As the only seismic survey company in Australia and as a geotechnical engineering firm on the Gold Coast, we believe it is important to contribute to the industry standard by drawing on our wide  experience to aid others in the industry. We are proud to lead Australian research in seismic subsurface imaging technology.

Bibliography

• Fredericks, J., & Clarkson, A. (2010). The Use of Seismic Technology In Geotechnical Applications. Gold Coast Local Engineers Group June 20th 2010. Griffith University: Earthsolve.
• Fredericks, J., & Tayler, J. (2012). Design Raft Slabs In Diluvium Clay Soils Using MASW Seismic Investigation Procedures . Australian Structural Engineering Conference 11th-13th July (pp. 1-8). Perth: Earthsolve.
• Fredericks, J., & Tayler, J. (2012). Site Control Using Seismic Technology. ANZ Conference 15th-18th July 2012. Melbourne: Earthsolve.
• Fredericks, J., Gratchev, I., & Tayler, J. (2012). Seismic Technology For Slope Stability Investigation In Yeppoon North Queensland. Young Geotechnical Professionals Conference (pp. 1-6). Melbourne: Earthsolve.
• Tayler, J. (2011). Introduction To Use Of Seismic Technology To Solve Common Geotechnical Problems In Small Lots . International Conference On Advances In Geotechnical Engineering 7th-9th November 2011 (pp. 1-8). Perth: Earthsolve.
• Tayler, J. The Use of Seismic Technology to Solve Common Geotechnical Problems. National Foundation and Footing Society November 2010. Brisbane: Earthsolve.
• Tayler, J. (2010). The Use of Seismic Technology To Solve Common Geotechnical Problems. Northern Engineering Conference (NEC 2010) (pp. 1-5). Townsville: Earthsolve.

The post Conference Papers appeared first on Earth Solve.

1. Introduction

To date site investigations using borehole and probing methods have provided engineers with 1D geotechnical data, which has been accepted as being satisfactory. Investigations using seismic technology provide 2D and 3D sub-surface imagery which reveals the severe limitations of analoguemethods. Also many geotechnical problems are best solved by using seismic technology.

2. Geotechnical Model

The first step in understanding the sub-surface conditions on any site is to develop a geotechnicalmodel for that site. Geological maps are helpful. To understand the geology is to understand the processes that resulted in the formation of a particular site. Next a seismic survey can produce 2D and 3D imagery for the site. Once this model is established it is the writers’ view that the more accurate the model, errors of judgment as to the foundation options available to a particular site are greatly reduced.

Figure 1. Rockline has been imaged as a foundation for screw piering

3. What is Seismic Technology

Seismic technology is the use of sound waves to interpret different sub-surface materials. It is nonintrusive. In broad terms, the manner seismic waves travel through a material can give information to resolve sub-surface material properties.The author knows of three different seismic technologies used to image sub-surface materials.

1. 1. Reflection
2. 2. Refraction
3. 3. Multi channel analysis of surface waves (MASW)

In this paper, discussion will be limited to two methods, being refraction and MASW. The refraction method will be mentioned but our main discussion will be towards the MASW method. Refraction has disadvantages.It cannot:

• Image soft layers within a stronger soil matrix,
• Depth of image can be restrictive
• The method cannot image below a water table.

The MASW method is not restricted by any of the above limitations. MASW can image voids, boulders in a softer matrix, soft layers in a harder matrix. Depth of image, as it applies to urban geotechnical work, is effectively limitless. Using either the refraction or MASW method, we are able to produce a 2D section through the block in consideration. Typically a section may be 50m long and 10m deep using refraction or 50m deep using MASW. By taking at least two sections through a block, we can input the data into an imagingsoftware, to produce a 3d image of sub-surface layers.

4. Material Strength Evaluation

An equation for Young’s modulus has been calculated with a relationship to surface wave velocity Vs.

1. 1. Vs= [E/2(1+�)�]1/2
2. 2. Vs= velocity of surface wave in m/sec
3. 3. E= Young’s modulus in kPa
4. 4. �= poisons ratio
5. 5. �=density of material in gm/cm3

Formula was presented in a paper: Data Acquisition and Analysis of Active and Passive SurfaceWaves; SAGEEP 2003 Short Course, by Koichi Hayashi [1]. PoIsson’s ratio and material density are known values for ranges of soil and rock types. Therefore Young’s modulus E, can be easily calculated for any sub-surface material. The allowable bearing capacity of various types of sub-surface materials in various consistency or density are known. Therefore surface waves velocity can be equated to an allowable bearing capacity. Surface wave velocity has also been related to N value, as resultant from the Standard Penetrometertest (SPT). Various authors have proposed different equations. We submit an equation correlated by Mr. Koya Suto. [3]. This formula currently is correlating well with screw pier test data.

6. Vs=37.5N0.6It is generally acknowledged that a Vs =150m/sec is at least 100Kpa allowable bearing capacity androck equates to a Vs= 250m/sec. We usually use Vs=350 for rock of qu ~1Mpa, and our experience isthat screw piers mostly found at Vs=350-375 m/sec. This is an area for further research.

