The load-settlement relationship for plate loading of an isolated stone column in soft clay was predicted prior to field testing. The column was constructed by vibro-replacement and, after the test, was excavated to check its dimensions. The theory used was proposed on the basis of laboratory model tests by Hughes and Withers (1974). The purpose was to verifv the theory on a field scale. A standard site investigation supplemented by Cambridge and Ménard pressuremeter tests provided basic soil parameters. The ultimate column load depends on the friction angle of the gravel used to form the column, the size of the column formed and the restraint of the clay on the uncemented gravel. To predict the load-settlement curve the essential radial stress-strain data for the clay were obtained from a Cambridge pressuremeter. The prediction is excellent if allowance is made for transfer of load from column to clay through side shear and correct column size. Accurate estimation of column diameter is the major factor influencing the calculation of ultimate load and settlement characteristics. The column improved substantially the bearing capacity of the natural soil. La relation charge-tassement pour un essai à la plaque d'une colonne en Pierre, isolée, dans l'argile molle, a été prévue avant l'essai in situ. Suite à l'essai, la colonne construite par la méthode de vibro-remplacement, a été excavée afin de vérifier ses dimensions La théorie utilisée est à la base d'essais sur modèle de laboratoire, d'après Hughes et Withers (1974). L'objectif a été de vérifier cette théorie à l'échelle du chantier. Une reconnaissance classique du terrain, complétée par des pressiomètres de Cambridge et Ménard, ont fourni les caractéristiques du sol. La charge limite de rupture d'une colonne, dépend de l'angle de frottement du gravier qui la forme, de sa taille et de la contrainte de l'argile sur le gravier non-cimenté. Afin de prévoir la courbe de charge-tassement, l'information essentielle de contrainte-déformation, a été obtenue à l'aide du pressiomètre de Cambridge. La prévision est excellente, si l'on tient compte du transfer de la charge, de la colonne à l'argile, grâce au cisaillement latéral et la taille exacte de la colonne. Une estimation précise du diamètre de la colonne, est le facteur principal pouvant influencer le calcul de la charge de rupture et du tassement. La colonne a considérablement amélioré la force portante du sol réel.

Stone column foundations are often incorrectly regarded as piles and tested accordingly. For single columns load tests are appropriate. However full scale field tests in which stresses on and between columns were measured clearly demonstrate how the performance of columns under widespread loads depends on loading circumstances which cannot be simulated by small scale tests on single columns. Such tests are misleading for contract purposes. A more cost effective approach is to concentrate on high grade site investigation and control during design and construction; supplemented by instrumentation to monitor response to initial loading of the prototype.

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The Paper was first presented in 1984 at the Centenary Celebrations of the Glasgow and West of Scotland Association and was published in the centenary volume. The paper is based on Mr Burbidge's Masters thesis' in which he assembled an enormous number of case records, each of them meticulously presented and analysed.122. As mentioned in 116 no attempt was made to compare the results of the correlations derived in the Paper with predictions of other methods as this would have required a case-by-case comparison. One of my students carried out comparisons of nine of the case histories for which the soils data were well established (Maail 1985).90 The observed settlements p, were compared with the calculated settlements p, using six methods including the one given in the Paper. The results are given in Fig. 34 where the observed settlements are plotted against pJp, and pJp, for each method. A point lying above the horizontal axis represents an overestimate of settlement (conservative) whereas a point lying below the axis represents an underestimate of settlement (unconservative). The number against each point refers to the case number given in Appendix-1.123. The predictions given by Terzaghi and Peck7 are given in Fig. 34(a) and can be seen to be generally very conservative. Meyerh~f'.'~ increased the allowable bearing pressure given by Terzaghi and Peck by 50% and dispensed with the correction for the water table. As can be seen from Fig. 34(b) some of the predictions have become very unconservative. Peck, Hanson and Thornburn (1974) proposed a revised set of charts with an overburden correction applied to the SPT results.'' Tomlinson*' uses the original Terzaghi and Peck charts with an overburden correction applied to the SPT results. These two methods give results which are plotted in Figs 34(c) and (d) respectively. It can be seen that in both cases the agreement between observed and calculated settlements is reasonable. Fig. 34(e) shows the comparison with Parry's method.33 For most of the cases the calculations are unconservative. As a check Fig. 34(f) shows the comparison with the method proposed in the Paper. It is encouraging that most of the observations lie within one standard deviation of the best estimate of the settlement. Mr E. J. Wilson, Consultant Engineering GeologistThe Terzaghi and Peck7 chart for estimating settlement is arguably the most used and almost certainly the most misused chart in soil mechanics; but it was produced in 1948 by Terzaghi and Peck on the basis of a study of plate bearing tests, some standard and non-standard penetration tests, and probably no actual settlement measurements on structures. They made it clear that this was a very conser-

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