What Does Bulk Density Test Mean?
The bulk density test is a classification test carried out for cohesive soils. Bulk density is the mass of the soil sample per unit volume including voids, or the weight of the soil sample for a given volume.
It is also used to measure the degree of soil compaction and to calculate the porosity and void ratio when the specific gravity of the sample is known.
Bulk density depends on organic matter present in the soil, the texture of the soil, the density of soil minerals, and their packing arrangement. Soils rich in organic matter and soils that are loose, well aggregated, and porous have lower bulk density. Sandy soils have higher bulk density, as the total pore space in the sand is lesser than the pore spaces in silty or clay soil. The bulk density of soil increases with depth as lower layers are more compacted, have lesser aggregation, and have a lesser amount of organic matter.
High bulk density indicates low soil porosity and soil compaction. The bulk density of soil and its porosity indicates how the particles and voids in a soil sample are arranged and reflect its size and shape. It varies with moisture content since soils that swell with moisture will reduce in volume when it is dried, hence the moisture content of the soil should be determined at the time of sampling.
This test is performed as per ASTM D 7263 and can be used to convert the water fraction of soil from mass basis to volume basis, and degree of soil compaction. Bulk density is the mass of sample per unit volume including voids. Since bulk density varies with moisture content, the water content is determined at the time of sampling.
Trenchlesspedia Explains Bulk Density Test
The bulk density of a soil core sample can be determined using a standard test method for which a known volume of the soil sample is air-dried and weighed.
The apparatus for the test includes:
- A cylindrical steel core cutter of standard length, diameter, and wall thickness with one end beveled.
- A steel dolly.
- 10 kg steel mallet or drop/ sledgehammer.
- Drying oven.
The test can be conducted as follows:
- Expose and clean the soil layer to be tested.
- Position the steel dolly on top of the cutter and using the hammer, force the cutter into the soil till the cutter’s top edge is a few millimeters below the soil surface.
- Repeat the procedure nearby with other cores to get the desired number of soil samples.
- Dig out the core samples without damaging them and trim the ends level with the cutter edge using a spatula or straight edge.
- Samples that are not completely filled with soil should be discarded.
- To take samples of subsurface soil, the same procedure has to be followed but after digging a pit to the required depth.
- Transfer the samples to the laboratory.
Once transferred to the laboratory, the following steps have to be followed:
- Weigh the cutter containing the wet core to the nearest gram.
- For freely moving soil, extrude the core to a tray and dry to a constant weight at 105°C, which may take several days.
- If the soil does not easily come out of the cutter allow it to dry in the cutter.
- Weigh the dry soil with the cutter and then weigh the cutter separately.
- Measure the internal volume of the cutter to the nearest 0.5mm.
Bulk density is given as ρ = m / V
‘ρ’ is the bulk density given in mg/m3
‘m’ is the mass of the dry soil sample in gram (g), and
‘V’ is the volume of the core (cm3)