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Three-dimensional characterization of desiccation cracking behavior of
compacted clayey soil using X-ray computed tomography
Article in Engineering Geology · April 2019
DOI: 10.1016/j.enggeo.2019.04.014
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Tang, C.S., Zhu, C., Leng, T., Shi, B., Cheng, Q. and Zeng, H., 2019. Three-dimensional characterization of
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desiccation cracking behavior of compacted clayey soil using X-ray computed tomography. Engineering
Geology. 255, 1-10. doi.org/10.1016/j.enggeo.2019.04.014
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https://doi.org/10.1016/j.enggeo.2019.04.014
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If you need the published version for personal study only, please send
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email to: tangchaosheng@nju.edu.cn
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Three-dimensional characterization of desiccation cracking
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behavior of compacted clayey soil using X-ray computed
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tomography
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Chao-Sheng Tang*1, Cheng Zhu2, Ting Leng3, Bin Shi4, Qing Cheng5, Hao Zeng6
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1. School of Earth Sciences and Engineering, Nanjing University, 163 Xianlin Avenue, Nanjing
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210023, China. E-mail: tangchaosheng@nju.edu.cn
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2. Department of Civil and Environmental Engineering, Rowan University, 201 Mullica Hill Road,
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Glassboro, New Jersey 08028, USA. E-mail: zhuc@rowan.edu
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3. School of Earth Sciences and Engineering, Nanjing University, 163 Xianlin Road, Nanjing
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210023, China. E-mail: nju_langting16@163.com
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4. School of Earth Sciences and Engineering, Nanjing University, 163 Xianlin Avenue, Nanjing
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210023, China. E-mail: shibin@nju.edu.cn
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5. School of Earth Sciences and Engineering, Nanjing University, 163 Xianlin Avenue, Nanjing
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210023, China. E-mail: chengqing@nju.edu.cn
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6. School of Earth Sciences and Engineering, Nanjing University, 163 Xianlin Avenue, Nanjing
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210023, China. E-mail: MG1729094@nju.edu.cn
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*Corresponding author:
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Chao-Sheng Tang
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Professor at Nanjing University
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E-mail: tangchaosheng@nju.edu.cn
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Abstract: The shrinkage potential of clayey soils dominates their volumetric deformation and
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microscopic cracking behavior during desiccation processes and has profound practical
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consequences. To investigate the evolution of desiccation crack network in the compacted clayey
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soil, we apply a noninvasive approach integrating X-ray computed tomography (CT) and digital
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image processing technique. Representative geometrical parameters including soil mass area,
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shrinkage strain, crack ratio, average crack width, total crack length, and crack segment number are
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acquired from image analysis results to quantify the morphological evolution of desiccation crack
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patterns. Experimental observations show that cracks initiate at surface, propagate both laterally and
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downward into the soil body, and transit into massive networks due to coalescence and bifurcation.
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Quantitative analyses reveal the strong dependence of geometrical parameters on water loss through
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evaporation. The shrinkage and cracking potential of the clayey soil specimen attenuate with the
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decreasing water content, which significantly reduces the growing trend of geometrical parameters
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during the posterior stage of desiccation. The spatial variation of crack network evolutions under
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drying results from intrinsic soil microstructural heterogeneity and suction capillary-induced local
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stress intensity. The proposed integrated approach is of great significance to characterize
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three-dimensional soil desiccation crack patterns and brings new perspectives into the study of the
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hydro-mechanical behavior of clayey soils.
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Keywords: desiccation crack, compacted clayey soil, X-ray computed tomography (CT),
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quantitative analysis, image process, shrinkage behavior
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1. Introduction
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Clay-rich soils exhibit significant volumetric deformation when exposed to water content changes.
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The presence of clay minerals such as montmorillonite contribute to the intrinsic high swelling
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potential and low hydraulic conductivity and make clayey soils a favorable choice in waste
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containment as a buffer and backfill materials (Qu et al., 2002; Ye et al., 2011; Tripathy et al., 2015;
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Villar et al., 2018). However, the resulting development of desiccation cracks in clayey soils under
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dry and heated conditions may considerably alter their mechanical and hydraulic performance
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(Corte and Higashi, 1960; Tang et al., 2011a; Costa et al., 2012; Sanchez et al., 2013; Zhang et al.,
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2016). These may cause negative impacts on the integrity of geotechnical facilities, the mitigation
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of geo-environmental problems, and the stability and serviceability of earth structures (Rodrí
guez et
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al., 2007; Tang et al., 2008; Peron et al., 2009; Lakshmikantha et al., 2012; Li et al., 2017; Ledesma
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et al., 2018). The formation of desiccation cracks reduces the mechanical strength and increases the
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compressibility of soil, resulting in potential structure tilt and cracking (Morris et al., 1992; An et al.,
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2018). The hydraulic conductivity of cracked soils increases by orders of magnitude, compared to
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that of intact soil (Albrecht and Benson, 2001; Wang et al., 2016), which weakens the functionality
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and sealing effect of impervious barrier layers used for geotechnical facilities such as landfill lining
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system and nuclear waste repository. Moreover, desiccation cracks create preferential pathways for
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rainwater infiltration, which further deepens the infiltration depth and accelerates soil instabilities,
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resulting in landslide and other geo-hazards (Yao et al, 2001; Tang et al., 2011b; Wang et al., 2018).
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Therefore, the study of crack initiation and propagation in soil due to drying is crucial to the
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fundamental understanding of desiccated soil behaviors.
