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NEC2013
Engineers Australia, Queensland
Geotechnical Quality Assurance by Using Geophysical Seismic Methods
Jeremy Fredericks and James Tayler (Authors)
Earthsolve
171 San Fernando Drive Worongary Qld, Australia
Email: [email protected]
Abstract— Geotechnical testing is mostly dominated by
discrete methods such as drilling boreholes, different types of
penetrometer testing and common compaction test
methods. These methods have served the engineering
profession for over a hundred years and widely accepted.
New geophysical methods of testing are now available. These
geophysical methods are revealing deficiencies in our current
physical methods as well as providing geotechnical insight
into common geotechnical problems on building sites. These
geophysical methods are revealing how to view the site as a
whole as opposed to the discrete snapshot view testing
methods now being used. These geophysical methods are
proving to be quick and thus economical as well as
maintaining accepted geotechnical accuracies. Based on
research, it is possible to obtain material properties from
seismic results. It will take time for geotechnical engineers to
become familiar with these new methods of testing, but
results using these geophysical methods indicate that the
time has arrived.
Keywords-MASW, refraction, filling, pier, codes
I.
INTRODUCTION
What is seismic technology
Seismic technology is the use of seismic (‘sound’) waves to
detect different materials sub-surface. Each different material
has characteristic seismic velocities. It is non-intrusive and in
broad terms, the faster a ‘sound’ waves moves through a
material, the stiffer the material.
A.
Refraction
To employ the refraction method, one needs a souce of
‘sound’. The best is a 10kg sledge hammer. There are others
such as a shot gun blast or dynamite. The sledge hammer is
the cheapest and most convenient (but not necessarily the
most exciting).
However sound of penetration is usually only about 10metres
below ground surface. This is usually adequate for urban
work.
The refraction method (using the compression Vp wave) is
based on the fact that when ‘sound’ travels from one
medium to another, the waves refracts. A ‘sound’ is made
with a sledge hammer and a ‘sound’ travels through the
various soils, travel times are automaticly recorded as they
are detected by geophones. A simple mathematics procedure
is followed, resulting in the depths of the various soil and rock
layers sub-surface.
Disadvantages of the method is that (a) a water table can
negatively affect results (but as a result we use refraction to
detect the water table), (b) the method relies on the fact that
material layers are getting stiffer therefore stronger with
increased depth. Therefore any soft layers are hidden from
the interpretation. A noisy site also causes difficulty. A
common result from a refraction survey is a cross section
equal to the length of the array and about 10m depth.
In this paper, discussion will be limited to two methods. One
is the refraction method and the other is MASW method,
(multi analysis of surface waves). There is another method
called reflection, but its application is not so applicable for
house size sites and will not be discussed in this paper.
Fig. 1 Typical Refraction Wave Results
B.
MASW
The MASW method involves using surface waves (Vs). These
waves are slower than Vp waves. Most new development in
NEC2013
Engineers Australia, Queensland
seismic is currently in the MASW method. The ‘sound’ waves
are obtained from active and passive sources. Active sources
can be by using a sledgehammer and passive sources
secondly by listening to the natural ‘sound’ waves in the subsurface generated from tidal motion, thunder and cultural
‘sound’ from traffic noise. Typically, the analysis of the upper
layers depends on the use of ‘sound’ from a sound source, eg.
A sledgehammer, but analysis of the deeper layers relies on
listening to the natural ‘sound’s sub-surface. There is an
advantage over the refraction method, because the result is
not limited by the strength of the ‘sound’ from the active
sound source. The depth of penetration of the survey is
usually about half the length of the array, using the active
method, but can be about equal to the length of the array
using the passive method. A typical result from a MASW
survey is as below. In the cross section graph, the length of
the array long and a depth are about equal. So if the array is
30m long, one can detect about 30m deep, using the passive
method. The actual depth depends on the site materials. The
active method is acknowledged to provide better resolution,
especially in the upper layers. The MASW results are
unaffected by water table and also show any soft layers.
Voids and boulders can be imaged as well. A noisy site is not a
major problem using MASW the method. Noise can assist
when using the passive method for deep layers. One
disadvantage is that the site levels must not vary by more
than about 10%. Output is conveniently often shown as flat
surface but level variations may be introduced using imaging
software.
