Gabion faced embankments are helping a new water treatment plant blend into the landscape. Report and photographs by Adrian Greeman
At the risk of upsetting Scottish nationalists, the latest project from Scottish Water could be said to resemble an 18th century fortress.
Grass embankments slope up to steep “moats” in the Midlothian landscape.
Deep within these, behind the canyon walls, the entrances leading to secure interiors and personnel are hidden away.
But this is no Redcoat stronghold to keep the kilted clans at bay.
The purpose of the revetments and grassed over operations rooms is to make Edinburgh’s new water treatment plant blend into the surrounding landscape.
It will be one of the biggest in the country, serving 450,000 people, “but from most angles you will struggle to pick it out from the countryside, and even then only if you are fairly close”, says John Marshall, Black & Veatch contracts manager for the design and build project.
“Minimal visual impact was a planning condition of the whole project.” It is some countryside.
The 175M litres per day capacity plant lies in farmland in the picturesque Pentland hills to the south-west of the capital, a favourite hiking spot and a minor treasure of the Scottish landscape.
Building what is effectively an industrial plant in this area of outstanding natural beauty, albeit for a clean water purpose, required sensitivity.
The plant is needed here because of the location of three separate dammed reservoirs that feed two older treatment plants, and which are to be decommissioned.
The supplies are all at a high level, the highest at 300m and the lowest at 220m.
“The overall project is a £130M scheme”
The treatment works, at 208m, can therefore be fed by gravity and again use gravity to feed new mains to Edinburgh 8km away and another 30m lower.
Apart from reducing the energy use, this will also allow the plant to run a small hydroelectric plant of 275kW output, serving all of its daily needs, and should tariff rules be amended, even feed to the grid.
So to hide away the main buildings for the water intakes, the main treatment filters and tanks, plus a huge storage tank holding 90M litres of processed water, they will be grassed over, in the process creating one of the largest planted “green-roof” buildings in Europe.
Then the buildings and the site service roads from the nearby main road are all to be effectively sunk into the landform, since each is being surrounded by sloping embankments blended into the roll of hills.
To make these embankments, but leave space clear within the site for various delivery roads, unloading courtyards and service access by the buildings, they will end with steep, almost vertical internal facings up to 9.5m high, created using gabion walling and geotextile reinforcement in the embankment.
These are being installed by subcontractor Maccaferri Construction, in one of the largest jobs it has tackled in the UK.
Altogether, its work is worth around £600,000.
“The overall project is a £130M scheme,” says Marshall.
This comprises the plant, and laying 15km of pipes including four 1,200mm diameter intake pipes and twin 1,200mm processed water pipelines running on to the city.
The plant is being built in an area in the greenfield site from which 300,000m3 of fill has been moved.
Within this an intake building, a 16m high main treatment building, an intermediate flow building and the main storage tank are under construction.
The plant will use a hybrid water treatment using combined gravity filtration and air flotation, all in one tank system, to remove impurities.
The so-called CoCoDAF method, developed by Thames Water, has a lower footprint than usual, helping keep the plant size to a minimum.
“Much of the construction is eventually below ground level, most of the main tank, and 9m height of the main processing building,” says Marshall.
But the rest, above ground, will not be visible either because of the high backfill slopes around them.
These are made behind the initial gabion faced earthfill walls.
Macafferri’s job comes in four sections of embankment wall. Two are very large, stretching 296m and 198m respectively and varying in height between 2m and 9.5m, changing according to the lie of the ground.
Two lower stretches are just 1m high and extend to 280m and 412m.
The subcontractor designs, builds and finishes these, working in conjunction with the earthfill contractor who moves spoil from the temporary holding areas as required.
For the lower walls a simple gravity gabion is used, explains Alan Rice, business development manager for Maccaferri Construction, the firm’s contracting arm.
Baskets formed with the firm’s double twisted galvanised wire are 1m deep and 1m wide and have a 1.5m length along the wall.
They are filled with a red basalt brought in from a local quarry, the 100-200mm long rock pieces tipped in by excavator initially and then stacked at the front of the basket byhand to give a fair finish to the wall.
Behind this neat front facing, the rest of the basket is filled by excavator.
“It is a glacial boulder clay with lumps of rock big enough to crush a car so you have to choose what you use”
The same applies for walls above 2m.
But for these the design uses Maccaferri’s Terramesh system, which combines the gabion front with a grid layer in the embankment behind, creating a reinforced ground embankment. “Essentially each basket has a ‘tail’ projecting 3m behind it,” explains Rice.
The tail is the last part of a 6m length of mesh that wraps around to form the top, front and base of the basket and then continues back into the ground behind.
The mesh is galvanised and coated in PVC for corrosion resistance and durability.
Like the gravity units, the basket contents are stacked partly by hand to achieve a consistent front facing for the wall.
“The baskets are bigger, 2m long and sufficient to be able to work easily inside them,” says Rice. An excavator finishes the fill.
Baskets are strengthened at one-third and two-third full points with connector wires across to help maintain the shape and wall profile.
Each new layer is set back slightly by about 100mm, which gives an overall slope of 6 degrees, just enough off the vertical to prevent a visually oppressive “sheerness” to the facings.
Once in place a granular fill goes in for the half metre behind the basket and then the 3m width of embankment behind that is built up with selected material from the cut-and-cover stockpiles, brought in by the earthmover.
“It is a glacial boulder clay with lumps of rock big enough to crush a car so you have to choose what you use,” says Rice.
Maccaferri does the carefully controlled compaction of this material to achieve a reinforced ground wall around the mesh.
Behind that the earthmover gradually builds up the sloping ground with the rest of the fill, also compacted in layers but less rigorously than for the facing embankments.
“This basic design has to be supplemented with another geogrid for the higher walls,” says Rice.
Any sections of the wall over about 6m use a reinforced ground width of 7m rather than 3m.
To achieve this, a supplementary grid of polyesther Paragrid 80-05 is laid, which Maccaferri also supplies.
This grid is embedded in the fill along with the gabion “tails”.
As on the lower walls, as the embankments rise, the ground is built up behind them by the earthmoving contractor.
Later, the slopes will be seeded.
Meanwhile, the main contractor will be finishing the 95m by 200m top of the 7m deep storage tank with a grassed area; it is built up with a drainage material unrolled over the concrete and then topsoil before turf is laid.
The main filtration building also gets a turfed area, this time on a steel frame structure “which will be Scotland’s largest green roof”, believes Marshall.
It comprises a steel undersheet and roof insulation layer with a Kalzit roof above.
“Over that goes a drainage layer and a fibre mesh before you spread topsoil,” says Marshall.
Turf, made just off site while the rest of the work was under way, finishes the whole, which will then appear from a distance to be nothing more than part of the local grassland.