Study On Seepage Deformation Characteristics Of Silt And Design Of Dewatering Structure

With the continuous development and utilization of underground space,the engineering and environmental problems caused by groundwater become increasingly prominent.The quaternary strata in the Yellow River alluvial plain are mostly interbedded silt and silty clay,and the groundwater level is high,so it is difficult to dewater the foundation pit.The silty soil layer does not have the characteristics of the sandy soil layer with large permeability,easy drainage and consolidation,and does not have the characteristics of the viscous soil with good water-proof performance,not easy to be washed away by water.In the foundation pit engineering,it shows poor engineering properties,and the precipitation effect is difficult to guarantee,and it is easy to occur soil permeability deformation.Therefore,it is necessary to study the permeability and deformation characteristics of typical strata in the middle and lower reaches of the Yellow River.In addition,curtain and pipe well dewatering are important measures to control groundwater in this kind of stratum.It has important engineering guiding significance to analyze and explore the relationship between the depth of hanging curtain and the permeability deformation of foundation pit and the reasonable filter material specification of pipe well dewatering.Based on a project in the bridge resettlement area of Jinan Prior Zone For Replacing Old Growth Drivers With New Ones,this paper analyzes the physical and mechanical properties and particle size distribution characteristics of silt with the help of indoor soil tests.Rely on theoretical research and model tests,the paper focuses on three key problems: the permeability and deformation characteristics of silty soil,the relationship between the depth of pensile curtain in soil and the foundation pit seepage deformation,and the filter material specification of tube well in silty soil.The main research contents of this paper are as follows:(1)The physical and mechanical properties of silty soil are comprehensively analyzed through indoor soil tests,and the silty soil particle size grading curve was drawn by combining the screening method and densitometer method,so as to obtain the non-uniformity coefficient and grading characteristic value of the silty soil,and analyze the content percentage of clay,silt and sand in the silty soil particles,so as to determine that the silty soil was sandy silty soil.It lays a foundation for the subsequent theoretical calculation and model test in this paper.(2)By reviewing the literature at home and abroad,the theory of seepage deformation and the discriminant methods of seepage deformation under different applicable conditions were summarized,and the generation conditions,the non-uniform coefficient method,the gradation curve method,the comparison method of pore diameter and fine particle size and the fine particle content method were used to discriminate the silty soil would occur the fluid soil type failure.Through the theory of critical hydraulic gradient of soil layer,the critical hydraulic gradient of silt layer is preliminarily calculated.In addition,the seepage deformation tests of silty soil under different seepage directions were carried out with a self-made model box,and the critical hydraulic gradient and failure hydraulic gradient of silty soil under vertical and horizontal seepage were obtained,and the complete disaster process of the occurrence,development and final failure of the silty soil layer was analyzed.(3)The paper summarizes the influence of the hanging curtain’s depth on the foundation pit,and preliminarily determines the quantitative relationship between the pensile curtain’s depth and the difference of water level inside and outside the foundation pit through the theory of soil stability.The self-made seepage model box was used to simulate the groundwater environment of foundation pit excavation,and six groups of groundwater seepage tests were designed according to different penetration depths of different curtain and without curtain,to explore the relationship between the penetration depth of curtain and the difference of water head inside and outside the foundation pit when penetration failure occurred inside the foundation pit.(4)According to the arrangement relationship between aquifer particles and filter material,the filter material arranged into a cube formed the largest pore,arranged into a ruse body formed the smallest pore,so as to deduce the theoretical range of the filter material filling ratio η.A model test of filter material specifications in silty aquifer tubewell was carried out through a self-made seepage device,and 8 groups of filter material with different levels were designed to deeply explore the influence of filter material specifications on seepage water yield and silt particle loss.It is proved that when the ratio η is 7～9,a large number of silt particles will not be lost and good drainage effect can be guaranteed.(5)Taking a block project in the bridge resettlement area of Jinan Prior Zone For Replacing Old Growth Drivers With New Ones as the engineering background,combined with theoretical analysis and model test research results,in-depth analysis of the causes of the local collapse accident of the foundation pit,mainly in the following three aspects:Continuous rainfall results in excessive hydraulic pressure inside and outside the foundation pit,without pensile curtain is set to block the hydraulic connection,and inappropriate filter material causes tubewell silting failure.And put forward the specific accident solution,for the future similar foundation pit projects to provide reference.