NEC2013 Engineers Australia, Queensland
Earthsolve
171 San Fernando Drive Worongary Qld, Australia
Email: earthsolve@bigpond.com
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.
What is seismic technology
I NTRODUCTION material, the stiffer the material.
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
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.
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.
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 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.
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
Engineers Australia, Queensland 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. 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 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) A refraction Vp wave section.
2) A graph showing the average SPT N values over the length of the array line.
3) A graph showing a section similar to the refraction section, but with the layers established from surface waves.
4) A graph similar to the above showing a section but converting the surface waves speed to SPT N values,
Engineers Australia, Queensland 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
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.
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.
Fig. 6 Final installed screw pier depths and final torque readings are plotted on a seismic section to check foundation material.
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.
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.
C ONCLUSION
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
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
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.
Engineers Australia, Queensland