4.2 Current Geotechnical Practices

The most common method of sub-surface investigation is by using boreholes to drill, extract soil, observe and record the nature of the materials encountered. The strength of the material is then assessed often by probing. Results from probing are correlated to provide an indication of the strengthof the material. Soil strength on house blocks, may be assessed using a DCP (dynamic conepenetromter); an instrument that has achieved widespread use. This is because the tool is economicalto purchase and easy to use by one man with out the need of machinery. Correlation to allowable bearing capacity is usually achieved by reference to a paper called ‘Determination of Allowable Bearing Pressures under Small Structures’ by Stockwell 1977 [4]. Both borehole and DCP methodsprovide 1D data at the point of test. The results obtained are subject to the interpretation of thetechnician who did the work. Accuracy of strength of material is based on accuracy of the empirical correlation. Most engineers would opine that the borehole and probe method are the only methods available and that the empirical correlation provide all the foundation information required, without further thought. When we review the enormous cost of rectifying foundation failure every year in housing, it should be reasoned that perhaps current soil test investigation methods are at least partially at fault. The importance of this matter to society is considerable. So important is the matter that most advanced societies have building authorities to monitor domestic building activity, because housing foundation failure and related cost is important to the public.

• Housing is usually very cost sensitive.
• Housing mostly represents the biggest financial investment most people make during their lifetime.
• Foundation failure affects the value of that investment.
• Foundation failure is often financially expensive to repair.
• The money mostly has to come out of personal savings and there is often no way to commercially defray the costs through tax deductibility.

Therefore if the construction industry can find a new economical method of foundation characteristicevaluation, it should be attract consideration.

4.3 Current Soil Test Investigation Deficiencies

By comparing the current borehole testing regime with the seismic method, the deficiency in the current borehole soil test method and the efficency of the seismic method becomes increasingly apparent. Advantages of the seismic method are listed below

1. 1. Greater depth of investigation survey.
2. 2. Provides a whole section rather than a 1D view
3. 3. The section data can transformed to a 3D image.
4. 4. Seismic method can highlight areas for further investigation.
5. 5. Seismic method can detetect subtleties in the sub-surface matrix.
6. 6. It easier to detect strength variations in the sub-surface material matrix.

Disadvantages of the borehole method:

1. a. Investigation is limited by price and therefore limited depth of bores and number of boreholes.
2. b. Boreholes are the interpretation of the driller.
3. c. Borehole may not represent the soils on the site.
4. d. Borehole method does not highlighting sub-surface trends well.

Note: Further to item a) above, depth of investigation is very relevant. On reactive clay sites, there is extra consideration for reactivity calculations required for deep clay sites and with fill or weak soil sites, it important to locate a suitable founding depth. It is very common for soil investigation to finish far tooshallow to gain full appreciation of the site by the geotechnical engineer.

4.4 Seismic Refraction Method

This method has been used about 40 years. Despite certain limitations, it is an old method but a good method, and can be used where MASW can not be used. One such peculiar use is where there is rockfill overlying rock. Refraction method also reveals the water table effectively.

Figure 2. Typical refraction wave results

A 10kg sledge hammer is commonly used as the sound source for the refraction method. The sledgehammer is the cheapest and most convenient.The seismic waves from the sledge hammer can penetrate about 10metres depth sub-surface. The sledge hammer usually adequate for urban work. To gain deeper depths a stronger seismic wavesource is required, such as a large weight drop, a shotgun blast or use of dynamite.

The refraction method uses the fact that when seismic waves travels from one medium to another, the waves refract.

4.5 Seismic MASW Method

The MASW method involves using surface waves. These waves are slower than Vp waves. Most new development in seismic is currently in the MASW method. The seismic waves are obtained fromactive and passive sources. Active sources can be by using a sledge hammer. Passive sources are natural sound waves in the sub-surface generated from tidal motion, thunder and cultural sound from traffic noise.The MASW active method is useful to image soil profiles to 20m depth, whilst the passive method is useful to image sub-surface below 20m. To date we have imaged to 110m depth. The MASW seismic –(passive method) advantage over the refraction method is because the result is not limited by the strength of the sound from the sledge hammer. A typical result from a MASW survey is shown below.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 can even assist, particularly in detection of deep layers.Both refraction and MASW methods have a disadvantage in that the site should not vary in level by more than 10% between geophones.

Figure 3. MASW –Surface Wave graph.

Rock would be considered to be the green layer.

4.6 Criticism of the Seismic Method

Criticism is often cast towards the seismic results as engineers compare the results with analogueborehole results. A question often asked is

1. a) whether the equipment requires calibrating.
2. b) whether the engineer may have been better advised to spend the money on extra good boreholes.
3. c) How do the results compare with analogue probe results?

Answers:

1. a) We are advised that re-calibration of the seismograph is not required.
2. b) One does not know when one has good borehole data.
3. c) Seismic results are real results showing the waves vibration properties of soils and rocks in-situ.

