Seismic Vulnerability Analysis of Major Ethiopian Dams with Emphasis on Dam Safety Evaluation
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Date
2020-02-01
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Addis Ababa University
Abstract
Dams are critical facilities, which require special consideration to ensure their longterm
safety. Among the safety concerns for large storage dams, seismic safety plays
an important role, particularly for dams located close to the seismically active regions
like the East African Rift System. Large dams in such seismically active regions must
be capable of resisting severe earthquake ground motion expected at the dam site
without uncontrolled release of water impounded in the reservoir. This can be
achieved by conducting a comprehensive site-specific seismic hazard analysis, proper
seismic design, adequate construction quality control, and appropriate operation and
timely maintenance for upgrading any seismic deficiency, particularly for older dams.
The main factor contributing to the risk of large storage dams is the water stored in
the reservoir. Some of the reservoirs in Ethiopia are very large. In this dissertation, the
seismic safety evaluation of large Ethiopian dams is analysed, which includes the
review of previous works, site-specific seismic hazard evaluations, seismic risk
analysis and detailed seismic safety analysis.
A review of the seismic design criteria used for large dams in Ethiopia shows that
different criteria were considered and, in some cases, the poorly known seismic
activity in the project region was ignored. In some dams, modern design and safety
criteria were used whereas in other projects out-dated seismic design guidelines and
codes were employed. As a result, the existing, under construction and planned dams
require detailed seismic safety reviews to comply with modern seismic safety criteria.
The dam sites are located in variable geological and tectonic settings, which are
responsible for the spatial variability of the seismicity at dam sites. The main tectonic
structure in Ethiopia is the Main Ethiopian Rift (MER), which is characterized by its
extensional tectonic nature, seismically active faults and major fractures that affect the
safety of dams and may cause water losses from the reservoirs. This requires an
extensive geological investigation beyond the footprint of the dam.
For the seismic hazard study, the seismogenic source zones were modelled by
integrating the information developed from the regional geology, tectonics, seismic
energy release map, and observed seismicity. The seismic hazard analyses were
conducted based on the probabilistic approach and a seismic hazard map is developed
for the horizontal component of the peak ground acceleration (PGA) for a return
period of 10,000 years. Six seismic zones are delineated in this map. In addition, for
the different seismic zones, an estimation of the future power and irrigation potential
of Ethiopia is made. Moreover, the dam sites are ranked according to the PGA-values
for return periods of 10,000 years. These results are used as input for the seismic risk
analysis.
The seismic risk of 30 large Ethiopian dams was evaluated. In the risk analyses, the
levels of seismic hazard for which the dam is exposed, the vulnerability of the dam,
and the consequences in the case of uncontrolled release of water from the reservoir
were considered. Based on the risk analysis results, the following five dams Gibe
GERD Saddle dam, Gidabo, Tendaho, and Tekeze dams were selected for detailed
site-specific hazard evaluation and seismic safety analysis. In the site-specific seismic
hazard analyses, multiple earthquake effects were taken into account. For the
nonlinear stress, deformation and stability analyses acceleration time histories were
used, which match the acceleration response spectra obtained from the seismic hazard
analysis.
Gibe III dam is an RCC gravity dam with a height of 243 m. It is the world's highest
RCC dam. The dam site is located at the border of the seismically active MER.
Seismic stability analyses are carried out to check the response of the dam for the
updated ground motion parameters of the safety evaluation earthquake (SEE). The
static and dynamic analyses are performed using a two-dimensional (2D) plane stress
finite element model of the highest cross-section of the dam. First, a linear-elastic
dynamic analysis is carried out followed by the dynamic sliding stability analysis of
different detached concrete blocks. The foundation rock is assumed massless, which
implies that only the kinematic interaction effects are considered. The hydrodynamic
pressure acting on the upstream face of the dam was represented by an added mass
according to Westergard, assuming incompressible water in the infinite reservoir. The
spectrum-matched acceleration time histories are used as input in the dynamic
analysis. All dynamic analyses are done by direct time integration of the equations of
motion.
