PAPER

chalk relative density and degree of cementation to the cone resistance and friction ratio. Power (1982) carried out CPTs at Mundford and cor- related the results with the well-known Mundford classification (Ward n",er rreI:a",ion et al, 1968 and Wakeling, 1969). Bracegirdle et al (1990) reported the use of CPTs to identify soft zones in the area of a cofferdam and concluded that it provided "a 's~:a",ic quick and effective means of profiling the chalk and assessing the o cone extent of cavities or zones of disturbed material". Finally, Illingworth and Chantler (1999)used cone tests to assess depths of weathering on a site at Reading, and converted cone resistance to an equivalent SPT N value to carry out pile design. )ene:rai:ion It seems likely that the identification of solution features and other variations is the main purpose for which CPTs are used in chalk. Since the cone test produces continuous numerical output, it is considered that it should also be valuable for shallow foundation and pile capacity ~es:sine~a c design. However, before it is possible to carry out such design with con- fidence, it is necessary to have a better idea of how chalk can be classi- fied from CPTs. ByAKC Smith, Webber Associates, formerly of Fugro. This paper presents results from a number of sites across the chalk outcrop, and shows how the relationship between end resistance and friction ratio can vary. It also presents some examples showing the dif- Infrotluc5on ficulties of interpreting cone test results in chalk and some examples During the last 35 years, there have been considerable developments in of the lateral variation of cone resistance resulting from weathering the understanding of foundations in chalk, as described for example by and solution features. CIRIA (1994). However, there is still considerable uncertainty There is very little published information to enable cone test results concerning design parameters, particularly if only routine site to be used directly for design, apart from a few correlations with SPTs. investigation methods are employed (and in particular if values of The paper discusses how such information might be obtained. design parameters are based on the standard penetration test (SPT)). The static cone penetration test (CPT) has a number of advantages Classlflcaflon of chalk from CPTs over the standard penetration test. It is not nearly so operator-depen- Previous work dent and is much faster than shell and auger boring. The CPT gives a Since the development of the Mundford classification (Ward et al, 1968 continuous profile of end resistance and sleeve friction, rather than and Wakeling, 1969),it has been widely used despite warnings (eg Hobbs discrete values of blow count as with SPTs, and the effects of flints on and Healy, 1979and Burland, 1990)pointing out its specific purpose and the test can more easily be identified and discounted (although some- single location of origin. times flints prevent further penetration). Many authors (eg Montague, CIRIA (1994) reports that a major reason for this use (and misuse) is 1990, Mortimore et al, 1990 and CIRIA, 1994) discuss various short- the fact that nothing has supplanted the Mundford classification. comings of SPTs in chalk. Similarly, the correlation between Mundford grade and SPT N value There is very little published information on interpreting the cone derived by Wakeling (1969)and that between Mundford grade and stat- resistance for design purposes. Hodges and Pink (1971)correlated cone ic cone end resistance derived by Power (1982) are also widely used. resistance with end bearing capacity and shaft friction for driven steel These relationships are summarised in Table 1. pipe piles at Portsmouth. Searle (1979) presented expressions relating Power (1982) carried out cone tests adjacent to about 20 of the auger holes that had been made at Mundford and used to derive the original chalk classification. He also carried out a limited number of tests that included measurements of sleeve friction (f,). He pointed out that chalk usually exhibits a characteristically variable trace (eg Figure 1) for both cone end resistance and sleeve friction. Power found that the fric- tion ratio (R,) trace exhibited relative uniformity, showing a steady increase with depth and improving quality of the chalk. The sharpest peaks in cone resistance probably result from the pres- ence of flints, but Power attributes the generally variable nature of the traces to the manner in which the penetration resistance builds up and is then followed by grain crushing and/or closure of discontinuities. This is consistent with the deformation behaviour of chalk as dis- cussed by Holloway-Strong and Hughes (2001). Variability in density, degree of cementation and jointing and fissuring could also contribute to the variation —CIRIA (1994) points out that the dry density of chalk can vary by 0.1Mg/m over 0.1 m depth. For correlating the cone resistance with the Mundford classification, Table 1:Cenelagens behaeen ttnndfonl Chalk grade and penebngen test resntts. Power recommended averaging results over about 1m. The site-specific correlation he derived for both end resistance and sleeve friction is summarised in Table 1 and shown on Figure 2(a). Searle (1979) presented expressions giving degree of cementation in terms of cone resis- tance and friction ratio. The envelopes given by these expressions are shown on Figure 2(b). Lunne et al (1997) quote an unpublished report by Powell and Quarterman (1994), who subsequently examined the use of the CPT on a number of chalk sites. Their results fol- lowed similar trends to Power's in that both q, and f, increased with improving chalk grade, but the ranges were different, even for the Mundford site. The correlations they obtained are shown on Figure 2(c). Data from recent investigations During the last few years, Fugro has carried Table ra Laeagens et sites. out a number of projects that have included

