Risk and reliability in gas protection design – 20 years on: Part 1

Geoff Card, GB Card and Partners; James Lucas, The Environmental Protection Group; and Steve

Wilson, EPG

Supporting online Appendices

Appendix 1 - Case study of diffusive gas flow in East Midlands

Buildings over an old landfill site in the East Midlands had cracked concrete floor slabs and designed

joints with poor sealing. There was evidence of settlement of some slabs which were ground

bearing. Landfill gas ingress was occurring through some of the cracks (up to 1% flammable gas but

only within the cracks).

Although the waste is indicated to have been deposited between 1961 to 1970 experience suggests

that it is possible that waste was unofficially deposited up to 1978. The buildings were constructed in

the 1980s after closure, and it is known that when landfills were operational at that time unofficial

deposition of waste was common. The buildings have ground bearing slabs most likely located

directly on the waste without any capping layer present.

The maximum depth of waste below the site is 4.3m. The waste mass across the site is variable.

However, it was typically noted to include abundant wood, plastic/plastic bag/polythene and metal

(bicycles, box section, steel bars, mattress springs etc), generally in a matrix of sandy ash with brick,

coal, slag and concrete. evidence of green waste (topsoil, roots, stems, wood and garden waste) was

noted at depth in some locations below the estate. The waste moisture is variable with areas that

are dry and other areas where it is wet. This gives variable gas conditions. There is evidence of

hydrocarbon contamination in the waste materials.

The gas conditions are:

• Methane - variable up to 87.5%v/v;

• Carbon dioxide - variable up to 16.0%; and

• Flow variable from -8litres/h to 8litres/h with no evidence of sustained gas pressure in the

ground.

Although sub slab monitoring was not completed it is considered there are elevated gas

concentrations immediately below the slab causing gas migration by diffusion into the building.

The buildings had been in use in this condition for over 40 years without any incidents. Monitoring of

gas ingress at the cracks over a five year period showed that although gas was migrating through the

cracks it was at a very slow rate that was easily diluted by ventilation in the buildings.

A maximum of 100ppm flammable gas was recorded in ambient air, discounting results that were

caused by the internal operation as an engineering works (use of solvents and paint spraying which

caused readings up to 300ppm). At joints or perimeter crack of the slab and at electric cable entry

points concentrations up to 10,000ppm flammable gas were recorded with a flame ionisation

detector (FID) immediately at the point of ingress (0mm). This attenuated over a short distance for

example to 3000ppm at 10mmfrom the crack and 300ppm at 20mm from the crack which

represents ambient conditions (ambient readings due to paint spraying were 300ppm).

At no point did the ambient concentration in the occupied space exceed 100ppm (0.01%v/v) unless

it was caused by internal operations such as paint spraying. Thus, even with an excessively cracked

floor slab and waste immediately below, the attenuation factor between soil gas and the ambient

internal air concentration is at least 1000. Internal sources such as boilers, use of solvents, etc often

gave higher internal levels of flammable gas than that measured around the cracks.

The CSM for the site is shown in figure A1.

Figure A1.1 - 1 CSM for the site

Appendix 2 - Summary of borehole standpipe gas concentrations and predicted and measured

surface emission rates on various sites

Site Source of gas Gas concentration

(%)

Surface emission rate of methane

(l/h/m2)

Methane Carbon

dioxide

Maximum predicted

from borehole

hazardous gas flow

measurements and

Pecksen (1985)

assumption

Measured

in flux

chamber

Australia,

NSW

Engineered fill

material

comprising:

- reworked natural

soils, sandy clay,

clayey sand and

shale. TOC 0.1% to

0.9%

- sand with clay/silt,

traces shale and

sandstone gravel,

and traces of metal,

plastic, concrete

and wood

fragments); and

- sandy clay, with

shale and sandstone

gravel, and traces of

metal, plastic,

concrete and wood

fragments. TOC

0.1% to 1.1%

0.1 to

39.7

10.9 to

50.4

0.7 <0.0003

Remediated

landfill in

Reading, UK

Engineered fill

material

comprising:

- reworked natural

soils, sandy gravelly

clay TOC 0.05% to

0.98%; and

- clay with

occasional paper,

timber, plastic,

metal textile and

brick.

TOC 2.43% to 3.74%

0.1 to

76.8

0.1 to

36.1

1.53 0.0006 –

0.071

Site Source of gas Gas concentration

(%)

Surface emission rate of methane

(l/h/m2)

Methane Carbon

dioxide

Maximum predicted

from borehole

hazardous gas flow

measurements and

Pecksen (1985)

assumption

Measured

in flux

chamber

Remediated

landfill with

thick

capping

layer,

Sussex, UK

Engineered fill

material

comprising:

- capping layer 2m

thick comprising

clay with TOC <1%

overlying

- residual landfill

material up to 7m

thick including

wood and trapped

reservoir of

historically

generated gas

0.1 to

91.8

0.1 to

6.9

2.2 0.001 to

0.21

Old landfill

site,

Midlands,

UK

1960’s landfill

material comprising

ash, slag, bricks,

glass, paper, wood

and metal.

Hydrocarbon

contamination

0.1 to

63.3

8.7 to

19.0

0.012 <0.00022

to 0.001

Notes 1. Monitoring in underfloor voids of building detected mean (sustained) methane concentration between 0.07% and 0.1% which is well below the design value of 0.25% allowed for Green by NHBC, 2007.

