best practice in ground‐gas investigation and risk

Best practice in ground‐gas investigation and risk assessment

Matt Askin – Senior Consultant, GGS

15th January 2020

Presentation Content1. The ground-gas hazard

2. Key properties of ground-gases

3. Monitoring strategy - Spot Vs continuous monitoring

4. Data interpretation – understanding site specific gas regime

5. Risk assessment & protective measures

6. Case study – Gas regime modification

Ground‐Gas Hazards

Loscoe Explosion, 1986

3 people injured

Gorebridge Incident• New housing estate built in 2009

• Sept 2013 council tenants overcome by gas and taken to hospital. 22 people required medicalattention. Residents moved to alternative accommodation

• April 2014 IMT set up

• 64 Homes demolished in 2016

• Guidance on investigations for ground gas – Permanent gases and Volatile Organic Compounds (VOCs)

• Links together key recent documents into a full British Standard

• Monitoring and sampling of ground gases

• Does not cover Radon

• Updated from the 2007 standard and first published June 2015

• Heavily used by industry (including regulators)

• Revised to: BS8485:2015+A1:2019

• Modified the Points Scoring System

• Amended some criteria for membranes

BS 8485:2015 + A1:2019

Consequence

Like

lihoo

d

Risk Management Matrix

Do Nothing

Manage

Avoid

CS1CS2CS3CS4CS5CS6

Characteristic Situations

Consider source factors:• Gas concentrations?

• Gas flow rate?

• Source generation rate?

• Source volume?

Licenced landfill

Peat deposits

Coal fields

Former landfills Brownfield sites

Breaking the Pathway

Consider the receptors

Residential properties and residents Business properties and office workers

Commercial properties and workers Construction site workers

Preliminary Conceptual Site Model

BS8576 requires cross‐sections for assessing risk from permanent gases

Solid Liquid Gas

Ground‐gas monitoring challenges

Viscosities at STP:• Water 8.9 x 10‐4 kg/(ms)• Air 1.8 x 10‐5 kg/(ms) 50 times lower

Gas migrations and permeability anisotropy

Permeability anisotropy and borehole monitoring

Pressure as a Migration Driver

Massmann J. and Farrier D.F. 1992. Effects of barometric pressure on gas transport in the vadose zone. Water Resources Research, Vol.28, No. 3. 777‐791.

• 2D finite element analysis ‐ 25mb pressure fall over 24 hours • 45m lateral migration within medium sand

Silt – 11m

Fine Sand – 25m

Medium Sand – 45m

Dissolved gases in groundwater

Solubilities at STP:• Methane 25 mg/l• Carbon dioxide 1,450 mg/l 58 times more soluble!

Taken from NHBC guidance

MonitoringWell Design

Monitoring wells are unique scientific instruments

X

Monitoring Well Installation

Made Ground

Topsoil

Natural Ground

Bedrock

Turf

Reduce uncertainty

Spot Monitoring

31 Jan 02 Feb 04 Feb 06 Feb 08 Feb 10 Feb 12 Feb

When the frequency of monitoring exceedsthe frequency of change of the measured parameter, the monitoring can be termed ‘continuous’

Continuous Monitoring

Gas Sentinel® • British designed & built by specialists

for specialists

• Small, light & smart

• Telemetry enabled

• Continuous flow

• Discrete & secure

GGS has the largest fleet of continuous monitoring devices in the UK

0

10

20

30

40

50

60

08 Jul 10 Jul 12 Jul 14 Jul 16 Jul 18 Jul 20 Jul 22 Jul 24 Jul 26 Jul 28 Jul 30 Jul

Met

hane

Con

cent

ratio

n (%

v/v

)

Weekly monitoring on these dates shows almost no methane

Weekly monitoring on these dates shows falling methane

Weekly monitoring on these dates shows rising methane

‘Spot’ & ‘Continuous’ monitoring

Methane

Environmental Correlations

Multi‐parameter continuous data…

Identify or eliminate correlations with the environment

Identify or rule out ground‐gas drivers

Atmospheric Pressure as a Ground‐Gas Driver

985

990

995

1000

1005

1010

1015

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

27‐Dec 28‐Dec 29‐Dec

Pressure (m

illibars)

Metha

ne (%

v/v)

Methane

Atmospheric Pressure

930

940

950

960

970

980

990

1000

Pres

sure

(mba

r)

Atmospheric Pressure

05

101520253035

20 Dec 22 Dec 24 Dec 26 Dec 28 Dec 30 Dec

Con

cent

ratio

n (%

v/v

) Methane

Atmospheric Pressure as a Ground‐Gas Driver

25

30

35

40

45

50

55

16 Aug 23 Aug 30 Aug 06 SepCon

cent

ratio

n (p

pmv)

TVOC

05

10152025303540

Tem

pera

ture

(ºC

)

Temperature

Temperature as a Ground‐Gas Driver

2.502.602.702.802.903.003.103.203.30

Wat

er le

vel (

mbg

l)

Water Level

0

5

10

15

20

25

26 Aug 31 Aug 05 Sep 10 Sep 15 Sep 20 Sep 25 Sep

Con

cent

ratio

n (%

v/v

) O₂CH4

Water Level as a Ground‐Gas Driver

Continuous Monitoring ‐ Time Series Graphs

Solubility, the piston effect and flow readings

Hydrostatic Head

Pressurised Headspace

1. Low pressure weather system passes over site

2. Atmospheric pressure drops

3. Small volume of methane degasses and builds up in headspace

4. Rainfall percolates to water table which rises

5. Hydrostatic head builds up

6. Headspace is pressurised

7. Spot monitoring records:

a) High methane concentration

b) High borehole flow

Risk Profile Changing?

Be careful what you wish for!

Risk Profile Changing?

Get the balance right, including FoS

Gas regime modificationGas monitoring and risk assessment is carried out during a discrete period of time.

What happens after the SI is not always known or considered?

The gas regime may be changed by: • Further SI boreholes

• Site re‐profiling

• Drilling and grouting works

• Piling e.g. vibro‐piles

• Service trenches

Gas regime modification ‐ Case Study

Proposed housing development close to former landfill and coal miningDetailed site investigation, gas monitoring and risk assessment carried out

SI demonstrates high concentrations of methane in:• Former landfill• Former mine workings• Sandstone

• Monitoring wells represent migration pathway

• Over‐drill and grout removes pathway P2


• Site is reprofiled to cap landfill• Mine workings are drilled and grouted• Phase 2 public open space and more housing area created


Service trenches for sewers now penetrate through Boulder Clay to sandstone• Service trenches need to be sealed• R1 house again need high levels of gas protection


CL:AIRE TB18 ‐ 2019

• Best practice guide built on over 12 years experience of continuous monitoring

•Over 500 projects reviewed

•How to integrate continuous data into your CSM

•Develops the ‘Lines of Evidence’ approach

“Ground‐gas – a geological problem with an engineering solution”

Thank you

Matt Askin – 07841 009 343matt.askin@ggs‐uk.comwww.ggs‐uk.com

best practice in ground‐gas investigation and risk

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