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Effects of microstructure on desiccation cracking of a compacted soil
Journal Pre-proof Effects of microstructure on desiccation cracking of a compacted soil
Qing Cheng, Chao-Sheng Tang, Hao Zeng, Cheng Zhu, Ni An, Bin Shi PII:
S0013-7952(19)31742-9
DOI:
https://doi.org/10.1016/j.enggeo.2019.105418
Reference:
ENGEO 105418
To appear in:
Engineering Geology
Received date:
16 September 2019
Revised date:
23 October 2019
Accepted date:
15 November 2019
Please cite this article as: Q. Cheng, C.-S. Tang, H. Zeng, et al., Effects of microstructure on desiccation cracking of a compacted soil, Engineering Geology (2019), https://doi.org/ 10.1016/j.enggeo.2019.105418
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© 2019 Published by Elsevier.
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Effects of microstructure on desiccation cracking of a compacted soil Qing Cheng1 , Chao-Sheng Tang*2 , Hao Zeng3 , Cheng Zhu4 , Ni An5,6 , Bin Shi7
Information of the authors 1. Assistant Professor, School of Earth Sciences and Engineering, Nanjing University, Nanjing,
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Jiangsu Province, China. E-mail: [email protected]
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2. Professor, School of Earth Sciences and Engineering, Nanjing University, Nanjing, Jiangsu
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Province, China. E-mail: [email protected]
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3. MPhil student, School of Earth Sciences and Engineering, Nanjing University, Nanjing, Jiangsu
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Province, China. E-mail: [email protected]
4. Department of Civil and Environmental Engineering, Rowan University, 201 Mullica Hill Road,
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Glassboro, New Jersey 08028, USA. Email: [email protected]
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5. School of Earth Sciences and Engineering, Nanjing University, Nanjing, Jiangsu Province, China.
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E-mail: [email protected]
6. Geoenvironmental Research Centre, Cardiff School of Engineering, Cardiff University, Newport Road, Cardiff CF24 3AA, UK
7. Professor, School of Earth Sciences and Engineering, Nanjing University, Nanjing, Jiangsu Province, China. E-mail: [email protected] *Corresponding author: Prof. Chao-Sheng Tang School of Earth Sciences and Engineering, Nanjing University, Nanjing, Jiangsu Province, China. E-mail: [email protected]
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ABSTRACT Desiccation cracking has a significant influence on the hydro- mechanical behaviour of soils. Most previous studies focus on the desiccation cracking of slurry soil samples, whereas little attention has been paid to compacted soils. This study aims to investigate the effects of microstructure on the desiccation cracking of a compacted lean clay. Five soil samples are mixed with different water
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contents including 12.5%, 14.5%, 16.5% (optimum water content), 18.5%, and 20.5%, compacted to
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generate different initial soil microstructures. After compaction, the soil samples are subjected to
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saturation and then the same drying process. The pore size distribution of each soil sample is
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characterized by performing mercury intrusion porosimetry (MIP) test. The change in water content
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and the evolution of surface crack pattern during the drying process are continuously monitored. Experimental results show that the addition of water content during soil compaction significantly
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influences the microstructure and desiccation cracking behaviour of soils. With increasing
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compaction water content from the dry side to the wet side of the optimum water content, soil
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microstructure transits from an aggregate structure to a dispersed structure, resulting in the change of pore size distribution from bimodal to unimodal. For soils with aggregate structures, the desiccation cracks initiate simultaneously and distribute uniformly throughout the soil body. With decreasing water content, crack geometrical parameters such as surface crack ratio and crack density increase almost linearly. Comparatively, for soils with dispersed structures, more localized growth of primary and secondary cracks are observed and the crack geometrical parameters show a two-stage linear growth during drying. This study provides microstructural interpretations to important aspects of desiccation cracking in compacted clayey soils and may guide the design of clay materials for geotechnical engineering applications.
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Keywords: microstructure; desiccation cracking; compacted soil; crack pattern; water content
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1. Introduction During drying process, water evaporation may result in desiccation cracking of soils. The development of cracks has a significant influence on the mechanical and hydraulic properties of soils, leading to a variety of geotechnical and geological engineering problems (Morris et al., 1992; Miller et al., 1998; Yesiller et al., 2000; Albrecht and Benson, 2001; Rayhani et al., 2007; Tang et al., 2008;
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Lozada et al., 2015; Chaduvula et al., 2017; An et al., 2018). For example, in landfill engineering, the
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desiccation cracking in barrier soils may result in more serious rainfall infiltration and landfill gas
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emission, further contaminating surrounding ground water and air (Li et al., 2019; Tang et al., 2019).
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In pavement engineering, the desiccation cracking in subsoil has a negative influence on the
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long-term serviceability of roadways and endangers the safe and quality rides for the travelling public (DeCarlo and Shokri, 2014; Guo et al., 218). In slope engineering, the desiccation cracking
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et al., 2018; Jiang et al., 2019).
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may increase the risk of rainfall infiltration and further reduce the factor of safety of the slope (Wang
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In the past few decades, many researchers paid attention to the qualitative and quantitative analysis of crack patterns by using different methods, such as image analysis and fractal analysis (Miller et al., 1998; Vogel et al., 2005; Tang et al., 2008; Lakshmikantha et al., 2009; Li et al., 2011; Liu et al., 2013; Lu et al., 2016; Zeng et al., 2019). Many efforts have also been devoted to investigate various influencing factors of the desiccation cracking of soils. The influencing factors can be separated into two main groups, namely soil internal factors and external environmental factors. The internal factors influencing desiccation cracking mainly include mineral composition, fine content, water retention characteristics, tensile strength, size and thickness of soil samples (Morris et al., 1992; Yesiller et al., 2000; Mitchell and Soga, 2005; Tang et al., 2008; Lakshmikantha
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et al., 2012; Shorlin et al., 2000; Tollenaar et al., 2017). Temperature, boundary condition, relative humidity, wetting-drying cycles and other external environmental factors also affect desiccation cracking (Yesiller et al., 2000; Rodríguez et al., 2007; Groisman and Kaplan, 2007; Tang et al., 2010; DeCarlo and Shokri, 2014; Lakshmikantha et al., 2018; Levatti et al., 2019). In most of the previous studies, the experiments were conducted on slurry samples. However, in terms of engineering
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practice, compacted soils, instead of slurry soils, are more often used as construction materials,
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which highlight the importance of this study and its potential benefits in guiding future engineering
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applications.
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Geotechnical properties of compacted soils can be significantly influenced by the initial
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compaction state characterized by compaction water content and dry density (Nowamooz et al., 2013; Wang et al., 2014; Otalvaro et al., 2016; Jiang et al., 2017 ). This is because different compaction
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states may result in different soil microstructures and further affect the hydro- mechanical behaviour
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of compacted soils (Delage et al., 1996; Mitchell and Soga, 2005; Romero and Simms, 2008 ). In
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terms of effects of compaction water content on soil microstructure, a major conclusion from previous studies is that an aggregate structure is well developed in the soil sample compacted on the dry side of the optimum water content. For the wet sample, clay particles forming a matrix envelop the silt grain and fill the inter-granular voids (Delage et al., 1996; Oualmakran et al., 2016; He et al., 2017; Zhang et al., 2018; Yuan et al., 2019). In terms of effects of dry density, a larger dry density may refer to the compression of inter-aggregate pores whereas the intra-aggregate pores almost remain unchanged (Delage and Lefebvre, 1984; Griffiths and Joshi, 1989; Wang and Xu, 2007; Yu et al., 2016). Obviously, desiccation cracking of compacted soils can also be affected by soil microstructure. However, so far, few of previous studies has investigated the effects of
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microstructure induced by different compaction water contents on desiccation cracking of compacted soil samples. The principal objective of this study is to investigate the effects of different microstructure induced by various compaction water contents on desiccation cracking of a compacted soil. A series of desiccation cracking tests were carried out on compacted soil samples with various compaction
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water contents. Moreover, mercury intrusion porosimetry (MIP) technique was used to investigate
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the pore size distribution (PSD) of each soil sample. Effects of soil microstructure on crack evolution
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and final pattern of desiccation cracking were analysed and quantified.
