Sur submarine slide, Monterey Fan, central California

The Sur debris slide and associated debris flow cover more than 1000 km² of the eastern part of Monterey submarine fan and extend from the base of the continental slope near the apex of the fan to the Monterey fan valley. The flow is generally confined between the continental slope and remnants of an older channel (Monterey East fan valley). The hummocky surface of the debris slide and diffuse echo returns from the surface of the nearly acoustically transparent debris flow, as seen in 3.5 kHz profiles, are common to large submarine slides described from passive-margin continental-slope and rise deposits. Piston cores from the Sur slide recovered contorted and sheared fan turbidite units as well as clast- and matrix-supported mudlump debris flows. The cores show that the slide debris is overlain by 0.23–0.87 m of mud and turbidite sand that limit the age of the slide to latest Holocene, although more than one stage of emplacement is possible.     

A large-scale colluvial landslide caused by multiple factors: mechanism analysis and phased stabilization

A case study of a large-scale colluvial landslide is described in this paper. This instability initiated in December 2003 in the Southwestern region of China and affected construction schedule and safety of Xiaowan project, which was the first large dam in Lantsang River. Geological investigation and deformation monitoring were carried out with severe safety and stabilization design. As it was impossible to reach the control level of FOS (factor of safety) under long-term condition before rainy seasons, the stabilization works of the colluvial slope were designed by a “phased hazard mitigation” method to satisfy the safety requirement in rainy seasons. LEM (limit equilibrium method) was used for determination of the shear strength parameters of the sliding zone and instability mechanism analysis. The study found that the whole colluvial slope included three potential sliding masses, and the instability appeared to be a progressive retrogressive-type of slope movements. Inappropriate cutting, rainfall, and blasting were found to be the main causes of the colluvial instability. On the basis of the potential hazards and the importance of the project, the control level of FOS, design cases, and corresponding calculated conditions for design analysis were defined in each stabilization phase. The stabilization works were designed by using LEM. The performance of the engineered slope after additional reinforcement was assessed based on the monitoring data.     

Deterministic and Probabilistic Stability Analysis of Soil Slope – A Case Study

This paper deals with the soil slope stability analysis along the road section (National Highway-05) near Luhri area, Himachal Pradesh, India. The area constantly experiences local as well as regional slides which enhances the siltation, reduce carrying capacity of the Sutlej river with time and further increases the water level on the banks of the slope. High traffic congestion puts the area at high risk zone and therefore, stability analysis was done using probabilistic as well as deterministic approach under static conditions through limit equilibrium method (LEM) and finite element method (FEM). FEM was used for stability analysis, mainly to obtain deformational characteristics, under static conditions and compared with LEM results. The results of both the methods help in identifying the zones of vulnerability in the case study. Further, the values of safety factor obtained from both the methods are in close approximation. The study also reveals that the slope is critically stable and a remedial measure has been suggested by modifying the geometry of the slope, which needs proper protection from any external factors like rainfall and earthquakes that may cause unforeseeable circumstances.     

广州飞碟训练中心高边坡稳定性分析

以广州飞碟训练中心人工高边坡工程为背景,讨论了人工高边坡设计中的若干关键技术问题,介绍了边坡设计方案。基于相同物理模型和边界条件,先后运用不同极限平衡理论分析该高边坡稳定性,分析了不同极限平衡法计算结果的差异和原因,以及极限平衡法在边坡计算中的限制。运用有限元强度折减法进行相同条件下的计算,并对不考虑锚索加固和考虑锚索加固的边坡计算结果进行比较,分析边坡变形特征和破坏机制,指出有限元折减强度法的优势。     

Finite element modeling approach to assess the stability of debris and rock slopes: a case study from the Indian Himalayas

