EPG was appointed to assess permanent gas risks associated with a suite of offices located in Epsom, Surrey. The office accommodation was constructed in 1990, on the site of a former clay pit which was subsequently utilised as a landfill for the disposal of commercial and domestic waste. Ground conditions underlying the office accommodation comprised approximately 9m of waste material. When constructed the gas protection strategy adopted for the building comprised mechanical ventilation of the sub-floor void, linked to a continuous gas monitoring and alarm detection system within the building which incorporated 29 No. monitoring heads. A major Client concern was the number of low and high level gas alarms which had been detected by the gas monitoring system in recent months.
The works implemented by EPG included an inspection of the building and gas detection alarm system, together with a full and thorough review of all archive information pertaining to the site (including information held by the building owner and the Local Authority). These works indicated that the gas detection system was obsolete and had been subject to little maintenance and calibration since installation. The reason for the recent low and high level gas alarm events was quickly identified as being associated with an erroneous detector head within the gas alarm system.
EPG utilised the information obtained from the building / alarm detection system inspection, in conjunction with the documentation review, to inform the development of a robust Conceptual Site Model for the qualitative assessment of permanent gas risks. The Conceptual Site Model was then utilised to inform quantitative risk assessment via gas migration modelling, including; gas generation modelling, calculation of vertical gas migration by advection and calculation of anticipated equilibrium gas concentrations within the building.
Utilising this modelling EPG were able to conclude that gas risks were minimal and that instead of replacing the now obsolete gas detection system (which would have been very expensive and required on-going maintenance / management and building evacuation procedures to be in place) the active gas detection and alarm system could be safely shut-down. Some minor elements or work were highlighted to be required associated with the existing sub-floor void ventilation, such that going forward it operates as a passive system thus requiring no on-going servicing, management or running costs.
In this instance the building owner was looking to divest the site and was finding that issues associated with permanent gas risks were causing concern to future potential buyers. EPG were able to provide a prompt, robust, scientific and cost effective assessment of this risk in the form of an updated Environmental Assessment Report, which was provided to future buyers to facilitate divestment of the office accommodation. EPG was able to save the current site owner considerable money and time by working with existing data and utilising this to inform a robust decision making framework, based on mathematical modelling and quantitative risk assessment compliant with current UK best practice and guidance.