GROUND ENGINEERING SEPTEMBER 2001 PAPER

Cone resistance, q (MPa) Friction ratio, Rf ( lo) 0 10 20 30 40 0 4 8 0

E a 5- ID CI Chalk

0.00 0.25 0.50 0.75 1.00 Sleeve friction, f (MPa)----

ABOVE - Figure 1: T))plea l static cone test prof Re ln Middle Chalk at Mundford.

RIGHT -Figure 3: The Chalk Outcrop In England (after Harrtson et al, 1991)wHh cone test sites.

(a) Power (1982) (b) Searle (1979) (c) Powell and Quarterman (1994) 100—

C Q.

rrO a

Qo cO 10- ' a o Ql N C N og E (I F7 .' oo O RIGHT -Figure 2: 0c C Relatfonshlps O

between fr@Ron 1 rago and cone 0.1 10 0.1 10 0.1 10 end resistance Friction ratio, R, (%) Friction ratio, R, (%) Friction ratio, R, (%) ln chalk cone tests in chalk. The results from 10 of these have been selected to Figure 5 summarises the findings from all the sites in terms of illustrate some aspects of cone behaviour in chalk across the outcrop. graphs of cone resistance against friction ratio. The results have been The site locations are listed in Table 2 and shown on Figure 3. Most of averaged over 0.3m, which has been found to give more representative the sites are in Upper Chalk, reflecting the fact that this material has the results than the 1m length recommended by Power, and also appears most extensive outcrop, but the site at is in Lower Chalk and more reasonable compared to the dimensions of the cone (36mm diam- that at South Humber Power Station in the Flamborough Chalk. The eter and friction sleeve length of 130mm). classification of the chalk of southern England is currently being Some of the sites give results similar to those obtained by Power, but revised (Rawson et al, 2001), but for reasons of continuity the former some are very different. The one site in the Lower Chalk (Snodland) tripartite classification is used here. The tripartite classification is gives a different envelope ft'om all the others. The two sites at Reading untenable for the deposits north of the Wash, and the classification give very similar results to each other. presented by Harrison et al (1991)is used here. The three locations in North (, CTRL Contract 330 and Cone test profiles from three of the sites are presented on Figure 4 Isle of Grain) present an interesting contrast between results from (overleaf). It can be seen that they all demonstrate the same pattern of sites geographically close together and stratigraphically similar, being very variable end resistance and sleeve friction that Power observed. very close to the top of the chalk. At the Isle of Grain, the overlying However, the friction ratio varies considerably from site to site. Thanet Sand was also penetrated (Figure 6). The first profile, from Reading, is similar to that observed by Power At Dartford the chalk was covered by alluvial deposits, and at at Mundford, with the friction ratio generally in the range 1 o to 2 o Channel Tunnel Rail Link Contract 330 the chalk was covered by Head and increasing slightly when the chalk quality improves. The second, Deposits and solution feature infill (see below). However, the friction from Norwich, gives friction ratios generally in the range 3 'o to 4 'o. ratio at the Isle of Grain is significantly higher than at the other two The third profile is from the Lower Chalk at Snodland, and friction sites. Clayton (1990)describes how chemical decomposition can change ratios are mostly in the range 4'o to 8"'o. It seems likely that the the mechanical properties of the chalk, particularly where it is over- high friction ratio results from the much more clayey nature of the lain by granular material, and it is considered possible that this has Lower Chalk. occurred differently at the Isle of Grain from the other two sites.