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2. Materials and methods 2.1 Testing materials
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Xiashu soil taken from Nanjing, China is tested. It is an aeolian sediment and widely distributed
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in the Middle-Lower Yangtze Plain. The fractions of sand, silt and clay are 2%, 76% and 22%,
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respectively. The plastic and liquid limits are 19.5% and 36.5%, respectively. Index properties of the soil are summarized in Table 1. It is classified as a lean clay (CL) according to the Unified Soil Classification System (ASTM D-2487, 2017).
2.2 Sample preparation and testing procedure The tested soil was air-dried, crushed using a rubber pestle and passed through a 2-mm sieve. De-aired water was added to the sieved, air-dried soil to reach the target water contents of 12.5%, 14.5%, 16.5%, 18.5% and 20.5%, respectively. Then, the soil was sealed inside a plastic bag and kept for 48 hours to ensure the moisture equalization. Five soil samples (160 mm × 160 mm × 5 mm)
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were prepared at different water contents. Static compaction was conducted using a loading machine, in which the axial displacement rate is controlled and fixed at 1.0 mm/min. The dry density is prepared to be 1.7 Mg/m3 by controlling the compaction height. After compaction, a desired amount of de-aired water was added to the compacted soil sample for saturation. The sample was then sealed using a plastic wrap and kept for another 48 hours for moisture equalization. After that, the plastic wrap was removed and the soil sample was setting up in the testing apparatus and air dried under
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room temperature (20±2o C). In addition, a slurry sample with an initial water content of 80% was
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also prepared for comparison in terms of the crack propagation manner.
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Moreover, small cubes with side length of 5 mm were prepared for the MIP tests to investigate
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effects of compaction water content on pore size distribution. For the MIP tests, freeze-drying method was adopted (Delage et al., 1996; Tang et al., 2011c; Yu et al., 2016). The small cubic
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samples were submerged in liquid nitrogen for a few minutes. Then, the frozen samples were put into
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a freeze-dryer with a temperature of -50℃ for 48 hours. During the MIP test, an absolute pressure p
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is applied on mercury to enter the soil pores (Romero and Simms, 2008). According to Washburn equation (Washburn, 1921), the pore diameter d can be calculated as follows: 𝑑=−
4𝜎𝐻𝑔 𝜃𝑛𝑤 𝑝
(1)
where 𝜎𝐻𝑔 is the surface tension of mercury (0.485 N/m at room temperature); 𝜃𝑛𝑤 is contact angle between mercury and the pore wall (162°, as suggested by Penumadu and Dean (2000)).
2.3 Testing apparatus The experimental setup used in Tang et al. (2010) was adopted in this study. The soil sample was placed on a balance to measure the weight with an accuracy of 0.01 g for monitoring change in water
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content during the drying process. A digital camera was fixed above the sample to capture the images of surface crack pattern evolution with elapsed drying time.
2.4 Cracking image analysis The images taken by the digital camera were analysed using the software Crack Image Analysis
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System (CIAS, available at www.climate-engeo.com). First of all, the image pre-processing
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including grayscale (Fig. 1(b)), binalization (Fig. 1(c)), denoising (Fig. 1(d)), skeletonising (Fig. 1(e))
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and crack identification (Fig. 1(f)) is carried out (Fig. 1). Then, the next step is quantitative analysis.
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Geometric indexes (e.g., length and width of each crack segment, angle of each intersection), general
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indexes (e.g., number of intersections and crack segments, average crack length, average crack width, surface crack ratio) and statistical indexes (e.g., probability density function of crack length, crack
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width and intersection angle) can be obtained through this step. In this study, some quantitative
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indexes are reported. Note that only the central part (150 mm × 150 mm) was used for the image
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analysis in order to minimize the boundary effects on cracking observations. The edge shrinkage area of each soil sample is counted when determining surface crack ratio (Zeng et al., 2019). More details of the procedure of the quantitative cracking image analysis are available in Tang et al. (2008) and Liu et al. (2013).
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Experimental results and discussion
3.1 Microstructure of soil samples with different compaction water contents Fig. 2 shows the pore size distributions obtained from MIP tests for soil samples compacted at various water contents. As can be seen from the figure, for the soil samples compacted at water
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contents of 12.5% and 14.5% (on the dry side of the standard Proctor optimum water content) and at water content of 16.5% (standard Proctor optimum water content), bimodal pore size distributions with two distinct pore domains can be observed. The two distinct pore domains are named as micro pore (or intra-aggregate pore) and macro pore (inter-aggregate pore) (Delage et al., 1996; Romero et al., 2011; Ng et al., 2016). On the dry side and at optimum water content, clay particles assemble
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together and an aggregate structure is well developed in the soil. Both micro pores and macro pores
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can be detected by the MIP technique. The corresponding diameter at the peak point of the micro
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pores is about 0.6 μm and that of the macro pores is in the range from 10 to 30 μm. With increasing
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compaction water content, the amount of micro pores increases whereas the amount of macro pores
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decreases because of the reduced aggregation of clay particles (Delage et al., 1996). However, for the soil samples compacted on the wet side of the standard Proctor optimum water content (at water
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contents of 18.5% and 20.5%), the pore size distributions show a unimodal form. This is attributed to
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the assembly of clay-particle matrix around silt grains and the gradual formation of a dispersed
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structure (Delage et al., 1996). The corresponding diameter at the peak point is about 0.8 μm and there is no obvious separation between micro pores and macro pores. At various compaction water content levels, the soil samples with different microstructures are obtained. According to Alonso et al. (2013), a simple microstructural state variable ξm is adopted to quantitatively describe the microstructure of soil samples: 𝜉𝑚 = 𝑒𝑚 ⁄𝑒
(2)
where e is the total void ratio and em is the microstructural void ratio. The total void ratio e consists of the microstructural void ratio em and the macrostructural void ratio eM (Romero et al., 2011). In this study, the delimiting pore diameter, at which the derivative of PSD curve becomes zero, is used
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to separate micro pores and macro pores (Ng et al., 2016). The delimiting pore diameters are found to be 1.9, 1.9, 2.4, 5.8 and 5.8 μm for the soil samples compacted at water contents of 12.5%, 14.5%, 16.5%, 18.5% and 20.5%, respectively. Fig. 3 shows the microstructural state variable ξm of each soil sample compacted at various compaction water contents, which is calculated based on the pore size distributions shown in Fig. 2. It can be found that with increasing compaction water content, the
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microstructural state variable increases, corresponding to a less aggregated structure of clay particles
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(Delage et al., 1996). In this study, the MIP tests were conducted on the compacted soil samples. For
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a lean clay, the saturation process performed under zero stress does not significantly alter the PSD
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(Oualmakran et al., 2016).
3.2 Crack initiation and propagation in compacted samples with different microstructure
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Under a drying condition, pore water evaporates from the surface of saturated samples. As the
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water content decreases, matric suction is developed and the suction-dependent tensile stress
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increases. Cracking occurs when the tensile stress exceeds the tensile strength (Tang et al., 2011a). With the elapsed drying time, the desiccation cracking further propagates. Fig. 4 shows the crack initiation and propagation of each soil sample with different microstructure. For the soil samples compacted on the dry side and at the optimum water content, crack initiates evenly on the soil surface and then widens gradually (Fig. 4 (a)-(c)). Despite of small crack width, the initiation of cracks occurs randomly and almost simultaneously on the surface of soil sample. In contrast, cracks evolve progressively in slurry samples, with the onset of primary cracks followed by the branching of sub-cracks (Fig. 5). This is because cracks usually initiate at the surface defects where shrinkage distortion and stress concentration occurs (Zabat et al., 1997; Weinberger, 1999; Tang et al., 2011b).
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The soil samples compacted on the dry side and at the optimum water content have an aggregate structure, as discussed in previous section. During evaporation, soil volume shrinks as the inter-aggregate pores shrink and there is a rearrangement of aggregates (Nowamooz and Masrouri, 2010; Burton et al., 2015; Li et al., 2018). The strain energy build-up prior to cracking within a soil element subjected to tensile stress can be given as (1.299σ2 S2 t)/E, where σ is the tensile stress, S is
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the pore diameter, t is the layer thickness and E is the Young’s modulus (Costa et al., 2013). The
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strain energy is proportional to the pore size. The macro pores between aggregates has a larger pore
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size, leading to higher strain energy prior to cracking. Thus, the places between aggregates can be
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regarded as the surface weak zones, resulting in crack initiation. Several crack initiation points
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appearing almost at the same time, as marked by the red circles in Fig. 4 (a)-(c), can be observed. However, the slurry samples in the literature (Tang et al., 20 11a) have a more homogeneous
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microstructure and shrink uniformly during the drying process. Desiccation cracking and strain
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energy concentration/release process can be maintained in an orderly manner (Tang et al., 2008) (Fig.
