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Clearing the air

TECHNICAL NOTE

Computer modelling has answered important questions about the Air Wall gas barrier system. Stephen Rice reports on the latest developments.

The Air Wall system is a method of controlling gas migration, developed with assistance from the European Regional Development Fund (Ground Engineering May 2002 and others - see references).

Its principles as used to control ground gas flows from a landfill are illustrated in Figure 1. The system is also appropriate for dealing with hazards from other gas sources such as contaminated land, leaking pipelines and hydrocarbon deposits.

All sites are thoroughly investigated before an Air Wall is recommended. Potential gas flow paths are identified using pulse pressure testing and, where appropriate, tracer gas tests.

The system is provable: on-site monitoring can demonstrate performance and any residual leakage can be rectified by increasing air input pressure and/or installing fill-in wells.

There are a number of benefits over conventional barriers and vent systems:

The air pressure bulb extends down to twice the depth of the input well, ensuring a good seal with the groundwater table.

During commissioning, the system is proven using tracer gas testing to ensure pressure balance.

Continuous monitoring of vent pipe gases with active control of input pressures is available.

To complement prototype and field studies during development, a number of computer simulations have been made to investigate details of equipment performance and gas flows in the ground.

Most of the work has been undertaken using the finite element program Compass (Code for modelling partially saturated soils) developed by the Geoenvironmental Centre at Cardiff University of Wales. Some of the later work has been carried out using Modflow, a commercially available program for assessing two-phase flow problems.

Modflow does not overtly allow for fluid compressibility but calibration with the more rigorous Compass package shows that it gives good results for the low gas pressure gradients typically associated with near surface ground gas flows.

The analyses took several weeks of computer time and the examples presented show a summary of the work for isotropic permeability. Analyses with an anisotropic permeability ratio of kh = 10kv give very similar results to the isotropic case.

Four key questions have been addressed:

How does a slotted inlet pipe interact with the ground when an internal pressure is applied? What arrangement and size of slots is required to ensure effective flow continuity between the pipe and the surrounding ground?

Can an Air Wall be generated - ie is it possible to arrange the inlet pipes in a row so that the pressure bulbs generated by each pipe overlap give a reliable impediment to gas flows?

The installed Air Wall has a finite depth and there is a length of blank unslotted pipe at the top to avoid direct escape of the input air to the atmosphere. Is there a risk of residual gas flows escaping above or below the Air Wall?

How does the pressurised part of the Air Wall interact with the vent wells?

How does a slotted inlet pipe interact with the ground?

About 10 different slot arrangements were analysed.

Figure 2 illustrates that an inlet well with six slots provides a uniform pressure source with an effective pressure about 85% of the nominal inlet pressure.

Can an Air Wall be generated?

Many combinations of permeability, well size and well spacing were analysed. Figure 3 shows that, with isotropic permeability, a uniform pressure wall can be achieved with an effective pressure of about 80% of the nominal inlet pressure.

Note that edge effects mean the wall pressure achieved reduces at the ends and this shows that careful thought must be given to edge details.

Of course, this comment applies equally to conventional barriers and vent systems.

Is there a risk of residual gas flows escaping above or below the Air Wall?

Conventional barriers used to isolate a gas source inevitably produce local increases in gas pressure and the risk of leaks past the barrier is always present.

Conventional vent trenches are limited in depth and cannot guarantee to intercept all gas flows.

The Air Wall offers a major advantage in this respect. Figure 4 shows that it extends to about twice the physical depth of the inlet well with an effective pressure of about 65% of the nominal inlet pressure.

Methane is practically insoluble in water at normal temperatures and pressures. Hence the groundwater table acts as a lower boundary for methane migration.

The Air Wall can accommodate significant fluctuations in groundwater level while still providing an effective barrier.

The length of blank unslotted pipe at the top of the well is typically less than 1m. It can be seen that, on this basis, the low pressure zone is restricted to some 200mm immediately below the ground surface.

In practice, this zone is usually occupied by a vented gravel trench containing the air input pipework.

How does the pressurised part of the Air Wall interact with the vent wells?

Figure 5 illustrates the combined effect of a row of inlet wells and the matching vent wells.

Figure 6 shows the same analysis with equipotentials and flow lines. The results indicate that there is significant air flow downstream of the wall - ie away from the gas source hazard.

This element of flow might be considered wasted. However monitoring of field installations suggests that it contributes to oxidation of methane in the ground.

Hence it has a role in helping to purge the hazardous gases from the development site.

Figure 7 shows a similar behaviour when a field gas pressure gradient is applied.

Conclusion

The results demonstrate the Air Wall provides a robust means of controlling ground gas flows.

The active control may be perceived as incurring a maintenance liability, but in practice it provides assurance of continuing effectiveness.

The running costs are less than those associated with ongoing environmental monitoring usually required at such sites.

Acknowledgements Particular thanks are due to Professor H Thomas of the Geoenvironmental Centre at Cardiff University of Wales for his valuable contribution to modelling.

References August 2000, Air Wall Research Project Report. Earth Science Partnership, Cardiff.

October 2001, Gas Protection, Farnborough Road, Castle Vale, Post Remedial Gas Monitoring. Geotechnical Developments (UK), Warwickshire.

May 2002, Air on a shoestring. Ground Engineering. Britten, Rice and Eaton.

September 2002, Airwall www.earthsciencepartnershiplimited. com Stephen Rice is director of Earth Science Partnership.

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