The Misuse of SPTs in Fine Soils and the Implications of Eurocode 7 Reid Taylor

8/16/2019 The Misuse of SPTs in Fine Soils and the Implications of Eurocode 7 Reid Taylor

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TECHNICAL NOTE

28 GROUND ENGINEERING JULY 2010

The misuse of SPTs

in fine soils and theimplications of Eurocode 7

The standard penetration test(SPT) is one of the most com-monly used insitu tests to deter-

mine density and subsequently theinsitu strength of granular soils foruse in bearing capacity analysis.

Due to its simplicity and costeffectiveness the use of the SPTas part of insitu testing for groundinvestigations of fine soils has become common practice in theUK. Several researchers, (Stroud1974; Stroud & Butler 1975; Charles2005; Tomlinson 2001) state that anapproximation for the undrainedshear strength and the coefficientof volume compressibility can

be obtained from a standardpenetration test N value correlatedto the plasticity index of the soil.

Consequently, it has becomecommon to multiply the SPT Nvalue by a factor of five (Charles2005) to provide a very approximatevalue of undrained shear strength.While this correlation is widelyused in geotechnical engineeringthe geographical spread of thedata from which the correlationwas derived is significantly limitedas the research is predominantlyEnglish based.

However, the use of themultiplying factor of five has now become standard in the geotechnicalindustry throughout the UK. Theuse of this rule of thumb may partlyexplain why to date the use ofalternate sampling has been regardedas the general site investigationmethodology.

Research carried out by Reid(2009) has revealed that there islittle if any relationship betweenSPT N values and the undrainedshear strength or the coefficientof volume compressibility forfine soils within the geographicalarea of South Lanarkshire.Consequently the continued use

of historical empirical correlationsis questioned.

Historical empiricalcorrelationsIn 1974 Stroud published findingsthat the SPT was a reliable testthat could provide a means ofestimating insitu properties of clay.Stroud investigated the relationship between SPT N value and undrainedshear strength (c) and found a“simple correlation” of the form

c = f 1 x N

where f 1 is an independentmultiplication factor (Stroud 1974,p374). The f 1 values were found torange from 3.1 to 7.6kN/m2 and were

plotted against plasticity index (seeFigure 1). Stroud also investigatedthe relationship between SPT N

value and the coefficient of volumecompressibility (mv) and found adirect relationship of

mv = 1/(f 2 x N)

where f 2 is an independentmultiplication factor. The f 2 valueswere found to range from 350to 800kN/m2 and were plottedagainst plasticity index (see Figure2), and a “best fit” correlation off 2 = 440kN/m

2 was found to apply.Both the f 1 and f 2 values werefound to increase with decreasingplasticity index.

A lack of statistical analysis is

noted throughout the paper with nomention of a correlation coefficientfor the relationship between f 1 and f 2

with plasticity index. Furthermore,the best fitting trend lines are notdefined in terms of their type andfunction. In addition, the paper doesnot state if corrections were appliedto the N values.

A year later Stroud & Butler (1975)examined relationships betweenSPT N values with undrained shearstrength and the coefficient ofvolume compressibility of glacialmaterials.

The f 1 values were found to rangefrom 4.4 to 7.0kN/m2 with f 2 valuesranging from 350 to 750kN/m2.The f 1 and f 2 values were found toincrease with decreasing plasticity

index. The f 1 and f 2 values wereplotted against plasticity index alongwith the previous cases provided byStroud (1974) and similar chartswere produced. Again there is a lackof statistical analysis and provisionof details.

Limited information isavailable on the correlation between the coefficient of volumecompressibility and SPT N value.An f 2 value of 450kN/m

2 wasrecommended for materials ofmedium plasticity and 600kN/m2

for materials with a plasticity indexof


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GROUND ENGINEERING JULY 2010 29

a statistical point of view, takingaccount of test types and SPTcorrections.

The authors indicate that the useof the current correlations withoutmaking corrections to the “field” Nvalues can lead to “erroneous resultsand designs” (Sivrikaya & Togrol2006, p54). In addition the authorsmake mention that no statisticalanalysis has taken place to establishthe significance of the correlationspreviously published.

Sivrikaya & Togrol’s analysis ofthe relation between SPT N valuesand undrained shear strength

highlights that, in most cases,f 1 increases with an increase inplasticity of clay supporting thefindings by Sowers (Sowers 1979after Sivrikaya & Togrol 2006) butopposing the findings by Stroud(1974). The comparison of studiesin Table 1 displays the similaritiesand differences of the value of f 1 inrelation to soil type.

