Advanced Ground Investigation Techniques to Help Limit ... · • GPR is a continuous scanning method where as the antenna travels over the ground surface scans are recorded at set

Advanced Subsurface Investigations

Advanced Ground Investigation

Techniques to Help Limit Risk or

Examine Failure


Overview• Introduction

• What is geophysics?

• Why use it?

• Common Methods

• Seismic

• Ground Radar

• Electrical

• Case Studies

• Conclusion


Introduction

What is Geophysics?

“A Section of Earth Sciences that Employs the Principles of Physics”

Large scale – The Universe

Small Scale – Your sites!


Why use it?

• Imaging subsurface

• Geotechnical

information

• Cover large areas

rapidly / cost

effectively


Imaging of Subsurface Features

• Layer profiles

• Bedrock delineation

• Overburden calculations / volume

• Location of buried objects

• Location of geo-hazards


Determination of Geotechnical Parameters

• Profiling of bedrock depth and material hardness for rippability,

tunnelling and piling hardness.

• Assessment of layer stiffness, elastic moduli, liquefaction potential.

• In-situ electrical properties for earthing design soil layer resistivity.


Common Methods

• Seismic

• MASW

• Refraction

• Ground Radar / Geo Radar / GPR

• Electrical methods

• EM Conductivity

• Resistivity


MASW

• Multi channel Analysis of Surface Waves (MASW)

1. Reasonably new technique (6 – 7 yrs old)

2. Gaining recognition and acceptance as a Geotechnical tool

3. Can be collected as a continuous method

4. Provides information of layers and layer stiffness as 1d or 2d Vs profiles

5. Can use to calculate Poisson ratio and densities of layers

6. Is only good for investigation to approximately 30m


Time to learn something!!




1d Vs profiles


2d Vs profiles


CPT correlation using MASW


example from Pilbara Rail line Culvert Failure


Ground Penetrating Radar – What is it?

• A shallow geophysical investigation method some 40yrs old.

• Is as it sounds ! Radio waves or EM energy of specific

frequencies are pulsed into the ground 1000’s of times per

second.

• The energy transmits through the ground and is reflected back

to a receiver on the surface when there is a change in the

electrical properties of the material eg: soil to metal or plastic

pipe.


Tx Rx

GPR Antenna

Ground surface

Energy

-1 0 1

Time

GPR Trace

Impulse Radar (GPR)


-1 0 1

GPR Trace

Tx Rx

GPR Antenna

Ground surface

Energy

Time

Impulse Radar (GPR)


-1 0 1

GPR Trace

Tx Rx

GPR Antenna

Ground surface

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Time

Impulse Radar (GPR)


-1 0 1

GPR Trace

Tx Rx

GPR Antenna

Ground surface

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Time

Impulse Radar (GPR)


-1 0 1

GPR Trace

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GPR Antenna

Ground surface

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Time

Impulse Radar (GPR)


-1 0 1

GPR Trace

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GPR Antenna

Ground surface

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Impulse Radar (GPR)


-1 0 1

GPR Trace

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GPR Antenna

Ground surface

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Impulse Radar (GPR)


-1 0 1

GPR Trace

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GPR Antenna

Ground surface

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Time

Impulse Radar (GPR)


-1 0 1

GPR Trace

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GPR Antenna

Ground surface

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Impulse Radar (GPR)


-1 0 1

GPR Trace

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GPR Antenna

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Impulse Radar (GPR)


-1 0 1

GPR Trace

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GPR Antenna

Ground surface

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Impulse Radar (GPR)


-1 0 1

GPR Trace

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GPR Antenna

Ground surface

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Impulse Radar (GPR)


-1 0 1

GPR Trace

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GPR Antenna

Ground surface

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Impulse Radar (GPR)


-1 0 1

GPR Trace

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GPR Antenna

Ground surface

Energy

Time

Impulse Radar (GPR)


-1 0 1

GPR Trace

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GPR Antenna

Ground surface

Energy

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Impulse Radar (GPR)


-1 0 1

GPR Trace

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GPR Antenna

Ground surface

Energy

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Impulse Radar (GPR)


-1 0 1

GPR Trace

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GPR Antenna

Ground surface

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Impulse Radar (GPR)


-1 0 1

GPR Trace

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GPR Antenna

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-1 0 1

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GPR Antenna

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-1 0 1

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GPR Antenna

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-1 0 1

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GPR Antenna

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-1 0 1

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GPR Antenna

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Impulse Radar (GPR)


-1 0 1

GPR Trace

Tx Rx

GPR Antenna

Ground surface

Energy

Time

Impulse Radar (GPR)


-1 0 1

GPR Trace

Tx Rx

GPR Antenna

Ground surface

Energy

Time

Impulse Radar (GPR)


• Depth and resolution of subsurface targets is frequency dependant.

