Response Process Of Soil Moisture Content To Tunnel Drainage And Its Effect

With the development of tunnel construction technology and the improvement of route alignment requirements,more and more tunnels are used in the construction of railways and highways in China.While overcoming the obstacles of line elevation and shortening the mileage,the construction and operation of tunnel engineering will inevitably have an impact on the ecological environment of the tunnel site;for example,the tunnel drainage breaks the balance of the seepage field in the aquifer,causing the groundwater level to drop,which in turn causes the moisture content in the overlying soil is reduced,which affects the growth and development of vegetation.In order to study the effect of tunnel drainage on soil moisture content,a soil column test simulation device and a box test simulation device were constructed,by changing the tunnel drainage and groundwater level,the response law of seepage field and water field after tunnel drainage was explored,it is expected to provide a theoretical basis for the ecological environment impact assessment of tunnel water gushing.Both soil column test and box test show that the rising height of capillary water in soil is a function of power,and the rising rate of capillary water gradually decreases with time;for example,the rising rate of capillary water in soil column test was the fastest at the initial stage(0~5000min was between 0.087~0.055 cm/min),and then gradually decreases(down to 0.00077 cm/min at 50,000 min);the moisture content in the soil profile gradually decreases as the capillary rise height increases.The soil column test device has a total height of 200 cm,a lower 40 cm thick aquifer and a 160 cm thick upper soil.The aquifer was continuously subjected to three times of water depth reduction of 10 cm,and the response range of soil moisture content was gradually increased from the top of the phreatic aquifer.During the test period,the moisture content of the observation points from 1 to 9# decreased from 53.6% ~ 34.3% to 51.5% ~ 33.3%;the moisture content of the 10# observation point increased from 6.5% to 33.1% and then decreased to 30.7%.The drop of groundwater level has a certain influence on the moisture content of the overlying soil,but the change range is not large during the experimental observation period,it is not excluded that the long-term soil moisture content will decrease to the vegetation withered water content,at this time,the basic living conditions of vegetation can not be guaranteed.According to the change of seepage field at different times(0min,10 min,60min,120 min,200min and 350min)during tunnel drainage.10 min after the tunnel is drained,the depth of the diving surface is between 2 and 6.5 cm.As time goes by,the tunnel drainage volume gradually decreases,and the diving surface falls gradually.After 350 minutes of tunnel drainage,the tunnel drainage is equal to the lateral recharge of the aquifer,and at this time,the diving surface is reduced by 15 cm and the seepage field reaches a new equilibrium.According to the comparative analysis results of aquifer flow field and soil water field at the same time(0min,10 min,120min,350 min,1000min,7200min)after tunnel drainage.Within 10~350 minutes after the tunnel began to drain,the seepage field reached a new equilibrium,and the diving surface decreased by 15 cm.The response range of soil water field expanded from 0~16 cm at 10 minutes to 0~30 cm,and the decrease range of soil water field was between 0.1% and 0.5%.After the tunnel was drained for 350 to 7200 minutes,the seepage field no longer changed,and the moisture content in the soil moisture field continued to decrease,with the decrease ranged from 0.4% to 1.2%.