Notes:

1. i. We propose that interpreted properties are drained values whereas current in-situ soildata are un-drained values which rely on empirical correlation to achieve materialproperties.
2. ii. We propose that seismic technology has solved the problem of testing of soils in-situ. All other analogue methods of soil testing impose a force on the soil, (causing soil disturbance) which mobilizes an un-drained soil strength condition, which is then measured. The theory of effective stress in soils assumes that pore water is non compressible. This assumption would not be true for seismic testing, which uses soundwaves, which cause the sub-surface materials including pore water to respond in the elastic range.
3. iii. In some soils, particularly silty clays and gravels, DCP (dynamic cone penetrometer)results can be an inaccurate representation of the soil strength. The reason we use DCP and SPT (Standard penetrometer test) is because it is ‘cheap’, compared to other methods like CPT (cone penetrometer test) and DMT (Dilatometer Test). Considering the quality and quantity of information that the seismic method produces, seismic method is fast and with output that is more user friendly, including production of a 3D visualization. Particularly on problem sites, it is often more economical.

5. SCREW PIER FOUNDATION DEPTHS

5.1 The Screw Pier Story

In the 1990, the screw pier came on the construction scene in Australia. Screw piers have revolutionized construction foundations and gained a ready market in the housing construction industry.The reason is because they are unaffected by clay soil reactivity, the water table, 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.

5.2 Current Soil Testing for Screw Piers

Consider that when a soil tester is engaged to soil test a site, the soil testers view is to minimally comply with current standards of soil testing. A minimum specification is provided because of contract price. In the housing sector, the soil test is aimed at slab on ground solutions. However the engineer who engaged the investigation requires sub-surface founding information in order to design a suitable foundation system for a building. What the design engineer and the soil tester is trying to achieve may not be the same, particularly if the foundation design involves screw piers. In a foundation design using screw piers, soil reactivity becomes irrelevant, but bearing capacity at depth becomes relevant. A common method to interpret soil strength is by the DCP. DCP results are used, because it is very economical. On many sites DCP data is not available or accurate because of gravel, cobbles or boulders, as well as silty clay in the sub-surface matrix. DCP results in Silty Clays vary depending on the in-situ moisture content. DCP shaft friction is thought to contribute significantly to the observed unreliability of results.

5.3 Site Investigations

Currently soil testing using analogue borehole and probe methods are the norm for a site investigations. The author considers that in the future as geophysical methods become more well known, methods such as seismic will be at least an alternative and even the preferred method. The reason will be because of quantity and quality of information for price. With one survey the engineer will obtain a section through the block with no compromise on depth of investigation. Seismic data should not necessarily be supported by borehole data if the soil conditions of the area are well known. Borehole data is helpful to indentify layers imaged in the seismic output. Sometimes borehole data will be required to meet code requirements. Site investigation codes need to be updated to include geophysical methods.

5.4 Seismic Software Computer Output

Figure 4. 3D model of sub-surface contour map of the rock line; Vs= 350m/sec.

Once data is collected from the site, it is entered into seismic computer software programs. The output can be in the form of four outputs.

1. 1) A 1D graph showing the average SPT N values over the length of the array line.
2. 2) A Vp or Vs wave section.
3. 3) A graph similar to the above showing a section but converting the surface waves speed to SPTN values, so soil and rock strength can be indentified across the section.
4. 4) Computer output can be loaded into a 3D mapping program. Output can be to produce a 3D model. Also if a particular soil or rock layer is of interest, then a sub-surface contour map can beimaged.

6.0  CASE HISTORIES

6.1 Slope Stability

A major considerations when considering slope stability is the presence and location of colluvium. Another is the location of the rock line and any seepage layers in the soil profile. Using analogue methods, conclusive data is often difficult to obtain. Often one is confronted with boulders in the soil matrix and if an excavator is used, penetration depth is limited. A 20 tonne excavator will achieved only about 5 metres excavation depth. Using seismic, the colluvium layer often is so easily identifiableand adequate penetration depth is achieved.

Figure 6.

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 underthe earth flow.

6.2 Filling

A common feature of construction is filling. Regulatory bodies are applying pressure for engineers to prove filling compaction. Fill compaction is currently verified by a compaction test and a supervisionprogramme. Many filling platforms are showing signs of poor 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 seismic grid. Seismic cross sections can be created and these can be inserted into a imaging program to produce a 3D sub-surface image of the filling compaction.

Figure 7. MASW –Surface Wave section graph showing different degrees of compaction in the filling. Fill depth is about 2.5m. Blue is softness in filling compaction. Black colour is bedrock.

6.3 Difficult Blocks

Certain blocks are difficult to soil test using augers because the site contains considerable rock fillor cobbles and boulders. Seismic can penetrate these sites and provide a good result.

