| Earthquake is the most powerful and destructive natural disaster on earth.It is well known that the earthquake not only directly causes the loss of people and property,but it also induces a tremendous number of co-seismic landslides,which cause widespread concern due to their great potential in causing serious losses especially in alpine canyon region.Meanwhile,the large-scale co-seismic landslide may block the river channel,forming a landslide dam and creating a high likelihood of downstream flooding in the event of landslide dam failure.However,there is a lack of successful cases in reducing the risk of dam failure after integrated emergency mitigation measures.Furthermore,it is more difficult to carry out the detailed design of emergency mitigation measures and sustainable development and utilization of the landslide dam.On 3rd August 2014,a moderate earthquake(Ms 6.5,Mw 6.2)struck Ludian County,located in Yunnan Province,southern China.Due to the complexity of its seismogenic fault and rupture process,the Ludian earthquake showed abundant and unique dynamic geological phenomenon,which not only produced co-seismic surface ruptures,but also induced a large number of co-seismic landslides.In addition,this moderate earthquake induced such a large-scale Hongshiyan rock mass failure.The debris of the Hongshiyan landslide slid into the Niulan river bed and covered the distal limit of an older pre-existing landslide located on the opposite side of the valley forming a 95 m-high landslide dam.After the formation of the landslide-dammed lake,the water level rose sharply.The failure potential of the dam created an elevated risk to population and infrastructure downstream.Fortunately,after effective emergency mitigation measures by taking full advantage of existing hydropower projects downstream and upstream of the Hongshiyan landslide dam were carried out,the risk of dam break was reduced significantly.In addition,engineering and economic considerations pointed to the potential transformation of the Hongshiyan landslide dam into a dual-purpose water conservancy and hydroelectric project.In order to ensure the safety of later operation of the project,the geomorphic features,material composition,geological structure and seepage characteristics of the Hongshiyan landslide dam have become the focus of attention.Disaster not only brings us painful lessons,but also provides us with valuable research opportunities.The Hongshiyan landslide dam includes the problems of focal mechanism,earthquake-induced landslide,landslide runout,seepage characteristics of landslide dam,emergency measures and sustainable utilization of landslide-dammed lake,which provides valuable scientific research material for the co-seismic landslide damming event.Based on the abundant and unique dynamic geological phenomena in the Ludian earthquake and the comprehensive treatment method of the landslide-dammed lake,the present paper focuses on the large-scale Hongshiyan rockslide damming event.The precious and complete geological data obtained by means of traditional in-situ surveys and remote sensing techniques,was used for integrating remote sensing,image,and numerical analysis.The present paper reveals the distribution of seismic intensity and co-seismic landslides of the Ludian earthquake with the complex faulting process,the initiation mechanism and motion process of weak base Hongshiyan high-speed shortrunout landslide,and aims at improving the risk control ability of the landslide dam triggered by the earthquake.The main research contents and contributions of the present paper are as follows:(1)Given the stress state of each fracture in the ideal Riedel shear model,NNWstriking Y shear ruptures and NWW-striking R shear ruptures in the north of Wangjiapo Village indicate an NWW-SEE principal compressive stress acting on the sinistral strike-slip Baogunao-Xiaohe fault.Under the present tectonic stress,a NEE-striking dextral strike-slip fault plane(R' shear)could be developed at the left step-over of the NNW-striking sinistral strike-slip Baogunao-Xiaohe fault near Longtoushan Town.(2)On the basis of the spatial distribution and sensitivity analysis of influencing factors of co-seismic landslides,the 2014 Ludian earthquake could be with a complex faulting process,i.e.,NNW-striking sinistral strike-slip fault plane(Y shear)and NEEstriking dextral strike-slip fault plane(R' shear).The correlation between the co-seismic landslides and the distance to the seismogenic fault is much stronger than the distance to the epicenter,showing obvious “fault effect”.In addition,landslide risk assessment by means of mathematical and mechanical models reveal that the large difference of ground motion intensity in the EW and SN direction has a great influence on the distribution of the co-seismic landslides triggered by the Ludian earthquake.(3)Based on field investigation and kinematics analysis,the geological setting of the Hongshiyan slope consists of an upper strong limestone slab and an underlying weak silty mudstone with persistent bedding plane dipping obliquely into the slope.Upper strong soluble carbonate slab overlying weak mudstone provides a geological foundation for the Hongshiyan landslide.The seismic activities,optical satellite imagery,and UDEC analysis suggest that the Hongshiyan slope was in a state of marginal static stability pre-earthquake as a result of pre-failure progressive deformation controlled by the shear strength of the mudstone.The marginal pre-earthquake stability of the Hongshiyan slope,coupled with the strongest seismic loading,which the Hongshiyan slope has been historically subjected to,are the reasons why this moderate earthquake induced a large-scale catastrophic rock slope failure.(4)Based on the above results in terms of the seismogenic fault and seismic intensity distribution of the Ludian earthquake,the most relevant ground motion parameters are used in discrete element numerical simulation to reveal the failure mechanics of the Hongshiyan slope.The numerical results show that the rupture process does not occur along discontinuities within the mudstone,but through the underlying weak mudstone,forming a curved rupture surface subjected to the horizontal seismic force.As a result,failure in tension progressively develops along the high-angle joint(JS2)not only from the top to bottom but also from the basal weak layer upwards.(5)The mass movement and accumulation process of the Hongshiyan high-speed short-runout landslide is successfully back analyzed by the continuum model.The numerical results show that sliding masses with low moisture content are thrown along the basal sliding surface to impact the dry river bed under earthquake loading,and then crash with the old landslide deposits composed of loose bulks on the opposite side of the valley in large angle,are the reasons why the Hongshiyan landslide is short-runout,and the formation mechanisms of large-scale landslide dam with a height of 95 m.(6)The geomorphic features of the Hongshiyan landslide damming event and internal geological structure of landslide dam are revealed by using data obtained by means of traditional in-situ surveys,pre-earthquake SRTM-DEM and post-earthquake high-resolution DEM.To take full account of the scale and danger of the Hongshiyan landslide-dammed lake,and the severity of the loss due to the landslide dam break,the risk level of the Hongshiyan landslide-dammed lake is divided into the extremely highrisk in class ?.(7)The present paper provides a comprehensive review of successfully reducing the risk of Hongshiyan landslide dam break by taking full advantage of existing hydropower projects downstream and upstream of the landslide dam;The finite element analysis suggests that relatively gentle slope of dam faces and low permeability of lower deposits(Qdel-2)with basically continuous grading and high density,play significant roles in maintaining the stability of Hongshiyan landslide dam;In addition,the existing of old landslide deposits(Qdel-1)in the Hongshiyan landslide dam can improve the stability of landslide dam to a certain extent,and further ensures its stability in the process of reducing the risk of the landslide dam.If the diaphragm wall can effectively prevent the internal seepage induced by the difference between upstream and downstream water levels,the stability of the Hongshiyan landslide dam can be greatly improved,so that it can meet the basic conditions of the transformation into a dualpurpose water conservancy and hydroelectric project. |
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