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carried out at nine sites across the UK (both working sites as well asBuilding Research Establishment test bed sites) to clarify the usefulnessand reliability of the software analysis and correlations for use in the UK.

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le ancaic~ lI weicl ht

)enetroiiIeI:er I'orsoil invesI:iclaI:ionanc iiIoniI:orinclAlaI ',erla

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xso penetration due to one blow of the hammer (90'one)Aistheareaof theconeM is the striking massP is the struck massVis the velocity of impact (of the hammer)

by DD Langton, Soil Solution

AbstractDr Roland Gourves, the principal lecturer in soil mechanics at CUST,Blaise Pascal University, Clermont-Ferrand, France has designed andbeen developing the Panda (a lightweight handheld dynamic conepenetrometer for testing soils and materials) since 1991.It has becomewidely used and accepted across France, parts of central Europe and insmall numbers around the world.

Since its initial development, the Panda has been continuouslydeveloped to be used to test the compaction of fill in earthworks, throughsoftware analysis as well as in site investigation. Trials have been

It should be noted that the expression for energy used in the formula(I/2M%) is for kinetic energy, as the energy input is variable because itis delivered manually by the blow of a hammer. The test values recordedby the microprocessor can be transferred to a computer to plot the valuesof cone resistance against depth using the Panda Windows software(Figure 2).

Although there are many applications of the Panda, they can beseparated into two main categories. First, in monitoring the compaction

of materials used as fill both intrenching works and to structures aswell as cut and cover or ground

2 improvement operations. Second,Variable(measured) the Panda can be used for all types of

2.16 site investigation and is especially2 4 10 useful where access is restricted, for

90 90 90 example for slope stability studies16 22.5 35.7 along road and railway cuttings and1.1 1.6 2.5 embankments and for testing the

0.586 strength of the pavement and0.5 permanent way. Other notable14 applications include investigations

for low-rise developments andtemporary structures and in thesafety of heavy plant. The Panda hasthe capability of reaching between4m and 6m in soils of up to 20MPa to30 MPa resistance and even deeperwhere circumstances allow.

50500

181590

43.721.9

61

35

o,lnsall er J

4

$

Cone resistance (Mpa)

0 2.5 5 7.5 10 12.5 '15ammer

The principle of the PandaThe Panda is a lightweight (20kg) dynamic cone penetrometer, which usesvariable energy and can be operated by one man to test soils in almostany location to a depth of 6m. Table 1 shows the dimensions of thePanda compared to specification for DPH and SPT according toBS1377:Part9:1990.

The blow from a hammer to the head of the tool provides the energyinput (Figure 1).A unique microprocessor records two parameters foreach blow of the hammer, the speed of impact and the depth of conepenetration, by measuring the output from two sensors. Anaccelerometer on the head of the tool measures the speed of impact ofthe hammer while the depth of penetration is measured by a retractabletape. The dynamic cone resistance (qd) and current depth are thencalculated and displayed in real time data on the screen of themicroprocessor. q„ is calculated using the Dutch formula (Cassan M,1988) which is the most appropriate formula for this apparatus and hasbeen modified to the following:

Pandmicroproc

Head

Rod

—Measurementof depth

Rgwe t (LEPTJ:The Panda dwlng

r 1 9 taslhlg

Rgwe 2:Etmmtfleof a test owstodoat lnhtose sandand soltsgL

Interpretation of the Pandaresults for monitorilng thecompaction of fillInterpretation of results from aPanda test when monitoring thecompaction of fill can be done easilyusing a simple method available inthe Panda software. The method,which is the result of a long period ofscientific research at CUST, BlaisePascal University in ClermontFerrand (Zhou 1997) involveschoosing the appropriate material(used during the fill operation) froma list available in the software basedupon a simple classification systemusing plasticity and grain sizedistribution.