Landslides and slope failures are recurrent phenomena in the Indian Himalayas. The study area comprises the hill slopes along a road stretch of 1.5 km at a distance of 9 km from Pipalkoti on Chamoli–Badrinath highway (NH-58) in the Garhwal Himalayas, India. Based on the field survey, contour map, and the hillshade, the study area has been divided into different zones. Three different zones/slopes in this study area including one potential debris slide, one stable debris slope, and one potential rock slide have been undertaken for investigation and modeling. Field mapping, data collection related to slope features and soil/rock sample collection, and discontinuity mapping for all the slopes have been carried out in field. Soil samples have been tested in the laboratory to determine the physico-mechanical properties. These properties along with some material properties from the literature have been used as input parameters for the numerical simulation. To investigate the failure process in the debris/rock slides as well as stable debris slope, the slopes were modeled as a continuum using 2D finite element plain strain approach. Shear strength reduction analysis was performed to determine the critical strength reduction factor. The computed deformations and the stress distributions, along the failure surface, have been compared with the field observations and found to be in good agreement. The analysis results indicated rock/debris slide slopes to be highly unstable. The debris slide modeling depicted failures both above and below road levels as observed in field. The rock slide modeling could depict the exact pattern of failure involving 3 sets of discontinuities simultaneously as observed in real-field scenario which is a major limitation in case of limit equilibrium analysis. The field-observed stable slope comes to be stable through FE analysis also. Based on these analyses, landslide hazard assessment of the study area could be done.     

Debris slide hazards in the Adirondack Province of New York state

More than 400 debris slide scars have been identified in the Adirondack Province through aerial photographic interpretation. Debris slides are concentrated in the high relief, east-central portion of the province, although some isolated scars occur in the north and southwest. Intense rainfall, relief, slope angle, slope form, and slope exposure are recognized as factors in Adirondack slide formation and distribution. Vegetational, pedological, and geological characteristics also influence slope stability conditions, but complexities in after-the-fact analyses obscure their exact function in slide activity. On the basis of slope angle and slope form, more than 600 major valley systems which exhibit slide potential were identified in the Adirondacks. Approximately 25 percent of these sites of potential slides are on private land. The prospect for higher land-use in these areas warrants serious consideration of the hazards of debris slides. Debris slides cannot be prevented, or even accurately predicted, but certain measures may possibly be taken to avert extensive economic damage or loss of life.     

物质点强度折减法及其在边坡中的应用

物质点法适用于模拟连续介质大变形,如边坡失稳全过程。在物质点法中应用强度折减法,用于边坡稳定性评价。与极限平衡法相比,二者安全系数计算值、滑动面位置结果基本一致;与有限元强度折减法相比,物质点法失稳评价标准的物理意义明确。利用物质点法大变形计算优势,评价边坡失稳后的破坏后果,通过算例说明其评价不同安全系数下的滑坡堆积形态及滑移距离的能力,尤其是评价滑坡对临近建筑物的影响程度的能力。物质点强度折减法可用于边坡稳定性评价及边坡破坏后果评价。     

Application of the Swiss Federal Guidelines on rock fall hazard: a case study in the Cilento region (Southern Italy)

On December 15, 2008, a rock slide of approximately 4,000 m³ occurred near San Severino di Centola (Cilento, Province of Salerno). The landslide occurred along very steep bedding planes intersected by tectonic joints affecting a rocky cliff. The landslide debris, formed by several boulders, came to a halt along the gentler slope below. Many blocks reached the slope base including the national road n° 562 and injuring a motorist on the road. Furthermore, the road suffered heavy damages causing a halt in traffic for many weeks. Several signs of the impacts were detected along the slope and on the road; also, the volumes of some boulders were measured. The area affected by block trajectories stretch down slope about 4,900 m². By means of 2D and 3D trajectory codes using the “lumped-mass” method, it was possible to estimate more suitable impact energy restitution and rolling friction coefficients, to reconstruct rock fall trajectories, and to calculate total kinetic energies. Extending to the surrounding slope (surface about 2 ha) these data, a hazard scenario was carried out, displaying the pattern of iso-energy distribution curves calculated along 11 critical profiles, and zones exposed to 70% and 30% of frequencies of block transits and endpoints. Successively, with reference to the Swiss Federal Guidelines, another scenario was prepared on the basis of rock fall intensity values and the return period of the 2008 landslide event. In this way, it was possible to define areas of high, moderate and low hazard. The degrees of hazard are assigned according to their consequences for construction activity and outline zones where additional protection is required. The construction of a retaining embankment was believed suitable for local risk mitigation.     