Figure 8. MASW N value image, shows boulders in the fill matrix (blue). Rock is blue. Natural soil line can be discerned at 3.5m depth. 20.018.016.014.012.010.0 8.0 6.0 4.0 2.0-0.0-2.0 Depth (m) 0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 20.0 22.0 24.0 26.0 28.0 30.0 32.0 34.0 Distance (m)LINE 4 – MASW VS PROFILE(m/sec)S-wave velocity406080100120140160200240280330400500 Scale = 1/178

6.4 Preliminary Site Investigation

A long length seismic scan across a large block can be used to identify location for borehole testing.

Figure 9. Refraction image over 110m. Weathered rock rises close to surface (yellow layer).

Installed borehole confirms rock at 1.0m depth.6.5 Screw Pier Foundation Depth

Seismic technology is finding a very ready application for establishment of screw pier foundationdepth. Seismic technology is quick and economical where foundation depths are deep. Once the MASW seismic has established the depth to rock, it is good practice to follow up with at least one borehole.

7. FUTURE DIRECTIONS

7.1 Engineering Standards

Engineering standards need to be updated to include geophysical methods of site investigation.

7.2 Familiarization with Seismic

Engineering professionals need to become familiar with geophysical methods such as seismic, so thata site investigation does not necessarily mean borehole testing.

8. CONCLUSION

Geophysical methods such as seismic offer a digital solution to common urban engineering problems.These methods are offering solutions and insight where traditional analogue methods may not beoffering clear indication of the sub-surface profiles. These methods will gain increasing acceptance within the engineering profession by providing a better and economical result for the public.

9.  REFERENCES

1. 1. Hayashi, Koichi, “Short Course: Data Acquisition and Analysis of Active and Passive SurfaceWaves”; SAGEEP 2003.
2. 2. Look, Burt, “Handbook of Geotechnical Investigation and Design Tables”, ISBN 13:978-0-415-43038-8, 2007.
3. 3. Suto, Koya, “About the Relationship between the S wave velocity and the N-value”, 2010.
4. 4. Stockwell, “Journal of New Zealand Institution of Engineers”, ‘Determination of Allowable BearingPressures under Small Structures’, Vol 32 No. 6; 15thJune 1977.

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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. 1) A refraction Vp wave section.
2. 2) A graph showing the average SPT N values
3. over the length of the array line.
4. 3) A graph showing a section similar to the refraction section, but with the layers established from surface waves.
5. 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

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FORENSIC INVESTIGATION OF SUBSIDENCE TO DWELLING

December 2007

James Tayler

Consulting Engineer MUDGEERABA Queensland

ABSTRACT

Investigation of house subsidence is always fraught with difficulty, because one doesn’t ever know the full information of the foundation soils, the method of construction or the quality of construction. This report illustrates the importance of undertaking a desktop study and obtaining the origins of the soils to confirm the bore hole logging in order to obtain a correct diagnosis of the likely cause of subsidence.

1) BACKGROUND

The owner of a house noted severe cracking in his house following the construction of a road in front of the house. The owner complained to the contractor who passed the claim on to their insurer. The insurer engaged a suitably experienced geotechnical (Geotechnical engineer #1) to write a report to identify the cause of cracking. The geotechnical engineer engaged an experienced soil testing company (Soil tester #1) to conduct soil testing around the dwelling.The diagnosis by Geotechnical Engineer #1 was that cracking of the house finishes resulted from the house being founded on clay soils which had cracked as a result of the recent drought conditions in the Gold Coast region. The owner had been in possession of the house for about twenty years and was convinced that the cracks only appeared after the construction of the road in front of their dwelling, which involved the use of a compaction roller.

2) DWELLING TYPE

The house is a single storey masonry veneer dwelling with a concrete tile roof and on grade concrete floor slab constructed in the early 1980s, prior to the advent of the first Australian Standard 2870-1986, Residential Slabs and Footings. Damage to masonry walls and concrete floors was significant (with reference to Table A1 and A2 AS2870-

1986).

3) DROUGHT

The Gold Coast had experienced drought in the mid 2000s and the local Hinze Dam reached critical low levels in early 2004. July 2006 was a record dry month.

4) BORELOGS FROM SOIL TESTER #1

The general soil profile recorded using a 100 mm diameter power drilling rig was residual clay soils to at least a depth of 2.5 m depth. Shrink/swell laboratory testing indicated that the clays soils had significant shrink swell potential.

5) CONCLUSION BY GEOTECHNICAL ENGINEER #1

Based on the results of the soil testing, Geotechnical Engineer #1 reported that the clay foundation soils were susceptible to drying shrinkage from the recent drought, causing settlement of the dwelling resulting in cracked finishes. The conclusion was that the cracking was not caused by the road making process in front of the owner’s house.

6) ENGAGEMENT OF GEOTECHNICAL ENGINEER #2

The owner engaged wanted a second opinion. Another geotechnical engineer (Geotechnical Engineer #2), considered that the initial investigation was extensive and seemingly thorough. Therefore it was mentioned to the owner that proceeding with another investigation that may produce similar results. Upon insistence from the owner that cracking occurred after the road making process, the commission was accepted based on the following considerations.