GROGNI> ENGINEERING SEPTENIBER 2001 31 PAPER

Cone resistance, Friction ratio, Cone resistance, Fnction ratio, Cone resistance Friction ratio, q, (MPa)— RI (%) q (MPa) RI (%) q (MPa) RI (%) 0 10 20 30 40048 10 20 30 400 4 8 10 20 30 400 4 8 0

10-

E E 15 eco ro CI

20-

25-

30 0.00 0.25 0.50 0.75 1.00 0.00 0.25 0.50 0.75 1.00 0.00 0.25 0.50 0.75 1.00 Sleeve friction, Sleeve friction, Sleeve friction, .--- f (MPa) f (MPa) .""" f (MPa) -"--"

(s) Oracle Development, Reading (b) John lnnes Centre, Norwich (c) Blue Circle South East Works, Snodland

Figure 4:t)ftflect cene test prellles from three sites.

Chalks from north of the Wash tend to be much less weathered than Much of the material at Dorchester was found difficult to interpret those in southern Fngland. This can be seen in the envelope of results from the cone tests alone. It was decided to classify it as chalk, but to from the South Humber site, for which the minimum value of cone include an alternative where this was considered possible (as on resistance measured is of the order of 6MPa. Figure 7 —again, this material is classified as "possible chalk" on Unfortunately, it has been found necessary to assess the sites in iso- Figure 5). Subsequently, the client supplied borehole logs, which indi- lation, without reference to other data. Even at sites where borehole cated much of this material to consist of weathered chalk with much information was available, the only quantitative data available for cor- clay infill in the joints relation were SPT results (see below). Similarly, identification of the nature of infill in solution features is not always straightforward. Figure 8 shows profiles from two cone Identification of chalk from cone test profiles tests 3m apart on the Channel Tunnel Rail Link near Gravesend. The This variability of the friction ratio from site to site means that basis of the soil descriptions was agreed with Rail Link Engineering, interpretation of cone tests in chalk can be difficult without borehole based on exposures it had observed nearby. The chalk surface was data for correlation. An example of this is shown in Figure 6, which found to be characterised by very deeply etched chemically weathered shows a cone test profile from the Isle of Grain. This test found the features that were filled in the post-Cretaceous period with a struc- following sequence of materials: tureless residue of clay with flints. The fringe of these weathered fea- 0 - 0.8m Made Ground (Railway Ballast) tures, which may be 15m to 20m deep and as little as 3m across, com- 0.8 - 1.8m Hard sandy CLAY (Made Ground —Railway prises clay with flints. Embankment) The chalk was generally indicated by a rapid increase in cone Very stiff CLAY (Made Ground or former Topsoil) resistance and decrease in friction ratio. Immediately above the Interbedded SAND and CLAY (I.ambeth Group) chalk there was usually a layer characterised by spikes in the cone Very dense silty SAND (Thanet Sand) resistance (probably resulting from flints) and an extremely variable CHALK friction ratio. This material was described as a sandy clay. It is probable that it represents the fringe of clay with flints described The cone test profile for the Lambeth Group shown in Figure 6 above, although it is considered possible that it represents completely appears very similar to profiles for chalk (Figure 2), with very weathered chalk. Results from this material are designated by open variable cone resistance, friction and friction ratio profiles, reflecting circles on Figure 5. the layering of sand and clay. The relationship between friction ratio Finally, the material encountered from the ground surface on Figure and cone resistance is also similar to that for many of the chalk sites 8 generally exhibited a comparatively uniform cone resistance, (Figure 5). although with a high and variable friction ratio, generally in the range The main differences between the cone test profiles for the Lambeth 4 ro to 6 Io. The results of particle size distribution tests and plasticity Group and typical profiles for chalk are that the friction ratio for the tests are shown on Figures 9 and 10. They show a high proportion of Lambeth Group is not quite so spiky as for the chalk, and that the peaks sand size particles (50'o to 60"o) and plasticity results below the A-line. in friction ratio for the Lambeth Group correspond closely to troughs The material has been described as "sandy very silty clay", although it in cone resistance, indicating clay bands. In this case, distinguishing is believed to be reworked Thanet Beds. between the two is straightforward because of the very obvious layer of The variability of the friction ratio could lead to the material being Thanet Sand between the Lambeth Group and the chalk. identified as chalk (perhaps completely weathered at the low cone

