| China has vast ocean areas,and is one of the countries where typhoon lands most frequently and losses due to typhoon are most severe.Volcanic rock develops extensively in the eastern part of Zhejiang province,where the weathering crust with thickness of more than ten meters widely distributed in hilly and mountainous area.Typhoon rainfall with high frequency and high risk always causes a large number of landslides which have the characteristics of wide distribution,small size,high frequency,high density and great harm.These landslides severely threat the safety of life and economy of the society and have been listed as the emphasis of disaster prevention and mitigation by local government.Due to the differences on rainfall intensity and time effect,deformation characteristics and failure mechanisms of landslides induced by typhoon rainfall are different from those induced by general rainfall.Determining the hydrological response and deformation characteristics of the slope at different stages and threshold value of rainfall through numerical and physical modeling,is very significant for early warning and forecast.Based on the data of several landslides induced by typhoon,this paper selects the typical landslide to study the characteristics and influencing factors of landslides induced by typhoon rainfall through statistics,prospecting,numerical and physical modelings.The paper has preliminarily built the conceptual model of failure mechanisms of landslide induced by typhoon rainfall,and proposed the failure mechanisms and threshold value of rainfall density of landslide induced by typhoon rainfall from aspect of internal and external processes,based on changing laws of main parameters including rainfall,water content,water table,displacement and strain.The paper finally obtains the main conclusions as follows:(1)Intensity and period of typhoon rainfall have been analyzed by statistics of the typhoon rainfall data in Zhejiang Province.Effect of rainfall on landslide is mainly explained in three aspects: rainfall amount,rainfall intensity and rainfall duration.Typhoon always could cause heavy and extremely heavy rainfall.The maximum daily precipitation that affects the landslide in coastal areas of Zhejiang province is generally larger than 200 mm/d.The landslides induced by typhoon rainfall concentrate mainly during heavy rainfall period and no delay than the rainfall.The threshold value of rainfall in 24 hours to induce the landslides is between 180~200 mm/d.(2)The distribution characteristics and influential factors of landslides inducing by typhoon rainfall have been explained based on the landslides data from seven times typhoon rainfall which influenced Zhejiang province during 1999-2009.Typical landslide is chosen to build engineering geological model and two kinds of conceptual models of failure mechanisms of landslide induced by typhoon rainfall.The landslides induced by typhoon mainly are usually small shallow soil landslides which can be classified into failing landslide and slow moving landslides.Failing landslide has the features of small size,thin thickness,and sudden occurrence.This kind of landslide is mainly superficial moving model.Slow moving landslide has the features of large size,large thickness,great harm and creep movement.The failure model mainly performs as sliding along surface that cuts the stratum,and source material is mainly weathered rocks.Mechanisms of the second kind of landslide are the focus of this thesis.(3)In order to study the seepage characteristics in Zhonglincun landslide,this paper describes hydrological response and deformation characteristics from two aspects: first,simulate the deformation laws due to water content change in slope during typhoon rainfall period with physical modeling.Second,analyze the deformation laws due to water table change in slope during typhoon rainfall period with numerical modeling.The numerical and physical modelings are based on the conceptual engineering geological model of the Zhonglincun landslide obtained with prospecting and monitoring.(4)Based on the Zhonglincun landslide in the Wencheng County,this paper uses BOTDA fiber sensing technology to conduct laboratory physical modest experiment.Mechanisms of landslide initiation and deformation are revealed from the variation of water content in slope soil and strain distribution of landslide under action of typhoon rainfall.The water content of the slope soil and the distribution of strain in slope are intensively correlated with rainfall intensity,the water content and the strain of the slope increases quickly with rainfall intensity between 50~200 mm/d,which is the disaster gestation period.The slope appears large deformation when rainfall exceeds 200 mm/d,which is the critical value of rainfall.The position of sliding surface accords well with position of mutation of water content.(5)The changing laws of water table and displacement distribution were obtained using numerical modeling of Zhonglincun landslide with Geostudio software.The seepage characteristics in the slope under heavy rain and rain with gradual changing density are basically the same.The water table responses quickly to the rainfall.The water table could rise to the contact surface between strongly and medium weathered layers,and ground water could flow out of the slope at the slope toe.At the stage of rainfall with gradual changing,stability factor of the slope is weakly related with water table,displacement of deformation is comparatively small.The slope is under creep deformation,the small cracks only appear on the slope surface.At stage of heavy rainfall,after the water table rises into strongly weathered layer,stability factor decreases quickly.After the water table becomes stable,rate of decreasing of stability factor decreases and displacement of the slope suddenly increases.The slope is at the stage of deformation and failure.The displacement is larger and the sliding surface is deeper under the first condition than the second condition.(6)Through numerical simulation and physical experiment,two kinds of geological and mechanical models are established for the failure and deformation of landslide caused by typhoon rainstorm: creep-cracking and sliding-tension-creep.The landslide deforms and fails mainly according to the second model,while it deforms and fails according to the model between the first and the second models.(7)The simulation results reveal the process of the deformation and failure and the mechanisms of landslide induced by the typhoon rainstorm.The process can be divided into two stages: disaster-gestation and failure stages.Antecedent rainfall of extreme heavy rain during the typhoon period belongs to the stage of disaster-gestation.At this stage,the rainfall infiltration causes the increasing of water content and unit weight,and decreasing of matrix suction,which lead to the deformation and failure of slope soil.In this stage,the landslide has smaller displacement and is in the creep state.Tension and shear cracks appear on the slope surface and the depth of the sliding surface is small.The extremely heavy rain during the typhoon period belongs to the stage of deformation and failure of the landslide.In this stage,the hydrodynamic pressure of the groundwater increases,and the intensity and the velocity of the groundwater seepage increase accordingly.The seepage could carry a large number of soil grains and voids are formed,leading to collapse of upper part of the slope and sliding with large displacement and the formation of secondary deep sliding surface accordingly.(8)The threshold value of rainfall at the stage of gradual changing density to induce the creeping deformation is 100 mm/d.The countermeasures at this stage include landfill in the cracks,drainage system and deformation monitoring.As the rainfall density exceeds 200 mm/d,severe deformation and failure will occur in the landslide,and evacuation should be the main measure at this stage. |
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