The Initating Mechanism And Runout Pattern Of Typical Seismic Loess Landslides

The Loess Plateau of Northwest China is located in the northern margin of the North South seismic belt, and historical earthquake induced a lot of loess landslide which caused heavy casualties and economic losses, thus earthquake landslide reaearch becomes the focus in geotechnical engineering. Tianshui has been chosen as investigation target area. The methods of field investigation, remote sensing interpretation, dynamic triaxial test, electronscanning, centrifugal shaking table test and numerical simulation, etc. are employed in this paper. Loess landslide distribution regularity and characteristics were cataloged, dynamic characteristics and damage effect of Loess and Mudstone were analysed from micro perspective, slope ground motion response and failure characteristics were also studied. Taking above research as foundation, the ground motion process of loess-mudstone landslides were simulated, so as to explore the initating mechanism, movement process and runout mode of seismic landslides. The main results are as follows:1 Seismic loess landslides distribution regularity and characteristics were cataloged and studied.(1) According to material composition, sliding surface, the shear exit, and the erosion effect after earthquake, landslides in this area could be divided into four types: Seismic cracking and sliding, vibrating shearing, creeping after earthquake and the three mixed mode. (2)The weak intercalated layer developed in Neogene soft red rock is the key point to control the large landslides. There are five types:① completely shearing-wrinkling cleavage, ②completely shearing-shearing broken, ③ completely shearing-shearing disarranging, ④ Incompletely shearing-cracks, and ⑤Incompletely shearing-crack tracking. (3) There is a good correlation between large landslide homotactic friction factor and their maximal elevation, horizontal distance, area and volume. (4)The earthquake landslides database in Tianshui was established preliminarily, and landslides distribution was also cataloged and analysed. The landslides mainly range from epicenter highly seismic region sheeted, along active fault and river system distributed linearly, distributed in the altitude of 1300 m-1700 m area, slope degrees of 9-27°area, and the slope aspect roughly facing the South, etc..2 The dynamic characteristics and damage effect of Loess and Mudstone were analysed.(1) The dynamic elastic modulus of loess and mudstone decreases with the increase of dynamic strain, but increases with the increase of confining pressure. Also, the trend of dynamic stress and strain of loess and mudstone increases with the increasing of confining pressure, and it needs more vibration time to reach the failure criterion. The lower confining pressure it is, under the same failure vibration time, the easier to damage the rock and soil mass it is. When the dynamic shear stress ratio is larger than a certain range, the rock and soil mass can reach the corresponding failure under a certain cycle dynamic stress. The intensity of earthquake and its own material property influence the dynamic strength of rock and soil mass. (2) The formation of particles on the shear failure surface of mudstone under dynamic loading is a little out-of-flatness. When the rock and soil mass strength is much higher, the formation of particles on the shear failure surface tend to be messy and much loose, final it caused shearing dilation. While the rock and soil mass strength is much lower, the particles are much flattening and tight, finally it caused shearing shrinkage.3 Ground motion response characteristics of loess slope were studied.(1) The horizontal and vertical acceleration responses of the slope are nonlinear. The acceleration amplification effect increases with the elevation, and the horizontal direction is larger than the vertical, usually presented much larger on the slope surface. (2) With the increase of the slope height and falling gradient, the ground motion response is gradually extended to the deep part on the top and the shallow surface of the slope. (3) With the increase of seismic wave amplitude, eigenperiod and duration, the response range and magnitude interior slope increased gradually. (4) Lithology has a significant influence on the seismic response. The slope with soft interlayer can absorb some seismic waves, which caused internal response in the soft interlayer very large.4 The deformation and failure characteristics and mechanism of loess slope under earthquake are analyzed in both micro and macro perspectives.(1) The microscopic deformation and failure mechanism of loess slope under strong earthquake should be:the reduplicative vibration caused damage to the rock and soil mass, which reduced the rock and soil mass bond strength, changed its porosity, and finally decreased its shear strength. (2) The macroscopic deformation and failure mechanism should be: ①earthquake caused the rock and soil loose, and which arose stress concentration in the soft layer. Rock and soil vibration in the slope is not synchronous; ②Tensile failure gradually formed on the top of the slope, and at the toe of the slope formed shear failure. The failure has progressivity; and ③When the shear strength is more than slope affordability, the progressive failure accelerated. Connected shear zone formed. There is a certain initial speed, and it is about to slide and then form landslide.5 Large loess-mudstone landslides initating mechanism, movement process and final runout mode were explored.(1) Large loess-mudstone landslide motion process under earthquake can be divided into five stages: Progressive failure, initialtion, violent movement, keeping speed movement and shift down movement. (2) There are three mechanisms and six effects in the process: ①initialtion and violent movement mechanism: fluctuation and vibration effect, progressive failure effect, slide bed peak and residual strength reduction accelerative effect, and slide bed elastic concussion effect; ②route high velocity mechanism: slip mass accelerating effect and maintaining speed effect; and ③final deceleration effect. (3) Large loess-mudstone landslide runout pattern showed: low shearing exit at slope toe, high speed motion, a long runout and causing a wide range of disasters.