GROUND ENGINEERING SEPTEMBER 2001 PAPER

Probable chalk Possible chalk

CTRL Contract 330 Dartford Isle of Grain Mundford (after Power, 1982) 100 100 100 100

mI'l II@ 10 iv,Qg„ I

no -'ambeth Group / / o 1 1 1 Hemel Hempstead Norwich Stratton, Dorchester Saffron Walden 100 100 100

10 As O 1 1 1 Bear Wharf, Reading Oracle, Reading South Humber Snodland 100 100

10

1 1 1 0 2 4 6 8 0 2 4 6 8 0 2 4 6 8 0 2 4 6 8 Friction ratio, R, (%) Friction ratio, R, (%) Friction ratio, R, (%) Friction ratio, R, (%)

ABOVE- Cone resistance, Friction ratio, Soil type Rgnre6: (MPS) (%) q RI Emrelopes of 0 10 20 30 40 0 4 8 frlcgon ratio Raifway ballast and cone reslshnce LEFT- Rgnre 6: from different tntedutdded SAND and CLAY (Lambeth Group) Static cone test sites. profge from the Isle of Grain.

10- BELOW- Flgnre 7:Static cone test profge from Stratton, E Dorchester. 15 Q tp O Cone resistance, Friction ratio, Soil type CHALK q, (MPS) RI (%) 0 10 20 30 400 4 8 20- 0 Sand aaPSOS) o s tabes CHALK (possibly loose to madlufll dense vefy silty SAND) CHALK (possibly soft E sandy CLAY with some 4.S gravel) 25- 5 ~ CL CHALK tb c)

30 10 0.00 0.25 0.50 0.75 1.00 0.00 0.25 0.50 0.75 1.00 Sleeve friction, Sleeve friction, f (MPa) " ". f (MPa)