GROUND ENGINEERING SEPTEMBER 1999 33

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Rgluu 3:Thetwo Nnes oftolerance rehdoto the chosendensNy of amaterial.Rgme 4 (FAR

RIRNT): Model

for conereslstsnce anddepth ln anhonle gee sonssoll ate ghnm

hand ofcompacgen.

00

0.1.0.2-

0.3-E

0.4-

oe 05-

0.6-

Cone resistance (Mpa)

2 3 4 5 678910 20 30 405060

zc

0.7-q

02), qd2: is the cone resistance at the cntical depthzc: is the cntical depth from the start depth

0.9

How are the lines of tolerance defined?These lines of tolerance can be defined because a relationship existsbetween the degree of compaction )li (or experimental dry density as apercentage of maximum dry density) and the dynamic cone resistancefor a given material. Following research (Zhou 1997) a model has beendeveloped for cone resistance and depth, in a homogeneous soil at agiven level of compaction (see Figure 4). It was found that soil of givenproperties compacted evenly to a given density, consistent values of zc,

qd, and q„scan be defined.

qd,: is the cone resistance at the surfaceq .,:is the cone resistance at the critical depth, which is the maximum

value of cone resistance for a given material at a given level ofcompaction.

zc: is the critical depth measured from the surface at which the

With 18 different natural soils to choose from, three different levels ofcompaction (95'/I of optimum Proctor, 98.5% of optimum Proctor and97% of modified Proctor), and a choice of moisture contents (either wetor dry of the optimum moisture content), over 130 different choices ofmaterials are available in the software. After the appropriate materialhas been chosen the software then plots two "lines of tolerance" on thegraph which relate to a chosen density for that material (Figure 3).

The two lines are known as the line of reference (green line) and theline of failure (red line). The green line represents the chosen level ofcompaction for the material and hence values plotting to the right of thisline are greater or equal to that level of compaction. The red linerepresents a 25/I to 3% margin of error below which the compaction ofthe material should not fall. The area between the two lines is known asthe area of tolerance.

Cone resistance maximum cone resistance q„n isfound.

aIDo

Using this technique soils andmaterials from across theclassification spectrum can betaken and compacted to varyingdensities and the average valuesof zc, q„, and qdz can easily becalculated and hence the lines ofreference and failure defined foreach material at each differentlevel of compaction.

Figures 5 to 8 show examplesSoil: 83 compaction: 03 of compaction monitoring from

different sites. Figures 5 and 6show tests carried out with the

Amec/Tarmac joint venture at Manchester Airport second runway.Figure 5 shows the results of a test carried out in cohesive fill used inthe cut and cover operation, the green line in this case represents 95 o

maximum density for a medium to high plasticity clay. The layerboundaries are well defined by a reduction in cone resistance (indicatedby dashed lines); the increase in cone resistance at 0.15and between 0.3mand 0.5m is due to modification of these layers using quicklime.

Figure 6 represents a test carried out on the same site in granularbackfill to the new A538 tunnel. The shape of the plot indicates thematerial was well compacted using the correct layer thickness as layerboundaries are poorly defined. The green line of reference represents98% maximum density, which remains straight, as the test did not gobeyond the critical depth (qd,). Nuclear Density Gauge results showedthe material was between 98,o and 100 I maximum density.

Figures 7 and 8 are both good examples of where oversized layershave been used when compacting a material. Figure 7 is a test carriedout on the clay lining of a landfill site using a high plasticity claycompacted wet of the optimum moisture content. Laboratory tests onsamples taken showed the density of the material to be between 95;6 and98% of the maximum. The green line of tolerance plotted on the graphagain represents 98% maximum density for a high plasticity clay wet ofthe optimum moisture content. On this particular site however, thedensity of the material is less important than the permeability andintegrity of the lining, which was found to be sufficient.

