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Transitional behavior in well-graded soils: An example of completely decomposed granite
Engineering Geology 253 (2019) 240–250
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Transitional behavior in well-graded soils: An example of completely decomposed granite Elsayed Elkamhawya,b, Bo Zhoua, Huabin Wanga, a b
T
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School of Civil Engineering and Mechanics, Huazhong University of Science and Technology, Wuhan 430074, PR China Faculty of Engineering, Zagazig University, Zagazig 44519, Egypt
A R T I C LE I N FO
A B S T R A C T
Keywords: Completely decomposed granite Transitional behavior Uniqueness Convergence
This study presents the results of a series of isotropic compression tests and drained and undrained triaxial shearing tests on naturally graded and reconstituted completely decomposed granite soils to investigate their compressive response and transitional behavior. Naturally graded soils exhibited unique isotropic compression and critical state lines within the stress levels studied. Soils reconstituted by increasing the sand content by 5% and 10% revealed non-unique isotropic compression lines; however, unique critical state lines were present in the e-log p' space. The degree of non-convergence increased as the sand content increased. The value of the m parameter, which is used to quantify the transitional mode and is defined as the ratio between the changes in the final and initial specific volumes (i.e., m = Δv2/Δv1; see Fig. 1), increased from 0 in the case of naturally graded soil to 0.3 and 0.73 for reconstituted soils with increased sand content of 5% and 10%, respectively. For naturally graded soils, the initial void ratio affects only the initial compression response, and its influence does not extend to the normal compression line. In the case of reconstituted soils, the fabric that results from various compaction efforts exerts a significant effect on the soil′s compressive response. However, this fabric cannot withstand large strain during the shearing path and is completely destroyed, producing a unique fabric in the critical state. Both natural and reconstituted soils revealed isotropic responses during the loading and unloading compression paths with unrecoverable volume change. The transitional behavior is no longer limited to gap-graded soils, and grading is no longer the only dominant factor in this behavior. The transitional mode occurs due to relatively inconsiderable changes in soil mineralogy, grading, and microstructure.
1. Introduction Completely decomposed granite (CDG) soils are abundant in southeastern China, and they are used extensively as fill materials in various engineering applications, such as back-filling materials for retaining structures, levees, roads, and embankments. As China develops rapidly, it is urgent to study and understand the behavior of these soils. Many studies have been conducted on CDG, including coarse and finegraded soils, well-graded soils that are classified as well-graded sand according to the unified soil classification system (Lee and Coop, 1995), and the well-graded soils SC-CL (Yan and Li, 2012) and SC-CH (Ng et al., 2004) (SC is clayey sand, and CL and CH are low and high plasticity clay, respectively). The main feature of the previously mentioned CDG soils is that the isotropic compression lines (ICLs) and critical state lines (CSLs) merge into unique lines. This means that a unique void ratio–effective stress state relationship exists with normally consolidated soils and that the soils obey Rendulic's principle, in which
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the state boundary surfaces of various initial void ratios (i.e., various densities) could be unique. Soil should be elastic when its state is below the state boundary surface and plastic after reaching the yield once the state boundary surface has been reached. A critical state framework is applicable in cases in which the soil has a unique CSL. However, the intact CDG soils studied by Wang and Yan (2006) exhibited non-unique compression lines, implying that compression lines depend robustly on the initial densities; this behavior is termed “transitional behavior.” Been and Jefferies (1985) found that the behavior of clean sands can be described in a manner similar to that of clayey soils within the critical state framework because both compression lines and CSLs merge into unique lines. Because clays and clean sands define unique compression lines and are considered the extremes of the whole range of soils, a transitional mode in the soils' compression behavior must exist between these extremes (Martins et al., 2001). Nocilla et al. (2006) defined transitional behavior as the behavior between clays and clean sands. The transitional mode is particularly characterized by the non-
Corresponding author. E-mail address: [email protected] (H. Wang).
https://doi.org/10.1016/j.enggeo.2019.02.027 Received 17 September 2018; Received in revised form 22 February 2019; Accepted 26 February 2019 Available online 28 February 2019 0013-7952/ © 2019 Elsevier B.V. All rights reserved.
