Crack Development For Grantic Weathered Soil And Its Influence On Wall Collapse Of Benggang

Benggang erosion is a severe environmental issue that hinders the ecological safety and sustainable economic development in the low mountainous regions of southern China.The formation and development of slope erosion are influenced by the characteristics of rock and soil as well as environmental factors,with slope collapse being the main driving and accelerating force.Granite,as the primary parent material for slope erosion,possesses unique structural composition and strength characteristics that fundamentally contribute to slope instability.The favorable hydrothermal conditions in southern China facilitate the occurrence and development of fractures within weathered granite soils,which severely disrupt the integrity and stability of the soil structure,leading to slope collapse and overall instability.Research on the instability of slope erosion focuses on the effects of external environmental and internal geological factors,while the mechanisms from localized fracture development to overall deformation and failure of the soil are still unclear.Therefore,it is of great significance to investigate the characteristics of fracture development in weathered granite soils and clarify the mechanisms by which fractures contribute to slope collapse,to reveal the erosion mechanism of Benggang erosion.In this study,Benggang in four typical areas of Benggang erosion,namely Tongcheng County in Hubei,Gan County in Jiangxi,Changting County in Fujian,and Wuhua County in Guangdong,were studied.The distribution characteristics of crack development in the collapsing wall were investigated through field surveys and structural visualization technology.The instability characteristics of collapsing wall deformation were quantified based on in-situ monitoring and its response relationship with changes in water content,guiding the simulation experiment of indoor reshaping of soil cracks.The development characteristics of cracks under dry-wet alternating driving and their influence on soil structure and strength were analyzed through indoor experiments.The erosion amount of the collapsing wall was obtained based on point cloud models,clarifying the relationship between crack distribution characteristics and slope collapse amount,and combined with numerical simulation analysis to elucidate the influence of cracks on slope instability.The main research results are as follows:(1)The distribution characteristics of cracks in the collapsed wall have obvious spatial heterogeneity.Based on the main causes and occurrences of the cracks,the cracks in the collapsed wall can be classified into two types: unloading cracks and seepage cracks.Unloading cracks mainly occur in the upper and lower parts of the collapsed wall,while seepage cracks mainly occur in the middle and upper parts of the collapsed wall.Since the development of cracks is affected by stress unloading and changes in moisture content,the cracks in the collapsed wall can be classified into slope top,slope middle,and slope foot cracks according to their occurrence sites.The distribution characteristics of the cracks show that slope top cracks are the deepest,with an average depth of 1.07 meters and an average length of 0.94 meters.Slope middle cracks are the shallowest,with an average depth of 0.25 meters,but they have the largest proportion,accounting for 53%.Slope foot cracks are fewer and difficult to observe.Combining CT scanning processing,voids with a sphericity greater than 5 are defined as microcracks,and the results show that all microcracks in the weathered granite soil are voids with an equivalent aperture greater than60μm,with an average proportion of microcracks of 68.08%.The degree of microcracks in the collapsed wall shows a trend of first decreasing and then increasing with increasing depth of the profile.There is a significant power function relationship between void connectivity and microcrack degree,while there is a significant power function relationship between clay content and equivalent average aperture.The microcrack degree in the middle part of the slope is small,but it has the largest proportion in the voids and the best crack connectivity.The distribution characteristics of microcracks in the collapsed wall are specifically manifested as follows: the most microcracks occur in the slope top,but their average diameter is small;there are fewer microcracks in the slope middle,but they are concentrated and have a larger diameter;there are more voids with an equivalent aperture greater than 60μm in the slope foot,but the development of microcracks is scattered,and the crack connectivity and average diameter are smaller.(2)The spatial and temporal variations of soil moisture characteristics are closely related to the instability process of the landslide.The instability of the landslide is manifested as continuous deformation at the crest,no apparent deformation in the middle slope,and long-term slow and short-term rapid deformation at the toe.The maximum displacement of the landslide is concentrated at the crest,with a maximum total displacement of 6.06 mm in three years,and the monthly displacement rate increases with the increase of the coefficient of variation(CV)of soil moisture.The maximum monthly displacement at the toe occurs in the month with the most dramatic change in water content,and landslides occurred in that month.The variation of soil moisture content in the landslide mainly occurs at the crest,and the soil moisture content decreases with the depth of the profile.The variation of monthly soil moisture content is closely related to the seasonal variation of rainfall,showing a clear alternation between dry and wet periods.Using the soil moisture change parameters and cumulative rainfall parameters in the random forest model has high accuracy in predicting landslide displacement.The monthly coefficient of variation of soil moisture content and the maximum water content change between two months have the highest explanatory power for landslide displacement,and the absolute value of monthly displacement at different depths has a significant power function relationship with the monthly coefficient of variation of soil moisture content at that location.