FORENSIC INVESTIGATION OF SUBSIDENCE TO DWELLING J TAYLER

34 Australian Geomechanics Vol 44 No 1 March 2009

a) It was noted that a power drilling rig had been used for soil testing. Forensic testing using a power drilling

machine is not recommended because detection of soil layer interfaces can be missed and soil structure can

be destroyed. Thus the soil profile may have not been recorded accurately.

b) If the soil foundation was fill, vibration from a compaction roller could initiate further settlement resulting

in cracking of finishes and structural elements.

c) Other houses in the street had also cracked.

The commission included a new soil testing programme using hand augers (Soil tester #2) conducted in late 2007.

7) RESULTS OF SOIL TESTER #2

The results of the soil testing programme (Soil tester #2) resulted in a general soil profile of brown clayey fill to depths of 1.2 m and 2.2 m-see Table 1. The filling comprised of SILTY SAND and SANDY CLAY in different layers.

Table 1: DEPTH OF FILLING

Test Location #BH1 Test location #BH2

2.2 m 1.2 m

The Fill was above a layer of black alluvial clay with root fibres, over grey alluvial clay with rounded river gravel (20-30

mm). The bore hole locations are shown on Figure 1.

Figure 1: Site plan shows location of investigation holes.

8) DESKTOP STUDY BY GEOTECHNICAL ENGINEER #2

A desk top study, including observation of geological maps (BEENLEIGH 1:100,000), soils maps (Moreton Region

Land Resource Areas map 1:250,000) and Google earth aerial photography, revealed that the development had been constructed on the Nerang River flood plain and that the origin of natural soil type in the area was alluvium.

FORENSIC INVESTIGATION OF SUBSIDENCE TO DWELLING J TAYLER

Australian Geomechanics Vol 44 No 1 March 2009 35

9) CONCLUSION BY GEOTECHNICAL ENGINEER #2

· Based on the results of Soil tester #2, the house block had been filled with sandy and clayey filling overlying alluvial soils to form a flat building block.

· As the bore logs by Soil tester #1 did not mention ‘alluvial’ the material logging might be incorrect

· It was considered that vibration from a compaction roller nearby could initiate compaction and settlement in the filling. Clay filling could settle if the clays had desiccated as a result of the drought.

· Settlement could result in cracking of the masonry and concrete structural components and finishes to the house.

· It was not discounted that the house may have experienced some settlement cracking as a result of some

drying shrinkage of the clay soils, even before the construction of the road.

· It was concluded that cracking of masonry and concrete structural components and finishes arising from

settlement of the fill foundation soils as a result of use of compaction equipment nearby was plausible and very likely.

10) CONCLUSION

Lessons learnt:

a) Don’t assume bore hole logging by others is correct.

b) Listen to the report of eye witnesses.

c) Forensic soils investigation should be conducted using hand augers wherever practical, or other means that provide a method of identifying soil structure and origin.

d) Bore hole logging must accurate or wrong conclusions will result.

e) It is important to check origins of soils using a desk top study (e.g. geological maps).

In this case the error in the bore logs of Soil tester #1 may have been avoided if bore hole logging had been accurate and results checked against available maps such as geological, soils and aerial photographs (Google earth) which would have indicated that the likely natural soil type was of alluvial origin.

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Slope instability has become one of the most significant geotechnical risks in recent times. Unfavourable geologic and climatic conditions, combined with anthropogenic activities, result in thousands of slope failures each year. Standard methods of geotechnical investigation such as boreholes and SPT-tests are not consistent in providing the geotechnical data for the site as a whole. Leaving engineers to assume the soil properties for most of the site, a disadvantage that may significantly increase the cost of remediation measures. Geophysics, in particular seismic surveying can provide alternative methods to solve this geotechnical problem. Although seismic surveys are widely used throughout the mining industry, it has not been applied to slope stability problems in Australia. This is mostly due to a lack of awareness, availability and acceptance/application of modern science. This paper seeks to establish the advantages of seismic methods in resolving slope instability issues.

This paper illustrates the advantages of using seismic technology by presenting a case study, seismic technology for slope stability investigation in Yeppoon North Queensland. After significant rain in February 2008 a medium sized landslide occurred at the top of an embankment in the road reserve at Statue Bay, Yeppoon. The landslide regressed into the rear of the private properties above and therefore remediation measures were required. This is a reasonably standard geotechnical task, but when combined with steep vegetated slopes and very limited access, standard methods were not adequate. Seismic refraction and Multichannel Analysis of Surface Waves (MASW) surveying methods were used to generate 2D & 3D sub-surface images. These images distinctively illustrated the sub-surface material profiles, including the rock line and areas of slip prone material.

1 INTRODUCTION

Slope instability has become one of the most significant geotechnical risks in recent times. In Australia the Thredbo Landslide in 1997 (Tayler, 2010) in which 18 people lost their life, is probably the most well known. This landslide in particular, highlighted to councils and engineers throughout Australia the importance of undertaking an appropriate slope instability investigation.

Landsides occur when the downward forces, including the self weight exceeds the shear strength holding the soil on the slope. Shear strength loss can be attributed to excess pore water pressure, weathering of the soil profile and anthropogenic activities. The most important information required when conducting a slope stability investigation is soil depth, soil profile, soil strength and groundwater conditions (Varnes, 1978).

