Introduction to Use of Seismic Technology to Solve Common Geotechnical Problems in Small Lots

Synopsis: Despite all the array of new analytical methods to resolve geotechnical problems, the age old question remains, to decisively establish the properties of foundation soils and rocks. Seismic technology provides 2D and 3D method for interpretation of sub-surface soil and rock profiles.

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. Reflection
2. Refraction
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. Vs= [E/2(1+ï�µ)ï�²]1/2
2. Vs= velocity of surface wave in m/sec
3. E= Young’s modulus in kPa
4. ï�µ= poisons ratio
5. ï�²=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. Greater depth of investigation survey.
2. Provides a whole section rather than a 1D view
3. The section data can transformed to a 3D image.
4. Seismic method can highlight areas for further investigation.
5. Seismic method can detetect subtleties in the sub-surface matrix.
6. It easier to detect strength variations in the sub-surface material matrix.

Disadvantages of the borehole method:

a. Investigation is limited by price and therefore limited depth of bores and number of boreholes.
b. Boreholes are the interpretation of the driller.
c. Borehole may not represent the soils on the site.
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

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

Answers:

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

Notes:

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