Maccaferri works closely with Galliford Northern on remediation and site formation works to prepare the site

21/08/2002

Maccaferri Ltd, specialists in retaining structures, slope reinforcement and erosion protection, is working with main contractors Galliford Northern (a GallifordTry company) and their Consultants, Joynes Pike & Associates to complete the extensive groundworks required to prepare a large brownfield site in Halifax for re-development as a retail park. The location of a former wire works which was abandoned 18 months ago, the 20,000m² site was never de-commissioned, and as a result, extensive remediation and ground works were required prior to construction.

During initial consultations between the joint developers, AWG Developments and City & Northern Ltd, the main contractor Galliford Northern and consulting engineers competing for the work, a number of proposals were considered, with three being carried forward to the tender process. A key issue was the presence of a 35-metre high unstable ash slope immediately behind one of the old buildings. One of the tendered solutions was discounted at an early stage but both the remaining solutions to this problem were closely considered, either a contiguous bored pile wall or a gabion/terramesh wall.

This second option, proposed by Doncaster-based consulting engineers, Joynes Pike & Associates, with materials supplied by Maccaferri, was selected by Galliford Northern.

Work on site commenced earlier this year with the de-commissioning of the site and the removal of chemicals and asbestos by licensed contractors to registered waste facilities.

On completion, the remaining warehouse structures were demolished and, under the supervision of Joynes Pike & Associates, all materials were crushed on site to form a selection of engineered granular materials, which could be re-used on-site and thereby reduce the need for imported material.

Site preparation was a major element in the construction programme. Located on uneven ground, the terrain gently rises from Charlestown Road to the toe of an abandoned quarry below Beacon Hill Road, which bounds the site at the top. This quarry, created when the site was occupied by a brickworks had been backfilled with ash, creating a steep, inherently unstable slope rising some 35 metres at a typical angle of 40-45°.

Achieving the correct factor of safety

Adopting the temporary works solution proposed by Jonathan Saunders, Associate at Joynes Pike & Associates, Galliford Northern’s earthworks sub-contractor Webfell Construction installed temporary sheet piles at the top of the site to stabilise the road and adjacent dwellings. Pre-augured piles were used, penetrating 6 metres into the rock strata and instrumentation installed to monitor any movement.

Over a four week period, 90% of the ash (40,000 tons) was removed from the site, and a temporary platform constructed using site-won material. Working to the Maccaferri design, specialist sub-contractor P C Construction installed a 9-metre high Gabion Terramesh® rendered soil wall 80 metres in length across the foot of the slope. This reinforced block was subsequently surcharged with granular material forming a slope grading up to 270 to achieve the correct long-term factor of safety.

Commenting on the completion of this phase of the groundworks, Rob Todd, project manager at Galliford Northern said: "The schedule for this project is very tight and it is therefore important that we have fast and effective solutions for each stage of the works. We looked at a number of options for the groundworks, but it was the Maccaferri gabion terramesh® wall that offered the most practical advantages. It is much quicker to construct than the piled wall alternatives and this allowed us to make significant savings."

Environmental considerations were also key, as Rob Todd continues: "By using the Maccaferri system we were able to reduce the amount of contaminated ground materials on the site. Ash backfill was replaced with site-won granular material, the content of which had been checked and validated by Joynes Pike & Associates in accordance with the Method Statement previously approved by both the Environment Agency and Local Authority. This would not have been possible with the alternative piled wall solution."

Sustainability and cost control

The remainder of the site has been re-engineered to achieve load-bearing requirements for the proposed development. Approximately 53,000m³ of material has been excavated to carry out this operation with tests undertaken every 500m³ to ensure no contaminants remained within the ground. The works were supervised and validated by Joynes Pike & Associates.

Materials from the demolished warehouse buildings have been recycled where possible, reducing the requirement to import stone to the site. Stonework has been reclaimed for re-use and timber carted away for shredding and re-use in the manufacture of chipboard. The 40,000 tons of ash recovered from the old quarry was sold to a block manufacturing company for recycling.

The use of recycled materials has allowed Galliford Northern to make substantial savings, while also ensuring that the former brownfield site conforms to requirements for use as a retail park. The site will house three retail units and a fast food outlet with associated car parking.

The units will be constructed using steel portal frames, vertical and roof cladding and re-constructed stone walling, with the Maccaferri gabion terramesh wall providing an attractive natural stone background.

Maccaferri Gabion Terramesh® is a retaining wall system combining a box gabion front face with a continuous mesh tail forming the anchorage element. Delivered to site in ‘flat pack’ form, each unit is fabricated from a double-twist, hexagonal mesh of PVC and Galfan galvanised steel wire.

Measuring 2m x 1m x 0.5m with a fixed diaphragm at 1m, units are laced together using continuous 2.2mm diameter lacing wire, or alternatively high tensile steel fixing rings can be attached using a special pneumatic assembly tool. Once assembled, each unit is filled with natural local stone to create a flat or steeply stepped retaining wall.

Maccaferri Ltd specialises in the supply of gabions, geotextiles, geogrids and geomats. In addition to a full range of products for retaining structures, slope reinforcement and erosion protection, the company offers an in-house design and consultancy service.

Related Pages