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In the past decades, investigations on the desiccation cracking behavior of soil have been carried
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out through a number of observational tools such as optical microscope (Shin and Santamarina,
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2011), electron microscope (Wang et al., 2015), and digital camera (Tang et al., 2008; Tollenaar et
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al., 2017). To study the underlying mechanisms of crack formation, digitizing the crack pattern and
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performing quantitative assessment become important. Recent development of computer science
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and digital image processing techniques provides a solid technical support for such purposes (Tang
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et al., 2008; Li et al., 2010; Li et al., 2011; Tollenaar et al., 2017; Wang et al., 2018). A set of
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geometric and morphological parameters (e.g., surface crack ratio, number of nodes, crack length,
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crack wide, crack density, clod area and number of clods) was proposed to quantify the evolving
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desiccation crack patterns. However, most previous research efforts were dedicated to
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two-dimensional analyses of surface desiccation crack networks, with limited attention paid to the
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characterization of three-dimensional crack networks. The initiation and propagation process of
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desiccation cracks below the soil surface remains unclear. Poor understanding of the internal
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cracking process or three-dimensional crack characteristics would hinder the accurate evaluation of
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the hydro-mechanical behavior of soil.
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To overcome the intrinsic disadvantages of two-dimensional microstructural characterizations,
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various three-dimensional evaluations have been applied to explore the full soil structure.
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Comparing to other three-dimensional characterization techniques such as laser scan (Sanchez et al.,
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2013) and electrical resistivity tomography (Jones et al., 2014; Tang et al., 2018), X-ray computed
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tomography (CT) is an effective and nondestructive technique that allows high-resolution
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visualization of the internal structure of objects (Mees et al., 2003). This visualization captures the
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density contrast of the object, made possible by emitting X-ray beams, letting them penetrate the
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object along multiple directions, and measuring the progressive attenuation (Phillips et al., 1997;
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Robert et al., 2006). The first computed tomography scanner was designed and set up by electronics
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engineer G. N. Hounsfield from EMI company in 1972. It was originally introduced to the field of
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medical radiology (Hounsfield, 1973) and then applied as a new technique to study other materials.
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In the field of soil science and geology, Petrovic et al. (1982) conducted a pioneering X-ray
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CT-based study and revealed the linear relationship between soil bulk density and X-ray attenuation.
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Since then, the past few decades have witnessed extensive applications of X-ray CT in
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characterizing geological materials. Researchers first examined the correlation between X-ray
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energy level and the relative attenuation of X-ray passing through soil minerals (Carlson et al. 2000;
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Van Geet et al. 2000; Heck and Elliot, 2006). The significant difference between attenuated X-ray
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passing through soil pores and soil solids enables the application of X-ray CT in the quantification
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of porous soil microstructure. X-ray CT-based research have been carried out to study the
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volumetric and geometrical characteristics of the pore and crack networks in soil, such as porosity
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(Anderson et al. 1990), pore diameter (Peython et al. 1992), perimeter and area (Gravers et al. 1989),
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circularity (Gantzer et al. 2002), and crack network density (Perret et al. 1999). Given the
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increasing concern in the morphological analyses for crack networks, researchers have worked on
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the numerical density of crack networks and connectivity (Perret et al., 1999) and the exploration of
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pore objects and their links through a series of quantification models (Delerue et al., 2003). Cracks
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initiate, propagate and coalesce in soil during various physical processes, resulting in a complicated
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cracking network. The imaging features of X-ray CT technique enable qualitative descriptions of
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crack networks, which provides new insights into soil microstructural changes and underlying
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failure mechanisms. For instance, the innovative experimental setup that integrates X-ray CT scan
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and mechanical tests such as triaxial compression or bending tests has been applied to investigate
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the dynamic deformation and structure damage process of soils (Chen et al., 2006; Mukunoki, 2014;
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Otani et al., 2014; Wang et al., 2009). Although X-ray CT technique has been widely applied in
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studying soil mechanical behaviors, integrated qualitative and quantitative investigations on soil
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desiccation cracking processes remain scarce.
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The objective of this research is to systematically characterize the spatiotemporal evolution of
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desiccation cracks in compacted clayey soil based on X-ray computed tomography. We performed
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seven scans during the desiccation process and resorted to laboratory-owned image analysis tools to
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quantify geometrical crack parameters. The structure of this paper is organized as follows. Section 2
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introduces the testing material and methodology of this research. Section 3 presents the CT scan
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results and discusses both qualitative and quantitative analysis results. Section 4 summarizes major
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findings in this study and points out future research directions.
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2. Materials and Methods
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2.1 Materials
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The physical and mechanical properties of the soil collected from Nanjing China are summarized in
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Table 1. According to the USCS classification, it is a fat clay (CH) (ASTM, 2011). The
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mineralogical composition of the clay component is dominated by interstratified illite (about 74%)
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and montmorillonite (about 18%).
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2.2 Specimen preparation
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Natural soil retrieved from the field was oven-dried, crushed and sieved through a 2-mm mesh sieve
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in the laboratory. The soil powder was first homogeneously mixed with water to reach an initial
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water content of 25% and then sealed in an air-proofed plastic bag for 48 hours to allow a uniform
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distribution of moisture within the soil. 25% water content is chosen to ensure a relatively
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homogeneous state of soil mixture and fabric. If water content is insufficient, it would be
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experimentally difficult to reach the homogeneous state. Soil compacted at a relatively low water
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content level usually presents an aggregated structure, whereas soil compacted at a relatively high
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water content level presents a homogeneous structure (Delage et al., 1996). After homogenization,
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the required mass of moist soil was placed in the mold of 50 mm inner diameter and statically
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compacted to the target dry density d = 1.3 Mg/m3. The final height of each sample was 100 mm,
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compacted through four equal layers with each layer 25 mm thick. Compacted samples in molds
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were submerged in water for one week under confined condition. Filter paper and porous stone
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were placed at both ends to provide solid support while allowing free passage of water (Fig. 1). A
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vacuum pump was connected to the container to facilitate the saturation process. To access the
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degree of saturation, we monitored the formation of air bubbles from the soil body during the
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vacuum process. The soil specimen was considered fully saturated if no air escaped from the
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container after 4 hours. We calculated the degree of saturation based on the measurement of water
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content. After full saturation, the measured water content of the specimen was about 42.5%.
