Gas & Groundwater Monitoring
Monitoring wells can be installed within boreholes to enable subsequent monitoring of the soil atmosphere and\or recording of gas flows and groundwater levels. Representative samples of gas and\or groundwater can also be recovered from such installations.
Lithos engineers are able to specify well design in the field, after consideration of: the strata encountered; groundwater strikes; appropriate well materials; the required response zone etc.
Determination of porewater pressure in strata of low permeability is important for geotechnical design (eg slope stability assessment, retaining walls, deep basement excavations etc), but requires the installation of piezometers, rather than wells.
Hazardous gas is considered to be any mixture of potentially explosive, toxic or asphyxiating gases, but the most notorious ground gases are methane, carbon dioxide and radon.
Assessment of potential risks associated with hazardous gas is based on a review of data obtained during the desk study.
Where the desk study indicates a potential risk from hazardous gas, Lithos are able to review monitoring data and undertake a risk assessment in accordance with current guidance.
Where the proposed development comprises low rise residential housing, the gas regime is characterized in accordance with a “traffic light” methodology. For sites classed as Amber 2, validation is required to confirm that protection measures have been installed correctly.
Other forms of development are characterized in accordance with “Wilson & Card” methodology. Both methodologies are outlined in CIRIA Report C665.
On completion of the risk assessment, we provide advice with respect to appropriate gas protection measures (membrane and sub-floor ventilation).
Long-term monitoring of standpipes or piezometers is always recommended if water levels are likely to have a significant effect on earthworks or foundation design.
Monitoring of groundwater level is also required to comply with guidance provided in CIRIA Report C697:2007 which recommends that soakaways should not be constructed ‘in ground where the water table reaches a level within 1m below the base of the soakaway at any time of the year’.
Where groundwater is considered vulnerable to contamination, or may already have been impacted, monitoring wells are usually installed in order to obtain representative samples.