Characteristics and formation mechanism of a catastrophic rainfall–induced rock avalanche–mud flow in Sichuan, China, 2010

A catastrophic rock avalanche–mud flow was triggered by the heavy rainfall in Sichuan, China, on July 27, 2010. A mass of strongly weathered basalts with a volume of ∼480,000 m³ was initiated from a valley side slope and then moved downstream along the valley, entraining a large amount of unconsolidated substrate and bilateral materials and colluviums. The entrainment increased the volume of slide to ∼1.0 million m³ and may also enhance the mobility of the landslide. Approximately 30 min after the first failure, the deposits of the rock avalanche in the steepest part of the valley started to creep slowly down as a mud flow. It reached a small town at the foot of the slope after several hours, causing the damage of 92 houses and the urgent evacuation of 1,500 people. The field investigation, mapping, grain size test, and aerial photo interpretation were applied to analyze the dynamic process and the formation mechanism of the landslide. The strongly weathered and fragmented basalts as well as the most vulnerable combination of joint sets were revealed to be the most contributive factors. The antecedent torrential rainfall is the direct trigger, which affected the slope stability in three aspects: induced debris flow that scoured the toe of the sliding surface of rock avalanche; caused the increase of the slope unit weight, and penetrated into the steep joints reducing the strength of the materials.     

Small landslide types and controls in glacial deposits: Lower Skagit river drainage,northern cascade range,Washington

Observations of 167 small, shallow landslides spanning a 22-year period on extensively logged slopes of Quaternary terraces in the lower Skagit and Baker Valleys, Washington, shows that there is a relationship between the common slope failures in this area and the slope angle, stratigraphy, and logging practices. Landslide frequency increases upvalley, as do mean annual precipitation and the frequency of perched water tables. Debris slides are most common, occur on steep slopes (>50%) composed of sand and gravel, and are most abundant in areas previously logged by the clear—cut method. Debris flows occur on shallower slopes (>30%) where the stratigraphy leads to perched water tables. Debris flows larger than 600 m² in area appear to be unrelated to logging practices. Slump flows, described here for the first time, occur on similar slope angles and stratigraphic situations as debris flows. They differ mainly by the presence of semiconsolidated material, usually till, at the slide head. Where till is breached—commonly along road cuts—water infiltration is increased, saturating underlying fine-grained deposits, which then fail by debris flowage. Secondary slumping of till happens when the slope steepens during debris flow failure. Small landslides surrounding Lake Shannon may contribute up to 80% of the total particulate matter yield to the fluvial system at present, increasing lake sedimentation by a rate of 5 mm/yr.     

Investigation of the failure mechanism and stabilization of a landslide in weathered tuffite,Giresun, northeastern Turkey

