ChapterPDF Available

Abstract

The Saddle Dam D at the Hydroelectric Power project Xe-Pian Xe-Namnoy in the Lao People’s Democratic Republic failed on July 23, 2018. An Independent Expert Panel (IEP) was established to investigate into and report on the failure of the embankment dam. The Executive Summary of the IEP final report (Anton J. Schleiss, Jean-Pierre Tournier, and Ahmed F. Chraibi, Report of Independent Expert Panel (IEP), Xe-Pian Xe-Namnoy Project - Failure of Saddle Dam D, Final Report. 20 March 2019) is presented here.
Failure of Saddle Dam, Xe-Pian Xe-Namnoy
Project: Executive Summary
Ahmed F. Chraibi1(B), Anton J. Schleiss2, and Jean-Pierre Tournier3
1Dam Consultant, PO Box 21514, Rabat Annakhil, Rabat, Morocco
ach@damtech.ma
2Ecole polytechnique fédérale de Lausanne (EPFL), Lausanne, Switzerland
anton.schleiss@epfl.ch
3Hydro-Québec Équipement & Services partagés, 855, rue Ste-Catherine Est - 10e étage,
Montreal, QC H2L 4P5, Canada
Tournier.Jean-Pierre@hydro.qc.ca
Abstract. The Saddle Dam D at the Hydroelectric Power project Xe-Pian Xe-
Namnoy in the Lao People’s Democratic Republic failed on July 23, 2018. An
Independent Expert Panel (IEP) was established to investigate into and report on
the failure of the embankment dam. The Executive Summary of the IEP final
report (Anton J. Schleiss, Jean-Pierre Tournier, and Ahmed F. Chraibi, Report of
Independent Expert Panel (IEP), Xe-Pian Xe-Namnoy Project - Failure of Saddle
Dam D, Final Report. 20 March 2019) is presented here.
Keywords: Dam failure ·Embankment dam ·Tropical residual soils
This final report on the failure of Saddle Dam D of Xe-Pian Xe-Namnoy hydropower
project summarizes the findings of the Independent Expert Panel (IEP) based on the
available supporting information and the observations made by the IEP during the site
visits carried out in the beginning of October 2018 and end of November 2018. Fur-
thermore, it considers the results of the recommended geotechnical investigations, made
available in January 2019, and the numerical sensitivity analysis of the dam stability.
According to the available sequence of photographs and the reporting of the event,
the observed movements of the sliding mass are of complex geometry. Nevertheless, the
IEP is convinced that the main evidence of the incident at its beginning is a rotational
sliding involving the lateritic foundation. The most important weakness in the foundation
triggering deep sliding has developed along the deepest area of the saddle, respectively
the highest section of the dam. Thus, the foundation of the Saddle Dam D was without
doubt involved in its failure.
According to the site visit observations, the monitoring data analysis and the review
of the available photographs, before, during and after the failure, as well as the results
of geotechnical investigations, the IEP considers that the root cause of the incident is
related to the high permeability of the foundation. The high permeability was above
all favored by the presence of canaliculus interconnected path having high sensitivity
to erosion. In fact, the geotechnical investigations revealed, that the foundation of the
Saddle Dam D is very heterogeneous with a predominance of clayey sandy-silty soils.
© Springer Nature Switzerland AG 2020
J.-M. Zhang et al. (Eds.): ICED 2020, SSGG, pp. 24–26, 2020.
https://doi.org/10.1007/978-3-030-46351-9_2
Failure of Saddle Dam, Xe-Pian Xe-Namnoy Project 25
Numerous passages rich in sand and even gravel leading to low core recoveries and
higher permeability values have been observed.
The mechanism of failure of the Saddle Dam D was most probably triggered by the
following successive sequences:
1. Due to the presence of high permeability horizons in the foundation, as confirmed
by the investigations, groundwater level at the downstream toe was close to the sur-
face generating resurgence in the vegetated area where topography declines rapidly.
This hypothesis is supported by the observation made downstream of the very sim-
ilar Saddle Dam E, where evidence of resurgence with some internal erosion was
observed.
2. With continuing resurgence in the vegetated area downstream of the dam toe, regres-
sive erosion has developed in the foundation resulting in the formation of ducts that
collapsed from time to time, especially in the deepest section of the saddle where
the highest seepage gradients occur. The resulting softening of the laterite triggered
the speeding up of the settlement and the appearance of the first cracks on the dam
crest.
3. When the erosion and softening in the foundation reached a certain extent, the static
dam stability was no longer ensured and a deep rotational sliding at the highest
section of the embankment developed. Simultaneously, converging embankment
movements occurred from the lateral border of the sliding mass towards the middle,
resulting in a bumping up of the downstream embankment face and the subsidence
of the track in front of the dam toe.
4. When the remaining thin upstream edge of the embankment crest breached, the
embankment was overtopped and the catastrophic uncontrolled release of water
from the reservoir washed away the central section of the Saddle Dam D and its
foundation.
Even if July 2018 was the wettest month over the record since 2008, with some
1350 mm falling up to the 29th of July, and the highest daily rainfall event occurring on
July 22 with 438 mm, the flood event at the spillway operating with some 680 m3/s just
before Saddle Dam D failure was only in the range of a 10 to 20 years flood. Yet, the
reservoir was still well below the maximum operation level at the failure incident and
the embankment has to withstand safely the probably maximum flood event. Thus, the
failure incident cannot be considered as “force majeure”.
The failure could have been prevented by an appropriate treatment of the foundation
aiming at providing the required water tightness, filtration and drainage. Furthermore,
an early and correct interpretation of the monitoring data and a reinforced detailed visual
inspection in the downstream toe region of the embankment, would have allowed to take
actions trying to save the Saddle Dam D and/or at least trigger the warning earlier.
