Engineering Geological Study on Daguangbao Landslide
Landslide disaster almost occurred at every mountainous area where there are human habitation and engineering activities around the world.Landslide disaster,with the characteristics of causing many casualties and huge economic losses,suddenness,high frequency,group-occurring,lasting graded effect,has a prominent position among the natural disasters.With the extension of human activities space range and the increase of engineering construction scale,coupled with the influence of the global extreme weather conditions,landslide disaster is increasingly frequent and become the second common geological disaster type,after the earthquake.The seismic landslide has a low frequency,but once happened,it is far stronger and more severe than landslides triggered by other factors,not only in term of disaster scale,area,but the induced losses.The seismic landslide happens suddenly and is difficult to predict as same as the earthquake.The seismic landslide research mainly focus on the post-disaster research,and the reliability of the research results is highly affected by the study degree on the before landslide prototype.Therefore,carrying out the seismic landslide research has important theoretical and practical significance.Although the landslide disaster has been widely studied and the increasingly mature as a natural science,there are still some deficiencies,especially the lack of system engineering geological research of the seismic landslide with large complex and unique formation mechanism and movement process and extensive attention.However,it tends to be indispensable to do system engineering geological research of single landslide as a typical case study of landslide.This dissertation focus on the largest landslide triggered the "5.12" wenchuan earthquake — Daguangbao landslide.Through the collection of the terrain and geological data before sliding,in-situ system engineering geological survey,exploration and geophysical prospecting,detection and test after sliding,the engineering geology environment condition,landslides landform,structure of the landslide body,the position the sliding surface and the properties of the sliding zone are ascertained.Based on the field investigation of the characteristics of landslide movement and the analysis on data summary,combined with the physical simulation experiment of the movement process of the landslide,the deformation and failure process of the Daguangbao landslide are analyzed and validated.On the basis of the above research results,the formation mechanism of the Daguangbao landslide is analyzed and discussed,and the system engineering geological profiles of this landslide are established.The research results not only record a credible historical documentary of landslide event,but also as a typical case of earthquake triggeredlandslides,have made some progress in the approach of landslide research and systemic research,mainly including:(1)A series of engineering geological survey maps of the Daguangbao landslide were conducted,and a set of relatively complete quantitative data of the essential factors of the landslide were built.Through engineering geological survey mapping with a scale of 1:2000,coupled with geophysical exploration,pitting and boreholes,a series of landslide engineering geological maps were developed.The plane and spatial shape of the landslide,the landform,the pre-and post-failure structures,the slip surface position and its characteristics were found out.A set of relatively complete quantitative data of the essential factors of the landslide were achieved.Those results revealed the background conditions for landslide occurrence,landform characteristics of deposition area,vegetation distribution,lithology distribution and structure character and formation mechanics of rock mass.The failure mode of the Daguangbao landslide is considered to be a giant wedge failure controlled by the mega earthquake and specific topographic and rock structural condition,which had spent only two minutes from starting to deposition.(2)Based on the movement and deposition processes,combined with characteristics of the elements and terrain of the landslide,a commonly used index system for landslide engineering geological zoning was established.Landslide engineering geological zoning contributes to enhancing the knowledge of the landslide and has significance on the statistics of landslide movement parameters.Based on the engineering geological survey mapping of topography and landform and deposition structure of the landslide,and the analysis on the failure and movement processes,the landslide masses were divided into three large regions including landslide cliff area,main deposit area and subordinate deposit area,and subdivided into ten small regions(3)The corresponding relationship between landslide movement phenomena and landslide motion parameters is established,which has opened up a new ways and means for the study of the landslide kinematics.Through investigation and analysis the slickenside,position change of iconic landmarks,vegetation toppling,flat stone trend,slide cracks,and other topic features of the landslide motion,the movement characteristic parameters are determined,inluding the sliding direction,distance,velocity,the apparent angle and so on.(4)New landform elements and their concept are proposed by combining the geomorphology and dynamics.According to the typical geomorphic features including deposition structure features of the Daguangbao landslide,the dynamic mechanism of the formation of those features are analyzed,and the landslide ridge hillock,landslide "nuclear",landslide extrusion mound,landslide groove,detachment sliding surface and other new concept of landslide elements are proposed,that aregeomorphological performance of landslide movement and deposition.(5)Integrating the description about the landslide process from the victims,the geology survey of landslide motion characteristics and the physical simulation test results of landslide movement process,we suggest that the front locked segment of rock mass had experienced a shear break and burst out,with some unique such as huge sound,edge shock wave,landslide dust and the deposition characteristics due to“suddenly braking effect”,and the deformation and failure process of the landslide has distinct stages.According to the deformation mechanic analysis of the previous typical geological phenomena,the whole process of deformation and failure of the Daguangbao landslide are divided to six main stages,including the stage of shattering tensile cracking and edge formation,the stage of shear break of the locked segment of rock mass,the stage of high-speed sliding of the main landslide body,the stage of“suddenly braking” and napping motion and deposition,the stage of detaching sliding and flowing,and the stage of secondary sliding of the scarp cliff;among those stages,the former five stages are the deformation and failure process of the main landslide body,the last stage is the failure and movement process of the secondary landslide body.(6)Based on the investigation and analysis of the geological conditions and typical movement phenomena of the Daguangbao landslide,the physical and mechanical test results of slip soil,and the generalization and summarization of former research achievement about landslide dynamics,the unique dynamical mechanics of the Daguangbao landslide that starts up quickly,slide with high-speed,and brakes suddenly is revealed.The mechanical mechanism of the sudden initiation of the Daguangbao landslide can be summarized as seismic load,pore water pressure and locking section effect.The “peak-residual strength drop effect”,“pore water and gas pressure effect”,“Self lubricating effect” and “rolling friction effect” are the primary causes of high-speed.The “suddenly braking” can be summarized as collision and detaching sliding.The mechanisms of starting up quickly,sliding with high-speed,braking suddenly include multiple processes or effects,that work together to make the unique dynamical phenomena of the Daguangbao landslide.