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5). Moreover, from one crack initiation point, three cracks propagate in three directions with an angle of 120° simultaneously, results in a “Y” shape crack intersection. This can be explained by adopting the concept of Griffith’s fracture energy balance. The “Y” shape crack intersection is the most energy efficient, where the crack length required is the minimum for expending the same amount of energy (Kodikara et al., 2000; Costa et al., 2013). With increasing compaction water content, soil microstructure transits into a less aggregated structure, resulting in a more localized cracking pattern. Furthermore, in soil samples compacted to the dry side of the optimum water content, a minor extent of sub-crack growth is observed (Fig. 4(a)-(c)), which is different from the slurry samples in the literature (Tang et al., 2011a) as well. This is likely because the intra-aggregate pores are almost
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unchanged during drying (Nowamooz and Masrouri, 2010; Burton et al., 2015; Li et al., 2018). Moreover, no new cracks form and the additional strain is accommodated by further opening of existing cracks when a certain crack intensity is reached (Bai et al., 2000, Tang et al., 2006; Yin, 2010). For the soil samples compacted on the wet side with a dispersed structure, primary cracks initiate first (Fig. 4 (d)-(e)). The primary cracks of the soil sample compacted at water content of
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18.5% initiate both at the inner surface and the edge, as shown in Fig. 2(d). With a higher
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compaction water content of 20.5%, the cracks initiates from the edge of soil sample, as shown in
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Fig. 4(e). This is because with a higher compaction water content, the microstructure is closer to the
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slurry samples. Therefore, the crack propagation manner of the soil samples with a dispersed
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structure is more similar to that of the slurry samples (as shown in Fig, 5). After the primary cracks initiating, sub-cracks initiate at existing primary cracks and almost perpendicular to the primary
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cracks (Fig. 4 (d)-(e)). This can be explained by adopting the maximum stress release criterion
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(Lachenbruch, 1962; Morris et al., 1992). When the primary cracks initiate, the internal tensile stress
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perpendicular to the existing primary cracks are released and the newly maximum tensile stress is parallel to the existing primary crack. Hence, the sub-cracks grow at the local maximum tensile stress and are perpendicular to the primary cracks. Quantitative analysis is carried out for the images showing development of desiccation cracking during the drying process. Fig. 6 shows the change of surface crack ratio Rsc with water content during drying. The surface crack ratio is defined as the ratio of the surface area of cracks to the total surface area of soil sample. It should be noted that the water content at the onset of cracking (indicated by the intersection point between surface crack ratio curve and water content axis) is defined as the cracking water content ωc (Tang et al., 2011b). It is found that with increasing
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compaction water content from 12.5% to 20.5%, the cracking water content decreases from about 25% to 19%. The cracking water contents of the samples compacted on the dry side and at the optimum water content is higher than those of the samples compacted on the wet side. This is likely because the cracks in the samples compacted on the dry side and at the optimum water content appears in the void between aggregates while the cracks in the samples compacted on the wet side appears in the
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void between soil particles, as discussed previously. According to the Young-Laplace equation, there
4𝑇𝑠 cos 𝜃 𝑑
(3)
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𝑠=
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is an inverse relationship between pore diameter d and suction s:
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where Ts is the surface tension of water (mN/m); θ is the contact angle (°). Larger pore diameter
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develops smaller suction, resulting in higher amount of water retained in the soil. As a result, a higher cracking water content is observed in the soil samples compacted on the dry side. Moreover, it
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can be seen from the figure that with decreasing water content during drying, the surface crack ratios
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of all the soil samples with various microstructure show an increasing trend. Specifically, for the soil
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samples compacted on the dry side and at the optimum water content with bimodal pore size distributions, Rsc increases almost linearly with decreasing water content. This is likely because for the soil samples compacted on the dry side and at optimum water content, all the cracks initiate almost simultaneously and the generated cracks gradually widen with further drying. For these three samples, the changing rate of Rsc and the final Rsc after reaching an equilibrium state decreases with decreasing compaction water content. The final Rsc of soil sample with compaction water content of 12.5% (0.056) is about twice than the soil sample with compaction water content of 16.5% (0.029). However, for the soil samples compacted on the wet side with unimodal pore size distributions, the relationship between surface crack ratio and water content before reaching the equilibrium state can
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be regarded as consisting of two approximately linear segments. The changing rate of the first segment is much smaller than that of the second one. Actually, the first segment corresponds to the initiation of primary cracks and sub-cracks. The second segment corresponds to widening the existing cracks after the geometric crack pattern tends to stabilize (Fig. 4). Fig. 7 shows the change of crack density with water content during drying. Note that the crack
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density (/cm2 ) represents number of crack segments per unit area. In general, the crack density
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increases with decreasing water content before reaching the equilibrium state. Similarly, for the soil
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samples compacted on the dry side and at the optimum water content with bimodal pore size
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distributions, the crack density increases almost linearly from the cracking water content to water
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content of about 15%. For the soil samples compacted on the wet side with unimodal pore size distributions, the crack density increases slightly in the water content range from cracking water
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content to 15%. With further drying to water content of 10%, a more dramatic increase in the crack
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density is observed. The corresponding water content at the intersection point is larger than that in
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Fig. 6 because the initiation of sub-cracks result in a significant increase in the crack density while do not count much in the surface crack ratio. The corresponding water content when the crack density becomes stabilised is larger than that for surface crack ratio (Fig. 6). This is because when the geometric structure of the crack network tended to stabilize and no new cracks were initiated, cracks become widen with further drying (Tang et al., 2011b). Moreover, the final crack density at the equilibrium state decreases with increasing compaction water content.
3.3 Final crack pattern of compacted samples with different microstructure Fig. 8 shows the final crack pattern of each soil sample. It can be found that there exist more fine
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and thin cracks in samples with lower compaction water contents and vice versa. With increasing compaction water content, the geometric crack pattern tends to be simpler. This represents that there are less weak zones exist in a soil sample with a higher compaction water content. Compacted samples in this study exhibit similar geometrical shapes of soil clods (e.g., quadrangle, pentagon, and hexagon) as those observed in desiccated slurry samples (Yesiller et al., 2000; Rodríguez et al., 2007;
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Groisman and Kaplan, 2007; Tang et al., 2010; DeCarlo and Shokri, 2014; Lakshmikantha et al.,
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2018; Levatti et al., 2019). In previous studies investigating desiccation cracking of slurry samples
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(Vogel et al., 2005; Péron et al., 2009; Tang et al., 2011a), the researchers get a common view that
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the crack segments are smooth and the soil surface is split into separate clods by the crack network.
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However, as shown in Fig. 8, the crack segments are rough and the clods of compacted samples are not fully separated. This can be attributed to the homogenous structure of slurry samples and
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heterogeneous structure of compacted samples.
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The final crack pattern of each sample is quantitatively analysed. The relationships between
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change of crack density as well as average width and microstructural state variable are plotted in Fig. 9. With increasing microstructural state variable, the crack density decreases almost linearly whereas the average width show an opposite trend. Moreover, it is found that on the wet side, with a given change in microstructural state variable, the change in average width is about 4.5 times more than that on the dry side. Crack width is one of the important indices to characterize the crack pattern. It is closely related to the crack propagation. For example, the widest cracks at the end of the drying process are the primary cracks (Tang et al., 2011b). Fig. 10 shows the probability-density function of crack width of each soil sample at the end of the desiccation test. Generally, with increasing compaction water
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content, the probability density function of crack width changes from unimodal to bimodal, indicating that the grading characteristics of primary cracks and sub-cracks becomes more obvious. It can be seen from the figure that for the two samples compacted on the dry side (with water contents of 12.5% and 14.5%), the range of the crack width is relatively narrow, primarily distributed in the range from 0 to 0.10 cm. This confirms that for the soil sample with an aggregate structure, there is
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no sub-cracks. For the soil sample compacted at optimum water content of 16.5%, the majority of
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crack width is still distributed in the range from 0 to 0.10 cm. However, a fraction of crack width is
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wider (0.10~0.25 cm), which is also observed in Fig. 8. This is likely because even though the soil
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sample compacted on the optimum water content has less obvious aggregates and a similar coating
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clayey fraction to that found on the dry side (Delage et al., 1996). For the soil samples compacted on the wet side (with water contents of 18.5% and 20.5%), the crack with is mainly distributed in the
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range from 0 to 0.20 cm and the distribution is relatively average. Extra-wide cracks (with width
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over 0.30 cm) appear on the surface of the sample compacted at water content of 20.5%. The
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observed phenomenon also confirms that both wide primary cracks and relatively thin sub-cracks exist in soil samples with a dispersed structure. Crack intersection angle is another important index for describing the crack pattern and also influenced by the crack propagation. The probability-density function of crack intersection angle is presented in Fig. 11. The probability-density function of crack intersection angle of each soil sample is basically a normal distribution. For the soil samples compacted on the dry side and at optimum water content, the intersection angles are mainly distributed in the range from 100 to 140°. This corresponds to the observed phenomenon that for soil samples with an aggregate structure, only primary cracks exist and they are formed simultaneously at the beginning stage of desiccation.
