Western Access Earthworks (Southall Waterside)

Robin Slade

Associate Director

Tel: 01483 310600

Email Robin

Client

Clark Smith Partnership
(on behalf of FM Conway)

 

Services

  • Ground Investigation
  • Earthworks Specification
  • Geotechnical Design Report to HD22/08
  • Materials Management Plan
  • Temporary works design
  • Supervision, compliance testing and verification

Geotechnical Design Reports and Geoenvironmental Strategy

The Southall Waterside development will create a new neighbourhood alongside the Grand Union Canal in West London. The 45 hectare site, which housed the Southall Gasworks until 1973, will be accessed partly via the Western Access which comprises a road over three new embankments and two integrated bridges, constructed within a 0.75 hectare area of brownfield land adjacent to the main site. The earthworks will support a new three lane carriageway with designated cycle lane and footways spanning the Yeading Brook and Grand Union Canal.

CGL has undertaken geotechnical and geoenvironmental services on behalf of the main contractor from a Pre-Construction Services Agreement (PCSA) phase of their design and build contract, through to detailed design phase and construction. Early engagement has provided the opportunity for CGL to value engineer throughout the design development.

During the PCSA phase, CGL carried out targeted rounds of ground investigation to confirm the ground model for geotechnical design and establish the extent of contamination to inform materials management. CGL undertook a controlled waters Detailed Quantitative Risk Assessment (DQRA), liaising closely with the Environment Agency, and produced a Remediation Scheme Addendum which included chemical acceptability criteria to maximise the volume of material that could be reused within the site.

CGL produced outline-stage and detailed design-stage Geotechnical Design Reports in accordance with the Design Manual for Roads and Bridges HD22/08. This included the production of Specification Appendices defining the acceptability criteria for materials to be used within the earthworks. CGL identified design risks associated with poorly compacted fill and specified supplementary investigation to develop a solution that comprises a strengthened earthwork behind integrated bridge abutments, requiring integration with the project structural design engineers.

During enabling works and initial earthworks construction phases, CGL undertook a site supervision and watching brief role on behalf of the main contractor. This included testing of site won and imported materials against our agreed chemical screening criteria, identification of unexpected contamination, compliance testing of materials and guidance on application of the specification.

Where out of specification material has been identified, CGL has undertaken further assessment to identify opportunities for use of materials within the site, reducing the need for offsite disposal. The scheme is anticipated to directly reuse over 60,000m³ of site won material, with further volumes made available through chemical and geotechnical treatment using cement additives.

During the ongoing construction phase, CGL is carrying out further activities including groundwater monitoring and data assessment, temporary works design and onsite support, review of the remediation contractor’s methodology and works verification.

Due to the location of the scheme it has necessitated close engagement with a number of key stakeholders including two Local Authorities, Transport for London, Network Rail, the Environment Agency plus the Canal and River Trust.