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m parameter is calculated from the initial and final specific volumes v1 and v2, respectively. The parameter m can be defined as the ratio of the changes of the final and initial specific volumes (i.e., m = Δv2/Δv1), as shown in Fig. 1b. The parameter P can be also calculated from the initial specific volumes and the intersect of the CSLs with the specific volume axis at log P′ = 1 (Γ), as indicated in Fig. 1d (i.e., P = ΔΓ/Δv1). The soil behavior is considered to be completely transitional when m and/or P equal 1, as the ICLs are in parallel and the CSLs also exhibit parallelism. In the case of unique ICLs and CSLs, the values of m and P equal 0. Values between 0 and 1 represent various degrees of convergence from uniqueness to parallelism. As mentioned above, the transitional mode was observed in soils with fine to coarse grading, and several factors dominate this mode. Plastic fine content was found to be a paramount factor (Nocilla et al., 2006; Shipton and Coop, 2015), and grading and mineralogy are also key factors (Shipton and Coop, 2012). The sample reconstitution method, over-consolidation ratio, and stress levels have no influence on this mode (Martins et al., 2001; Shipton and Coop, 2015). Xu and Coop (2017) showed that the transitional mode was more pronounced in intact soils than in reconstituted soils. For CDG saprolite soils, disturbance and compression paths were considered to be substantial factors, as Wang and Yan (2006) found nonconvergence of ICLs in intact samples. Yan and Li (2012) found unique ICLs for recompacted soil samples. They also conducted conducted anisotropic compression paths with various stress ratios and found that the compression lines merged into a unique line in samples compressed at the same stress ratio, whereas the samples that were compressed to different stress ratios showed nearly parallel compression lines. In contrast, CDG soils exhibited unique CSLs regardless of whether they were disturbed or intact, which indicates that the initial fabric was completely destroyed throughout large strains during shearing, producing a unique fabric at the critical state. Because of the abundance of CDG soils in China and their applications, there is an urgent need to survey and understand their
uniqueness of normal compression lines and/or CSLs (Altuhafi et al., 2010; Ferreira and Bica, 2006; Nocilla et al., 2006; Ponzoni et al., 2014). This mode of soil behavior cannot be described within a critical state framework, and as such, these soils do not follow Rendulic's principle. Many studies have shown that transitional behavior is more common than previously thought; it had been limited to gap-graded soils (Ferreira and Bica, 2006; Martins et al., 2001). Transitional behavior has also been observed in well-graded silty clayey soils (Nocilla et al., 2006; Ponzoni et al., 2014; Xu and Coop, 2017), well-graded sands (Altuhafi et al., 2010; Altuhafi and Coop, 2011), well-graded coarse granular soils (Xiao et al., 2016), and gap-graded soils (Ferreira and Bica, 2006; Martins et al., 2001; Shipton and Coop, 2015). Non-unique compression lines have been observed in both odometer and isotropic compression tests (Altuhafi et al., 2010; Altuhafi and Coop, 2011; Ferreira and Bica, 2006; Martins et al., 2001; Nocilla et al., 2006; Ponzoni et al., 2014; Shipton and Coop, 2012; Shipton and Coop, 2015; Wang and Yan, 2006; Xiao et al., 2016; Xu and Coop, 2017). Although the compression lines do not merge into a unique line, Altuhafi et al. (2010) clearly observed a unique CSL; Wang and Yan (2006) found the same response in the case of intact CDG, whereas nonunique CSLs were observed in other studies (Ferreira and Bica, 2006; Nocilla et al., 2006; Ponzoni et al., 2014; Shipton and Coop, 2015). Two explanations have been given for both the uniqueness and non-uniqueness of CSLs. First, for unique CSL, the fabric responsible for nonunique ICLs is weak and is completely destroyed in the shearing stage, producing a unique fabric at the critical state. However, in many cases, the fabric responsible for non-unique ICLs is extremely sturdy and resists damage to an extent that allows its effects to persist even at the critical state, which results in the non-uniqueness of CSLs (Baudet and Stallebrass, 2004; Ferreira and Bica, 2006; Shipton and Coop, 2015). Ponzoni et al. (2014) proposed two parameters m and P to quantify the transitional mode via the quantification of the degree of non-convergence for normal compression lines and CSLs. Fig. 1 shows how the
Fig. 1. Quantification of the convergence of the ICLs and CSLs: a) schematic illustration of ICLs; b) calculation of parameter m; c) schematic illustration of CSLs; d) calculation of parameter P (Ponzoni et al., 2014). 241
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Fig. 2. Site of the soil used in this study.
