Results of previous studies on using the fall-cone test to determine the plastic limit of soils show that further research is warranted. The present investigation devotes itself to modification of the specimen preparation technique, examination of the relationship between depth of cone penetration and water content, and estimation of the plastic limit using the penetration depth against water content relationship. Specimen rings are made and used to replace the specimen cup so as to facilitate the preparation of specimens. Data of 26 soils, covering a wide range of liquid limits from 30% to 526%, show that the logarithm of depth of cone penetration against logarithm of water content relationship is linear. Therefore, a linear equation with two soil-dependent parameters is developed for estimation of the plastic limit. For a soil, the value of the soil-dependent parameters can be determined from as little as four fall-cone tests and substituted into the equation to estimate the plastic limit. The computed plastic limits of the soils analysed are within 0·8–1·2, with an average value of 1, times the measured plastic limits. The effects of strain rate and undrained strength ratio on the estimation of the plastic limit are evaluated and found to be limited.
A linear logarithm-logarithm model for the fall cone penetration depth versus water content relationship (flow curve) has been developed based on the results of an experimental study using the British fall cone apparatus. The fall cone flow curve is expressed by a simple equation with parameters m and c, which represent the slope of the flow curve and the water content at a penetration depth of 1 mm, respectively. For a soil, the flow curve can be determined by applying a linear regression analysis to at least four data points with penetration depths approximately evenly distributed between 25 and 3 mm. It is shown in this paper that both the liquid limit and the plastic limit determined from the linear logarithm-logarithm flow curve are in close agreement with those determined from conventional methods. A one-point method for determination of the liquid limit is developed from the model and is verified by applying statistical analysis to a large volume of experimental data.Résumé : Un modèle linéaire log-log pour la relation entre la profondeur de pénétration du cône tombant et la teneur en eau (courbe d'écoulement) a été développé en partant des résultats d'une étude expérimentale utilisant l'appareil britannique du cône tombant. La courbe d'écoulement du cône tombant est exprimée par une simple équation avec les paramètres m et c, qui représentent respectivement la pente de la courbe d'écoulement et la teneur en eau à la profondeur de pénétration de 1 mm. Pour un sol donné, la courbe d'écoulement peut être déterminée en appliquant une analyse de régression linéaire comprenant au moins quatre points correspondant à des profondeurs de pénétration distribuées à peu près uniformément entre 25 mm et 3 mm. Il est montré dans cet article que tant la limite de liquidité que la limite de plasticité déterminées par la courbe d'écoulement log-log linéaire se situent très près des valeurs déterminées par les méthodes conventionnelles. Une méthode à point unique a été développée en partant du modèle pour déterminer la limite de liquidité et a été vérifiée an appliquant une analyse statistique sur un grand volume de données expérimentales.
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