The Gardens at Crathes is a luxury housing development on the banks of the River Dee, near the famous Crathes Castle. Many of the 45 detached properties have views looking out over the river.

Client Stewart Milne Homes
Project Luxury housing development
Outline scope of works Construction of rising main incorporating trenching technology. Roads, tracking for utilities, foundations and external works on the housing plots.
Dates Jan 2015 to April 2015
Headlines Amendments to design of pumping main to improve performance in light of ground conditions

Use of trenching technology to minimise disruption to road users

Foam concrete used to accelerate reinstatement

W M Donald undertook the enabling works on the site up to, and including, external works on the plots themselves. Activities included landscaping, construction of access roads, creation of tracking for services such as gas, water and electric, and building the foundations.

Fundamental to the viability of the site was the construction of a 4km pumping main from Crathes to Banchory. Working with the consulting engineer, W M Donald redesigned the pumping main to optimise its performance and simplify buildability.

W M Donald also won a contract to build the sewers and infrastructure on the development site.

W M Donald identified an opportunity to use trenching for a significant proportion of the pumping main construction programme. Willie Merson was the W M Donald project manager and these are his reflections on how the job went:

"I have to be honest and admit I was sceptical about trenching before the project began. We’d always achieved good results with the traditional open cut and fill approach and the outputs were known. Trenching was new to me and the rest of the team and I suppose there was an element of concern about the ‘unknown’.

The first step was to dig some trial holes. If these identified any services – water, gas, electricity, telecoms – along the route then the plan was to revert to open cut. In the event, the route was clear and so we pressed ahead with a trenching approach and started the first cut on January 19th.

For the first few days it looked as though my reservations were well-founded. We immediately came across boulder clay and had to progress slowly. We only achieved trenching rates of 35m-50m per day, similar to what we would have achieved using open cut. The reduction in trenching speed was to ensure that the boulders in the clay were removed safely. At this point there were discussions as to whether we should abandon the approach.

As soon as we had standard ground conditions, however, the trenching really started to pay off. We started to hit rates of 150m to 250m per day – 3 to 5 times what we would expect from a standard digger. As we trenched we laid a 90mm polyethylene pipe. The pipe was delivered on coils to minimise jointing. To maintain productivity we used self-levelling foam concrete as the fill material. This is a more expensive approach than traditional aggregates but it ensured that at the end of a trenching shift, the new road surface could be laid overnight. We employed two 8 wheeler lorries to ensure we had enough tar on site. A further benefit of the foam concrete was that it eliminated the need for compactors which was a plus from a health and safety perspective.

In total we trenched about 2,000m in the carriageway. Where the ground conditions are right, I’m a convert to the trenching approach. The ability to get in, do the job quickly and effectively, and minimise disruption to the general public were a huge plus. I’m sure we’ll use trenching again on future projects."

Willie Merson - Project Manager

Scope of Works

  • Bulk earthworks cut and fill
  • Offsite rising main
  • Widening of existing carriageway
  • Sewers
  • Sewage pumping station
  • Co-ordination and installation of services

Engineering Commentary

"The Crathes project is typical of many of our works. Our knowledge of sewers and pumping allowed us, working with the consulting engineer, to redesign the pumping main. This improved its performance and made it more straightforward to build.

We did a lot of homework before trialling the trenching. Crathes proved that, in the right conditions, the use of a trenching machine definitely has its merits. The ability to reinstate the road surface on top of the foam concrete overnight meant we used every metre of the 300 metre traffic management zone we were allowed.

Ewan Riddoch – Technical Director

Learning Points

Viewed in isolation, trenching is more expensive than cut and fill. To take advantage of the faster speeds, foam concrete is used for backfill rather than as-dug materials. The as-dug materials may then incur a disposal charge if a use cannot be found for them elsewhere.

On the plus side, output is far quicker, the narrow width of the trench ensures a neater reinstatement finish, and the quick curing of the foam concrete means reinstatement of the road surface can proceed within 6-8 hours.

The key to success is to ensure you have done your homework before you start work on site. Key factors to take into account are suitable ground conditions, absence of buried services, and the development of a detailed traffic management strategy.

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