real results showing the seismic vibration properties of soils
and rocks in-situ. Question like ‘How do the results compare
with real DCP (dynamic cone penetrometer) results or SPT
(standard penetration test) results’ need consideration. The
consideration is whether DCP or SPT results are accurate. The
best you can say is that DCP & CPT (cone penetrometers test)
results are an indication of soil strength, usually based on
ruptured soil correlations. In marine and diluvium clays, DCP
results are usually inaccurate representation of the soil
strength (because of shaft friction). The reason we use DCP
and SPT is because it is cheap, compared to other methods
like CPT and DMT (dilatometer). Seismic results, represent
drained soil properties. The amount of information that the
seismic method produces is considerable, and except for
minimalistic testing program, the seismic method should be
very cost effective, is faster user friendly, with easy to
understand graphics. One more note, it needs to be firmly
understood that MASW output is averaged vertically and
horizontally and interpolated. Compare with DCP and the like,
where results are discrete. Three dimensional imaging is
highly interpolated and the imaging can be misleading. We
always recommend borehole testing to verify the seismic
results. MASW is deemed to not have systemic problems, but
depth of penetration can be variable depending on the subsoil profile.
II. SCREW PIER FOUNDATION DEPTHS
The screw pier story
Every so often, something new changes the way we build
things. In the 1960’s, it was the invention of the gang nail
plate. This product changed the way we build roofs by
making roof trusses economical to build. In the 1990, the
screw pier came on the scene in Australia. Screw piers have
become very popular for construction of foundations. They
have gained a ready market in the housing construction
industry. The reason is because they are unaffected by clay
soil reactivity, as well as being fast to install and cost
effective. The advent of the screw pier has created a need to
obtain suitable foundation data. Seismic technology is well
placed to fulfill that need.
B.
Fig. 2 MASW –Surface Wave graph-showing shelf rock
D.
Criticism of the Seismic Method
Criticism is often cast towards the seismic results as
engineers compare the results with analogue (borehole)
results. A question often asked is whether the equipment
requires calibrating. Another question is whether the
engineer may have been better advised to spend the money
on extra good boreholes. Our response is that one does one
know when one has good borehole data. Seismic results are
AS2870-2011
AS2870 is a standard that considers foundations with respect
to soil reactivity. Soil reactivity is mainly irrelevant to screw
piers and this standard does not mention seismic technology
as a testing method. I do not know of any Australian codes
that include MASW seismic technology. Seismic is mentioned
in EURO codes.
C.
Current Soil testing
Consider that when a soil tester is engaged to soil test a site,
the soil tester’s view is to comply with current Australian
standards of soil testing. However, the engineer who engaged
the investigation wanted sub-surface information in order to
design a suitable foundation system for a building. This is the
best we can do currently, but it needs to be understood that
NEC2013
Engineers Australia, Queensland
what the engineer and the soil tester is trying to achieve may
not be the same. If the foundation design involves screw
piers, soil reactivity becomes irrelevant, but bearing capacity
at depth becomes relevant. A common method to interpret
soil strength is by the DCP. The DCP provides an indication of
the strength of soils only. In certain soils like saturated clays,
the method is unsuitable. DCP results are affected by gravels,
but the method is used, because it is cheap. Soil testers will
submit a soil test where penetration was not achievable or
DCP data was not available because of gravel, cobbles or
boulders in the sub-surface matrix.
D.
Site Investigations
Currently soil testing is the norm for a site investigation. To
the future, the writer expects seismic methods will be at least
an alternative and even the preferred method. The reason
will be the depth and quantity of information made available
for price. With one seismic survey, the engineer will get a
section through the block and no compromise on depth of
investigation. Seismic data should be supported by borehole
data if the soil conditions of the area are not well known.
With housing, borehole data will be required to meet code
requirements of AS2870. For the future we hope that codes
will be updated to include geophysical methods.
E.
Limitations of the Borehole Method
The borehole method is the norm, and it is usually supported
by DCP data. SPT, CPT and DMT usually cost much more and
therefore not used except for the more difficult jobs. The
engineer is reliant on the expertise of the driller. Positioning
of the boreholes, interpretation of the soils and execution of
the DCP or SPT are all dependent on the driller. Penetration
using the borehole method is not always possible, because of
gravels or boulders and DCP results can be greatly inaccurate
(because certain soils stick to the the DCP shaft and the DCP
reading is a reflection of soil friction). Most engineers would
have had the experience ‘late Friday afternoon’ job where the
driller has totally missed correct interpretation of the soil
profile.
F.
Seismic software computer output
Once data is collected from the site, it is entered into seismic
computer software programs. The output is in the form of
four graphs.
1)
2)
3)
4)
A refraction Vp wave section.
A graph showing the average SPT N values over the
length of the array line.
A graph showing a section similar to the refraction
section, but with the layers established from surface
waves.
A graph similar to the above showing a section but
converting the surface waves speed to SPT N values,
so soil and rock strength can be indentified across the
section.
III.
