Articulo sobre comparación del uso del piezocono en arcillas lacustres

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Hird, C. C. & Springman, S. M. (2006). Ge otechnique 56, No. 6, 427–438

427

Comparative performance of 5 cm2 and 10 cm2 piezocones in alacustrine clay

C . C . H I R D * a n d S . M . S P R I N G M A N †

A piezocone investigation has been carried out in a deepdeposit of glacial lacustrine clay using piezocones withcross-sectional areas of 5 cm2 and 10 cm2. The piezoconetests formed part of a larger soil characterisation study,but this paper focuses on the relative performance of thepiezocones in profiling the clay. It is shown that using a5 cm2 piezocone rather than a 10 cm2 one significantlyimproved the detection of relatively thin silt layers withinthe lacustrine, and occasionally varved, clay. The feasibil-ity of detecting silt layers as thin as 2–4 mm and of matching the piezocone responses associated with siltlayers at different test locations, thereby allowing anassessment of their continuity, is demonstrated. There

were no significant differences between the magnitudes of the cone resistance and excess pore pressure recorded inthe clay with 5 cm2 and 10 cm2 piezocones. Pore pressuredissipation test results were variable but, in a regionwhere no silt layers were detected, similar results wereobtained with piezocones of each size.

KEYWORDS: clays; fabrics/structure of soils; in situ testing;site investigation

Nous avons effectue une etude au piezocone dans un depotprofond d’argile lacustre glaciaire en utilisant des sectionstransversales de 5 cm2 et de 10 cm2. Les essais au piezo-cone faisaient partie d’une plus vaste etude de caracterisa-tion du sol mais cet expose se concentre sur laperformance relative des piezocones dans le profilage del’argile. Il est montre qu’en utilisant un piezocone de5 cm2 plutot que de 10 cm2, on ameliore de manieresensible la detection de couches de limon relativementminces dans l’argile lacustre et occasionnellement varvee.Nous demontrons la faisabilite de detecter des couches delimon d’une epaisseur de 2 a 4 mm et de faire corre-spondre les reponses des piezocones associes avec les

couches de limon dans diverses zones d’essai, permettantainsi une evaluation de leur continuite. On n’a constateaucune difference significative entre les magnitudes desresistances de cone et la pression interstitielle excessiveenregistree dans l’argile avec des piezocones de 5 cm2 et de10 cm2. Les resultats des essais de dissipation de pressionde pore etaient variables mais dans une region ou aucunecouche de limon n’a ete detectee, des resultats similairesont ete obtenus avec des piezocones de chaque dimension.

INTRODUCTIONPiezocone testing is a mature ground investigation technique

that has been substantially standardised. However, althoughthe international reference test procedure for piezocone test-ing (ISSMGE, 1999) refers to a piezocone with a cross-sectional area of 10 cm2, the use of either a larger or asmaller piezocone is permitted (within area limits of 5 cm2

and 20 cm2), and may be advantageous. For example, alarger piezocone is more robust and, depending on the load cell arrangement and specification, can give more accuratecone resistance data in soft soils; a smaller one can give better detection of thin layers, via the pore pressure response(Lunne et al., 1997; Tumay et al., 2001). Specially designed pore pressure probes, with relatively small diameters, havealso been employed for the latter purpose (e.g. Torstensson,1977; Kolk & Wegerif, 2005) but these do not fall within

the scope of the ISSMGE (1999) test reference.When different sizes of piezocone are employed, the

question of scale effects inevitably arises. For piezoconesranging in area from 5 to 15 cm2, the usual assumption, based on experience summarised by Lunne et al. (1997), isthat scale effects are negligible in soil layers of sufficientthickness relative to the cone diameter: that is, quantitiessuch as the cone resistance and excess pore pressure do notdepend on the size of the piezocone. However, in highlyinterbedded soils, significant scale effects are to be expected.

This issue was studied theoretically by Vreugdenhil et al.(1994) for cone resistance, and a scale effect on pore

pressure was demonstrated experimentally by Hird et al.(2003), who compared the results from 1 cm2 and 5 cm2

piezocones in specially constructed soil models.The enhanced detection of layering detail that can be

achieved using a smaller piezocone can lead, for example, toan improved characterisation of clay deposits containing thinsilt or sand layers. These layers, if sufficiently numerous,continuous and permeable, may influence or substantiallycontrol the rate of drainage and consolidation of the depositunder applied loads. The obvious benefit of more reliable predictions of the rate of consolidation, based on better knowledge of layering, is that such predictions will enablemore cost-efficient management of construction processes.

