What are soil compaction and soil compaction tests?

Soil compaction is the major area of geotechnical engineering. Soil is composed of multiple pores present on its surface, while the soil is mainly composed of gravel, sand, rocks, etc. The arrangement of these rocks and gravels is such that they build up voids (spaces) between them, which allows spaces for the roots of the plants and crops to spread and absorb the required amount of nutrients. Soil compaction is a major problem primarily caused by human activities, micro-organisms, and other multiple factors.

Soil compaction is caused when the load applied to the soil induces internal stresses to the soil, this causes internal air and water to escape out of the pores present on the soil surface. This causes soil densification resulting in compacting of the soil. The spaces between the rocks and the gravels are reduced. Soil compaction also occurs when micro-organisms consume the majority of the organic matter, this reduces voids and prevents roots from extending for the nutrients they need. Hence, soil compaction is also soil fertility.

From the engineering point of view, soil compaction increases the strength of the soil but decreases soil compressibility and permeability. These factors are the key input for many civil engineering applications like the building of dams, structures, and embankments. The knowledge of the engineering properties of soil is very crucial to make the structure efficient and stable in all forms of environmental influences.

Estimation of soil compaction is necessary as an initial site preparation step before the construction of structures and buildings. This is usually achieved by performing different laboratory tests. The data of which can be utilized to increase the load-bearing capacity and stiffness of in-situ soils.

Proctor soil compaction test

This is the most common laboratory-focused test used to determine the soil compaction extent. This test simulates the in-situ compaction process generated by construction equipment during the construction of dams and embankments. Proctor soil compaction test focuses on the change in the moisture content of soil and determines the optimum water content of the soil.

In this test, a cylindrical mold of 0.95-liter volume is considered. Soil mass is filled into this cylindrical mold. Soil is compacted into three layers or five layers, depending upon measurement standards. The compaction is done by dropping loads of 2.5 kg, twenty-five times from a height of 45 cm. Sometimes loads of 4.5 kg are also used in place of 2.5 kg depending upon the test standards. This procedure is repeated for 5 moisture contents that give the optimum water content.

The following are the test equipments used in the proctor soil compaction test:

• Cylindrical compaction mold of 10 cm diameter
• Proctor rammer of 2.5 Kg and 4.5 Kg depending on the test standards
• Number 4 sieve
• Steel straightedge
• Moisture containers
• Graduated cylinders
• Mixer
• Controlled oven
• Metallic tray and scoop

In the proctor soil compaction test, first, the dry unit weight of the soil is determined by the following formula:

$W_d=\frac{w-w_m}{\left(1+w\right)\times v}$

Where $W_d$ is the unit dry weight of the soil in kg

$w$ is the weight of the mold and soil mass together in kg

$w_m$ is the weight of the mold

$v$ denotes the volume of the mold in $m^3$

Next, the value of the zero voids curve is calculated and the data are plotted in a graph with abscissa as the compaction water content and ordinate as the dry unit weight. The zero void curve is obtained by the following formula:

${\lambda }_d=\frac{G_s\times {\lambda }_w}{\left(1+w\right)\times G_s}$

Where ${\lambda }_w$ is the saturated weight of the soil in $\frac{kN}{m^3}$ and $G_s$ is the specific gravity of the soil particles.

Sand cone test method

This method is popularly used as field density testing for soil compaction measures during highway construction. This test includes dry density testing to conclude the soil compaction values.

In this test an empty sand cone is considered, its weight is measured, and filled with fine-grained dry sand. Now, the weight of the sand cone along with the added sand is measured for the second time. A hole-based plate is placed on the spot where the compaction of soil needs to be measured. A certain amount of soil is dogged from the area using the hole-based plate and its weight is measured, the sand cone is then placed exactly over the hole-based plate. A control valve lets the sand from the cone fill the area of the hole base plate.

Soil permeability test

Soil permeability refers to the ability of soil to allow the passage of water. Soils are excellent in possessing the property of permeability due to their grain structure and spacings caused due to placement of rocks and other materials. The permeability of soil is signified by the co-efficient of permeability given by $k$. One of the comely used tests to determine soil permeability is the constant head permeability test. This test makes use of an apparatus that has a reservoir at its top and contains de-aired water. The bottom of the apparatus contains the water, which is permeated through the soil sample used. Data acquired through pressure and volume measurements, along with height and cross-sectional areas of the soil sample are made to determine the coefficient of permeability of the soil. The tests are generally repeated three to four times to ensure accurate results. 

Compaction Percentage

Compaction percentage of soil is the ratio between the dry unit weight of soil and the unit weight value obtained from the laboratory measurement. It is expressed as,

$\omega =\frac{Dry unit weight of soil}{Unit weight obtained from laboratory}$

Where $\omega$ represents the compaction percentage.

For instance, a 95% soil compaction indicates that the soil has been compacted to 95% of the density as achieved in the laboratory. 

Context and Applications

This topic is extensively taught in many undergraduate and postgraduate degree courses of:

• Bachelors in Civil Engineering
• Masters in Civil Engineering
• Masters in Soil Engineering

Practice Problems

1. Which of the following tests make use of a sand cone?

1. Proctor soil compaction test
2. Soil permeability test
3. Sand cone method
4. None of these

Answer: Option c

Explanation: The sand cone method makes use of sand cones.

2. Which of the following is the test equipment used in the proctor soil compaction test?

1. Mixer
2. Controlled oven
3. Graduated cylinders
4. All of these

Answer: Option d

Explanation: The major equipment used in the Proctor soil compaction test are a mixer, controlled oven, graduated cylinders, and so on.

3. What is the primary cause of soil compaction?

1. Reduction of spaces between the rocks beneath the soil
2. Escaping of water and air out from the soil pores
3. Both a and b
4. None of these

Answer: Option c

Explanation: Due to externally applied load, the water and air present beneath the soil surface escape out, this results in the decrease of voids and spaces present between the rocks.

4. Which of the following weights are used in the proctor soil compaction test?

1. 2.5 Kg
2. 4.6 Kg
3. 3.5 Kg
4. All of these

Answer: Option a

Explanation: Weights of 2.5 Kg are generally used in the Proctor soil compaction test.

5. Which of the following constitutes the top part of the apparatus used for testing soil permeability?

1. De-aired water
2. Water permeated through the soil
3. Sand
4. Soil with moisture

Answer: Option a

Explanation: The top part of the apparatus used for testing soil permeability contains de-aired water.