This study investigates the causes and failure mechanism of the Aksu landslide that occurred during the construction of the Giresun–Espiye road between KM: 1 + 030–1 + 170 in northern Turkey and recommends proper stabilization techniques. For the purpose of investigating the causes and mechanism of this slope failure, engineering geological mapping, geotechnical investigation and rock mass characterization were performed. From top to bottom, weathered tuffite, tuffite, flysch, and dacitic tuffite were the major units in the study area. The disturbance of the slope by the excavations performed at the toe of the slope (i.e., due to the foundation excavation for the Tünel restaurant building and for the road cut) led to a “translational slide”. The “translational slide” occurred in completely weathered tuffite due to the disturbance of the stability of the slope by the excavations performed at the toe of the slope, particularly for the foundation excavation of the Tünel restaurant building and for the road cut along the Giresun–Espiye road. The rise in the groundwater level was also another important factor that has contributed to the occurrence of the landslide. After establishing the geometry of the landslide in detail, the shear strength parameters of the failure surface were determined by the back analysis method. Sensitivity analyses were performed and landslide failure mechanisms were modeled to quantify the contributing factors that have caused the formation of the Aksu landslide. The influence of an earthquake was investigated through pseudostatic slope stability analysis. Toe buttressing, ground water drainage, and surface water drainage alternatives were considered for stabilizing the slope.     

复合地基加固路堤的稳定性分析

通过对复合地基加固路堤稳定性的极限平衡法和有限单元法的计算结果进行对比分析,揭示出当路堤的稳定性由复合地基决定时,极限平衡法和有限单元法的计算结果存在较大差异。应用岩土有限元软件Plaxis中的强度折减法可得到合理的路堤稳定性分析结果。对复合地基中桩体破坏模式的分析认为,桩体发生非剪断破坏之外的弯曲、转动、拉伸等破坏模式是极限平衡法与有限单元法计算结果产生差异的根本原因。由此进一步指出,当处理存在土与结构物相互作用的边坡稳定问题时,极限平衡法的分析结果可能会高估了边坡的稳定性,应慎重判别其合理性。     

Characteristics and remediation of a landslide complex triggered by the 2008 Wenchuan, China earthquake-case from Yingxiu near the earthquake epicenter

During the 2008 Wenchuan earthquake, the river valley from Yingxiu to Wenchuan experienced numerous landslides and became a prominent area of landslide complexes. The present large landslide complex near the earthquake epicenter consisted of Laohuzui slide 1, Laohuzui slide 2 and Douyaping slide. The scale, geology, morphology, sliding process, and failure mechanism of the landslide complex are analyzed by means of field investigation, aerial photograph and stereographic projection technique. Characteristics of these three slides including seismic response of slope, landslide debris, damage and potential failure are discussed: the convex slope and the upslope of fractured granitic rock at high altitude are highly prone to landsliding under earthquake; the high source altitude and long travel path determine grain sizes and the deposit angle of the slide debris; the landslide complex completely buries the G213 roadway and dams up the Minjiang River in these sections; after the earthquake, rainfall, aftershocks and river erosion may retrigger new failures, such as retrogressive slide of weathered fractured rock, colluvial landslide, debris flow, embankment failure and rockfall. The following are presented as suggested remedial measures to protect the roadway and stabilize the slope: the removing and trenching, protective concrete/rock blocks against erosion, retaining structure, rockfall stopping wall, rockfall restraining net, rock bolt, and the planting of vegetation.     

基于边界元法的边坡矢量和稳定分析

矢量和法物理力学意义明确,计算简单,且能根据边坡当前的应力分布状态合理地评价其整体稳定性状态。其中边坡的应力状态通常是采用有限元法来求解。由于边界元法具有研究问题降阶、离散化带来的误差值仅产生在边界以及计算量小等优点,在工程中得到了广泛应用;对于平面问题,以源点作为原点,以所积分单元的切向和法向为坐标轴建立局部坐标系,对于线性单元可以得到所有积分的解析解。因此,可以得到计算区域内部任意点的场变量的解析解,这就保证了位于边界附近区域场变量的精度。利用边界元法得到二维边坡体内连续的应力分布状态,使用矢量和法对该边坡进行稳定性分析,并且与基于有限元的矢量和法、极限平衡法进行对比分析。边坡圆弧滑面和折线滑面的计算结果表明,基于边界元法得到的矢量和安全系数和基于有限元的矢量和法、极限平衡法基本一致;边界元法对应的矢量和安全系数对边界单元尺寸不敏感。     

