Project background

Following the 2012 Olympics in London, parts of the Queen Elizabeth Olympic Park are undergoing redevelopment into residential developments, schools and infrastructure. The proposed redevelopment area contains landfill material and contaminated subgrade soil, so the risks associated with ground gases and vapours at depth need to be investigated. RSK was contracted to provide the developers with cost-effective and sustainable solutions for potential ground gases and vapours. In 2013, RSK investigated a site in the northern area of the park and provided recommendations for its safe redevelopment for residential buildings.

Challenges

Our preliminary risk assessment confirmed the presence of a potentially gassing former landfill on an area known as phase 1 and the presence of fill material on the areas marked phases 2 and 3. RSK completed a comprehensive investigation to fully understand the ground gas beneath the site and to assess where relevant protection measures might be needed. A high level of ground gas protection was expected for the proposed buildings.

Ground gas site investigation

Initial ground gas monitoring confirmed

  • methane concentrations of 25.5% in phase 1 and significantly lower concentrations in phases 2 and 3
  • that the former virgin landfill was the source of ground gas in phase 1 and processed landfill material was the source in phases 2 and 3
  • that the entire site falls within amber 2 (townhouses) and CS3 (flats) (CIRIA 665, Table 8.5 Wilson and Card classification and Table 8.7 NHBC traffic light system for 150 mm voids).

The assessment was based on the worst-case scenario (phase 1). RSK recommended further monitoring to fully characterise the ground gas regime beneath phases 2 and 3 and thus to reclassify these areas of the site. In 2015, supplementary ground gas monitoring on all phases included

  • installing new monitoring boreholes and considering the coverage, depth and installation details of the boreholes
  • detailed/near-forensic soil and source descriptions during site work
  • chemical testing of soil for total organic content and oil organic matter
  • spot ground gas monitoring visits
  • continuous ground gas monitoring
  • bulk gas sampling and testing
  • a ground gas recovery nitrogen purge trial.
RSK Geosciences Projects - Olympic Park Development - image 4
RSK Geosciences Projects - Olympic Park Development - image 3

Outcome

A comprehensive reassessment of all the ground gas monitoring data reclassified the site from amber 2/CS3 (entire site) to amber 1/CS2 on phases 2 and 3 (two-thirds of the site), which affected the design and costs of the ground gas protection measures. The initial design included ground gas protection measures necessary to achieve a score of 4.5, as specified in BS 8485:2015+A1:2019. However, because of the reclassification of phases 2 and 3, the design for the proposed high-rise buildings required only 3.5 points, which comprised a ground-gas-resistant membrane (2 points) and a reinforced, cast-in-situ, suspended floor slab with minimal penetration (1.5 points). The design changes included phasing out the ventilation element of the design, which meant significant cost savings for the client.

School development

RSK investigated the former chemical works area in the park in relation to its safe redevelopment as a school. The soil and groundwater beneath the site contained volatile hydrocarbon residues that could potentially migrate upwards into the proposed buildings. A soil gas investigation and associated assessments confirmed the need for vapour protection measures.

The principal on-site contractor had difficulty developing a design that would adhere to the relevant legislation: specifically, the selection of a vapour-resistant membrane and the provision of a ventilation system that would enable sufficient dilution of specific vapours.

Following RSK’s advice, specialist subcontractors compiled an effective vapour protection design. Several membrane products were proposed for the design, but some were unsuitable. RSK reviewed the data sheets of all the proposed products and helped to select appropriate membranes. Additionally, RSK verified the design and calculated the vapour dilutions in the proposed geocomposite void-former blanket and submitted the design for proper approval. Ground gas investigation results and assessments feed into the scoring system, which governs the site-specific design details of ground gas protection measures developed for buildings constructed on brownfield sites.

RSK Geosciences Projects - Olympic Park Development - image of person with soil
RSK Geosciences Projects - Olympic Park Development - image of person with soil