Study On Properties And Modification Of HeFei Expansive Soil

With the vigorous development of China’s infrastructure construction,the capital construction projects in various regions have reached a peak.Many projects will encounter expansive soil foundation with complex properties in actual construction.Because of its poor engineering properties such as water absorption and expansion,water loss and contraction,many engineering problems and economic property losses have been brought in actual construction.Therefore,it is important to fully understand the modification of expansive soil.In this paper,the expansive soil problem is mainly studied from the following aspects:(1)A series of basic physical and mechanical properties tests of expansive soils are carried out,such as limit water content,specific gravity,compaction and shear.The slow shear tests of expansive soils are mainly expounded.It is concluded that the slow shear tests can be divided into three stages:attenuation strain,constant velocity strain and acceleration strain.The instantaneous deformation of soil sample increases with the increase of horizontal stress.Under the same stress path,the larger the vertical load,the smaller the deformation.When the vertical pressure and shear stress are the same,the instantaneous displacement increases with the increase of water content.When the shear stress is less than 50 kPa,the instantaneous displacement is dominant in the two kinds of soil samples with different water content.When the shear stress continues to increase,the larger water content will lead to failure.(2)The mechanical properties of expansive soil are improved by using fibers,and the effects of different fibre lengths and fibre contents on the shear properties of expansive soil are explored.The results show that the expansive force of expansive soil is weakened after adding fibers.Under the same fibre length,the expansive force decreases with the increase of fibre content.When the fibre content reaches 0.3%,the expansive force of expansive soil is restrained.The effect of force is the strongest.The cohesion of expansive soil is improved by adding fibers in direct shear test.When the content of fibers reaches 0.3%,the cohesion of expansive soil reaches the maximum,and the cohesion of expansive soil increases with the increase of the length of fibers.The optimum content of fibers is 0.3%.It is concluded that the fiber content and fiber length have no significant effect on the internal friction angle,but generally reduce the internal friction angle of expansive soil.(3)The influence of different cationic concentration on expansive soil expansibility is analyzed.The free expansibility,unloaded expansibility and loaded expansibility of expansive soil are tested after adding cations.The results show that different K~+concentration can inhibit the expansive properties of expansive soil.For the free expansibility and unloaded expansibility,when K~+concentration is 3 mol/L,the inhibition effect is the best,when the concentration exceeds this value.The inhibition effect was weakened.The existence of K~+shortens the time for expansive soils to reach expansive stability for expansive soils.For the loading expansion rate,the expansion rate decreases obviously in the initial stage of loading.Because of the existence of vertical load,K~+has more significant inhibition effect on expansive soil,but when the vertical load reaches a certain value,the expansive soil will collapse.(4)Using scanning electron microscope(SEM)to join the cationic micro analysis of soil samples,after tests is to join the cationic appeared after the soil internal crystal between micro aggregate distribution in the soil.Soil makes the connection between the micro aggregate more solid,reduced the occurrence of soil pore and fracture,the nμmber of micro pores of soil increased.When the ion concentration is 3 mol/L the maximμm pore size is less than the proportion of 1μm~2 74.1%,increased by 19.3%,and the nμmber of macropore get decreases,and the expansion of the than normal soil specimen decreases by 8.9%.In addition,the nμmber of pores with a diameter less than or equal to 1μm reached 66.7%when the cationic concentration was 3mol/L,the corresponding nμmber of pores with a diameter greater than 5μm was reduced,and the nμmber of pores with a diameter less than or equal to 5μm reached 74.4.Finally,it can be seen that the three parameters of pore area,pore diameter and pore circμmference of expansive soil have basically the same trend of change.