| There are a large number of unsaturated expansive soils in Pingdingshan area.Unsaturated expansive soil contains a large number of clay minerals with strong hydrophilicity,such as montmorillonite,illite,kaolinite and so on.Therefore,it is very sensitive to the change of humidity.With the development of urban construction in Pingdingshan area,in order to maximize the use of limited land resources,the larger the buildings,the deeper the foundation pit,and inevitably encounter the problem of deep expansive soil.At present,the commonly used supporting methods of expansive soil foundation pit in Pingdingshan area include slope setting,soil nailing wall supporting,row pile supporting or various supporting structures.Due to various reasons,some deep foundation pits with expansive soil have a long construction period,sometimes even more than one year,which will lead to long-term exposure of expansive soil on the wall of the pit.When the adverse conditions such as rainfall,ice and snow melting water,leakage of underground water pipes and so on are encountered,longterm immersion of pit bottom will occur.Due to the water sensitivity of expansive soil on the pit wall,it is easy to soften the expansive soil and cause deformation pressure.This additional lateral pressure may lead to the destruction of the supporting structure.For example,the local deformation of expansive soil foundation pit supported by sloping and soil nailing wall is too large,which may even lead to the collapse of the pit wall.The main problems of expansive soil foundation pit supported by row piles are the collapse of some retaining walls between piles,serious or even the collapse or fracture of row piles,which leads to the collapse of the foundation pit.This paper takes the deep foundation pit of Xihu Pearl Expansive Soil in Pingdingshan New City as the research object.Through field investigation,laboratory test and theoretical calculation,the permeability characteristics of unsaturated expansive soil in the pit wall of deep foundation pit under immersion condition,the distribution law of volume water content of unsaturated expansive soil in the side wall under immersion condition,and the distribution law of soil pressure in the side wall under immersion condition of pit bottom are studied.The soil nailing support under the condition of water immersion at the bottom of the pit is studied.The results are as follows:(1)Through the seepage test of expansive soil water movement and the test results of unloaded expansion rate,the distribution law of volume water content of soil in 7 days of seepage shows that the volume water content of soil surface will rapidly increase from the initial value(24.75%)to a maximum value(43.23%)close to the saturated volume water content in a very short time after water is applied to the soil surface.? The wetting front will move forward with infiltration,and the distribution curve of water content will gradually change from steep to relatively flat.(2)Combined with soil-water characteristic curve,the hydraulic conductivity of expansive soil with initial volume water content of 24.75% and dry density of 1.65g/cm3 is calculated quantitatively by instantaneous profile method.The hydraulic conductivity of unsaturated expansive soils is a function of water content(or matrix potential)and an exponential function,which increases with the increase of water content(or matrix potential).(3)According to the infiltration solution of the one-dimensional linearization equation of water movement in unsaturated soil,the distribution law of water content in the pit wall after 60 days' immersion under the known water content at the bottom surface of the pit is calculated.It is found that with the increase of infiltration depth in the horizontal and vertical directions of the pit wall,the volume water content increases gradually,and the increasing range is smaller,and the increasing rate is also higher.The more slowly it comes,the less it will increase until it is equal to the initial moisture content of the soil.(4)When the water content of expansive soil increases,the force that restricts its free expansion is named as the wet expansion force.From the test results of wet expansion force,it can be seen that the wet expansion force of expansive soil increases with the increase of water content under the condition of initial water content and initial dry density.According to Rankine's earth pressure theory,the active earth pressure on the side wall of foundation pit is regularly triangular or trapezoidal distribution without considering the influence of expansive soil after 60 days of immersion.Considering the strength reduction and wet swelling force of expansive soil,the strength of active earth pressure will increase in expansive soil layer,especially in the area where the water content of expansive soil increases,the strength of active earth pressure will increase sharply and irregularly.(5)Through theoretical calculation,the design parameters of soil nailing support for expansive soil foundation pit after 60 days of immersion in pit bottom are studied.Expansive soil is regarded as ordinary cohesive soil in the design of soil nailing support,only considering the strength reduction of expansive soil,and considering the effect of the strength reduction of expansive soil and the wet swelling force caused by water absorption expansion.In the design of foundation pit soil nailing support,the length of soil nailing considering only the strength reduction of expansive soil is longer than that of soil nailing without considering the influence of expansive soil,especially the length of soil nailing in expansive soil layer increases obviously,and the reinforcement bar also needs to be coarsened.At the same time,the design length of soil nailing considering the strength reduction and wet swelling force of expansive soil is longer than that of soil nailing considering only the strength reduction.This paper has 27 figures,46 tables and 101 references. |
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