Soil stabilisation is used extensively to improve the physical and mechanical properties of soils to achieve the desired strength and durability properties. During the design process, laboratory investigation is conducted firstly to obtain an enhancement in soil strength and stiffness, in addition to the type and amount of binder required. The methods of preparing and curing specimens of soil–binder mixtures directly influence the properties of the stabilised soils. The most common laboratory protocols used for preparing and curing the specimens of stabilised soil are presented in this short review. The review focuses on several aspects such as homogenisation of the natural soil, mixing type and duration, mould type, moulding techniques and curing time and condition. This review can assist various construction projects that deal with soil improvement to choose an appropriate method for preparing and curing a soil–binder mixture to simulate the field conditions as much as possible and obtain uniform soil–binder mixtures.
Introduction
For soil stabilisation applications, the specimens of a soil–binder mixture are prepared in a laboratory according to a standard procedure, which in principle should simulate the field conditions. These procedures vary between different countries; in addition, there are variations between different testing companies (Åhnberg and Holm, 2009; BRE, 2002; BSI, 2005; Carlsten and Ekström, 1997; Kitazume, 2012). In Sweden, the specimens of soil–binder mixtures are prepared according to the common procedure described by the Swedish Geotechnical Society (SGS) (Carlsten and Ekström, 1997) and the Building Research Establishment (BRE, 2002). In Japan, the specimens are prepared according to the Japanese Geotechnical Society standard (Kitazume and Terashi, 2013). These variations are related to differences in soil type, type and procedure of soil stabilisation in the field and differences in traditional laboratory testing in general (Åhnberg and Holm, 2009).
Generally, the specimen of a stabilised soil is prepared in a laboratory according to a standard protocol, which normally consists of several steps. Firstly, natural soil is homogenised, and then a cementitious binder is added in dry or in slurry form, and the mixture is blended by hand or an electric blender for a certain time. Then, the soil–binder mixtures are gradually filled as layers in a mould or tube according to the specified technique. Usually, five different moulding techniques can be used or combined to prepare a specimen, as summarised in the following (Kitazume et al., 2015).
Tapping. For each layer, the mould is tapped (hit) against a table or the floor for a specified number of times until the specimen height is subsequently filled.
Rodding. For each layer, the mixture is slowly tamped down for a specified number of times using a rod to compact/smooth out each layer.
Dynamic compaction. Each layer is compacted by using a Proctor hammer for specified drop height, weight and number of blows to achieve standard compaction energy (600 kJ/m3) or according to the specified compaction energy.
Static compaction. Each layer is compressed by using a specified static load for a certain time.
No compaction. The soil–binder mixture is filled in the mould by either pouring or placing.
Preparing and homogenising the natural soil before adding the cementitious binders represent the most common concern for obtaining a uniform soil–binder mixture. Disaggregating natural soil prior to treatment has many effects such as homogenising the soil, reducing the variation in water content and obtaining smaller-sized particles by separating the agglomerated particles. This process could assist in obtaining a uniform distribution of the cementitious binders around soil particles. Table 1 summarises the most common procedures for homogenising natural soil prior to treatment, specimen preparation methods and curing conditions. The most common step in these procedures is that natural soil has to be disaggregated and homogenised by remixing it alone before adding the stabiliser. Most of the standards do not specify the time required for the disintegration and homogenisation process because it can be influenced by several factors, such as the type and gradation, consistency limits, water content and organic content of the soil (Åhnberg and Holm, 2009; Bhadriraju et al., 2007; BRE, 2002; Bruce et al., 2013; Carlsten and Ekström, 1997).
For the effect of mixing time after adding the cementitious binder on the obtained uniform soil–binder mixture, several investigations have shown that the mixing time significantly influences the properties of stabilised soils. Several factors control the uniformity of soil–binder mixtures, such as mixing time, type of mixer used and the characteristics of the original soil, in addition to the type, amount and form of the added binder (in dry or in a slurry form). Kitazume (2005) pointed out the influence of mixing time and form of binder used on the unconfined compressive strength of the stabilised soil. These results were based on the laboratory mixing tests by Nakamura et al. (1982). The laboratory tests were conducted according to the Japanese standards for preparing the laboratory specimens (JGS, 2005) but using different mixing times. Portland cement was added to the soil in either a dry form or a slurry form with a water-to-cement ratio of 100%. The results showed that the unconfined compressive strength of the stabilised soil significantly decreased, as the mixing time was decreased to shorter than 10 min, particularly for the case of when low binder amounts were used. The results also showed that adding the binder in a dry form required a longer mixing time compared with adding the binder in slurry form. The recommended mixing time to mix the soil and binder is set as 10 min according to Japanese and British standards (BSI, 1990a, 1990b; JGS, 2005). In contrast, in Sweden, the recommended mixing time is set as 5 min and the produced mixture should be visually homogenised (Åhnberg and Andersson, 2011; Carlsten and Ekström, 1997).
