A marginal brownfield site in County Durham is being transformed into a housing development that is terraced in more ways than one.
A terraced housing development on a marginal site in Stanley, County Durham avoided expensive civil engineering works by using a retaining wall system.
The combination of topography, soils and difficult access in Tees Crescent would normally have required piled foundations or heavy reinforced concrete walling.
Broseley Homes, acting as client, design engineer and main contractor, is building a range of two- and threebedroom houses on 15 plots across an area of 2800m 2. The site has a 8m diagonal fall.
Access was restricted by two roads and a nearby housing estate, ruling out the use of heavy plant.
Broseley opted for Tensar International's Tensartech TW1 Wall System as a practical, efficient approach that would save time and money.
Part of the site had been used as a council storage yard, and the remainder was undeveloped.
A site investigation found the soil was 0.2m to 1.2m of made ground overlying a glacial till of sandy, gravelly clays with low bearing capacity.
'To build the homes and provide gardens, two levelled platforms were required, ' says Broseley Homes managing director Len Worsfold. 'Conventional solutions were lengthy and costly. To achieve a feasible alternative, Tensar's technical design department worked closely with us to develop a design and supply package.
'There were time savings through the rapid installation without concrete curing, allowing quick access to site when complete, no hire costs for heavy plant and minimal imported fill. With mostly site-won material for the fi ll, this was also an environmentally preferable solution.'
In Tees Crescent, 32 individual retaining walls form two levels. The houses sit on the upper level and the gentle slope down to the lower level will be topsoiled and seeded to create gardens. Steps will lead from the top level to car parking space at the bottom.
Site workers built the terraces from fi stabilised with geogrid and faced with TW1 block walling, which is a cast concrete masonry block, dry laid and anchored to the geogrid. Tensar says this meant no costly foundation was required below the walls, despite the poor ground conditions.
The construction method was to bench excavate the existing bank.
Broseley Homes could then cast a 150mm thick concrete levelling strip along the wall face, onto which the base block was bedded.
It then built the wall structures using compacted site-won fill, reinforced with horizontal layers of Tensar high strength uniaxial geo grid reinforcement. The modular blocks forming the face connect to the geogrid using polymer connectors.
Tensar area civil engineer Colin Thompson says: 'Our internally developed design software for wall and embankment installations enabled us to model the solution for the Stanley site.
'This approach can save more than 40% of the cost of an alternative concrete solution. It has been extensively proven in practice on infrastructure projects all over the world and has British Board of Agrément (BBA) certification, but has only recently been introduced for housing development projects.'
The project is due to finish next spring.
Three of a kind
At a similar project in Crumpsall, Greater Manchester, developer Redrow Homes asked Tensar International to supply a system for three development platforms.
The 2.4ha site at Charminster Drive has planning permission for 68 houses and 100 apartments. Lying close to the River Irk valley, it was infested with weeds and fly-tipped debris was strewn over its wet claybased soils. It also features a height difference across the ground of about 17m, with a valley up to 7m deep running through the middle.
Redrow Homes engineering manager, John Roocroft says: 'The cut and fill contract was carried out in two phases, to provide the three development platforms, with a maximum height difference between the platforms of 3m to 4m.
'I was concerned with the quality of the ground, and the potential cost of a conventional solution, which would have involved costly, time consuming deep piling and extensive formed concreting.
'We originally intended using the locally won site clays for fill with the geogrids, but very heavy weather conditions and the exceptionally poor quality would have added time delays, so imported granular fill was used. Despite this, the costs were still significantly better than with 'hard' solutions.'
Site workers built the platform embankments using geogrids and aggregate, anchored to split face, dry laid facing blocks.
Although a high quality cast stone block was used on one of the walls, it proved too heavy to handle easily, so lighter Tensar facing blocks were brought in.
The Crumpsall development should complete in March next year.