Geotechnical Engineering

DHARM

INDEX PROPERTIES AND CLASSIFICATION TESTS 39

obtaining the weight of the soil. The compaction is achieved by applying vibration and a

compressive force simultaneously, the latter being sufficient to compact the soil without breaking

individual grains. The extent to which these should be applied depends on experience and

judgement. More efficient packing may be achieved by applying the vibratory force (with the

aid of a vibratory table as specified in IS-2720 (Part XIV)–1983)* in the presence of water;

however this needs a proper drainage arrangement at the base of the cylinder used for the

purpose, and also the application of vacuum to remove both air and water. It should be noted,

however, that it is not possible to obtain a zero volume of void spaces, because of the irregular

size and shape of the soil particles. Practically speaking, there will always be some voids in a

soil mass, irrespective of the efforts (natural or external) at densification.

In the dry method, the mould with the dry soil in it is placed on a vibratory table and

vibrated for 8 minutes at a frequency of 60 vibrations per second, after having placed a stand-

ard surcharge weight on top.

In the wet method, the mould should be filled with wet soil and a sufficient quantity of

water added to allow a small quantity of water to accumulate on the surface. During and just

after filling, it should be vibrated for a total of 6 minutes. Amplitude of vibration may be

reduced during this period to avoid excessive boiling. The mould should be again vibrated for

8 minutes after adding the surcharge weight. Dial gauge readings are recorded on the sur-

charge base plate to facilitate the determination of the final volume.

The wet method should be preferred if it is found to give higher maximum densities

than the dry method; otherwise, the latter may be employed as quicker results are secured by

this approach.

Other details are contained in the relevant Indian Standard, and its revised versions.

The minimum unit-weight (or maximum void ratio) can be determined in the laboratory

by carefully letting the soil flow slowly into the test cylinder through a funnel. Once this task

has been carefully performed, the top surface is struck level with the top of the cylinder by a

straight edge and the weight of the soil of known volume may be found in this state, which is

considered to be the loosest. Oven-dried soil is to be used. Even the slightest disturbance may

cause slight densification, thus affecting the result.

If proper means are available for the determination of the final volume of vibrated sand,

the known weight of sand in the loosest state may itself be used for the determination of the

void ratio in the densest state. In that case the sequence of operations will change.

Thus, it may be understood, that there is some degree of arbitrariness involved in the

determination of the void ratio or unit weight in the densest as well as in the loosest state.

The concept of Density Index is developed somewhat as follows:

Assuming that the sand is in the loosest state:

emax =

V

V

v

s

max

min

,

for which the corresponding value of density index is taken as zero.

*“I.S.–2720 (Part XIV)–1983 Methods of Test for Soils–Part XIV Determination of Density Index

(Relative Density) for Soils” gives two approaches–the dry method and the wet method for the determi-

nation of the maximum density.