Guangdong province, in southeastern China. CDG soils are reddish brown with some white spots, as shown in Fig. 2. Core stones are not found at the soil site, which implies that the parent rock has been completely decomposed (Alavi Nezhad Khalil Abad et al., 2016). The parent granitic rock of the soils is monzonite granite (retrieved from www.geodata.ngac.cn). The decomposed granite soil is classified as grade V based on the Geotechnical Engineering Office classification system of decomposed rocks (GEO, 2017). The x-ray diffraction (XRD) was used to analyze the soil composition and showed large quantities of clay minerals (e.g., kaolinite, montmorillonite, and illite) and quartz. Table 1 shows a statistical summary of the percentages of soil minerals. The mean value of quartz is obviously the maximum value, probably because quartz is the most stable mineral and is inert to chemical weathering; thus, it was observed extensively in all soil samples. Kaolinite was the most abundant clay mineral, at ratios of 33% to 43%; this can be attributed to the intensive weathering environment and the unimpeded drainage conditions, such as well-drained hill slopes, joints, and fractures in the parent rock. Other clay minerals such as montmorillonite and illite were observed in small amounts, possibly because of the removal of limited amounts of cations in a stagnant condition near the bottom of the slope (Irfan, 1996; Ng et al., 2001). XRD analysis also showed no trace of feldspar in the CDG soil samples, as feldspar is considered to be the most vulnerable mineral to chemical weathering and eventually forms clay minerals (Irfan, 1996; Zauyah et al., 2010).
Table 1 Statistical analysis of minerals for CDG soil. Soil minerals Montmorillonite Illite Kaolinite Quartz
Mean (%) 1 2.5 38.25 58.25
Minimum (%) 2 3 33 50
Maximum (%) 2 7 43 67
Range 0 4 10 17
mechanical characteristics and investigate the transitional mode. The presence of a transitional mode in soil behavior requires a reconsideration of terms such as the unique compression line, the unique critical state line, and the state parameter proposed by Been and Jefferies (1985). Thus, a new framework is needed to describe soils that show a transitional mode. The essential objective of this study is to illuminate the compressive response and transitional mode of CDG soils that originate from the decomposition of monzonitic granite rock via a series of isotropic compression tests and drained and undrained shearing tests —in particular, to investigate the ICL patterns and quantify the degree of transitional behavior with the parameter m proposed by Ponzoni et al. (2014). 2. Test materials 2.1. Mineralogical features The CDG soils used in this study were obtained from a site in Bozhi, 242
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additional amount of sand fraction was added from the sand retained on each sieve, as shown in Fig. 4. The reconstituted soil B10 was prepared in the same manner. According to the unified soil classification system, the soil can be classified as high plasticity clayey sand (SC-CH). Table 4 summarizes the main physical properties of both the natural and reconstituted soils. The specific gravity (G.s) was found to be 2.67 for naturally graded soil. The liquid and plastic limits for naturally graded soil were 55.84% and 27.85%, respectively, with (1.51 g/cm3) maximum dry density corresponding to the optimum moisture content (25.25%). The maximum dry densities of the reconstituted soils were 1.572 g/cm3 and 1.564 g/ cm3 after the sand content was increased by 5% and 10%, respectively. The maximum dry density increased when the sand content was increased by 5% and then decreased when the sand content was increased by 10%; this can be explained by the binary-mixed soil overall void ratio model proposed by Lade et al. (1998). Where, the overall void ratio decreased as the sand content increased by 5%, and an increase by 10% led to an increase in the overall void ratio; thus, the maximum dry density increased and then decreased again. The trend of the maximum dry density shows that the total sand content obtained by adding 5% to the sand content represents a threshold. Ham et al. (2010) observed the same trend between the maximum dry density and gravel contents. As expected, the optimum moisture content decreased as the sand content increased.
Fig. 3. Gradation curves.
