Isakbek Torgoev

Geophysical investigations of seismically induced surface effects: Case study of a landslide in the Suusamyr Valley, Kyrgyzstan

In summer 1998, a geophysical survey including seismic profilesand electrical tomography has been carried out in the Suusamyr valley, Kyrgyzstan. The scope wasto investigate surface effects induced by the Ms = 7.3 Suusamyr earthquake, the 19th of August, 1992. Inthis paper, special attention is paid to the case study of a debris slide triggered by the earthquake.Seismic data are analysed by P-wave refraction technique and by surface wave inversion. Electrical tomographicprofiles are processed by 2D-inversion.Using geotechnical and geological information, P-velocity modelsand resistivity sections are interpreted in terms of geological materials, in order to build a geological3D model. On the basis of the latter, we carried out static finite element computations as well as staticand pseudo-static calculations with Janbu’s method. Newmark displacement was computed, considering or notthe influence of the shallow soft deposits. The results are compared to the real displacementobserved in the field and conclusions are drawn about the mechanism of the landslide.

Impact of Uranium Mining and Processing on the Environment of Mountainous areas of Kyrgyzstan

In this report the results of analysis of modern geo-ecological situation in areas of uranium mining and milling in the territory of Kyrgyzstan are presented. Major threats for the mountain environment and citizens which are connected to mining and processing of the uranium are due to the contamination of the hydrosphere by radio nuclides, and due to the stimulation of dangerous natural and technological processes (landslides, mudflows, destruction of the tailings and dumps) with unfavorable ecological consequences.

A Review of Recent Case Studies of Landslides Investigated in the Tien Shan Using Microseismic and Other Geophysical Methods

This paper reviews recent case studies completed on mass movements (and landslide dams) with probable (post-) seismic origin or susceptible to seismic failure in the Tien Shan, Central Asia. Geophysical investigations and seismological and microseismic surveys presented here were carried out on the Kainama earthflow in the Southern Kyrgyz Tien Shan, on loess landslides in Tajikistan as well as on unstable slopes and a landslide dam in the Central Kyrgyz Tien Shan. The latter investigations were completed in summer 2011 using electrical tomography combined with seismic refraction and microseismic measurements as well as earthquake recordings. For all sites complex 3D models were built (with the GOCAD software). For some sites 2D numerical modelling of seismic slope stability allowed us to make accurate assessment of their susceptibility to seismic failure. Modelling of the seismic amplification potential of various slopes confirmed the results obtained from the seismological surveys: strongest amplifications generally affect the upper parts of the slope close to the crest of the mountains—not only due to topographic effects but also due to the presence of weaker rocks or thicker soft deposits in these areas. The results of the dynamic modelling further show that a combination of seismic and hydrologic factors (pore pressure build-up during the seismic shaking and post-seismic rise of groundwater level) was necessary to trigger the failure.

Mass Movement in the Waste Dump of High-Altitude Kumtor Goldmine (Kyrgyzstan)

The movement of waste rocks dumps together with ice, stored on the glaciers of the high-altitude Kumtor gold mine is reviewed in this paper. This mining site is located in the central part of the Tien Shan (Kyrgyzstan). There are no other cases around the world, when the waste rocks and removed ice were stored on the active mountain glaciers. The landslide has formed in the area of dumps, stored on the Davydov glacier, and the displacement velocity has dramatically increased in spring 2013. Due to the accelerated movement of rock-ice mass, a number of Kumtor mine infrastructural units was destroyed with economic losses of about USD 100 million. This article presents the results of monitoring of the landslide displacements and provides analysis of the causes for its acceleration. The potential environmental impact of further development of the gravitational mass movement and impact on the Kumtor gold mine facilities were assessed.

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.

Tien-Shan Landslides Triggered by Earthquakes in Pamir-Hindukush Zone

Sudden landslides cause big problems all over the world. These landslides often take place in the fault areas marked by a high seismic activity. Here, we study the impact of teleseismic events, located in the Pamir-Hindukush source zone, on the stability of the landslide-prone slopes in Tien-Shan. The Pamir-Hindukush zone is a unique seismic region of the Earth, where the strongest earthquakes occur at depths of 200–250 km. Some of those earthquakes have a magnitude of M > 7. Overall, 89 perceptible earthquakes (M > 5) occurred from 1991 to 2010 (according to the available data) with 35% of them occurring in springtime. In the mountainous regions of the Tien-Shan, such earthquakes cause liquefaction on water saturated slopes with accompanying slumping and cracking. This paper describes mechanisms of sudden landslide movement using the example of Sary-Bulak landslide which failed in 1976 after the earthquake in the Pamir-Hindukush zone.

Rockslides in the Open Pit of Kumtor Goldmine (Kyrgyzstan)

The high-elevation Kumtor mine is located in the Central Tien Shan in the permafrost and active glaciers area. This area is characterized by extreme continental climate. The average annual temperature in the mine area is −7.8 °C with fluctuations in the average minimum temperature of −28.8 °C in January up to 1.7 °C in July. The average annual precipitation is 350–400 mm, a quarter of which falls as rain and the rest in the form of snow. Harsh climatic conditions in the area, the presence of mountain glaciers and permafrost rocks, diversity of terrain, rock types, seasonal freezing and melting of the rocks and soil together with global warming and large-scale anthropogenic impact cause active development of cryogenic gravitational processes. The most dangerous among them are repeated slumping and sliding of the north-eastern wall of the Central pit. The report presents the results of geotechnical studies and analysis of the main reasons for the failure of the pit walls, which include melting of the permafrost rocks at the top of the pit and glacial water inflow into the tectonic disruption zone.

