Geological Risk Assessment

Ground Gas Risk Assessments

Ground gas risk assessments are a major part of site investigations due to large areas of land being affected by hazardous ground gases. Many sites are either underlain directly by made ground of significant thickness with the potential to generate large quantities of ground gases, or are within close proximity to landfill sites and other sites associated with waste disposal in particular historical landfill sites that pre date current legislation of the requirement to be lined properly to prevent contaminant and ground gas migration. In addition mine gases can also present a risk to human health and to buildings.

The risks from ground gases are asphyxiation suffocation and explosion. Explosion can occur when concentrations of methane build up and become trapped such as beneath a ground floor slab at a development, when methane is between 5% and 15% within the atmosphere it becomes flammable. these limits are know as the Lower Explosive Limit (LEL) and the Upper explosive Limit (UEL).

A good ground gas risk assessment would involve gas monitoring of boreholes installed with gas monitoring standpipes at a spacing and frequency over a period of time all recommended within CIRIA 665 current guidance. The monitoring should also be carried out over varying atmospheric pressures in particular at falling pressure. this enable a worst case scenario to be considered.

GeoCon use the latest gas analyser on the market to provide an accurate indication of the levels of ground gases at a site and ion particular gas flow rates. Typically we monitor for Carbon Dioxide (CO2), Methane (CH4), Carbon Monoxide (CO), Hydrogen Sulphide (H2S) and of course Oxygen (O).

In addition to the above some sites may require monitoring for hydrocarbon vapours. This is done by using either a Photo-ionization Detector (PID) or a Flame Ionisation Detector (FID)

Human Health Risk Assessments

Human health risk assessments are probably the most common type of risk assessment as far as contaminated land is concerned. Local Authority planners across the country now insist that as part of a planning condition the developer needs to ensure that the site does not pose a risk to any human end users of the site. The condition usually states

“No development shall take place until an investigation and risk assessment into contamination at the site in accordance with the scheme to be approved in writing by the local authority has been carried out. The investigation and risk assessment shall include recommendations for remedial action and the development shall not be occupied until these recommendations have been implemented”.

There can also be further conditions imposed regarding remediation.

Developers should bare in mind that reports downloaded from the internet and other generic reports are NOT sufficient to satisfy any local authority, and will not be accepted as a phase I preliminary site investigation. This can result in delay to projects and also wasted money.

GeoCon create a Conceptual Site Model (CSM) as part of any phase I preliminary site investigation to identify any potential sources of contamination and any pathways where a source can come in to contact with a potential receptor such as human being living at a property developed on a site. This is know as a “pollutant linkage”, and if present further action would be required.

Further action would then involve ground investigation, chemical sampling, analysis and risk assessment.

GeoCon use the latest national and international guidance to perform an initial Generic Quantitative Risk Assessment (GQRA) for human health by comparison of the soil analytical results against the published guidance criteria. These can be classified as Generic Assessment Criteria (GAC) and are widely referred to by consultants, Regulatory Authorities and other professionals within the industry.

In some cases further more detailed risk assessment may be required based on the findings of the phase II site investigation and would include additional ground investigation, chemical sampling and analysis at closer spacing and frequencies to the phase II site investigation and would then involve a Detailed Quantitative Risk Assessment (DQRA).

Controlled water risk assessment

As part of our site investigation service, when there is a requirement, GeoCon will carry out controlled water risk assessments for both surface water courses and ground water bodies / aquifers.

The requirement would initially be outlined during the phase I preliminary site investigation based on any pollutant linkages identified. A surface water risk assessment would involve water monitoring on site and of the local water body, chemical analysis and risk assessment using modelling software. A groundwater risk assessment would largely involve the same processes but would focus on deeper water bodies within the underlying soils at a site, the geology, the presence of aquitards and aquifers groundwater flow patterns, and groundwater modelling.

GeoCon assesses the risk to any water bodies local to a site from contaminated soils and groundwater at the site, and provide subsequent advice on what actions to take to satisfy any regulators that pollution from the site will not occur such as removal and / or remediation.

Flood Risk Assessments, Reports

(FRA) are required for sites that lie within in an area at risk of flooding from either rivers or the sea. Generally a phase I preliminary site investigation would identify the requirement for an FRA by identifying if a site lies within a flood risk area. Once identified a level one Flood Risk Assessment would then be recommended. This involves collating data and information from sources such as the Environment Agency (EA) modelled flood levels, the British Geological Survey, existing and proposed developments and topographic levels on site.

The FRA would identify the risk of flooding at a site, how the development may affect flooding and provides recommendations as to how the risk of flooding to the site can be avoided. Additionally consideration is given to other sources of flooding which including groundwater, surface water run off and sewer flooding.

Flood Risk Assessments are required to be completed according to Planning Policy Statement PPS 25: Development and Flood Risk, which was created to establish the key role of the planning system in managing flood risk. Flooding can cause damage to property and endanger lives.

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GeoCon are members of the British Geotechnical Association, AGS, CHAS Accredited and registered with ISO 9001, 14001 and 18001.

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GeoCon specialise in Ground Engineering and Engineering Geology focusing on site investigation for all sizes of projects.

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GeoCon SIte Investigations have six regional offices located in London, Manchester, Birmingham, Newcastle-Upon-Tyne, and Bristol.