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Sub-cracks normally show a “T” or “+” intersections with an intersection angle of about 90° (Vogel
pr
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et al., 2005; Péron et al., 2009; Tang et al., 2011a).
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3.4 General effects of microstructure on desiccation cracking of compacted soil
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Based on the abovementioned phenomenon and interpretation, the crack propagations of soil samples with an aggregate structure and a dispersed structure show different manners. Fig. 12 and
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Fig. 13 provide the schematic view of the desiccation cracking process in soils with aggregate and
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dispersed microstructures, respectively. For soil samples with an aggregate structure, the cracking
i.
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propagation can be summarized into two stages: Independent “Y” shaped cracks between aggregates initiate evenly on the soil surface with soil shrinkage during evaporation (Fig. 12 (a)-(b)). These cracks extend gradually until reaching the edge or intersecting with each other and the geometric structure of the crack pattern is formed (Fig. 12 (b)-(c)). ii.
The geometric structure of the crack pattern tends to stabilise when the water content continues to decrease. The existing cracks keep widening until the desiccation ceased (Fig. 12 c)-(d)).
For soil samples with a dispersed structure, desiccation cracking takes place in three stages:
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i.
During the drying process, water evaporates and soil shrinks. Independent cracks between soil particles initiate at the inner surface or the edge (Fig. 13 (a)-(b)) and propagates until reaching the edge or intersecting with each other (Fig. 13 (b)-(c)). These initial independent cracks are considered as primary cracks.
ii.
Perpendicular to the existing primary cracks, some thin cracks initiate and propagate when
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the water content keeps decreasing (Fig. 13 (c)-(d)). These post-developed cracks are
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regarded as sub-cracks. The primary cracks and the sub-cracks form the geometric structure
With further decreasing water content, the geometric structure of the crack pattern stays
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iii.
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of the crack pattern.
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nearly unchanged while the primary cracks and the sub-cracks widen until the desiccation
Conclusions
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4
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ceased (Fig. 13 (d)).
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Effects of soil microstructure on desiccation cracking of a compacted lean clay are investigated experimentally in this study. Soil samples with different microstructures induced by various compaction water contents are tested. The water content evolution, surface crack initiation and crack propagation are monitored during the drying process. Quantitative analysis of crack evolutions with the elapsed drying time leads to the following major conclusions: 1.
The pore size distributions of soil samples compacted on the dry side and at optimum water content are bimodal because of the aggregate structure, whereas the pore size of soil samples compacted on the wet side follows a unimodal distribution due to the dispersed structure. With increasing compaction water content, the microstructural state variable increases.
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2.
For soil samples with an aggregate structure, independent cracks initiate evenly on the soil surface at the same time and then widen gradually. These cracks mostly intersect at an angle of 120°. For soil samples with a dispersed structure, primary cracks initiate first followed by the growth of sub-cracks along the orthogonal directions. The primary cracks formed “Y” shapes with intersection angles of 120° and the sub-cracks show “T” shapes with intersection angles of
The cracking water contents of the samples compacted on the dry side and at optimum water
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3.
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90°.
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content is higher than those compacted on the wet side. Because the crack in the samples with an
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aggregate structure is likely to appear in the void between aggregates while the crack in the
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samples with a dispersed structure appears in the void between soil particles. The macro pore has a larger diameter and develops smaller suction, resulting in higher amount of water retained
During the process of desiccation cracking, the surface crack ratio and the crack density of soil
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4.
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in the soil.
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samples with an aggregate structure increase almost linearly with decreasing water content before stabilising. Soil samples with dispersed structures show a two-stage linear growth with decreasing water content. 5.
In terms of final crack pattern, with increasing compaction water content, the crack pattern of the soil sample becomes less complicated. There are more fine and thin cracks for the soil sample with a smaller microstructural state variable and vice versa.
Acknowledgements This work was supported by National Natural Science Foundation of China (Grant No.
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41572246, 41772280, 41322019, 41902271), Natural Science Foundation of Jiangsu Province (BK20171228, BK20170394), and the Fundamental Research Funds for the Central Universities.
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873-878. Tang, C.S., Shi, B., Liu, C., Suo, W.B., Gao, L., 2011b. Experimental characterization of shrinkage and desiccation cracking in thin clay layer. Applied Clay Science 52 (1-2), 69-77. Tang, C.S., Shi, B., Liu, C., Zhao, L.Z., Wang, B.J., 2008. Influencing factors of geometrical structure of surface shrinkage cracks in clayey soils. Engineering Geology 101 (3-4), 204-217.
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Volgel, H.J., Hofmann, H., Roth, K., 2005. Studies of crack dynamics in clay soil I: Experimental methods, results, and morphological quantification. Geoderma 125 (3), 203-211. Wang, L.L., Tang, C.S., Shi, B., Cui, Y.J., Zhang, G.Q., Hilary, I., 2018. Nucleation and propagation mechanisms of soil desiccation cracks. Engineering Geology, 238, 27-35. Wang, Q., Cui, Y.J., Tang, A.M., Li, X.L., Ye, W.M., 2014. Time- and density-dependent microstructure features of compacted bentonite. Soils and Foundations 54 (4), 657-666. Wang, Y.H., Xu, D., 2007. Dual porosity and secondary consolidation. Journal Geotechnical and Geoenvironmental Engineering ASCE 133 (7), 793-801. Washburn, E.W., 1921. A note on a method of determining the distribution of pore sizes in a porous
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Figure captions Fig. 1 Crack image processing procedure Fig. 2 Pore size distribution of soil samples with various compaction water contents Fig. 3 Microstructural state variable determined for soil samples with various compaction water contents Fig. 4 Crack patterns captured at varying desiccation stages in soil samples compacted at (a)
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ω0 =12.5%, (b) ω0 =14.5%, (c) ω0 =16.5%, (d) ω0 =18.5% and (e) ω0 =20.5%
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Fig. 5 Progressive growth of primary cracks and sub-cracks in a slurry sample
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Fig. 6 Change of surface crack ratio with water content during drying
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Fig. 7 Change of crack density with water content during drying Fig. 8 Final crack pattern of soil samples with various compaction water contents
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Fig. 9 The evolution of crack density and average crack width with microstructural state variable in
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five soil samples
water contents
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Fig. 10 Probability-density function of crack width for five soil samples with different compaction
Fig. 11 Probability-density function of crack intersection angle for five soil samples with different compaction water contents Fig. 12 Schematic drawing of the development of desiccation cracks in the soil sample with an aggregate structure Fig. 13 Schematic drawing of the development of desiccation cracks in the soil sample with a dispersed structure
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Table 1. Index properties of test soil
Property
Value
Specific gravity, Gs
2.73 2
Silt content (%)
76
Clay content (%)
22
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Sand content (%)
Liquid limit, ωl (%)
36.5
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Plastic limit, ωp (%)
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Shrinkage limit, ωsl (%)
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Plasticity index, Ip (%)
19.5 17.0 10.5 16.5
Maximum dry density (Mg/m3 )
1.7
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Optimum water content, ωopt (%)
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Compaction water content affects microstructure and desiccation cracking of soils.
With increasing compaction water content, the microstructural state variable increases.
For soils with aggregate structures, the desiccation cracks initiate simultaneously and distribute uniformly.