Figure 8 is a plot of the results of a test carried out in a trench in acarriageway for one of the utility companies operating in the Midlands.In this case the green line of tolerance represents 95% maximumdensity and this level of compaction appears to have been achieved,however layer thicknesses of over 300mm have been used, leaving weaklayers which could lead to settlement and ultimately lead to defects in

0.25-

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Cone resistance (Mpa)

2 3 4 5 6 78910 20 3040 5060 0.1

0.05-

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0.25-

Cone resistance (Mpa)

0,5 I 2 3 4 56 810 20 304050Clockwise from tap lefh

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Figures:lust la granular NeatManchester AlrporL

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0.1

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0.8-0.91,0-

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Cone resistance (M pa)

05 I 2 3 456 810 0.1

0.1-0.2-0.3-0.4-E

$ 0.50.6-0.7-0.8-0.9-

Cone resistance (Mpa)

Or5 I 2 3 456 810 20 30

34 GROIINI) KNGINKKRIN(i sKRTKhlBKR 1999

PAPER

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2022191916344246524835

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the reinstatement.

Interpretation of the Pandaresttits for site

BRE test sites qd (MPa)

0 2 4 6 8 10 12 14 16 18 20 00qd (MPa)

1 2 3 4 5 6 7 8 9 10

investigationThe output of the Panda isgiven in MPa, which, althoughdirectly related to values ofstatic cone resistance, requiressome correlations to othermore conventional designparameters such as SPT, othertypes of DCP, CBR andundrained shear strength.Correlations to these para-meters already exist thanks toexperience and researchcarried out to date in France,however some clarification ofthese correlations was requiredin typical soils found in the UK.

E 2-

6-

qd (MPa)

0246810121416 18 20 22 24 26 28 30

Crust

Rgnie Br~I eongnnens sog yrollies at BREseeL

2-

'5-

qd (MPa)

0 1 2 3 4

Stoney Clay till

5 6 7 8

Bttildi Resca rghEshbiigmtntent test bed sIesThe Building ResearchEstablishment (BRE) has anumber of well-documentedtest bed sites across thecountry, representing varioussoil types in differentgeographical areas (Table 2and Figures 9 to 13,Butcher etal, 1995). Typical continuoussoil profiles have beenobtained at each of the sitesusing the Panda (Figure 9) tocompare it to the informationalready collected from thesesites by the BRE.

Data was also collected atother non-BRE sites courtesyof Structural Soils and MurrayRix. It must be noted that whentesting with the Panda, anover-sized hole is produced toavoid the effects of rodfriction, although this doesbecome unavoidable at depth.

2-E

Rgnre 11:Canons Perh qd so deyth.

ravel

workedn Clay

theredn Clay

Rgnre13:Bsg Barstonqd so deyth.

E 2-

4

00 1 2 3qd (MPa)

4 5

Rgnre 13:golda)near qd so deyth

ured silty clay

6 7 8 9 10

Seasonallyweathered

n-grey mottled

clay erratics(1-30mm)

(rRo(INI) KNCINKKRIN( sKPTK5(IIKR 1999 3g

PAPER

However, while testing at each of the sites the rod friction was assessedevery 0.5m and the test terminated when it was considered that any rodfriction was adding to the values of cone resistance.

Analysis of the resultsThe data available from the BRE included static cone resistance (CPT)and dynamic sounding tests (DPL, DPM and DPH). The results from thestatic cone tests can be plotted directly against the Panda results as theyeffectively give the same parameter. The results from the other tests canbe plotted either as blows/10cm or as dynamic cone resistance (qd) usingthe Dutch formula, although the energy input for the equation is forpotential energy (MgH) and not kinetic energy.

p0.1

0.5-1.0

E 15

k 2:5

3.0-

qd (MPa)

0.5 1 2 3 4 567810 20 304050 Rsuru td:10tests ourledout tocontamlrudodsreund treateduslns theColmlx process.

CowdenThe overconsolidated glacial tills at Cowden have been studied for over20 years and their properties are well documented (Marsland A andPowell JJM, 1985). For the depths tested, the soil consists of a stiff tillwith gravel of intermediate plasticity. Figure 10 shows profiles to adepth of Sm for each of the apparatus, each showing a similar profilewith gravel layers at 2.2m and 3m.