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2.3 Apparatus
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The basic principle of X-ray CT is to measure the progressive attenuation of X-ray due to
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absorption and scattering when the beam penetrates a sample situated in its pathway. The X-ray CT
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scanner (Somatom Sensation 40, manufactured by Siemens Corporation) located in the Yangtze
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River Scientific Research Institute of China was utilized. This apparatus consists of an X-ray source
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that moves along the entire circumference of a 70-cm-diameter circle at 0.37s per cycle in tilt of
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+/-30°and emits X-rays towards the soil object positioned at the center of the circle and an array of
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X-ray detectors that is opposite to the source and collects attenuated X-ray beams. The data
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acquisition system contains 40 slices per-rotation, 1344 channels per slice and 4640 projections per
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360°. The scanner resolution is 0.06 mm/pixel at a tube voltage of 140 kV and a tube current of 500
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mAv (Table 2). The raw X-ray intensity projections captured by those detectors were processed by
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CT reconstruction software in the CT scanning system to produce the 3D structure of soil.
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2.4 Test procedures
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After saturation, the specimen was taken out from the container and oven dried at a constant
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temperature of 30 °C. During drying, it was taken out for X-ray CT scanning at different time
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intervals. We scanned the specimen seven times in total, one under initially saturated state as the
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reference scenario, one immediately after surficial crack initiation (after about 5 hours drying), and
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five others after another 2, 4, 9, 14 and 24 hours respectively. Before each scanning, we weighed the
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specimen to determine its accumulated water loss and present water content, and measured
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geometrical properties such as diameter and height (Table 3). Note that a total of four positions
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were measured and the average values of diameter and height were used. The spatial heterogenous
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distribution of water content throughout the specimen can be captured through the evolution of local
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cracks. CT scan generates a set of raw data, with each corresponding to a stack of 2D slices along
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the longitudinal direction. Each slice has a thickness of 0.06 mm and contains 512 × 512 pixels
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with a pixel size of 0.147 × 0.147 mm. The voxel size of the resulting 3D reconstruction was
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0.001 mm3.
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2.5 CT image processing
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To achieve more accurate visualization and characterization of desiccation cracking pattern based
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on CT results, we resorted to an integrated image processing tool – CIAS, which was developed in
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our previous study (Tang et al., 2008). It is equipped with various functions such as graying,
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binarization, noise reduction, and morphological analysis and allowing simultaneous processing of
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multi-slice stack. User-defined functions in CIAS enhances its capabilities of crack recognition and
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quantitative assessment. During image processing, we transformed the stack of grey scale 2D slice
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images (Fig. 2 (a)) into binary ones (Fig. 2 (b)) (white pixels represent soil mass, and black pixels
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inside the white area represent cracks), extracted the cracking network, and performed quantitative
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analysis on the crack skeleton (Fig. 2 (c)) and crack segments (Fig. 2 (d)).
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In order to quantify the crack patterns, the following parameters are calculated for each slice of
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CT images: (1) The area of soil mass Asm (in mm2). It is calculated by the total number of white
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pixel as illustrated in Fig. 2 (b) and the physical resolution (e.g. one pixels per 0.147 mm) of the CT
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images. (2) The soil mass shrinkage ratio Ssm, defined as follows:
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𝑆𝑠𝑚 = (𝐴𝑠𝑚−0 − 𝐴𝑠𝑚−𝑛 ) ∗ 100/𝐴𝑠𝑚−0
(1)
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where Asm-0 is the original area of soil mass determined from the first CT scanning sequence (w =
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42.5%) and corresponds to the bulk mass including water, pores, and soil solids. Asm-n is the area of
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soil mass determined from the subsequent CT scanning sequence n (n = 2,3,4,5,6 and 7, Table 3).
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Note that the concept of soil mass in this study accounts for both soil solids and pores. (3) The crack
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ratio Rc, defined as the ratio of crack area to the total area of each slice. In general, Rc reflects the
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extent of cracking in soil. (4) Average crack width wav. The crack width is determined by calculating
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the shortest distance from a stochastic point on one boundary to the opposite boundary of a crack
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segment. In this investigation, a total of 2000 stochastic points are selected from each crack pattern.
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(5) Total crack length ltot. It is determined by counting the total number of black pixels after the
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image is skeletonized, as shown in Fig. 2 (c). (6) Number of crack segments Sn. The elements
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between two adjacent intersections are defined as one crack segment. The structure of the crack
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network can be decomposed into a series of crack segments that connect with each other, as
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indicated by different colors in Fig. 2 (d). All these processes can be operated automatically using
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CIAS, with more details available in Tang et al. (2008). To the best of our knowledge, the area of
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soil mass Asm and soil mass shrinkage ratio Ssm were first introduced in this study to describe the
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crack patterns in soil CT images. The variables of crack ratio Rc, average crack width wav, total
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crack length ltot and the number of crack segments Sn were first defined by Tang et al. (2008).
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3. Results and Discussion
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3.1 CT scanning of soil desiccation cracking
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During the oven-drying process, desiccation cracks initiate, propagate and coalescence in the
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cylindrical soil matrix. Seven CT scanning sequences conducted at different water contents (Table 3)
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capture the evolving crack pattern: (1) middle horizontal profile of the specimen at 5 cm depth (Fig.
