MAXIMUM DRY DENSITY AND OPTIMUM MOISTURE CONTENT

Updated: May 1, 2020

AIM

To determine the maximum dry density and the optimum

moisture content of soil using heavy compaction as per IS: 2720

(Part 8) - 1983.

APPARATUS



CYLINDRICAL METAL MOULD

i) Cylindrical metal mould - it should be either of 100mm dia.

and 1000cc volume or 150mm dia. and 2250cc volume and

should conform to IS: 10074 - 1982

ii) Balances - one of 10kg capacity, sensitive to 1g and the

other of 200g capacity, sensitive to 0.01g

iii) Oven - thermostatically controlled with an interior of noncorroding material to maintain temperature between 105 and

110oC

iv) Steel straightedge - 30cm long

v) IS Sieves of sizes - 4.75mm, 19mm and 37.5mm

PREPARATION OF SAMPLE

A representative portion of air-dried soil material, large enough to

provide about 6kg of material passing through a 19mm IS Sieve

(for soils not susceptible to crushing during compaction) or about

15kg of material passing through a 19mm IS Sieve (for soils

susceptible to crushing during compaction), should be taken.

This portion should be sieved through a 19mm IS Sieve and the

coarse fraction rejected after its proportion of the total sample

has been recorded.

Aggregations of particles should be broken down so that if the

sample was sieved through a 4.75mm IS Sieve, only separated

individual particles would be retained.

PROCEDURE

A) Soil not susceptible to crushing during compaction –

i) A 5kg sample of air-dried soil passing through the 19mm IS

Sieve should be taken. The sample should be mixed

thoroughly with a suitable amount of water depending on the

soil type (for sandy and gravelly soil - 3 to 5% and for

cohesive soil - 12 to 16% below the plastic limit). The soil

sample should be stored in a sealed container for a

minimum period of 16hrs.

ii) The mould of 1000cc capacity with base plate attached,

should be weighed to the nearest 1g (W1 ). The mould

should be placed on a solid base, such as a concrete floor

or plinth and the moist soil should be compacted into the

mould, with the extension attached, in five layers of

approximately equal mass, each layer being given 25 blows

from the 4.9kg rammer dropped from a height of 450mm

above the soil. The blows should be distributed uniformly

over the surface of each layer. The amount of soil used

should be sufficient to fill the mould, leaving not more than

about 6mm to be struck off when the extension is removed.

The extension should be removed and the compacted soil

should be levelled off carefully to the top of the mould by

means of the straight edge. The mould and soil should then

be weighed to the nearest gram (W2).

iii) The compacted soil specimen should be removed from the

mould and placed onto the mixing tray. The water content

(w) of a representative sample of the specimen should be

determined as in Para 5.1.

iv) The remaining soil specimen should be broken up, rubbed

through 19mm IS Sieve and then mixed with the remaining

original sample. Suitable increments of water should be

added successively and mixed into the sample, and the

above operations i.e. Para ii) to iv) should be repeated for

each increment of water added. The total number of

determinations made should be at least five and the

moisture contents should be such that the optimum

moisture content at which the maximum dry density occurs,

lies within that range.

B) Soil susceptible to crushing during compaction –

Five or more 2.5kg samples of air-dried soil passing through the

19mm IS Sieve, should be taken. The samples should each be

mixed thoroughly with different amounts of water and stored in a

sealed container as mentioned in Para A) i), above. Follow the

operations given in Para A) ii) to iv), above.

C) Compaction in large size mould –

For compacting soil containing coarse material upto 37.5mm

size, the 2250cc mould should be used. A sample weighing

about 30kg and passing through the 37.5mm IS Sieve is used for

the test. Soil is compacted in five layers, each layer being given

55 blows of the 4.9kg rammer. The rest of the procedure is the

same as in Para A) or B), above.


REPORTING OF RESULTS

Bulk density γ in g/cc of each compacted specimen should be

calculated from the equation,

γ = (W2-W1)/V

where, V = volume in cc of the mould.

The dry density γ d in g/cc = 100γ/(100+w)


The dry densities, γ d obtained in a series of determinations

should be plotted against the corresponding moisture contents,

w. A smooth curve should be drawn through the resulting points

and the position of the maximum on the curve should be

determined. A sample graph is shown below:



The dry density in g/cc corresponding to the maximum point on

the moisture content/dry density curve should be reported as the

maximum dry density to the nearest 0.01.

The percentage moisture content corresponding to the maximum

dry density on the moisture content/dry density curve should be

reported as the optimum moisture content and quoted to the

nearest 0.2 for values below 5 percent, to the nearest 0.5 for

values from 5 to 10 percent and to the nearest whole number for

values exceeding 10 percent.

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