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The Usoi Dam “The Geological Engineering Involved in Assessing the Safety Concerns for the
World’s Tallest Dam”
A report by Brian Shams for
CE 281 Engineering Geology
University of California, Berkeley
December 10, 2012
Photo taken by Martin Mergili
Purpose The original concept of this report was to present a geotechnical case history. The Usoi dam was a unique choice in that the structure is not man-‐made and therefore not engineered. Despite this the author feels that given the significance of the structure and the sheer scale it is worth presenting in this report. The existence of numerous documented analysis and research on the potential hazards of the Usoi Dam has accounted for the basis of this report.
Abstract The Usoi Dam is one of the world’s most amazing structures. The dam, which is the world’s tallest, formed naturally in 1911 from the 7.4 magnitude Sarez earthquake. The initial slide is characterized as the world’s largest recorded non-‐volcanic landslide and immediately buried the village of Usoi along with all the inhabitants. The lake that formed subsequently after the slide, Lake Serez, now contains roughly 1/9 the volume of Lake Tahoe in California. With a lake elevation of almost 11,000 feet this dam stores a significant amount of potential energy. If a dam failure were to occur the resulting damage would affect millions and would also impact multiple countries in the region. Although massive, Usoi Dam poses a big challenge to geotechnical investigation. This report gives an overview of the geotechnical and geological challenges in assessing the safety of the Usoi Dam in Tajikistan.
Executive Summary In addition to the original investigation made on the Usoi Dam by the former Soviet Union recent analysis have also become available. The most significant of these is the report by the United Nations International Strategy for Disaster Reduction (2000) (1). The general findings of these analyses indicate that the probability of a complete dam breach of the Usoi Dam is unlikely. However an additional analysis by the USGS (2006) (7) suggests that the calculated wave heights from a landslide on Lake Sarez were previously unconservative, and the new results still suggest an unlikelihood of complete dam breach. Regardless all analyses seem to indicate a desire for more research and that the worst-‐case scenario of a complete dam breach should not be ignored. In general, the threat of overtopping and dam erosion is a real threat to the safety of all inhabitants downstream. The more likely scenario is a dam overtopping flood that would not erode Usoi Dam enough to cause lake draining. This would affect roughly 35,000 people, being mostly village inhabitants along the Murgab River.
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Table of Contents
Setting……………………………………………………………………………………………3
Local Faults & Seismicity…………………………………………………………………4
Structural Integrity of Dam….…………………………………………………………..5
Geological Records…..……………………………………………………………………...5-‐6
Permeability & Water Filtration of Dam…………………………………………...6
Modes of Dam Failure……………………………………………………………………...6-‐7
Landslide Concerns…………………………………………………………………………7-‐8
Conclusions…………………………………………………………………………………….9
References..…………………………………………………………………………………….10
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Setting The Usoi Dam is situated deep within the western Pamir Mountain Range in eastern Tajikistan. The dam itself was formed by a massive landslide triggered during the 7.4 magnitude Sarez Earthquake in 1911. The slide immediately dammed up the Murgab River and instantly buried the village of Usoi along with all its inhabitants. Two lakes were quickly formed, Shadau Lake and Sarez Lake, Sarez being the larger of the two. The dam is roughly composed of 6 billion tons of rock. The 10,725 ft. elevation Lake Sarez contains roughly 4 cubic miles of water, equivalent to about 1/9 the volume of Lake Tahoe in California.
The potential for lose of human life and negative economic impacts can be quite significant in the event of a partial or complete failure of the dam. The Murgab River eventually flows into the greater Amu Darya River, which is the main source of existence for much of the settlements along the borders between Afghanistan with Tajikistan and Turkmenistan with Uzbekistan. A major flood on the Amu Darya would easily affect millions of people (1). In fact the original motivation for the study of this problem was initiated by the Soviet Union because of the concern for loosing its important agricultural fields in Uzbekistan.
Satellite Image of the Pamir Mountain Range (NASA 2003), (lower left corner) zooming on Lake Serez, in red.
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Local Faults & Seismicity To understand the local seismic conditions one must understand the greater geology of the area. The entire Pamir Mountain Range is situated at the closest points between the Tien Shan Range and the Himalayan Range. As the Indian Plate and Eurasian Plates are colliding the Pamir Mountains are being sheared and also pushed upward. This is causing an unevenly spread uplift force throughout the Pamir Range which has resulted in the many faults and larger shear zones. The Usoi Dam is located within 20 km of the Rushan-‐Pshart Thrust Fault Zone. Much of this seismic activity results in large earthquakes that frequent the Pamir Range and the nearby mountain ranges. It is also very important that a local southwest to northwest trending wrench fault is present in the innermost corner of the wedge that forms the failure surface (3).