Clays deposited in glacial lakes may well be varved, with

seasonal layering of the silts and clays on the scale of a fewmillimetres each (van Husen, 1987). Otherwise, the exis-tence, thickness and composition of any silty layers withinthese clay deposits will vary, even within a single deposit,according to the location and conditions during the sedimen-tation process. Influential factors will include the size and shape of the catchment, the sediment supply, and the ratesof water inflow and outflow, with seasonal variations due torunoff and meltwater. In these circumstances, site character-isation requires the best possible investigation of layering.This is true, for example, for significant areas in the ‘Mittel-land’ region of Switzerland, between the Jura mountains and the Alps, where the advance and retreat of the glaciers hasoffered opportunities for lacustrine clays to sediment out in

pro- or post-glacial lakes. Although Heil et al. (1997),Springman et al. (1999) and Trausch-Giudici (2004) haveemployed 10 cm2 piezocones at several sites in this region,to the authors’ knowledge no corresponding experience of

Manuscript received 11 August 2005; revised manuscript accepted 13 April 2006.Discussion on this paper closes on 1 February 2007, for further

details see p. ii.* Department of Civil and Structural Engineering, University of Sheffield, UK.† Institute for Geotechnical Engineering, ETH Zurich, Switzerland.



using smaller piezocones has yet been reported. One of thedifficulties of using a smaller piezocone in these particular deposits is the risk of damage if underlying hard layers (e.g.moraine) are penetrated, or embedded coarse material isencountered. Such material may be embedded by landslidesor by the glacial deposition of ‘erratics’, which may be boulders composed of granite, limestone or marl (Gerber &Kopp, 1990; Gerber, 1994).

As part of a larger study of the effectiveness of a varietyof ground investigation techniques in such soils, a compari-son has recently been made of the performances of 5 cm2

and 10 cm2 piezocones in a thick deposit of Swiss lacustrineclay at Wauwil, about 70 km SW of Zurich. At this location,a 4.7 km2 lake formed behind a terminal moraine after theretreat of the Reuss glacier from its maximum extent c.22 000 years ago during several periods of advance and retreat until c. 12 000 years ago. The purpose of this paper is to report the piezocone investigations and to draw conclu-sions about the use of 5 cm2 piezocones in similar deposits.The results of the larger study will be reported separately.

GROUND CONDITIONSIt is believed from surface-based seismic refraction and

reflection surveys carried out at the Wauwil test site that thedepth of the lacustrine deposits is at least 70 m (Maurer,2004, private communication). The ground conditions to adepth of 31 m are summarised in Fig. 1. Above 8 m depth,the soil below the topsoil is a calcareous sandy silt, and atdepths of up to 3 m the content by weight of calcium

carbonate (as dolomite: CaMg(CO3)2) is over 70%. Thecalcareous content drops to around 44% at 5 m depth and to36% at 7 m depth. In the Unified Soil Classification Systemthis soil is classified as MH or ML, whereas below 8 m thesoil is classified as CI. Fig. 1(b) shows that the soil possesses unusually high plasticity at shallow depths, prob-ably because it contains some organic material. At 1.5 mdepth the organic content is medium, as determined inaccordance with the Swiss code (SNV, 1999) and at 3 mdepth the soil is lightly organic. Below this, the soil wasfound not to be organic. Between about 8 m and 21 m depththe Atterberg limits are reasonably uniform, whereas be-

Fig. 1. Ground conditions at Wauwil: (a) soil description; (b) natural water content and Atterberg limits; (c) vertical stresses; (d)overconsolidation ratio

428 HIRD AND SPRINGMAN



tween about 21 m and 30 m there is a slight reduction in plasticity. This is consistent with an overall increase in thesilt content as varving becomes more apparent and indivi-dual silt laminae attain a greater thickness. A profile of totalvertical stress, based on the measured water content and specific gravity together with an assumption of full satura-tion, is plotted in Fig. 1(c).

The test site lies in the centre of a laterally extensive flat

area just behind the furthest projection of the Reuss glacier into the Wigger valley. Following the glacial retreat, the post-glacial lake gradually drained as an outlet through theterminal moraine was eroded by the outflowing stream, untilthe area became an uncultivated marshy wetland. However, aland drainage and pumping system was installed in 1965 tocreate a thin desiccated crust and permit agricultural use.The groundwater level at the time of the tests was onlyabout 0.25 m below the ground surface on average, but itmust have been lowered to a depth of around 1.25 m atsome previous time for long enough for consolidation tooccur. This statement is based on the evidence of desiccationin the cone resistance profile, presented below. In a soft soildeposit with a fluctuating water table, Parry (1970) showed that, provided desiccation extends to the lowest level of thewater table, the resulting undrained shear strength (and hence cone resistance) of the soil reduces with depth and reaches a minimum at that level (i.e. 1.25 m depth in thiscase). From the present and previous groundwater levels, a profile of overconsolidation ratio due to groundwater fluctua-tion can be constructed, and this is shown in Fig. 1(d). It isnot known to what extent ageing processes may have in-creased the apparent degree of overconsolidation, but the

effect is not thought to be large. This point is discussed again later. In calculating effective vertical stresses, hydro-static conditions were assumed to exist below the presentand previous water tables. This is consistent with boreholeobservations and the results of piezocone dissipation testsreported below. The current effective vertical stress profile isshown in Fig. 1(c).

