Study Of The Fissures And Modification Of Expansive Soil In Hefei Under Wetting-drying Circulation

Expansive soil has the characteristics of strong swell-shrink, developing fissures andover consolidation.Among them,the first two characteristics have great influence on itsengineering geological properties. Under dry-wet circulation conditions, the fissures candestroy the integrity of the soil and decrease its strength, at the same time,the repeatingexpansion and contraction of expansive soil caused by volume changes are the mainreason for the fissures generated.Studying on the evolvement rules of fissures and theswell-shrink indicator of expansive soil can help us revealing the mechanism of theslope and roadbed destroyed, and provide exact evidences and suggestions on themodification of expansive soil.This paper is aimed at the fissures behavior and improvement of the Hefei expansivesoil. It focuses on the change rule of the expansive soil under dry-wet circulationconditions. Field observation test and laboratory simulation test are carried out for theresearch on the fissures behavior. As for the former, field measurements are carried outto research the fissures quantitatively. As for the later, raster images vector conversiontechnology is adopted to carry out quantitative analysis for the evolvement rule of thefissures. As for the research on the expansive soil modification, lime is used as the mainfactor to test the swell-shrink indicator of expansive soil with indoor soil test andcomment the improving effects. Moreover, many wetting-drying tests are carried out toresearch the stability of the improved expansive soil.A piece of side slope in a building area under construction in Hefei is selected tocarry out the field observation test for fissures. The test results show that when the soilmoisture content increases, the widths of both primary and secondary fissures decrease;when the soil moisture content decreases, the average width of primary and secondaryfissures increases and the variation tendencies of both primary and secondary fissuresare the same, which reflects that there is a negative correlation between the change ofexpansive soil fissures and the soil moisture content. Under dry-wet circulationconditions, the shrinking effect of fissures weakens and the swell speed becomes faster.The width of the primary fissures will not increase without limits. It is limited by thesurrounding soil. However, secondary fissures may develop to primary fissures with thedevelopment of drying and watering circulation.Through the indoor simulation wetting-drying test, the following conclusions can be gained: the production mechanism of fissures is not evenly related to thedehumidification rate of the internal and external soil. The gradient change of soilmoisture content leads to the changes in the internal and external stress of the soil. Thearea ratio and length ratio of Fissures and the average width of Fissures increase rapidlywith the decrease of soil moisture content in the early period while maintain a stablecondition after decreasing with the decrease of soil moisture content after the moisturecontent reaches the peak. Under dry-wet circulation conditions, the area ratio and lengthratio of Fissures and the average width of fissures increase with the increase of thecirculation time, which is reflected in the increase of the total area and length of thefissures. However, the effect of wetting-drying circulation is limited. fissures do notdevelop without limitations. During the dry-wet circulation of expansive soil sample,the shrinking percentage of the sample increases as well. It has certain inhibiting effecton the development of fissures. However, the effect is non-significant.Through the characteristic test of improving expansive soil with lime and stabilitytest under dry-wet circulation conditions, the following conclusions are gained: the bestlime-additive ratio is6%～8%. This ratio can ensure good effect and maintain stabilityfor the improved expansive soil under dry-wet circulation conditions.