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Interpretation of static cone penetration tests in chalk

PAPER

Introduction

During the last 35 years, there have been considerable developments in the understanding of foundations in chalk, as described for example by CIRIA (1994). However, there is still considerable uncertainty concerning design parameters, particularly if only routine site investigation methods are employed (and in particular if values of design parameters are based on the standard penetration test (SPT)).

The static cone penetration test (CPT) has a number of advantages over the standard penetration test.

It is not nearly so operator-dependent and is much faster than shell and auger boring. The CPT gives a continuous profile of end resistance and sleeve friction, rather than discrete values of blow count as with SPTs, and the effects of flints on the test can more easily be identified and discounted (although sometimes flints prevent further penetration). Many authors (eg Montague, 1990, Mortimore et al, 1990 and CIRIA, 1994) discuss various shortcomings of SPTs in chalk.

There is very little published information on interpreting the cone resistance for design purposes.

Hodges and Pink (1971) correlated cone resistance with end bearing capacity and shaft friction for driven steel pipe piles at Portsmouth. Searle (1979) presented expressions relating chalk relative density and degree of cementation to the cone resistance and friction ratio. Power (1982) carried out CPTs at Mundford and correlated the results with the well-known Mundford classification (Ward 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 quick and effective means of profiling the chalk and assessing the 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.

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 design. However, before it is possible to carry out such design with confidence, it is necessary to have a better idea of how chalk can be classified from CPTs.

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 difficulties of interpreting cone test results in chalk and some examples of the lateral variation of cone resistance resulting from weathering and solution features.

There is very little published information to enable cone test results to be used directly for design, apart from a few correlations with SPTs. The paper discusses how such information might be obtained.

Classification of chalk from CPTs Previous work Since the development of the Mundford classification (Ward et al, 1968 and Wakeling, 1969), it has been widely used despite warnings (eg Hobbs and Healy,1979 and Burland,1990) pointing out its specific purpose and single location of origin.

CIRIA (1994) reports that a major reason for this use (and misuse) is the fact that nothing has supplanted the Mundford classification. Similarly, the correlation between Mundford grade and SPT N value derived by Wakeling (1969) and that between Mundford grade and static cone end resistance derived by Power (1982) are also widely used. These relationships are summarised in Table 1.

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 s). 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 friction ratio (R f) 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 presence 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 discussed 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 3over 0. 1 m depth.

For correlating the cone resistance with the Mundford classification, 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 resistance 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 followed similar trends to Power's in that both q cand f sincreased 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 out a number of projects that have included cone tests in chalk. The results from 10 of these have been selected to illustrate some aspects of cone behaviour in chalk across the outcrop. The site locations are listed in Table 2 and shown on Figure 3. Most of the sites are in Upper Chalk, reflecting the fact that this material has the most extensive outcrop, but the site at Snodland is in Lower Chalk and that at South Humber Power Station in the Flamborough Chalk. The classification of the chalk of southern England is currently being revised (Rawson et al, 2001), but for reasons of continuity the former tripartite classification is used here. The tripartite classification is untenable for the deposits north of the Wash, and the classification presented by Harrison et al (1991) is used here.

Cone test profiles from three of the sites are presented on Figure 4 (overleaf ). It can be seen that they all demonstrate the same pattern of very variable end resistance and sleeve friction that Power observed.

However, the friction ratio varies considerably from site to site.

The first profile, from Reading, is similar to that observed by Power at Mundford, with the friction ratio generally in the range 1% to 2% and increasing slightly when the chalk quality improves. The second, from Norwich, gives friction ratios generally in the range 3% to 4%. The third profile is from the Lower Chalk at Snodland, and friction ratios are mostly in the range 4% to 8%. It seems likely that the high friction ratio results from the much more clayey nature of the Lower Chalk.

Figure 5 summarises the findings from all the sites in terms of graphs of cone resistance against friction ratio. The results have been averaged over 0. 3m, which has been found to give more representative results than the 1m length recommended by Power, and also appears more reasonable compared to the dimensions of the cone (36mm diameter and friction sleeve length of 130mm).

Some of the sites give results similar to those obtained by Power, but some are very different. The one site in the Lower Chalk (Snodland) gives a different envelope from all the others. The two sites at Reading give very similar results to each other.