2.2. Physical characteristics Fig. 3 shows the gradation curves of the CDG soils and other decomposed granite soils from Hong Kong and South Korea, and Table 2 summarizes the main features of these soils. Compared with decomposed granite soils from Hong Kong (Ng et al., 2004; Wang and Yan, 2006; Yan and Li, 2012) and South Korea (Lee and Coop, 1995), the studied soils are well graded, and the CDG soils are typically characterized as well graded (Chiu and Ng, 2014; Rocchi and Coop, 2016). The soils in this study are predominantly fine well graded, like those that studied by Wang and Yan (2006). High fines content can be imputed to the severe circumstances of the weathering environment and/ or to the relatively greater vulnerability of the soil to chemical weathering (Lee and Coop, 1995; Ng et al., 2004; Yan and Li, 2012). Although all of the soils reported in Fig. 3 are CDG soils, the grading varies substantially, perhaps due to differences in mineral composition resulting from different weathering environments and the structure of the parent rock. The reconstituted soils were prepared by increasing the sand content by 5% (B5) and 10% (B10); thus, the whole sand content reached 36.9% and 41.9%, respectively. Due to increasing sand content, the gravel and fines content decreased, and their new ratios were calculated based on the predetermined ratio of sand (i.e., 36.9% and 41.9%). For example, for the reconstituted samples B5, the natural grading of sand fractions were 16.2%, 4.7%, and 11% retained by 0.5-mm, 0.25-mm, and 0.075-mm sieves, respectively; the reconstituted grading of sand fractions were then calculated as 18.74%, 5.44%, and 12.72%. Then, 1 kg of the oven-dried natural soil was weighted, and the required additional weight of sand to reach the 36.9% was then calculated and found to be 79.23 g. This additional 79.23 g was then added with the pre-calculated new ratios (i.e., 40.24, 11.67, and 27.32 g were added from the sand retained by the 0.5-mm, 0.25-mm, and 0.075-mm sieves, respectively), as indicated in Table 3. It is worth noting that the
3. Specimen preparation and test program The tests in this study were performed with an automated stress path triaxial testing system. The triaxial device was manufactured by Global Digital Systems (GDS) Ltd. in the UK. The GDS triaxial testing system (GDSTTS) is a fully automated triaxial testing system designed principally for stress path testing. The system is based on the classic Bishop and Wesley stress path triaxial cell, which controls stress on the sample directly. The triaxial apparatus was equipped with three controllers (cell, back pressure, and axial stress and axial displacement controllers) in addition to an external linear variable displacement transducer to measure the axial strain. Back pressure was applied from the top cap, the pore water pressure was measured from the base of the specimen, and the volume change was measured via the back-pressure controller. The triaxial specimens were prepared with the moist tamping method. The specimens were compacted inside a split mold at each soil's optimum moisture content in three layers, with scratching between the adjacent layers to limit the effects of layering. Various densities were obtained by controlling the weight of the soil in each layer and the applied compaction effort. After compaction, the mold was dismantled, and the specimen was encapsulated with a rubber membrane. The soil samples have to be saturated to avoid the complication of pore air pressure that differs from the pore water pressure and the accompanying measurement difficulties. To improve the degree of saturation, the soil specimens were submerged in a water tank subjected to vacuum pressure for 24 h. The sample was installed in the GDS device and the back and cell pressures were then increased simultaneously to maintain constant effective mean stress throughout the saturation process. A saturation check was then conducted to inspect the degree of saturation using Skempton's B-value (Skempton, 1954), and in all tests
Table 2 Main features of the natural CDG studied soil and other reported soils. Study Soil in this study Yan and Li (2012) Wang and Yan (2006) Ng et al. (2004) Lee and Coop (1995)
Mean dia. (D50), mm 0.025 0.30 0.01 0.70 2.0
Fine %
silt%
Clay%
Soil classification based on USCS
61.1 35 65 25 5
36.3 25% 40% 10% _
24.8 10% 25% 15% _
SC-CH SC-CL SC-CH SC-CH SW
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Max. dry density (g/ cm3) 1.51 1.71 _ 1.65 1.794
Slope of ICL(ξ)
Slope of CSL (λ) in e-log p'
0.1 0.091 0.076 0.13 0.087
0.083 0.066 0. 072 0.093 0.087
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Table 3 Soil content of the natural and the reconstituted soils and the additional amounts of sand. Soil ID B0
B5
B10
Dia. (mm) Soil fraction % of Pass % of soil contents B0 Sand sizes (mm) % of sand % of Pass % of soil contents B5 Sand sizes (mm) % of sand Amount of additional sand/1 kg = 79.23 g % of pass % of soil contents B10 Sand sizes (mm) % of sand Amount of additional sand/1 kg = 135 g
10.0 Gravel 100 7% – – 100 6.5% – – – 100 5.97% – – –
5.0
2.0 Sand 93.0 31.9% 2–0.5 16.2 93.5 36.9% 2–0.5 18.74 40.24 94.03 41.9% 2–0.5 21.28 68.55
99.5
99.5
99.5
0.5
0.25
76.8
72.1
0.5–0.25 4.7 74.75
0.25–0.075 11 69.32
0.5–0.25 5.44 11.67 72.75
0.25–0.075 12.72 27.32 66.58
0.5–0.25 6.17 19.90
0.25–0.075 14.45 46.55
0.075 Fines 61.1 61.% – – 56.6 56.6% – – – 52.13 52.13% – – –
0.005
0.002
33.2
24.8
30.75
22.97
28.32
21.16
Note: B refers to site name Bozhi; 0, 5 and 10 after B refer to the additional amount of sands.
