Study On The Trigger Mechanism Of Loess Landslide

Positioned in Shaanxi’s emerging economic zone and the heart of key water conservancyprojects, loess landslide at downstream of the Jing River in the south bank occurs frequently,leaving severe property loss and casualty and restriction of the local economic developmentand the key water conservancy projects construction. Under on-site inspection, the textsummaries and analyzes the development type, formation condition and distribution law ofloess landslide in this area. Loess mechanical behavior and slope deformation-failure lawunder different trigger condition were systematically studied in laboratory test, as well asnumerical analysis. In addition, by quantitative analysis, the specific landslide controlmechanism with landslide development features was described. On the basis ofmicro-and-macro analyses stated above, the text discussed the trigger mechanism of loesslandslide systematically and profoundly. Finally we arrive at the following review.⑴In this area, the loess landslide can be divided into5types, namely irrigation flowslidetype，irrigation slide type, lateral erosion slide type, lateral erosion slump type and excavationsubslide type. Irrigation trigger type is the most modal and destructive one. Lateral erosionslide type, with the characteristics of thick sliding mass and long latency, once slides, willblock the river channel and greatly affect the running of the line side water conservancyprojects.⑵Loess landslide in this area shows the temporal characteristics of a growing numberand high risk in spring, the spatial characteristics of clustering and variation. Plus, irrigationtrigger type also shows the spatial and temporal characteristic of multiphase.⑶CTC (Conventional Triaxial Compression Test) suggests that in research area, Q2loesssample is characterized by strain softening. With the rising of moisture content, shear strengthof the sample obviously reduces. Under high deviatoric stress, sample tends to shrink volumeand produce high pore water pressure. By fitting steady state line and locus line of peak inplane p′-q′, the latent instability area with the possibility of liquefaction was determined. ⑷CSD (Triaxial Drained Test at Constant Shear Stress) suggests that under this stresspath, incomplete drained shear failure caused by pore pressure booming is the main loesssample failure mechanism. According to the stress-strain behavior, the failure process can bedivided into boost creep and liquefaction paroxysm. When pore pressure gets to0.70, it tendsto liquefy.⑸RTC (Reduced Triaxial Compression Test) suggests that under this stress path, loessfailure mechanism works when the rising deviatoric stress resulting from the decrease ofconfining pressure damages the loess under stress. Under this stress path, sample also showsthe strain softening characteristic, but shear-resistant strength is much lower than the CTCresult, explaining that stress path greatly affects the soil’s constitutive relation. Underundrained shear, sample tends to be destroyed when confining pressure is close to0.55.⑹Consolidated undrained triaxial creep test suggests that with larger deviatoric stress,more load steps, higher water content, there will be more evident creep deformation. Underthe low stress, sample is characterized by linear creep. With the increase of stress, non-linearcreep characteristic becomes obvious. Self-built empirical modeling is one kind of idealempirical creep model with simple formula and easy access to parameter. Generalized Kelvinmodel with5elements can precisely describe the linear viscoelasticity during the decayingand creep process of loess.⑺What the text has got from FLAC3D numerical simulation experiment are as follows.Firstly, irrigation trigger type is retrogressive landslide with thin sliding mass andsubhorizontal shear opening. The basic reason for landsliding if this kind is that constantrising water level results in saturated slope toe and thicker loess layer and therefore soil shearstrength is gradually weakened. Secondly, lateral erosion trigger type is retrogressivelandslide with great thickness, straight sliding mass in posterior line and deep formation shearin the anterior line. The basic reason for landsliding if this kind is that lateral erosionconstantly changes the slope deformation, leaving the slope toe with concentrated stress andno lateral bracing. Thirdly, user-defined creep model owns better effect in promoting landslidedevelopment, compared with M-C (Mohr-Coulomb). With more creep time， higher water level and deeper lateral erosion, deformation failure characteristic will be more evident in theslope.⑻Landslides in the research area is characterized by the coexistence of individuality andgenerality and largely affected by landslide control element. Their main manifestations are asfollows. It gets controlled by liquefaction degree in sliding distance, controlled bygeomorphic element in sliding types and by interaction among landslides groups.⑼The text profoundly discussed the trigger mechanism of loess landslide and providedstatic liquefaction instability mechanism for irrigation trigger type of landslide, creep crackinstability mechanism for lateral erosion trigger type and strength damage slide promotionmechanism based on creep.