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3); (2) middle vertical profile of the specimen (Fig. 4); (3) 3D reconstruction of the specimen
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structure (Fig. 5). The originally saturated specimen (the 1st scan) was intact, verified by the
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observations in both 2D (Figs. 3 (a) and 4 (a)) and 3D (Fig. 5 (a)) conditions. When water content
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decreases from 42.5% to 37.3%, several preliminary single cracks initiate on sample surface. While
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surficial observation was not able to reveal crack propagation orientation (Fig. 5 (b)), X-ray CT
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indicated that most preliminary cracks propagated perpendicularly from the circumference of each
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slice towards the inner core (Figs. 3 (b) and 4 (b)). Water evaporation before crack initiation
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induced only 5.2% water content decrease (Table 3). With the ongoing drying and the gradual loss
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of water, cracks propagated and widened, in directions deviated from previous orthogonal path
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(Figs. 3 (c) and 4 (c)). Continual desiccation aggravated the growth and the deviation tendency of
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those cracks, and even caused bifurcations at some crack tips, observed both inside the soil matrix
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(Figs. 3 (d) and 4 (d)) and on the soil-air interface (Fig. 5 (d)). Comparing to those primary cracks
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that first initiated at the edge and then propagated towards the center of each slice, secondary cracks
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were branches of primary cracks and contributed predominantly to the development of the internal
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crack network (Figs. 3 (e) and 4 (e)). These secondary cracks initiated at a depth of about 1-1.5 cm
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from specimen surface, and they gradually connected to each other from both lateral and
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longitudinal directions. As a consequence of the increasing damage, the soil specimen lost its
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original cylindrical shape and formed a more anisotropic structure (Figs. 3 (f), 4 (f) and 5 (f)).
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Toward the end of drying, extensive internal and external cracking was observed, with crack
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skeleton and geometrical characteristics shown in Figs. 3 (g), 4 (g) and 5 (g). A slight crack
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narrowing can be observed from the 7th scan. This is attributed to the overall volumetric shrinkage,
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resulting in significant reduction of average specimen height from 99.34 mm to 93.84 mm and
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average specimen diameter from 49.92 mm to 46.92 mm (Table 3). It is also noted that most cracks
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are limited to the external zone with a thickness of about 1-1.5 cm, whereas the inner soil body
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remains intact. If the specimen size and thickness increase, different desiccation cracking behaviors
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may appear, as reflected by the crack formation time and final crack pattern. This size effect has
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been investigated in many previous studies, such as Rodrí
guez et al. (2007) and Lakshmikantha et
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al. (2012). This originates from two main factors: Thicker specimens will make it more difficult for
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cracks to propagate deep into the soil body, since it takes time for enough suction to build up inside
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the soil body. Larger specimens possess more structural heterogeneities, which contributes to the
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possibility of having more local stress concentrations and the ultimate formation of a complex crack
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network. For a given soil thickness, geometrical characteristics of the crack pattern would finally
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reach a relatively equilibrium state with the increasing soil surface area.
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3.2 Quantitative CT image analysis
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Based on CT scan results, we carried out quantitative analysis on the spatiotemporal evolution of
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desiccation cracking pattern, characterized by the geometrical parameters introduced in Section 2.5
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(Fig. 6). Geometrical parameters obtained from those slice images provided a new insight into the
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cracking network within the soil matrix, which was advantageous over the traditional 2D analysis
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based on surficial cracks only. Area of soil mass Asm generally decreased with water content and
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transited from relatively uniform to highly fluctuated profile due to crack-induced inhomogeneity
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(Fig. 6(a)). The spatial distribution of soil mass shrinkage ratio Ssm indicated a general trend that
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soil volume decreased more at both ends (Fig. 6 (b)). In early stage when soil water content was
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above 25.0% (scanning 1-4), we observed an overall increasing trend in all geometrical parameters
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of desiccation cracks throughout the overall specimen depth, especially within the upper part from 5
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to 35 mm and lower part from 65 to 85 mm (Figs. 6 (c)-6 (f)). The spikes in the soil mass shrinkage
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ratio Ssm, crack ratio Rc, average crack width wav, total crack length ltot, and number of crack
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segments Sn profiles (Figs. 5 (b-f)) within the depths of 5-35 mm and 65-85 mm are due to the rapid
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growth of local lateral cracks, as observed from the specimen surface (Fig. 7) as well as internal
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profiles (Fig. 4). In the presence of crack coalescence, the increasing trends of all parameters were
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significantly perturbed when the water content was less than 25.0%. Partial profiles of these
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parameters obtained at the water content of 15.6% even lie below the previous scanning results,
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implying a significant contribution of local shrinkage and crack closure within the sample. This is
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consistent with our previous observations through the middle vertical profile (Fig. 4 (g)) and the 3D
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reconstructed soil structure (Fig. 5 (g)).
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3.3 Drying induced cracking behavior
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Water evaporation plays an important role in the onset and growth of cracks during soil desiccation
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processes. The continual removal of evaporated water yields the formation of water-air meniscus
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among clay particles and the capillary suction in the external layer of the particles. The capillary
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suction and effective stress between clay particles increase with desiccation, resulting in soil
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consolidation and shrinkage. As soil possesses a heterogeneous fabric conditioned by mineralogical
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composition, boundary and environmental factors, cracks are prone to initiate at surface defects
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where accumulating suction induced tensile stress exceeds soil tensile strength first. This interprets
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the phenomenon that cracks usually initiated on the surface and propagated towards the inner soil
256
body.