Fig. 4. Separated soil content. Table 4 Physical characteristics of the studied CDG soils. Soil ID B0 B5 B10
G.S 2.67 2.65 2.64
P.L (%) 27.85 – –
L.L (%) 55.84 – –
γdry max (g/cm3) 1.51 1.572 1.564
P·I (%) 27.99 – –
Optimum W.C (%) 25.25 23.5 22.4
Soil classification SC-CH – –
Note: B refers to the site name Bozhi; 0, 5 and 10 after B refers to the additional amount of sand.
the B-values were higher than 0.95. The loading and unloading rates during isotropic compression were 10 kPa/h and 20 kPa/h, respectively. No noticeable excess pore water pressure was triggered by these rates. The drained and undrained shearing tests were conducted in a strain-controlled mode using axial strain rates of 0.02%/min and 0.05%/min, respectively, following Head (1992). The isotropic compression tests and the drained and undrained shearing tests were performed on normally consolidated specimens, as summarized in Tables 5, 6 and 7. Many different methods are used to calculate the void ratios of soil samples, and the two methods discussed in detail by Verdugo and Ishihara (1996) are the most common. In the first method, the specimen's initial dimensions are measured before the test begins to calculate the specimen's initial void ratio, and the void
Table 5 Experimental details for the isotropic compression test. Test ID
Compaction ratio
Load cycle, P′ kPa
B0
85% 87.5% 90% 85% 87.5% 90% 85% 87.5% 90%
20–600 20–250–150–450 20–200–50–400–50 20–600 20–250–150–450 20–200–50–400–50 20–400 20–250–150–450 20–200–50–400–75
B5
B10
244
Initial void ratio 0.982 0.935 0.916 0.915 0.855 0.768 0.877 0.848 0.817
Final void ratio 0.686 0.709 0.741 0.641 0.644 0.647 0.665 0.634 0.653
Slope of ICL(ξ) 0.1
–
–
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determined using a scanning electron microscope (SEM). The SEM photos shown in Fig. 6 indicate that kaolinite and illite minerals were present in flake and leaf shapes. These shapes are characterized by high deformability and significantly affect soil compressibility even if existed in small quantities (Lee et al., 2007; Madhusudhan and Baudet, 2014). However, the volumetric change during the unloading path was almost insignificant, which indicates that the volume change was unrecoverable. The slope of the ICL (ξ) of the naturally graded soil was 0.1, and those of other CDG soils were ξ = 0.091 in Yan and Li (2012) and ξ = 0.087 in Lee and Coop (1995). The high compressibility of the naturally graded soil can be clearly seen compared with other CDG soils. Although the fines content in the studied soils was high, the slope of the compression line was lower than that in the CDG soils reported by Ng et al. (2004), with ξ = 0.13. This difference can be explained by the presence of crushable feldspar in the soil studied by Ng et al. (2004). Wang and Yan (2006) studied intact CDG soil, which showed parallel compression lines with ξ = 0.076 even though its grading was finer than those of the soils in this study; this can be illustrated by the fabric and microstructure of the parent rock in the intact soil samples. Particle rearrangement and particle crushing are the two mechanisms responsible for volume changes. Yamamuro and Lade (1997) concluded that the mechanism of volume change is due to particle rearrangement under low consolidation stress levels and that particle crushing is the dominant mechanism during high consolidation stress levels. However, both mechanisms can occur simultaneously under low stress levels for CDG soil in case of presence weak minerals in a soil composition, such as crushable feldspar (Ng et al., 2004). Yan and Li (2012) concluded that the volumetric change was due to particle rearrangement because the consolidation stress level was not high (400 kPa) and the soil grading did not change noticeably after the isotropic compression test. Lee and Coop (1995) included crushable feldspar in their soil and used a high stress level (10,000 kPa), and they attributed the mechanism of volume change to particle breakage. The XRD analysis of the studied soil showed no crushable feldspar in the soil minerals, the stress levels were not high (450 kPa), and the fines content of the studied soil was much greater than that in the soil reported by Yan and Li (2012), and clay particles serve as a pillow for sand grains, cushioning them from stresses (Nocilla et al., 2006). Consequently, the mechanism of volume change can be attributed to particle rearrangement rather than particle breakage. As mentioned before, the high fines content, particularly of plastic fines, led to a steepening of the initial compression path without a distinct yield stress. This also emphasizes