Civil Engineering


A96 Filter Drain Recycling

Lhanbryde Bypass and Inverurie

Carnell worked in collaboration with BEAR Scotland, the trunk road operating company for Transport Scotland’s North East unit, to undertake filter drain refurbishment at two locations on the A96 in July 2022.

Using the StoneMaster process to recycle the filter drain media in-situ, provided several carbon, safety and customer benefits over the traditional dig-out and replace method.
The combined 3,750m length of filter drain was brought back to specification in 11 night shifts, 5 fewer than planned. By only taking away non-compliant material, 787 HGV journeys were removed from the network, while the reduction of materials and haulage alone led to a reduction in CO2e emissions of over 54 tonnes.

Using StoneMaster on the A96 to recycle the filter drain in-situ resulted in:

  • 3,750m of safer road.
  • Re-use of 2201m3 aggregate.
  • 11,412 fewer HGV miles transporting materials.
  • Completion 5 shifts ahead of programme.
  • Reduction of 30,266m3 CO2e. The equivalent volume of of 11 hot air balloons!

Resilient roads through a sustainable process

In producing a fully functioning filter drain which will extend the life of the carriageway, StoneMaster removed a tonne of waste every 4.3 linear metres. That included 450 tonnes of surface vegetation and detritus removed at all verge run-off locations.
Cost and programme savings were made through using a ‘lift & shift’ traffic management methodology. Safety was improved through reduced roadworker exposure, while the risk to road users was minimised through a 79% reduction in HGVs movements in and out of the work zone.
The StoneMaster process is resilient in the face of record fuel and aggregate prices and the shortage of HGV drivers.

Mission Zero

StoneMaster significantly reduces the production of green house gases (CO2e), therefore supporting Mission Zero and the ambitious target to reduce transport carbon emissions by 75% by 2030.

Our calculations use DEFRA GHG factors for input into the Carbon Management System (CMS) and measurement of emissions from Transport Scotland’s maintenance operations.

Think Safe, Home Safe

Collaborations with the traffic management & client teams, helped identify and report instances of vehicles jumping the red lights. This led to measures being put in place to keep the site safe at all times.

Filter drain maintenance supports the Climate Change Risk Assessment for Scotland, as we expect to see ‘increased rainfall on the wettest days of the year’.