Canons ParkAll of the testing carried out at Canons Park (Figure 11)resulted in verysimilar profiles, all reflecting dense gravel overlying London Clay (highplasticity firm to stiff clay). The dynamic soundings all reflect thedifferent soil profiles well, with the more lightweight equipmentlogically giving the highest blow counts and more detail. Although boththe depth of the base of the gravel layer and the values for coneresistance vary considerably within the gravel, this can be expected insuch a variable material, and it is known that the depth of the base of thegravel varies locally.

More importantly, a great deal of consistency in cone resistance forthe clay above and below the gravel is shown by all the different types ofequipment. The increase in cone resistance at around 5m is also veryconsistent, this marking the boundary between reworked London Clayand undisturbed weathered London Clay.

BothkennarThe site at Bothkennar is located on the south bank of the River Forthclose to Kincardine Bridge. The soil at the site is a recent marine depositand consists of a 1.5m clay crust overlying 17m of a medium to highplasticity very soft silty clay (Powell, J.J.M.and Quarterman RST, 1995).

Blow counts from this site generally gave counts of between 0 and 3,which accounts for the erratic plots (Figure 12) when blows areconverted to cone resistance (note 1DPH blow = 2MPa). This sitehighlights the lack of definition shown by heavier equipment in verysoft soils. It must be noted that according to BS 1377 and recommendedtesting procedures, values of less than 3 for DPH are invalid. The DPMgives a higher blow count than the DPH because the annulus between therods and the cone size is smaller, causing it to suffer greatly from rodfriction. The Panda data and the static cone data did however give verysimilar results to a depth of 4.5m below which the Panda began to sufferfrom rod friction.

PentreThe site consists of 3m to 4m of alluvium (intermediate plasticity stiffsilty clay) overlying normally consolidated very silty clays of lowplasticity (Lunne T, Robertson PK and Powell JJM, 1997).Unfortunatelythere are no dynamic soundings available from the site at Pentre andinsufficient static cone resistance data from shallow depths to make anyuseful comparisons. However, some of the basic soil properties aredetailed in Table 1along with values for undrained shear strength.

The transition from the alluvium to the silty clay can be seen in thesoil properties by a 50% reduction in clay content at around 4m and afurther 50% reduction at 5m which are accompanied by reduced shearstrength and plasticity. This change is also visible in the profile of thePanda data (Figure 9) with a gradual increase in cone resistance from2MPa to 4 MPa between 3.5m and 5m.

BRE GarstonLocated 30km north west of London, the BRE site at Garston (Figure 13)consists of 12m of chalky boulder clay (Marsland A and Powell JJM,1989). It must be noted that the tests carried out with the Panda werecarried out some distance from the tests carried out by the BRE usingDPH and static cone resistance, which may account for the difference inprofiles in the first 2m. The profiles between 2m and 4m are moreconsistent.

Vale ofYork

Data has also been used for correlation purposes from a location in theVale of York, which consists of very sandy glacial till with some gravel.

The Panda for ground ImprovementThe benefit of using the Panda for ground improvement is that it can beused purely as a comparative tool without the need to correlate the datato other parameters. Figure 14 shows a composite plot using the Pandasoftware of 10 tests carried out in contaminated ground which had beentreated by Bachy Soletanche using its Colmix process to preventcontaminants leaching into groundwater. The tests were carried outseven days after the ground had been treated to allow time for the groutto set. A minimum resistance of 1MPa for the treated ground wasrequired and the plot shows the variation in cone resistance across thesite after treatment was consistently between 2 and 10MPa.

Summary

Soil type Typical Panda qd

Very soft claySoft to firm clayFirm to stiff claySand and gravel

0-1MPa1-2MPa2-3MPa4-30MPa

Correlations to SPT and OCPThe profiles of DCP data converted to cone resistance are very similar tothose of static cone resistance and the Panda and hence a goodrelationship between all the different pieces of apparatus must exist.Correlations between DPH N,p and SPT N (N = 8N» -6, Butcher et al, 1995and N = 5 x N» AP Butcher, internal BRE report) have already beenidentified by the BRE for stiff clays. Hence, a correlation from Panda toDPH N Ip would allow a tentative correlation to SPT to be calculated.

