Sduty On Engineering Properties Of Expansive Soil By Resistivity In Situ Test

Expansive soil contains a large number of hydrophilic mineral.The water changes make noticeable swelling-shrinkage of expansive soil,which does great harm to the project.Thus,it is necessary to investigate and evaluate expansion potential of soil before the construction of the project in site.At present,the engineering evaluation of expansive soil site mainly predicts swelling deformation of expansive soil site by drilling,sampling and laboratory tests.This method is usually cumbersome and time-consuming.Therefore,it is of importance to study a convenient and quick way to evaluate the engineering properties of expansive in-situ soil.Soil resistivity is the basic parameters of the soil.The main effect indicators include water content,density,saturation,porosity,particle structure,composition and concentration of pore fluid ions in the pore characteristics,mineral composition,etc.The changes of these indexes will cause the engineering characteristic indexes and resistivity changes of expansive soil to a certain extent.Therefore,it is of practical and theoretical significance to study the relationship between the resistivity index and the engineering properties of expansive soil to establish the in-situ evaluation of the engineering properties of expansive soil.This paper is based on the rapid expansion project of African black cotton soil.The research was performed by the laboratory test,theoretical analysis,model test and field test.The relationship between the resistivity index and the engineering properties of expansive soil,as well as the in-situ test method based on resistivity,are studied.The main research achievements are as follows(1)The engineering properties of natural and manual expansive soil(bentonite-kaolin compound sample)are studied by the indoor tests.The test results show that for the manual expansive soil,the free expansion ratio,the free expansion coefficient increased with the ratio of bentonite increasing.The unloaded linear expansion ratio increased with the moisture content increasing,increased by the exponential function with the ratio of bentonite increasing.The clay content increasd but the sand content decreasd with the ratio of bentonite increasing.The liquid limit and plasticity index increased obviously and the plastic limit was stable with the free expansion ratio increasing.PH increasd with water soil ratio incrasing and tended to be stable,decreased linearly with the free expansion ratio increasing.For the natural xpansive soil,the unloaded linear expansion ratio increased by the exponential function with the free expansion ratio increasing,pH increased linearly with water soil ratio increasing.(2)The relationship of natural and manual expansive soil between electrical indicators and engineering properties is studied by the indoor electrical tests.The test results show that the resistivity of natural and manual expansive soil decreased by the exponential function with moisture content,density,saturability,the free expansion ratio,the liquid limit,plasticity index,the clay content,pH increasing and increased by the exponential function with the porosity increasing.All the fitting is good.(3)The resistivity probe testing in situ is developed.This kind of probe is adopted with the test method of four electrodes vertical electric sounding and applies to test the resistivity of the soil within 50 meters under the ground.(4)The resistivity of the expansive soil is tested by the indoor model tests and field tests.The test results show that the varirtion of the resistivity by the indoor model tests is the same with that by Wenner four electrodes test method.That is to say,the resistivity deceased with the moisture content and the free expansive ratio increasing.The resistivity by the field tests decreased with the depth and the free expansive ratio according to the tested resistivity has fit in nicely with the free expansive ratio according to the indoor tests.