For soils with dispersed structures, localized growth of primary and secondary cracks are
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With increasing microstructural state variable, the final crack pattern of the soil sample becomes
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less complicated.
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observed.
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Qing Cheng, Chao-Sheng Tang, Hao Zeng, Cheng Zhu, Ni An, Bin Shi PII:
S0013-7952(19)31742-9
DOI:
https://doi.org/10.1016/j.enggeo.2019.105418
Reference:
ENGEO 105418
To appear in:
Engineering Geology
Received date:
16 September 2019
Revised date:
23 October 2019
Accepted date:
15 November 2019
Please cite this article as: Q. Cheng, C.-S. Tang, H. Zeng, et al., Effects of microstructure on desiccation cracking of a compacted soil, Engineering Geology (2019), https://doi.org/ 10.1016/j.enggeo.2019.105418
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Effects of microstructure on desiccation cracking of a compacted soil Qing Cheng1 , Chao-Sheng Tang*2 , Hao Zeng3 , Cheng Zhu4 , Ni An5,6 , Bin Shi7
Information of the authors 1. Assistant Professor, School of Earth Sciences and Engineering, Nanjing University, Nanjing,
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Jiangsu Province, China. E-mail: [email protected]
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2. Professor, School of Earth Sciences and Engineering, Nanjing University, Nanjing, Jiangsu
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Province, China. E-mail: [email protected]
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3. MPhil student, School of Earth Sciences and Engineering, Nanjing University, Nanjing, Jiangsu
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Province, China. E-mail: [email protected]
4. Department of Civil and Environmental Engineering, Rowan University, 201 Mullica Hill Road,
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Glassboro, New Jersey 08028, USA. Email: [email protected]
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5. School of Earth Sciences and Engineering, Nanjing University, Nanjing, Jiangsu Province, China.
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E-mail: [email protected]
6. Geoenvironmental Research Centre, Cardiff School of Engineering, Cardiff University, Newport Road, Cardiff CF24 3AA, UK
7. Professor, School of Earth Sciences and Engineering, Nanjing University, Nanjing, Jiangsu Province, China. E-mail: [email protected] *Corresponding author: Prof. Chao-Sheng Tang School of Earth Sciences and Engineering, Nanjing University, Nanjing, Jiangsu Province, China. E-mail: [email protected]
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ABSTRACT Desiccation cracking has a significant influence on the hydro- mechanical behaviour of soils. Most previous studies focus on the desiccation cracking of slurry soil samples, whereas little attention has been paid to compacted soils. This study aims to investigate the effects of microstructure on the desiccation cracking of a compacted lean clay. Five soil samples are mixed with different water
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contents including 12.5%, 14.5%, 16.5% (optimum water content), 18.5%, and 20.5%, compacted to
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generate different initial soil microstructures. After compaction, the soil samples are subjected to
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saturation and then the same drying process. The pore size distribution of each soil sample is
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characterized by performing mercury intrusion porosimetry (MIP) test. The change in water content
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and the evolution of surface crack pattern during the drying process are continuously monitored. Experimental results show that the addition of water content during soil compaction significantly
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influences the microstructure and desiccation cracking behaviour of soils. With increasing
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compaction water content from the dry side to the wet side of the optimum water content, soil
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microstructure transits from an aggregate structure to a dispersed structure, resulting in the change of pore size distribution from bimodal to unimodal. For soils with aggregate structures, the desiccation cracks initiate simultaneously and distribute uniformly throughout the soil body. With decreasing water content, crack geometrical parameters such as surface crack ratio and crack density increase almost linearly. Comparatively, for soils with dispersed structures, more localized growth of primary and secondary cracks are observed and the crack geometrical parameters show a two-stage linear growth during drying. This study provides microstructural interpretations to important aspects of desiccation cracking in compacted clayey soils and may guide the design of clay materials for geotechnical engineering applications.
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Keywords: microstructure; desiccation cracking; compacted soil; crack pattern; water content
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1. Introduction During drying process, water evaporation may result in desiccation cracking of soils. The development of cracks has a significant influence on the mechanical and hydraulic properties of soils, leading to a variety of geotechnical and geological engineering problems (Morris et al., 1992; Miller et al., 1998; Yesiller et al., 2000; Albrecht and Benson, 2001; Rayhani et al., 2007; Tang et al., 2008;
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Lozada et al., 2015; Chaduvula et al., 2017; An et al., 2018). For example, in landfill engineering, the
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desiccation cracking in barrier soils may result in more serious rainfall infiltration and landfill gas
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emission, further contaminating surrounding ground water and air (Li et al., 2019; Tang et al., 2019).
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In pavement engineering, the desiccation cracking in subsoil has a negative influence on the
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long-term serviceability of roadways and endangers the safe and quality rides for the travelling public (DeCarlo and Shokri, 2014; Guo et al., 218). In slope engineering, the desiccation cracking
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et al., 2018; Jiang et al., 2019).
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may increase the risk of rainfall infiltration and further reduce the factor of safety of the slope (Wang
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In the past few decades, many researchers paid attention to the qualitative and quantitative analysis of crack patterns by using different methods, such as image analysis and fractal analysis (Miller et al., 1998; Vogel et al., 2005; Tang et al., 2008; Lakshmikantha et al., 2009; Li et al., 2011; Liu et al., 2013; Lu et al., 2016; Zeng et al., 2019). Many efforts have also been devoted to investigate various influencing factors of the desiccation cracking of soils. The influencing factors can be separated into two main groups, namely soil internal factors and external environmental factors. The internal factors influencing desiccation cracking mainly include mineral composition, fine content, water retention characteristics, tensile strength, size and thickness of soil samples (Morris et al., 1992; Yesiller et al., 2000; Mitchell and Soga, 2005; Tang et al., 2008; Lakshmikantha
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et al., 2012; Shorlin et al., 2000; Tollenaar et al., 2017). Temperature, boundary condition, relative humidity, wetting-drying cycles and other external environmental factors also affect desiccation cracking (Yesiller et al., 2000; Rodríguez et al., 2007; Groisman and Kaplan, 2007; Tang et al., 2010; DeCarlo and Shokri, 2014; Lakshmikantha et al., 2018; Levatti et al., 2019). In most of the previous studies, the experiments were conducted on slurry samples. However, in terms of engineering
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practice, compacted soils, instead of slurry soils, are more often used as construction materials,
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which highlight the importance of this study and its potential benefits in guiding future engineering
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applications.
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Geotechnical properties of compacted soils can be significantly influenced by the initial
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compaction state characterized by compaction water content and dry density (Nowamooz et al., 2013; Wang et al., 2014; Otalvaro et al., 2016; Jiang et al., 2017 ). This is because different compaction
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states may result in different soil microstructures and further affect the hydro- mechanical behaviour
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of compacted soils (Delage et al., 1996; Mitchell and Soga, 2005; Romero and Simms, 2008 ). In
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terms of effects of compaction water content on soil microstructure, a major conclusion from previous studies is that an aggregate structure is well developed in the soil sample compacted on the dry side of the optimum water content. For the wet sample, clay particles forming a matrix envelop the silt grain and fill the inter-granular voids (Delage et al., 1996; Oualmakran et al., 2016; He et al., 2017; Zhang et al., 2018; Yuan et al., 2019). In terms of effects of dry density, a larger dry density may refer to the compression of inter-aggregate pores whereas the intra-aggregate pores almost remain unchanged (Delage and Lefebvre, 1984; Griffiths and Joshi, 1989; Wang and Xu, 2007; Yu et al., 2016). Obviously, desiccation cracking of compacted soils can also be affected by soil microstructure. However, so far, few of previous studies has investigated the effects of
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microstructure induced by different compaction water contents on desiccation cracking of compacted soil samples. The principal objective of this study is to investigate the effects of different microstructure induced by various compaction water contents on desiccation cracking of a compacted soil. A series of desiccation cracking tests were carried out on compacted soil samples with various compaction
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water contents. Moreover, mercury intrusion porosimetry (MIP) technique was used to investigate
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the pore size distribution (PSD) of each soil sample. Effects of soil microstructure on crack evolution
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and final pattern of desiccation cracking were analysed and quantified.