Hazard mitigation for landslide dams in Mailuu-Suu valley (Kyrgyzstan)

The main reasons of mountain slopes instability in this area are the geologic conditions and geodynamic peculiarities of the region. Seismic, climatic and man-caused factors play a role of rather a trigger mechanism for landslide initiation and development. Among the general geologic factors that lead to the intensive development of landslide processes in the area besides the MailuuSuu valley specific topography and erosion processes are the following: - composition, characteristics and bedding conditions of rocks and soils on the slopes, specifically the most active landslides are developed in loess-like loamy soils of the Quaternary period (Q 1 - Q3), as well as in slightly consolidated Mesozoic-Cenozoic sediments, layered by strata of weakly stable fissured limestones and sandstones alternating with interlayers of clays and siltstones; - watering of soils and rocks which are components of unstable slopes either by large quantity of atmospheric precipitation infiltration or by groundwater impact including the ones coming along tectonic failures from the remote mountainous feeding areas of the Boobash-Ata mountain range (Fergana ridge); - structurally-tectonic and geodynamical features of the area, namely tectonic dislocations and folds. The studied region is characterized by high seismicity (M»7) associated with modern geodynamic activity and continuous orogenic processes. Contemporary recent horizontal (up to 10 mm/year) and vertical (up to 5 mm/year) earth crust movements are actively developed here. Besides, the area of the development of numerous and active landslides in the eastern mountain framing of Fergana valley coincides spatially with the zone of the intensive compression caused by the south-north pressure from the Pamirs side (WETZEL et alii, 2000). Along the Mailuu-Suu River valley edges and its main tributaries 40 landslides at a stage of the main movement or their preparatory phase have been registered. The most hazardous from them with relation to landslide river blockage with possible catastrophic effects are the large-sized Koi-Tash and Tectonic landslides located close by (Figure 1). These landslides generation started in 1954 on the left edge of the valley within the zone of the regional Central (Arslanbob) fault and the Main anticline. The detailed description of these and other landslides of considered area is presented in a series of papers (c.f. TORGOEV et alii, 2001).

Integrated Geophysical-Geological 3D Model of the Right-Bank Slope Downstream from the Rogun Dam Construction Site, Tajikistan

In summer of 2015 we had completed a geophysical survey complemented by borehole drilling near the right-bank slope of the Rogun Dam construction site, Tajikistan. These data were first processed and then compiled within a 3D geomodel. The present paper describes the geophysical results and the 3D geomodel generated for an ancient mass movement located immediately downstream from the construction site. The geophysical survey included electrical and seismic profiles and ambient vibration measurements as well as earthquake recordings. The electrical and seismic data were processed as tomographic sections, the ambient vibrations as horizontal-to-vertical spectral H/V ratios, and the earthquake data mainly in terms of standard spectral ratios. By estimating the average shear wave velocities of the subsurface, we computed the local soft layer thickness from the resonance frequencies revealed by the H/V ratios. Three seismic stations had been installed for ten days along a profile crossing the intermediate plateau. Standard spectral ratios inferred from ten processed earthquake measurements confirmed the presence of a thick soft material layer on the plateau made of weathered rocks, colluvium, and terrace deposits, which produce a medium-level amplification at about 2 Hz. The 3D geomodel was first built on the basis of new topographic data, satellite imagery, and a geological map with two sections. Then, the various electrical resistivity and seismic refraction tomographies were inserted in the geomodel. The soft layer thickness information and borehole data were represented in terms of logs in the model. The site is crossed by the Ionakhsh Fault that could be modeled on the basis of the geological inputs and of a lateral resistivity gradient found on one electrical profile along the steep lower slope. The integrated interpretation of all results reveals that probably only a relatively small part of the ancient giant mass movement is really exposed to slope instability phenomena.

Geophysical Monitoring of Artificial Landslide Dam of Kambarata Hydro Power Plant-2 (Kyrgyzstan)

One of the main stages of the Kambarata HPP-2 construction, which is situated on Naryn River (Kyrgyzstan), was the design of a unique dam. This dam was formed under the difficult seismic-tectonic conditions of Central Tien-Shan. The applied method of dam construction was not a traditional one, but one which used explosive energy. An oriented large-scale explosion (around 2,600 tons of explosives) blasted a rock mass from the right slope of Naryn river basin, downstream from Kambarata river mouth. The big explosion initiated the failure of an artificial landslide which dammed Naryn River—the volume of landslide dam was around 1.8 mln. m3 and its height was 35 m. Since the explosion did not produce the designed dam properties (60 m of height and 3 mln. m3 of volume) there was a need to apply traditional methods of dam construction in order to reach the planned elevation of 961 m. The monitoring of the dam and the surrounding rock massif was set up study their geotechnical properties and filtration anisotropy. Water inflow in the dam and the surrounding rock massif was mapped on the basis of an electrical resistivity survey and regular piezometric observations. The latter started in 2010 and revealed the gradual clogging of dam’s body. Additional measures to insure the impermeability of the dam include the construction of a continuous concrete screen along the whole face of the dam. The interest of this study for landslide dam research is related to the fact that here we provide an insight to the internal structure of a dam that partly consists of landslide material.