that particle rearrangement is the dominant mechanism governing the volume change. The compression curves in Fig. 5 reveal that transitional behavior is present in the CDG soils. Although the ICLs of naturally graded soil merged in a unique compression line, the reconstituted soils prepared by increasing the sand content exhibited non-unique compression lines under the stress levels used in this study. An identical response was reported in Italian silt by Nocilla et al. (2006): the reconstituted soil with 45% clay content revealed a unique compression line, and as the clay content decreased, the compression lines gradually diverged to become perfectly parallel at a clay content of 3.5%. The presence of a transitional mode in CDG soils, which was characterized by its wellgraded nature, confirmed that transitional behavior was much more prevalent and not limited to gap-graded soils (Ferreira and Bica, 2006; Martins et al., 2001). Ponzoni et al. (2014) proposed the m parameter to quantify the transitional mode during the one-dimensional compression test, as indicated in Fig. 1b. The changes in specific volume and void ratio are equal, Δv = Δe, for the same range of mean effective stress; thus, in this study, the m parameter was adopted in the isotropic compression tests in terms of the void ratio rather than the specific volume. Values of the parameter m range from 0 to 1: when m equals 1, soils exhibit a full transitional mode and the compression lines are parallel, whereas when m equals 0, soils exhibit a unique compression
Table 6 Experimental details for the drained triaxial compression shearing test. Test ID B0
B5
B10
Compaction ratio, % 89.58 89.74 87.31 88.43 86.93 87.20 85.76 89 89.10 86.07 87.56 86.85
Consolidated stress P′(kPa) 100 200 300 400 100 200 300 400 100 200 300 400
Void ratio before shearing 0.814 0.781 0.745 0.706 0.787 0.732 0.687 0.666 0.766 0.712 0.664 0.655
Final void ratio 0.757 0.698 0.663 0.627 0.684 0.641 0.602 0.592 0.665 0.627 0.583 0.567
Table 7 Experimental details for the undrained triaxial compression shearing test. Test ID B0
B5
B10
Compaction ratio, % 87.57 89.04 88.28 89.16 86.96 85.87 88.18 89.36 88.34 88.41 86.25 86.31
Consolidated stress P′(kPa) 100 200 300 400 100 200 300 400 100 200 300 400
Final void ratio 0.816 0.780 0.744 0.706 0.788 0.730 0.695 0.660 0.768 0.693 0.670 0.657
ratio during the test path is then calculated from the volume change measurements. The roughness of the specimen's surface may produce errors in the initial dimension measurements, so the initial void ratio calculation is not accurate. In the second method, the specimen's water content is measured after the test is finished, and by assuming that the sample is fully saturated, the void ratio is back-calculated from the final void ratio and volume change. This second method was used in this study because of its reliability and accuracy. 4. Compression behavior 4.1. Isotropic compression Table 5 shows the details of the isotropic compression loading and unloading tests. To produce specimens with various initial densities, three different compaction efforts and different amounts of soil were used in each layer. Fig. 5 depicts the isotropic compression responses of both the loading and unloading paths. The number after the soil type represents the compaction ratio. For example, in the sample symbolized by (B0-85), B0 means that the soil was natural soil and 85 denotes that the sample was compacted to 85% from the maximum dry density. The ICLs of naturally graded soil (B0) merge in a unique compression line, which represents a state boundary of the soil regardless of the initial density and void ratio. However, the other two reconstituted soils prepared by increasing the sand content exhibited non-unique compression lines. The soils showed high compressibility via isotropic compression with an indistinct yield point, as the high plastic fines content steepened the initial compression lines. The high compressibility during the loading path can be attributed to the well-graded nature and high fines content (Yan and Li, 2012). The total fines content was 61.1%, including 24.8% clays (see Table 2), and the XRD analysis showed that the soil content of the clay minerals was high, 41.75% (see Table 1). These results can also be attributed to the loose structure induced by the moist tamping technique used for sample preparation (Ng et al., 2004). The microstructure of the natural soil was 245
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Fig. 5. Isotropic compression loading and unloading paths; a) Natural soil B0, b) reconstituted soil B5, and c) reconstituted soil B10.
