DHARMINDEX PROPERTIES AND CLASSIFICATION TESTS 41
Table 3.2 Representative values of Density Index and typical unit
weights (Mc Carthy, 1977)Descriptive condition Density index, % Typical range of unit
weight, kN/m^3Loose Less than 35 Less than 14
Medium dense 35 to 65 14 to 17
Dense 65 to 85 17 to 20
Very dense Greater than 85 Above 20Depending upon the texture, two sands with the same void ratio may display different
abilities for densification; hence the density index gives a better idea of the unit weight than
the void ratio itself.
The density index concept finds application in compaction of granular material, in various
soil vibration problems associated with earth works, pile driving, foundations of machinery,
vibrations transmitted to sandy soils by automobiles and trains, etc. Density index value gives
us an idea, in such cases, whether or not such undesirable consequences can be expected from
engineering operations which might affect structures or foundations due to vibration settlement.
In.-Situ Unit Weight
The in-situ unit weight refers to the unit weight of a soil in the undisturbed condition or of a
compacted soil in-place.
Determination of in-situ unit weight is made on borrow-pit soils so as to estimate the
quantity of soil required for placing and compacting a certain fill or embankment. During the
construction of compacted fills, it is standard practice to make in-situ determination of a unit
weight of the soil after it is placed to ensure that the compaction effort has been adequate.
Two important methods for the determination of the in-situ unit weight are being given:
(i) Sand-replacement method.
(ii) Core-cutter method.3.7.1 Sand-replacement Method
The principle of the sand replacement method consists in obtaining the volume of the soil
excavated by filling in the hole in-situ from which it is excavated, with sand, previously cali-
brated for its unit weight, and thereafter determining the weight of the sand required to fill
the hole.
The apparatus* consists of the sand pouring cylinder (Fig. 3.7), tray with a central cir-
cular hole, container for calibration, balance, scoop, etc.
*“IS–2720 (Part XXVIII)–1974 (First revision)–Methods of Test for Soils–Determination of in-
place density by sand-replacement method” contains the complete details of the apparatus and the
recommended procedure in this regard.