Study On Seepage Of Foundation Pit And Engineering Application

Many projects are related with movement of water in the soil in each field of civil engineering. Stress, deformation, strength and stability of soil are related with movement of water and seepage, so seepage has great effect on civil engineering. Especially pit is excavated in the region with high underground water level and large permeability coefficient, instability of excavation slope created by seepage failure of soil is major or crucial reason for fatal accident of foundation pit engineering. The destination goal of seepage calculation is solving hydraulic factor of seepage field such as seepage discharge, head pressure, compression force and hydraulic slope, analyzing seepage stability, selecting rational impervious design and drainage, or reinforcement design, effectively controlling seepage. The state of saturated line of excavation slope is necessary information for seepage calculation and slope stability verification. Seepage slope of each location of excavation slope and pit foundation should be less than allowable value of soil, otherwise, soil would be seepage failure.The large foundation pit excavation of Jantan hydro-junction power plant, which is located Huizhou city, Guangdong province, its depth and area is large, and located in river bed with high underground water level and large permeability coefficient. So seepage is the central problem in the cause of excavation. Taken the pit excavation as an engineering example in this paper, pit seepage is studied including saturated line and seepage discharge, and excavation slope stability is discussed induced by seepage. For the research, the main studying content is listed as follows.1. Based on the classical theory of seepage, borrowing ideas of seepage analysis methods from sands soil dam, and using classical subsection-combination method, simplified model I is obtained for analyzing seepage of foundation pit. Considering the effect of soil strata and using seepage refraction at the interface between two different soils, model II is obtained on the basis of model I. Seepage discharge is calculated respectively by solving the two models under the condition of normal and maximum water levels. The results indicate that, the seepage discharge of pit calculated by model II is coincident with actual better than that of model I, so model II is better under the normal underground water level.2. Types of seepage deformation and methods of calculation corresponding critical hydraulic slope of soil are discussed. Based on these methods, and used relational parameters offered by actual project and the calculated results of excavation, seepage deformation is evaluated that of power plant foundation pit. The results indicate that, the pit is unstable and unsafe without watertight structure; when setting impervious wall, the hydraulic gradient on slope steps down to the value lower than 0.05, the probability of seepage deformation is small when setting impervious wall because the hydraulic gradient reduces to very small value.3. Used limit equilibrium theory to analyze stability of sandy slope consideration in seepage, treated slide mass as a whole, considered force equilibrium and moment equilibrium at the same time to ascertain the minimum safety factor and the state of slide. Based on the magnitude of natural repose angle of soil and slope angle, earth slide is divided into shallow slide and deep slide, and respectively discussed. Combined with engineering, the slide safety factor is calculated and accorded very well with practice.4. Based on geological information of hydro-junction engineering, and in the presence of Geo-CAD software, the power plant strata are visualized in 3-Dimension including each soil, rock layer and direction and magnitude of fault. That is supplied necessary information for setting strata parameters, constructing rational calculation model and modeling pit excavation.5. Selecting two representative sections, respectively named 1-1, 2-2, recovered the shortcoming of modeling in FLAC^3D, two sections are meshed into perfect grid. In the cause of modeling, four operating mode are considered of section 1-1, namely initial equilibrium, setting open-end caisson, excavation and backfilling; three operating mode are considered of section 2-2, namely initial equilibrium, excavation, setting open-end caisson and backfilling; For excavation and backfilling modes of each section, two results are calculated and compared between coupling seepage and uncoupling seepage. The results indicate that, considering the effect of seepage coupling, the total stress of soils declines and so does of pit's rebound displacement, yield region shows past yield. Open caisson and impervious soleplate laid on the floor of the pit have obvious effect on reducing seepage discharge, and seepage discharge is coincident with calculated by model II under the condition of without watertight structure.