Dunnottar Park is a housing development being built close to the A90 dual carriageway in the south of Stonehaven.

Client Stewart Milne Homes
Project Housing development of 101 properties on a site with complex access and environmental challenges.
Outline scope of works Construction of access road incorporating a BEBO bridge structure, surface water management, earthworks management, construction of temporary haul road and bridge for site access, redirection of electricity cables.
Dates Start: February 2015. Full site completion forecast for 2019.
Headlines

Construction of a 32m long BEBO (pre-cast reinforced concrete arch) bridge with complex geometry

Designed, obtained approval from SEPA for, and implemented surface water management measures in an environmentally sensitive area

Redirection of 3 x 11kV electricity cables using directional drilling techniques

 

The site is bounded on two sides by waterways – Carron Water and the smaller Burn of Glaslaw – and had been on the radar of housing developers for several years, but the challenging access and environmental complications had frightened them off in the past. The first challenge was to provide construction access to the site. The existing bridge on to the site was solely for agricultural use and was deemed unsuitable for construction activities. A temporary bridge with a 70t load capacity was constructed in conjunction with a temporary haul road.

Permanent vehicular access to the site required the construction of an inclined access road over Carron Water. The key feature of this element of the project was the construction of a 32m long BEBO bridge. BEBO – a contraction of the German word ‘BEtonBOgen’ meaning concrete arch – is a modular bridge building system consisting of pre-cast reinforced concrete arch, spandrel and wing wall elements. The system is well-suited to large span, low-rise, high overfill applications such as that at Carron Den. The bridge required the construction of substantial foundations and 1.5t sand bags were used to provide a natural reinforcement for the embankment (see ‘Engineering Commentary’ below).

The site has a clay subsoil and surface water management was critical to ensure water quality in the river and the burn were not compromised alongside the need to create stable ground conditions for building works to proceed. W M Donald developed a natural alternative to flocculation to prevent ‘dirty’ water from entering the waterways (see ‘Learning Point’ below) and obtained approval from SEPA to implement this approach. Other surface water management activities included the construction of lagoons.

 

Other activities on site included the use of directional drilling to allow the diversion of an 11kV overhead cable, and the construction of a pumping station and rising main to exit into a public sewer.

W M Donald is working collaboratively with Clancy Docwra, the utility contractor on site, to ensure the effective implementation of gas, electricity and water connections to properties on site.

Scope of Works

  • Offsite rising main
  • BEBO Bridge
  • Sewers
  • Roads
  • Plot works
  • Services diversion
  • Services installation
  •  Bulk earthworks
  • SUDS detention basin
  • Sewage pumping station

Engineering Commentary

"Fundamental to the successful construction of the BEBO bridge were measures designed to ensure the toe of the arches were held firmly in place. This had to be achieved without impacting on the water quality in the river.

A key element of the approach adopted was to use one tonne builders’ bulk bags filled with sand to support the embankment."

Ewan Riddoch – Technical Director

Learning Points

The Dunnottar Park site has a clay subsoil leading to high surface water run-off which needs to be controlled and pumped away from the site. Site activities such as earthworks disturb the ground and lead to clay particles, behaving like colloids, in the surface water. The clay particles do not settle and cause turbidity (cloudiness) in the water.

A standard treatment to remove such particles is flocculation. Flocculation involves the addition of a chemical – flocculant – to the water which makes the particles aggregate – come together, become larger and heavier – to form flocs. The flocs can then be removed using a variety of techniques including filtration.

At Dunnottar Park the proximity to sensitive water courses during the fish spawning season meant, as a precautionary measure, the addition of flocculants was not permissible. The solution was to drill holes to provide even distribution of water across a large area of grassland.

 

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study as a PDF

 

Web Links

  BEBO bridge system -
www.beboarch.com/article/artFile/17-111128-Bridges.pdf

Stewart Milne
www.stewartmilnehomes.com/future-developments-carron-den.aspx

Scottish Environment Protection Agency (SEPA)
www.sepa.org.uk