An overview on the compaction characteristics of soils by laboratory tests

Highlights

• Several data have been considered regarding the compaction properties of soils.

• Correlations exist between maximum dry density (MDD) and optimum water content.

• Correlations exist between optimum water content (OWC) and plastic limit (PL).

Abstract

Soil compaction involves a densification and a relative variation of physical and mechanical characteristics of soils. Evaluating laboratory compaction parameters such as maximum dry density (MDD) and optimum water content (OWC) is a very important task to control field compaction for all earth-works structures. In this note over 400 publications and newly laboratory tests have been reviewed to give an overview regarding the correlations among maximum dry density, optimum water content, plastic limit, optimum degree of saturation and degree of compaction. The review confirms that viable correlations exist among maximum dry density versus optimum water content, maximum dry density against plastic limit and between optimum water content plotted against plastic limit. The correlation between optimum degree of saturation and optimum water content shows that OWC is lower for soils with larger maximum dry density. Besides with optimum water content less than 40% the relationship between optimum degree of saturation and OWC depends very much on soil type and particle size. This research comprises of a large number of soil types, various compaction energy level and methods and operators over the world.

Introduction

During the construction of road embankments, railway subgrade, earth-dams, or compacted clay liners for waste disposals, it is necessary to compact the fill material, to reduce the intergranular voids so as to reach a high density (Raviolo, 1993; Shimobe and Spagnoli, 2020). In this way, the compressibility (i.e. settlement) of the soil decreases, the shear strength increases, the permeability and the tendency to absorb water are reduced. The compaction of soils is not a fundamental property of the soils, though (Carter and Bentley, 1991). As a matter of facts, it is influenced by mineralogy, gradation, classification, water content, and mechanical variables as type as well as degree of compaction (Korfiatis and Manikopoulos, 1982). The construction of structures on fine-grained soils is considered as one of the challenging tasks in geotechnical engineering, as structures have to be built on good soils in order to be safe (Prasanna et al., 2017) and for waste disposal compacted clay liners had to provide low permeability and relatively high undrained shear strength (Rubinos et al., 2015). Besides, optimum water content (OWC), w_opt, and maximum dry density (MDD), ρ_dmax, are necessary prior to the field compaction. These two parameters are very important, because they help to determine the maximum dry density, i.e. where soils have a larger bearing capacity due to the maximum soil densification, throughout the optimum water content. The optimum water content of soil is the water content at which a maximum dry density can be achieved after a given compaction effort. It is in fact minimum amount of water required to form a film of water on the surfaces of the soil particles, which is just sufficient to support sliding movement of the soil particles. Compressing a soil to its greatest theoretical density means discharging all the gases from within the soil through the voids (Ren et al., 2015). In general, the hydraulic conductivity under a specified compactive effort takes its minimum value at the wet side of w_opt (e.g. Mitchell et al., 1965; Kimura and Kusakabe, 1993), although Colombo and Colleselli (1996) report the permeability to be at lowest at w_opt.

Several correlations were performed in the past among optimum water content, maximum dry density and Atterberg limits. Wesley (2003) and Ramesh et al. (2010) showed, for instance, that Atterberg limits and laboratory compaction have been found to be reliable in order to characterize and explain the behavior of fine-grained soils for engineering applications, because both test methodologies depend on the same factors such as type and proportion of clay mineral, shape and grain size distribution (e.g. Carter and Bentley, 1991; Mitchell and Soga, 2005). Atterberg (or consistency) limits, have considerable effects on the compaction characteristics (e.g. Zhang and Frederick, 2017). Several studies were performed in order to relate the Atterberg limits and compaction parameters for fine-grained soils, particularly natural soils (e.g. Jumikis, 1946; Davidson and Gardiner, 1949; Joslin, 1959; Morin and Todor, 1977; Blotz et al., 1998; Sivrikaya et al., 2008; Di Matteo et al., 2009; Terzaghi and Peck, 2010). For instance, Gurtug and Sridharan (2002) proposed correlations based on plastic limit (PL) to determine optimum water content and maximum dry density at varying compaction energy, showing that ρ_dmax was 0.98 times the dry density at PL and w_opt was 0.92 times PL. Sridharan and Nagaraj (2005) predicted that PL shows better correlation with compaction parameters than liquid limit, LL, and plasticity index, PI, whereas Noor et al. (2011) suggested correlations for predicting compaction parameters from specific gravity, G_s, PI and LL for 106 fine-grained soils from India. Farooq et al. (2016) proposed predictive curves for quick estimation of maximum dry density and optimum water content based on LL, PI and compaction energy without performing laboratory compaction tests. Recently Tatsuoka (2015) and Tatsuoka and Gomes Correia (2018) also considered the compaction characteristics taking into account matric suction and physical properties of compacted soils controlled by the degree of saturation. They analyzed a large amount of results from laboratory and field compaction tests, CBR tests and laboratory stress-strain (shear strength, cyclic strength and elastic shear modulus) and permeability tests of compacted soils and found out the importance of controlling the degree of saturation in soil compaction connected to soil structure design such as backfill and fill dam. In addition, they proposed the soil structure compaction control based on dry density and degree of saturation measured at compaction.

The main driver for the scientific interest in correlating the compaction characteristics with Atterberg limits, is due to the fact that laboratory determination of compaction parameters requires considerable time which can be saved through the use of empirical correlations during early stages of a project (e.g. Gurtug and Sridharan, 2002; Farooq et al., 2016; Prasanna et al., 2017; Khalid and ur Rehman, 2018).

Besides, several correlations exist between maximum dry density and optimum water content of soil (e.g. Mori, 1962; Ekwue and Stone, 1997; Sivrikaya et al., 2008; Shimobe, 2012). Mori (1962) obtained the following relation considering 380 different soils (from clays to sandy soils including gravels less than 4.76 mm particle size).

$${\rho }_{\mathit{dmax}=\frac{1}{0.0107w_{\mathit{opt}}+0.400}}$$

which corresponds to the typical relational expressions among various conventional proposed equations. Blotz et al. (1998) studied the relationship between compaction effort and maximum dry density describing a method combining LL. Sivrikaya et al. (2008) proposed the relation of maximum dry unit weight, γ_dmax, to w_opt for 156 fine-grained soils:

$${\gamma }_{\mathit{dmax}}=23.45e^{-0.018w_{\mathit{opt}}}\left(\text{in}\mathrm{kN}/{\mathrm{m}}^3\right)$$

where γ_dmax is the maximum dry unit weight and can be converted to ρ_dmax (=γ_dmax/9.81). From the engineering point of view, the correlation between the degree of saturation, S_r, in the optimum compaction state, referred as S_opt and the optimum compaction state (w_opt, ρ_dmax) is interesting, whereby optimum degree of saturation of soil is generally said to be 85%≦S_opt≦95%, 10%≦air porosity, n_a≦20% (Inaba et al., 1976; Oka et al., 2015).

In this paper data from several sources in literature and novel test results provided by the authors, are discussed considering the relation between w_opt, ρ_dmax, PL and the optimum degree of saturation, referred as S_opt, in order to provide a general overview regarding the relationships occurring between maximum dry density, optimum water content and plastic limits of different soils.