Several investigators have shown that different curing procedures such as curing time and curing temperature significantly influence the strength and stiffness properties of stabilised soils. For the effect of curing time, Kitazume (2005) pointed out the influence of curing time and soil types on the unconfined compressive strength of lime-stabilised soil based on the results from Terashi et al. (1977). The results showed that the strength properties of lime treatment are dependent on the soil type (lime is more effective in clay) and the unconfined compressive strength increases almost linearly with the logarithm of the curing time. For lime treatment, 50–75% of the final shear strength is obtained after 1–3 months of curing, respectively, while 90% of stabilised soil shear strength is expected to be obtained after 1 year of curing (Broms, 2004: p. 263). For cement treatment, the improvement in soil strength and stiffness increases as the cement content and curing time increases, and the major improvement in soil strength occurs during the first 28 d of curing (Hassan, 2009; Ho et al., 2017; Kang et al., 2017; Lorenzo and Bergado, 2006; Sariosseiri and Muhunthan, 2009; Subramaniam et al., 2016). For the effect of curing temperature, Kitazume (2005) mentioned that a higher strength can be obtained under a higher curing temperature during short-term curing, and almost the same impact can be obtained at a longer curing time for different soil–binder treatments.
For soil stabilisation applications, choosing an appropriate laboratory method for preparing and curing the specimens of soil–binder mixtures is considered highly important to simulate the field conditions as much as possible. For instance, in shallow soil stabilisation applications such as road projects where the stabilised soil in the field is usually compacted as layers using a compactor to obtain certain compacting efforts, the dynamic compaction method is the most appropriate laboratory procedure to prepare the soil–binder mixture to simulate the desired compaction efforts. In contrast, for deep soil stabilisation, the compaction efforts are less important compared with those for road projects; therefore, the tapping or static compaction technique can be used. Moreover, choosing the appropriate method also depends on soil type, initial water content and the type and amounts of binder used.
For deep soil stabilisation, a group of researchers studied the influence of different laboratory moulding techniques on the wet density and the unconfined compressive strength of stabilised soil (Kitazume et al., 2015). The study was a part of an international collaboration between four organisations, the Tokyo Institute of Technology, the Sapienza University of Rome, the University of Coimbra and the Swedish Geotechnical Institute. Details of their studies are presented in the first four methods mentioned in Table 1. Regardless of the soil type and the type and amount of binder used, they observed that the modelling technique considerably influenced the wet density and the unconfined compressive strength of stabilised soil. The liquidity index and the undrained shear strength of the soil–binder mixture after treatment were used as indices to evaluate the results. They found that the tapping and rodding techniques were highly applicable when the undrained shear strength was less than 10 kPa or the liquidity index was larger than 1. The rodding technique was highly applicable when the undrained shear strength ranged from 10 to 20 kPa or the liquidity index ranged between 0·5 and 1·0. Moulding with rodding and dynamic compaction were highly applicable when the undrained shear strength was larger than 20 kPa or the liquidity index was smaller than 0·5.
Conclusions
This short review presents the most common laboratory procedures used to prepare and cure the specimens of soil–binder mixtures. The aspects of the various laboratory procedures presented include homogenisation of the natural soil, blending time, mould types and moulding techniques and curing conditions (time and temperature). Different moulding techniques and curing conditions considerably influence the properties of the soil–binder mixture. For soil stabilisation applications, choosing the appropriate method for preparing and curing the specimens of soil–binder mixtures is considered highly important to simulate the field conditions as much as possible, which subsequently reflect the strength and stiffness of stabilised soil in the field site. Dynamic compaction and rodding methods are more applicable for shallow soil stabilisation such as road projects or when the soil shear strength is greater than 20 kPa. Static compaction and tapping methods are more applicable for deep soil stabilisation or when the soil shear strength is lower than 10 kPa.
Acknowledgements
The authors would like to acknowledge the Iraqi Ministry of Higher Education and Scientific Research and the University of Babylon for offering the opportunity to pursue this study through their financial support.
