Geotechnical Engineering

DHARM

44 GEOTECHNICAL ENGINEERING

Rammer

Dolly

Core-cutter

Cutting

edge

Fig. 3.8 Core-cutter apparatus
The core-cutter is 10 cm in diameter and 12.5 cm in length. The dolly is 2.5 cm long. The
bottom 1 cm of the ring is sharpened into a cutting edge. The empty weight (W 1 ) of the core-
cutter is found. The core-cutter with the dolly is rammed into the soil with the aid of a 14-cm
diameter metal rammer. The ramming is stopped when the top of the dolly reaches almost the
surface of the soil. The soil around the cutter is excavated to remove the cutter and dolly full of
soil, from the ground. The dolly is also removed later, and the soil is carefully trimmed level
with the top and bottom of the core-cutter. The weight of the core-cutter and the soil is found
(W 2 ). The weight of the soil in the core-cutter, W, is then got as (W 2 – W 1 ). The volume of this
soil is the same as that of the internal volume of the cutter, V, which is known.
The in-situ unit weight of the soil, γ, is given by W/V. If the moisture content, w, is also
found, the dry-unit weight, γd, may be found as γd = γ/(1 + w).

This method* is suitable for soft cohesive soils. It cannot be used for stiff clays, sandy
soils and soils containing gravel particles, which could damage the cutting edge.

In an alternative approach, the volume, V, of a clay soil sample which can be trimmed
into a more or less regular-shaped piece, can be obtained by coating it with paraffin and then
immersing it in a graduated jar filled with water. The rise in water level in the jar gives the
volume of the sample together with the paraffin. The volume of the paraffin can be got by
dividing the weight of paraffin by its known unit weight. It can then be subtracted from this to
obtain the volume of the soil sample. The weight of the soil sample, W, would have been ob-
tained earlier before coating it with paraffin. The weight of the paraffin can also be got as the
increase in weight of the sample on coating it with the paraffin. The in-situ unit weight of the
soil may now be got as γ = W/V.

*“IS: 2720 (Part XXIX)–1975–Methods of Test for Soils–Determination of in-place density by the

core-cutter method” contains the complete details of the apparatus and the recommended procedure in

this regard.