Site Investigation – Guernsey

Summary

Omnia were commissioned by our client to assist with an intrusive site investigation being undertaken on an industrial estate in St Peter Port, Guernsey. Our client requested Omnia’s support in supervising the proposed intrusive investigation and producing a Phase II Geo-Environmental and Geotechnical report to support a planning application for the construction of five industrial units and associated infrastructure. 

The site comprised a number of buildings used for commercial/industrial purposes and was located within the footprint of a historical clay pit for a brickwork located nearby. As such, the ground beneath the site was assumed to comprise Made Ground overlying natural bedrock deposits. 

Scope of Works

The intrusive site investigation was undertaken in September 2024 during which the following works were undertaken: 

• Eleven window sampling boreholes were advanced to a maximum depth of 6.00m below ground level to allow for the logging of shallow soils, undertaken in-situ SPT testing and collection of samples for chemical and geotechnical testing. 

• Four trial pits were advanced to a maximum depth of 1.50m below ground level to allow for the logging of shallow soils and the collection of samples for chemical and geotechnical testing. 

• Follow on soakaway testing was undertaken within two of the trial pits. 

• Follow on gas monitoring within four monitoring wells which were installed during the intrusive investigation. 

Outcome

An interpretive report was produced for the client detailing the ground conditions encountered during the site investigation and the results of the in-situ and laboratory testing. Made Ground was encountered within all locations across the site associated with the historical clay pit and subsequent infilling. The full thickness of the Made Ground beneath the site was recorded in only five of the locations and was recorded up to a maximum thickness of 5.40m. 

As part of the report, Omnia undertook a Tier 1 Risk Assessment based on the proposed commercial end use which identified levels of asbestos fibres in the Made Ground which had the potential to cause significant harm to future site users. Omnia recommended that remediation measures are undertaken, following a remediation method statement, to make the site suitable for use. 

Based on the results of the ground gas monitoring undertaken, a potential risk to future site users was identified. In order to determine the extent of the risk and the magnitude of any protective measures required on site Omnia have been commissioned by our client to undertake a further ground gas monitoring visits including the analysis of the collected data and recommendations for any protective measures. 

Additionally, preliminary foundation options were provided to the client based on the ground conditions encountered and SPT N value collected during the intrusive works.