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2. Materials and methods 2.1 Testing materials
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Xiashu soil taken from Nanjing, China is tested. It is an aeolian sediment and widely distributed
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in the Middle-Lower Yangtze Plain. The fractions of sand, silt and clay are 2%, 76% and 22%,
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respectively. The plastic and liquid limits are 19.5% and 36.5%, respectively. Index properties of the soil are summarized in Table 1. It is classified as a lean clay (CL) according to the Unified Soil Classification System (ASTM D-2487, 2017).
2.2 Sample preparation and testing procedure The tested soil was air-dried, crushed using a rubber pestle and passed through a 2-mm sieve. De-aired water was added to the sieved, air-dried soil to reach the target water contents of 12.5%, 14.5%, 16.5%, 18.5% and 20.5%, respectively. Then, the soil was sealed inside a plastic bag and kept for 48 hours to ensure the moisture equalization. Five soil samples (160 mm × 160 mm × 5 mm)
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were prepared at different water contents. Static compaction was conducted using a loading machine, in which the axial displacement rate is controlled and fixed at 1.0 mm/min. The dry density is prepared to be 1.7 Mg/m3 by controlling the compaction height. After compaction, a desired amount of de-aired water was added to the compacted soil sample for saturation. The sample was then sealed using a plastic wrap and kept for another 48 hours for moisture equalization. After that, the plastic wrap was removed and the soil sample was setting up in the testing apparatus and air dried under
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room temperature (20±2o C). In addition, a slurry sample with an initial water content of 80% was
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also prepared for comparison in terms of the crack propagation manner.
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Moreover, small cubes with side length of 5 mm were prepared for the MIP tests to investigate
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effects of compaction water content on pore size distribution. For the MIP tests, freeze-drying method was adopted (Delage et al., 1996; Tang et al., 2011c; Yu et al., 2016). The small cubic
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samples were submerged in liquid nitrogen for a few minutes. Then, the frozen samples were put into
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a freeze-dryer with a temperature of -50℃ for 48 hours. During the MIP test, an absolute pressure p
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is applied on mercury to enter the soil pores (Romero and Simms, 2008). According to Washburn equation (Washburn, 1921), the pore diameter d can be calculated as follows: 𝑑=−
4𝜎𝐻𝑔 𝜃𝑛𝑤 𝑝
(1)
where 𝜎𝐻𝑔 is the surface tension of mercury (0.485 N/m at room temperature); 𝜃𝑛𝑤 is contact angle between mercury and the pore wall (162°, as suggested by Penumadu and Dean (2000)).
2.3 Testing apparatus The experimental setup used in Tang et al. (2010) was adopted in this study. The soil sample was placed on a balance to measure the weight with an accuracy of 0.01 g for monitoring change in water
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content during the drying process. A digital camera was fixed above the sample to capture the images of surface crack pattern evolution with elapsed drying time.
2.4 Cracking image analysis The images taken by the digital camera were analysed using the software Crack Image Analysis
f
System (CIAS, available at www.climate-engeo.com). First of all, the image pre-processing
oo
including grayscale (Fig. 1(b)), binalization (Fig. 1(c)), denoising (Fig. 1(d)), skeletonising (Fig. 1(e))
pr
and crack identification (Fig. 1(f)) is carried out (Fig. 1). Then, the next step is quantitative analysis.
e-
Geometric indexes (e.g., length and width of each crack segment, angle of each intersection), general
Pr
indexes (e.g., number of intersections and crack segments, average crack length, average crack width, surface crack ratio) and statistical indexes (e.g., probability density function of crack length, crack
al
width and intersection angle) can be obtained through this step. In this study, some quantitative
rn
indexes are reported. Note that only the central part (150 mm × 150 mm) was used for the image
Jo u
analysis in order to minimize the boundary effects on cracking observations. The edge shrinkage area of each soil sample is counted when determining surface crack ratio (Zeng et al., 2019). More details of the procedure of the quantitative cracking image analysis are available in Tang et al. (2008) and Liu et al. (2013).
3
Experimental results and discussion
3.1 Microstructure of soil samples with different compaction water contents Fig. 2 shows the pore size distributions obtained from MIP tests for soil samples compacted at various water contents. As can be seen from the figure, for the soil samples compacted at water
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contents of 12.5% and 14.5% (on the dry side of the standard Proctor optimum water content) and at water content of 16.5% (standard Proctor optimum water content), bimodal pore size distributions with two distinct pore domains can be observed. The two distinct pore domains are named as micro pore (or intra-aggregate pore) and macro pore (inter-aggregate pore) (Delage et al., 1996; Romero et al., 2011; Ng et al., 2016). On the dry side and at optimum water content, clay particles assemble
f
together and an aggregate structure is well developed in the soil. Both micro pores and macro pores
oo
can be detected by the MIP technique. The corresponding diameter at the peak point of the micro
pr
pores is about 0.6 μm and that of the macro pores is in the range from 10 to 30 μm. With increasing
e-
compaction water content, the amount of micro pores increases whereas the amount of macro pores
Pr
decreases because of the reduced aggregation of clay particles (Delage et al., 1996). However, for the soil samples compacted on the wet side of the standard Proctor optimum water content (at water
al
contents of 18.5% and 20.5%), the pore size distributions show a unimodal form. This is attributed to
rn
the assembly of clay-particle matrix around silt grains and the gradual formation of a dispersed
Jo u
structure (Delage et al., 1996). The corresponding diameter at the peak point is about 0.8 μm and there is no obvious separation between micro pores and macro pores. At various compaction water content levels, the soil samples with different microstructures are obtained. According to Alonso et al. (2013), a simple microstructural state variable ξm is adopted to quantitatively describe the microstructure of soil samples: 𝜉𝑚 = 𝑒𝑚 ⁄𝑒
(2)
where e is the total void ratio and em is the microstructural void ratio. The total void ratio e consists of the microstructural void ratio em and the macrostructural void ratio eM (Romero et al., 2011). In this study, the delimiting pore diameter, at which the derivative of PSD curve becomes zero, is used
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to separate micro pores and macro pores (Ng et al., 2016). The delimiting pore diameters are found to be 1.9, 1.9, 2.4, 5.8 and 5.8 μm for the soil samples compacted at water contents of 12.5%, 14.5%, 16.5%, 18.5% and 20.5%, respectively. Fig. 3 shows the microstructural state variable ξm of each soil sample compacted at various compaction water contents, which is calculated based on the pore size distributions shown in Fig. 2. It can be found that with increasing compaction water content, the
f
microstructural state variable increases, corresponding to a less aggregated structure of clay particles
oo
(Delage et al., 1996). In this study, the MIP tests were conducted on the compacted soil samples. For
pr
a lean clay, the saturation process performed under zero stress does not significantly alter the PSD
Pr
e-
(Oualmakran et al., 2016).
3.2 Crack initiation and propagation in compacted samples with different microstructure
al
Under a drying condition, pore water evaporates from the surface of saturated samples. As the
rn
water content decreases, matric suction is developed and the suction-dependent tensile stress
Jo u
increases. Cracking occurs when the tensile stress exceeds the tensile strength (Tang et al., 2011a). With the elapsed drying time, the desiccation cracking further propagates. Fig. 4 shows the crack initiation and propagation of each soil sample with different microstructure. For the soil samples compacted on the dry side and at the optimum water content, crack initiates evenly on the soil surface and then widens gradually (Fig. 4 (a)-(c)). Despite of small crack width, the initiation of cracks occurs randomly and almost simultaneously on the surface of soil sample. In contrast, cracks evolve progressively in slurry samples, with the onset of primary cracks followed by the branching of sub-cracks (Fig. 5). This is because cracks usually initiate at the surface defects where shrinkage distortion and stress concentration occurs (Zabat et al., 1997; Weinberger, 1999; Tang et al., 2011b).
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The soil samples compacted on the dry side and at the optimum water content have an aggregate structure, as discussed in previous section. During evaporation, soil volume shrinks as the inter-aggregate pores shrink and there is a rearrangement of aggregates (Nowamooz and Masrouri, 2010; Burton et al., 2015; Li et al., 2018). The strain energy build-up prior to cracking within a soil element subjected to tensile stress can be given as (1.299σ2 S2 t)/E, where σ is the tensile stress, S is
f
the pore diameter, t is the layer thickness and E is the Young’s modulus (Costa et al., 2013). The
oo
strain energy is proportional to the pore size. The macro pores between aggregates has a larger pore
pr
size, leading to higher strain energy prior to cracking. Thus, the places between aggregates can be
e-
regarded as the surface weak zones, resulting in crack initiation. Several crack initiation points
Pr
appearing almost at the same time, as marked by the red circles in Fig. 4 (a)-(c), can be observed. However, the slurry samples in the literature (Tang et al., 20 11a) have a more homogeneous
al
microstructure and shrink uniformly during the drying process. Desiccation cracking and strain
rn
energy concentration/release process can be maintained in an orderly manner (Tang et al., 2008) (Fig.
