Practical Considerations when Using the Swedish Fall Cone

This paper presents the results of Swedish fall cone tests and Casagrande liquid limit tests conducted on saline Champlain Sea clay samples from Lachenaie, Quebec. The main objective was to study a few hitherto unanswered practical questions regarding these testing methods. Penetration range is found to affect the Hansbo's relationship used in fall cone experiments, while the mass and the bluntness degree of the cone have no effect on it. A direct relationship between thixotropic regain in shear strength and sensitivity is found. When measuring the liquid limit, if only the first penetration depth is recorded, results are up to 5% smaller than those obtained when following the standard procedure of CAN/BNQ-2501-092. With this standard, the average of the first two penetration depths within 0.3 mm of each other is recorded. These penetrations usually follow the bulk of the thixotropic shear strength regain. The Swedish fall cone was compared to the traditional Casagrande apparatus for liquid limit determinations. The two methods yielded identical results in the studied conditions (saline Lachenaie clay with liquid limit between 44% and 75%). An incorrect calibration of the height-of-drop of 1.4 mm led to a mean error of 6 liquid limit points. This error is greater than the theoretical error obtained by assuming that the number of blows is proportional to the square of the height-of-drop.

Mots clés

Clay, fall cone, liquid limit, undrained shear strength, Casagrande apparatus, thixotropy