A Seismic Hazard Assessment was performed on June 2000 on Lake Sarez by GIBB Ltd. The conclusion was that a maximum credible earthquake of a magnitude of 7.9 would correspond with a return period of slightly above 130 years. However the likely hood is lower for the earthquake to occur near the location of the initial 1911 Sarez Earthquake (3).
North to South cross section of the different continental blocks of the Pamir Range. The Indian plate is colliding with Eurasian plate causing the entire fractured Pamir Range to be uplifted (6).
Top: The Epicenters to previous large earthquakes (red dots) and traces of active faults (red lines). Bottom: Spatial distribution of epicenters of previous large earthquakes proportional to amplitude (blue circles) (2).
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Structural Integrity of the Dam In most cases landslide dams are composed of a heterogeneous mass of unconsolidated earth, which is not suitable for a functioning dam. However given the massive dimensions of the Usoi Landslide Dam it does not fall under these normal conditions. In analyzing the stability of the dam with respect to frictional resistance to the horizontal hydrostatic pressure of the lake it was deemed that the Dam would have a factor of safety of 9 (1). And the resistance to a large earthquake brought down the factor of safety to 4.5. It should be noted that the internal structure of the dam is unknown and it can only inferred by the surrounding geology, so in the case of analyzing the dam it was assumed to act as a CFRD (concrete-‐faced rock fill) dam where the water pressure would act on this imaginary impervious concrete face (1).
The conclusion of the report by the United Nations was that there is no danger of the dam failing from lateral stresses even in the worst-‐case scenario of a massive earthquake inducing a horizontal acceleration of .5g (1). The main concern for this dam is the possibility of another massive landslide that would cause a wave large enough to overtop the dam and potentially erode the dam in the process. This hazard will be discussed in more detail on the section regarding landslides of this report.
Geological Records The geological records from the region indicate the presence of many landslides and many landslide dams like Usoi Dam. There is also extensive evidence that many of these dams survived for many hundreds of years until finally the lake that was contained would eventually completely fill up with sediment. This adds support to the theory that the Usoi dam is stable from a structural perspective.
A diagram representing the massive size of the Usoi Dam in Comparison to the Oroville Dam (1).
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Currently the main body of the Usoi Dam is composed primarily of quartzite, sandstone, and schist from the Sarez Formation and the northern portion close to the original failure surface is composed primarily of relatively weaker marble, shale, anhydrite, and dolomite (3). This interface of geologic units suggests that it is likely the slip surface of the original failure.
Permeability & Water Filtration of Dam To get an idea about the immense quantity of water moving through the Usoi Dam system one must know that during the first few years after the Usoi Dam was formed the level of Lake Sarez (currently 37 miles long) increased by roughly 250 ft. annually (7). Measurements of the seepage output of the Usoi Dam began in earnest in 1943 and have been monitored on a regular basis since. The seepage has averaged a constant 45 m3/s (1). The annual variation varies between 35-‐80 m3/s (1). This suggests that as the lake level increases more water is exiting and therefore there is a relative equilibrium in water inflow and outflow. Despite this an annual rise of lake level has been observed to be around 7.3 in. (7). The culprit for this rise in lake level can be due to the large quantities of sediment that flow into the lake on a regular basis. No concrete analysis has been performed on the possibility of piping through the dam however it is assumed to be minimal because of the relatively constant rate of seepage annually (1). All the springs occur at roughly the same elevation, roughly 450 feet below lake level. This indicates that the upper part of the dam is very permeable whereas the lower part has a very low permeability (1).
Modes of Dam Failure Usoi Dam is unlikely to fail catastrophically, however on the unlikely chance that it would occur it would have a very large impact on several countries in the region (7). The only real mode of failure that could cause water to flood through the dam is from overtopping as results of a landslide on Lake Serez. And within this one failure
A diagram representing the water pressure on the Usoi Dam and flow of permeating water (1).
