Project Sheet: Slab Bridge Cottage, adjacent to Shropshire Union Canal

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Client

  • British Waterways

Contractor

  • Condor Projects Ltd (sub-contractor to Morrison Construction)

Works Undertaken

  • Design of anchored retaining walls
  • Design of the gabion wall
  • Assessment of revised soil parameters when lacustrine clays were encountered
  • Design of sheet piles to protect the canal edge

Description of Project

Following a landslide adjacent to the Shropshire Union Canal, British Waterways appointed the Contractor to undertake remedial works, as there were concerns that Slab Bridge Cottage may be at risk.

A detailed design was undertaken by others which comprised a soil nailed slope at a constant gradient. However, Condor projects were concerned about the Health & Safety issues associated with working on a steep slope and the long-term issues of anchor heads being present in a garden.

GroundSolve Ltd were asked to review the proposals and provide an alternative solution.

The alternative solution comprised forming a terrace of anchored walls in front of Slab Bridge Cottage, which provided an aesthetically pleasing finish, as they were timber faced. Safe access for the residents to their barge mooring adjacent to the canal was also maintained and the scheme minimised the requirement for plant and materials required on the slope during the construction period.

Buildability also needed to be given careful consideration as access onto the site for plant was very difficult and there were weight limitations on plant working adjacent to the canal. GroundSolve Ltd worked closely with Condor Projects as the alternative scheme was developed.

Site works commenced in August 2008 and was completed early in 2009.

Installation of the anchors for the retaining walls
Installation of the anchors for the retaining walls
Formwork construction for the retaining walls
Formwork construction for the retaining walls
Formwork construction for the retaining walls and base for the gabion baskets
Formwork construction for the retaining walls and base for the gabion baskets

Value Engineering

GroundSolve Ltd. were responsible for the design of the works.

The supervision was carried out on a visiting basis to minimise costs, whilst still allowing the constant updating of the ground model.

The alternative design reduced the number of soil nails required by 60%.
However, during soil nail testing several of the soil nails failed. Subsequent investigation revealed the logging of the soils from the original site investigation to be inaccurate. Further site investigation proved the ground conditions to be varved glacial clays with a lower horizontal then vertical strength meaning the nails were failing along the soil laminations.

Consequently, further redesign of the works was required.

Environmental and Sustainability Issues

The site was adjacent to a section of the Shropshire Union canal which is frequented by tourists and locals. Therefore, it was vital that works blended in with the surrounding area.

Due to the sensitive environmental nature of the site the following measures were implemented:

  • The concrete walls were clad with timber from a sustainable source.
  • The solution minimised the quantity of excavation works which were required.
  • All the materials excavated during the works was re-used on site for landscaping.
  • Due to the rural nature of the site, lorry movements were minimised as the number of soil nails were reduced considerably.
  • Significant landscaping was undertaken.
  • Since the walls were anchored, the founding depth was only 0.5m, therefore this minimised the volume of concrete required.
The construction works in progress
The construction works in progress