| In order to prevent water and soil loss caused by Benggang collapsed,this paper studies the Bengang collapsed mechanism from the perspectives of the characteristics of crack and expansion-shrinkage.Taking the typical Benggang of Wuli Town,Tongcheng County,Hubei Province as the research object.Conducting indoor experiments under controlled soil hight-to-diameter ratio conditions and dry-wet cycles conditions,a total of two cameras are set up on the top and side of the soil specimen to take pictures at a fixed time and fixed site,combined with digital image processing technology,analyzing qualitatively the crack development,expansion-shrinkage deformation rules of the four layers(the topsoil layer,the laterite layer,the transition layer,the sandy layer).The research results can provide a scientific basis for improving the stability of Benggang soiland preventing the occurrence of collapse.The main results are as follows:(1)Through a single dehumidification experiment,the crack development is divided into three stages: the initial gestation stage,the rapid growth stage,and the stable stage.Among the four layers of soil,the transition layer cracks are the most prominent,with a shrinkage cracking crack degree of 18.78%,and the sandy layer cracks are not obvious,with a shrinkage cracking crack degree of 5.41%.The two layers belong to adjacent soil layers on the vertical section of the Benggang soil,and the greater difference in the development of cracks between the two will destroy the stability of the soil and promote the occurrence of collapse.(2)The dehumidification experiments are carried out under the condition of controlling the height-to-diameter ratio of the soil.The results showed that: under the same height-to-diameter ratio,the cracks developed similarly,and the crack parameters are not much different.Under different height-to-diameter ratios,with the increase of the heightto-diameter ratio,the radial shrinkage area gradually increases,and the cracked area gradually decreases until no cracks develop.The deformation of the soil morphology changes from shrinkage cracked soil(crack development accompanied by radial shrinkage)to only radial shrinkage soil.In this process,the specific critical values of the height-to diameter ratio of the four layers are respectively: 0.147～0.160,0.160～0.180,0.160～0.180,0.134～0.147.When the height-to-diameter ratio is 0.147,the shrinkage cracking crack degree of the topsoil layer,laterite layer,and transition layer all reach the maximum:17.79 %,21.85 %,22.44 %,and when the height-to-diameter ratio is 0.234,the sandy layer reaches the maximum value of 5.90 %.(3)The shrinkage cracked soil specimens and the radial shrinkage soil specimens were subjected to the indoor dry-wet cycles experiments respectively.The results showed that:As the number of dry-wet cycles(T),for the shrinkage cracked soil specimens,with the progress of the dry-wet cycles,the crack morphology of the topsoil layer and sandy layer is basically unchanged,the laterite layer and transition layer tends to become more complicated.In the crack parameters,the degree of shrinkage cracking crack,average crack width gradually decrease and become stable.The total length of cracks,the number of crack strips,the number of crack intersections,the number of divided soil lumps first increase and then decrease,all crack parameters stabilized after the third dry-wet cycle.The radial shrinkage phenomenon no longer occurs,and the axial shrinkage rate gradually decreases and stabilizes after the fourth dry-wet cycle.For the radial shrinkage soil specimens,the radial shrinkage rate and radial expansion rate gradually decrease and become stable after the third dry-wet cycle.The radial expansion ratte and axial expansion rate of the soil specimens in the two states have the same changing trend.The radial expansive rate gradually decreases and stabilizes after the third dry-wet cycle,the axial expansive rate first increases,then decreases,and finally stabilizes,reaching its maximum value in the second dry-wet cycle.(4)The force measurement during the dehumidification process was carried out by a Flexiforce sensor.The results show that the shrinkage cracked soil specimens’ radial force is mainly affected by radial tensile stress and crack development,in the process of dehumidification,it reaches the maximum about the cracking water content;and the axial force is mainly affected by gravity,axial tensile stress and crack development,the overall trend is decreasing in the process of dehumidification.The radial shrinkage soil specimens’ radial force is mainly affected by radial tensile stress,and the axial force is mainly affected by gravity and axial tensile stress.Among them,the radial shrinkage rate of soil is mainly affected by the radial tensile stress,and the axial shrinkage rate is mainly affected by gravity.(5)Combine the observation results,quantitative analysis results,and force measurement results,the soil deformation process in the dehumidification process can be divided into five stages:(1)axial shrinkage;(2)the upper soil body shrinks radially,and then the axial shrinkage speed slows down;(3)the cracks develop,the whole soil shrinks radially.At this time,both the radial and axial shrinkage speed slows down;(4)The crack skeleton develops and stabilizes,the crack deepens and widens,the radial shrinkage speed slows down,and axial shrinkage stops;(5)The crack develops and the shrinkage deformation stops. |