The three locations in North Kent (Dartford, CTRL Contract 330 and Isle of Grain) present an interesting contrast between results from sites geographically close together and stratigraphically similar, being very close to the top of the chalk. At the Isle of Grain, the overlying Thanet Sand was also penetrated (Figure 6).

At Dartford the chalk was covered by alluvial deposits, and at Channel Tunnel Rail Link Contract 330 the chalk was covered by Head Deposits and solution feature infill (see below). However, the friction ratio at the Isle of Grain is significantly higher than at the other two sites. Clayton (1990) describes how chemical decomposition can change the mechanical properties of the chalk, particularly where it is overlain by granular material, and it is considered possible that this has occurred differently at the Isle of Grain from the other two sites.

Chalks from north of the Wash tend to be much less weathered than those in southern England. This can be seen in the envelope of results from the South Humber site, for which the minimum value of cone resistance measured is of the order of 6MPa.

Unfortunately, it has been found necessary to assess the sites in isolation, without reference to other data. Even at sites where borehole information was available, the only quantitative data available for correlation were SPT results (see below).

Identification of chalk from cone test profiles

This variability of the friction ratio from site to site means that interpretation of cone tests in chalk can be difficult without borehole data for correlation. An example of this is shown in Figure 6, which shows a cone test profile from the Isle of Grain. This test found the following sequence of materials:

0 - 0. 8m Made Ground (Railway Ballast) 0. 8 - 1. 8m Hard sandy CLAY (Made Ground - Railway Embankment) 1. 8 - 2. 4m Very stiff CLAY (Made Ground or former Topsoil) 2. 4 - 7. 5m Interbedded SAND and CLAY (Lambeth Group) 7. 5 - 17. 2m Very dense silty SAND (Thanet Sand) 17. 2 - 21. 0m CHALK The cone test profile for the Lambeth Group shown in Figure 6 appears very similar to profiles for chalk (Figure 2), with very variable cone resistance, friction and friction ratio profiles, reflecting the layering of sand and clay. The relationship between friction ratio and cone resistance is also similar to that for many of the chalk sites (Figure 5).

The main differences between the cone test profiles for the Lambeth Group and typical profiles for chalk are that the friction ratio for the Lambeth Group is not quite so spiky as for the chalk, and that the peaks in friction ratio for the Lambeth Group correspond closely to troughs in cone resistance, indicating clay bands.

In this case, distinguishing between the two is straightforward because of the very obvious layer of Thanet Sand between the Lambeth Group and the chalk.

Much of the material at Dorchester was found difficult to interpret from the cone tests alone. It was decided to classify it as chalk, but to include an alternative where this was considered possible (as on Figure 7 - again, this material is classified as 'possible chalk' on Figure 5). Subsequently, the client supplied borehole logs, which indicated much of this material to consist of weathered chalk with much clay infill in the joints.

Similarly, identification of the nature of infill in solution features is not always straightforward. Figure 8 shows profiles from two cone tests 3m apart on the Channel Tunnel Rail Link near Gravesend. The basis of the soil descriptions was agreed with Rail Link Engineering, based on exposures it had observed nearby. The chalk surface was found to be characterised by very deeply etched chemically weathered features that were filled in the post-Cretaceous period with a structureless residue of clay with flints. The fringe of these weathered features, which may be 15m to 20m deep and as little as 3m across, comprises clay with flints.

The chalk was generally indicated by a rapid increase in cone resistance and decrease in friction ratio. Immediately above the chalk there was usually a layer characterised by spikes in the cone resistance (probably resulting from flints) and an extremely variable friction ratio. This material was described as a sandy clay. It is probable that it represents the fringe of clay with flints described above, although it is considered possible that it represents completely weathered chalk. Results from this material are designated by open circles on Figure 5.

Finally, the material encountered from the ground surface on Figure 8 generally exhibited a comparatively uniform cone resistance, although with a high and variable friction ratio, generally in the range 4% to 6%. The results of particle size distribution tests and plasticity tests are shown on Figures 9 and 10. They show a high proportion of sand size particles (50% to 60%) and plasticity results below the A-line. The material has been described as 'sandy very silty clay' although it is believed to be reworked Thanet Beds.

The variability of the friction ratio could lead to the material being identified as chalk (perhaps completely weathered at the low cone resistances measured), but could also result from sandy layers within the clay. Similar friction ratio profiles have been observed in tills in Scotland (Adam, 1985) and in loess in Kazakhstan (Fugro, unpublished report).

