DETERMINATION OF RELATIVE DENSITY OR DENSITY INDEX OF SAND

DETERMINATION OF RELATIVE DENSITY OR DENSITY INDEX OF SAND (IS-2720-PART-15)

AIM

To determine the relative density or density index of cohesionless soil

THEORY

Relative density is also known as density index. It is defined as the ratio of difference between the void ratio of a cohesionless soil (i.e. sand) in the loosest state and any given void ratio to the difference between its void ratios in the loosest state and in the densest state.

APPARATUS

• Vibratory table
• Moulds
• Surcharge weights
• Dial gauge
• Pouring device
• Mixing pans
• Weighing scale
• Steel straight edge
• Metal hand scoop

TEST PROCEDURE

The test procedure to determine the relative density of soil involves the measurement of density of soils in its loosest possible state (γmin) and densest possible state (γmax). Knowing the specific gravity of soil solids (Gs) the void ratios of the soil in its loosest state (emax) and densest state (emin) are computed. The density of soil in the field (γ) is used to compute the void ratio (e) in the field. After obtaining the three void ratios (i.e. minimum, maximum and natural) the relative density is computed. The two moulds (3000 cm3 or 15000 cm3) are used depending upon the maximum size of soil particle present.

The first step is to calibrate these moulds. Then the possible minimum and maximum densities of the soil are obtained as explained below.

CALIBRATION OF MOULDS

To calibrate, the mould should be filled with water and a glass plate should be slide carefully over the top surface of the mould in such a manner as to ensure that the mould is completely filled with water. The volume of the mould should be calculated in cm3 by dividing the weight of water in the mould by the unit weight of water.

PREPARATION OF SOIL SAMPLE

A representative sample of soil should be selected. The weight of soil sample to be taken depends upon the maximum size of particles in the soil as given in the table below. The soil sample should be dried in an oven at a temperature of 1050 to 1100 c. The soil sample should be pulverized without breaking the individual soil particles and sieved through the required sieve.

Weight of soil sample to be taken for the test (As per IS-2720-Part-15)
Maximum size of soil particles (mm)	Weight of soil sample required (kg)	Pouring device to be used in the test for the determination of minimum density	Size of the mould to be used (cm3)
80	45	Shovel or large scoop	15000
40	12	Scoop	3000
20	12	Scoop	3000
10	12	Pouring device (25 mm dia spout)	3000
4.75	12	Pouring device (12 mm dia spout)	3000

 PROCEDURE FOR THE DETERMINATION OF MINIMUM DENSITY

1. The pouring device and mould should be selected according to the maximum size of particle as indicated in the table above. The mould should be weighed and weight recorded. Oven dry soil should be used.
2. Soil containing particles smaller than 10 mm should be placed as loosely as possible in the mould by pouring the soil through the spout in a steady stream. The spout should be adjusted so that the height of free fall of the soil is always 25 mm. while pouring the soil, the pouring device should be moved in a spiral motion from the outside towards the centre to form a soil layer of uniform thickness without segregation. The mould should be filled approximately 25 mm above the top and leveled with the top by making one continuous pass with the steel straight edge. If all excess material is not removed, an additional continuous pass should be made. Great care shall be exercised to avoid jarring during the entire pouring and trimming operation.
3. The mould and the soil should be weighed and the weight recorded.
4. Soil containing particles larger than 10 mm should be placed by means of a large scoop (or shovel) held as close as possible to and just above the soil surface to cause the material to slide rather than fall into previously placed soil. If necessary, larger particles may be held by hand to prevent them from rolling off the scoop. The mould should be filled to overflowing but not more than 25 mm above the top. The surface of the soil should be leveled with the top of the mould using the steel straight edge in such a way that any slight projections of the larger particles above the top of the mould shall approximately balance the large voids in the surface below the top of the mould. The mould and the soil should be weighed and the weighed and the weight recorded.

PROCEDURE FOR THE DETERMINATION OF MAXIMUM DENSITY

The maximum density of soil may be determined by either dry or wet method.

1. DRY METHOD

1. The guide sleeve should be assembled on top of the mould and the clamp assemblies tightened so that the inner surfaces of the walls of the mould and the sleeve are in line. The lock nuts should be tightened. The third clamp should be loosened, the guide sleeve removed, the empty mould weighed and its weight recorded.
2. The mould should then be filled with the thoroughly mixed oven dry soil in a loose state.
3. The guide sleeves should be attached to the mould and the surcharge base plate should be placed on the soil surface. The surcharge weight should then be lowered on to the base plate using the hoist in the case of the 15000 cm3 mould.
4. The mould should be fixed to the vibrator deck. The assembly of mould fixed on to the vibrating table is shown in the figure below. The vibrator control should be set at its maximum amplitude and the loaded soil specimen should be vibrated for 8 minutes.
5. The surcharge weight and the guide sleeves should be removed from the mould. The dial gauge readings on two opposite sides of the surcharge base plate should be obtained and the average recorded. The mould with the soil should be weighed and its weight recorded.

Relative Density Test

2. WET METHOD

1. The wet method may be conducted on dry soil or wet soil from the field.
2. The mould should be filled with wet soil. Sufficient water should be added to the soil to allow a small quantity of free water to accumulate on the surface of the soil during filling. During and just after filling the mould, it should be vibrated for a total of six minutes. During this period the amplitude of the vibrator should be reduced as much as necessary to avoid excessive boiling or fluffing of the soil, which may occur in some soils. During the final minutes of vibration, any water appearing above the surface of the soil should be removed.
3. The guide sleeve, surcharge base plate and surcharge weight should be assembled as described in dry method.
4. The specimen with the surcharge weight should be vibrated for 8 minutes. After the vibration, the surcharge weight and the guide should be removed from the mould. Dial gauge reading should be obtained on two opposite sides of the base plate and recorded. The entire wet specimen should be carefully removed from the mould, dried to constant weight, weighed and the weight recorded.

OBSERVATION AND CALCULATION

Observation and calculations are recorded and done as per the table shown below.

Determination of minimum density
Observation	Trial-1	Trial-2	Trial-3
Weight of mould, g
Weight of soil+mould,(W) g
Calibrated volume of mould, (Vc), cm3
Minimum density, γmin=W/Vc, g/cm3

Determination of maximum density
Observation	Trial-1	Trial-2	Trial-3
Weight of mould, g
Dial gauge reading , left
Dial gauge reading, right
Average gauge reading, Df
Initial gauge reading, Di
Surface area of soil sample, (A), cm2
Volume of soil Vs=Vc-(Df-Di)A, cm3
Weight of dry soil+mould, g
Weight of dry soil, (W), g
Maximum density, (γmax)=W/Vs, g/cm3

Computation of relative density
Observation	Trial-1	Trial-2	Trial-3
In place density, γd, g/cm3
γmax, g/cm3
γmin, g/cm3
γd-γmin
γmax-γmin
Realtive density Dr={[(γmax)( γd-γmin)]/[γd(γmax-γmin)]}*100