Second Opinion Delivers Significant Cost Savings

by | Sep 6th 2019

Second Opinion Delivers Significant Cost Savings

6th September 2019

Geoenvironmental • Geotechnical • Specialist Services

Fresh samples, statistical analysis and a more pragmatic approach led to a revised risk assessment for a housing developer in the South East.

Our Client

The client, specialises in new build residential developments. Its portfolio spans towns, villages and several rural exception sites, across the South East of England.

Preliminary-Assessment

The preliminary assessment identified the potential for Made Ground in the south of the site. Mounds of demolition spoils were also noted to be present in the southern part of the site during a site walkover. The first phase of the ground investigation identified Made Ground comprising gravelly silty clay to depths of between 0.80m and 2.00m in the southern part of the site. The gravel fraction comprised variable quantities of flint, brick, chalk, charcoal-like and concrete. In the northern part of the site, away from any potential sources of contamination identified by the preliminary assessment, Made Ground was not encountered. In this area a shallow layer of topsoil was noted directly overlying the head deposits. Within a separate area in the central part of the site, within the outline of a former pond identified by the preliminary risk assessment, a shallow depth of Made Ground (up to 0.80m depth) was recorded.

 

Our Challenge

Ground & Water was commissioned to provide a developer and its designers, with information regarding the ground conditions underlying a site located to the north-east of Lewes in East Sussex, to assist them in preparing an appropriate scheme for a 38-dwelling housing development.

A previous site investigation had been carried out in 2017 by others. The developer was seeking a second opinion. Ground & Water’s preliminary assessment concurred there was the potential for Made Ground in the south of the site as a result of landfill activities and the back-filling of ponds.

The Ground & Water Approach

The previous risk assessment, undertaken by others, highlighted elevated concentrations of lead and polycyclic aromatic hydrocarbon (PAH) compounds and an isolated detection of chrysoltile asbestos fibres within the Made Ground encountered. Recommendations from the previous assessment advised that, as a whole, the Made Ground recorded to be present across the southern and central part of the site posed an unacceptable level of risk to the proposed end users, were it to remain within areas of private garden or soft landscaping.

The previous advice was remediation was necessary in the private garden areas, comprising the removal of Made Ground to a depth of 600mm. It was recommended that the void produced should be backfilled with a high visibility geotextile over lain by a geogrid and the remainder back filled with certified clean sub-soil and Topsoil.

Additional investigation, chemical laboratory testing and risk assessment was undertaken by Ground & Water in August 2019. Further machine excavated trial pits were undertaken targeting the proposed private garden areas of the residential housing in the southern and central portions of the site.

By undertaking up to date risk assessments on the bundles of asbestos fibres encountered and CLAIRE statistical analysis, it was concluded that no remediation was considered necessary to protect end-users. A minimum thickness of 300mm of certified clean topsoil was recommended placed within all proposed back garden areas and 150mm in soft landscaped areas to form a growing medium.

The results of the ground-gas assessment undertaken by the previous contractor suggested that the site fell into a Characteristic Situation 1 (CS1), where no precautions to the ingress of ground-gas were considered necessary. However further monitoring was recommended.

“A pragmatic approach to ground gas risk assessment for the 21st Century” was used by Ground & Water, to determine if the site fell into CS1. This is instead of undertaking potentially expensive longer term ground-gas monitoring from the wells installed onsite.

 

The Outcome

Ground & Water’s re-evaluation of the site, resulted in revised advice. It was considered that expensive remediation of the Made Ground in the southern and central areas of the site, was not necessary. This would equate to a substantial saving for the developer.

Second opinions are well worth consideration, albeit on most occasions to simply provide greater confidence around risk assessment. We would love to hear from you if you feel your site needs a second look.

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