These results all indicate the need for some borehole information or other means of obtaining site-specific correlations for cone tests in the chalk. However, once this has been achieved, cone testing has the significant advantage over boreholes of speed of investigation. At CTRL Contract 330, for example, it was found possible to achieve average testing rates of the order of 150 linear metres per truck per day.

This is a considerable advantage when it is necessary to carry out tests at close centres to search for solution features. For example, the cone tests on Figure 8 were only 3m apart, but the difference in penetration and interpreted chalk depth was about 10m. This result shows the variability that can occur in chalk level as a result of solution features and the value of the static cone test in identifying such features.

Similar large variations over short horizontal distances were observed at the site at Hemel Hempstead, sometimes with very low cone resistances being measured below material with significantly higher resistance.

Use of data for design CIRIA (1994) describes the difficulties of designing both shallow and piled foundations in chalk. It points out that considerable care is required when carrying out plate bearing tests for estimating settlements of shallow foundations and that the expense of this would not necessarily be justified for most structures.

It also quotes a number of shortcomings associated with the use of the Standard Penetration Test for pile design in chalk. Nevertheless, the SPT is still extensively used for pile design, as evidenced for example by Matthews et al (2000). Illingworth and Chantler (1999) found that the CPT required correlation against SPT N values for pile design. However, they also found that the CPT proved to be more effective than the SPT for assessing the very variable weathering profile at their site, because of the CPT's greater speed of operation and the fact that it gives a continuous profile.

Power (1982) correlated the CPT and the SPT for a number of sites, obtaining the relationship:

qc= 0. 4N However, there was a great deal of scatter in his results, reflecting both the natural variability of the chalk and the uncertainties associated with the SPT. Illingworth and Chantler (1999) derived the same relationship at Reading, based on cone tests adjacent to boreholes. Similar relationships were observed for two of the sites considered in this paper (South Humber and Stratton).

Since the CPT has a number of advantages over the SPT, as listed in the introduction to this paper, it would seem preferable to use CPT results directly for design. Indeed, the fact that different sites give different relationships between friction ratio and cone end resistance indicates that it is most unlikely that a unique relationship between SPT N value and pile shaft resistance is obtainable.

It is proposed to carry out static cone tests in conjunction with a programme of pile tests to be carried out by CIRIA wtih the Federation of Piling Specialists. It is hoped that this programme will lead to greater understanding of the relationship between cone end resistance, side friction and pile end and shaft resistance, and thereby enable more economical design to be safely achieved.

Conclusions

This paper has described results of static cone tests at a number of sites in chalk. It has demonstrated how the results may vary, although the reasons for this variation are not yet known. Standard site investigations are unlikely to provide the information to explain the variation.

Cone tests give more reliable and repeatable information than standard penetration tests, and can also provide this information much more quickly. However, site-specific correlation with boreholes is usually necessary to aid the interpretation of cone tests in chalk.

The paper has also demonstrated how effective the CPT can be as a tool for investigating solution features in chalk. These can be a major hazard for shallow foundations because of the possible variability in chalk level over a very short distance.

It is likely that the CPT could be a much more effective tool for design of piles and shallow foundations than the SPT. However, at present not enough information is available for CPT results to be used directly for foundation design, and they need to be correlated with SPT N values. Further work is proposed to provide the necessary information for CPT results to be used directly for design.

Acknowledgements

Work described in this paper was carried out while the author was employed by Fugro, which carried out all the cone tests described. The paper is published by kind permission of the directors of Fugro. Cone test results at specific sites are included by kind permission of the following: Bear Wharf, Reading Westpile Oracle Development, Reading Westpile and Hammerson Properties Saxonfield Development, Stratton Village Morrish Builders Channel Tunnel Rail Link Contract 330 Alfred McAlpine AMEC Joint Venture and Rail Link Engineering Dartford Area Resignalling Balfour Beatty Rail Projects Blue Circle South East Works Arup Geotechnics and Blue Circle Industries John Innes Centre Albury Site Investigations and John Innes Centre Genome Foundation and Exploration Services, Oscar Faber and The Wellcome Trust South Humber Power Station Alstom Power Generation The author also wishes to thank PT Power and NR Ramsey of Fugro, AM Barwise, Lankelma, formerly of Fugro and DR Illingworth of Westpile for helpful discussions during the preparation of the paper.

References

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