Soil Compaction Test

Introduction

Compaction of soils is a procedure in which a soil sustains mechanical stress and is densified.  Soil consists of solid particles and voids filled with water or/and air. A more detailed explanation of the three-phase nature of soils is provided in Soil as a three-phase System. When subjected to stress, soil particles are redistributed within the soil mass and the void volume decreases resulting in densification. The mechanical stress may be applied by kneading, or via dynamic or static methods. The degree of compaction is quantified by measuring the change of the soil’s dry unit weight, γ_d.

Within the framework of engineering applications, compaction is particularly useful as it results in:

• An increase in strength of soils
• A decrease in compressibility of soils
• A decrease in permeability of soils

Those factors are crucial in structures and engineering applications such as earth dams, embankments, support of pavements, or support of foundations.

The degree of the compaction depends on the soil properties, the type and amount of energy provided by the compaction process and the soil’s water content. For every soil, there is an optimum amount of moisture for which it can experience its maximum compression. In other words, for a given compactive effort, a soil is reaching its maximum dry unit weight (γ_d,max), at an optimum water content level (w_opt).

The compressibility of a relatively dry soil increases as water is added to it. That is, for water content levels dry of optimum (w_opt), the water acts as a lubricant, enabling soil particles to slide relative to each other, thus leading to a denser configuration. Beyond a certain water content level (wet of optimum, w>w_opt), excess water within the soil results in pore water pressure increase that pushes the soil particles apart. A typical correlation between the dry unit weight and the water content is presented in Figure 1. Also, it is worthwhile to note that, as it can be seen in Figure 2, for a given soil, the highest strength is achieved just dry of optimum (Figure 2a), while the lowest hydraulic conductivity is achieved just wet of optimum (Figure 2b). The effect of the compactive effort on the maximum dry unit weight (γ_d,max), and the optimum water content level (w_opt) can be observed in Figure 4. With increased in compactive effort, γ_d,max increases, while w_opt decreases. That is, a smaller water content level is sufficient to saturate a denser sample.

Figure 1: Effect of water content on the dry unit weight during compaction of a soil

Figure 2: Effect of water content on soil a) strength, and b) hydraulic conductivity

Proctor Compaction Test

The most common laboratory test for soil compaction is the Proctor compaction test.

The Proctor test was invented in the 1930s by R. R. Proctor, a field engineer for the Bureau of Waterworks and Supply, in Los Angeles, California. The process, which simulates the in-situ compaction processes typically performed during construction of earth dams or embankments, is the most common laboratory test conducted to derive the compressibility of soils.

The type of compaction and the provided energy for a given soil volume are standard and, thus, the test focuses on the change of a sample’s moisture content to derive the optimum water content (w_opt).

The standard Proctor test includes a 0.95-liter volume cylindrical mold in which the soil mass is placed and compacted in 3 layers. Each layer is compressed by dropping 25 times a 2,5 kg weight falling from an elevation of 30 centimeters.

A modified version of the test was introduced after World War II, in the 1950’s, when heavy machinery could result in higher compaction. In the new approach, the cylindrical mold remains the same, however, the drop weight is increased to 4,5kg and the dropping height to 45 centimeters. In addition, the soil is compacted in 5 layers with 25 blows per layer.

The test is conducted for 5 moisture contents to obtain the optimum water content (w_opt), for which the value of the dry unit weight is maximum (γ_d,max).

Test Equipment

The equipment utilized to conduct the test includes: 

• 10-centimeter diameter cylindrical compaction mold equipped with a base and a collar
• Proctor rammer weighing 2,5 kg or 4,5 kg depending on whether the standard of the modified test is conducted
• No.4 Sieve
• Steel straightedge
• Moisture containers
• Graduated cylinder
• Mixer
• Controlled oven
• Metallic tray and a scoop

Typical cylindrical compaction molds and rammers are shown in Figure 3.

Figure 3: Proctor molds and rammers (ASTM/AASHTO) by Controls Group (for more information, click here)

Test Procedure

The procedure of the Proctor Compaction Test consists of the following steps:

1. Obtain about 3 kg of soil.
2. Pass the soil through the No. 4 sieve.
3. Weight the soil mass and the mold without the collar (W_m).
4. Place the soil in the mixer and gradually add water to reach the desired moisture content (w).
5. Apply lubricant to the collar.
6. Remove the soil from the mixer and place it in the mold in 3 layers or 5 layers depending on the method utilized (Standard Proctor or Modified Proctor). For each layer, initiate the compaction process with 25 blows per layer. The drops are applied manually or mechanically at a steady rate. The soil mass should fill the mold and extend into the collar but not more than ~1 centimeter.
7. Carefully remove the collar and trim the soil that extends above the mold with a sharpened straight edge.
8. Weight the mold and the containing soil (W).
9. Extrude the soil from the mold using a metallic extruder, making sure that the extruder and the mold are in-line.
10. Measure the water content from the top, middle and bottom of the  sample.
11. Place the soil again in the mixer and add water to achieve higher water content, w.

Calculations

First, the compaction water content (w) of the soil sample is calculated using the average of the three measurements obtained (top, middle and bottom part of the soil mass).

Subsequently, the dry unit weight (γ_d) is calculated as follows:

where:     W = the weight of the mold and the soil mass (kg)

                W_m = the weight of the mold (kg)

                w = the water content of the soil (%)

                V = the volume of the mold (m³, typically 0.033m³)

This procedure should be repeated for 4 more times, given that the selected water contents will be both lower and higher from the optimum. Ideally, the selected points should be well distributed with 1-2 of them close to the optimum moisture content.

The derived dry unit weights along with the corresponding water contents are plotted in a diagram along with the zero-voids curve, a line showing the dry unit weight correlation with the water content assuming that the soil is 100% saturated. No matter how much energy is provided to the sample, it is impossible to compact it beyond this curve. The zero-voids curve is calculated as follows:

where:    G_S = the specific gravity of soil particles (typically, G_S~2.70)

               γ_W = the saturated unit weight of the soil (kN/m³)

Typical curves derived from the Standard and Modified Proctor tests, as well as the zero air voids curve are presented in Figure 4.

Figure 4: Typical curves derived by the Standard and Modified Proctor tests. The zero air voids curve is also shown