Iraq
Mosul Dam Rehabilitation Project
Mosul Dam is an earth fill dam, located in Northern Iraq, on the Tigris River, approximately 40 km northwest of Mosul. The construction of the dam was completed in 1984. It is one of the largest multi-purpose dams in the Middle-East, and the largest Dam in Iraq. The dam is a multi-purpose dam providing flood control, irrigation, power generation, and water supply. Through its 113 m height and 3.65 km length, the reservoir has a storage capacity of 11.1 billion m³ of water. The main hydroelectric power station has 4 turbines for an installed capacity of 750MW. The discharge system foresees two bottom-outlet tunnels (10 m in diameter), a five-gate service spillway, and an emergency fuse-plug spillway.
A problematic geological setting
The dam is founded on a layered sequence of rocks including marls, chalky limestone, gypsum, anhydrite, and limestone. A feature of the geology is the occurrence of karstic limestone and the development of solution cavities within the gypsum and anhydrite layers. Dissolution phenomena result in the formation of fissures and voids that allow considerable water percolation, due to the strong hydraulic gradient imposed by the Dam. The karst development extends to a depth of 100 m below the dam’s foundation. The under-seepage phenomena and the associated risks were identified during the design for the dam construction, and actions were taken in an attempt to mitigate the possible outcomes. To this aim, a deep grout curtain was designed to be installed all along the dam centerline, from a grouting gallery. The grouting gallery was constructed on top of before placing the embankment; the gallery was specifically designed also to allow repeated grouting efforts to continue over the lifetime of the dam. The original specialized subcontractor initiated an aggressive grouting program in 1984, but the impoundment of the reservoir began whilst the curtain grouting was in progress. Afterward, the Iraqi Ministry of Water Resources (MoWR) has continued grouting ever since 1989, using the equipment and technique left at site by the subcontractor. In spite of the actions taken, over the years the structural integrity of the dam and its operational capacity were cause for concerns, mainly because of the evident phenomena of dissolution of the gypsum and anhydrite layers. The rate of subsurface dissolution was increased by the presence of the reservoir; above a pool elevation of 318 m, the rate of subsurface dissolution increased markedly, leading to the recommendation that the pool not be raised above this level. The low reservoir level impacted the power generation and the agricultural irrigation. In addition, the geopolitical instability in Northern Iraq almost interrupted the ordinary operations of maintenance grouting (managed by MOWR) between 2014 and 2015; and it is believed to have further impacted the condition of the Dam foundation.
The tender for the remedial works
In 2016, the U.S. Army Corps of Engineers (USACE) performed a risk-assessment analysis and identified various Potential Failure Modes at the highest risk level, all related to the risk of internal erosion through the foundation bedrock. The risks for Mosul Dam were classified as very high, mainly due to the dam location and the number people living in along the Tigris Valley: if the risks were not addressed, failure can result in catastrophic loss of life, economic damage, and geopolitical instability. Owing to the critical situation, the Government of Iraq awarded the Mosul Dam Rehabilitation Project to the Italian firm Trevi S.p.A. (Trevi Group), through an international bid launched in October 2015. The Contract between the Iraqi Ministry of Water Resource (MoWR) and TREVI was signed in early March 2016. The Government assigned the roles of Engineer and Contract Administrator to the USACE, monitoring Mosul Dam since May 2015. The contract included all the activities aimed at repairing the grout curtain, with the supply of all the equipment and materials necessary to execute the drilling and grouting intervention. The contract included as well the works for the repair and maintenance of structures associated with the bottom outlet tunnels.
| Owner | MINISTRY OF WATER RESOURCES OF IRAQ |
| Engineer | U.S. Army Corps of Engineers |
| Main Contractor | Trevi S.p.A. |
| Duration of Works | 02/03/2016 - 12/08/2019 |
Camp and site preparation, and infrastructure upgrading
Mobilization and site preparation faced logistic problems related to the site location, with the war conflict against ISIS at 13 km from the Dam. The setting up of the site began in July 2016, with the construction of a secure base camp facility to provide living and working accommodations for the approximately 1400 people on site. Also, it was started the installation of six new office buildings, of a new repair-maintenance shop, of the grout and mortar mixing plants. Concurrently, activities were started for the replacement of the entire grouting infrastructure and the construction of new networks for electric-power, ventilation, communication and internet, water and wastewater systems. Drilling and grouting works The drilling and grouting activity for the Dam refurbishment started at the end of October (2016). The drilling and grouting works were carried out with the goal of injecting mixes able to intercept and plug the fissures and voids (mainly derived from dissolution and karst phenomena). The end purpose of the emergency grouting program was to install a grout curtain along the full length of the grouting gallery (2200 m) and at the East and West Abutments (490 and 220 m). A further piece of curtain was installed from the crest of the Main Dam in the 280 m immediately right of the spillway, to grout the rock between the base of the embankment and the top of the grouting tunnel extrados (where the grouting gallery enters the foundation rock). Owing to the stratigraphy and design considerations, it was necessary to drill and inject holes to a depth of 150 m and over. As a result of the water level in the reservoir, the gallery activities were further complicated by the need to operate in the presence of a significant water-head (up to 100 m).
Grouted quantities and results
The drilling and grouting works were performed 24 hours/day, six days/week. From November, 2016 through July, 2019, Trevi completed 403,100 m of grouted holes injecting over 40,800 m³ of grout mixes, with an average grout take of 102 l/m. The activity involved 667,800 m of rotary drilling/redrilling, 85,000 m of coring, 35,450 m predrilling in the embankment. The boreholes in the lower part of the gallery were drilled and grouted in challenging conditions, due to very high reservoir level. Here, the artesian pressure and water flow required significant efforts to complete the grout curtain and to install the upstream piezometers. As expected, the largest grout takes were recorded in the vuggy limestone and GB layers of the Lower Gallery. In general, the downstream line (the first injected) recorded the highest grout takes. Within each line, the grout takes significantly decreased from the primary holes towards the secondary, tertiary and quaternary holes. The decreasing trend in terms of grout take is a clear signal of the void filling and gradual reducing of the residual permeability. In addition to the holes inspected by OPTV, check holes were cored-drilled and investigated by water pressure tests to evaluate the grouting efficacy. The 81% of the tested stages measured values LU ≤ 1 and the 98% LU ≤ 3. In general, the result of the Lugeon tests was evaluated as a great improvement. Besides the drilling and grouting works, activities were performed to install and renew the inclinometer and piezometer array, both downstream of the dam and into the gallery. These instruments allowed the monitoring of the hydraulic pressures in the dam foundation bedrock, and their behavior as the grouting activity progressed. The readings showed an evident amelioration of the hydraulic conditions, with noticeably increasing of the hydraulic gradient between upstream and downstream piezometers. A new risk analysis downgraded the risk to a level significantly lower than the one evaluated in 2016. The general safe conditions of the Dam allowed for the water to rise again at levels of 2005 and for the reopening of the spillway.