PIEZOCONE TEST PROCEDURES Piezocones

The dimensions of the piezocones used in the study areshown in Fig. 2. There were two 5 cm2 instruments, piezo-cones F1 and F2, each of which could measure pore water pressure at four positions, as indicated by the filter elements(Fig. 2(a)). The measured pore pressure will be denoted bythe symbol u1, u2, u3 or u4 depending upon the filter position, as indicated. The vertical force on the end of thecone was measured with a load cell of 10 kN capacity. Thecombined force on the end of the cone and on a frictionsleeve of area 100 cm2 was similarly measured, and theforce on the friction sleeve was calculated by subtraction.The piezocone was connected to 10 cm2 driving rods by atapered connector, with a taper starting 365 mm above theu4 filter.

Two 10 cm2 piezocones were also used, piezocones ETH1and ETH2 (Fig. 2(b)). One of these had three pore pressuremeasurement positions (giving u1, u2 and u3) and the other just one (giving u2). The loads on the cone and the 150 cm2

friction sleeve were measured independently by load cellswith capacities of 50 kN and 7.5 kN respectively.

Fig. 2. Piezocone dimensions: (a) 5 cm2 piezocones F1 and F2 (*bracketed dimensions applyto cone F2); (b) 10 cm2 piezocones ETH1 and ETH2 (†filters absent on cone ETH2). Alldimensions in mm

COMPARATIVE PERFORMANCE OF PIEZOCONES IN A LACUSTRINE CLAY 429



Each piezocone, with filters attached, was saturated in thelaboratory by placing it in a chamber under a high vacuumfor at least 30 min before allowing previously de-aired saturation fluid to enter the chamber until the filters weresubmerged. The fluid used for the 5 cm2 piezocones wasglycerine, which was pre-heated to about 708C in order toreduce its viscosity. A 1:1 mixture of glycerine and water atroom temperature was used for the 10 cm2 piezocones. After

the fluid had entered the chamber, the vacuum was main-tained for at least 1 h. On completion of the saturation procedure, it was possible to check its effectiveness byquickly altering the pressure in the chamber and thencomparing the form of the cone resistance and pore pressureresponses. Typically, a pressure increase of 300 kPa wasachieved in about 0.7 s. When normalised by the final load or pressure change, the responses should, ideally, be identi-cal. In reality, small lags in the pore pressure responserelative to the cone load, generally of only a few milli-seconds but exceptionally as large as 50 ms, were observed.Several checks of this type were carried out prior to the startof the field testing, and these confirmed the reliability of thesaturation procedure. However, the transfer of the datalogging system to site meant that the saturation could not bechecked in the laboratory in every case.

As mentioned later, there was evidence of inadequatesaturation in one field test with a 5 cm2 piezocone, but thiswas after an unrehearsed attempt was made to modify thefilter arrangement on the piezocone by fitting a filter at itsapex. In this case, the saturation had not been checked inthe laboratory. Very occasionally, it appeared that saturationhad been lost from a filter on a 10 cm2 piezocone during itstransportation to site and installation in the ground. It is probable that this was due to leakage of fluid as or after arubber membrane (intended to maintain saturation) was placed over the filters.

Data recording The signals from the piezocone transducers were ampli-

fied, either within the cone (piezocones F1 and F2) or externally (piezocones ETH1 and ETH2), before being passed to the data acquisition system. During penetration of the piezocones, including the pauses for the addition of driving rods, data were saved at the rate of 25 readings per second. In order to reduce electrical noise, each saved read-ing was the average of at least 80 readings recorded at amuch higher rate. A variable rate was adopted during pore pressure dissipation tests to capture sufficient data to definethe dissipation curve adequately.

Driving systemA highly portable, relatively lightweight, hydraulic driving

system was employed. This had to be anchored to theground using two 0.46 m diameter screw anchors, installed vertically to a depth of about 1.1 m. Anchors were screwed in at the corners of a 0.8 m square at each test location, inorder to optimise the use of four available anchors. Using pairs of anchors in turn, a piezocone test could then beconducted at the mid-point of each side of the square, sothat four closely grouped, individual tests were possible.However, as the minimum distance between the tests wasonly about 0.6 m, it was recognised that there was a risk of interference between adjacent tests due to lack of verticalityof piezocone penetration. From the readings of inclinometers

placed behind or within the piezocones, it was observed thatthe direction of driving deviated, in general, by up to about28 from the vertical. Theoretically, two adjacent test pathsdeviating towards one another at a combined angle of 48

could intersect at a depth of just over 8 m. Unfortunately, asreported below, interference did occur in one instance (at adepth of about 16 m in Test B208).