Fig. 6. SEM photos for clay minerals of the natural soil; a) irregular flake and page shapes of kaolinite, and b) leaf-shape of illite.
However, it would be interesting to investigate the behavior of the reconstituted soils under high stress levels to check whether the soils would have a unique void ratio–effective stress relation and reach unique ICL. The stiff response increased as the initial density increased based on the compaction effort. However, the initial density affected only the initial compression response, and its influence did not extend to the
line. Increasing the sand content significantly decreased the degree of convergence. The value of m increased from 0 in the case of naturally graded soil (B0) to 0.3 for reconstituted soil when the sand content increased by 5% (B5) and to 0.73 for reconstituted soil when the sand content increased by 10% (B10). The consolidation stresses were selected to simulate the loading conditions of the fill slopes in southeast China, and the available triaxial device is limited to low stress levels. 246
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strain was greater than the axial strain for a compaction ratio of 85% (i.e., B10-85), and the axial and radial strains were almost identical for a compaction ratio of 87.5% (i.e., B10-87.5). For reconstituted soils prepared by increasing the sand content by 10%, the axial strain increased gradually as the compaction effort increased to exceed the radial strain, as depicted in Fig. 8. The stress-strain curves show that the axial and radial strains were parallel during the loading and unloading paths of the isotropic compression tests, which implies that the soil behavior was isotropic. The relationship between axial and radial strains in this context was plotted in Fig. 9 to shed more light on the soils' volumetric response. The gradient of the axial and radial strain relationship for the CDG soils used in this study emphasizes the soils' isotropic behavior, as the slope of the axial–radial strain relationship almost equals 1:1.
normal compression line in the natural soil B0, as indicated in Fig. 5. XRD analysis showed that quartz was the dominant mineral (58.25%) in the natural soil (B0) (see Table 1). Because the sand is present in a form with high quartz content, any increase in sand content changes the composition of soil minerals, and the induced fabric differs from the fabric of the natural soil regardless of a slight change in the soil grading (see grading curves). Because the soil grading changes slightly but is still classified as a well-graded soil, the transitional mode is not limited to gap-graded soil, which indicates that not only is grading the responsible for the transitional mode but also that the soil fabric is the paramount factor governing this behavior. Although the re-compacted CDG soils reported by Ng et al. (2004) and Yan and Li (2012) exhibited unique ICLs under low stress levels, the intact CDG soil studied by Wang and Yan (2006) showed parallel ICLs in the same stress levels. These observations closely match the conclusions of Xu et al. (2018), because the transitional mode is more pronounced in intact soils than in reconstituted soils, which indicates that fabric is a powerful factor in transitional behavior (Baudet and Stallebrass, 2004). Nocilla et al. (2006) attributed the lack of convergence of the compression lines of soil samples in different initial states to the inability of the compression path to erase the differences in their initial fabrics to reach a unique relationship between the void ratio and effective stress. Xiao et al. (2016) also attributed the non-uniqueness of CSLs to the robust fabric, where the shearing path was not sufficient to destroy it. Under the same conditions as the compression test, the compression lines merged into a unique line in naturally graded soil, whereas the reconstituted soils exhibited non-unique compression lines, and as the sand content increased, the degree of non-convergence increased. In other words, the transitional mode became more noticeable as the sand content increased. It can therefore be concluded that the fabric induced by increasing the sand content of the reconstituted soils was more robust than that in naturally graded soil and that the compression path was not sufficient to erase the difference in the initial induced fabric, hence the lack of convergence of the compression lines. Nocilla et al. (2006) showed that it is not possible to identify visually the differences via SEM photos due to the complexity of the soil fabric. Thus, it would be interesting to use an industrial computed tomographic (CT) scanner coupled with a triaxial device to scan soil samples horizontally and vertically during the isotropic compression at different mean effective stresses to monitor the fabric of the soil sample step by step.
