The effect of soil fixation and anti-scour instability of slope vegetation generally depends on the strength and anti-disintegration ability of slope soil due to increase of root system. Therefore, it is particularly necessary to study the disintegration characteristics of expansive soil related to slope instability under acidic conditions (simulated acid rain). In this paper, the response surface method (RSM) was used with the pH value, root diameter, root length, root coefficient, and distribution as independent variables, and the disintegration amount of root-soil (DARS) after 60min as the response value. Then X-ray diffractometer (XRD) was used to analyze the mineral composition changes of the sample under this environment. Simultaneously, the plasticity index of expansive soil at different values of pH was studied to discuss the disintegration mechanism of root compound expansive soil in an acid environments. The results show that the root system improves the anti-disintegration characteristics of the root-soil, and the effects of various factors on the amount of disintegration were as follows: root length > pH value > root distribution > root amount > root diameter. The DARS with a length of 20mm increased by 26.67% and 41.56% compared to the 30mm and 40mm. Compared to the horizontal distribution and horizontal + slant distribution, the DARS with slant distribution was increases by 11.39% and 20.24% respectively. The DARS with 2 roots is increased by 9.92% and 16.75% compared to 4 and 6 roots respectively. The 1mm diameter DARS is 6.65% and 15.49% higher than the 2mm and 3mm, respectively. In addition, an acidic environments can lead to an increase in the amount of disintegration or rate of disintegration. The disintegration at pH = 4.2 was increased by 11.4% and 22.4% compared to pH = 5.6 and pH = 7, respectively. The acidity affects soil disintegration is due to the hydrophilic minerals in the expansive soil react with H+ ion in the acid solution to form soluble salts. Due to the dissociation and leaching of free quartz and metal oxides in the soil to varying degrees, the ability of expansive soil to accumulate is reduced. The intensity of erosion and leaching decreases with increasing pH. In addition, the pH value can affects the plasticity index of the soil, which increases with the increasing pH, thus affects the disintegration properties of the expansive soil.
The expansive soil slope is mainly characterized by the decline of slope integrity caused by shallow expansive soil cracking and the destruction of internal soil structure, which seriously affects the overall stability of expansive soil slope. To study the effect of the combination of geogrid reinforcement and slope vegetation on inhibiting the development of expansive soil cracks, six groups of test models were made. The natural dry–wet cycle was simulated, and the crack image was binarized by using image processing technology. The crack characteristic parameters such as crack ratio, crack width, and crack length were extracted, and the effect of various reinforcement methods on inhibiting the development of cracks was comprehensively evaluated. The basic situation of the development of crack indexes in each group with the development of multiple dry–wet cycles was obtained, and the fluctuation changes of crack indexes in different stages were different under different reinforcement methods and dry–wet cycles. At the same time, the influence of different reinforcement methods on the crack development of expansive soil is obtained. It is considered that planting vetiver grass + geogrid backpacking has a good effect on inhibiting the crack development of expansive soil.
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