Theory Of Seepage Deformation Applied In Foundation Pit Dewatering

It has become an inevitable trend that the pit progresses toward large depth and area, with the incensement of the high and super-belt high structures, and the underground structures. It not only considers the reliability and economy of support measure, but also the dewatering effect when the deep pit is digging under the complex Hydrogeology condition, in order to make sure the stability of foundation and construction carried through smoothly, and to prevent the soil deformation of infiltration related to groundwater. The deep study of seepage deformation and the analysis seepage flow are of great importance to the effect mechanism for the foundation pit dewatering and stability in both engineering and actual value. The seepage deformation is applied to optimum design of deep pit dewatering. It has important practical significance for the cost savings of pit engineering and the achievement of best results. It also has important academic value for the improvement of current design theory of pit dewatering by approach.This paper considers the theory of seepage deformation with specific examples, based on the experience of deep foundation pit dewatering in Wuhan City. It focuses on the effect mechanism of seepage flow on stability of pit soil by numerical modeling method. The parameters during the raining process are calculated and analyzed, such as permeability, pumpage and waterproof curtain, etc. The connection between these parameters and dewatering effect is analyzed. The characteristic of foundation pit seepage field is discussed under different soil condition. The current method of anti-uprush for foundation pit is improved.First of all, this paper summarized the current theory of seepage deformation. The permeability of soil and its impact factor are researched. The mechanism of seepage deformation during dewatering is analyzed. The common problems of seepage deformation are described, such as soil flow, piping and uprush. The mechanisms of deep pit foundation deformation are discussed, with the probable type of seepage deformation and discrimination standard. Countermeasures for preventing pit foundation deformation are proposed. Seepage stability for real pit soil is analyzed, and effect of dewatering on pit foundation engineering is summarized.Second, numerical modeling is applied by Ritz finite element method based on three-dimensional unstable seepage function. The Visual Modflow software is used to calculate three-dimensional unstable seepage. The results are compared with analytical solution and monitored values, and they are calculated and verified by the real pit foundation dewatering engineering of shield well in Wuhan Yangtze River Tunnel. Third, the calculation model of real example is simplified in order to facilitate further comparison. The common condition of pit foundation dewatering engineering is set to be standard situation in Wuhan. Large number of pit foundation dewatering is calculated through variation of many pit foundation parameters (permeability, pumpage, edge distance and depth of pumping well, waterproof curtain, etc). The curves between progresses of pit foundation dewatering and various parameters are derived. The factors and mechanism affecting the dewatering are analyzed using the theory of seepage deformation. Under the circumstances of maintaining the pumping discharge, the smaller pumping wells are selected mostly. With appropriate depth and larger boundary of well, the proper position of waterproof curtain in semi permeable layer has great active influence on fit foundation dewatering effect. The conclusion can be referenced by real pit foundation dewatering.Fourth, the characteristic of pit foundation seepage under different soil condition is analyzed by the SEEP/W module of GeoStudio software. It reflects the equipotent line of pit foundation seepage flow field, velocity vector and hydraulic gradients in different discipline of soil condition. The influence of seepage flow on stability of pit foundation soil seepage is compared. The probable unfavorable factors in engineering are discussed. The analysis indicates that the hydraulic gradient is largest near the corner and overflow point in homogeneous and isotropic un-layered soil, where they are easiest to deform in the whole foundation. The equipotent line has the trend to aggregate towards soil layer of small permeability under layered soil condition. The comparison imply that the fit foundation easy to be deformed when the level permeability is larger than vertical permeability.Finally, the function of seepage force is derived using the theory of seepage deformation. The mechanism is analyzed. The checking method for anti-uprush of pit foundation is obtained considering seepage force, which is the balance method of seepage force, namely method two. This method is compared with classical pressure balance method, namely method one. The relation between these two methods is discussed from the aspects of principle, expression and application range. It indicates that method one tends to be security when calculating safety coefficient, but the method two is comparatively economic. Owing to the existence of soil pore, seepage of groundwater is common. The layer above the confined aquifer is considered as semi-permeable layer compared with it, so the role of seepage can't be ignored. The method two is more reasonable in calculating safety factor under taking account of seepage force. In real engineering, the concrete condition of pit foundation soil should be analyzed according to the theory of seepage deformation. Different checking method of anti-uprush of pit foundation should be selected. In addition, this paper referenced standard (SL274-2001) in the slope stability factor on the simplification of Bishop and the value principles of Sweden method. The proposed method is method two to calculate. The minimum safety factor is more than 1.1, but it should be less than the new norm, the lower limit of which is 1.5.At the same time, the paper investigates the calculation method of pit foundation dewatering and drawdown considering seepage force, based on the analysis of checking method of anti-uprush. The water drawdown calculated is smaller considering the role of seepage flow compared with the original method in the same initial groundwater level. It indicates that the difference is related to the safety factor and the thickness of overlapping layer. In the drawdown design of seepage force, the scope of application is broad when the overlapping layer regarded as a semi-permeable. It includes limitation of safety factor, in accordance with safety requirements of norms. It ensures the stability of pit foundation anti-uprush. It enhances economic benefit because the water level drawdown of design decreases. Therefore, it has the attribute of objectivity, university and practicality, and more economical and reasonable, considering the seepage force in anti-uprush checking and water level drawdown of design.