Formation Mechanism Analysis Of Gradually Retrogressive Loess Landslides In South Jingyang,China

The loess landslides induced by irrigation and other factors in the South Jingyang Plateau were found to be featured by the characteristics of frequent occurrence,group-occurrencing and catastrophe.Such loess landslides occur sometimes on the edge of the Plateau,causing the Plateau surface gradually to retreate.Furthermore,sliding events are responsible for the serious loss of land resources,seriously affecting the sustainable development of the local region's society and economy.The study provides an experimental basis for the prediction and controlling of landslides in the region and provides a theoretical reference for landslide disaster prevention in the Loess Plateau,with important engineering significance and scientific value.Supported by the national basic research program of China: Occurrence and evolution mechanism and control theory of major disaster and disaster chain in loess,the retrogressive loess landslides in South Jingyang were chosen as an object to reveal the formation mechanism of such loess landslides based on the research needs and status of loess landslides.Firstly,the geological conditions of the retrogressive loess landslides in the south Jingyang platform were investigated by using field surveys.And the important internal and external factors affecting the formation of such landslides were summarized.The ring shear tests with different initial moisture content,normal stress and shear rate were conducted on loess samples to explore the effect of influencing factors on the mechanical behavior of loess.Furthermore,a series of conventional triaixal shear tests were conducted on intact loess samples and fissured loess samples respectively,to clarify the role of joint fissures in the formation and evolution of the geological structure and landslide hazards in the South Jingyang plateau.Based on the field investigation and experimental results,the stability of the retrogressive loess landslides in the south Jingyang Platform was analyzed by using numerical simulation.Finally,the formation mechanism of the retreating loess landslides in the South Jingyang platform was proposed as well.This study provides effective ideas for the follow-up study of the catastrophic loess landslide.The main research results of this article are drawn as follows:(1)By analyzing the engineering geological conditions and geological environment background of the study area,it is found that most of the loess bodies on the edge of platform are almost upright and vertical,which represents the typical river terrace landforms,providing good conditions for the starting and movement of landslides in this area.Loess landslide groups in the study area have obviously gradual retrogressive characteristics.This is likely caused by the rise of groundwater level in slopes near the platform edge due to the long-term irrigation activities in the south Jingyang tableland.As groundwater level rises,soil at the slope foot is prone to collapse upon wetting,and the pore pressure of soil would gradually increase,which eventually results in a reduction in shear strength of soil.(2)Undrained ring shear tests with different normal stress,shear rates and moisture contents were conducted on loess samples obtained from the South Jingyang platform.The test results revealed that: as the normal stress increases,both the peak shear strength and residual shear strength increase,while the friction coefficient decreases.The peak shear strength,residual shear strength and friction coefficient decreased with the increasing of shear rate and it is noted that under a higher shear rate,the stress fluctuation of the shear stress-shear displacement curves becomes greater.It is clear that an increase in the moisture content apparently softens the loess.The influences of three factors including normal stress,shear rate and moisture content on the peak shear strength,residual shear strength and residual friction coefficient of the loess sample were quantitatively summarized.Moreover,at higher shear rates,the “grain crushing” of big particles on the shear plane results in the accumulation of of fine particles at the shear plane.Consequently,the shear strength of loess sample decreases due to the liquefaction of fine particles which accumulate with water on the shear plane.The ring shear test can well explain the high mobility of the landslide in the study area.(3)The triaxial shear tests were conducted on fissured loess samples and the results revealed that: with a relatively low confining stress pressure,the stress-strain curves of fissured specimen with different inclination angles show strain-softening tendency.However,loess shows strain-hardening characteristics with high confining stress pressure,indicating that the fissured loess specimen with lower confining stress is more prone to failure.The strength of fissured loess decreases with increasing of inclination angel of fissure plane,the resistances of fissured loess with fissure plane inclination angle of 60 degree are smallest under all confining pressures.Loess samples with moisture content below a critical value,cohesion decreases with increase of water content,whereas only little change in the friction angle,demonstrating that the increase in water content has a greater influence in the cohesion than the friction angle within the fissured loess samples.However,with the water content above a critical value,the friction angle decreases sharply,and little changes in the cohesion value.(4)Based on the field surveys and laboratory tests,the Geostudio software was adopted to investigate the effect of the groundwater levels,crack depth,distance of the crack from the slope edge and the irrigation time on the seepage and stability of a typical slope in the study area.It was found that the stability factor of the slope gradually decreases as the groundwater level rises.The dominant infiltration phenomenon becomes particularly obvious as the crack depth increases.As the distance of the crack from the slope edge decreases,the phenomenon of dominant infiltration becomes more and more obvious,and the area of the uneven infiltration area formed by the water infiltration into the soil along the crack development location becomes larger and larger.Moreover,the stability factor of the slope shows a decreasing trend with the decreasing of the distance of the crack from the slope edge.As the irrigation time increases,the minimum pore water pressure in the slope gradually increases,the area of the saturated area inside the slope gradually increases,and the pore water pressure in the same position of the slope gradually increases.With the increase of irrigation time,the reduction in the strength of the soil inside the slope is more and more obvious,resulting in a decreasing trend of the stability factor of the slope.(5)Based on the filed investigation and experimental tests,the occurrence of the gradual retrogressive loess landslides groups in South Jingyang tableland is attributed to the combination effect of strata lithology,slope shape,irrigation or precipitation,neotectonic movement and human activitites.Among these factors,the long-term variation of pore pressure due to irrigation return flow is the main factor triggering GRLL in this area.This is likely caused by the rise of groundwater level in slopes near the platform edge because of long-term irrigation practice in the south Jingyang tableland.As groundwater level rises,soil at the slope foot is prone to collapse upon wetting,and the pore pressure of soil would gradually increase,which inevitably results in a reduction of effective stress of soil.When the slope mass attached to the slope foot is incapable of providing the required supporting force,slide will occur in the slope and the mass movement of slopes enters a new developmental cycle.In this cycle,the back part of the landslide continues to retrogress,leading to soil on the platform edge to retreat progressively.Based on our field investigation and analysis,the dynamic evolutional process of gradual retrogressive loess landslides groups can be divided into three stages,i.e.,a creeptensional stage,a localized liquefaction stage and a sliding stage.Among these stages,the fissures acting as preferential infiltration channels and infiltration induced by irrigation caused the landslides prone to fail.Furthermore,landslides eventually triggered by the expansion of liquefaction area in loess in turn promote the development of fissures at the platform edge and triggers a new sequence of landslides,which eventually lead to the formation of GRLL in the study area.