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Mosul Dam safety milestone achieved by Trevi

Trevi mosul

The Italy-based ground engineering specialist Trevi has met another milestone at the Mosul Dam in Iraq.

Trevi Group has announced that 5M man hours have been worked on the dam with no time lost accidents.

The stabilisation of Mosul Dam through drilling and grouting works started in 2016 after the US Embassy in Iraq raised concerns that the dam faces a “serious and unprecedented risk of catastrophic failure” with little warning.

It said at the time 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, who won the £215M contract in 2016, is currently operating with 14 drilling rigs and 19 grouting pump units, with a workforce of up to 800 people. So far Trevi has completed 3,700 boreholes, equivalent to about 300,000 lm of grouted holes and injected 37,000m³ of grout mixtures in the ground.

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

Trevi said: “Despite all those hindrances and the stringent technical specifications of the project, the 700 workers deployed by Trevi from a dozen different countries, operated professionally and safely through 24/7 operations, allowing the achievement of 5M man-hours worked with no lost time accidents.

“This great achievement is the result of the commitment and efforts made by Trevi since the beginning of the project, guiding all workers to improve their responsiveness towards the culture of safety at the construction site.”

The firm has implemented a pathway of training which includes personal protective equipment, risk management and accident prevention. According to Trevi this has allowed staff to be more knowledgeable and understand the value of working safely.

Trevi extended its Contract with the Ministry of Water resources of Iraq on 5 July 2018. The maintenance grouting works are now planned to end in August 2019.

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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