Factors Affecting Compaction by a Compaction Tester

Factors Affecting Compaction by a Compaction Tester

The basic components of a compaction apparatus are the compaction cylinder and the compaction hammer. The former is used to prepare soil samples, while the latter is used to compact these samples. Depending on the particle size of the soil to be compacted, two different specifications of compaction cylinders are employed. The main parameters of the compaction test methods and corresponding equipment should conform to the table below. Compaction tests are divided into light compaction and heavy compaction. Light compaction tests are suitable for soils with particle sizes not exceeding 20 mm, whereas heavy compaction tests are suitable for soils with particle sizes not exceeding 40 mm. Under identical compaction conditions, the compaction characteristics of different soils vary. Soils with well-graded particle size distributions exhibit higher dry densities and lower moisture contents, thereby meeting the requirements for strength and stability.

Soil quality control is an essential component for ensuring the quality of embankment construction. Therefore, before embankment filling begins, a particle-size analysis test should be conducted to select soil that meets the design requirements. The dry density of the compaction curve varies with changes in moisture content. When the moisture content is low, the water film surrounding the soil particles is thinner, and the cohesive forces between the soil particles are stronger, which can partially offset the compaction effect exerted by the compaction equipment. As a result, the soil particles are less likely to undergo relative movement or compaction, leading to a lower dry density of the soil. On the other hand, if the soil moisture content is too high, free water fills the pores while some air becomes trapped. Under the action of compaction loads, it becomes impossible to expel the excess water and trapped air from the soil, causing the pore water pressure to rise continuously. This counteracts part of the compaction effort, thereby reducing the overall compaction effectiveness and resulting in a lower dry density of the soil. When the moisture content ω approaches the optimum moisture content ωAP, the appropriate moisture level allows water to act as a lubricant within the soil. The cohesive forces and frictional resistance between soil particles are reduced, and the opposing effects of pore water pressure and trapped air within the soil are also minimized. Consequently, the soil particles can move and pack more easily, increasing the initial dry density of the soil. Under the same compaction effort provided by the compaction equipment, the soil particles tend to arrange themselves more closely, thus achieving a higher dry density. For cohesive soils, the moisture content typically approaches the plastic limit of the cohesive soil, which can be approximated as ωAP = ωp + 2. The figure above shows the dry densities of soils at different moisture contents and their corresponding saturated states, yielding the saturation curve at Sr = 100%. As shown in the figure, the experimental compaction curve gradually approaches the saturation curve on the right side of its peak and remains essentially parallel to the saturation curve without intersecting it. This is because, at any given moisture content, the fill soil will never reach a fully saturated state; a certain amount of trapped air always remains within the soil, meaning the fill is never completely saturated. The results indicate that under compaction conditions (at the peak point of the compaction curve), the degree of saturation of cohesive soils generally hovers around 80%.

In the compaction test using a laboratory compaction apparatus, when the hammer weight and drop height are fixed, the magnitude of compaction effort can also be expressed in terms of the number of hammer blows, n. For the same soil, a smaller compaction effort will result in a lower dry density, whereas a greater compaction effort will lead to a higher dry density. The relationship between moisture content, however, is opposite: the less the compaction, the higher the moisture content; conversely, the greater the compaction effort, the lower the moisture content. It is important to note that the compaction effect diminishes as the compaction energy increases. Therefore, simply increasing the compaction energy to boost the soil’s dry density is not economically viable; instead, a comprehensive approach should be adopted.