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We calculated the spatial average values of crack geometrical parameters, including crack ratio,
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average crack width, total crack length, number of crack segments and crack area over the specimen
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height, and compared their evolutions with the decreasing water content (Fig. 8). These parameters
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showed highly similar trends that they increased significantly with decreasing water content from
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42.5% to 25.0% and then decreased slightly afterwards. This implies that clayey soils tend to crack
262
more easily at high water content and the shrinkage and cracking potential attenuate with decreasing
263
water content. Previous study on clayey soil also leads to similar observations that crack
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development is more active at relatively high water content range (Tang et al., 2011b; Cao et. al,
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2014). The formation of desiccation cracking pattern associated with crack initiation, propagation
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and coalescence contributed predominantly to the increase of all geometrical parameters in the first
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six scanning. Nevertheless, these curves declined past the 6th scanning at water content of 25.0%,
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which could be originated from two major factors, the gradual stabilization of cracking pattern and
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the closure of desiccation cracks due to extensive soil volumetric shrinkage. More details about the
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volumetric shrinkage characteristics will be discussed in the following sections.
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3.4 CT scan-based volumetric shrinkage assessment
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The volumetric shrinkage and the ensuing cracks in clayey soil are closely related to the amount of
274
clay minerals and their strong hydrophilicity. The hydration film covering clay particle surface gets
275
thinner during soil desiccation. The consequent buildup of capillary suction decreases the film
276
thickness, reduces the inter-particle pore space, and rearranges the particle fabric, reflected as
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overall volume reduction at macroscopic scale. Increasing clay content especially montmorillonite
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significantly adds to such shrinkage behavior of soil (Kleppe et al., 1985; Volgel et al., 2005; Tay et
279
al., 2011). As the dominated clay compositions of the tested soil are montmorillonite and illite,
280
strong volumetric shrinkage behavior was observed during the desiccation process. In this study, we
281
adopted three approaches to calculate soil volume, which enables assessing shrinkage
282
characteristics from different perspectives. The first approach estimates the soil bulk volume Vb
283
directly from geometric measurements of diameter and height (Table 3). The second approach
284
integrates the volume in each CT image slice to find the bulk volume Vb-CT. As the slice thickness is
285
0.6 mm, soil volume in each slice can be obtained as the product of cross-sectional area (Fig. 3)
286
times the slice thickness. Unlike the first two approaches targeting bulk volume, the third one
287
focuses on the soil mass volume Vs-CT, calculated as the product of soil mass area Asm (Fig. 6 (a))
288
times slice thickness. We know that volumetric shrinkage in soil provides space for the formation of
289
cracks in a given domain (Tang et. al, 2010a). CT scan results indicated that cracks initiated after
290
the 1st scan. Separating crack volume from soil mass volume is necessary for the better
291
understanding of soil volumetric shrinkage.
292
Fig. 9 (a) presents the change of bulk and soil mass volume with respect to water content 𝑤
293
using three approaches. When 𝑤 = 42.5% and soil remained undamaged, three approaches led to
294
the same volume, validating the calculation of soil volume from CT data. As 𝑤 decreased, volume
295
results estimated from three approaches exhibited large deviations. Comparing the calculation of
15
296
bulk volumes from Vb and Vb-CT, Vb was consistently higher. Note that both Vb and Vb-CT showed
297
only minor reductions when 𝑤 decreased from 42.5% to 25.0%. This slow-decreasing trend is
298
primarily produced by the opening of cracks that compensates drying-induced volumetric shrinkage
299
(Fig. 8). Interestingly, although massive cracks developed in the soil body, the macroscopic
300
geometry of the specimen stayed almost unchanged (Table 3), implying that most cracks resulted
301
from local volumetric shrinkage. Diameter and height presented in Table 3 are average of multiple
302
measurements from different external locations. As the cylindrical shape was distorted during
303
cracking, convex deformation was observed at parts of external boundaries, causing higher
304
measurement results in Vb than Vb-CT. Beyond the threshold of 𝑤 = 25.0% , soil volume
305
experienced significant shrinkage as indicated by the rapid decrease of Vb and Vb-CT. The
306
contraction of crack space due to volumetric shrinkage led to decreasing crack parameters in Fig. 8.
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On the contrary, Vs-CT decreased monotonically with water content, reflecting an expected denser
308
soil fabric during drying. Therefore, taking crack space into account yields a more reliable and
309
accurate volumetric assessment for cracked soils, without which the shrinkage potential of the soil
310
would be significantly underestimated. Fig. 9 (b) depicts the volumetric shrinkage strain evolution
311
derived from three approaches. Using s-CT as a reference, b and b-CT neglected crack volume
312
change and thus underestimated the overall volumetric shrinkage, up to 90% and 73% at 𝑤 =
313
25.0%, respectively. The underestimation increased with decreasing water content and increasing
314
crack amount in soil. These comparisons highlight the significance of the proposed technique that
315
integrates CT scan and digital image processing, which plays a key role in quantifying 3D cracking
316
features and enhancing the reliability and accuracy in soil shrinkage analyses.
317
Soil shrinkage characteristic curve (SSCC), correlating void ratio and water content in soil, has
318
been used extensively to characterize soil shrinkage properties (Groenevelt and Grant, 2004;
16
319
Cornelis et al., 2006; Krisdani et al., 2008). For natural soils, a typical SSCC can be decomposed
320
into four stages (Stirk, 1954): structure shrinkage, normal shrinkage, residual shrinkage, and zero
321
shrinkage. Structure shrinkage covers the duration when the decreased soil volume is less than the
322
volume of water loss. Transition from structure shrinkage into normal shrinkage occurs when the
323
reduced volume of soil becomes the same as the volume of water loss. During normal shrinkage
324
stage, soil is usually saturated. The stage of residual shrinkage begins with the air entry into pore
325
space, where soil transfers from saturated to unsaturated state. Once the water content reaches the
326
shrinkage limit of soil, further desiccation will not alter the pore volume, which can be
327
characterized as the zero shrinkage stage (Cornelis et al., 2006; Krisdani et al., 2008). In this study,
328
we calculated void ratio e and degree of saturation Sr through:
329
330
𝑒=
𝐺𝑠 𝑉𝑠−𝐶𝑇 (1+0.01𝑤)
𝑆𝑟 =
𝑚
𝑤𝐺𝑠
𝑒
(2)
(3)
331
in which 𝑚 is soil weight (Table 3), 𝐺𝑠 = 2.71 (Table 1), and Vs-CT is soil mass volume (Fig. 9a).