Most geotechnical engineers at some point have been left inadequately equipped to undertake slope stability investigation when supplied with a common AS2870 & AS1726 compliant geotechnical reports. These are based on standard test methods. These methods give results at the test point. However, even with numerous test points, the engineer is left to assume the soil profiles in-between these points. These assumptions regularly leave the engineer at risk of missing vital sub-surface data, such as the presence of colluvium, soft soils, a spike and or a depression in the rock line.

Seismic technology has been applied to many geotechnical problems, such as bearing capacity evaluation, earthquake site classification, compaction certification, subsidence investigation and now slope stability investigation. It gives the geotechnical engineer another tool to compile the geotechnical model for the site. Combined with standard methods the seismic results can join the test points, reducing assumptions and the giving the engineer confidence in the sub-surface profile. This paper will show how seismic technology can be used to aid in slope stability investigations.

2 CASE STUDY

Figure 1: A view of the landslide from the top of the bluff.

This site is located on the top of a bluff in Statue Bay, Yeppoon North Queensland. It consists of two residential sized blocks with a 27m high steep vegetated embankment at the rear (Figure 1). The embankment was cut by the military in the 1930s to construct a road around the base of the bluff. The construction drawings show that the natural slope was about 30-40° before the cut and that the cut batters vary from about 60-65° in the sedimentary rock, to vertical in the tuff. The road below is considered a main road that connects outlying communities to the city centre of Yeppoon.

The residents advised that the landslide was activated during heavy rain in February 2008. This was confirmed by the rain records where 385mm of rain fell in 3 days, over 30% of the yearly total (Bureau of Meteorology).

The landslide occurred in the road reserve below and extended about 1.5m into the rear of the private properties above (Figure 1&3). The toe of the landslide also blocked the road below and it remained closed for a number of days during the cleanup operation. The complex debris slide (Varnes, 1978) was about 16 x 9 x 3m deep and had a volume of approximately 285m3 (Figure 2&3). To ensure the safety of the road users and the residents remediation measures were required.

This is a reasonably standard geotechnical task, but when combined with steep vegetated slopes and very limited access, standard methods were not considered adequate. Therefore seismic MASW and refraction surveying methods were utilised to give additional information on the sub-surface material.

 2.1 FIELD INVESTIGATION

The site was tested by augering four boreholes to provide information on the soil profile in accordance with

AS1726-1996, Geotechnical Site Investigations. Due to the site access limitations this was undertaken using a small power auger to about 1.5m then hand augers to greater depths. These results were augmented by the use of four seismic surveying lines, five dynamic cone penetrometer tests and shear vane testing. Disturbed and undisturbed samples were also collected for laboratory testing. Tests selected were Atterberg limits and Shear box testing. A brief site level survey was also carried out to enable computer aided slope analysis.

The boreholes indicated a variable soil profile. Generally the profile consisted of a clay layer overlying extremely weathered rock. The clay layer varied in thickness from 1m at test location (TL) number 4 to 3m at TL

2. Two examples of the borelogs are included in Tables 1 & 2.

 2.2.2 DCP RESULTS

The standard DCP results indicate variable stregnth material througout the soil profile. As previously stated, these results are only relevant at the test point. In contrast, the seismic velocities are averaged over about four meters. If these averaged seismic velocities are converted to DCP results, using the empical formula by Karai (1966), the profile is more consistent and generally increasing in strength with depth. Figures 4&5 show how the standard DCP results compare to the converted DCP results, for test locations 1&3. These figures show a remarkable correlation between the standard DCP results and the converted DCP results.

 2.2.3 LABORATORY TESTS SUMMARY

The laboratory results indicate that the clay across the site has similar properties and parameters. The values of cohesion appear higher than expected. This was considered to be due to the strain rate being too fast to allow the pore water to dissipate resulting in higher cohesion values.

It is worth noting that, that the internal friction angle is generally the maximum slope that the soil can remain stable at in the long term. In this case, the natural slope was between 30-40°, which is similar to the measured internal friction angle. After the road was cut, the embankment was steepened to about 60-65°, well above the internal friction angle. This significantly increased the risk of instability of the embankment. This combined with unfavorable climatic conditions and high plasticity clays were some of the main causes of the landslide.

 2.2.4 SEISMIC RESULTS

 There are many correlations from seismic velocities to geotechnical properties. The two most notable are the empirical relationship to SPT–N value and the mathematical relationship to Young’s Modulus. Surface wave velocity (Vs ) has been correlated by numerous authors to the SPT N values. The correlation used in this paper is Vs =19(N60 )0.6  by Karai (1966). The mathematical relationship to Young’s Modulus is relevant at small stain. If the strain is higher a correlation factor is required to a factor down the calculated Young’s Modulus. In this case it was used to assess the strength of the underlying rock. This was considered to have low stain and the correlation factor was not required. The formula used in this case study is as follows:

Vs= [E/{2(1+u)r}]1/2 (1)

Vs=Seismic surface wave speed, as obtained on site by testing (m/sec)

E= Young’s modulus at small strain (kPa)

u= Poisson’s ratio usually as tabled in numerous text books (eg Look 2007)

r= in-situ weight of material, usually as tabled in numerous text books (g/cm2)

Note: To assess E, u and r text book values are usually adequate.

