The interpretation of in situ soil tests

Abstract

The purposes of in situ testing are set out, and the difficulties of the interpretation of the observations are emphasized. These difficulties are due to the complex behaviour of soils together with the lack of control and of choice of the boundary conditions in any field test. One notable exception is the pressuremeter test, from which soil properties can be derived directly without recourse to empirical correlations. The discussion is concentrated on the measurement of undrained shear strength. The results obtained from different tests (triaxial, plane strain, direct simple shear, pressuremeter and vane) are compared by expressing them in terms of the undrained strength ratio su/σv0′ as a function of the friction angle φ. Special attention is paid to tests in which the principal axes of stress and of strain increment are free to rotate. In such tests, uncertainty exists regarding the definition of failure and the planes of maximum stress obliquity. To derive these functions Matsuoka's failure criterion is used. As a consequence a theoretical hierarchy of strengths is established which agrees qualitatively with experimental evidence. The importance to a designer of this variety of strengths is emphasized. A study is made of the piezocone and the interpretation of the pore pressures in terms of the overconsolidation ratio of the clay tested. A plea is made for the standardization of the equipment, the operation and the interpretation of in situ tests to obtain maximum benefit from them. L'article décrit les buts des essais in situ et souligne les difficultés de l'interprétation des mesures. Ces difficultés sont dues au comportement complexe des sols, combiné avec le manque de contrôle et de choix des conditions limites qu'on a dans n'importe quel essai in situ. Une exception importante est l'essai pressiométrique—à partir duquel on peut evaluer directement des propriétés du sol sans être obligé de recourir à des correlations empiriques. La discussion se concentre sur la mesure de la résistance au cisaillement non-drainé. On compare les résultats obtenus à partir des différents essais (triaxial, déformation plane, cisaillement simple direct, pressiomètre et scissomètre) en les exprimant en fonction du rapport su/σv0′ de la résistance dans l'état non-drainé et en fonction de l'angle de friction φ. On étudie plus particulièrement les essais dans lesquels les axes principaux de l'augmentation de la contrainte et de la déformation peuvent tourner librement. Dans de tels cas il existe une incertitude concernant la définition de la rupture et des plans de l'obliquité maximale des contraintes. Afin de trouver ces fonctions on emploie le critère de rupture de Matsuoka. On établit par conséquent une hiérarchic théorique des résistances qui s'accorde qualitativement avec les résultats expérimentaux. On souligne l'importance pour le projeteur de ces diverses résistances. On étudie aussi le piézocône et l'interprétation des pressions interstitielles en fonction du rapport de surconsolidation de l'argile testée. On recommande que l'appareillage, l'exécution et l'interprétation des essais sur place soient uniformisés afin d'en tirer l'avantage maximal.

Discussion: Angles of friction and dilatancy of sand

“…9 (17) for these samples have been plotted by assuming ö cv to be equal to the ultimate friction angle measured in cylwests. Note that the plotted points are nearly as close to the corresponding representation of equation (17) as the relationship suggested by Wroth (1984). These results also indicate that cylwests may advantageously be used for the measurement of ö ps p in sands in the absence of more intricate apparatus for this purpose.…”

“…The dashed line in Fig. 9 represents the`approximate' relation between ö ps p and ö tr p obtained by Wroth (1984) on the basis of theoretical considerations on failure criteria and an assumed associated¯ow rule. Equation (17) can be represented by a family of parallel lines, such as those shown in Fig.…”

Undrained cavity expansion analysis with a unified state parameter model for clay and sand

“…In this section, the analytical solution using CASM is applied to simulate the self-boring pressuremeter tests, which have been widely used to obtain fundamental soil properties, as one of the best in situ tests for geotechnical investigation (Wroth, 1984;Clarke, 1995;Yu, 2000). The pressure/ displacement response from the inflated cylindrical membrane of the pressuremeter is analysed to determine soil parameters (e.g.…”

New model of vane shear strength testing in soils

“…A new theoreti-cal vane shear model has been derived from that suggested by Wroth (1984), modified to account for experimental effective stress data collected by other researchers, notably Matsui & Abe (1981) and Kimura & Saitoh (1983).…”