The findings by Sivrikaya &Togrol show that the correlation

between SPT N value andundrained shear strength are ofmedium significance and can beused in relation to soil type for the“preliminary” design stage only.

Limited information is availableon correlations between thecoefficient of volume compressibilityand SPT N value. In addition, thecorrelations all too often refer tounconfined compressive strength(Terzaghi & Peck, 1948) as opposedto undrained shear strength anddo not differentiate betweenuncorrected and corrected N values.

Furthermore, the f 1 values are

noted to vary between 2 to 17.5 andthe correlations differ with respectto the factor f 1 as to whether or notit will increase or decrease withdecreasing plasticity index (Stroud& Butler 1975; Sowers 1979 afterSivrikaya & Togrol 2006). Moreover,the lack of statistical informationand correlation coefficients withinmost of the literature means thesignificance of the correlationscannot be determined.

Comparisons and differencesin the correlationsThe historical correlations:n do not differentiate between

uncorrected and corrected Nvalues

n all too often refer to unconfinedcompressive strength as opposedto undrained shear strength

n do not show statistical analysesto substantiate the publishedassociations

n consider the correlationssufficient to provide reliableresults notwithstanding that thedatabase is limited geographically

n differ with respect to the factorf 1 as to whether or not it willincrease or decrease withdecreasing plasticity index.

The correlations by Sivrikaya &Togrol:n do differentiate between

uncorrected and corrected Nvalues

n do relate the uncorrected andcorrected N values to undrainedshear strength

n do provide statistical analysis tosupport their findings

n do agree with the work of Sowersthat the value of f 1 increases withan increasing plasticity index

n recommend the use of thecorrelations for “preliminary”design work only.

Given the lack of statisticalinformation and correlationcoefficients within most of theliterature the significance of thecorrelations must be questioned.

ResultsThe statistical analysis undertakenof ground investigation data held by South Lanarkshire Council usedmultiplying factors A1 and A2. Thefactors used were required to change

the corresponding uncorrected SPTN value into an undrained shear

strength and coefficient of volumecompressibility respectively.

The known values were obtainedfrom triaxial and oedometer testscarried out on undisturbed samplesobtained by a U100 sampler(following the procedure of Stroud& Butler).

The A1 and A2 multiplying factorsdo not equate directly to the factorsf 1 and f 2 as described by Stroud &Butler (1975) as these latter valuesare linked directly to the individualplasticity index of each sample.In contrast Reid formed threeindividual datasets comprising low,

intermediate and high to extremelyhigh plasticity values respectivelywith a further dataset comprising allthree datasets.

The descriptive statistical analysisundertaken of the undrained shearstrength datasets found the mean ofA1 to be approximately 4. This meanvalue is similar to the mean f 1 valuesof 5 and 5.5 provided by Stroud& Butler (1975) and Sivrikaya &Togrol (2006) respectively.

However, as the A1 values werefound to range from 0.18 to 19.30,further analysis was undertaken todetermine whether or not actualcorrelations or an association could be found between the parameters.

The relationship betweenuncorrected N value and undrainedshear strength is illustrated in Figure3. Hypothesis testing indicates thatthere is little, if any, correlationof significance between these twoparameters. The R 2 value of lessthan 0.2 indicates that there is nosignificant association between theuncorrected N value and undrainedshear strength.

The descriptive statistical analysisundertaken of the coefficient ofvolume compressibility datasetsfound the mean value for the

multiplier A2 to be approximately500, again comparing favourably

0

f 2 ( k N / m 2 )

PI%

10 20 30 40 50 60 700

200

400

600

800

1,000

Figure 2: f 2 plotted against plasticity index(Stroud & Butler 1975, p129)

0

f 1 ( k N / m 2 )

PI%

10 20 30 40 50 60 700

2

4

6

8

10

Figure 1: f 1 plotted against plasticity index(Stroud & Butler 1975, p128)

0

U n d r a i n e d s h e a r s t r e n g t h

( k N / m )

Uncorrected N value

5 10 15 20 25 30 35 40 450

350

300

250

200

150

100

50

y=9.1314x

R2=0.1846

Figure 3: Uncorrected N value versus undrained shear strength


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30 GROUND ENGINEERING JULY 2010

with the average value of 450published by Stroud & Butler.

However, as the A2 values werefound to range from 175 to 1,370,further analysis was carried out todetermine whether or not actual

correlations or associations could be found between the parameters.With an R 2 value of 0.2018, nosignificant association could befound between uncorrected Nvalue and the coefficient of volumecompressibility, as indicated byFigure 4.

The use of hypothesis testing andcorrelation coefficients to analysethe data for the South LanarkshireCouncil area found that themajority of relationships betweenthe various parameters are of littleor no significance.