Appendix 3 - Implications from monitoring of gas flow from compacted fill

If flow rates and gas concentrations in monitoring wells are sustained and do not vary with

barometric pressure then pressure driven flow from the source is likely to be dominant. When gas

concentrations and flows respond to changes in barometric pressure there is not likely to be any

underlying pressure driven flow (ie gas flow towards the surface is limited by diffusion) and

barometric pumping may influence gas concentrations and flow rates measured in monitoring wells.

Sladen et al (2001) suggested that rapidly varying concentrations of gas in the ground are indicative

of low gas emission rates. The conceptual model for this site is shown in figure 3.1. The results of gas

monitoring are shown in figures 3.2 and 3.3. the wells were installed in compacted fill materials The

fill material was a stony cohesive Class 2C material (table 6/2 Specification for Highway Works)

compacted to a method specification in accordance with the Specification for Highway Works

(275mm layers and 9 passes of the roller). The majority of the material was compacted at a moisture

content wet of optimum. The overall total organic content of the material (TOC – takes account of

wood and paper based on forensic descriptions and TOC tests on the fine soil fraction) varied from

1.12% to 7.74% with a mean of 2.5%.

Elevated monitoring well borehole flow rates (ie total ground gas flow) were initially recorded and

there was no variation with atmospheric pressure. The flow rates are caused in part by excess pore

water and air pressure in the compacted clay fill as well as short term gas generation. As the excess

pore pressure dissipated, the fill settled and short term gas generation reduced, the flow rates

reduced and eventually dropped below 5 l/h. After this the flow rates remained below 5l/h but were

variable. Importantly after the flow rates had reduced to less than 5l/h it was noted that bubbling of

gas out of water in the wells had stopped. This was considered to mark the change from pressure

driven to diffusive flow.

Figure A3.1 - Conceptual site model

Figure A3.2 - Change from pressure driven to diffusion driven flow

Figure A3.3 - Change from constant to variable gas concentration at change from pressure driven

to diffusive flow

At the same time as the borehole flow rates (total gas flow) dropped below 5litres/h the methane

concentration which had been consistently at around 60% started to vary significantly (as did carbon

dioxide which had been around 40% consistently). Additionally, the behaviour of the gas

concentrations during well purging changed once flow rates dropped below 5litres/h. When flow

rates were high, methane concentrations did not reduce over 10 minutes of purging. After flow rates

dropped to less than 5litres/h there was a significant reduction in concentration over 5 minutes

sampling (figure A3). The declining trend reversed for the last three monitoring rounds simply

because of the much lower gas concentrations that were initially being recorded.

Furthermore, surface emissions surveys, flux chamber tests and monitoring of underfloor voids all

showed that the emissions from the ground were minimal.

Figure A3 - Change over time of reduction (%) of methane concentration during pumping

Appendix 4 - Barometric pumping

The mechanisms for barometric pumping are discussed in Claire Technical Bulletin TB17 (Wilson et al

2018). Analysis of barometric pumping in a shallow unsaturated zone of 2m depth has been

completed following the method described by Hemp (1994). This shows that a sustained pressure

difference between the soil atmosphere and fresh air is only likely to develop in less permeable soils

with a permeability less than 1x10-6m/s (figure A4.1). More permeable soils provide less resistance

to air or gas flow and a differential pressure cannot develop. In less permeable soils a pressure

difference can develop. Thus, in less permeable soils there may be a correlation between flow rates

and atmospheric pressure. This may be incorrectly attributed to gas flow increasing during periods of

falling atmospheric pressure. The main mechanism that causes the variation can be dilution of gas in

the unsaturated zone when atmospheric air is forced into the soil and dilutes the gas that is present

in the soil pore space. Thus, variations in gas concentration with barometric pressure indicates low

flow and low surface emission rates.

a) Pressure lag K=10-7m/s

b) Pressure lag K=10-6m/s

c) Pressure lag K=10-5m/s

Figure A4.1 - Variation of soil pressure in response to barometric pressure, 2m deep unsaturated

zone

In much of the technical guidance it is stated that worst case gas emissions occur during low or

falling atmospheric pressure. Auer et al (1996) suggested that the direct advective effect of changes

in atmospheric pressure has comparatively little effect on the rate of contaminant transport out of

the ground. Therefore, the reason falling atmospheric pressure is associated with ground gas issues

may be because it causes ground gas to expand and come out of the ground (as described in the

main text regarding the incident at Loscoe - Ryan et al, 1988).This is only likely to occur via open

pathways such as open fractured rock, highly permeable gravel, mine shafts or similar. The ground

gas will only expand if it is affected by a reduction in pore pressure caused by the reduction of

atmospheric pressure. The extent to which such a reduction in pore pressure occurs is a function of

the soil’s permeability. If it is affected, then there will be resulting advective flow. Falls in

atmospheric pressure are not likely to cause significant gas emissions from soils that contain small

volumes of gas and have relatively low permeability. Auer et al did find that the effect of barometric

pumping could increase diffusive flow of gas towards the surface.

Analysis of soil gas expansion from a 2m deep unsaturated zone in response to variations in

atmospheric pressure is shown in figure A4.2. The analysis is based on the paper by Auer et al (1996)

which assumes that as barometric pressure changes the ground “breaths” which corresponds to an

up and down motion of the soil air. The figure shows the displacement of theoretical markers placed

at various depths below ground level in response to changes in barometric pressure. The markers

represent the layers of gas at various depths in the soil. Gas emissions from the ground will only

occur if the layer at a given depth rises and intersects the 0m line. The analysis for a soil with

k=1x10-6m/s shows that the layer of soil air at 0.1m depth does not intersect the 0m line and thus

gas surface emissions would not occur via flow through the soil matrix.

Figure A4.2 - Depth of soil gas displaced from the ground during barometric pumping

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