Alexander Strom

LANDSLIDES FROM MASSIVE ROCK SLOPE FAILURE AND ASSOCIATED PHENOMENA

Landslides from massive rock slope failure (MRSF) are a major geological hazard in many parts of the world. Hazard assessment is made difficult by a variety of complex initial failure processes and unpredictable post-failure behaviour, which includes transformation of movement mechanism, substantial changes in volume, and changes in the characteristics of the moving mass. Initial failure mechanisms are strongly influenced by geology and topography. Massive rock slope failure includes rockslides, rock avalanches, catastrophic spreads and rockfalls. Catastrophic debris flows can also be triggered by massive rock slope failure. Volcanoes are particularly prone to massive rock slope failure and can experience very large scale sector collapse or much smaller partial collapse. Both these types of failures may be transformed into lahars which can travel over 100 km from their source. MRSF deposits give insight into fragmentation and emplacement processes. Slow mountain slope deformation presents problems in interpretation of origin and movement mechanism. The identification of thresholds for the catastrophic failure of a slow moving rock slope is a key question in hazard assessment. Advances have been made in the analysis and modeling of initial failure and post-failure behaviour. However, these studies have been retrodictive in nature and their true predictive potential for hazard assessment remains uncertain yet promising.

The Formation and Behaviour of Natural and Artificial Rockslide Dams; Implications for Engineering Performance and Hazard Management

The formation and behaviour of natural and artificial rockslide dams are reviewed to update the well-known work of Costa and Schuster [1]. Rockslide dams block surface drainage to form upstream lakes. They may occur naturally due to landslides or as a result of engineered rock slope failure. As evidenced by the 2010 Hunza event (Pakistan), the stability of rockslide dams is a major consideration in landslide risk assessment in mountain terrain, particularly with respect to the possibility of a destructive downstream flood resulting from a breach of the dam. The damming of a river by a rockslide may require immediate engineering response to mitigate the hazard. However, failure by breaching is less frequent than long-term stability. These issues are examined with reference to nine case histories of rockslide dams and rockslide-dammed lakes; Gohna (1894), Rio Barrancas (1914), Condor-Sencca (1945), Mayunmarca (1974), La Josefina (1993), Tsao-Ling (1999), Yigong (2000), Tangjiashan (2008), and the Hunza (2010). The case histories also illustrate the utility of digital terrain data (especially the SRTM-3 data set obtained in February 2000) and remote sensing imagery to obtain accurate estimates of the impoundment volumes and other geomorphic data on rockslide-dammed lakes. Methods of estimating peak breach discharge and downstream flood effects exist but are still largely empirical in nature. Measures to mitigate hazard associated with rockslide-dammed lakes include the construction of a spillway over the rockslide debris, a by-pass tunnels through the abutments of the debris dam, the implementation of dam and lake-level monitoring and failure warning systems to mitigate downstream damage. A review of some well-documented examples show that these measures have had been applied with mixed success in the past. Natural rockslide dams are commonly used for foundations for conventional constructed dams. Artificial rockslide dams are created by rock slope failure induced by large-scale explosion (blast-fill dams). The largest blast-fill dam yet constructed is the Medeo Dam, a debris flow retention structure near Alma-Ata, Kazakhstan. Rockslide dams and their geomorphic effects may create an important legacy in the landscape through massive accumulations of lake sediments, impact on river channels, and effects on river long-profiles.

The classification of rockslide dams

It is suggested that rockslide – rock avalanche dams are more complex than is recognized in existing landslide dam classifications. A classification system is proposed emphasizing the three-dimensional relations of the landslide deposit to valley morphology. This system combines a three-step classification taking account of: (A) the plan view distribution of the deposit in relation to valley morphology and the impounded water bodies, (B) the cross valley profile of the landslide deposit as it relates to the buried valley morphology, and (C) the profile of the rockslide debris and the underlying substrate along the thalweg of the valley. It is argued this serves to better describe the dams with respect to the hazard of outburst floods, impoundment histories and the subsequent morphological evolution of mountain stream valleys.

Landslides in Cold Regions in the Context of Climate Change

Catastrophic slope failures that occur in glaciated zones of mountain ranges at high altitudes can be considered as landslides in cold regions, since ice plays an important role in their origination and emplacement. Case studies of the XX Century rock avalanche that fell onto the glacier and of the extraordinary prehistoric ice-rock avalanche are described briefly. They demonstrate that presence of large quantities of ice in the glaciated zones of high mountains results in significant masking of the origin of debris accumulations that could be found either on glaciers or at the feet of heavily glaciated slopes.

International Summer School on Rockslides and Related Phenomena in the Kokomeren River Basin, Kyrgyzstan

ICL Annual Summer School on Rockslides and Related Phenomena in Kyrgyzstan started in 2006, being supported by the IPL ProjectsM111,M126 and C106-2. This training course aims to familiarize students and young landslide researchers with various types of large-scale bedrock landslides (rockslides), with geological factors favourable for their origin and with their primary and secondary effects such as river damming and subsequent outburst floods. The Kokomeren River basin is characterised by a unique variability of rockslides that have occurred in different rock types, in confined and unconfined conditions and have formed both high dams and long-runout rock avalanches. Some of the dams remain intact while others have been deeply eroded, exposing their interiors. Particular emphasis is given on those peculiarities of rockslides’ morphology and internal structure that can clarify their motion mechanism(s) and explain the extreme mobility of rock avalanches.