Seismic-induced rockfalls and landslide dam following the October 30, 2016 earthquake in Central Italy

On October 30, 2016, a seismic event and its aftershocks produced diffuse landslides along the SP 209 road in the Nera River Gorge (Central Italy). Due to the steep slopes and the outcropping of highly fractured and bedded limestone, several rockfalls were triggered, of which the main event occurred on the slope of Mount Sasso Pizzuto. The seismic shock acted on a rock wedge that, after an initial slide, developed into a rockfall. The debris accumulation blocked the SP 209 road and dammed the Nera River, forming a small lake. The river discharge was around 3.6 m³/s; the water overtopped the dam and flooded the road. By a preliminary topographic survey, we estimated that the debris accumulation covers an area of about 16,500 m², while the volume is around 70,000 m³. The maximum volume occupied by the pre-existing talus mobilized by the rockfall is about 20% of the total volume. Besides blocking the road, the rockfall damaged a bridge severely, while, downstream of the dam, the water flow caused erosion of a road embankment. A rockfall protection gallery, a few hundred meters downstream of the dam, was damaged during the event. Other elastic nets and rigid barriers were not sufficient to protect the road from single-block rockfalls, with volumes around 1–2 m³. Considering the geological and geomorphological conditions, as well as the high seismicity and the socioeconomic importance of the area, a review of the entire rockfall protection systems is required to ensure protection of critical infrastructure and local communities.     

Geotechnical characterization and rockfall simulation of a slope: a practical case study from South Tyrol (Italy)

Rockfalls occur often along the state road SS241 Val d'Ega in the Val d'Ega Valley (South Tyrol, Italy). In order to protect the road and the traffic, stabilization works are necessary. Detailed geological and geomorphologic mapping along structural and geomechanic sections was carried out, and experimental rockfall trails were examined on the slope above the state road between km 3.550 and 3.830. Rockfall simulation, rock mass classification and an interpretation of field data, especially concerning joints, wrapped up the investigations. They helped find the specific need for stabilization work and its positioning along the slope. The stabilization works were completed on February 2002. Another important aspect concerns ongoing geotechnical investigations and stabilization works urgently realised during autumn 1999.

This paper is a practical approach in solving rockfall problems along public roads. It should be considered as a workbook for university personnel and also for professional geologists.     

小湾电站深切河谷边坡演化动力学过程及机制研究

小湾水电站坝址位于高山峡谷区,深切峡谷形成演化的动力学过程决定了坝区边坡目前的形态特征、岩体结构特征及稳定性状况,由此深刻影响边坡设计方案和建基面的选择。基于现场工作和分析测试成果,得出主要结论有:(1)由于河谷形成过程中的卸荷作用及风化作用,边坡浅表层改造强烈,产生了大量的中缓倾角裂隙,坡面附近近SN向陡裂张开、扩展;浅表层发育了大量近EW向挤压带(面);(2)岸坡浅表层中缓倾角结构面的产生机制,包括沿原构造节理扩展和新生裂隙;坡体下部的中缓倾角裂隙会因差异卸荷回弹而继续扩展,坡体中上部岩体质量劣化,在地应力场调整过程中,因剪切滑移继续扩展,坡体逐渐进入时效变形阶段。(3)对河谷边坡进行了地质—工程分类,指出各类边坡可能变形失稳模式包括倾倒变形、平面滑动、阶梯状滑动、楔形滑动和堆积体滑动等5种。     

Heavy-rainfall-induced catastrophic rockslide-debris flow at Sanxicun,Dujiangyan, after the Wenchuan Ms 8.0 earthquake

