2/17 Standard Pénétration Test and Compressibility of Soils L ’essai de pénétra

2/17 Standard Pénétration Test and Compressibility of Soils L ’essai de pénétration standard et la compressibilité des sols by Edgar S c h u l t z e , Professor Dr.-Ing. and E. M e n z e n b a c h , Dr.-Ing, Technische Hochschule Aachen., Germany D.I.C. Summary Considerable information has already been published relating the number of blows of a standard penetrometer with several soil properties, particularly relative density, based on laboratory tests. So far there has been no systematic investigation on the link between the number of blows and the compressibility of soils tested. The results of a many confined compression tests with undisturbed samples were used to establish by statistical methods a correlation between the modulus of compressibility and the results of standard penetration tests, carried out in the field above and below the site where the sample had been extracted. Alto­ gether, twelve groups of soils with 225 pairs of values have been investigated. Although the results scatter more than with laborat­ ory tests, good correlations in the form of straight line relation­ ships were found for sand, gravel and dry silty soils. For cohesive soils the relationship becomes less reliable the finer the soil. This is due to the influence of pore water pressure. For clays, no relationship could be established. For cohesionless soils, the number of blows decreases under the water table for the same modulus of compressibility. Particularly clear differences are observed in fine sands and sands which are oosely compacted. Further more a new statistical formula was derived between the relative density and the number of blows for sandy soils including the influence of the overburden pressure. 1. Introduction Reports on experience with penetrometers were presented in the “ Proceedings of the Fourth International Conference on Soil Mechanics and Foundation Engineering”, London 1957, Volume I, page 249. Invesfigations on the evaluation of penetration tests — in particular of the standard penetra­ tion test — have since been continued. The following evaluations of standard penetration test results have so far been published : I. (a) by H vo rslev (1949), page 120. (b) by Peck-Hanson-Thornburn (1953), page 109. (c) by M eyerh of (1956), page 17. An approximate estimate has been given of the relationship between the number of blows and the relative density of strata in non-cohesive soils, where the relative density of strata was not expressed in numbers. (d) Systematic Investigations of the “ Bureau o f R ecla ­ mation (1953)” s.a. Gibbs and H o lz (1957), page 35. Sommaire En se basant sur des recherches de laboratoire quelques publi­ cations ont déjà indiqué les relations existant entre le nombre de coups du pénétromètre standard ou d’instruments similaires et quelques constantes du sol, parmi lesquelles surtout la compacité relative. Mais jusqu'à présent on n’a pas entrepris de recherches systématiques sur le rapport existant entre le nombre de coups de la sonde et la compressibilité du sol. C’est pourquoi les résul­ tats d’un assez grand nombre d’essais de compression sur des échantillons non remaniés, effectués séparément pour chaque sol, ont été comparés au point de vue statistique, avec le nombre de coups de mouton standard sur le chantier, et effectués aux abords immédiats des lieux où l’on a prélevé les échantillons. 12 groupes de sols avec au total 225 couples de valeurs : nombre de coups-coefficient de compressibilité, ont été ainsi examinés. Malgré une dispersion des résultats plus grande que dans les recherches de laboratoire, on a observé que l’on suivait d’assez près une relation linéaire entre le nombre de coups et le coefficient de compressibilité dans des sols sableux, graveleux, et dans des sols limoneux non saturés d’eau. Plus le sol est à grains fins, plus le rapport est faible pour les sols cohérents. Ce phénomène est dû à l’influence de la pression de l’eau intersticielle. Pour l’argile il n’a pas été possible de trou­ ver de relation. Pour les sols submergés le nombre de coups diminue dans les sols non cohérents, le coefficient de compressibilité restant le même. On observe des différences particulièrement sensibles dans le sable fin et dans le sable meuble. En outre on a pu déduire une relation statistique entre le nombre de coups et la densité des sols sableux, en fonction de la profondeur du sondage. These authors give the relationships between the number of blows of the standard penetration test and the relative density of strata in the case of fine and coarse sand respectively. (e) K olbuszew ski (1957), page 126. The distribution of grain size in sands influences the rela­ tionships between the number of blows and the relative dens­ ity of strata, apart from degree of saturation, surcharge and the size of sand grains. (f) Investigations by Schubert (1955/56), page 49. These are comparable with the above-mentioned investigations which apply to the standard penetration test. II. (a) Terzaghi and Peck (1948), page 423. (b) Peck-Hanson-Thornburn (1953), page 223. These authors give the relationships between the number of blows and the permissible soil pressure for a settlement of one inch. Since these are based on a permissible settlement the soil pressures depend not only on the number of blows but also on the width of the foundation. 527 (c) S o w e r s (1954), p a g e 11. Sowers quoted the permissible soil pressure for different types of soil, apart from the width of the foundation, but with the reservation that they are only valid for buildings of light weight. III. (a) P e c k - H a n s o n - T h o r n b u r n (1953), p a g e 222. (b ) M e y e r h o f (19 5 6 ), page 17. Here the relationships between the number of blows and the angle of friction bas been developed, and the factors affecting load bearing capacity have been considered. None of these investigations deal with the direct relationship between the number of blows of the standard penetration test and the modulus of compressibility Es = l/w r, as calcu­ lated during compression tests with undisturbed soil samples. The latter is however of special importance for the examination of foundations. An attempt was therefore made to clarify these relationships in the course of a research programme carried out at the Technische Hochschule in Aachen ( M e n - z e n b a c h , 1959). of blows and other soil properties; this was also observed during these investigations. For this reason, the following soil groups were introduced, because they can be recognized by merely feeling them in the hand, so that laboratory exam­ ination is not necessary. A subdivision according to whether the soil was above or below water level was only possible in the case of fine sand, so far as non-cohesive soils were concer­ ned. For the rest of sandy and gravelly soils usually no suitable undisturbed samples were available. For this reason, the investigations of the non-cohesive soils are valid essentially for soils above the ground water table. For cohesive soils samples have been taken under and above the water table. Samples having a degree of saturation less than 85 per cent have all been extracted from above the water table. 4. Correlation between the number of blows and the modulus of compressibility The plotting of the values N and Ss produced a straight line which with the aid of statistical methods could be calcu­ lated as follow (Figs. 1-4) : 2. Relationship between the number of blows and compress­ ibility Most of the above investigations are concerned with rela­ tionship between the number of blows and the relative density of soil strata. Since the relative density of various types of soil is related to compressibility ( M o u s s a , 1960) there is a direct relationship between compressibility and the number of blows, because, the soil displaced by the cone of the penetro­ meter as it penetrates the ground must be taken up by adjacent areas which are compressed. In contrast to former experiments carried out in experimen­ tal containers using carefully placed soil at Aachen the results of penetration tests were evaluated from a large number of tests carried out on building sites. These were compared, using statistical methods, with the results of laboratory experiments on undisturbed soil samples which had been taken from the immediate neighbourhood of the penetration test. They are thus based on natural and undisturbed layers of various soils from different depths. Samples from great depths are particularly important because most devices for penetration tests in the laboratory apply only to limited depths. On the other hand, relative density in the field is not nearly as regular as it is in test cylinders, so that results will show considerable deviation. In order to obtain comparable pairs of value, the number of blows N was interpolated from two penetration tests-which had always be;n made at distances of one metre — uploads/Science et Technologie/ spt-and-compressibility.pdf

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