Jo u
5). Moreover, from one crack initiation point, three cracks propagate in three directions with an angle of 120° simultaneously, results in a “Y” shape crack intersection. This can be explained by adopting the concept of Griffith’s fracture energy balance. The “Y” shape crack intersection is the most energy efficient, where the crack length required is the minimum for expending the same amount of energy (Kodikara et al., 2000; Costa et al., 2013). With increasing compaction water content, soil microstructure transits into a less aggregated structure, resulting in a more localized cracking pattern. Furthermore, in soil samples compacted to the dry side of the optimum water content, a minor extent of sub-crack growth is observed (Fig. 4(a)-(c)), which is different from the slurry samples in the literature (Tang et al., 2011a) as well. This is likely because the intra-aggregate pores are almost
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unchanged during drying (Nowamooz and Masrouri, 2010; Burton et al., 2015; Li et al., 2018). Moreover, no new cracks form and the additional strain is accommodated by further opening of existing cracks when a certain crack intensity is reached (Bai et al., 2000, Tang et al., 2006; Yin, 2010). For the soil samples compacted on the wet side with a dispersed structure, primary cracks initiate first (Fig. 4 (d)-(e)). The primary cracks of the soil sample compacted at water content of
f
18.5% initiate both at the inner surface and the edge, as shown in Fig. 2(d). With a higher
oo
compaction water content of 20.5%, the cracks initiates from the edge of soil sample, as shown in
pr
Fig. 4(e). This is because with a higher compaction water content, the microstructure is closer to the
e-
slurry samples. Therefore, the crack propagation manner of the soil samples with a dispersed
Pr
structure is more similar to that of the slurry samples (as shown in Fig, 5). After the primary cracks initiating, sub-cracks initiate at existing primary cracks and almost perpendicular to the primary
al
cracks (Fig. 4 (d)-(e)). This can be explained by adopting the maximum stress release criterion
rn
(Lachenbruch, 1962; Morris et al., 1992). When the primary cracks initiate, the internal tensile stress
Jo u
perpendicular to the existing primary cracks are released and the newly maximum tensile stress is parallel to the existing primary crack. Hence, the sub-cracks grow at the local maximum tensile stress and are perpendicular to the primary cracks. Quantitative analysis is carried out for the images showing development of desiccation cracking during the drying process. Fig. 6 shows the change of surface crack ratio Rsc with water content during drying. The surface crack ratio is defined as the ratio of the surface area of cracks to the total surface area of soil sample. It should be noted that the water content at the onset of cracking (indicated by the intersection point between surface crack ratio curve and water content axis) is defined as the cracking water content ωc (Tang et al., 2011b). It is found that with increasing
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compaction water content from 12.5% to 20.5%, the cracking water content decreases from about 25% to 19%. The cracking water contents of the samples compacted on the dry side and at the optimum water content is higher than those of the samples compacted on the wet side. This is likely because the cracks in the samples compacted on the dry side and at the optimum water content appears in the void between aggregates while the cracks in the samples compacted on the wet side appears in the
f
void between soil particles, as discussed previously. According to the Young-Laplace equation, there
4𝑇𝑠 cos 𝜃 𝑑
(3)
pr
𝑠=
oo
is an inverse relationship between pore diameter d and suction s:
e-
where Ts is the surface tension of water (mN/m); θ is the contact angle (°). Larger pore diameter
Pr
develops smaller suction, resulting in higher amount of water retained in the soil. As a result, a higher cracking water content is observed in the soil samples compacted on the dry side. Moreover, it
al
can be seen from the figure that with decreasing water content during drying, the surface crack ratios
rn
of all the soil samples with various microstructure show an increasing trend. Specifically, for the soil
Jo u
samples compacted on the dry side and at the optimum water content with bimodal pore size distributions, Rsc increases almost linearly with decreasing water content. This is likely because for the soil samples compacted on the dry side and at optimum water content, all the cracks initiate almost simultaneously and the generated cracks gradually widen with further drying. For these three samples, the changing rate of Rsc and the final Rsc after reaching an equilibrium state decreases with decreasing compaction water content. The final Rsc of soil sample with compaction water content of 12.5% (0.056) is about twice than the soil sample with compaction water content of 16.5% (0.029). However, for the soil samples compacted on the wet side with unimodal pore size distributions, the relationship between surface crack ratio and water content before reaching the equilibrium state can
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be regarded as consisting of two approximately linear segments. The changing rate of the first segment is much smaller than that of the second one. Actually, the first segment corresponds to the initiation of primary cracks and sub-cracks. The second segment corresponds to widening the existing cracks after the geometric crack pattern tends to stabilize (Fig. 4). Fig. 7 shows the change of crack density with water content during drying. Note that the crack
f
density (/cm2 ) represents number of crack segments per unit area. In general, the crack density
oo
increases with decreasing water content before reaching the equilibrium state. Similarly, for the soil
pr
samples compacted on the dry side and at the optimum water content with bimodal pore size
e-
distributions, the crack density increases almost linearly from the cracking water content to water
Pr
content of about 15%. For the soil samples compacted on the wet side with unimodal pore size distributions, the crack density increases slightly in the water content range from cracking water
al
content to 15%. With further drying to water content of 10%, a more dramatic increase in the crack
rn
density is observed. The corresponding water content at the intersection point is larger than that in
Jo u
Fig. 6 because the initiation of sub-cracks result in a significant increase in the crack density while do not count much in the surface crack ratio. The corresponding water content when the crack density becomes stabilised is larger than that for surface crack ratio (Fig. 6). This is because when the geometric structure of the crack network tended to stabilize and no new cracks were initiated, cracks become widen with further drying (Tang et al., 2011b). Moreover, the final crack density at the equilibrium state decreases with increasing compaction water content.
3.3 Final crack pattern of compacted samples with different microstructure Fig. 8 shows the final crack pattern of each soil sample. It can be found that there exist more fine
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and thin cracks in samples with lower compaction water contents and vice versa. With increasing compaction water content, the geometric crack pattern tends to be simpler. This represents that there are less weak zones exist in a soil sample with a higher compaction water content. Compacted samples in this study exhibit similar geometrical shapes of soil clods (e.g., quadrangle, pentagon, and hexagon) as those observed in desiccated slurry samples (Yesiller et al., 2000; Rodríguez et al., 2007;
f
Groisman and Kaplan, 2007; Tang et al., 2010; DeCarlo and Shokri, 2014; Lakshmikantha et al.,
oo
2018; Levatti et al., 2019). In previous studies investigating desiccation cracking of slurry samples
pr
(Vogel et al., 2005; Péron et al., 2009; Tang et al., 2011a), the researchers get a common view that
e-
the crack segments are smooth and the soil surface is split into separate clods by the crack network.
