This document discusses the use of non-invasive geophysical techniques to build 3D ground models and reduce the cost and risk of site investigations. It provides examples of using techniques like MASW, CSWS, GPR to map soil properties and depth to bedrock in order to develop detailed geotechnical models. Case studies describe creating models for a sewer project in Langat, Malaysia and a landfill project in Western Australia, allowing identification of problem areas without extensive drilling and reducing costs. The document concludes that geophysical techniques provide a cheap and quick way to gather valuable site information.

1. WE PUT YOU ON SOLID GROUND
www.infratecheng.com
19th Southeast Asian Geotechnical Conference & 2nd AGSSEA Conference (19SEAGC &
2AGSSEA) Kuala Lumpur 31 May – 3 June 2016
The use of Non-Invasive Geophysical
Techniques to build 3-Dimensional ground
models and reduce the cost and risk of site
investigation
Sam Setchell, Muhammad Azrief bin Azahar,
Qusanssori Noor bin Rusli and Nick Lowe

2. Presentation Outline
 Site Investigations
 Geophysical Techniques
 Towards 3D models
 Case Studies
 Langat Sewer, KL
 Meru Landfill, Western Australia
 Cominco Phosphate Project, Republic of the Congo

3. Site Investigations
 Geotechnical Hazards present a major project
risk.
 Site Investigation – Relies on expensive drilling to gain
information on ground conditions
 ‘Point’ data means a conservative approach much
be taken
 More Data = Better geotechnical model = Lower
Risk
 Non-Invasive techniques – to reduce risk

4.  Geophysics - ideally suited to
geotechnical site
investigations
 Provides Engineering
Parameters without the need
for drilling
 Quickly and cheaply maps
variability – allows refined
and reduced drilling
 Can be converted into 3D
ground models
Geophysical Site Investigation

5.  Multi Channel Analysis of Surface
Waves – MASW
 Measures 2D profile of
compressibility / strength of soils
and rocks
 Continuous analysis of surface
waves – CSWS
 Measures 1D profile of
compressibility in high resolution
 Ground Penetrating Radar – GPR
 High resolution 2D scans that
quickly highlight depth to
features as well as voids.
Geophysical Techniques pt. 1

6.  Gravity Techniques
 geological features, man-
made structures and
voids
 Electrical Resistivity
Tomography (ERT)
 Measures 2D profile of
resistivity of sub surface
materials
 Ground Conductivity – EM
 Magnetics
Geophysical Techniques pt. 2

7. MASW Technique
 The MASW Technique utilises surface
waves elastic condition (stiffness) of the
ground for geotechnical engineering
purposes.
 A 2D Map of ground stiffness – in terms of
sear-wave velocity (Vs) is created

8. 3D models of ground conditions can be
created by non invasive techniques

9. Multiple lines used to create a 3D
survey

10. Development of Geotechnical Model

11. Calibrating the Model
Compressibility
High Medium Low
SPT N-Values from
subsequent drilling
investigation
Some lag, but otherwise good agreement between MASW and
SPTn
Compressible
layers lead to
rapid reduction
in SPT n

12. Depth to Bedrock
Use of CSWS technique
allowed identification of
variable depth to
bedrock under
residential development
in Fremantle.
Surface represents
inferred top of bedrock.
This was then verified
with just 1 borehole

13. Detailed Case Study – Langat Sewer
 Study to measure the
depth to bedrock along the
alignment of a proposed
sewer
 Directional drilling to be
undertaken → strength of
soil very important
 CSWS and SASW used to fill
in gaps from pre-existing
bore hole data.

14. Langat Sewer
Advantages of CSWS/SASW over
drilling:
 Data could be collected from
the actual pipeline alignment –
drilling had been taking place
up to 50m away.
 10 tests a day – much greater
coverage of data
 Little disturbance to traffic
 Cheaper for the client

15. Example Profile

16. Example Report Output
Profile
Location of tests on
aerial photo
Location of tests in 2D
plan view (red line
shows profile)

17. Detailed Case Study – Langat Sewer
 Profiles were produced for
the whole sewer alignment
 Amount of data collected
allowed client to make
decisions and reroute some
areas of pipeline
 Over all a successful
application of a
geophysical method!

18. Meru Landfill – Using
geophysics to plan location
 Limestone outcrops had been found on the site of proposed landfill
development
 The previous landfill cell encountered unforeseen rock at shallow levels,
resulting in a significant cost overrun (for excavation)
 A 4 month drilling campaign was proposed to identify the extent of this
limestone with an estimated budget of A$650,000.00 (~2 million ringgit)
- almost 3 times what was allowed.
 The critical aspect was to get complete (or close to complete) 3D
coverage across the site.
 The MASW technique (Multi Channel Analyses of Surface Waves) was
chosen by Infra Tech.
 Completed within a budget of A$75,000.00. (~232,000.00 ringgit)

19. Detailed MASW Survey – 13
lines

20. 2D MASW output
Evidence of
some rock
material
(known to be
limestone)
shown in
Black/Red

21. 2D to 3D conversion
Fence diagram
shows clear
band of rock
material

22. Blocks representing
‘limestone’ can be isolated
and an estimation of volume
of limestone material can be
made – all before drilling!
3D Block Model

23. Presentation Conclusions
 Inadequate site investigations can lead to project
failure (or worse)
 Geophysical techniques provide a cheep and quick
way to gather information
 Add value by:
 Reducing Risk – better site understanding
 Reducing drilling cost
 Creating more accurate ground models

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FASTER, STRONGER, CHEAPER, SAFER, GREENER