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Balfour Beatty Utility Services – Exeter (civils)

Background

Balfour Beatty Utility Solutions (BBUSL) were the principal contractor for South West Water, tasked with delivering up-to-date sewer infrastructure installation for the new Cranbrook development close to Exeter, which was conceived to be the first new stand alone, self sufficient, low-carbon development in Devon since the Middle Ages.

Problem

The sewer scheme produced 4000m3 of extremely wet arising consisting of sand and clay which was initially intended to be taken off site for disposal at land fill. Once this material had been disposed of, approx 8500 tonnes of primary or recycled aggregates would then be imported for use as both backfill and sub-base layers for the road make up, once the sewer pipe had been installed.

Due to the extremely high moisture content of the sand and clay arisings, local landfills and disposal sites would not accept the material. In addition, the arisings would be extremely difficult to transport as the moisture could potentially seep or leak out of trucks during transport.

Solution

In order to overcome this problem in a sustainable manner, BBUSL turned to SMR partner Conroys to deploy its Stabiltech Mobile Batching Plant to the site, to combine the waterlogged material with the SMR proprietary binder at half dose through a blending / mixing bucket. The material was then able to be re used 24 hours later as the backfill layer being laid and compacted in place of quarried aggregates following which it was tested for performance.

Benefits

Reduced Carbon Footprint – Lorry Movements: By carrying out the stabilisation process in-situ using a mobile Batching Unit meant it was not necessary for any material to be transported either on or off site. The entire 4000m3 of wet sandy and clay material would have been taken off site and disposed of at landfill. Primary aggregate would then have been imported to site to construct the backfill and sub-base layers for the sewer scheme. It was calculated that around 430 lorry journeys were negated; thus removing significant road miles from the project, wear and tear on the vehicles, driver costs and potential, traffic congestion and clean-up costs – all having a significant impact on the carbon footprint, environmental and budget elements of the project.

Reduction of Landfill: By using the mobile Batching Unit in conjunction with the SMR Proprietary Binder all excavated arisings were re-used and converted into a recycled capping / fill material and used as the backfill layer. This resulted in zero material being sent to landfill and zero use of primary aggregates.

Reduction of use of Quarried Aggregates: The 4000m3 of wet sandy clay material was processed through the mobile Batching Unit with the SMR Proprietary Binder into a recycled stabilised capping material – subsequently used as the backfill layer – which would previously typically have been constructed using imported primary aggregates.

Reduced Costs: By re-using the excavated material on-site this eliminated the need for sourcing primary aggregates and negated all tipping fees – saving significantly on project costs.

Reduced Job Time: As the 4000m3 of wet sandy/clay material was recycled on-site, this reduced the job time as there were no requirements for muck away transportation and quarried aggregate delivery.