Theory and Practice of Heavier Drop-Cones for Index Property and Strength Determinations of Fine-Grained Soils

The laboratory fall-cone (FC) test method is widely adopted for liquid limit LLFC determination, and in undrained shear strength su measurement of very soft to firm fine-grained soils. Additionally, it is used to establish the PL100 parameter (i.e., the water content w mobilizing a 100-fold su increase relative to the LLFC strength), typically determined using LLFC data extrapolation approaches. This paper presents a theoretical and data-reanalysis investigation into the application of drop-cone (DC) methods for su measurement of firm to stiff fine-grained soils, and their PL100 determination. Here, a 30° or 60° cone of modest mass M, released from a drop height hd, contacts the test-specimen surface with an impact velocity, and penetrates into the soil, before coming to its rest position for a cone penetration depth d. Based on previously reported experimental FC and DC d:w data for ten different fine-grained soils, the authors examined the effects of (i) frictional resistance between the stem of the falling DC and its guide bearing, and (ii) the DC’s higher soil deformation rate, on the deduced PL100 water content. An adjusted cone-factor framework for DC su testing, which considers the hd/d ratio, and higher soil deformation rate, is presented. Bearing friction has the effect of deducing slightly higher su and PL100 values. Small calibration adjustments to the DC setup, namely marginally increasing (i) the cone mass M, (ii) the drop height hd, or (iii) both M and hd, can eliminate systematic overpredictions. Theoretical 30° and 60° DC M:hd combinations are identified for PL100 determination. Finally, LLFC data extrapolation for PL100 determination, employing the bi-logarithmic log10d:log10w model, was found to achieve moderate levels of agreement between predicted PL100 values and those directly measured using an 8 kg/30°–0 mm contacting cone for d = 20 mm, raising possible concerns about the generalizability of the extrapolation approach.