Since the bottom outlet has only a small capacity mainly for the release of environ-
mental flow, there was no immediate possibility to control or to lower the level of the
reservoir when the first signs of failure were observed. There remained only the possi-
bility of removing by blasting and breaching the spillway labyrinth wall reaching almost
6 m height. Without having any control on the reservoir level during operation, at least
in the most upper part by spillway gates eventually together with the powerhouse, such
26 A.F.Chraibietal.
a concept is not acceptable according to the best international practice. In view of the
catastrophic consequences in case of failure, this is particularly important for reservoirs,
which volume is contained by several large embankment dams like for the Xe-Namnoy
reservoir.
Saddle Dams E and F as well as the part of Xe-Pian Dam founded on lateritic
soil, have similar foundation conditions compared to the failed Saddle Dam D. From
the monitoring assessment, they already exhibit a comparable sudden acceleration of
settlement and increase in the downstream hydrostatic pressures which are linked to
the foundation quality. These dams have to be reviewed and appropriate rehabilitation
measures have to be defined to ensure the safety requirements preventing any undesirable
behavior.
The evolution of the groundwater level in any topographical depression present in
the near downstream region of both Xe-Pian and Xe-Namnoy dam has to be monitored
by piezometers or at least included in the visual inspection program.
The timely (re-)construction of new Saddle Dam D and rehabilitation of the afore-
mentioned dams is of paramount importance in order to allow a safe reservoir filling
during the next rainy season. The concept and design of the new saddle dam has to be
robust in view of the very limited construction time, uncertainty of foundation and safety
requirements. Furthermore, the new Saddle Dam D should be equipped with two high-
capacity outlets which allow to control the reservoir level at least in its most upper 20 m
during wet season. The reinforcement of Saddle Dams E and F as well as Xe-Pian dam
requires relevant information on the depth and the quality of their lateritic foundation.
Thus, prior to the detailed definition of the reinforcement works to carry out, specific
and thorough investigations are necessary.
Lessons learnt from the incident comprise, among others:
The delicate and very heterogeneous nature of Lateritic soils: they may contain highly
permeable and erodible horizons, canaliculus conveying water on a long distance, they
soften when saturated and may be sensitive to significant settlement. Since Laterite
formations are residual soils, which may even have a potential of collapse when they
are not permanently saturated. Investigations in lateritic soil should include large and
deep open trenches. Positive cutoff is the most adapted seepage control arrangement
in this type of foundation;
During the first reservoir filling, highly experienced dam engineers should be mobi-
lized on the site (or in permanent contact with) to carry out immediate interpretation
of monitoring data and to inspect the dam and its surroundings. Experienced eyes are
very important in early detection of undesirable behaviors;
Easy access paths and vegetation-free space has to be ensured downstream of the dam
in order to allow a comfortable visual inspection and early intervention in case of
danger.
The IEP recommends that all large hydropower and dam projects are reviewed during
the design and construction phases by an independent international panel of experts.
Furthermore, a dam safety concept should be put into operation in Laos PDR by creating
a dam safety supervisory authority based on a legal framework in the country.
... Figure 2 shows the statistics of dam failure studied by the Chinese National Committee on Large Dams (CHINCOLD). Among the 5109 dam failure cases globally, earth-rock dam failure accounts for 85.94% of the cases (or 4390 cases), and concrete dam failure accounts for only 2.31% of the cases [4]. ...
... Many famous dams, such as the Hoover arch dam and the Three Gorges gravity dam, were built with concrete materials. Concrete dams are not prone to overtopping failure, but they account for less than 5.00% of dams in China due to their strict foundation requirements and high cost [4]. The development of roller compacted concrete (RCC) dam construction has promoted more projects to adopt gravity dams or arch dams because of their economic and high efficiency. ...
Article
Full-text available
Keywords cemented material dam stress-strain model intelligent quality control cemented low-strength rock cemented weathered materials model test against flood overtopping Cemented material dam (CMD) can fully utilize local sand, gravel, and rock materials for dam construction. It has many advantages such as flood overtopping without failure, environmental friendliness, economic savings, rapid construction , and so on. It has been rapidly popularised and applied to domestic and foreign projects since it was put forward by the first author. This paper illustrates the reasons for the proposal of CMDs, puts forward the stress-strain model of the cemented mixture and the intelligent system of production quality control, presents the model test results for CMD against flood overtopping, and introduces the practice of dam construction with low-strength soft rock and weathered materials.
... Figure 2 shows the statistics of dam failure studied by the Chinese National Committee on Large Dams (CHINCOLD). Among the 5109 dam failure cases globally, earth-rock dam failure accounts for 85.94% of the cases (or 4390 cases), and concrete dam failure accounts for only 2.31% of the cases [4]. ...
... Many famous dams, such as the Hoover arch dam and the Three Gorges gravity dam, were built with concrete materials. Concrete dams are not prone to overtopping failure, but they account for less than 5.00% of dams in China due to their strict foundation requirements and high cost [4]. The development of roller compacted concrete (RCC) dam construction has promoted more projects to adopt gravity dams or arch dams because of their economic and high efficiency. ...
Article
Full-text available
Cemented material dam (CMD) can fully utilize local sand, gravel, and rock materials for dam construction. It has many advantages such as flood overtopping without failure, environmental friendliness, economic savings, rapid construction, and so on. It has been rapidly popularised and applied to domestic and foreign projects since it was put forward by the first author. This paper illustrates the reasons for the proposal of CMDs, puts forward the stress–strain model of the cemented mixture and the intelligent system of production quality control, presents the model test results for CMD against flood overtopping, and introduces the practice of dam construction with low-strength soft rock and weathered materials.
ResearchGate has not been able to resolve any references for this publication.