The length of each driving rod was only 0.5 m. Althoughthe anchors provided sufficient reaction, some vertical move-ment of the anchors and the rig was observed during therelease and reapplication of driving force at the rod changes.Clearly, this was not desirable. However, the net effect of

the movements on the measurements of depth made duringdriving was very small. The driving system was designed todeliver a standard rate of penetration of 20 mm/s. In prac-tice, the rate was generally close to 22 mm/s. When a pore pressure dissipation test was initiated, the hydraulic power was cut off but the rods remained clamped to the drivingsystem. Potentially, the compliance of the driving system,coupled with friction on the driving rods in the ground, hasan influence on dissipation test results. It is believed thatfriction on the driving rods was the dominant factor incontrolling the vertical position of the piezocones during thedissipation tests carried out in this investigation.

EXPERIMENTAL RESULTSThe tests were carried out at two locations, identified as A

and B, 30 m apart. The elevations of the ground surface atA and B, as measured by GPS, were within 0.1 m of eachother and so the difference of elevation may be considered to be negligible. Two tests were performed with a 5 cm2

piezocone at each location, and two with a 10 cm2 one. Asummary of the tests is given in Table 1, where the locationA or B is incorporated in the test identifier. Only three testsare listed at position B, because of obvious signs of inade-quate saturation of the 5 cm2 piezocone filters in one test,coupled with data logger malfunctions. Therefore no datafrom that test will be presented. Pore pressure dissipationtests were conducted at various depths, as indicated in Table2. The penetration tests all terminated at about 30 m depth,

which was the limit of what could be achieved withoutincurring excessive movement of the screw anchor system.

The data obtained during the rod changes have beenomitted in order to clarify the graphical presentation of theresults. Typically, there were significant reductions of coneresistance, sleeve friction and u1 after the driving force wasreleased, and noticeable temporary increases in u3 and u4

(where applicable). As the driving force was reapplied, the previous values of all these quantities were restored, albeit atdifferent rates. The data omitted for each rod changespanned the interval from the time that the cone resistancestarted to reduce until the time that its value was restored.

Cone resistance and sleeve friction profilesProfiles of cone resistance with depth are shown for six

tests in Fig. 3. The data from Test B207 were erratic (seeTable 1) and are not therefore included. In Fig. 3, the coneresistance has been corrected for the area ratio effect(Campanella et al., 1982)

qt ¼ qc þ u 1 að Þ (1)

where qt is the corrected cone resistance, qc is the uncor-rected cone resistance, u is the pore pressure at the base of the cone, and a is the cone area ratio. Values of a wereobtained experimentally for each cone, following the proce-dure recommended by Lunne et al. (1997), and are given inTable 1. The value of u was assumed to equal the measured

value of u2.Figure 3 shows that, generally, very consistent cone

resistance profiles were obtained, and that variations inmeasurements made with each size of piezocone were as




large as those in measurements taken with different sizes of piezocone. Profiles of corrected cone resistance show a moreor less linear increase with depth below the former deepestgroundwater level. This is characteristic of previous piezo-cone investigations in lacustrine deposits at Kreuzlingen(e.g. Amann & Heil, 1995; Heil et al., 1997; Springman et al., 1999), Birmensdorf (Panduri, 2000) and Wauwil (Heil,2005, private communication). However, the result from Test

B208 deviates from the rest between about 16 m and 22 mdepth, and this is attributed to the intersection of the piezo-cone with soil that had been disturbed by a previousadjacent test (a risk already discussed). The 5 cm2 piezo-cones recorded larger peak resistances when passing throughsome silt layers, especially below 25 m depth. Althoughdissipation tests using the 10 cm2 piezocone were targeted atsome of the silt layers below 25 m depth, this does notaccount for the lower peaks shown in Fig. 3. At shallowdepths, in very soft soil, it is clear that the precision of measurement is only of the order of 30%.

The results for sleeve friction (Fig. 4) display significantvariation at all depths. Values are very low, and small errorsin the datum reading at the start of Tests A303 and B305led to slightly negative sleeve friction values being recorded at depths of 4– 10 m. Hence no data of sleeve friction areshown from these tests. Ideally, frictional resistances should be corrected for the effect of unequal pore pressures on theends of the friction sleeve (e.g. Lunne et al., 1997) but, because the pore pressure at the upper end of the sleeve wasnot always measured, correction was possible for only twoof the tests (Fig. 4(c)). The area ratios given in Table 1 wereused for the correction, which was considerable for the5 cm2 piezocone. The friction values from the 10 cm2 piezo-cone appear systematically larger than those for the 5 cm2

piezocone (Fig. 4(c)). However, if the two other 10 cm2

piezocone test results, shown in Fig. 4(b) to be somewhatsmaller, had been similarly corrected, they would have beenin better agreement.