This paper uses the catastrophic rockslide at Sanxicun village in Dujianyan city as an example to investigate the formation mechanism of a rapid and long run-out rockslide-debris flow of fractured/cracked slope, under the application of a rare heavy rainfall in July 2013. The slope site could be affected by the Wenchuan Ms 8.0 Earthquake in 2008. The sliding involved the thick fractured and layered rockmass with a gentle dip plane at Sanxicun. It had the following formation process: (1) toppling due to shear failure at a high-level position, (2) shoveling the accumulative layer below, (3) forming of debris flow of the highly weathered bottom rockmass, and (4) flooding downward along valley. The debris flow destroyed 11 houses and killed 166 people. The run-out distance was about 1200 m, and the accumulative volume was 1.9?×?10⁶ m³. The rockslide can be divided into sliding source, shear-shoveling, and flow accumulative regions. The stability of this fractured rock slope and the sliding processes are discussed at four stages of cracking, creeping, separating, and residual accumulating, under the applications of hydrostatic pressure and uplift pressure. This research also investigates the safety factors under different situations. The double rheological model (F-V model) of the DAN-W software is utilized to simulate the kinematic and dynamic processes of the shear-shoveling region and debris flow. After the shear failure occurred at a high-level position of rock, the rockslide moved for approximately 47 s downward along the valley with a maximum velocity of 35 m/s. This is a typical rapid and long run-out rockslide. Finally, this paper concludes that the identification of the potential geological hazards at the Wenchuan mountain area is crucial to prevent catastrophic rockslide triggered by heavy rainfall. The identified geological hazards should be properly considered in the town planning of the reconstruction works.     

Large,prehistoric clay slides revealed in road excavations in Buvika,mid-Norway

Several excavated sections along new roads in Buvika, mid-Norway, display records of large, prehistoric clay slides. Slightly undulating but otherwise intact laminated clay, with minor amounts of unsorted or sorted coarser-grained debris, appears in the sections. Folding, shearing and internal slide surfaces have also been recorded, and the deposits are interpreted as slide debris. Slide-scar morphologies are diffuse or absent for most of these deposits, and the inferred slide mechanism is translation and rotation of blocks on a thin layer of remoulded quick clay. One section cuts through a slide block inside a classic, morphologically well-defined quick-clay slide scar. Varying results from ¹⁴C-datings of organic material in the deposits give only maximum ages of the slide events. The style of sliding reflected in several sections differs from slide events in the area known from historical records. These younger slide events seem to be characterised by collapse and remoulding of thicker quick-clay layers. The present study of slide deposits gives information on processes and deformation history of the slides. It is suggested that the gradual formation of quick clay has an important impact on long-term landscape degradation and on the character of the present day landscape. There may be far more signs of slide activity in Norwegian valleys than solely indicated by slide scars and also in the relatively high-lying parts of the valley fill, such as interfluve areas.     

The Todagin Creek landslide of October 3, 2006, Northwest British Columbia, Canada

The Todagin Creek landslide is located at 57.61° N 129.98° W in Northwest British Columbia. A seismic station 90 km north of the landslide recorded the event at 1643 hours coordinated universal time (UTC; 0943 hours Pacific daylight time (PDT)) on October 3, 2006. The signal verifies the discovery and relative time bounds provided by a hunting party in the valley. The landslide initiated as a translational rock slide on sedimentary rock dipping down slope at 34° and striking parallel to the valley. The landslide transformed into a debris avalanche and had a total volume estimated at 4 Mm³. An elevation drop of 771 m along a planar length of 1,885 m resulted in a travel angle (fahrb?schung) of 21.3°. The narrowest part of the landslide through the transport zone is 345 m. The widest part of the divergent toe of the landslide reaches a width of 1,010 m. Landslide debris impounded a lake of approximately 32 ha and destroyed an additional 67 ha of forest. The impoundment took 7 to 10 days to fill, with muddied waters observed downstream on October 13. No clear linkage exists with precipitation and temperature records preceding the landslide, but strong diurnal temperature cycles occurred in the days prior to the event. The Todagin Creek area appears to have an affinity for large landslides with the deposits of three other landslides >5 Mm³ observed in the valley.