Ground gas flow and continuous monitoring

06Feb / 2015

Ground gas flow and continuous monitoring

Ground-gas flow and continuous monitoring

I’m often asked: Does GasClam® measure flow?

This article attempts to answer this question and highlights the reasons why flow measurements may sometimes be misleading.

“No…”

GasClam® does not measure flow. In order to measure flow directly, a valve must be released which allows ground-gases (or air) to move from borehole to atmosphere, or vice versa. GasClam® is sealed into a monitoring well throughout monitoring and therefore no gas flow passes from ground to air. Options are available to intermittently release gas to the air, rather than recirculate to the borehole, but flow is not recorded as it passes through.

Infrastructure required to measure flow

Bespoke equipment can be used in conjunction with GasClam®, allowing flow to be periodically measured on sites where continuous monitoring is used. Site visits can be undertaken and flow can be measured in the usual way with a handheld monitor, while GasClam® remains in situ. This data can supplement continuous ground-gas concentration measurements.

What does flow tell us?

Flow rates are used in Gas Screening Value (GSV) calculations, which in turn are applied in generic ground-gas risk assessments to determine the level of ground-gas protection required in buildings. This is done as flow rates can be an indication of ground-gas generation potential.

A common observation is that flows can sometimes be positive, sometimes negative and sometimes neutral. It is not always clear how to apply sporadic flow readings to GSV calculations, and it is sometimes reasonable to come to the conclusion that sporadic flow readings are not caused by gas generation.

When sporadic flow rates are measured, this can be a result of ‘barometric pumping’. This phenomenon is caused when ground pressure does not equalise quickly with air pressure due to the ground surface being sealed or of low permeability. During these ‘lag’ periods, air pressure is elevated above ground pressure, or vice versa (dependent on direction of air pressure change).

The graph below illustrates this lag, as well as the type of flow reading which would be measured at various times.

Barometric pumping graph

Graph 1: Barometric pumping and flow measurements

What’s the alternative?

The graph above shows that flow readings would be measured on this site; however flows would have come about due to barometric pumping, not due to ground-gas generation.

Perhaps a more appropriate line of evidence in risk assessment is differential pressure, where differential pressure = ground pressure – air pressure.

Continuous monitoring allows differential pressure to be plotted (see graph below).

Differential pressure

Graph 2: Differential pressure fluctuations (ground pressure – air pressure)

On those sites where differential pressure fluctuates around zero, barometric pumping is often observed. On sites where differential pressure is consistently elevated above zero, we may be looking at the more risky sites where gas is being actively generated.

Summary

  • GasClam does not measure flow readings, this requires ground-gas or air to pass from borehole to atmosphere, or vice versa.
  • Flow can be measured while GasClam® is in situ, using bespoke equipment
  • Flow is not always an indication of generation, but a sign of barometric pumping
  • Differential pressure can demonstrate whether barometric pumping is occurring, or gas is being generated. This is potentially a useful line of evidence in ground-gas risk assessment.
  • Instead of a GSV calculation, consider carrying out a Purge & Recovery Test to provide a calculated ground-gas ‘flux’

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