Pr
However, as shown in Fig. 8, the crack segments are rough and the clods of compacted samples are not fully separated. This can be attributed to the homogenous structure of slurry samples and
al
heterogeneous structure of compacted samples.
rn
The final crack pattern of each sample is quantitatively analysed. The relationships between
Jo u
change of crack density as well as average width and microstructural state variable are plotted in Fig. 9. With increasing microstructural state variable, the crack density decreases almost linearly whereas the average width show an opposite trend. Moreover, it is found that on the wet side, with a given change in microstructural state variable, the change in average width is about 4.5 times more than that on the dry side. Crack width is one of the important indices to characterize the crack pattern. It is closely related to the crack propagation. For example, the widest cracks at the end of the drying process are the primary cracks (Tang et al., 2011b). Fig. 10 shows the probability-density function of crack width of each soil sample at the end of the desiccation test. Generally, with increasing compaction water
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content, the probability density function of crack width changes from unimodal to bimodal, indicating that the grading characteristics of primary cracks and sub-cracks becomes more obvious. It can be seen from the figure that for the two samples compacted on the dry side (with water contents of 12.5% and 14.5%), the range of the crack width is relatively narrow, primarily distributed in the range from 0 to 0.10 cm. This confirms that for the soil sample with an aggregate structure, there is
f
no sub-cracks. For the soil sample compacted at optimum water content of 16.5%, the majority of
oo
crack width is still distributed in the range from 0 to 0.10 cm. However, a fraction of crack width is
pr
wider (0.10~0.25 cm), which is also observed in Fig. 8. This is likely because even though the soil
e-
sample compacted on the optimum water content has less obvious aggregates and a similar coating
Pr
clayey fraction to that found on the dry side (Delage et al., 1996). For the soil samples compacted on the wet side (with water contents of 18.5% and 20.5%), the crack with is mainly distributed in the
al
range from 0 to 0.20 cm and the distribution is relatively average. Extra-wide cracks (with width
rn
over 0.30 cm) appear on the surface of the sample compacted at water content of 20.5%. The
Jo u
observed phenomenon also confirms that both wide primary cracks and relatively thin sub-cracks exist in soil samples with a dispersed structure. Crack intersection angle is another important index for describing the crack pattern and also influenced by the crack propagation. The probability-density function of crack intersection angle is presented in Fig. 11. The probability-density function of crack intersection angle of each soil sample is basically a normal distribution. For the soil samples compacted on the dry side and at optimum water content, the intersection angles are mainly distributed in the range from 100 to 140°. This corresponds to the observed phenomenon that for soil samples with an aggregate structure, only primary cracks exist and they are formed simultaneously at the beginning stage of desiccation.
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f
Sub-cracks normally show a “T” or “+” intersections with an intersection angle of about 90° (Vogel
pr
oo
et al., 2005; Péron et al., 2009; Tang et al., 2011a).
e-
3.4 General effects of microstructure on desiccation cracking of compacted soil
Pr
Based on the abovementioned phenomenon and interpretation, the crack propagations of soil samples with an aggregate structure and a dispersed structure show different manners. Fig. 12 and
al
Fig. 13 provide the schematic view of the desiccation cracking process in soils with aggregate and
rn
dispersed microstructures, respectively. For soil samples with an aggregate structure, the cracking
i.
Jo u
propagation can be summarized into two stages: Independent “Y” shaped cracks between aggregates initiate evenly on the soil surface with soil shrinkage during evaporation (Fig. 12 (a)-(b)). These cracks extend gradually until reaching the edge or intersecting with each other and the geometric structure of the crack pattern is formed (Fig. 12 (b)-(c)). ii.
The geometric structure of the crack pattern tends to stabilise when the water content continues to decrease. The existing cracks keep widening until the desiccation ceased (Fig. 12 c)-(d)).
For soil samples with a dispersed structure, desiccation cracking takes place in three stages:
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i.
During the drying process, water evaporates and soil shrinks. Independent cracks between soil particles initiate at the inner surface or the edge (Fig. 13 (a)-(b)) and propagates until reaching the edge or intersecting with each other (Fig. 13 (b)-(c)). These initial independent cracks are considered as primary cracks.
ii.
Perpendicular to the existing primary cracks, some thin cracks initiate and propagate when
f
the water content keeps decreasing (Fig. 13 (c)-(d)). These post-developed cracks are
oo
regarded as sub-cracks. The primary cracks and the sub-cracks form the geometric structure
With further decreasing water content, the geometric structure of the crack pattern stays
e-
iii.
pr
of the crack pattern.
Pr
nearly unchanged while the primary cracks and the sub-cracks widen until the desiccation
Conclusions
rn
4
al
ceased (Fig. 13 (d)).
Jo u
Effects of soil microstructure on desiccation cracking of a compacted lean clay are investigated experimentally in this study. Soil samples with different microstructures induced by various compaction water contents are tested. The water content evolution, surface crack initiation and crack propagation are monitored during the drying process. Quantitative analysis of crack evolutions with the elapsed drying time leads to the following major conclusions: 1.
The pore size distributions of soil samples compacted on the dry side and at optimum water content are bimodal because of the aggregate structure, whereas the pore size of soil samples compacted on the wet side follows a unimodal distribution due to the dispersed structure. With increasing compaction water content, the microstructural state variable increases.
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2.
For soil samples with an aggregate structure, independent cracks initiate evenly on the soil surface at the same time and then widen gradually. These cracks mostly intersect at an angle of 120°. For soil samples with a dispersed structure, primary cracks initiate first followed by the growth of sub-cracks along the orthogonal directions. The primary cracks formed “Y” shapes with intersection angles of 120° and the sub-cracks show “T” shapes with intersection angles of
The cracking water contents of the samples compacted on the dry side and at optimum water
oo
3.
f
90°.
pr
content is higher than those compacted on the wet side. Because the crack in the samples with an
e-
aggregate structure is likely to appear in the void between aggregates while the crack in the
Pr
samples with a dispersed structure appears in the void between soil particles. The macro pore has a larger diameter and develops smaller suction, resulting in higher amount of water retained
During the process of desiccation cracking, the surface crack ratio and the crack density of soil
rn
4.
al
in the soil.
Jo u
samples with an aggregate structure increase almost linearly with decreasing water content before stabilising. Soil samples with dispersed structures show a two-stage linear growth with decreasing water content. 5.
In terms of final crack pattern, with increasing compaction water content, the crack pattern of the soil sample becomes less complicated. There are more fine and thin cracks for the soil sample with a smaller microstructural state variable and vice versa.
Acknowledgements This work was supported by National Natural Science Foundation of China (Grant No.
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41572246, 41772280, 41322019, 41902271), Natural Science Foundation of Jiangsu Province (BK20171228, BK20170394), and the Fundamental Research Funds for the Central Universities.
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f
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oo
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Nowamooz, H., Masrouri, F., 2010. Relationships between soil fabric and suction cycles in
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Nowamooz, H., Jahangir, E., Masrouri, F., 2013. Volume change behaviour of a swelling soil
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Rodríguez, R., Sánchez, M., Ledesma, A., Lloret, A., 2007. Experimental and numerical analysis of
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desiccation of a mining waste. Canadian Geotechnical Journal 44 (6), 644-658.
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Romero, E., Della Vecchia, G., Jommi, C., 2011. An insight into the water retention properties of
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compacted clayey soils. Géotechnique 61 (4), 313-328. Romero, E., Simms, P.H., 2008. Microstructure investigation in unsaturated soils: a review with
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Figure captions Fig. 1 Crack image processing procedure Fig. 2 Pore size distribution of soil samples with various compaction water contents Fig. 3 Microstructural state variable determined for soil samples with various compaction water contents Fig. 4 Crack patterns captured at varying desiccation stages in soil samples compacted at (a)
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ω0 =12.5%, (b) ω0 =14.5%, (c) ω0 =16.5%, (d) ω0 =18.5% and (e) ω0 =20.5%
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Fig. 5 Progressive growth of primary cracks and sub-cracks in a slurry sample
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Fig. 6 Change of surface crack ratio with water content during drying
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Fig. 7 Change of crack density with water content during drying Fig. 8 Final crack pattern of soil samples with various compaction water contents
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Fig. 9 The evolution of crack density and average crack width with microstructural state variable in
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five soil samples
water contents
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Fig. 10 Probability-density function of crack width for five soil samples with different compaction
Fig. 11 Probability-density function of crack intersection angle for five soil samples with different compaction water contents Fig. 12 Schematic drawing of the development of desiccation cracks in the soil sample with an aggregate structure Fig. 13 Schematic drawing of the development of desiccation cracks in the soil sample with a dispersed structure
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Table 1. Index properties of test soil
Property
Value
Specific gravity, Gs
2.73 2
Silt content (%)
76
Clay content (%)
22
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Sand content (%)
Liquid limit, ωl (%)
36.5
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Plastic limit, ωp (%)
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Shrinkage limit, ωsl (%)
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Plasticity index, Ip (%)
19.5 17.0 10.5 16.5
Maximum dry density (Mg/m3 )
1.7
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Optimum water content, ωopt (%)
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Compaction water content affects microstructure and desiccation cracking of soils.
With increasing compaction water content, the microstructural state variable increases.
For soils with aggregate structures, the desiccation cracks initiate simultaneously and distribute uniformly.
For soils with dispersed structures, localized growth of primary and secondary cracks are
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With increasing microstructural state variable, the final crack pattern of the soil sample becomes
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less complicated.
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observed.
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