Utilisation of Data Derived from Large-Scale Experiments and Study of Natural Blockages for Blast-Fill Dam Design

Blast-fill and, especially, blast-rockslide technology allows construction of large dams with high safety factor and at the acceptable cost at those sites, which geological and seismological conditions complicate or even prevent construction of dams of other types. Large-scale field experiments and study of natural dams of the same size as the designed structures have been carried out to obtain reliable information about the processes that take place during large amount of blasted rock emplacement. Grain-size composition, density and filtration characteristics evolution of the experimental Burlykia dams’ body, and topographic and geological conditions of dam sites preferable for such dams construction are described. Besides, types of across- and along-valley distribution of rock avalanche debris, which, in turn, determines dams’ height and shape, are identified and factors determining either morphological type are proposed. Such critical parameters as dams’ permeability and erosion resistance in case of overtopping depends on blockages’ dual-zone internal structure with wide core composed of shattered debris and coarse carapace. High seismic stability of natural blockages exemplified by numerous case studies shows that large blast-rockslide dams could be considered as one of the most earthquake-proof type of water retaining structures.

Earthquakes, Landslides, Dams and Reservoirs in the Tien Shan, Central Asia

This paper presents an overview of seismic and mass movement hazards affecting major Hydropower-plants (HPP) and ongoing dam projects in the mountain regions of Central Asia. HPP cascades are located along the Naryn River in the Kyrgyz Republic and the Vakhsh-Surkhob valley fault zone in Tajikistan. The latter region hosting the presently and future tallest man-made dams of the world is very prone to earthquakes and various types of slope instabilities. The Naryn Valley hosting several dam sites is marked by the presence of ancient rockslides and a dense network of seismically active faults. In December 2009, Kyrgyz, Russian, Slovak and Belgian teams have monitored seismic ground motions and displacements induced by the blasting of a slope on the Kambarata 2 site producing a 35 m–high blockage on the Naryn River. This work is part of a NATO Science for Peace and Security project on landslide dam hazards in the Tien Shan.

The First Meeting of ICL Landslides in Cold Regions Network, Harbin, 2012

The First Meeting of ICL Landslides in Cold Regions Network and First Symposium on Landslides in Cold Regions were held in Harbin, China on 23–27 July 2012. The main goal of the network is to promote cooperation of scientists studying landslides in the permafrost regions and regions with extreme weather conditions. It will support joint comprehensive investigations carried out by geographers, geologists, geocryologists, and meteorologists from different countries and regions, landslide mechanisms study, distinguishing of landforms, provision of landslide hazard assessment, and elaboration of early warning systems. Such cooperation will enhance our understanding of hazardous phenomena in cold regions and the safety of people living there, their property, and infrastructure. This meeting included an international symposium “Landslide in Cold Regions,” 2-day field trip, discussion, and approval of the “Constitution of ICL Landslides in Cold Regions Network,” “2012–2016 Action Plan of ICL Landslides in Cold Regions Network,” and “Declaration of the First Meeting of ICL Landslides in Cold Regions Network.”

The Kambarata 2 blast-fill dam, Kyrgyz Republic:blast event, geophysical monitoring and dam structure modelling

BackgroundThe blast- and earth-fill dam of the Kambarata 2 hydropower station is situated in the seismically active Central Tien Shan region of the Kyrgyz Republic. More than 70% of the dam volume was produced during a blast event on December 22, 2009. In 2010–2011, dam construction was completed after earth filling on top of the blasted material and installing concrete and clay screens together with bentonite grouts. A geophysical survey had been completed in 2012–2013, mainly to monitor the resistivities inside the dam.ResultsThe geophysical survey completed on the Kambarata 2 dam site showed lower resistivity zones in the earth fill and relatively higher resistivities in the blast-fill material. Topographic, geophysical and piezometric inputs had been compiled within a 3D geomodel constructed with GOCAD software. This model was compared with the design structure of the dam in order to define the upper limits of the underlying alluvium, the deposited blast fill, earth fill and top gravel materials (represented by the dam surface). The central cross-section of this model was extrapolated over the full length of the main dam profile.ConclusionsOn the basis of a calibrated hydrogeological model and known geomechanical properties of the materials, dam stability calculations were completed for different scenarios considering different reservoir levels and varying seismic conditions. Some of these scenarios indicated a critical vulnerability of the dam, e.g., if impacted by a horizontal seismic acceleration of Ahu2009=u20090.3xa0g and a vertical seismic acceleration Avu2009=u20090.15xa0g, with an estimated return period of 475xa0years.As a general conclusion, it was noted that this case study can be used as an example for surveys on much larger natural – landslide or moraine – dams. A series of geophysical methods (e.g., electrical and electro-magnetic techniques, seismic and microseismic measurements) can be applied to investigate even very deep dam structures. These methods have the advantage over classical direct prospecting techniques, such as drilling, of using equipment that is much lighter and thus more easily transportable and applicable in difficult terrain. Furthermore, they can provide continuous information over wider areas. This specific application to a blast-fill dam allows us to better outline the strengths and weaknesses of the exploration types and geomodels as a series of investigated parameters can be verified more easily than for natural dams.

International Summer School on Rockslides and Related Phenomena in the Kokomeren River Valley, Tien Shan, Kyrgyzstan: IPL-106-2 Project and WCoE

The International Summer School on Rockslides and Related Phenomena in the Kokomeren River valley, Tien Shan, Kyrgyzstan is the annual field training course focused on identification and study of large-scale bedrock landslides. These phenomena pose a threat to communities living in mountainous regions all over the world and are characterised by the enormous amount of material involved, and its high mobility and ability to create natural dams. Since 2006 more than 50 students and young landslide researchers from 17 countries have been introduced to rockslides and rock avalanches of different morphological types, some of which have formed deeply eroded rockslide dams that allow study of their internal structure, as well as evidence of inundation and of catastrophic outburst floods, and impressive manifestations of recent tectonic phenomena.