The associations that are oflow or moderate significance onlyapply to soil groups of specificplasticity and as such negate theapplication of the correlation tothis geographical area. A furthershortened check on “corrected” Nvalues yielded findings with lowerR 2 values to those discussed above.

Conclusion from the reviewon SPT applicabilityThe use of such rules of thumbwithout a full understanding of theirrelationships in terms of significanceand underlying experimental errorwill lead to the incorrect applicationof these multiplying factors. The use

of such values may lead to erroneousdesign values when compared tolaboratory values obtained.

The findings confirm the guidancegiven in Eurocode 7 that the use ofthe SPT should be restricted to a“qualitative evaluation of the soilprofile” as there is “no generalagreement on the use of SPT resultsin clayey soil” (British Standard2007a, p50); the SPT is mainly usedfor the determination of strengthproperties in coarse soils.

Moreover, “if an empiricalrelationship is used in the analysis,it shall be clearly established that itis relevant for the prevailing groundcondition” (British Standard 2010,p23). Consequently, in September2009 South Lanarkshire Councilinstituted a continuous samplingmethodology for future groundinvestigation works.

The implications ofEurocode 7The use of continuous samplingallows undisturbed samples offine soils to be obtained by U100sampling via the standard shell andauger soil rig. Triaxial tests can becarried out on the undisturbedsamples and the resultant values

of undrained shear strength usedto classify the strength of soil in

terms of BS EN ISO 14688-2:2004(2007b).

Standard interpretative practiceof South Lanarkshire Council is toplot all data onto Excel spreadsheetsand charts to view the data and

determine if any correlations can be found, for example increasingundrained shear strength withdepth. This statistical practiceis reinforced by Eurocode 7 as ameans to demonstrate how oneobtains the characteristic value ofthe site and subsequently the designvalue (a cautious estimate of thecharacteristic value).

Experience within SouthLanarkshire has shown thatdetermining the characteristic valuecan be difficult. Typical data (Figure5) illustrates the lack of correlation between undrained shear strengthand depth. This is not unusual andindeed a site displaying a significantassociation between undrainedshear strength and depth has yet to be found.

For sites with this type of data,with a range of undrained shearstrengths at the same horizon,the key question is how to arriveat a suitable characteristic value:too low a value could negate thedevelopment of the site whiletoo high a value could result inoverestimated bearing capacities.Eurocode 7 does provide some helpin as much as local knowledge or judgement can be used. However,

while this may be feasible forCategory 1 developments it isunlikely to meet the test of Category2 or 3 developments.

Of greater concern is that, asEurocode 7 is now “live”, guidancewithin BS EN ISO 22475-1:2006(2007c), Table 2 (column 9 and 10,row 12) discloses that undisturbedsamples of fine soils obtained bya shell and auger U100 samplerwill only meet the specification ofCategory C or at best Category B,equivalent to soil quality class 4 orat best class 3.

The view is that undisturbedsamples obtained from CategoryC have been altered and must beregarded as disturbed samples.Consequently such samples areunsuitable for testing for shearstrength or for compressibility (class1) as provided in BS EN 1997-2:2007.

However, according to paragraph6.4.2.6.3 of the BS EN ISO 22475-1:2006 (2007c, p.25) “Thick-walledopen-tube samplers are mostlysuitable for stiff and dense soils andfor soils containing coarse particles(see line 2 of Table 3).

“For soil types that are difficultto sample, sample-retaining or

closure devices are necessary.”Furthermore, according to

paragraph 6.4.2.6.4: “The thick-walled open-tube sampler is usuallyregarded as a category B samplingmethod.”

It is also to be noted from Table3 that open-tube thick-walledsamplers are suitable for categoryB class 3, but may be suitable forcategory A class 2, if soil conditionsare favourable.

The table also states that open-tube thick-walled and thin-walledsamplers are unsuitable for usein “firm cohesive” soils. Thereappears to be some confusion in theterminology used in columns 5 and6 of Table 3 in that it is unclear ifthe phrase “firm cohesive” used incolumn 5 relates to strength whereasit is clear that “firm” used in column6 refers to consistency.

If column 5 refers to strengththen it is apparent that open-tube thick walled and thin walledsamplers are only suitable for soilswith an undrained strength lessthan medium with soils of greater

undrained strength requiringinvestigation by rotary cored

wireline drilling or similar.Notwithstanding the foregoing, it

is evident that the open-tube thick-walled sampling will not provideclass 1 samples and as such itwould seem, at least from a Scottishpoint of view, that rotary coredwireline drilling could becomethe “standard” replacement forthe shell and auger rig to providesamples suitable for strength andcompressibility testing.