( RO('Ni) I( g()IHI:I')RING SRVTR(v(HFR 2001 33 PAPER

resistances measured), but could also result from sandy layers within Power (1982) correlated the CPT and the SPT for a number of sites, the clay. Similar friction ratio profiles have been observed in tills in obtaining the relationship: Scotland (Adam, 1985) and in loess in Kazakhstan (Fugro, unpublished report). q, = 0.4N These results all indicate the need for some borehole information or other means of obtaining site-specific correlations for cone tests in the However, there was a great deal of scatter in his results, reflecting chalk. However, once this has been achieved, cone testing has the sig- both the natural variability of the chalk and the uncertainties associ- nificant advantage over boreholes of speed of investigation. At CTRL ated with the SPT. Illingworth and Chantler (1999) derived the same Contract 330, for example, it was found possible to achieve average test- relationship at Reading, based on cone tests adjacent to boreholes. ing rates of the order of 150 linear metres per truck per day. Similar relationships were observed for two of the sites considered in This is a considerable advantage when it is necessary to carry out this paper (South Humber and Stratton). tests at close centres to search for solution features. For example, the Since the CPT has a number of advantages over the SPT, as listed in cone tests on Figure 8 were only 3m apart, but the difference in the introduction to this paper, it would seem preferable to use CPT penetration and interpreted chalk depth was about 10m. This result results directly for design. Indeed, the fact that different sites give dif- shows the variability that can occur in chalk level as a result of solu- ferent relationships between friction ratio and cone end resistance tion features and the value of the static cone test in identifying such indicates that it is most unlikely that a unique relationship between features. Similar large variations over short horizontal distances were SPT N value and pile shaft resistance is obtainable. observed at the site at Hemel Hempstead, sometimes with very low It is proposed to carry out static cone tests in conjunction with a pro- cone resistances being measured below material with significantly gramme of pile tests to be carried out by CIRIA wtih the Federation of higher resistance. Piling Specialists. It is hoped that this programme will lead to greater understanding of the relationship between cone end resistance, side Use of data for design friction and pile end and shaft resistance, and thereby enable more eco- CIRIA (1994) describes the difficulties of designing both shallow and nomical design to be safely achieved. piled foundations in chalk. It points out that considerable care is required when carrying out plate bearing tests for estimating Conclusions settlements of shallow foundations and that the expense of this would This paper has described results of static cone tests at a number of sites not necessarily be justified for most structures. in chalk. It has demonstrated how the results may vary, although the It also quotes a number of shortcomings associated with the use of reasons for this variation are not yet known. Standard site the Standard Penetration Test for pile design in chalk. Nevertheless, investigations are unlikely to provide the information to explain the the SPT is still extensively used for pile design, as evidenced for exam- variation. ple by Matthews et al (2000). Illingworth and Chantler (1999)found that Cone tests give more reliable and repeatable information than stan- the CPT required correlation against SPT N values for pile design. dard penetration tests, and can also provide this information much However, they also found that the CPT proved to be more effective than more quickly. However, site-specific correlation with boreholes is usu- the SPT for assessing the very variable weathering profile at their site, ally necessary to aid the interpretation of cone tests in chalk. because of the CPT's greater speed of operation and the fact that it The paper has also demonstrated how effective the CPT can be as a gives a continuous profile. tool for investigating solution features in chalk. These can be a major

Cone resistance, Friction ratio, Soil type Cone resistance, Friction ratio, Soil type q, (MPa)— Rl (%) q (MPa)— Rf (%) 0 10 20 30 400 4 8 0 10 20 30 40 0 4 8 0

Very stlft sandy CLAY with flint gravel (possibly CHALK) CHALK 10-

E Finn to stiff loosey very 15 sort sandy CLAY with CL flint gravel (possibly s) CHALK with solution CI feature)

CHALK

20-

25-

30 0.00 0.25 0.50 0.75 1.00 0.00 0.25 0.50 0.75 1.00

Sleeve friction, Sleeve friction, f (MPa) "."" f (MPa) -"-".

(a) Test 46907E (a)Test 46907F

Figure 8:Prollles from adJacent cone hefts In solution feature at Gravesend (Channetltinnel ttaa Unh).

34 GR<)l'Nl) I'IiNGINEERING SERTEKIHER 2001 PAPER

LEFT-Rsure S: Parede she Clay Silt Sand Gravel Cobbles dlitrlb neon resuns for 100 soluson feature InRR,CIRL 80- Contract 33O. 0~0 (3I E I 60- e rtr 40- C oCl 3 2O-

0 1E-3 0.01 0.1 10 Particle size (mm)

0term ed i at e H i h V r h i h Extr m I h i h ~ ( g e y g e e y g p ""y plasticity f'plasticity plasticity plasticity 70 g 60. g 50. X 40-

g 3O K3 20. 10. IIISRT- Rsnre Ilk Plaatbdtytest 0 resuits forsolueon 0 10 20 30 40 50 60 70 80 90 100 110 120 foature Infsl, CTRL Limit Ceutract 33L Liquid (%)