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scenario there are two possibilities. The first is another earthquake triggered landslide would cause a wall of water to be pushed over Usoi Dam and thus overtopping it but not eroding the dam to the point of being lower than the level of Lake Sarez. The second and worst case scenario is that which a large earthquake would perhaps trigger one giant landslide or multiple landslides that would create a wall of water high enough to not only overtop Usoi Dam but to erode it to the point that Lake Sarez would begin to drain downstream. An analysis of the worst-‐case scenario with an erosion rate of 100 m/h indicates that the lake would drain in roughly 4 hours time (7).
Landslide Concerns The biggest concern for the structural integrity of the Usoi Dam is the possibility of an overtopping wave, which could only be caused by a large landslide on lake Serez. Virtually every party that has set out to explore the Usoi Dam has noticed with concern an area on the northern slopes of Lake Serez that looks likely to slide into the lake in the future. This area has been name the Right Bank Slope or Right Bank Landslide. A 2006 USGS study analyzed the volume of water that would overtop the Usoi Dam based on different volumes for the Right Bank Slope. The results of this analysis are presented in table 1 (7). It should also be noted that the natural setting of the slopes around Lake Serez is that of a very dry climate. The introduction of Lake Serez has added moisture to the already slide prone slopes. The water from the lake has the combined effect of lower the friction angle and adding buoyance to the surrounding slopes, both of which would reduce slope stability and a cause for concern (1).
Top: a diagram showing the overtopping of the Usoi Dam from a landslide. Bottom: a diagram showing a large overtopping event with significant surface erosion leading to a complete dam breach.
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Kinematics of a potential slide failure of the Right Bank Slope using the “Method of Kinematic Elements” of Gussmann (1982, 1992). The angle of internal friction between the elements is assumed to be 40° and the cohesion to be zero. It is further assumed that the angle of internal friction between the base of the dam and the underlying geologic material is 25° and the cohesion along this contact is 10 kN/m2 (1).
An image of the Right Bank Landslide area of concern (Photo, State Secretariat for Economic Affairs, Switzerland) (4).
A diagram showing the approximate location of the Right Bank Landslide are or Right Bank Slope (7).
Different overtopping flood volumes for different landslides masses. USGS (7).
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Conclusions A much more comprehensive look at the problems posed by the Usoi Dam are available from the sources used in writing this report. The purpose of this report was to simplify the analysis of the Usoi Dam to an easily presentable fashion and to highlight the more important aspects of the geotechnical and geological analysis. In general the conclusion of these analysis on the Usoi Dam is that the structure is stable from a structural perspective but does have the potential for failure in the event of a large landslide induced wave on Lake Sarez. Many warning systems and prevention techniques have been presented but non have been implemented and it not clear if they ever will be.
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References
Alford, D. K., Schuster, R. L., & International Strategy for Disaster Reduction. (2000). Usoi landslide dam and Lake Sarez: An assessment of hazard and risk in the Pamir Mountains, Tajikistan. New York: United Nations.
Bindi, D., Abdrakhmatov, K., Parolai, S., Mucciarelli, M., Grünthal, G., Ischuk, A., ... & Zschau, J. (2012). Seismic hazard assessment in Central Asia: Outcomes from a site approach. Soil Dynamics and Earthquake Engineering.
Ischuk, A. R. (January 01, 2011). Usoi Rockslide Dam and Lake Sarez, Pamir Mountains, Tajikistan. Lecture Notes in Earth Sciences, 133, 423-440.
Lacasse, S., & Nadim, F. (January 01, 2011). Learning to Live with Geohazards: From Research to Practice. Geotechnical Special Publication, 1, 224, 64-116.
Lacasse, S., Nadim, F., & Høeg, K. (2012). Risk assessment and mitigation in geo-‐practice. In Geotechnical Engineering State of the Art and Practice@ sKeynote Lectures from GeoCongress 2012 (pp. 729-‐764). ASCE.
Lohr, T. (2001). A Short Story About The Geological History of the Pamir.
Risley, J. C., Walder, J. S., Denlinger, R. P., United States., & Geological Survey (U.S.). (2006). Usoi dam wave overtopping and flood routing in the Bartang and Panj Rivers, Tajikistan. Reston, Va: U.S. Dept. of the Interior, U.S. Geological Survey.
Strom, A. (2013). Geological Prerequisites for Landslide Dams’ Disaster Assessment and Mitigation in Central Asia. Progress of Geo-‐Disaster Mitigation Technology in Asia, 17-‐53.
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