However, again there is conflictwith BS EN ISO 22475-1:2006as, according to Part 2 (2007a,p75), class 2 samples can be usedfor strength tests etc. Indeed, forcertain soil or special purposes,shear strength tests can be carriedout on reconstituted or remouldedspecimens (2007a, p74).

Baldwin & Gosling (2009) providean insight into the implicationsthat BS EN ISO 22475-1:2006 willhave for geotechnical sampling inthe UK. To comply with the newguidance engineers should specify

thin-wall samplers to ensure class1 samples are obtained during the

TECHNICAL NOTE

0 C o e f f i c i e n t o f v o l u m e c o m p r e s

s i b i l t y ( m 2 / M N )

Uncorrected N value

10 20 30 40 500

0.1

0.2

0.3

0.4

y=-0.0508Ln(x)+0.2794

R2=0.2018

Figure 4: Uncorrected N value versus the coefficient of volumecompressibility

0

D e p t h ( m )

Undrained shear stength (kN/m2)

20 40 60 80 100 120

6

1

0

2

3

4

5

y=0.0095x+2.0916

R2=0.0316

Figure 5: Typical undrained shear strength versus depth


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GROUND ENGINEERING JULY 2010 31

ground investigation.According to the article by

Baldwin and Gosling (GE , March2010) the development of a thin-wall open drive tube sampler thatcomplies with the requirements

stated in BS EN ISO 22475-1:2006is now available on a commercial basis. Subsequently, Baldwin& Gosling (2010) discussed thedevelopment and field trials of a thinwall open drive sampler (UT100).

The field trials were carried outin a variety of soil types, includingglacial tills, but were limited to sitesin England. The results allowedBaldwin & Gosling to concludethat “the UT100 sampler is capableof taking a sample that meetsthe standard as being suitable forstrength and compressibility testingin the laboratory”.

Baldwin & Gosling also state:“Down to about 9m the undrainedshear strengths determined from theUT100 and U100 without a liner aresimilar to each other, whereas thosedetermined on the U100 with a linerare much lower.”

Albeit the information may begeographically limited, it begs thequestion why the U100 withouta liner cannot be used at least insome areas. The article also pointsout that “undrained shear strengthsof between about 50kPa and220kPa have been measured in thelaboratory on the samples recoveredfrom the UT100”.

The higher values indicate soilsof very high strength, analogousto the former description of “verystiff” soils in terms of BS5930(2007d) and as such it is unclearas to whether or not the use of theUT100 sampler in these soils wouldcomply with Table 3 of BS EN ISO22475-1:2006.

In January 2010 SouthLanarkshire Council commencedlimited field testing of the ArchwayUT100 cutting shoe, following theguidelines provided in Eurocode 7.

To date testing has been verylimited and only carried out on

two sites. In both instances the sitescomprised medium-to-high-strength boulder clays.

The results on the cutting shoeare shown below. During the trialscrumpling and deformation of

the cutting shoe was noted. Theresults tend to indicate that, due tothe nature of Scottish glacial tills,which can include pre-consolidatedfine soils with gravel, cobble layersand or boulders, the use of thistype of shoe is unlikely to produceundisturbed samples that wouldcomply with Eurocode 7.

Notwithstanding that the testingwas limited, the indications are thatthe UT100 sampler is unlikely to besuitable for the geographical areaof South Lanarkshire. Indeed thecontractor involved in the testinghas been advised by Archway thatthe UT100 will not be suitable forthe Scottish area.

On this basis it would seem thatthe only alternative will be to utiliserotary cored wireline drilling withthe attendant increase in cost andpossibly time, unless clarificationof Tables 2, 3 and 4 of BS EN ISO22475-1:2006 can be produced bythe Technical Committee.

The experience in SouthLanarkshire is that, irrespective ofthe type of sampler used, there can be no guarantee that the quality ofdata when plotted on an Excel chartwill provide any better correlationsthan those that are typically

obtained.The problem is not so much about

quality of samples but about theinterpretation of the results. Afterall, a test is only a test carried out ona sample from a specific locality, it isnot definitive. It is the geotechnicalengineer who must arrive at a designvalue as design values obtained froma laboratory test.

The use of rotary cored boreholesto obtain suitable undisturbed soilsamples has major implications forthe Scottish market, not least incost and time. Inevitably there is theoption to scale back the extent of the

investigation to control costs, butthis is likely to lead to a reduction inthe amount of detail across the site.

Limiting the detail simply meansdecreasing the sample size ofthe data and, consequently,

increasing the margin of error. Inother words obtaining good qualitysamples and obtaining resultsthereof is of little use if you havean insufficient sample size uponwhich to statistically determinethe characteristic value andsubsequently the design value.