(3)The development of cracks in the landslide is influenced by the joint effect of drywet alternation and soil clay content,while the effect of dry-wet alternation leads to the attenuation of soil structural strength by affecting the development of cracks.Under the influence of dry-wet alternation,soil with 30% clay content shows a complex network of crack development features,and cracks stabilize after two cycles of dry-wet alternation.The crack ratio has a significant exponential function relationship with the soil cohesion when the crack ratio is greater than 6.9%,causing a significant decrease in soil cohesion.Soil with 20% clay content shows a development feature of a single main crack,and the crack development stabilizes after four cycles of dry-wet alternation.The crack ratio has a significant linear negative correlation with the soil cohesion.On the other hand,soil with10% clay content has no obvious surface crack development.The degree of microcrack increases under dry-wet alternation,eventually forming macroscopic cracks.The higher the degree of microcrack,the lower the soil cohesion and the consolidation coefficient.The intensity of dry-wet alternation mainly controls the degree of surface crack development,while the frequency controls the degree of microcrack development.The effect of crack ratio on water holding capacity is similar for soils with different clay contents,showing that the higher the crack ratio,the worse the soil water holding capacity.When the crack ratio is greater than 4.47%,water holding capacity decreases significantly.Redundancy analysis shows that crack width,crack length,and degree of microcracks explain the soil structural strength at the top,middle,and foot of the slope,respectively.The path analysis of the structural equation model shows that the effect of dry-wet alternation leads to the attenuation of soil structural strength by affecting the development of soil cracks.The soil strength parameter is greatly influenced by the development of surface cracks,while the water-holding capacity is greatly influenced by the degree of microcracks.In addition,the depth of crack development in soil with 30% clay content stabilizes after three cycles of high-intensity dry-wet alternation.The accuracy of the analytical solution for predicting the depth of crack development based on the elastic model is 12.81%.(4)The total length of the cracks on the collapsing wall is the main influencing factor on the rate of collapse erosion on the collapsing slope.The vertical development depth of the cracks affects the volume of the collapsing body at that location and the stability of the collapsing wall.In-situ monitoring shows that the total erosion of the collapsing slope tends to increase gradually from north to south,while the proportion of the collapse volume gradually decreases.The contribution of the collapse volume of different parts of the collapsing slope to the total collapse volume is shown as follows: the slope top > the slope foot > the slope middle,and the average depth of the cracks at different slope positions has a significant power function relationship with the collapse volume at that location.A partial least squares model considering the length of the cracks,terrain,climate,and clay content can accurately predict the collapse volume.The total length of the cracks on the collapsing wall and the clay content at the slope top are the main controlling factors of the collapse volume,and both have a significant power function relationship with the collapse volume.The elevation integral of the residual erosion volume of the collapsing wall(HI)dominates the regional distribution difference of the annual collapse volume.Finite element numerical simulations based on Flac3 D analyzed the stability of the seepage process of the collapsing wall under three conditions of crack existence.The results showed that the existence of cracks would lead to the formation of stress concentration zones inside the soil mass,and the slope top cracks had the greatest influence range on seepage,while the slope middle cracks had the smallest influence range,and the plastic failure surface would be affected by the presence of cracks above it when there were cracks at the slope foot.When the cracks at the slope top,slope middle,and slope foot developed at actual maximum depths of 1.5m,0.5m,and 0.5m,respectively,the safety factor calculation results were: slope top cracks <slope middle cracks < slope foot cracks.When the depth of the cracks was less than 0.5m,the safety factor was slope top < slope foot < slope middle,and the stability of the collapsing wall decreased with the deepening of the crack development depth,which was consistent with the monitoring results.In summary,this study shows that the alternation of wet and dry conditions leads to the development of cracks and a decrease in soil structure strength.The presence of cracks promotes soil water infiltration and further vertical development of cracks,ultimately causing the collapse of the soil along the crack surface.The development of wall collapse cracks is influenced by the content of soil clay particles and the alternation of wet and dry conditions,and the spatial distribution of cracks is caused by the spatial variation of wall soil properties and water content.Among them,the cracks at the top of the slope develop deeper and longer,resulting in a larger volume of the slope collapse.The cracks in the middle of the slope develop shallowly,resulting in a smaller volume of the slope collapse.The cracks at the foot of the slope are mainly internal micro-cracks in the soil and are the weakest,and collapse will occur once the micro-cracks are connected.In general,the total length of cracks is the main factor affecting the short-term erosion rate of slope collapse,while the depth of cracks in different parts determines the volume of collapse in that area and the way and degree in which it affects the stability of the slope.This study provides a theoretical basis for the study of the erosion mechanism of granite rocky desertification in southern China and the prevention and control of regional soil erosion.