Geotechnical News • March 2018
43
THE GROUT LINE
Geology
The foundation of the dam consists
of Pleistocene to recent age alluvial
deposits overlying rocks of the Lower
to Middle Miocene age Lower Fars
Group (the principal foundation rocks
below the dam causing the major
concerns) and the Oligocene to Lower
Miocene age Jeribe Formation. The
lower terraces along the river have
sandy, clayey silt up to 20 m thick that
was used for constructing the core of
the dam.
The dam is founded on a layered
sequence of rocks including marls,
chalky limestone, gypsum, anhydrite,
and limestone. A feature of the geol-
ogy is the occurrence of karstic lime-
stone and the development of solution
cavities within the gypsum and anhy-
drite layers. Four significant gypsum
units were identified during design
and construction varying in thickness
from 8 to 16 m and identified as GB0
(Gypsum Breccia 0), GB1, GB2, and
GB3 in ascending order. The dissolu-
tion and erosion of gypsum by water
seeping under the dam is believed to
be the main cause of the development
of voids requiring continuous grout-
ing. The erosion and dissolution rates
in gypsum are related to the seepage
velocities and hydraulic gradient.
Dam design
Concern for underseepage and the
associated risk of piping and dissolu-
tion of gypsum in the foundation was
identified during the original design of
the dam. To address the issue a wide,
relatively shallow, grout blanket was
installed beneath the dam core and a
deep grout curtain was installed from a
dedicated grouting gallery constructed
at the base of the dam along the dam
centerline. This 3.7 m tall and 3.0 m
wide grouting gallery also provides
access for continuous maintenance
grouting of the deep grout curtain.
The presence of karstic terrain was
confirmed during the site excavation
for the core. Voids and large caverns
were found under the river bed and
in the surrounding area. There is
evidence that the area may have high
permeability conduits that carry river
and ground water at varying velocities
through the bedrock. The 113 m tall
embankment dam core was con-
structed with locally borrowed, sandy,
clayey silt material. The embankment
design includes multiple filters, an
internal chimney drain and transition
zones. The embankment shoulders
consist of rock fill. A typical cross
section of the embankment is shown
in Figure 2.
Dam safety risk
Mosul Dam serves as a flood control
structure protecting more than 1 mil-
lion people residing in the flood plain
from imminent threat. A risk-based
study performed by the U.S. Army
Corps of Engineers (USACE) in 2016
considered the likelihood of dam
failure due to various Probable Failure
Modes (PFMs) related to the founda-
tion conditions combined with the
predicted loss of life to determine a
severe risk profile. If the failure modes
(Figure 3) are not addressed, failure
could result in catastrophic loss of life,
economic damage, and geopolitical
instability.
Emergency grouting
The Government of Iraq’s (GoI)
Ministry of Water Resources (MoWR)
has been performing maintenance
grouting since original dam construc-
tion. The geo-political instability in
northern Iraq interrupted maintenance
grouting between 2014 and 2015 and
it was believed to have impacted the
foundation conditions of the dam. This
led to the emergency grouting initia-
tive to stabilize the dam foundation
beginning in 2016. The main objective
of the emergency grouting program is
to install a double grout line curtain
along the full length of the grouting
gallery and connecting curtains from
the crest of the dam east of the spill-
way and west abutment.
The conditions of the project present
multiple challenges, the primary ones
are as follows:
• Need to replace entire grouting
infrastructure (mixing, batching,
Figure 2. Typical cross section.
Figure 3.
