Abstract:
Expansive soil is a kind of clay with high plasticity and sensitivity. It has strong characteristics of water-absorbing expansion and water-losing contraction, so its strength is easily affected by dry and wet circulation. Under the condition of dry-wet cycle, expansive soil tends to show certain softening characteristics, and its deformation resistance and strength will gradually decline, which will easily lead to the instability of slope, subsidence of roadbed, damage of irrigation and water conservancy facilities, aggravation of soil erosion, and impact on the agricultural ecological environment. In order to weaken damages to civil engineering facilities and ecological environment induced by expansive soils, the experimental study on the expansive soil mixed with coal gangue powder was carried out. The purpose of this paper was to study strength properties of solidified expansive soil with coal gangue powder and microscopic pore characteristics. Expansive soil and coal gangue used for testing were collected from Gao miaozi township, Xinghe county, Inner Mongolia, China. The combination of different ratios of coal gangue was used to treat expansive soil. The optimum dosage of coal gangue powder was determined according to the no-load swelling test, the loaded swelling test, and the contraction test. And then, we conducted the drying and wetting cycle test on expansive soil with the optimum dosage of coal gangue. In addition, the index of shear strength, including cohesion and internal friction angle, obtained from the shear strength test, and pore characteristic values, consisting of porosity, total pore volume, cumulative pore volume and pore density, achieved from the mercury intrusion test. We revealed the strength change mechanism in the view of microcosmic level. The test results showed that the expansion and contraction decreased after mixing coal gangue powder. The optimal dosage of coal gangue powder was 6%. Compared with unmixed soil sample, the no-load expansion rate, loaded expansion rate and shrinkage rate of improved soil sample were reduced by 7.7%, 36% and 40%, respectively. The dry-wet cycle reduced the cohesion and the angle of internal friction of expansive soil and decreases with the increase of dry-wet cycles. The cohesive force and the attenuation of internal friction angle of coal gangue powder soil were all smaller than that of plain expansive soil. While shear strength attenuation restrained by adding coal gangue powder. The pore density-aperture curves were bimodal distribution. With the increase of dry-wet cycle, pore diameters gathered to big pore of diameter, and granular structure emerged, which depressed the strength indexes. The pore density of big pore in samples with coal gangue powder decreased by about 35% compared with expansive soil without coal gangue, after the fifth dry-wet cycle. As the number of dry-wet cycle increased, the porosity and total pore volume increased, and the big pores also gradually increased. The expansive soil sample mixed with coal gangue powder restricts the formation of large pores, inhibits the damage of dry-wet cycle to the pore of expansive soil, and thus reduces the effect on the strength of expansive soil sample. In conclusion, coal gangue powder can prevent strength of expansive soil from reducing via restraining pores in samples.