Pore pressure profilesProfiles of pore pressures measured at the cone shoulder

(u2) are compared in Fig. 5. Discounting the effect of pore pressure dissipation tests, the same comments about consis-tency can be made as for the cone resistance. Once again,Test B208 deviates at mid-depth. The u2 filter was effec-tively connected to the u1 filter in Test A202 (see Table 1) but, nevertheless, the response appears to be in line withother u2 responses. A data logging malfunction led to someerratic data above 12 m depth in Test A304.

Figure 6 illustrates typical results from a 5 cm2 piezocone.This displays the expected hierarchy of pore pressure magni-

tudes (Lunne et al., 1997). The pore pressure responses, asshown in both Fig. 5 and Fig. 6, are strongly influenced bysilt layers below about 22 m depth. The detection of more permeable layers is considered in the following section.

Detection of silt layersFigure 7 shows an example of pore pressure and cone

resistance responses from a 5 cm2 piezocone, when passingthrough one of the more prominent silt layers just below24.5 m depth. It can be seen that the layer was detectablevia the pore pressure response at all four filters. The u2, u3

and u4 responses are all very similar, suggesting that rela-tively little distortion or smearing of the silt layer occurred

after it passed beyond the shoulder of the piezocone. Theinitial peak in the u1 response as the silt layer is approached can be attributed to an increase of total stress in the clayunder approximately undrained conditions (Hird et al., T

a b l e 1 . S

u m m a r y o f p i e z o c o n e t e s t s

T e s t

i d e n t i fi e r

C o n e a r e a

( c m 2 )

C o n e

i d e n t i fi e r

A r e a r a t i o s *

F i l t e r s fi t t e d

F i l t e r m a t e r i a l

C

o m m e n t s

C o n e †

S l e e v e t o p ‡

S l e e v e b o t t o m ‡

A 2 0 1

1 0

E T H 1

0 . 7 6

0 . 0 1 7

0 . 0 1 7

u 2

C e r a m i c

A 2 0 2

1 0

E T H 2

0 . 6 7

0 . 0 1 7

0 . 0 1 7

u 1 / u 2 / u 3

C e r a m i c

P

o o r i n t e r n a l s e a l : u 1

a n d u 2

fi l t e r s e f f e c t i v e l y c o n n e c t e d .

A 3 0 3

5

F 1

0 . 6 0

0 . 0 1 3

0 . 0 2 4

u 1 / u 2 / u 3 / u 4

H D P E

O

f f s e t e r r o r f o r s l e e v e l o a d .

A 3 0 4

5

F 2

0 . 6 1

0 . 0 1 3

0 . 0 2 4

u 1 / u 2 / u 3 / u 4

H D P E

D

a t a l o g g e r m a l f u n c t i o n : s o m e e r r a t i c u 2

d a t a a b o v e 1 2 m

d e p t h .

B 3 0 5

5

F 1

0 . 6 0

0 . 0 1 3

0 . 0 2 4

u 1 / u 2 / u 3 / u 4

H D P E

O

f f s e t e r r o r f o r s l e e v e l o a d .

B 2 0 7

1 0

E T H 2

0 . 6 7

0 . 0 1 7

0 . 0 1 7

u 1 / u 2 / u 3

C e r a m i c

P

o o r s a t u r a t i o n f o r u 1 .

u

3

t r a n s d u c e r f a i l e d .

R

i g t r a n s m i t t i n g v i b r a t i o n s t o d r i l l r o d s : s o m e e r r a t i c c o n e a n d

s l e e v e l o a d d a t a .

B 2 0 8

1 0

E T H 2

0 . 6 7

0 . 0 1 7

0 . 0 1 7

u 1 / u 2 / u 3

C e r a m i c

u

3

t r a n s d u c e r f a i l e d .

P

r e v i o u s l y d i s t u r b e d s o i l i n t e r s e c t e d f r o m

1 6 t o 2 2 m , c a u s i n g

r e d u c e d p o r e p r e s s u r e s a n d c o n e r e s i s t a n c e

.

H D P E : h i g h d e n s i t y p o l y e t h y l e n e .

* A s d e fi n

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a l .

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† M e a s u r e

d v a l u e s .

‡ V a l u e s c

a l c u l a t e d f r o m

d i m e n s i o n s .