3 DISCUSSION

The main cause that contributed to the landslide was that the embankment was significantly steeper than the internal friction angle of the soil. The trigger of this landslide, as with many, was the significant rain event that occurred in February 2008. This rain saturated the high plasticity clays causing an increase in pore water pressure and a decrease in effective stress, resulting in instability.

The seismic output gave an overall view of the sub-surface profiles and strengths, this made it easy for the engineer to construct the geotechnical model for the site and assess the geotechnical issues. In this case, the seismic survey gave a clear indication of how the rock and soil profiles varied over the site. It clearly shows that the rock profile is deeper within the slip zone (Figures 6-8), overlaid by weaker clays. The standard soil testing methods confirmed this result.

Even though the seismic results are averaged both vertically and horizontally the converted DCP results correlated well to the standard DCP results (Figures 4&5). The results did differ, however the differences can be associated to the averaging effect of the seismic survey.

The seismic survey indicated sub-surface strengths to about 12m. Due to the poor site access, this wasn’t possible using standard methods. The depth of penetration of the seismic survey enabled the geotechnical engineer to access the rock strengths to a greater depth as required. The seismic survey provided the engineer confidence to design an economical solution. If this survey had not been carried out the design engineer would not have the required information. He would have been forced to make assumptions which can lead to overly conservative or even worse an overly optimistic design.

4 CONCLUSION

In conclusion, the seismic survey was an efficient and economical way to test the site as a whole. When combined with standard test methods, it enabled the geotechnical engineer to gain a deeper understanding of the sub-surface profile and strength. This ultimately led to an appropriate design solution.

Soil investigations within Australia would greatly benefit by the application of seismic technology. Seismic surveying techniques offer the geotechnical engineer another tool to build the geotechnical model for the site. The depth of penetration and the relationships to sub surface material properties gives the engineer an overall view of the site. The seismic results reduce assumptions and give the engineer confidence to construct a geotechnical model for any site.

5 REFERENCES

1. AS1726, 1993, Geotechnical Site Investigations, Standards Australia

2. AS2870, 2011, Residential Slabs & Footings, Standards Australia

3. Varnes D. J, 1978, Slope movement types and processes, Schuster R. L. & Krizek R. J. Ed., Landslides,

analysis and control. Transportation Research Board Sp. Rep. No. 176, Nat. Acad. Sciences, pp. 11–33

4. Hayashi, Koichi, 2003, Short Course: Data Acquisition and Analysis of Active and Passive Surface

Waves, SAGEEP Conference Proceedings 2003.

5. Look, Burt, 2007, Handbook of Geotechnical Investigation and Design Tables, ISBN 13:978-0-415-

43038-8

6. Suto, Koya,, 2010, About the Relationship between the S wave velocity and the N-value,

7. Tayler, James, 2010,

The post Seismic Technology For Slope Stability Investigation in Yeppoon North Queensland appeared first on Earth Solve.

Landslide is common in most populated areas in Australia, but public awareness increased greatly after the Thredbo Landslide in 1997 where about 3500t of debris crashed down a ski slope, killing 18 people.

Landslides occur when the weight of the soil on a slope exceeds the strength of the soil to hold it in place on that slope. Loss of soil strength can occur following prolonged rainfall.

Landslide includes rock fall, deep seated slides, debris flows and shallow landslides. However, the general public generally do not understand the risks.

I remember a case where a resident complained about 100m³ of red-brown soil had moved onto his building pad. After it was explained to him that a landslide had occurred, he responded by saying that his property had never experienced landslide in the past. He was amazed when he was informed that his property had experienced many landslides in the past, evidenced by numerous s-shaped trees.

After his neighbour refused ownership or to pay for removal of the soil, the owner sold the soil to a landscaping company after which the neighbor decided he deserved some of the proceeds.

Mostly, the consequences of landslide can be far-reaching including loss of property, loss of life, stress and disruption in many forms. The cost of landslides is not covered by most insurance policies with the costs having to be borne by individuals and government.

Australia-wide, after every wet spell, there will be multiple rock falls and landslides along narrow roads leading to the mountain resorts, mostly not recorded.

Total costs to society in Australia, including loss of property and rectification could be at least of the order of $1 billion dollars in the last 100 years.

Recently, we were requested to identify safe building locations on a certain rural residential property. Certain safe locations were identified but one location which was identified as an old landslip zone, slipped again fairly dramatically following rains in 2009 as per the photograph below.

Landslide is common in most populated areas in Australia, but public awareness increased greatly after the Thredbo Landslide in 1997 where about 3500t of debris crashed down a ski slope, killing 18 people.

