| Through the investigation of loess landslideshappened during Haiyuan M 8.5 Earthquake in 1920, which triggered by earthquake liquefaction, there is a type of landslide, which has motion features such as low angle, long distance,and which also can cause considerabledamage.The genetic mechanism is a matter of great concern and controversy, and many issues need to be further discussed.Based on the Shibeiyuan loess landslide triggered by Haiyuan Earthquake, the author explored the landslide relics, tested the loess basic physical and mechanical property, finished the test of loess micro-structure through SEM, dynamic liquefaction test and large-scale ring shear test, etc. Aiming at the characteristics of longitudinal nonuniform distribution of Malan loess, the thesismade further research on the structural characters, static and dynamic features of loess, discussed the liquefaction characters of saturated loess and relative controlling factors, revealed the appearance and improvement of excess pore water pressure during the process of saturated loess liquefaction under the dynamic loading, also discussed the tendency of loess strength and deformation after the liquefaction. Based on theresearch above, the thesis analyzed the geological model of Shibeiyuan landslide, and demonstrated the genetic mechanism of this low angle, long distance, liquid state movement landslide caused by earthquake and loess layer liquefaction. Through all the research, draw conclusions preliminarilyas follow.(1) The exploration of Shibeiyuan landslide relics shows that, Shibeiyuan landslide has obvious characters distinguishing from other landslides. ①Accumulation body appears as block or wave that extends miles long, and which shows obvious feature of liquefaction. ②The landslide has an apparentfriction angle of 2.86 °~ 3.12 ° and unconspicuouslateral wall. ③ The slip zone is a thick soft-plasticstrata, which has no obvious scratch or polished surface.(2)On the basis of analysis about Shibeiyuan landslide geological structuretest, the thesis contrasted and analyzed theloess sample of different depth and various types, from the aspects of various material composition, structural character, strength parameter, unsaturated character and dynamic parameter. The research revealed the nonuniform of longitudinal distribution and mechanical properties of Malan loess in landslide zone. The quantitative relationship between loessdynamic parameters and influencing factors such as dry density, water content, consolidation ratio, vibration frequency and number of vibration cycle have been obtained respectively, whichmade an excellent scientific basis for the loess dynamic parameters.(3)The test revealed the regularity of pore water pressure increasing during the process of loess dynamic liquefaction, found out that the main control factors about loess liquefaction are grain composition, pore structure and saturation, and also analyzed quantitatively the influence mechanism of affection on liquefaction strength of different factors. Under the vibration condition, the lower clay content in the loess, the faster pore water pressure increasing,lower liquefaction resistance, the larger void ratio is, the more significant macropore andmesopore decrease and loess has lower liquefaction resistance. Saturation has tremendous influence on the state of cementing substances and structural strength. Loess has lower liquefaction resistance under higher saturation.(4)The steady state strength and steady state line have been obtained from the test, which revealed that the steady state strength has nothing to do with early stress paths, but was influenced by the following aspects: loess void ratio, confining pressure, saturation, shearing speed, drainage condition, etc. Within a certain range, the steady state strength will reduce along with the increase of loess void ratio, degree of saturation and shearing speed, and it will increase along with the increasement of confining pressure and drainage rate. The conclusion of the research shows well the failure mechanism of saturated loess formation being liquefied get to steady state deformation of long distance under the action of intensity earthquake.(5)This thesis provided a series of techniques and methods covering field sampling, test preparation, test program and results analysis, etc. for test research of loess slope dynamic response mechanism. Moreover, during the process of preparation of sample of saturated loess, CO2 cylinders, peristaltic pump, GDS pressure controller, MTS soil dynamic triaxial apparatus have been combined altogether, which optimized methods and procedures of loess saturation process. During the test of intensity and deformation law after loess has been liquified, controlment of rate for the dissipation of pore water pressure has been made.(6)The research on Shibeiyuan landslide genetic mechanism showed that, sandy loess and silty loess near groundwater level are the foundation of seismic liquefaction. Special hydrogeology condition and strong dynamic situation are the main trigger of the earthquake liquefaction, and the wave shape fluctuation landform and disturbance state in sandy loess layer are strong evidence of liquefaction flow. In conclusion, the genetic mechanism of Shibeiyuan landslide is that, loess structure is damaged under earthquake, excess pore water pressure accumulates in saturated zone, saturated loess is liquefiedand spread upward and sideward,overling loess formationtenses and cracks, liquefaction zone floats and the upper loess flows a long distance.Using the numerical simulation technology, the thesis modelled and reappeared the appearance, accumulation and development of saturated loess strata excess pore water pressurein Shibeiyuan under strong earthquake. Meanwhile the effective stress decrease and shear displacement increased gradually,and finally formed coalescence slip surfaces. This process revealed the genetic mechanism of low angle, long distance, liquid state movement landslide triggered by loess liquefaction caused by strong earthquake. |
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