332
The variations of soil void ratio and degree of saturation during desiccation are shown in Fig. 10.
333
When water content was higher than 25.0%, void ratio decreased linearly, while degree of
334
saturation was close to 100%, implying a normal shrinkage stage for the soil sample. The
335
phenomenon that soil remained saturated with the decreasing water content can be attributed to the
336
high shrinkage potential of the clayey soil. During drying, the reduction in water content was
337
compensated by the shrinkage of soil volume before the water content was smaller than 25%. The
338
sample reached residual shrinkage stage when water content decreased to 15.6% and degree of
339
saturation decreased to 82.2%. Since no data was collected in the ranges of 𝑤 = 15.6 − 25.0%
340
and 𝑤 < 15.6%, this study did not predict air entry threshold or shrinkage limit. The slight
341
fluctuation of the degree of saturation trend in Fig. 10 originates from the calculation of soil mass
17
342
volume Vs-CT using CT data, which can be minimized through higher-resolution X-ray scan.
343
344
3.5 Advantages and limitations of X-ray CT scan
345
X-ray computed tomography is a very useful tool in visualizing and characterizing the internal
346
structures of porous media. In this study, this technique was able to reveal microstructural changes
347
inside the soil body during drying, provide qualitative characterizations of the cracking process, and
348
enable quantitative assessment of soil volume changes. In addition to 3D volumetric investigations
349
discussed in this study, CT scan has great potentials in characterizing the spatial evolution of other
350
soil physical properties during desiccation, such as dry density and the degree of saturation. CT
351
scan provides a solid support to the quantitative analysis of internal microscopic crack propagation
352
and soil structure heterogeneity, and is vital to the accurate assessment of macroscopic soil
353
hydromechnical properties including permeability, compressibility and strength.
354
However, its performance could be considerably undermined by poor quality images containing
355
artifacts such as star-artifacts, beam-hardening and ring-artifacts, resulting in false prediction of the
356
object’s actual composition and big challenge for quantitative analysis (Alramahi et al., 2006). The
357
geometry of the scanned objects and the surrounding materials and their density contrast have a
358
great impact on the quality of the CT images. Beam hardening effect and partial volume should be
359
considered to ensure the accuracy of CT images. Higher X-ray intensity or micro-focus X-ray tubes
360
can improve the quality of CT images. However, high-resolution systems are more applicable to
361
smaller samples, e.g. <2 mm diameter for a 5-μm resolution in micro-focus CT scans (Mees et al,
362
2003), which imposes a strong limitation for larger scale characterizations.
363
364
4. Conclusions
18
365
The formation of desiccation cracks is a natural phenomenon in clay-rich soils subjected to drying.
366
Characterization of the damaged soil structure is critical to the integrity of geotechnical facilities,
367
mitigation of geo-environmental problems, and the stability and serviceability of earth structures. In
368
this study, we investigated the three dimensional desiccation cracking behavior of compacted clayey
369
soil using X-ray Computed Tomography. We resorted to an integrated image processing tool - CIAS
370
that was equipped with an integrated set of functions to process CT image stacks, reconstruct
371
three-dimensional soil structure, and quantify cracked soil structural evolutions. 2D cross-sectional
372
views and 3D porous structures produced at different moments were evaluated to track the
373
formation and development of desiccation crack pattern in the soil specimen. The spatiotemporal
374
evolutions of crack geometrical parameters including crack ratio, crack width, total crack length and
375
number of crack segments were correlated with continual water loss and volumetric shrinkage
376
processes. The following conclusions can be drawn:
377
(1) X-ray CT scan provides qualitative insights into the development of desiccation crack pattern
378
in the drying soil specimen. Cracks initiate at surface first and propagate both laterally and
379
downward into the soil body along an approximately orthogonal direction to the soil-air interface.
380
The propagation path gradually deviates as a result of the heterogeneous soil microstructure and
381
local stress intensity resulting from evaporation-induced suction capillary. Massive growth and
382
propagation of desiccation cracks cause crack coalescence and bifurcation during the latter stage,
383
resulting in aggravated destruction of the soil structure.
384
(2) Area of soil mass in each slice image decreased with time and transited from relatively
385
uniform to highly fluctuated profile due to crack-induced structural inhomogeneity. The spatial
386
distribution of soil shrinkage revealed larger volume reduction at the two ends of the specimen.
387
Across the specimen depth, the temporally growing trends of other geometrical parameters
19
388
including crack ratio, average crack width, total crack length and number of crack segments were
389
maintained first before significantly perturbed in the presence of crack coalescence.
390
(3) Clayey soils tend to crack more easily at relatively high water content and the shrinkage and
391
cracking potential attenuate with decreasing water content. The increasing rates of geometrical
392
crack pattern parameters were consistently higher during the early stage of water evaporation, and
393
decreased substantially afterwards with the continual loss of water.
394
(4) The volumetric shrinkage and the ensuing cracks in clayey soil are closely related to the
395
amount of clay minerals and their strong hydrophilicity. The volumetric shrinkage process
396
experienced by the tested specimen can be recognized as normal shrinkage followed by residual
397
shrinkage.
398
(5) Quantitative analyses reveal the strong dependence of crack network parameters on specimen
399
depth during sustained desiccation process, and also highlight the importance of accounting for
400
cracks in the assessment of soil shrinkage behavior and other physical properties.
