Compaction is the densification of soils through the expulsion of air from the soil mass under the application of mechanical energy (dynamic or static loads). Compaction improves the engineering properties of soils significantly. As a result of compaction, the permeability and compressibility of soils decrease substantially, and the shear strength increases. The compaction effort can be applied through rolling, tamping or vibration.
There are several factors that affect the compaction characteristics of soils. The main factors include the type of soil, moisture content and the compaction effort. Laboratory compaction tests aim to determine the moisture-density relationship of soils. The coordinates of the peak point in the moisture-density relationship corresponds to the maximum dry density and the optimum moisture content. The compaction mechanisms
for coarse grained and fine grained soils are different from each other (Foster 1962; Kim 2003).
When fine-grained soils are compacted in their dry state, soil particles can not easily achieve a dense state due to friction. As water is added to the soil mass and compaction is performed at low moisture contents, a thin water film is formed around the soil particles. These thin water films around the particles do not contribute to lubrication of particles. Addition of more water breaks these thin water films. Water then starts to act as a lubricant between the soil particles during compaction, facilitating the rearrangement of particles into denser states until the maximum dry density is achieved at the optimum moisture content. Beyond the optimum moisture content, water tends to separate and push the soil particles apart from each other rather than assisting them to come closer, and, accordingly, the soil dry unit weight values tend to drop. Therefore, the moisture-density relationship for fine-grained soils typically has a single peak (Foster 1962).
In the case of free-draining coarse-grained soils (sand and gravels), the compaction mechanism tends to be different and more complex than that of fine-grained soils. At moisture content values in which the soil is partially saturated, surface tension forces develop between the particles. During compaction, these surface tension forces prevent the soil particles from moving closer. Therefore, the maximum dry unit weight is typically observed at the dry state for free-draining soils. Addition of water tends to decrease the compacted dry unit weight until the surface tension forces break. After the surface tension forces break, further addition of water facilitates lubrication of the particles, and, accordingly, the dry unit weight values start to increase, typically reaching a maximum at a fully-saturated state. This mechanism commonly seen in granular soils is referred to as the “bulking” phenomenon (Foster 1962; Lambe and Whitman 1972; Kim 2003). This type of compaction curves has been observed not only for natural soils and but also for other free-draining geo-materials such as bottom ash and slag (Evans 2007;
Huang and Lovell 1990).
The shapes of compaction curves (moisture-density relation) for soils do not always necessarily fall into the two categories explained above. For some fine-grained and coarse-grained soils, somewhat irregularly-shaped compaction curves (with 1 and 1/2
peak, 2 peaks, and without distinct peaks) were also observed in laboratory tests and field trials. Figure 3.2 shows the typical shapes of regular and irregular compaction curves observed for different soil types. Several researchers have investigated the causes of irregularly-shaped compaction curves and discussed the effects of gradation, Atterberg limits, particle shape, mineralogy and surface tension on these kinds of curves (Lee and Suedkamp 1972; Lee 1976). Studies in the literature have indicated similar irregular (oddly shaped) compaction curves for some blast-furnaces slags. The Pennsylvania Department of Transportation studied the compaction characteristics of granulated blast-furnace slags and observed oddly-shaped curves with 1 and 1/2 and 2 peaks (Lee and Suedkamp 1972; Lee 1976). Occasionally, compaction may result in a substantial change in the gradation of materials that degrade easily. This particle degradation during compaction can also cause irregularity in the moisture-density relationship. Therefore, it is important to quantify and determine the effect of particle degradation on the compaction characteristics of a material (Hale et al. 1981).
Moisture content (%)
typical compaction curve for free draining granular material
typical compaction curve for free draining granular material
irregular compaction curve without a distinct peak
Figure 3.2 Typical shapes of regular and irregular compaction curves for different soil types (modified after Foster 1962; Lee and Suedkamp 1972)
Even the fine gradations of steel slag typically have a significant percentage of gravel-size particles and can be classified as a coarse-grained material. Therefore, the characteristic compaction curves for steel slags are typically more similar to those of coarse-grained soils rather than those of fine-grained soils. Studies on the compaction characteristics of steel slag are very scarce since properties of steel slag have not been explored for geotechnical applications. Very few researchers studied the moisture-density relationships of steel slags. Based on laboratory compaction tests on steel slag (type of steel slag not specified) Ghionna et al. (1996) reported a maximum dry unit weight of 26 kN/m3 and a corresponding optimum moisture content of approximately 4-6% The dry unit weight values measured in situ by these authors showed some scatter, with a mean value of 23 kN/m3. Raposo (2005) presented the compaction characteristics of BOF slag.
The compaction curves showed irregular shapes with two smooth peaks with a maximum dry unit weight of ~23-24 kN/m3 at moisture contents of approximately 4% and 12 %.
Rohde et al. (2003) presented standard Proctor compaction tests results on EAF slag samples with different gradations; both regular (single-peak) and irregular compaction curves were observed for different gradations of EAF slag. Optimum moisture content and maximum dry unit weight of EAF slag samples were in the range of 3-6% and 23-26kN/m3. Andreas et al. (2005) presented the standard Proctor compaction test results on ladle slag-EAF slag mixture that contained 35% EAF slag by weight. The dry unit weight and moisture content couples displayed a single peak compaction curve, with a maximum dry unit weight of 22 kN/m3 at approximately 13% moisture content. Due to its high specific gravity (typically above 3) and gradation (typically well-graded) the reported values for maximum dry unit weight of steel slags is higher than that of natural aggregates.