Continuous Ground Gas Monitoring Prevents Newly Built Houses from Demolition!
Ruddlesden geotechnical just finished working on a very interesting site with a ground gas problem.
In his wisdom, our client had omitted to carry out a Phase 1 Contamination Assessment prior to construction of a residential development of 15 houses. Our Phase 1 assessment concluded that Phase 2 investigation was required, primarily for ground gas assessment reasons.
Ground gas monitoring of borehole installations identified particularly high levels of methane and carbon dioxide. The installed radon protection measures would not be sufficient to protect against the recorded levels of methane and carbon. By the time of the Phase 2 investigation was undertaken, the development was near completion, with roofs complete and only internal finishes outstanding (!)
Continuous subfloor void monitoring was therefore undertaken to measure the gas concentrations beneath the subfloor (rather than at the source (discreet layer at 2m)). The continuous subfloor void monitoring, combined with internal ground gas sweeps, demonstrated that although elevated levels of ground gas were recorded in the boreholes, only background levels were present within the subfloor void. This was inferred to be because either significant volumes of ground gas were not migrating to the surface and/ or the ventilation was performing well.
The conclusions were able to be stated with confidence because the monitoring was continuous and so was able to show that the monitoring included periods of low and falling atmospheric pressure, i.e. likely worst-case conditions. No further ground gas assessment or protection measures were considered to be necessary (and no requirement to demolish the newly built houses to install a greater level of protection(!)).
All was approved by NHBC and the local authority. This was a perfect example of how extra investigation and risk assessment can result in development costs. The end result was a happy client and a satisfied consultant!