Dismantling and remediating a quarry

D&Ri speaks with DST (Domingos da Silva Teixeira) and Tagregados, part of DST Group, about its recent demolition and remediation of the Gouvães Quarry in Portugal.

The quarry plant and equipment filled 150, 40-ft shipping containers.

ALL PHOTOS: DST GROUP

Nestled away in Northern Portugal, there is a beautiful, green ‘lakeland’ habitat within the protected, 600sq km Alvão/Marão Natura 2000 site, that looks as if it has always been there.

But in fact, it is a reservoir – the Gouvães Dam Reservoir, and it has only been there for a year, having been created by the construction of the Gouvães Dam and the demolition and remediation of the Gouvães Quarry.

ABOUT GOUVÃES QUARRY

The quarry was built to enable energy giant Iberdrola to construct three hydroelectric dams on the Tâmega and Torno rivers, within the Douro river basin, which would collectively generate 6% of the country’s energy needs.

As such, its construction, operation and decommissioning were all subject to strict regulation and oversight from Portugal’s Directorate General of Energy and Geology, the Portuguese Environmental Agency, the Institute for the Conservation of Nature and Forests, and the Commission for the Coordination and Development of the Northern Region, were central to the project from the very beginning.

At the end of the quarry’s eight-year working life, a complex deconstruction procedure was undertaken to apply the approved environmental restoration plan.

SCOPE OF WORKS

The 27-ha quarry site comprised an 8-ha crushing plant incorporating ten screens, one jaw crusher, two cone crushers, vertical shaft crusher, 1.5 km of conveyor belts, and six storage silos.

Two sides of the plant site were surrounded by a giant cofferdam made of HDPE (high-density polyethylene) film, two layers of geotextile, soil and a rockfill retaining wall to protect the quarry from the reservoir’s waters.

While DST’s main scope of works had been constructing the quarry facilities and producing the aggregates for the construction of the hydroelectric dams, they were also responsible for closing the quarry at the end of the infrastructure project.

The clearing and restoring the site presented a unique set of challenges for main contractor DST.

The concrete slab was built to hold six silos, collectively containing 12,000 t of aggregates.

The upper part of slab that supported the silos contained over 350 t of steel and around 1,200 cu m of concrete.

Chief among these challenges was completing the works within just a threemonth timeframe. To achieve this, DST determined to carry out the demolition and remediation in two distinct phases.

The first phase would see the complete removal of all facilities, equipment, plant and structures on the site, with land restoration also carried out in the period. Then, following reservoir testing by Iberdrola, the second phase would comprise the removal of the cofferdam.

CLEARING THE SITE

DST began the first phase of the project by dividing the site into four distinct zones, so that it could remove the silos and clear the plant, equipment, infrastructure and support facilities, concurrently.

Nuno Faria, Project Manager at DST Group, says: “All the steel from the plant had to be carefully dismantled, as it was to be transported to Angola for use on another quarrying site.

“In total, the plant and equipment filled 150, 40-ft shipping containers. So, it was quite a logistical challenge.”

CONCRETE STRUCTURES

But an even bigger challenge was the demolition of the concrete slab, on which the quarry's six silos had been sat.

Once DST had removed the silos, DST Group’s specialist demolition division, Tagregados, had just two weeks to complete the task.

Diogo Fonseca, General Manager of Tagregados, says: “When you look at the photographs, it looks small. But it was not small.”

The slab measured 60 m long by 12 metres wide – with over 350 t of steel and around 1,200 cu m of concrete contained in the upper section alone.

“We did study different solutions before starting the demolition, including diamond wire cutting. But we didn’t know how the diamond wire would cope with that much steel, and we determined that it would also take too long to do,” explains Diogo.

“We also explored the use of explosives to create key cuts to lower the structure. But again, with the amount of steel inside the slab, this didn’t give us a good level of comfort to be certain of the result.

“To get the slab demolished in the two weeks we had, the only real option was to mechanically bring it down.”

Tagregados deployed six 36-t excavators, two 24-t excavators, a mobile jaw crusher and numerous hydraulic hammers, multiprocessors, and pulverisers for the task.

“The upper slab was held up by multiple sets of pillars, which supported arched-roof sections.

“At its thickest point it was 1.5 m thick and at its narrowest points it was 1 m thick, and so we initially thought that if we removed the first pillars, that this would be the breaking point,” says Diogo.

“But of course, because it had been designed to carry the 30-m-high silosand the 12,000 t of aggregates inside them, the structure was so strong that when we removed the first pillars, the upper slab didn’t move. It was still there. Because at its narrowest, 1-m-thick points, it had been reinforced with three layers of 32-mm rebar. “Only when we removed the second set of pillars, did we achieve the breaking point on the next arched-roof section at its narrowest point.

“And this pattern of breaking was repeated in every second section of the slab. So, it certainly proved that it was well-built. It was bullet-proof.”

Utilising a repetitive cycle of hammering, shearing, pulverizing and crushing, Tagregados completed the removal of the structure in just 12 days – well within the allotted two-week timeframe.

DISMANTLING A COFFERDAM

The successful slab demolition and the subsequent opening of the cofferdam, signalled the end of the project's first phase.

And, following three months of reservoir testing by Iberdrola, DST began the second and final stage of the restoration works; dismantling the cofferdam.

“When we resumed work on site, we again had only two weeks to remove the cofferdam,” says Nuno.

“And because the reservoir was now operational, the water level was rising and falling to certain levels.”

The cofferdam measured over 0.5-km long and 5-m high, with a base up to 30 m wide and a top width of 6 m.

It comprised soil, a rockfill retaining wall, two geotextile layers (1,200 and 300 gr) and an impermeable HDPE film, that protected its outer side.

Once the reservoir reached its lowest water level, the massive earthmoving process began and, in the end, the cofferdam was taken down two days ahead of schedule.

When the hydroelectric dam project was complete, this section of the once narrow Torno River had been transformed into a reservoir lake that, when at its peak water level, covers the majority of the former Gouvães Quarry site.

“In a few more years, you will see the entire area covered with vegetation. Trees, bushes, grasslands and the water is all you will be able to see,” Nuno concludes.

The Quarry won the European Sustainable Development Award, promoted by UEPG.