Table 2. Summary of pore pressure dissipation tests

Testidentifier

Cone area:cm2

Cone tipdepth: m

Filter position

Time for 50%dissipation: s

ch/( I r )0:5:

3 106 cm2/s

A201 10 25.7 u2 545 1431A202 10 25.5 u2 400 1949

u3 1650 266227.7 u2 340 2293

u3 3185 1379A303 5 12.2 u1 315 596*

u2 525 743*u3 2080 849u4 1780 1305

A304 5 11.8 u1 130 1444u2 370 1054u3 1010 1749u4 1620 1434

29.9 u1 60 3130u2 245 1591u3 1495 1182u4 1665 1395

B305 5 12.1 u1 260 722*u2 680 573*u3 1235 1430

u4 1970 117929.8 u1 215 873

u2 380 1026u3 430 4108u4 1050 2213

B207 10 12.1 u2 1380 565*29.9 u2 650 1200

B208 10 12.1 u1 620 606*u2 1355 575*

29.9 u1 190 1977u2 520 1500

* Subset of comparable values (see text).

Fig. 3. Corrected cone resistance profiles: (a) 5 cm2 piezocones; (b) 10 cm2 piezocones




2003). This increase is evident in the cone resistance (Fig.7(b)), which commences before the silt layer is contacted bythe u1 filter. The actual thickness of this silt layer is notknown, but it would probably have been similar to the depthincrement between the peak and trough of the u1 response:that is, about 11 mm.

Detection of some lesser silt layers by both 5 cm2 and 10 cm2 piezocones is illustrated in Fig. 8, where severalidentifiable layers are labelled. The effect of rod changescan still be seen, notwithstanding the omission of some data.

Consolidation of the soil around the tip of the piezoconeduring the rod change leads to a delay in pore pressurerecovery as penetration is resumed, an effect enhanced bythe presence of more permeable layers in the soil. Fig. 8

clearly shows the superiority of a 5 cm2 piezocone over a10 cm2 one in terms of thin layer detection via the fluctua-tions in the pore pressure response. It may be noted thatneither piezocone was sensitive enough to detect these lesser layers reliably via the cone resistance response. Data pre-sented in Fig. 9 further illustrate the difference between theamplitudes of the pore pressure responses of 5 cm2 and 10 cm2 piezocones encountering layers with greater per-meability.

The continuity of more permeable layers is obviously an

important issue for site characterisation. Pore pressure signa-tures of some silt layers obtained from the 5 cm2 piezocone,including the two most significant layers in the profile, at positions A and B, are compared in Fig. 10. Given the

Fig. 4. Sleeve friction profiles: (a) 5 cm2 piezocone (uncorrected); (b) 10 cm2 piezocones (uncorrected); (c) 5 cm2 and 10 cm2

piezocones compared (corrected). D, dissipation test

Fig. 5. Pore pressure (u2) profiles: (a) 5 cm2 piezocones; (b) 10 cm2 piezocones. D, dissipation test




similarity of the profiles, it is virtually certain that the samelayer sequence exists at each location and highly likely thatthese layers persist over the 30 m distance separating A and B. Interestingly, the layers at B appear to be about 0.75 mlower than at A. It is not known whether this is due todeposition on a slightly sloping lake bed or to differentialsettlement of underlying layers.

Pore pressure dissipation test resultsAn example of pore pressure dissipation curves obtained

with the 5 cm2 piezocone is given in Fig. 11(a). In Fig.11(b), the pore pressures have been normalised by firstsubtracting the steady state pore pressure, taken as hydro-

static with groundwater level at 0.25 m depth, and thendividing the resulting excess pore pressures by the valuerecorded at the start of the test, that is, immediately after penetration was stopped. This value was invariably close tothe last value recorded during steady penetration. A theor-etical curve (Teh & Houlsby, 1991) is fitted to each datasetindependently so that it passes through the point at which50% of the excess pore pressure has dissipated. This is

achieved by assuming a value for ch/( I r )0:5

, where ch is thecoefficient of consolidation with horizontal drainage and I r

is the rigidity index, defined as the shear modulus divided by the undrained shear strength. Although the theoreticaland experimental curves are quite well matched in the later stages of consolidation, there are some significant differ-ences in the early stages. The pore pressure falls morerapidly than predicted for pore pressure measured on thecone face (u1), and this is linked to the simultaneousreduction in cone resistance, shown in Fig. 11(a). It is possible that, because the driving system could not hold the piezocone rigidly in place, there was some vertical move-ment and relaxation of force on the cone face. A moreextreme example of such an effect, in a calibration chamber test, was reported by Sills & Hird (2005). However, theoverall trend of reducing cone resistance with time appearsto be normal. For pore pressures measured on the shaft (u3

and u4), an initial rise in pore pressure invariably occurred with both 5 cm2 piezocones and is attributed to the long-itudinal gradient of pore pressure along the shaft, as evi-denced in Fig. 6. It should be remembered that thesaturation of the filters was shown to be good by theresponses obtained when penetrating the silt layers (e.g. Fig.8(a)), and therefore a lack of saturation cannot be blamed for the apparent lag in the response. In Test A202, initialrises were similarly seen in u3 with a 10 cm2 piezocone but, because of a transducer failure, u3 results were not obtained from other 10 cm2 piezocone tests.

