Standard Proctor Test For Compacted Soil
- To obtain the relationship between water content and dry unit weight by using the standard proctor compaction test.
- To determine the maximum dry unit weight and optimum water content.
Standards For This Test
- AASHTO: T99-86,
- ASTM: D698-91,
- BS1377: Part 4; Clause 3.
Relation b/w Water content and ρdry
Compaction is the process of densification of soil by reducing air voids and the degree of compaction of a soil is measured in terms of its dry unit weight.
Determination of the dry density and moisture content of a soil under given compaction effort can be obtained from these equations shown below.
Bulk unit weight of the soil (ᵞbulk)
ᵞbulk = (solid weight of soil inside the mold + moisture weight)/ volume of the mold
(Mass of the air within the voids of soil is neglected)
Dry unit weight of the soil (ᵞdry)
ᵞdry = ᵞbulk / (1+ water content)
Water content = (Mass of water) / (dry mass of soil) %
Then the graph of the dry unit weight verses water content can be plotted. It gives a curve shown in figure 1, and from that curve the maximum dry unit weight and optimum moisture content can be found.
If all the air of soil could be expelled by compaction, the soil would become fully saturated or the soil is at zero air voids condition. Practically it is impossible to attain full saturation by compaction, the line showing the relationship between dry density and water content at saturation is called zero air void line or theoretical saturation line. Zero air void line is shown in figure 2.
The following apparatus are required,
- Moulds – There shall be cylindrical moulds conforming to the moulds described above. The mould of diameter 101.6 mm shall have a height of 116.4 mm, and therefore will be of a volume 944 cm3. The mould of 152.4 mm shall have a height of 116.4 mm, and therefore will be of a volume 2124 cm3.The moulds shall be fitted with a detachable base plate and a removable extension approximately 50 mm high.
- A metal Rammer – There shall be a metal rammer having a 50 mm diameter circular face, and weighing 2.49 kg. The rammer shall be equipped with a suitable arrangement for controlling the height of drop to 305mm. ( Alternatively there can be rammer of 2.5 kg weight with a drop 300 mm)
- Balances – A balance readable and accurate to 1 g ( with a capacity 20 kg) and a balance readable and accurate to 0.01 g,
- Sieves – A 75 mm sieve, a 19 mm sieve and a 4.75 mm
- Mixing tools – Miscellaneous tools such as mixing pan, spoon, trowel, spatula etc.
- Metal tray – A large metal tray ( 600 mm X 500 mm and 80 mm deep),
- Straightedge – A Steel straightedge, 300 mm long, 25 mm wide, and 3 mm thick with one beveled edge,
- Sample extruder – (Optional) An apparatus ( such as a jack) for extruding specimen from the mould,
- An oven – Thermostatically controlled oven to provide temperature 105 -110 Co,
- Cans – Cans to take samples for moisture content determination,
- Obtain approximately 3 kg of air – dried soil in the mixing pan, break all the lumps so that it passes the sieve given in method A, B, C and D
- Add suitable amount of water
- Determine the weight of the empty mould without the base plate and the collar (M1) to the nearest 1g
- Fix the collar and the base plate
- Compact the moist soil in to the mould in three layers of approximately equal mass
(Each layer shall be compacted by 25 blows in the case of 101.6 mm diameter mould and 56 blows in the case of 152.4 mm diameter mould. Blows must be distributed uniformly over the surface of each layer so that the rammer always falls freely. The amount of soil must be sufficient to fill the mould, leaving not more than 6mm to be struck off when the extension is removed.
- Detach the collar carefully without disturbing the compacted soil inside the mould and using a straight edge trim the excess soil leaving to the mould
- Obtain the weight of mould with the moist soil (M2) after removing the base plate
- Extrude the sample and break it to collect the sample for water content determination preferably at least two specimens one near the top and other near the bottom
- Weigh an empty moisture can, M3 and weigh again with the moist soil obtained from the extruded sample in step 8 (M4)
- Keep this can in the oven for water content determination
- Repeat step 4 to 10. During this process weight M2 increases for some time with the increase in moisture and decreases thereafter. Conduct at least two trials after the weight starts to reduce.
- After 24 hours get the weight of oven dried sample (M5)
The bulk density, ρ in kg/m3 of each compacted specimen shall be computed from the equation;
ρ = M2 – M1/ V
M1 is the weight of the mold and base, in kg
M2 is the weight of mold, base and soil, in kg
V is the volume of the mold in m3
Moisture content can be obtained from the equation;
w = M4 – M5 / M5 – M3
w is the moisture content of the soil as a fraction
M3 is the weight of an empty can
M4 is the weight of the moist soil obtained from the extruded sample with container
M5 is the weight of oven dried sample
Dry density can be obtained using the following relationship;
ρdry = ρwet
(1 + w)/100
ρwet is the wet density
The dry densities ρdry, obtained in a series of determinations is plotted against the corresponding moisture content, w. A smooth curve is drawn through the resulting points and the position of the maximum on this curve is determined. Thus the maximum dry density and the corresponding water content is obtained from the graph.
Compaction of soil is an important process, as it helps it of achieve certain physical properties necessary for its proper behavior under loading: for example proper compaction of an earthen dam or a highway Embankment reduces the chances of its settlement, increases the shear strength of the soil due to its increased density and reduces the permeability of the soil.
The proctor compaction test was carried out successfully and obtained a curve that satisfied the objectives. Clearly identified the relationship between the dry density and the water content of a soil and reasons to have that change was studied.
Analysis and Discussion
It can be observed that as the moisture content is increased and the same compaction effort is used, i.e. 25 blows per layer, the dry unit weight is increased. This happens because water acts as a softening agent which makes the soil particles slip and move to a more dense position (Das, 2010).
However, when certain moisture content is reached, any addition to the moisture content will decrease the dry unit weight of the soil. This occurs because the water takes the space that could have been taken by the soil particles (Das, 2010). The moisture content in which the maximum dry density is achieved is the optimum moisture content. It was determined that the optimum moisture content of the soil sample is 22.7% and the maximum dry density is 15.77 kN/m3.
It can be observed there is no point in the compaction curve to the right of the 100% saturation curve which is expected in accord to theory. Also, the shape of the wet side of the optimum of the compaction curve follows the shape of the 100% saturation curve.
Compaction also has effect on hydraulic conductivity and strength. As the soil increases its unit weight by compaction, the hydraulic conductivity decreases because the pore spaces are taken by soil particles. The minimum hydraulic conductivity occurs at the optimum moisture content.
Beyond the optimum moisture content, the hydraulic conductivity increases because the pore spaces are taken by water instead of the soil particles. Compaction increases the strength of soils if compacted on the dry side of the optimum because of the increase in dry unit weight, thus, its bearing capacity and decreases the potential settlement of structures built on the soil. The opposite happens if the soil is compacted on the wet side of the optimum.