Study On The Mechanisms For Geohazards Of Macro Loess Discontinuities

As a natural plane or interface that distributed in the surface and interior of the Loess structure,the macro Loess discontinuity is formed under the control of multiple geological forces.It is not only the embodiment of distinctly inhomogeneous,anisotropic,and nonlinear nature of Loess,but also the geometric and physical boundary of erosion and geohazards.Widely developing in the Loess Plateau,it is known for being activated by earthquake fault zones,gully erosion areas,geomorphic variation boundaries and human activity areas and causing serious disasters.In this study,the author mainly focused on the Loess Plateau as the research object.Based on a large number of field investigations,site explorations,geological compilation and remote sensing interpretation,the distribution of the macro Loess discontinuity and loess landslides on the Loess Plateau were summarized.Under combined effects of the tectonic movement,seismic activities,natural-geographical environment,Loess structure and human engineering activities,the author has studied the relationships among the macro Loess discontinuity,regional tectonics and loess landslides.On the basis of these studies mentioned above,the mechanism of group occurrence,spatial location,prototype control and internal disaster of loess landslides are obtained in this paper.The results are as follows:(1)According to the field investigations,eleven types of loess slope were found.13,798 groups of the macro Loess discontinuity were calculated,which occurred in the prone area of geohazards on the Loess Plateau.And seven geneses and eighteen genres of the the macro Loess discontinuity have been summarized.Also,the distribution,cutting types and geometric properties of various discontinuities are obtained.Besides,new division standards to measure the scale of the macro Loess discontinuity were given.Total five categories of macro Loess discontinuity was divided.In addition,based on the geometrical statistics of 7,495 tectonic joints,the author has compiled the rose diagram of tectonic joints in the Loess Plateau.And six groups of preferred joints were found.Moreover,the Holocene tectonic stress field was obtained according to the geometrical features of 40 conjugate tectonic joints distributed in different area.(2)14,544 loess landslides collected from the Loess Plateau of China are used to compile loess landslided istribution map.Besides,main factors considered in this research include geological tectonic,earthquakes,Loess properties,and the landslide density.The author has divided eight zones with high possibilities of landslides in the Loess Plateau and also summarized the characteristics of each zone.Based on the GPS data collected on the Loess Plateau and surrounding area,the author identified topographic deformation,stress-strain and tectonic stress field of Gan-qing Block,Ordos block and Fenwei graben by applying several groups of numerical simulation of these three blocks' tectonic movement.And the relationship between geological tectonic and the clustering occurrence of loess landslides has been elaborated in this paper and seven conceptual models depicting how the macro Loess discontinuity controll the occurrence of loess landslides under different tectonic conditions has been put forward.(3)Based on the systematical analysis of the distribution of 529 Margin fissures and 111 loess landslides occurred since the water diversion irrigation in 1971 on the South Jingyang Platform,it is concluded that the trend of margin fissures is controlled by the trend of slopes in the platform-edge and the slide direction is strictly controlled by the slope tendency of the platform.Based on the distribution of margin fissures,the distribution of unstable Loess structure and the evolution of margin fissures have been predicted.Besides,through the analysis of the geomorphic surface,water table,margin fissures and loess landslides in South Jingyang Platform,the mechanism of spatial location of loess landslides in loess platform has been obtained,that is “the combination of Loess discontinuities?unstable Loess structure?loess landslide?the openings of closed discontinuities?loess landslide group”.(4)In total,449 loess slopes have been investigated systematically.And the prototype controlling mechanism of loess collapse has been put forward,which is “initial?discontinuity?fall?and deposit”.By analyzing the examples of the loess landslides controlled by the Loess discontinuities of the typical platforms and gullies,the author has summarized nine types of landslides controlled by different Loess discontinuities and different slippery layers,and the prototype controlling mechanism of loess landslide has been put forward,that is the combination of loess discontinuities and slippery layer controls the prototype,thickness and scale of loess landslides.(5)Based on the different hydrogeological features of the Loess Plateau and the evolution process of the loess landslides,the author has put forward that the hydrostatic pressure and the hydrodynamic seepage pressure are the "principal murderer" of the loess landslide.Besides,through the comparison among different stages of the loess landslide,the internal cataclysm mechanism of the loess landslides has been revealed,that is "discontinuity?cave?gully ?sliding".Moreover,field investigation results has revealed that the macro loess discontinuity plays important roles in the different stages of the evolution of the loess landslide,including crack initiation interface,seepage dominant channel,erosion channel,water storage corridor,water barrier,parent separation surface,slope separation plane,sliding bearing surface,sliding expansion surface,and disaster amplification surface.