Study On The Influence Of Water Level Change Of Expansive Soil Foundation Pit On Pit Wall Load

Expansive soil is widely distributed in China.Foundation pit located in expansive soil area is prone to deformation and failure due to rainfall and water accumulation in the foundation pit during excavation.The instability of foundation pit often causes economic losses,even irreparable consequences such as casualties.There are many factors affecting the instability of expansive soil foundation pit.Rainfall and groundwater seepage on the side wall of expansive soil foundation pit are the key factors inducing the instability of foundation pit.Expansive soil collapses in water,destroying the structure of the soil,resulting in a reduction in strength;in addition,the expansive force generated will also have a negative impact on the support structure.Although people have done a lot of research on the problem of foundation pit,the failure mechanism of foundation pit instability has not been fully grasped,and the influencing factors need to be further studied.In this paper,the shear strength,expansive deformation and permeability of expansive soil are studied,and the stability of side wall is analyzed based on the problem of submergence at the bottom of pit.As a typical unsaturated soil,the strength of expansive soil can be expressed as effective shear strength under saturated state and adsorption strength caused by matrix suction.The increase of water content not only destroys the bonding state of soil particles,but also leads to the decrease of suction.Both of them have important influence on the strength of soil,so it is very important to study the law of soil strength changing with water content.In order to study the matric suction of unsaturated soils,it is necessary to combine the soil-water characteristic curve.According to the existing theory,unsaturated permeability coefficient and shear strength can be analyzed.In this paper,the concept of wet expansion rate is introduced,and the soil-water characteristic curve of expansive soil is measured by using soil tensiometer combined with the relationship between dry density and water content measured by this experiment.Based on the theoretical prediction model and saturated permeability coefficient,the relationship between permeability coefficient of unsaturated expansive soil and water content or matrix suction is predicted,which can be used to study the seepage of groundwater on the side wall of foundation pit.In recent years,finite element analysis software has been widely used in calculating seepage and stability of unsaturated soil foundation pit side wall.In this paper,Geostudo software is used to simulate the seepage field and stability of expansive soil foundation pit side wall,and the changes of pore water pressure distribution and side wall stability coefficient under normal conditions and after the pit bottom immersion are obtained.The main conclusions are as follows:(1)The shear strength index c and φ of expansive soil decrease with the increase of water content of soil sample.The larger the initial moisture content is,the larger the c value is and the smaller the φ value is.This phenomenon is more obvious when the water content is low.When the water content of soil sample increases to 30%,the c and φ values of soil decrease sharply.(2)The swelling rate of expansive soil increases with the increase of water content of soil sample.The larger the free swelling rate of soil sample,the higher the content of hydrophilic minerals,the larger the swelling rate,which can reflect the swelling force of soil after absorbing water to a certain extent.The swelling rate of expansive soil also depends on the initial water content of soil sample and the increment of water content based on it.The lower the initial water content is,the greater the swelling rate is when the water absorption reaches the same water content,and the greater the maximum swelling rate is after the final stability.(3)The soil-water characteristic curve of expansive soil can be measured reliably by tensiometer.On this basis,the unsaturated permeability coefficient of expansive soil is predicted based on theoretical model.The permeability coefficient decreases with the decrease of water content or the increase of matrix suction.The relationship between the two can be expressed by exponential function.(4)Based on the volumetric water content function and unsaturated permeability function,the SEEP/W module of Geostudio software is used to calculate the seepage after the water level rises and the bottom of the pit is immersed.After immersion,the pore water pressure of the soil increases,the suction of the matrix decreases and the water content increases in a certain range of the side wall,which leads to the decrease of the strength of the soil and the increase of the active earth pressure of the side wall.On this basis,the SLOPE/W module is used to analyze the stability of the side wall.The results show that the stability coefficient of the side wall decreases obviously after immersion,which indicates that the strength reduction caused by the increase of water content will have a negative impact on the stability of the side wall of the foundation pit.