An unprecedented task3 November 2011
Seepage has proved to be problematic at Iraq’s Mosul Dam since day one. In order to help them rehabilitate what has been described as the most dangerous dam in the world, an Iraqi delegation recently visited several US dams to share engineering solutions. Amanda Ellison reports.
The people of Iraq face challenges similar to those of Americans, and the one that recently brought a group of Iraqi visitors to south Florida was how to protect the lives of those who live and work near ageing structures.
Mosul Dam (formerly Saddam Dam) is Iraq’s largest dam and the Middle East’s fourth largest reservoir with a maximum storage capacity of 11.1x109m3. It is located on the River Tigris 72km north of the city of Mosul, Iraq’s third largest city with a population of 1.7M.
At over 130m high and 3.2km long, the earthfill dam is a critical energy source but it is also ageing and in desperate need of repair.
A dangerous dam
Hochtief Aktiengesellschaft (a German and Italian consortium) began construction of the dam in 1980 which was the first year of the Iran-Iraq war. Construction was completed in the spring of 1985 and the structure, with its 4x200MW turbines generating 320MW of electricity a day, is a key component in Iraq’s national power grid. Other functions also include flood control and water for irrigation and municipal supplies.
Mosul Dam’s foundations consist of alternating and highly variable units of gypsum, anhydrite, marl and limestone. Each of these is soluble in water under the environmental and hydrogeologic conditions of the dam and seepage has proved to be a consistent problem. Maintenance grouting has occurred continuously since the end of construction and more than 50,000 tons of material has been pumped into the dam.
According to a US Army Engineer Research and Development Centre report published in 2007, the pattern of regrouting in and between recently grouted sections of the dam shows that grouting at one location causes the flow path (seepage) of subsurface water to move to another location. It does not stop the actual seepage. In terms of the internal erosion potential of the foundations, the US Army Corps of engineers (USACE) has called Mosul dam the most dangerous dam in the world.
Iraq is fully aware of the challenges facing the ageing structure and the severity of the situation. Failure could lead to as many of 500,000 civilian deaths by inundating the cities of Mosul and Baghdad under water. Furthermore the Iraq Ministry of Water Resources estimates that the problem is costing half a million dollars a year.
The country is hard at work trying to determine the best path forward for rehabilitation. In 2007 USACE developed and executed a US$27M plan to help continue maintenance and repairs on the dam in the short term. The effort to rehabilitate the dam will be the largest ever attempted in the world.
At the request of the Ministry of Iraq, a five-member delegation from the Iraq Ministry of Water Resources recently completed a brief tour of the east coast of the US to learn about dam structures and cutoff walls. For most members in the delegation it was their first visit to the states. The tour included a visit to the US Army Corps of Engineers, Jacksonville District’s Herbert Hoover Dike at Lake Okeechobee as well as to Nashville District’s Wolf Creek and Center Hill Dams, plus Little Rock District’s Clearwater Dam.
The Wolf Creek Dam project is the most similar to Mosul Dam, and was an important stop on the itinerary. However, Wolf Creek doesn’t even come close to the size of the effort that will be required at Mosul. The deepest cutoff wall constructed to date at Wolf Creek was 122m. The cutoff wall to be attempted at Mosul Dam will be twice as deep at 244m. The Mosul Dam is ten times deeper than the Herbert Hoover Dike. In fact, equipment for the mammoth cutoff wall for Mosul is still under development.
“There is no precedence to what they are trying to achieve,” said David Paul, lead civil engineer for the US Army Corps of Engineers’ Risk Management Centre.
The goal of the tour for the Iraqi engineers was to learn how to secure contracts, from bid phase to completion; understand the critical need for construction management; and view different methods of technology being used during cutoff wall construction at various sites.
“The trip has created positive interaction,” said Paul. “They have a difficult project ahead of them.”
The engineers gained great benefits from understanding the history of the Corps’ problems with seepage, how the Corps evaluates those conditions, and the decision to use cutoff walls to mitigate seepage concerns.
“Our projects are very similar to what they have going at Mosul Dam,” said Paul. “They could directly relate their experiences to ours, and I think that was extremely valuable.”
The trip to Clearwater Dam provided an opportunity for the group to see actual cranes and rock mills in normal operation. They were also able to discuss crucial construction issues such as rock mill design and handling.
The success of the trip would not have been possible without the collaboration between several Corps districts. Members of the delegation expressed their sincere appreciation to the Corps for the time and effort that was put into ensuring their trip was informative and beneficial.
“All in all, I think it was a great moment spent collaborating and learning from fellow engineers from around the world,” said Bobby Van Cleave, geotechnical engineer at Clearwater Dam. “They have an enormous task at Mosul Dam, and maybe some information they gained from their visit will help them successfully complete that mission.”
Amanda Ellison can be contacted at the Jacksonville District of the US Army Corps of Engineers in Florida, US. Email: [email protected]
|Herbert Hoover Dike|
The Herbert Hoover Dike is listed as one of the US Army Corps of Engineers"™ top priority and is one of the US"™ dams in most need of repair. Since 2007, USACE has funded more work on the Herbert Hoover Dike (HHD) than for any other dam construction project in the nation. The HHD Rehabilitation Project received funding of: US$56M in 2008; US$74M in 2009; US$124M in 2010; US$107.8M in 2011.