Grand Ethiopian Renaissance Dam (GERD) is the largest hydropower station
currently under construction in Africa. The main dam is a 145 m high roller
compacted concrete (RCC) gravity dam. It will create a reservoir with a volume of
about 74 km3. Besides the RCC dam, there is a 5.2 km long saddle dam with a
maximum height of 65 m and a volume of 17 Mm3. It is one of the longest concretefaced
rockfill dams (CFRD) in Africa. The seismicity in the project area is assumed to
be low. However, the information on historical seismicity is scarce in the region.
Because of the size and importance of the GERD project, the seismic stability of the
saddle dam is checked for the ground motion with a return period of 30,000 years.
The dynamic analysis of the saddle dam is carried out by the equivalent linear method
using a 2D dam model of the highest cross-section. The results show that the dam is
safe under the worst-case earthquake loading and the crest settlement is insignificant
compared to the available freeboard.
Gidabo dam is a central core earth-fill dam with a height of 27 m. The project
includes an intake tower supported by a pile foundation in the upstream part of the
dam, which is connected with a diversion conduit laid on a weak compressible
foundation passing through the dam body. During dam construction (2016), the
conduit that was laid on the soil foundation settled, creating a vertical offset of more
than 50 cm at the joint between the pile-supported intake tower and the conduit due to
the static loads from dam construction. Moreover, the dam site is located in the
seismically active MER with several destructive earthquakes recorded in the past. The
seismic stability analysis was conducted to determine the maximum deformation of
the dam and settlement of the conduit when it is subjected to the SEE ground motion
with 10,000 years return period. The total settlement estimated during SEE is
tolerable. The dam is safe against overtopping, as sufficient freeboard is provided.
However, cracking of the clay core along the conduit due to differential settlement
may lead to internal erosion. Moreover, the offset of the conduit will increase and the
conduit may not be adequate for lowering of the reservoir after the SEE.
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Tendaho dam is one of the largest irrigation dams located in a region of high
seismicity in Ethiopia. Movements along tectonic faults and other discontinuities in
the footprint of the dam that can be activated by strong earthquakes close to the dam
are expected to be the worst-case seismic effects for the dam. The present study aimed
to check the seismic safety of the dam when it is subjected to both ground shaking and
tectonic fault movements. The dynamic analyses were carried out by a 2D model of
the highest cross-section using the equivalent linear analysis method. The results of
the dynamic analyses show that the maximum loss of freeboard is 1.89 m due to slope
movement, seismic densification of the embankment and fault displacement in the
footprint of the dam that can be accommodated by the available freeboard of 3 m.
Seepage along the fault in the dam foundation due to damage of the grout curtain and
erosion along the dam-abutment contact due to seepage are possible.
The 188 m high Tekeze dam is the highest arch dam in Africa. The project area is
characterized by undulating topography with steep slopes and deep valley, which is
different from the other dam sites. Therefore, mass movements into the reservoir that
generate impulse waves are possible during strong earthquakes. The seismic stability
of the critical slopes is checked for the horizontal component SEE ground motion
with a conventional pseudo-static procedure. The landslide is modelled as a solid
mass and three-dimensional (3D) free radial propagation of the impulse wave was
considered for estimating the wave generation and wave propagation parameters. The
parameters controlling the impulse waves on dams were computed and the size of
resulting impulse waves in the reservoir was determined. The maximum wave run-up
and the possibility of dam overtopping were estimated. The results show that the
maximum wave run-up under worst earthquake action can be accommodated by the
freeboard allowance of 5 m adopted in the design. As a result, there is no overtopping
risk expected from landslides generated impulse waves. However, the overall result of
the present study highlights the importance of reviewing the seismic safety of the dam
for the increased level of earthquake ground motion.
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Keywords
Seismic Vulnerability Analysis, Major Ethiopian Dams, Emphasis on Dam, Safety Evaluation