401
X-ray CT is proved to be an efficient and nondestructive technique for 3D desiccation crack
402
characterization in fine-grained soils. The proposed integrated approach based on X-ray CT and
403
image processing technique is of great significance to quantify the desiccation cracking behavior
404
inside soils and brings new perspectives into the study of the hydro-mechanical behavior of soils.
405
However, its performance could be considerably undermined by poor image quality or low density
406
contrast, resulting in false prediction of the object’s actual composition and big challenge for
407
quantitative analysis. Prospective study will focus on developing advanced CT image processing
408
techniques and using 3D surface reconstruction and 3D voxel reconstruction of CT images for
409
quantitative crack characterization to improve the fundamental understanding of desiccation
410
processes in soils.
20
411
412
Acknowledgement
413
This work was supported by National Natural Science Foundation of China (Grant No.
414
41572246, 41772280, 41322019), Natural Science Foundation of Jiangsu Province
415
(BK20171228, BK20170394), Key Project of National Natural Science Foundation of China
416
(Grant No. 41230636), and the Fundamental Research Funds for the Central Universities.
21
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215
31
216
Tables
217
Table 1 Physical and mechanical properties of the clayey soil tested in this study.
218
Table 2 Configuration features of the X-ray CT scanner used in this study.
219
Table 3 Geometrical parameters and water content of the sample measured during each scanning.
220
32
221
Table 1 Physical and mechanical properties of the clayey soil tested in this study.
Soil properties
Specific gravity
Consistency limit
Liquid limit (%)
Plastic limit (%)
Plasticity index (%)
USCS classification
Compaction study
Maximum dry density (Mg/m3)
Optimum moisture content (%)
Values
2.71
76.2
29.1
47.1
CH
1.69
18.3
222
223
33
224
Table 2 Configuration features of the X-ray CT scanner used in this study.
Features
Place of origin
Manufacturer
Aperture
Tilt
Rotational Speed
Detector
Data Acquisition System
Acquisition Mode
Power of X-ray Generator
Tube Voltage
Tube Current
Resolution
Details
Germany
Siemens
70 cm
+/-30°
0.37s
Adaption Array Detector based on UFCTM with 26880 elements in
40 rows
40 slices per rotation, 1344 channels per slice, 4640 projections per
360°
40*0.6 (0.6)
70kW
140kv
500mA
0.06 mm/pixel
225
226
34
227
Table 3 Geometrical parameters and water content of the sample measured during each scanning.
Scanning
sequence
1st
2nd
3rd
4th
5th
6th
7th
Weight m
(g)
353.24
350.15
347.89
341.99
329.52
318.98
294.88
Diameter D
(mm)
50
49.76
49.84
49.68
49.83
49.92
46.92
Height
(mm)
100
100.28
100.44
100.14
99.88
99.34
93.84
Water content w
(%)
42.5
37.3
36.4
34.1
29.2
25.0
15.6
228
35
229
Figure captions
230
Fig. 1 Schematic view of the cylindrical soil specimen under saturated conditions.
231
Fig. 2 Procedure of CT image processing, (a) original CT image; (b) binary image; (c) crack
232
skeleton; (d) crack segments.
233
Fig. 3 X-ray CT scan of cracking pattern at the middle horizontal profile of the specimen when
234
water content is (a) 42.5%; (b) 37.3%; (c) 36.4%; (d) 34.1%; (e) 29.2%; (f) 25.0%; (g) 15.6%.
235
Fig. 4 X-ray CT scan of typical cracking pattern at the middle vertical profile of the specimen when
236
water content is (a) 42.5%; (b) 37.3%; (c) 36.4%; (d) 34.1%; (e) 29.2%; (f) 25.0%; (g) 15.6%.
237
Fig. 5 Three dimensional reconstruction of the soil specimen during desiccation when water content
238
is (a) 42.5%; (b) 37.3%; (c) 36.4%; (d) 34.1%; (e) 29.2%; (f) 25.0%; (g) 15.6%.
239
Fig. 6 The spatiotemporal evolution of quantitative crack parameters along the longitudinal
240
direction of the soil specimen during desiccation: (a) area of soil mass; (b) soil mass shrinkage ratio;
241
(c) crack ratio; (d) average crack width; (e) total crack length; (f) number of crack segments.
242
Fig. 7 Spatial distribution of cracks along the longitudinal direction of the soil specimen at water
243
content of 29.2%.
244
.Fig. 8 The evolution of averaged quantitative crack parameters along the longitudinal direction
245
during desiccation.
246
Fig.9 Volume parameters estimated from geometrical measurement and X-ray CT results: (a)
247
specimen volume; (b) volumetric shrinkage strain.
248
Fig. 10 Variation of soil void ratio and degree of saturation during desiccation.
249
36
250
Figures
porous stone
saturation
container
sample
100 mm
porous stone
251
252
50 mm
Fig. 1 Schematic view of the cylindrical soil specimen under saturated conditions.
253
37
254
(a)
255
(b)
(c)
(d)
1 cm
256
Fig. 2 Procedure of CT image processing: (a) original CT image; (b) binary image; (c) crack
257
skeleton; (d) crack segments.
258
38
(a)
(b)
(c)
(d)
1 cm
(e)
(f)
(g)
259
260
Fig. 3 X-ray CT scan of cracking pattern at the middle horizontal profile of the specimen when
261
water content is (a) 42.5%; (b) 37.3%; (c) 36.4%; (d) 34.1%; (e) 29.2%; (f) 25.0%; (g) 15.6%.