hazard for shallow foundations because of the possible variability in References chalk level over a very short distance. Adam CH (1985). Static electric cone penetration testing in the glacial tills of central It is likely that the CPT could be Scotland. Proc Int Conf on on Glacial Tills and Boulder Clays, Engineering a much more effective tool for Technics Press, Edinburgh. design of piles and shallow foundations than the SPT. However, at pre- Bracegirdle A, Mair RJ and Daynes RJ (1990).Construction problems associated with an sent not enough information is available for CPT results to be used excavation in chalk at Costessey, Norfolk. Proc Int Chalk Symposium, Brighton, Thomas directly for foundation design, and they need to be correlated with SPT Telford, London, pp 385-391. Burland JB(1990).Preface Proc In t Chalk Symposium, Brighton, Thomas Telford, London, I<. N pp values. Further work is proposed to provide the necessary informa- CIRIA (1994).Foundations in Chalk. CIRIA Project report 11,London. tion for CPT results to be used directly for design. Clayton CRI (1990). The mechanical properties of chalk. Proc Int Chalk Symposium, Brighton, Thomas Telford, London, pp 213-232. Acknowledgements Harrison DJ, Hudson JH and Cannell B (1991).Appraisal of high-purity limestones in England and . British Geological Survey, Keyworth. Work described in this paper was carried out while the author was Hobbs NB and Healy PR (1979).Piling in chalk. Report PG6, CIRIA. employed by Fugro, which carried out all the cone tests described. The Hodges WGH and Pink S (1971).The use of penetrometer soundings in the estimation of paper is published by kind permission of the directors of Fugro. Cone pile bearing capacity and settlement for driven piles in the Portsmouth area as an alternative to site investigations borehole test results at specific sites are included by sampling and laboratory testing. Proc Roscoe by kind permission of the Mem Symp Cambridge. Foulis, pp 707-723. following: Holloway Strong M and Hughes SJ(2001).The influence of contact area on the deformation of chalk. Quarterly Journal of Engineering Geology and Hydrogeology 34(1),99-110. BearWharf,Reading Westpile lllingworth DR and Chantler JP (1999).CFA piles in chalk at the Oracle, Reading. Proc Int Symp on Tunnelled Construction and Piling, London, Hemming Group, pp 45-56. Oracle Development, Reading Westpile and Hammerson Properties Lunne T, Robertson PK and Powell J,IM (1997). Cone penetration testing in geotechnical practice. Blackie, London. Saxonfield Development, Stratton Village Morrish Builders Matthews MC, Clayton CRI and Bica AVD (2000). Tension pile tests in chalk. Quarterly Journal of Engineering Geology and Hydrogeology 33(3),201-212. Channel Tunnel Rail Link Contract 330 Alfred McAlpineAMEC Joint Venture and Montague, KN (1990). The SPT and pile performance in upper chalk. Proc Int. Chalk. Rail Link Engineering Symposium, Brighton, Thomas Telford, London, pp 269-276. Mortimore RN, Reading P and Smith N (1990). Discussion, Proc Int Chalk Symposium, Dartford Area Resignalling Rail Projects Brighton, Thomas Telford, London, p297. Power PT (1982), The use of the electric static cone penetrometer in the determination of Blue Circle South East Works Arup Geotechnics and Blue Circle Industries the engineering properties of chalk. Proc 2nd European Symp Penetration Testing, Amsterdam, 769-774. John Innes Centre Albury Site Investigations and Powell JJM and Quarterman RST (1994). A reappraisal of CPT testing in chalk. BRE John lnnes Centre RePort No. G/GP/9412. Rawson PF, Allen P and Gale A (2001). The Chalk Group —a revised lithostratigraphy. Genome Campus Extension Foundation and Exploration Services, Geoscientist, Vol. 11No. 1,January, p21. Oscar F'aber and The Wellcome Trust Searle IW (1979).The interpretation of Begemann friction jacket cone results to give soil types and design parameters Proc seventh European Conf Soil Mech Fdn Engng Brighton, 265.270. South Humber Power. Station Alstom Power Generation pp Wakeling TRM (1969).A compamson of the results of standard site investigation methods against the results of a detailed geotechnical investigation in Middle Chalk at Mundford, The author also wishes to thank PT Power and NR Ramsey of Fugro, Norfolk. Proc Conf on insitu investigations in soils and rocks. British Geotechnical Soc, London: 206-212. AM Barwise, I,ankelma, formerly of Fugro and DR Illingworth of Ward WH, Burland JB and Gallois RM (1968). Geotechnical assessment of a site at Westpile for helpful discussions during the preparation of the paper. Mundord, Norfolk, for a proton accelerator. Geotechnique, 18(4), 339-431.

GRQUND ENGINEERING sEPTEMBER 2001 35