Figure 16 shows values of DPH Nip plotted against Panda qd andresults in a correlation of 0.8DPHIp 1 MPa (qd), this gives a correlationof between 0.1and 0.2 SPT N per 1 MPa (qd). Further direct comparisonsbetween Panda MPa and SPT N values across a range of soil types wouldhelp to give confidence and improve this correlation.

Correlations to undrained shear strengthFigure 17 shows data from all the sites visited from horizons where thepercentage of clay was greater than 30%, It also includes data from alocation in the Vale of York (courtesy of Structural Soils), whichconsists of very sandy boulder clay with some gravel. The data gives acorrelation of Cu (kPa) = qd (kPa)/20. This agrees with the correlation

CorrelationsThe following correlations have been identified as a result of studiescarried out in France (Gourves and Barjot 1995),or as a result of a Pandaassessment by the Transport Research Laboratory.

1qd = 1MPa (CPT) (France)1qd (kPa) = Cu(kPa)/15 to 20 (France)logipCBR 0.352+1.057x log»qd(MPa) (TRL)TRL mm/blow = 100/qd (TRL)

Correlations to static cone resistanceThe plots of the BRE sites confirm that the Panda gives a goodapproximation to static cone resistance, as well as confirming that thePanda produces reliable values of cone resistance (qd MPa). Figure 15shows Panda data plotted against static cone resistance and includes anX = Y line which fits to data reasonably well.

This relationship has also been recognised by the BRE in stiff clays(Butcher et al, 1995).Establishing a relationship for soft clays, qt = 024 qd+0.14,will require further testing with the Panda.

36 GRQUND ENGINEERING SEPTEMBER 1999

PAPER

6-

a.

Ocx

CL

2-

4'n(nct)00~ (ups)

Rgnre18: Pandadafa againstslallc colNreslshnca 452ee ''O,O -; . R:H0%

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Trench1 Ave qd:37 CBR:102 %Trench2 Ave qd:31 CBR:84%Trench 3 Ave qd: 28 CBR'6 %Trench4 Ave qd:8 CBR:20%

Rgwnlgt ConerosbrbnnN

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10

~ The Panda has also been used to monitor the compaction of soils andmaterials in areas of fill, and road pavements. Correlations to the TRLDCP probe and CBR have recently been identified as a result of anassessment by the Transport Research Laboratory, however furthertrials using a range of materials is needed to establish if theserelationships exist in all circumstances.~ The results have also shown that the cone resistance values of thePanda are reliable and show good relationships to other methods oftesting.~ Correlations to static cone resistance, dynamic cone resistance,standard penetration test and undrained shear strength alreadyidentified by research in France have been verified for the UK.~ Further testing over wider range of different plasticities as well assands and gravels would clarify these correlations further.~ It must be noted that correlating from one parameter to anotherreduces the reliability of the value and hence correlations should bekept to a minimum and the values chosen to use in the correlationshould be chosen with care. However, with some care and experiencegood correlations can be made from one parameter to another.

6-

4-

4> 2-

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identified in France (Cu = qd/15-20) as well as being similar to thecorrelation between dynamic cone resistance and undrained shearstrength (Cu (kPa) = qd (kPa)/22) identified by the BRE for stiff clays.(Butcher AP, McElmeel K and Powell JJM, 1995).

The BRE has also discovered a second correlation for more sensitiveclays, such as very silty clays and very soft clays (cu = (qd/170)+20) whichthe results from greater than 4m at Pentre appear to follow, howeverthere is insufficient data to verify this correlation at present.