262
39
263
(b)
(a)
(c)
(d)
(e)
(f)
(g)
1 cm
264
265
Fig. 4 X-ray CT scan of typical cracking pattern at the middle vertical profile of the specimen when
266
water content is (a) 42.5%; (b) 37.3%; (c) 36.4%; (d) 34.1%; (e) 29.2%; (f) 25.0%; (g) 15.6%.
40
(b)
(a)
(c)
(d)
(e)
(f)
(g)
1 cm
267
268
Fig. 5 Three dimensional reconstruction of the soil specimen during desiccation when water content
269
is (a) 42.5%; (b) 37.3%; (c) 36.4%; (d) 34.1%; (e) 29.2%; (f) 25.0%; (g) 15.6%.
270
271
41
(a)
1400
0
1500
Area of soil mass Asm, mm
1600
1700
2
1800
1900
4
2000
(b)
272
Depth d, mm
Depth d, mm
20
40
60
6
100
(c)
8
10
12
14
16
18
20
22
24
0.0
26
0
0.4
0.8
Area of soil mass Asm, mm
40
1500
1600
60
Depth d, mm
Depth d, mm
20
(a)
16
18
20
22
24
26
w= 37.3%, t=5h
w= 36.4%, t=7h
w= 34.1%, t=9h
w= 29.2%, t=14h
w= 25.0%, t=19h
w= 15.6%, t=29h
0
w= 37.3%, t=5h
w= 36.4%, t=7h
w= 34.1%, t=9h
w= 29.2%, t=14h
20
w= 25.0%, t=19h
w= 15.6%, t=29h
(d)
1400
0
14
Average crack width wav, mm
Crack ratio Rc
6
12
60
100
4
10
w= 42.5%, t=0h
w= 37.3%, t=5h
w= 36.4%, t=7h
20= 34.1%, t=9h
w
w= 29.2%, t=14h
w= 25.0%, t=19h
w= 15.6%, t=29h
40
80
2
Soil mass shrinkage ratio Ssm
0
80
0
8
1.2
1.6
2.0
2.4
w= 37.3%, t=5h
w= 36.4%, t=7h
w= 34.1%, t=9h
w= 29.2%, t=14h
w= 25.0%, t=19h
w= 15.6%, t=29h
2
40
1700
1800
1900
2000
w= 42.5%, t=0h
w= 37.3%, t=5h
w= 36.4%, t=7h
w= 34.1%, t=9h
w= 29.2%, t=14h
w= 25.0%, t=19h
w= 15.6%, t=29h
60
80
80
20
273
100
100
Total crack length ltot, mm
0
Depth d, mm
20
40
60
80
40
100
0
(f)
60
40
60
Number of crack segments Sn
120 140 160 180 200 220 240 260
Depth d, mmn
(e)
20
Depth d, mm
0
80
80
10
20
30
40
50
0
w= 37.3%, t=5h
w= 36.4%, t=7h
w= 34.1%, t=9h
w= 29.2%, t=14h
20
w= 25.0%, t=19h
w= 15.6%, t=29h
w= 37.3%, t=5h
w= 36.4%, t=7h
w= 34.1%, t=9h
w= 29.2%, t=14h
w= 25.0%, t=19h
w= 15.6%, t=29h
40
60
80
100
274
275
100
100
w= 42.5%, t=0h
w= 37.3%, t=5h
w= 36.4%, t=7h
w= 34.1%, t=9h
w= 29.2%, t=14h
w= 25.0%, t=19h
w= 15.6%, t=29h
276
Fig. 6 The spatiotemporal evolution of quantitative crack parameters along the longitudinal
277
direction of the soil specimen during desiccation: (a) area of soil mass; (b) soil mass shrinkage ratio;
278
(c) crack ratio; (d) average crack width; (e) total crack length; (f) number of crack segments.
42
1cm
2cm
3cm
4cm
5cm
6cm
7cm
8cm
9cm
279
280
Fig. 7 Spatial distribution of cracks along the longitudinal direction of the soil specimen at water
281
content of 29.2%.
282
43
180
16
320
14
280
12
240
10
200
8
160
12
8
4
0
100
80
60
40
1.2
0.8
0.4
Crack ratio Rc, %
4
Total crack length ltot, mm
120
Average crack width wav, mm
80
Number of creak segment Sn
2
Crack area Ac, mm
2
20
0
6
Crack area Ac, mm
16
120
Average crack width wav, mm
20
140
Total crack length ltot, mm
Number of creak segment Sn
24
Crack ratio Rc, %
1.6
160
2
28
0.0
40
0
15
20
25
30
35
40
0
45
Water content w, %
283
284
Fig. 8 The evolution of averaged quantitative crack parameters along the longitudinal direction
285
during desiccation.
286
44
200
(a)
190
Volume, mm
3
180
170
160
150
Total bulk volume Vb by geometrical measurement
Total bulk volume Vb-CT by CT calculation
140
Total soil mass volume Vs-CT by CT calculation
15
20
25
30
35
40
45
Water content w, %
287
288
30
Volumetric shrinkage b determined by Vb
(b)
Volumetric shrinkage b-CT determined by Vb-CT
Volumetric shrinkage s-CT determined by Vs-CT
Volumetric shrinakge strain, %
25
20
15
10
5
0
15
289
20
25
30
35
40
45
Water content w, %
290
Fig.9 Volume parameters estimated from geometrical measurement and X-ray CT results: (a)
291
specimen volume; (b) volumetric shrinkage strain.
45
1.2
105
1.1
Void ratio e
1.0
95
0.9
0.8
90
0.7
Degree of saturation Sr
Void ratio e
Sketch trend line
Sketch trend line
0.6
0.5
15
20
25
30
35
40
85
Degree of saturation Sr, %
100
80
45
292
Water content w, %
293
Fig. 10 Variation of soil void ratio and degree of saturation during desiccation.
294
46
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