Soil Mechanics Lab: Compaction Test

Experiment: Soil Compaction Test (OMC & MDD)

1. OBJECTIVE

The primary objectives of the soil compaction test are:

• To establish the relationship between the moisture content and the dry density of a soil sample.
• To determine the Optimum Moisture Content (OMC) and the Maximum Dry Density (MDD) of the soil.

2. APPARATUS REQUIRED

• A soil sample of approximately 2.5 kg passing through a 4.75 mm IS sieve.
• A standard compaction mould, cylindrical in shape, with an internal diameter of 100 mm and a height of 127.3 mm.
• A standard rammer with a mass of 2.6 kg and a free-fall height of 310 mm.
• A weighing machine with an accuracy of 1 g.
• Moisture content containers (cans).
• A spatula for handling the soil.
• A measuring cylinder for water measurement.
• An oven for drying the soil samples.

3. THEORY

Soil compaction is the process of increasing the density of soil by forcing soil particles closer together, thereby reducing the volume of air voids. This is typically achieved through the application of mechanical energy, such as static or dynamic loads from rammers, rollers, or vibrators. Compaction significantly improves the engineering properties of soil.

Effects of Compaction:

• Increased Shear Strength and Bearing Capacity: Denser soil can withstand greater loads.
• Reduced Compressibility: The potential for future settlement under load is minimized.
• Reduced Permeability: Water flows less easily through densified soil.
• Increased Density: The mass per unit volume of the soil increases.

The degree of compaction is measured by the dry density (ρ_d), which is the mass of soil solids per unit volume. The relationship between bulk density (ρ) and dry density (ρ_d) is given by the formula:

where:

• ρ_d = Dry density of the soil
• ρ = Bulk density of the soil
• w = Water content (expressed as a decimal)

The relationship between water content and dry density is crucial. For a given compactive effort, the dry density of the soil increases with water content up to a certain point. The water acts as a lubricant, allowing soil particles to pack more tightly. The Maximum Dry Density (MDD) is the peak of the curve on a dry density vs. water content graph, and the corresponding water content at this peak is known as the Optimum Moisture Content (OMC).

4. PROCEDURE

1. Take about 2.5 kg of soil and pulverize it properly
2. Adjust the initial water content to 9% and mix uniformly with the soil
3. Grease the mould and base plate properly to ease removal after compaction
4. Attach the base plate to the mould and record its combined mass
5. Compact the soil in the mould in three equal layers:
   1. For each layer:
      • Place the soil in the mould
      • Compact using 25 blows from a rammer
   2. Scratch the top of each compacted layer with a knife to ensure bonding with the next layer
   3. For the third layer:
      • Attach the collar before compaction
      • Remove the collar carefully after compaction
6. Clean the mould and base plate after compaction and record the total mass
7. Take a small amount of soil from each layer, mix, and place in a container for water content determination
8. Break the compacted soil and add 3% additional water
9. Mix thoroughly and compact again
10. Repeat the process with further 3% water increments until the compacted mass starts to decrease compared to the previous value

5. OBSERVATION AND CALCULATIONS

• Weight of Soil = 2.5 kg = 2500 gm
• Height of Mould = 12.7 cm
• Diameter of Mould = 10 cm
• Volume of Mould (v) = 997.456 cc
• Weight of Mould with base plate (w_m) = 4.765 kg = 4765 gm
• Number of blows per layer = 25
• Number of layers = 3

Data Sheet

6. RESULT

From the compaction curve (graph of Dry Density vs. Water Content), the following results were obtained:

• Optimum Moisture Content (OMC) = 10.2 %
• Maximum Dry Density (MDD) = 1.862 gm/cc

7. DISCUSSION AND CONCLUSION

The laboratory test was conducted to determine the compaction characteristics of the given soil sample. By compacting the soil at various water contents, we observed the relationship between moisture and dry density.

The results show that as the water content was increased, the dry density of the soil also increased up to a certain point. The water initially acts as a lubricant, allowing the soil particles to pack more closely together, thus increasing the density. However, beyond a certain water content, the water starts to occupy space that could have been filled by soil particles, leading to a decrease in the dry density.

The peak of the compaction curve gives the Maximum Dry Density (MDD) and the corresponding Optimum Moisture Content (OMC). The determined values are 1.862 gm/cc and 10.2%, respectively. These values are critical for civil engineering applications, such as the construction of embankments, dams, and road bases, as they specify the conditions required to achieve the highest possible soil strength and stability.

In conclusion, the experiment successfully established the compaction curve for the soil sample and determined its Maximum Dry Density and Optimum Moisture Content, providing essential data for achieving proper compaction in the field.