The values of ch/( I r )0:5, derived by fitting Teh & Houlsby’s

theory, are included in the summary of test results (Table 2).These are sufficient to enable comparisons to be made, and there will be no attempt in this paper to predict ch byassuming values of I r . Unfortunately, as demonstrated byHird et al. (2003), the proximity of a more permeable layer can exert a strong influence on dissipation test results, and

Fig. 6. Multiple pore pressure profiles from a 5 cm2 piezocone(Test A303)

Fig. 7. Responses of a 5 cm2 piezocone encountering a silt layer: (a) pore pressures; (b) cone resistance (Test B305)




this is likely to account for much of the variation seen inTable 2 for a given piezocone size. Meaningful comparisonof results is therefore difficult. However, the results of thesubset of tests conducted at 12.1–12.2 m depth can perhaps be legitimately compared as the silt layers here were verythin (i.e. undetectable by piezocone). The results from the5 cm2 and 10 cm2 piezocones within the subset can be seento agree well when based on u1 or u2, with ch/( I r )

0:5 ranging

from 565 to 743 3 106

cm2

/s. No results based on u3 areavailable at this depth from a 10 cm2 piezocone, but results based on u3 and u4 from a 5 cm2 piezocone are distinctlyhigher than those based on u1 or u2.

DISCUSSIONThe tests reported above suggest that very similar results

may be expected from 5 cm2 and 10 cm2 piezocones inglacial lacustrine clays, except in relation to the detection of relatively permeable thin layers. However, the latter is vitallyimportant for optimal site characterisation and, in this re-spect, it has been shown that the smaller size of piezocone produces significantly better results (Figs 8 and 9). Although

this finding is in line with previous experience, there werefactors other than size that could have had an influence atthe limits of performance. These include the differences of filter material, filter height and saturation fluid. However, in

Fig. 8. Comparison of piezocone pore pressure responses: (a) 5 cm2 piezocone; (b) 10 cm2 piezocone. R, driving rod change; L,permeable layer; (L) permeable layer obscured by driving rod change

Fig. 9. Further comparison of piezocone pore pressure responses: (a) 5 cm2 piezocone; (b) 10 cm2 piezocone. R, driving rod change;L, permeable layer; (L), permeable layer obscured by driving rod change




the saturation checks that have been described, the ability of the 5 cm2 and 10 cm2 piezocones to respond to rapid changes of surrounding fluid pressure (within the limits of the test apparatus) was practically equal. Also, with regard to filter height, it is of interest that in Tests A303 and A304

very similar results were obtained from piezocones F1 and F2, which had u1 and u2 filters of significantly differingheight (Fig. 2). Taking account of all the evidence, it isconcluded that piezocone size was likely to have been the

main factor accounting for the superior performance of the5 cm2 piezocones.

The results obtained in the field using 5 cm2 piezoconesmay be compared with earlier results obtained in calibrationchamber tests, described by Hird et al. (2003). Fig. 12(a)shows a small portion of the profile of Fig. 8(a) adjusted sothat the depth of each filter, rather than the depth of the piezocone tip, is plotted at the instant of pore pressuremeasurement. Fig. 12(b) shows a similar plot for a calibra-tion chamber experiment, where silt layers of different and known thickness were interbedded with clay. Although thethickness of the individual silt layers in the field is unknown,

the comparison suggests that layers as thin as 4 mm, and probably as thin as 2 mm, were being detected. For such thinlayers, the use of multiple filters for pore pressure measure-ment gives added confidence in layer detection. The calibra-tion chamber experiments showed that detection was even better with a filter mounted at the apex of the piezocone, but this might not be practical under field conditions becauseof the risk of damage.

In the tests at Wauwil, detection of the thin silt layerswas significantly enhanced by increasing the data recordingrate to 25 readings per second, well above typical ratesused in commercial testing. With a lower rate, data processing would be easier but there could be a loss of detail. This is illustrated in Fig. 13(a), where the data of

Fig. 8(a) have been sampled periodically to simulate adata recording rate of 2.5 readings per second. A record-ing rate of 25 readings per second implies that a readingis taken at intervals of penetration of less than a milli-metre (with a penetration rate of 20 mm per second) incomparison with intervals of 8 mm, when the reading ratedrops to 2.5 readings per second. The resulting loss of detail with the lower rate is apparent when Figs 8(a) and 13(a) are compared, although most of the layers are stilldetected. However, a comparison of Fig. 13(b) with Fig.8(a) shows that a rate of 5 readings per second would have sufficed in this case.

