mentioned above,and its particle composition,structure,cause type,etc.,the general layer group characteristics of the deep overburden are obtained:from bottom to top,they are divided into muddy sand and gravel layers(?),Floating pebbles,muddy gravel,silty sand interlayer(?)and modern river floating pebble layer(?)and so on.(2)According to the completeness of the three-layer group structure of the overburden and its combination relationship,the construction of deep covering layer are divided into three types.On this basis,considering the water content and permeability of the deep-covering layer groups,according to the difference of the permeability coefficient,three main hydrogeological structure types of deep overburden in the southwestern region are proposed,including:uniform hydrogeological structure;stratospheric hydrogeological structure and composite hydrogeological structure.The stratospheric hydrogeological structure can be divided into mezzanine and interlayer types,and composite hydrogeological structure can be divided into near-universal composite and non-homogenous hybrid.(3)The corresponding conceptual models are established to simulate the natural seepage field of dam foundation and post-excavation seepage field of the foundation pit under the control of different hydrogeological structures.At the same time,the uniform hydrogeological structure and the dam foundation overburden seepage of the mezzanine hydrogeological structure under different upstream river level conditions were simulated.The results show that among the overburdens of the six hydrogeological structures,the layered hydrogeological structure and the near-universal composite hydrogeological structure have smaller dam foundation seepage flow or foundation pit water inflow due to the impermeable aquifer.And the effect of seepage control is better when the number of weak permeable layer is more and its closer to the top.Under the condition of high water level upstream,the control seepage effect of the weak permeable layer in the covering layer is obvious.(4)Take the dam foundation cover of Changheba Hydropower Station as an example of the project.According to the division basis of this paper,the deep covering layer of the Changheba dam foundation under natural conditions is divided into mezzanine hydrogeological structure,and the excavated dam foundation cover layer can be regarded as an incomplete stratospheric hydrogeological structure.Selecting a0.87km~2 area around the dam site to construct a conceptual hydrogeological model,according to the hydrogeological conditions,the model penetration coefficient can be divided into 6 areas,the(1)-(4)areas are deep overburden,and the(5),(6)areas are bedrock.The above-mentioned conceptual model is divided and simulated to seepage field under its natural conditions,and the model is identified and verified with early observation hole data and flow data.After revising the hydrogeological parameters of the model,according to the actual excavation of the dam foundation pit of Changheba Hydropower Station,three conditions including the completion of the construction of the cofferdam,the removal of the third layer of the overburden layer and the excavation of the foundation pit were simulated.And the two types of application seepage fields after considering the drainage of the base pit and the impermeability of the main and secondary wall were also simulated.The results show that the overall morphology pattern and characteristics of the seepage field of the deep overburden of the foundation of the Changheba Hydropower Station are basically consistent with the seepage field of the concept model of the above-mentioned laminated hydrogeological structure overburden.And because of the overall permeability of the dam foundation cover layer,the only layer of weak permeable sand layer was excavated after the foundation pit was excavated,and the permeability control ability of the cover layer was weak,so the amount of water inflow in the dam foundation pit has been large after excavation.After taking drainage measures,the amount of water inflow in the foundation pit has decreased to a certain extent.After the seepage prevention of the main and auxiliary walls,the total water inflow of the foundation pit has decreased,indicating that the anti-seepage wall has achieved a certain effect. |
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