Research On The Stability Of The Slopes On The Loess Terraces

Influenced by the uplift of the Qinghai-Tibetan Plateau and the down-cut of rivers,a series of fluvial terraces were formed in the western part of the loess plateau along the Yeller River and its branches.The terraces all have typical dual structure characters,which are formed with fluvial sand gravels,silty clay and aeolian loess,and they also have high steep slopes(the height could be tens to hundreds of meters)which were also formed between the terraces.Under the influence of increasingly fierce human activities,the slopes are more prone to generate landslides,namely the loess terrace landslides.Influenced by the special geohazard pregnant environment and triggering factors,Lanzhou city has become one of the capital cites plagued by landslides and debris flows,and it is also ranked as one of the top four cities that are the easiest to be attacked by landslides and mudflows(the other three cities are Vance Karan in Peru,Guatemala and Almaty in Kazakhstan).None of currently existing methods of evaluating regional stability have been applied to high heterogeneity terrace slope areas,Consequently,the stabilities of the slopes locate on the hundreds of meters high terraces areas in Lanzhou area have not been appropriately and quantitatively evaluated,not to mention the simulation under different triggering conditions.Therefore,it is urgent to study suitable methods to evaluate the stabilities of the slopes under different triggering conditions in such areas.The results can provide practical guidance and scientific basis for virescence irrigation methods and slope-cutting engineering optimization,and the prevention planning of the rainfall type landslide and earthquake.This study made a comprehensive analysis of the impact of natural factors and human activities on the stabilities of terrace slopes' with the aid of three-dimensional laser terrain scanner,three-dimensional images of the subsurface,geological survey,laboratory soil mechanics and other experiments Besides,quantitative evaluation was also conducted by using computer analysis and simulation tools,to reveal the stability of typical terrace slopes..Following that physically-based stability model was established by using distributed GIS,and validated by PS-InSAR(Synthetic Aperture Radar Interferometry Based on Permanent Scatterers)technology,In the end,a regional "ground-air" geological hazard assessment were achieved.At the same time,four main findings were summarized as follows:(1)Traditionally,the determination of analyzing units in estimation of regional slope stability have been taken little account into the material composition of the slopes.In this study,16 classes of typical terraces slopes were selected based on the systematic classification method of typical loess terraces slopes,which not only took into account the mechanical factors of the overlying loess and alluvial formation with sub-mechanical properties of silty clay(secondary loess)at different geological ages,but also considered the structural properties of underlying bedrocks and the geomorphic features around the slopes..(2)Current analysis on regional slope stability rarely considered the use of the multilayer heterogeneity of the basic physical properties and shear strength parameters.This paper systematically summarized the shear resistance parameters of loess that was formed in different historical stages.For the silty clay in all levels of the terrace,unsaturated ring shear test were conducted under eight different moisture conditions.Corresponding to residual moisture conditions,the residual cohesion and angle of internal friction were tested,respectively.At present,the hydrological model were usually coupled with the stability calculation model when carrying out regional slope stability calculation,but the results of the in situ high-density multi-phase electrical method showed that the infiltration of water in loess depth is limited.In the terrace area in Lanzhou,the maximum infiltration depth of the intact loess is no more than 5 m.When there aret cracks and sinkholes,they could work as advantage channels,the affects from infiltration process to the moisture content of the soil will become more complex.(3)At present,the computation on the slope stability in an area based on the heterogeneous of slopes elements and the physical mechanism is relatively rare.Limit equilibrium method of BS(Bishop,Bishop method),OF(Ordinary/Fellenius,Sweden circular slip method)and MP(Morgenstem-Price,Morgenstern-Price method)were used to calculate the slope stabilities under different water contents,different slopes and under various earthquake loading of 16 different typical slope units.The fitted regression equation shows that stability coefficient and slope water content present a negative correlation and the R2 are close to or larger than 0.8,with a strong correlation.In the GIS environment,the regression equation were promoted across the terrace slopes in the region,Several conditions were analyzed when conducting the regional estimation of terrace slope stabilities in Lanzhou area,and they include:current state(current slope and moisture),slope cutting under different conditions(different slopes),different irrigation and precipitation conditions(different water content)and under seismic loads.The results showed that:Under current conditions,vast majority of terraces slope are in a steady state according to the three limit equilibrium method,the number of unstable slopes OF>MP>BS.In current state,when water content in slopes exceeds 16.5%,the safty factor of the majority of slopes will be Less than one,the results of OF,BS,and MP methods show that the proportions of unstable slopes were:68.5%,50.2%and 57.4%respectively.In current water content,when increasing 6°,more than half of the terraces slopes in Lanzhou region will be in unstable state,the proportion of unstable slopes calculated by OF,BS,and MP were:68.5%,53.9%,50.8%,respectively.Then,under the seismic load of 0.2 g peak acceleration on the basis of conditions,most of the slopes in study area are in an instability state,the proportion of calculated unstable slopes by OF,BS,and MP were:69.4%,54.9%,50.5%,respectively.(4)As PS-InSAR technique could provide high-precision ground deformation monitoring data in a large area periodically,it has been developed rapidly.Its verified results could solve the accuracy assessment and timeliness problems in regional slope stability evaluation model better.The first using of PS-InSAR technology in a arid-semiarid region covered by a thick layer of loess,and through on-site verification proved its high applicability in monitoring and locating regional scale surface deformation.Compare the verified PS-InSAR data,existing landslide catalog and the FOS got by three different calculation methods,then bring in the new landslides as result of the irrigation and artificial slope cutting.The results show that on the stable surface,the vast majority of the detect radar target points is stable,while in Baita Mountain,Gaolan Hill,Fulongping slope instability was detected.During the computation of the stabilities of terraces slopes,the three different limit equilibrium methods showed different precision and accuracy:MP method achieved the highest accuracy rate with 75%.Stability coefficient obtained by BS method is tend to be larger,while the stability coefficient acquired by OF method were usually smaller.When tackling with the stability calculation of Terraces slopes with different water content and different slope cutting case,all the three limit equilibrium methods showed reasonable prediction results and had predicted the landslides that happened in Shixiakou and Baoensi in Lanzhou.But limited to the accessibility of newly happened landslides,the applicability of the three limit equilibrium methods have not been compared under conditions.