The results of pore pressure dissipation tests in stronglylayered soils will always be difficult to interpret. Although it

might be argued that the larger the scale of the measurementthe better, in principle the use of a 5 cm2 piezocone allowsthe test to be completed in half the time that would beneeded with a 10 cm2 instrument. This could be a significant

Fig. 10. Pore pressure (u1 ) responses demonstrating continuity of permeable layers: (a) location A; (b) location B

Fig. 11. Example of dissipation test results (Test A303, 12.2 mdepth): (a) pore pressure and cone resistance; (b) normalisedpore pressure




benefit commercially, where the layering can be seen fromthe penetration records to be less prevalent.

It is not the purpose of this paper to interpret the piezocone data in terms of the engineering properties or thestress history of the clay at Wauwil, but some brief comments on the latter will now be made. Fig. 14 showsa typical profile of the normalised parameter Qt ¼

qt v0ð Þ= 9v0, where v0 and 9v0 are the total and effec-

tive vertical stresses. The form of this profile is consistentwith the postulated overconsolidation represented by Fig.1(d). Below about 15 m depth, it can be seen that Qt

becomes approximately constant in a region where there is

probably relatively little variation of overconsolidation ratio(OCR) and the clay is probably close to being normallyconsolidated. A representative value of Qt in this region is3.5. For clays with a sensitivity of less than 15, whichincludes the clay at Wauwil, Karlsrud et al. (2005) suggestthe correlation OCR ¼ (Qt/3)1:2, giving a predicted OCR ¼1.20. This is a little higher than the values below 15 mdepth shown in Fig. 1(d), and may reflect an effect of

ageing. Because of sampling disturbance, it was not possible to determine apparent preconsolidation pressures reli-ably in oedometer tests, so the extent of ageing effectsremains speculative.

Fig. 12. Comparison of (a) field (Test B305) and (b) calibration chamber results. L, permeable layer

Fig. 13. Effect of data recording rate on pore pressure response (Test B305): (a) 2.5 readings per second; (b) 5 readings per second.R, driving rod change; L, permeable layer




CONCLUSIONSInvestigations of the performance of 5 cm2 and 10 cm2

piezocones in profiling a glacial lacustrine clay deposit arereported in this paper. It is concluded that a 5 cm 2 piezoconeis likely to outperform a 10 cm2 one significantly in detect-ing thin layers of silt or sand while providing essentially the

same values of cone resistance (qt) and pore pressure (u2) inthe clay, albeit with a possible loss of measurement accuracyin the case of cone resistance. The use of a 5 cm2 piezo-cone, preferably equipped to measure pore pressure on thecone face as well as at the shoulder, is therefore recom-mended to improve the future characterisation of suchdeposits in respect of layering detail. With an adequate datarecording rate (at least 5 readings per second) and carefulscrutiny of the data, it should then be possible to detect siltor sand layers as thin as 2–4 mm and to assess their continuity by matching responses at different piezocone test positions. Pore pressure dissipation test results can be expected to be variable but, in regions where no silt or sand layers are detected, similar results from 5 cm2 and 10 cm2

piezocones should be obtained. No underlying hard layers were encountered in this

research, but, in practice, the piezocone used must besufficiently robust to cope with such conditions. Therefore, before any particular 5 cm2 piezocone is used, its suscept-ibility to damage needs to be checked. If necessary, a preliminary test with a 10 cm2 piezocone should be carried out so that hard layers can be detected and then avoided.

ACKNOWLEDGEMENTSThe authors are most grateful for financial support from

the Royal Academy of Engineering in the UK and from theRectorate and Research Fund of ETH (Zurich) that allowed

them to collaborate. Invaluable technical support was pro-vided by Fugro Engineers BV, in adapting the 5 cm2 piezo-cones for field use, and by the technical staff of ETH

(Zurich), especially Mr Ernst Bleiker, Mr Marco Sperl and members of the Clay Mineralogy Laboratory. Mr SimonTanner gave equally invaluable organisational assistance. Mr Michael Heil is thanked for helpful technical discussionsand generous advice, as are Dr Hansruedi Maurer and Mr Felix Akeret for their collaboration with geophysical investi-gation. Without permission, enthusiastic support and localinformation from Mr Pius Marti and his colleagues at the

Wauwilermoos Strafanstalt, testing would have been impos-sible.

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Fig. 14. Typical profile of normalised cone resistance (TestA303)


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