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Trevi remediation enables Mosul Dam spillway to reopen



The main spillway on the Mosul Dam in Iraq has reopened a year after Italian engineering giant Trevi started repairs on the structure.

Last year the US Embassy in Iraq raised concerns that the dam faces a “serious and unprecedented risk of catastrophic failure” with little warning. It said that if the 11.1bn.m3 dam collapses, some areas could be inundated by up to 21m of water within hours. The risk came after a so-called Islamic State attack on the facility in August 2014 and the subsequent disruption of maintenance operations.

Trevi won the £215M contract for the dam repairs and work started around a year ago.

Work started with the modernisation of many of the existing plants. In the grouting gallery 250km of electric cable was installed along with 20km of tubes for grout mix, bentonite, water, concrete and dewatering system, and 2.5km of fibre optic cable.

According to Trevi, the Dam now has three external mixing plants, 20 Batching Grouting Units (BGU) and 13 drilling machines that operate 24 hours per day.

Some 6,000 tons of solid injections have been carried out. Trevi has had a bespoke automated injecting management system designed.

Now water levels at the dam are at the same level of 2005 and the spillway has reopened.

The residential area includes more than one thousand beds, 17 office buildings and even a pizzeria.

Background: an operationally and technically complex job

The Mosul Dam’s problems are thought to stem both from the original design of the dam and disruption to maintenance. When it was constructed, on ground consisting of gypsum, anhydrite, marl, dolomite and limestone, the hydraulic seal foundation included installation of a grout curtain, with holes up to 200m deep, as well as blanket grouting to provide a shield at the base of the dam. The grout curtain injections were carried out from a specially built 2,250m-long gallery. The grouting needed to be constantly maintained, which hasn’t happened.

The issue of water in the gypsum and anhydrite ground layers is behind the structural problems of the 1980s built dam. This has been compounded by the development of sinkholes in surrounding areas.

So the task for Trevi was to repair both the grout curtain and the dam’s bottom outlet, the latter of which will require specialised divers. The final aim was to achieve a grout curtain with a permeability lower than 10 Lugeon units.

Last year Trevi said the repairs would be carried out both from the grouting gallery and dam crest. In the small gallery, 3.7m-high and 3m-wide Soilmec SM-5 rigs were used. The work covered three areas, totalling 500m. On the dam crest, surface interventions were carried out using Soilmec SM-16 rigs with long stroke, for 420m towards the west of the spillway, and from the crest of the dam along the eastern wall of the spillway for 700m.




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