AcknowledgementsThe author would like to thank the BRE for allowing the use of its testbed sites and for supplying all the relevant data pertaining to those sites.Special thanks go to John Powell at the BRE. Soil Solution would alsolike to thank the AMEC/Tarmac joint venture at Manchester Airport,Murray Rix. Structural Soils, Bachy Soletanche and NorthumberlandCounty Council for theirhelp.

ReferencesAmor SJ, MH Burtwe)I and AS Turner (1999). Panda dynamic cone penetrometerassessment. Transport Research Laboratory, Crowthorne.BS 1377:Part 9:1990 Methods of testing soils for civil engineering purposes. BritishStandards institution. London.Butcher AP, McElmeel K & Powell JJM (1995).Dynamic probing and its uses In clay soils.Proceedings International Conference on Advances in Site Investigation Practice. ICELondon, March 1995.Thomas Telford. pp 383-395.Butcher AP. The use of dynamic soundings in clays, internal BRE report.Cassan M (1988). Les essais in situ en mechanique des sols Volume I realisation etinterpretation, Eyrolles, 1988,pp 146-151.Gourves R & Barjot R (1995).The Panda ultralight dynamic penetrometer. Proc 11th Euro.Conf. on Soil Mechanics and Foundation Engineering, 1995Copenhagen.Gourves R Panda ultralight dynamic cone penetrometer for soil investigation. Laboratoirede Genie Civil, CUST, Universite Blaise-Pascal de Clermont-Ferrand, France. BP 206 63174Aubiere.Jones C R and Rolt J (1991)Information note, Operating instructions for the TRRL dynamiccone penetrometer. Overseas unit information notes, Second edition. Transport ResearchLaboratory, Crowthorne.Lunne T Robertson PK and Powell JJM (1997).CPT in geotechn ical practice. Spon.Marsland A & Powell JJM (1985).Field and laboratory investigations of the clay tills at theBuilding Research Establishment test site at Cowden Holderness. Proceedings

International conference on construction inglacial tills and boulder clays. March 1985Edinburgh, pp147-168.Marsland A and Powell, JJM (1989).Field andlaboratory investigations of the clay tills at thetest bed site at the Building Research Station.Quaternary engineering geology, GeologicalSociety engineering geology specialpublication No7 pp229-238.Powell JJM and Quarterman, RST (1995),Engineering geological mapping of soft clayusing the ptezocone, Proc internationalsymposium on cone penetration testing,CPT'95. Linkoptng, Sweden. October 1995. Vol2, pp 263-268Zhou S (1997) Caracterisation des sols desurface a I'aide du penetrometer dynamicleger a energie variable type Panda.Formation Doctorale "Materiaux, structures.fiabilite en genie civil et genie mecanique"Laboratoire d'Accueil, LERMES/CUST,Un ivers ite Blaise Pascal.

Conclusions~ The results show.that the Panda is a lightweight effective tool fortesting undisturbed soils up to a depth of 6m and is especially usefulwhere access is restricted.

0.4

0.35-

y = 0.0092xRz = 0.948

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Correlation to TRL BCP and CBRFigure 18 shows data collected from an assessment of the Panda by theTransport Research Laboratory. The tests were carried out in a series oftrenches 900mm deep and backfilled to the surface with type 1 granularsub-base at varying levels of compaction and layer thicknesses. A seriesof tests were then carried out on each of the trenches with the Panda andthe TRL DCP. The data shows a linear relationship with a correlation ofTRL mm/blow = 100/qd.

The relationship between penetration in mm/blow can be related toCBR using the TRL Road Note 8 equation and has been adapted for usewith the Panda (Table 3).Using these two equations, CBR values havebeen calculated using data collected during the TRL assessment. Valuesfrom below 300mm was used as the strength of the material becomesconstant below this depth. The values obtained for CBR are comparablewith the exception of trench three. Trenches two and three both receivedcomparable levels of compaction and hence similar values of CBRwould be expected, such as those obtained from the Panda.

GROUND ENGINEERING SEPTEMBER 1999 37