CASE HISTORIES
Slope Stability
One of the first considerations with slope stability is the
presence and location of colluvium. Using analogue methods
is always difficult to obtain conclusive data. There are
problems with boulders and if an excavator is used,
penetration depth is limited. A 20tonne excavator will
achieved only about 5metres excavation depth. Using
seismic, the colluvium layer often is so identifiable and
penetration depth is no problem.
Fig. 4 Top (Pink) layer is a cross section through a shallow earth flow on a
slope of 25 degrees. Bottom (Green and Blue colour) layers are rock. Notice
the ancient watercourse under the earth flow. Site B
A.
Filling
A common feature of construction is filling. More emphasis is
being applied concerning consideration of the state of
compaction of the filling. Regulatory bodies, have policies
that require engineers to prove satisfactory compaction of
filling. A compaction certificate is not adequate. Seismic is
providing a role in providing an image of the filling and
showing graphical zones of weakness. A whole filling platform
can be analyzed by conducting a grid. Seismic cross sections
can be created.
C.
Subsidence
A certain building was constructed on the close proximity to a
drop-off (a scarp). The corner of the building had subsided
causing severe cracking in the masonry. Because of access
problems, conventional soil testing using boreholes would
have been very difficult. Using MASW seismic, it was imaging
was successful revealing that the rockline was depressed in
the corner of the building. The building could now be
underpinned.
NEC2013
Engineers Australia, Queensland
IV. FUTURE DIRECTIONS
A.
Australian standards
Australian standards such as AS1726 and AS2870 need to be
updated to include MASW seismic methods of site
investigation. Australian Standard AS1726-Site Investigations,
methods’ list includes Seismic Refraction but not MASW
Seismic. Although this standard makes provision for
alternative methods, the AS1726 standard could be updated
to include MASW seismic as a stated method.
Fig. 5 Shows rockline dropping off in corner
D.
Difficult blocks
Certain blocks are difficult to soil test using augers because
the site contains considerable rock fill or cobbles and
boulders. Seismic can penetrate these sites and provide a
usable result.
E.
Preliminary site investigation
A long length seismic scan across a large block can be used to
identify location for borehole testing.
With the high proportion of housing defects arising from
geotechnical problems, it is worthwhile to consider if our
testing regimes can be improved. Australian Standard AS2870
could include Refraction and Seismic as approved methods,
where clay or filling profiles are likely to be significant. An
example of deep clay profile is in alluvium/diluvium
(Quaternary) soil zones. Many newly developed building sites
have significant depth of filling. Home owners are poorly
informed about the ramifications of a Class ‘P’ site unless the
standard can direct them to further testing within code
approved testing envelope of methods. We recommend that
seismic be included as a method into AS2870, and highly
recommended where fill depth exceeds 2.0m depth.
B.
Familiarization with seismic
Engineering professionals need to become familiar with
Refraction and MASW seismic methods and how to
understand the results. Because of the speculative nature of
seismic test methods, results should be verified prior to
construction. This may include installation of boreholes.
Based on the results of seismic methods, the most
economical use of boreholes may be applied.
V. CONCLUSION
F.
Screw pier foundation depth
Seismic technology is finding a very ready application for
establishment of screw pier foundation depth. Considering
that foundation depths can be of the order of 20m, seismic
technology is quick and economical. Screw pier torque is
related to the soil load capacity for the screw pier. A graph
needs to be correlated for each screw pier design.
Geophysical methods such as Refraction and MASW seismic
offer a digital solution to common urban engineering
problems. These methods are offering solutions and insight
where conventional methods may not be offering clear
indication of the sub-surface profiles or access prohibits the
use of other methods. These methods should gain increasing
acceptance within the engineering profession and providing a
better understanding of sub-surface profiles that should
afford more cost effective building solutions.
ACKNOWLEDGMENTS
Fig. 6 Final installed screw pier depths and final torque readings are plotted
on a seismic section to check foundation material.
Thank you to Engineers Australia and the committees of the
Regional Engineering Conferences for making a platform
available for engineering practitioners like myself to make a
contribution to the advancement of better engineering
practices in Australia.
NEC2013
Engineers Australia, Queensland
Biographies
James Tayler was born in Brisbane in 1952,
is an RPEQ and chartered civil engineer
with Engineers Australia. For about 25
years, James has been the senior engineer
of a local engineering consultancy on the
Gold Coast specialising in the structural and
geotechnical disciplines, mainly in the
domestic building sector. Most recently,
James has focused in landslide risk
assessment and remediation.
James
first
worked
with
Frankipile/GroundTest in 1968 as a cadet
engineer and graduated from the QUT in
1976 with Bachelor of Technology-civil and later has worked with the Qld
state government and private consultancies on a wide range of structural
and geotechnical projects, including steel and concrete one and two storey
and high rise concrete buildings.