Study On The Characteristics Of Groundwater Seepage Field In Foundation Pit Engineering

Groundwater seepage control is an important security control in foundation pit engineering. When excavating in the deep aquifer, and unclosed curtain for cutting off drains is used, groundwater inside and outside of the foundation pit could not be partitioned, which can lower the groundwater level outside of the foundation pit. If there’s a building around the foundation pit, the drawdown should be considered to reduce impact of strata subsidence on the surrounding buildings. The groundwater water inflow and the foundation pit water level drawdown calculation method don’t consider the effect of unclosed curtain in Technical specification for retaining and protection of building foundation excavitions.For the porpose of studying the characteristics and law of groundwater seepage in foundation pit with unclosed curtain, this paper made the following works:1. Read extensively seepage research at home and abroad.2. The foundation pit seepage model test chamber is developed, and makes64seepage test condition.3. Seepage finite difference numerical calculation program is compiled by using vb language.By the aforementioned work, this paper discusses the law of influence on seepage field by insertion depth of curtain, thickness of water cross section, hydraulic head pressure difference. Groundwater seepage field of unclosed curtain foundation pit is calculated by two-dimensional profile difference method, and the program is developed, also comparating with various test conditions. The following conclusions can be made:1. The statistical analysis of the test results shows that in the experiment range of relative depth (D/△h=0.25-2.5, D/M=0.5-5), water level drawdown of the place outside the foundation pit near curtain decreases with the increase of the depth of the curtain, and the drawdown reduced with the increase of the depth of the curtain water level amplitude slowly. The drawdown relative to the head SA/△h is between0.1-0.4. Curtain depth is equal to1times the head, SA/△h is about0.2-0.4, the depth of the curtain is equal to2times the head, SA/△h is about0.1-0.3. The curtain insert a certain depth, absolute value of SA increases with the increase of the water head difference, but relative drawdown SA/△h decreases. Drawdown become large when the distance(X1/△h) become large, the opposite is also true. The test results can be used for determining hanging curtain depth in practical engineering design. 2. The statistical analysis of the test results show that in the experiment range of relative depth (D/△h=0.25-2.5, D/M=0.5-5), with the increase of the depth of the curtain, the seepage flow always reduced not so much. Curtain depth increases1times, total seepage flow decreased by10%-15%.3. By the seepage calculation and model test contrast with the flow net and the drawdown SA, they both agree with each other very well. The seepage calculation program in this paper can solve practical unclosed curtain seepage calculation of foundation pit.4. Through the model test, found that the coefficient of horizontal permeability and vertical permeability coefficient is different and the difference is bigger. The ratio of horizontal and vertical permeability coefficient kx/kz in the model test is about1.6-2.2. Using the numerical method for seepage calculation, horizontal and vertical permeability coefficient ratio of kx/kz has significant effects on the calculation of flow field. Therefore, in the actual project of groundwater seepage calculation, the anisotropy of permeability coefficient should not be ignored.5. Test shows that within the scope of the curtain of the test relative depth (D/△h=0.25-2.5, D/M=0.5-5), head loss outside the foundation pit take up50%～70%of the total head loss, and when the relatively curtain depth D/△h is larger head loss is smaller. The head loss inside the foundation pit is30%～50%of total water head loss, and when relatively curtain depth D/△h is larger, the head loss is larger. Off the bottom of the hydraulic slope is significantly greater than other parts of the small scale, water head change sharply, the head loss there alone accounts for10%～30%of total water head loss. Head changes slowly along the vertical under the the bottom of the curtain inside of the foundation pit.6. For unclosed curtain, the range of one times the curtain depth and above the bottom of the curtain inside the foundation pit, the basic seepage is in vertical direction and hydraulic gradient distribution is very uniform. In according to the analysis of the flowing soil damage, using area average hydraulic gradient judges more reasonable.