best practice in ground‐gas investigation and risk
TRANSCRIPT
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
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
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
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
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
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!
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
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
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
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
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
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
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
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
Gas regime modification ‐ Case Study
• Site is reprofiled to cap landfill• Mine workings are drilled and grouted• Phase 2 public open space and more housing area created
Gas regime modification ‐ Case Study
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
Gas regime modification ‐ Case Study
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