Desiccation and cracking behaviour of clay layer from slurry state under wetting–drying cycles
Highlights
► Soil water evaporation process is composed of two stages. ► Wetting–drying cycles result in aggregate formation. ► Wetting induces cracking of clay-rich soil. ► Soil reach equilibrium after a certain number of wetting–drying cycles. ► Image processing technique is an useful tool for quantitative analysis of crack pattern.
Introduction
The formation of desiccation cracks on soil surface due to loss of water is a common natural phenomenon, and can significantly affect the soil performance in various geotechnical, agricultural and environmental applications. For example, a cracked soil is more compressible than an intact one at the same water content and the overall mechanical strength is weakened due to the presence of cracks (Morris et al., 1992). The size (width, length and depth), tortuosity, spatial distribution and connectivity of cracks govern the rate and the velocity at which solutes and microorganisms are transported in the soil, and thus control the dispersal of substances in soil (Horgan and Young, 2000). Most importantly, the soil hydraulic properties are directly controlled by the desiccation crack networks (Chertkov, 2000, Chertkov and Ravina, 1999). Many previous studies have indicated that the hydraulic conductivity of cracked soils is several orders of magnitude greater than that of intact soils (Albrecht and Benson, 2001, Boynton and Daniel, 1985). This issue is therefore a major concern in design and construction of low permeability structures as clay buffers and barriers for nuclear waste isolation, liners and covers for landfill, etc.
Over the past decades, a number of field studies and laboratory experiments have been undertaken to investigate the initiation and propagation of desiccation cracks in soils (Corte and Higashi, 1960, Kleppe and Olson, 1985, Konrad and Ayad, 1997, Miller et al., 1998, Morris et al., 1992, Nahlawi and Kodikara, 2006, Tang et al., 2008, Tang et al., 2010, Velde, 1999). However, these investigations have been largely qualitative and most are limited in the description of desiccation cracking phenomena. More recently, techniques for quantifying the main features of the crack patterns have evolved from direct field measurement to more sophisticated analysis by image processing (Miller et al., 1998, Velde, 1999, Vogel et al., 2005a, Vogel et al., 2005b). Image analysis has proved to be a powerful tool by which several geometric and morphologic parameters such as crack width, length, area, angle and their distribution characteristics can be determined effectively. In addition, some modelling and theoretical studies on desiccation cracking have also been conducted (Abu-Hejleh and Znidarčić, 1995, Ayad et al., 1997, Chertkov, 2000, Chertkov, 2002, Chertkov and Ravina, 1998, Deng and Shen, 2006, Konrad and Ayad, 1997, Péron, 2008, Péron et al., 2009a). However, as soil is a highly complex material, the desiccation cracking behavior is governed by a large number of factors including mineral composition, clay content, relative humidity, temperature, layer thickness, boundary conditions etc. (Albrecht and Benson, 2001, Fang, 1997, Nahlawi and Kodikara, 2006, Rodríguez et al., 2007, Tang et al., 2007, Tang et al., 2008, Tang et al., 2010). It is therefore difficult to propose a rational model to describe this complex phenomenon with a reasonable number of parameters. The essential mechanism of desiccation cracking is still not well understood today and the prediction of cracks initiation and the associated crack network propagation also faces several challenges.
It is recognised that the soil in-situ is subject to diurnal changes and seasonal rainy and sunny weather, and undergoes periodical wetting–drying (W–D) cycles. Several studies have been performed to investigate the effect of wetting and drying on soil physical properties (Alonso et al., 2005, Nowamooz et al., 2009, Rao et al., 2001, Tang et al., 2011). However, the relationship between W–D cycles and desiccation cracking behaviour is still not clearly understood. In this study, the effect of W–D cycles on cracking behaviour was investigated by subjecting a group of initially saturated clay layers to five W–D cycles. The initiation and evolution of cracks on the specimen surface during the cycles were monitored. The geometric characteristics of surface crack patterns are described and quantified through image processing and the mechanisms involved are discussed.
Section snippets
Material
The Romainville clay was used in this investigation. The physical properties are presented in Table 1. This clay is a lagoonal-marine deposit which is part of the Paris Basin Tertiary (Oligocene) formations. It is widely distributed over the eastern region of Paris, and has been considered responsible for the large amount of damages to buildings due to the swelling–shrinkage and cracking phenomenon. Various studies have been undertaken to analyse the hydro-mechanical behaviour of the clay under
Evaporation, shrinkage and cracking process
The measured water content θ at various times t during the first drying path for the four separate specimens are shown in Fig. 3 (desiccation curve). Two distinct evaporation stages can be indentified: a constant evaporation stage during which water content decreases linearly with time; and a subsequent falling evaporation stage during which water loss slows down gradually until the residual water content of about 4.3% is reached.
The determined shrinkage curve (e versus θ) is presented in Fig. 4
Crack pattern and structure evolution after W–D cycles
Fig. 8 presents the typical crack pattern after each W–D cycle. It indicates that the specimen surface was split to separate clods by the crack networks. After the first W–D cycle (Figure 8 (a)), the shapes of the clods are relatively regular and most of the clods are close to quadrangles or pentagons; the crack segments are smooth and generally perpendicular to each other. This is consistent with the observations of Vogel et al., 2005a, Vogel et al., 2005b, Péron et al., 2009b, and can be
Conclusions
Desiccation and cracking behaviour of clay layers from a slurry state upon five wetting–drying (W–D) cycles were investigated through laboratory experiments. The process of water evaporation, surface cracks evolution, structure evolution and volume shrinkage behaviour were monitored and have been discussed here. The geometric characteristics of crack pattern after each W–D cycle were quantitatively analyzed by image processing. The following conclusions can be drawn:
- (1)
During the first drying
Acknowledgements
The authors would like to give special thanks for Dr. Sue Struthers to improve this paper in English writing. This work was supported by the National Natural Science of China (Grant No. 41072211), Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20090091120037), State Key Program of National Natural Science of China (Grant No. 40730739) and the College Graduate Student Innovation Program of Jiangsu Province (CX09B_011Z). It was performed within the project ANR-RGCU
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