Stability Analysis Of Loose Large-scale Deposits In Wcnchuan Earthquake Areas: Deformation And Failure Modes

The Wenchuan earthquake induced abundant loose deposits. These deposits arecharacterized by large scale and volume varying from few hundred thousand to billioncubic meters. Under the influence of rainstorm, earthquake and human engineeringactivities, the loose deposits tend to evolve to geological disasters, and then threatenseriously the safety of human’s life and properties. Also, these large deposits arecharacterized by wide distribution, massive quantity, complex formation mechanism,diverse disaster modes, high difficulty of identification, strong elusiveness, anddifficult prevention and countermeasures. Therefore, it is urgent to conduct scientificresearch on the deformation and failure modes as well as the stability analysis of suchlarge loose debris in Wenchuan Earthquake areas.In this thesis, the geological field surveys and field studies have been carried out.Firstly, the geological environmental conditions have been deeply investigated, and thecharacteristics of the development and distribution of the deposits have been analyzed.Then, the modes of deformation transferring failure and the evolution mechanism havebeen concluded. Meanwhile, three typical methods namely, engineering geologicalanalysis, numerical simulation and limit equilibrium methods are adopted to indicatethe deformation and failure modes and the stability state for the deposits. Finally,centrifugal tests are implemented to investigate the evolution regulation of the stressand deformation of the debris under different conditions. The main conclusions are asfollowing:(1) It is indicated that the development and distribution characteristics of thedeposits are significantly influenced by the geological environmental conditions. Inparticular the deposits exhibit banding shape, linear shape and cluster shapes along theLongmen Mountains Fault zone and rivers system; slope-sliding induced deposits aremainly distributed in regions of low hills lower than1000m of altitude, debris flowdeposits mainly occur from the regions of high mountains and canyons, and thecollapse type deposits are mainly observed in region of small formed small cuttingmountains under100m of longitude; the slope-sliding type deposits are developed insoil slopes, and the collapsing deposits are formed in rock slopes, but the debris flow deposits are principally formed in rock-soil slopes.(2) It is suggested that the main difference between the deposits induced byearthquake and the common deposits is the complexity of the geological structure andthe diversity of the particle structure. The deposits formed after the earthquake consistsof not only loose quaternary sediments but also ancient bedrock layers. The largedeposits after earthquake contains slope sliding deposits, collapsing deposits, as well asdebris flow deposits. The size distribution of the deposits particles is obviouslycharacterized by multilevel and fractal geometry. The particle varies from huge particleto coarse particle and to fine particle, exhibiting varying engineering properties andmultilevel characteristics. It exerts strong concentration phenomenon inbetween thesame or similar size particles; whereas, the huge particles and the coarse particles showproperties of loose intersection, lower compaction and relaxation when contact water.(3) Three typical types of deposits after earthquake have been identified. Basedon the forming sources, these three types are classified as sliding mode, collapse modeand scouring mode. According to the structure properties and failure modes, the typesof deposits consist of ancient slope activation mode, weak basement sliding mode,shallow regions of the thick and deep sediments sliding mode and the mountain typepeeling sliding mode. Following the collapse locations and morphologies the depositswere classified into block collapse (avalanche) mode, stagger and fracture collapsemode and shallow collapse mode. Furthermore, the scouring mode can be divided intokerve-scouring mode, dissolution induced toe collapse mode andblocking-diabrosis-cutting mode based on deformation and failure mode. Finally,according to the deformation and failure models, the scour models are classified asdissolution into.3major-categories and10sub-categories summarizing thedeformation properties and the evolution mechanisms of the deposits.(4) FLAC3Dsoftware and limit equilibrium methods are used to study the entiresliding stability of the deposits, the results of which show that, in the medium andbottom regions, subsidence is relatively large; but in the from regions, sheardeformation is the largest. The deformation is characterized by layering andclassifying phenomena, which indicates that the occurrence of deformation and failureof this type deposits is significantly influenced by the existing of weak layer orcrushed sliding surface. The deformation and failure is mainly of back-pullinglandslide. It is proposed that the entire sliding deposits belongs to the evolution modelof “trailing region deposits loading, middle region settlement and compaction, leadingedge shearing and pulling, then entire deposits slide along the bottom fracture sliding surface”.(5) Engineering geological analogy and elastic-plastic numerical simulation ofM-C model have been implemented to study the mechanical progress, deformationand failure characteristics of the collapsing deposits. By mechanism analysis, it showsthat the stability threshold angle of the entire sliding deposits is approximate35°.Large deformation appear in the medium and bottom regions of the deposits body, andthe largest deformation occurs in the onset regions of deposits. In the back-edgesliding regions, the shear strain is relatively large. The deformation and failuresurfaces don’t appear in the bedrock surface, but mainly in the regions of the frontfree surfaces, the medium onsets parts and or the back-edges of the deposits body.(6) The deformation and failure characteristics of the channel-scouring depositsmainly perform as dissolution-kerve, blocking-diabrosis and cutting, as well asscouring-silting up. Based on numerical analysis results, the stability of the depositswhen subjected to gravity and rainfall, it is calculated that the safety factors of thedeposits are all near2.5under natural state. When subjected to rainstorm or continuousrainfall, the increment of shear strain increases obviously, and corresponding safetyfactor decreases notably, and then consequently the deposits tend to instable. Thus,necessary measures must be adopted. It is investigated that the maximum deformationsoccur in inflection points and steeper regions. The comprehensive measures of“separate treating the water and gravels, consolidating the bottom and protecting theslope surface, retaining the silt and maintaining and detecting” is the effectivetreatments to cure the channel scouring deposits.(7) The results of the centrifuge tests that have been conducted to investigate theinfluence of rainfall capacity and the reinforcement effect of slide-resistance piles showthat the increase of water content can accelerate the deformation and failure of the loosedeposits, and the failure model is mainly of shallow pull-type. The steel reinforcedconcrete pile has obvious reinforcement effect on the deposits, resulting in the soilpressure along the pile being nonlinear along with the depth, and the stress statebetween the from piles and the back piles. The centrifuge tests verified a certain thereinforcement effect of the bottom consolidation and slope protection, as well as pilereinforcement.