| Yan’an City is located in the loss hilly gully district of the in Northern Shaanxi.In recent years,with the development of economy and society of Yan’an,the contradiction of the lack of space for urban development is very prominent.It is an urgent problem to realize the people’s dream of living in Yan’an old area and protect the historical and cultural heritage.In 2011,Yan’an city decided to build Yan’an New Area in the Qiaogou River Basin adjacent to the urban area by engineering and technical means,so as to break the land restriction bottleneck of Yan’an economic and social development,change the linear development pattern of Yan’an urban area,and realize the development space goal of relieving the old city.However,the implementation of large-scale ditch construction project in Yan’an new area will inevitably affect the hydrogeological environment of the site.If the drainage path is not given,the water level will rise,and new water bodies will be formed in the soil,which is easy to cause slope instability and loess collapse.It is not only related to the safety and stability of the construction,municipal administration,excavation and filling slope,but also related to ecological issues.This paper takes qiaoergou watershed,the first phase of Yan’an New Area(North area)project,as the research object.Based on the engineering geological and hydrological conditions of the research area,a numerical model of groundwater flow in the research area is established based on Visaul MODFLOW,and the changes of groundwater level under three different working conditions are predicted.The main conclusions or suggestions are as follows:(1)According to the four permeability tests,the final parameters are:under natural conditions,the permeability coefficient of Lishi loess is generally 0.005-0.02m/d;Malan loess is generally 0.02-0.08m/d;the permeability coefficient of bedrock is 0.013-0.037m/d;the permeability coefficient of diluvium and landslide accumulation is 0.07-0.165m/d.Under natural conditions,the precipitation infiltration coefficient of the study area is 0.028 in the non filled area and 0.01 in the backfilled area.(2)The hydrogeological numerical model was established by Visaul MODFLOW.Under natural conditions,the underground water level in Qiaoergou Basin was 960~1085m,decreasing from north to south successively,which was equivalent to the simulation results.The simulated calculated gullies excreted 412m3/d,which is basically equivalent to the measured flow rate of 387m~3/d,and can be used to predict the groundwater level after construction.(3)The channel in the backfill area must be filled with water guide layer to discharge groundwater in time.The water guide layer shall be laid continuously along the bottom of the trench and shall be laid at the intermediate groundwater discharge location of the aquifer.The branch channel water guide layer shall be gently connected with the main channel water guide layer.The material of water guide layer should be rounded pebble with good water guide,and filter layer should be set on the water guide layer.(4)In the absence of water diversion measures,the groundwater level in the study area presents an overall rising trend.In the case of water diversion and surface hardening measures,the rise of groundwater level can be effectively controlled. |
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