Lower the frequency the better the penetration but the larger the target

has to be to observe a reflection from and vis versa.

• GPR is a continuous scanning method where as the antenna travels

over the ground surface scans are recorded at set intervals of between

10mm to 500mm depending on what subsurface information is being

sought.

• Data is digitally recorded at a minimum 16 bit resolution, very large

files often result from individual profiles.

• Targets are identified by virtue of their shape, amplitude and phase as

recorded reflections.

• Depth to targets is calculated from the time taken for the energy to

travel too, and be reflected back from the target. This time is multiplied

by the velocity of propagation of the radio energy through the material

between antenna and target and divided by two to give a distance

below the antenna.


Darwin East Arm Wharf


Issues!

• Sheet pile structure with back fill local sands

• Failure of surface pavement

• Voids occurring around buried infrastructure

• Possible lack of compaction of material during

construction

• 250m section to investigate

• Used Ground Radar and MASW



Ground Radar Profiles


Results


MASW profiling


MASW Profile


Conclusions

• There is a defined low compaction layer directly under the

pavement and it is this that is moving around and appears in

some locations to be extending deeper under the structure.

• This suggests a possible issue with material loss below the

water line (tide range here is 7m) and movement through the

compacted back fill behind the steel sheet piles.

• Alternatively, the back fill material may not have achieved

compaction during the construction and has since settled and

created voiding that has allowed further movement.


Sid Enfield Drive

• Reinforced Earth wall supporting major road

• Concrete interlocking panels containing

compacted sand backfill supporting road

pavement,

• Sand leaking from behind panels

• How big are possible voids?




Noticeable increase in

amplitude and return

reflections – increased voiding

Vertical GPR Profile


Results


Conclusions

• The sand movement is occurring within the first 400mm

behind the concrete panels and is not effecting the competency

of the reinforced earth straps.

• The sand / fill material deeper in the structure is still

compacted and supporting the pavement above.

• The issue appears to be legacy from construction where

inadequate compaction was achieved right up against the rear

face of the panels. Over time vibration from the heavy traffic

above has loosed the sand which is finding its way out through

perished joints.


Dam Site Spillway, Qld


Issues


Construction



Local Geology


Investigation with Ground Radar

• Area of spillway was 125m x 80m = 10,000m2

• Slopes 1 in 10 and 1in 2

• Temperature +36°

• Cores limited across the site

• Need to locate voids and undertake investigation of weep drain

pipes with CCTV.

• Provide interpreted report and conclusions




CCTV


Results


Conclusions

• Weep drainage poorly designed

– Main long drains sitting above cross drains so do not continuously

drain. Water sits in drains and sediment not flushed

• Soft friable sandstone / mudstone bedrock possibly dissolving

under the gravel drainage layers – evident in drains.

• Removal of the bedrock allows gravel to settle as uncontained

causing voids under slabs and movement of clay pipe sections

with open joints.

• Voiding not excessive at present and does appear to be

predominantly in the 1 in 2 sloped section.


EM Conductivity Testing

• Method induces current into the ground through the use of an

electromagnetic field at set frequencies.

• Measures the variation in the recorded current caused by the effect of the

ground coupling and decay of conductive response.

• Variations in the sub surface materials poor spaces say from loose gravels,

sands to clays will effect a recordable response.

• Can be used to map large areas quickly to look for:

– buried material – landfill, drums.

– Geomorphology – karstic topo / sink holes

– Changes in lithology – sands and gravels with clays

• use multiple frequency system to create a profile from different depths to a

maximum of 10 – 15m


Rockhampton Levee Investigation



Ground Conductivity for soil assessment along

Levee, Design Route


Conclusions

• Technological advances in computing and modern mobile

electronics have never made it easier of cheaper to undertake

investigations in the field and get good repeatable information

recorded digitally for analysis and interpretation.

• Geophysical testing methods can provide a much “bigger

picture” investigation technique which, when tied with

physical testing for cross correlation is able to fill in the gaps

and provide more confident interpretation of subsurface issues.

• The methods discussed here are rapid and cost effective

solutions that can be applied to very large or very small scale

investigations.


Questions?

Documents

Advanced Ground Investigation Techniques to Help Limit ... · • GPR is a continuous scanning method where as the antenna travels over the ground surface scans are recorded at set