Experimental Study On Effect Of Microwave Heating On Properties Of Expansive Soil

Microwave heating has the characteristics of fast heating,uniform heating and strong penetration.These advantages inspire a new idea of improving expansive soil by microwave heating.In this paper,the industrial microwave oven was used to heat the expansive soil in a river bed at different times,and the influence of microwave heating time on the physical and mechanical properties and swelling characteristics of expansive soil was studied.The main conclusions are as follows.(1)Microwave heating will change the apparent characteristics and basic physical properties of expansive soil.With the extension of microwave heating time,the color of soil sample gradually becomes darker,from yellowish brown to brick red.The specific gravity of soil sample increases first and then decreases with the extension of microwave heating time,and its liquid plastic limit decreases with the extension of microwave heating time.(2)Microwave heating can obviously reduce the free swelling ration,load-free swelling rate and loaded swelling rate of expansive soil.With the increase of heating time,the three kinds of expansion rates all decrease,and are nearly linear with heating time.The effect of heating time on free expansion rate is the most significant.(3)In the shear strength index of consolidated undrained shear,the internal friction angle increases with the extension of microwave heating time,the cohesive force first increases and then decreases with the extension of microwave heating time,and the maximum deviatoric stress increases with the increase of heating time.The direct shear strength of expansive soil increases with the increase of heating time,and the internal friction angle and cohesive force increase with the increase of heating time.(4)The minerals in the soil sample undergo a two-stage physical and chemical reaction under the action of temperature,which leads to the change of the physical and mechanical properties of the soil sample.The temperature range of the first stage is100 ℃-200 ℃.This stage is mainly due to the escape of bound water between layers of clay minerals such as montmorillonite.A prominent endothermic valley will be formed on the differential thermal curve.When the temperature reaches 400°C,it enters the second stage,mineral decomposition or structural decomposition,the crystal lattice of montmorillonite mineral is destroyed,and montmorillonite decomposes into other minerals.The decrease in the content of montmorillonite is the cause of the decrease in its expansibility.(5)Scanning electron microscope was used to analyze soil samples with different heating time.The results show that microwave heating makes the connection between soil micro aggregates more firm,reduces the appearance of soil pores and cracks,and increases the number of soil micro pores.When the heating time reaches 15 min,the specific gravity reaches the maximum.The decrease of macropores makes the microstructure of expansive soil more compact,which is also the reason for the decrease of expansibility.