It could be argued that it is up tothe geotechnical engineer to specifywhat is required within the groundinvestigation – the contractormerely carries out the requirementsof the engineer.

Therefore there is the potentialthat, if the engineer does not complywith Eurocode 7 and damage to astructure is subsequently sustained,then the engineer could be heldliable for non-compliance.

Scottish soils are notoriouslyvariable and, as such, there is aneed for flexibility in obtainingsamples, in terms of number andcost. Accordingly there is a need fora comparable sampler to the U100which meets the standard set byEurocode 7.

The work to date carried out by Baldwin & Gosling is to becommended but it needs to beextended to other geographicalareas, after all Eurocode 7 is a

European guidance note.Further testing might reveal that

the UT100 is satisfactory for someparticular types of Scottish clays andwould provide some degree of analternative to rotary cored wirelinedrilling, subject to amendment ofTable 3.

However, it is unlikely that siteswill fall into a neat category to suitonly rotary cored drilling or pistonsampling etc; more likely sites willrequire a combination of methods,for example to allow pre-boring ofmedium-to-high-strength clays toallow insertion of a soil sampler tosample underlying clays of mediumstrength or less.

Availability of rotary or soilrigs may prove to be a problem, inturn extending the period normallyexpected for the site investigation.

While the objective behindEurocode 7 is to be welcomed itwould seem that there is a lackof awareness of the implicationsof the code by professionals andcontractors alike. One thing is forsure, Eurocode 7 is not going to goaway.

It would seem sensible for theindustry as a whole to gather now, before it gets too late, and work

as one to better understand theimplications of the new code.

A deformed (left) and new (right) Archway UT100 cutting shoeprovided by BAM Ritchies

Baldwin, M and Gosling, D (2009) Technical note – BS EN ISO 22475-1:Implications for geotechnical sampling inthe UK. GE , vol. 42, No. 8, pp28-31.

Baldwin, M and Gosling, D (2010)Technical note – Development of a thinwall open drive tube sampler (UT100). GE,vol. 43, No. 3, pp37-39.

British Standard (2007a) Eurocode 7 – Geotechnical design –Part 2: Ground investigation and testing.British Standard Institution, BS EN 1997-2:2007, ISBN 0 580 50569 0.

British Standard (2007b)Geotechnical investigation and testing– Identification and classification of soil– Part 2: Principles for a classification.British Standard Institution, BS EN ISO14688-2:2004, ISBN 0 580 47508 5.

British Standard (2007c)Geotechnical investigation and testing– Sampling methods and groundwatermeasurements – Part 1: Technical

principles for execution. British StandardInstitution, BS EN ISO 22475-1:2006,ISBN 0 580 5056 9.

British Standard (2007d) Code of practice for site investigations.United Kingdom, British StandardInstitution, BS 5930:1999, ISBN 0 58061622 8.

British Standard (2010) Eurocode 7 – Geotechnical design –Part 1: General rules. United Kingdom,British Standard Institution, BS EN 1997– 1:2004 – Incorporating corrigendumFebruary 2009, ISBN 978 0 58067106 7.

Charles, J.A. (2005) Geotechnics for building professionals.Watford: British Research Establishment.

Reid, A D (2009)

The use of the standard penetrationtest to estimate the undrained shearstrength and the coefficient of volumecompressibility of clay soils; MScdissertation, University of Glasgow.

Sivrikaya, O and Togrol, E (2006) Determination of undrained shearstrength of fine-grained soils by meansof SPT and its application in Turkey.Engineering Geology, 86,pp 52-69.

Stroud, M A (1974) The standard penetration test ininsensitive clays and soft rocks.In Proceedings of the EuropeanSymposium on Pentration TestingESOPT, Stockholm 1974. Stockholm,National Swedish Building Research,pp367-375.

Stroud, M A and Butler, F G (1975)The standard penetration test andthe engineering properties of glacialmaterials. In: Proceedings of theSymposium of glacial materials,University of Birmingham, April 1975.

Terzaghi, K and Peck, P (1948) Soil mechanics in engineering practice,1st ed. London: Chapman & Hall, pp299-300 & 430-431.

Tomlinson, M J (2001) Foundation design and construction. 7thed. England: Pearson Prentice Hall, p11.

Winn, K, Rahardjo, H andPeng, S C (2001) Characterization of residual soils inSingapore. Journal of the SoutheastAsian Geotechnical Society, Vol. 32, No.1. pp1-13.

References

Documents

The Misuse of SPTs in Fine Soils and the Implications of Eurocode 7 Reid Taylor