Landslides occur when the weight of the soil on a slope exceeds the strength of the soil to hold it in place on that slope. Loss of soil strength can occur following prolonged rainfall.

Landslide includes rock fall, deep seated slides, debris flows and shallow landslides. However, the general public generally do not understand the risks.

I remember a case where a resident complained about 100m³ of red-brown soil had moved onto his building pad. After it was explained to him that a landslide had occurred, he responded by saying that his property had never experienced landslide in the past. He was amazed when he was informed that his property had experienced many landslides in the past, evidenced by numerous s-shaped trees.

After his neighbour refused ownership or to pay for removal of the soil, the owner sold the soil to a landscaping company after which the neighbor decided he deserved some of the proceeds.

Mostly, the consequences of landslide can be far-reaching including loss of property, loss of life, stress and disruption in many forms. The cost of landslides is not covered by most insurance policies with the costs having to be borne by individuals and government.

Australia-wide, after every wet spell, there will be multiple rock falls and landslides along narrow roads leading to the mountain resorts, mostly not recorded.

Total costs to society in Australia, including loss of property and rectification could be at least of the order of $1 billion dollars in the last 100 years.

Recently, we were requested to identify safe building locations on a certain rural residential property. Certain safe locations were identified but one location which was identified as an old landslip zone, slipped again fairly dramatically following rains in 2009 as per the photograph below.

Builders also request landslip reports on moderate to steep slope sites. This is because the slope risk report completes their duty of care, limits their liability and provides information concerning geotechnical concerns of the area.

Slope risk reports are relatively inexpensive for house lots. Once it is explained that the necessity arises from outcomes from the Thredbo landslide disaster, there seems to be general acceptance and even enthusiasm for receipt of a positive report about their block.

The task is ongoing for engineering professionals involved in the geosciences is to continue ongoing research into landslide, to devise ever more accurate methods to understand landslide, to increase our knowledge of the geology of Australia and to mentor and set up an information passage to pass this knowledge through to the up-coming generation of geotechnical engineers. Legislators will have the task of ensuring the expertise makes it way to the public.

For the future, I expect greater use of geophysical methods to scan the subsurface for information of the soils and rock. This method is also less intrusive and explores to deeper depths, than conventional soil testing equipment.

Local authorities have always had a concern for the consequences of landslide, but since the Thredbo landslide, concern was highlighted. As well as duty of care and public safety, councils are concerned with their liability if they approve a development for construction, that may have landslide susceptibility.

This is an edited version of an article by James Tayler, a chartered structural and geotechnical engineer and manager of engineering consultancy Earthsolve.

» Click here to download a related file

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The article was published in Engineers Australia magazine in the December 2009 issue.

The tower had to be lightweight, and easily transported with small 4WD vehicles (on a roof rack) and erected without cranes. The tower was 24metres high with a 6metre wide base. It was a lattice tower designed of aluminium circular extruded tube. The tower was designed to be supported either on a concrete pad base or alternatively, on MEGA ANCHORS. Because the tower was designed to be erected in remote locations, soil testing was planned to use geophysical methods of Refraction and MASW seismic, with confirmation of the soil profile to rockline by using hand augers.

The post WIRELESS BROADBAND FOR REMOTE COMMUNITIES-ARNHEM LAND appeared first on Earth Solve.

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The post Geotechnical Quality Assurance by Using Geophysical Seismic Methods appeared first on Earth Solve.

Landslide is common in most populated areas in Australia, but public awareness increased greatly after the Thredbo Landslide in 1997 where about 3500t of debris crashed down a ski slope, killing 18 people.

Landslides occur when the weight of the soil on a slope exceeds the strength of the soil to hold it in place on that slope. Loss of soil strength can occur following prolonged rainfall.

Landslide includes rock fall, deep seated slides, debris flows and shallow landslides. However, the general public generally do not understand the risks.

I remember a case where a resident complained about 100m³ of red-brown soil had moved onto his building pad. After it was explained to him that a landslide had occurred, he responded by saying that his property had never experienced landslide in the past. He was amazed when he was informed that his property had experienced many landslides in the past, evidenced by numerous s-shaped trees.

After his neighbour refused ownership or to pay for removal of the soil, the owner sold the soil to a landscaping company after which the neighbor decided he deserved some of the proceeds.

Mostly, the consequences of landslide can be far-reaching including loss of property, loss of life, stress and disruption in many forms. The cost of landslides is not covered by most insurance policies with the costs having to be borne by individuals and government.

Australia-wide, after every wet spell, there will be multiple rock falls and landslides along narrow roads leading to the mountain resorts, mostly not recorded.

Total costs to society in Australia, including loss of property and rectification could be at least of the order of $1 billion dollars in the last 100 years.

Recently, we were requested to identify safe building locations on a certain rural residential property. Certain safe locations were identified but one location which was identified as an old landslip zone, slipped again fairly dramatically following rains in 2009 as per the photograph below.

The post LANDSLIDE ARTICLE IN ENGINEERS AUSTRALIA MAGAZINE 2009 appeared first on Earth Solve.