ABSTRACT: Every so often, there are technological events which shift construction methods. The screw pier is one such an example. While the screw pier has solved a problem, it has created a problem, being the need for soil testers to test subsurface to deeper depths. Site investigations are prone to finish at shallow depths because of concern of job cost over-runs. Seismic technology has been used in mining for decades to detect subsurface soil and rock profiles, but its use in urban construction has been slow, at least in Australia. The main reason is probably cost and lack of understanding by engineering practitioners. Every seismic scan can reach depths of 10-100m+ routinely, including rock, so there are no cost overruns. On every site tested using seismic technology, significant extra geotechnical information is achieved concerning the sub-surface conditions,and sometimes, unavailable by other methods. This paper should be seen as a basic introduction to seismic technology only.
1 INTRODUCTION
1.1 What is seismic technology;
Seismic technology is the use of seismic (‘sound’) waves to detect different materials sub-surface. It is non-intrusive and in broad terms, the faster a ‘sound’ wave moves through a material, the stronger the material strength. In this paper, discussion will be limited to two methods. One is the refraction method and the other is MASW method, (multi analysis of surfacewaves). The speed of propagation of ‘sound’ waves through sub-surface materials is a measure of the stiffness of the materials. This then equates as a measure of strength of the materials. 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.
1.2 Refraction:
To employ the refraction method, one needs a source of ‘sound’. The best is a 10kg sledgehammer. There are others such as a shot gun blastor dynamite. The sledge hammer is the cheapest and most convenient but not necessarily the most exciting.However depth of penetration is usually only about10 metres below ground surface. This is usually adequate for urban work.The refraction method (using the compression Vpwave) 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 automatically 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.D isadvantages of the method is that
(a) a watertable can negatively affect results,
(b) the method relies on the fact that material layers are getting stronger with depth. Therefore any soft layers arehidden from the interpretation. A noisy site also causes difficulty. A common result from are fraction survey is a cross section equal to the length of the array and about 10-20m depth.
1.3 MASW
The MASW method involves using surface waves (Vs). These waves are slower than Vp waves. Most new development in seismic is currently in the MASW method. The ‘sound’ waves are obtained from active and passive sources. Active sources can be by using a sledge hammer and passive sources secondly by listening to the natural ‘sound’ waves in the sub-surface 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 sledge hammer but analysis of the deeper layers relies on listening to the natural ‘sound’s sub-surface. There is an advantage here over the refraction method because the result is not limited by the strength of the ‘sound’ from the sledge hammer. The depth of penetration of the survey is usually about equal to the length of the array. A typical result from a MASW survey are two graphs. One graph shows an average SPT (standard penetration test) value per metre depth. The other is a cross section graph, the length of the array long and a depth equal to the length of the array. So if the array is 100m long, one can detect 100m deep. The MASW results are unaffected by water table and also do show any soft layers. Voids can show up as well as larger boulders. A noisy site is not a major problem, especially with deep layers. One disadvantage is that the site levels must be fairly non variant. Output is conveniently often shown as flat surface.
1.4 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. But when does one know when one has good borehole data. Seismic results are real results showing the waves vibration properties of soils and rocks in-situ. Question like ‘How do the results compare with real DCP results or SPT results’ need consideration. The consideration is whether DCP or SPT results are accurate. The best you can say is that DCP & CPT results are an indication of soil strength. In marine clays DCP results can be an inaccurate representation of the soil strength. The reason we use DCP and SPT is because it is cheap, compared to other methods like CPT and DMT. Considering the amount of information that the seismic method produces, it is arguable that the seismic method is cheaper, faster and more user friendly, including a nice pretty picture.
2 SCREW PIER FOUNDATION DEPTHS
2.1 The screw pier story
Every so often, something new revolutionized 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. Screw piers have revolutionised construction foundations and 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.
2.2 AS2870-1996.
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 seismic technology. Seismic is mentioned in EURO codes.
2.3 Current Soil testing
Consider that when a soil tester is engaged to
soiltest a site, the soil testers 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 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 subsurface matrix.
2.4 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 thepreferred 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.
2.5 Limitations of the Borehole Method
The borehole method is the norm, and it is usually supported by DCP data. SPT, CPT and DMT (dilatometer) 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.
2.6 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, so soil and rock strength can be indentified across the section.
3 CASE HISTORIES:
3.1 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 5 metres 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
3.2 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 like the QBSA are applying pressure for engineers to prove filling compaction. Seismic is providing a role in providing a scan 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.
3.3 Subsidence
A certain amenities building was constructed on a filling platform about 2.5m thick. This was founded on a compressible alluvial clay strata. The building was founded on screw piers to depth 14m below original ground level. A seismic survey detected that the rockline was at 18m below original ground level and this was confirmed by a borehole data from a bridge site up the road about 500m.
3.4 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 good result.
3.5 Preliminary site investigation
A long length seismic scan across a large block can be used to identify location for borehole testing.
3.6 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. There was one group that engaged a student to write a theses relating screw pier torque to Vp wave values. Screw pier torque is related to the soil load capacity for the screw pier. There are different types of screw piers designs and a graph needs to be correlated for each screw pier design
4 FUTURE DIRECTIONS:
4.1 Australian standards
Australian stands such as AS1726 and AS2870 need to be updated to include MASW seismic methods of site investigation.
4.2 Familiarization with seismic
Engineering professionals need to become familiar with MASW seismic methods, so that a site investigation does not necessarily mean borehole testing.
5 CONCLUSION
Geophysical methods such as MASW seismic offer a digital solution to common urban engineering problems. These methods are offering solutions and insight where analogue methods may not be offering clear indication of the sub-surface profiles. These methods will gain increasing acceptance within the engineering profession and providing a better result for the public.
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. Theses and dissertations Clarkson A. (2010). ” Using seismic methods to estimate the founding depth of screw piles.”
” IAP Student at Earthsolve in conjunction with Griffith
