GEONOR - Geotechnical - Structural

The Fall Cone Apparatus, first introduced in 1915, provides a rapid, simple and accurate method for determining empirically the undrained shear strength, the shear strength of remolded samples and thus the sensitivity of clays. The equipment is also used for determination of liquid WL (or fineness number WF ). Four cones of different weights and apex angles are provided.

PRINCIPLE

The shear strength of a soil is (s) is proportional to the weight of the cone (Q) and inversely proportional to the square of the penetration (h) of the cone into the sample: s=K·Q/h² (Hansbo, S (1957): A new approach to the determination of the shear strength of clay by the fall cone test. Swedish Geot. Inst. Pub. No. 14, Stockholm). The proportional constant K depends primarily on the angle of the cone and sensitivity of the clay.

The shear strength is measured by placing the sample underneath the cone holder, with the tip of cone touching the sample. After releasing the cone from the magnet holder and measuring the depth of penetration, the shear strength of the undisturbed or of the remolded sample is given in tables accompanying the apparatus.

DETERMINATION OF THE LIQUID LIMIT

The standard cone used to determine the liquid limit WL (see diagram) has a weight of 60 g and an angle of 60° (part no: 200400). The upper limit of plasticity corresponding to the liquid is defined as the moisture content at which the cone impression is 10 mm (Karlsson, R. (1961): Suggesting improvements in the liquid limits test, with reference to flow properties of remolded clays. 5th Ont. Conf. On Soil Mech, and Found. Eng., Vol 1. p. 171, Paris).

DETERMINATION OF THE SENSITIVITY

The undrained shear strength of an undisturbed sample is first measured. Thereafter, the sample is completely remolded and the new shear strength is measured with the same water content. The ratio of the undisturbed to remolded shear strength is defined as the sensitivity (S)