EU LIMASLiquefaction around marine structures

Workpackage 7, NTNU Trondheim

Development of soil sampler for measurement of gas content in soils

Methods for gas measurements

By associate professors Rolf Sandven (NTNU) and Mike Long (UCD)

EU LIMASLiquefaction around marine structures

Soil - water gas systems (Hrydech et al (1987))

Sr = 5-9 % Sr = 9 - 70 %

Sr > 70 %

EU LIMASLiquefaction around marine structures

Effect of gas on soil characteristics

Sample disturbance due to gas exsolution and expansion

Influence on soil parameters

– shear modulus– bulk modulus

– strength and pore pressure parameters

– consolidation parameters Influence on geophysical

properties– wave velocity– magnetism

EU LIMASLiquefaction around marine structures

Detection of gas in soil samples

Computer tomography (CT) scan imaging

Scanning Electron Microscope (SEM) techniques

Uni- or multisensor scanning devices– sonic wave velocity– gamma-ray– magnetism

Volumetric measurement during water absorption

EU LIMASLiquefaction around marine structures

X-ray computed tomography set-up (Desrues et al)

sample

EU LIMASLiquefaction around marine structures

CT scan results

Abegg et alVolume of gas bubbles

Wong et alSand sample with variable air saturation

EU LIMASLiquefaction around marine structures

Research team

Scanner Resolution Comment

3S-IMG ND 8000 medical

0.7 mm x 0.7 mm Resolution set 4 mm slice

CALGARY 0.75 x 0.75 mm 3 mm slice

BOULDER 0.2 mm x 0.2 mm 10 mm slice NASA equipment

KIEL Elscint CT Twin

1.0 mm x 1.0 mm 1.00 mm -> 0.85 mm slice

Research work also available from other institutions, e.g. DTU, Denmark

Examples of CT soil research data

EU LIMASLiquefaction around marine structures

Scanning electron microscope (SEM)– Impregnation of specimen

sections using special resin– Imaging of thin sections

using SEM techniques– Computerised image

analysis is used to measure void sizes and distributions

– Obtainable resolution in the m - scale

Soil containing gas bubbles (Sills et al)

EU LIMASLiquefaction around marine structures

Multisensor core logger Combines a number of

parameters to derive a picture of the core– gamma ray– shear wave and/or

compression wave velocities

Software processing of image

Geoscan II image

EU LIMASLiquefaction around marine structures

Multi-sensor core logger with output results V M

sample

EU LIMASThe Geotek autoclave logger system

EU LIMASLiquefaction around marine structures

Use of backpressure techniques

Core sample is pressurised in pressure cell before backpressure is applied

Sample can absorb de-aired water

Precision measurement of water absorption for increasing pressures indicates gas volume

EU LIMASLiquefaction around marine structures

In situ gas detection– BAT probe for

sampling of water and gas

– CPTU based modules• seismic CPTU• resistivity CPTU

– Spectral analysis of surface waves (SASW)

EU LIMASLiquefaction around marine structures

Gas detection Preliminary

conclusions

Transportable and flexible pressure systems emphasised for common use– measurement of water volume

change during application of back-pressure

Selected samples or projects– use of X-ray, CT or MRI

imaging methods – indirect methods based on

influence on soil parameters• shear wave or compression

wave velocity• needs soil specific

calibrations

EU LIMASLiquefaction around marine structures

Use of backpressure saturation technique

Core sample is pressurised in pressure cell before back-pressure and a flow gradient is applied

Sample can absorb de-aired water

Precision measurement of water absorption for increasing pressures

indicates gas volume

Back- pressure

supply

Inlet valve for deaired water

Scaled precision burette

Sample with gas bubbles

Ball valve closure