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Video: Grouting of outlet tunnels on Mosul Dam Project

Mosul Dam

A video has been released showing the repairs to the Mosul Dam in Iraq being undertaken by Italy-based ground engineering specialist Trevi.

For the first time, camera crews have filmed the safety operations being carried out at the Mosul Dam.

Under the Works Management of USACE (the US Army Corps of Engineers), commissioned by the Iraqi Water Resources Ministry, Trevi is carrying out the maintenance and training Iraqi workers on how to use them, through a training school created on site.

In 2016 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 dam which impound 11.1bn.m3  of water 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 in 2016.

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.

In 2017 Trevi reported that the Dam had three external mixing plants, 20 Batching Grouting Units (BGU) and 13 drilling machines that operated 24 hours per day.

Trevi has had a bespoke automated injecting management system designed and in 2017 the spillway reopened.

Background to 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.

In 2016 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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