5. Critical state The transitional mode is generally characterized by the non-uniqueness of ICLs and/or CSLs in the e-log p' space. The two conditions can be achieved together; nevertheless, despite the non-uniqueness of ICLs, a unique CSL can be also found. Wang and Yan (2006) found nonunique ICLs for intact CDG soils; however, a unique CSL has been achieved regardless of the initial states (i.e., initial density and void ratio). A unique CSL in the e-log p' space was robustly present in both the natural and reconstituted soils regardless of the drainage conditions and initial states, as indicated in Fig. 10. The uniqueness of CSLs is considered a common feature of CDG soils, as CSLs are considered to be an intrinsic feature independent of the initial density, drainage condition, and stress path (Fung, 2001; Lee and Coop, 1995; Ng and Chiu, 2003; Ng et al., 2004; Wang and Yan, 2006; Yan, 2003; Yan and Li, 2012). Although the reconstituted soil prepared by increasing sand content revealed non-unique ICLs, a unique CSL was present. The same response was also reported by Altuhafi et al. (2010), as non-unique ICLs were identified; however, a unique CSL was found in both the e-log p' and q-p' spaces. Altuhafi et al. (2010) suggested that the fabric of the soil that caused the lack of convergence of ICLs could be completely destroyed during shearing. Consequently, the unique CSL was attributed to a complete destruction of the induced fabric from the isotropic compression path during large strain-controlled shearing paths, which eventually led to a unique fabric at the critical state. In other words, the fabric responsible for non-unique ICLs was not sufficiently robust to withstand large strain-controlled shearing and was completely destroyed to reach a unique fabric in the critical state. It would be interesting to conduct a shearing path coupled with a CT scanning device to scan the soil sample vertically and horizontally at different axial strains to monitor the fabric evolution until the critical state is reached. Fig. 11 depicts the critical states in the deviator and the effective mean stress q-p' space. The critical states obviously lie on a unique line regardless of the initial state and drainage conditions. As expected, the reconstituted soil prepared by increasing the sand content by 5% (B5) exhibited the highest CSL gradient, followed by B10 and then the naturally graded soil (B0), where the gradients of CSL M equal 1.26, 1.21, and 1.03, respectively. The corresponding friction angles were 31.4°, 30.2°, and 26.1°, respectively. The trend of CSL gradients can be illustrated by the trend of the maximum dry density, as the maximum dry density obtained from the standard Proctor compaction test is considered an indicator of soil strength parameters.
4.2. Volumetric change The triaxial GDS apparatus was equipped with an external linear variable displacement transducer to measure the axial displacement, and the samples' volume change was monitored using the back-pressure transducer. To identify whether the soil's behavior is isotropic or anisotropic, the gradient of the relationship between the axial and volumetric strains must be 3:1 or 1:1 in the axial and radial strain relationship. In this context, the deformation of soil samples was accurately assessed during the isotropic compression loading and unloading paths. The axial and radial strains were plotted against the effective mean stress, as indicated in Fig. 7, in which a 90% compaction ratio was used to represent other compaction ratios. The radial strain was clearly higher than the axial strain for naturally graded soil (B0) and reconstituted soil (B5). This difference could be explained by the preparation methods; the samples were prepared with the moist tamping method, and when the soil sample was compacted in one direction, that direction became stiffer. Thus, its contractive response became very low, whereas the behavior became softer and more contractive when the sample was loaded in parallel to the compaction planes (Ham et al., 2012). The same behavior was observed in both intact and re-compacted CDG soils (Wang and Yan, 2006; Yan and Li, 2012). The reconstituted soil sample B10-90 revealed a reverse trend in which the axial strain exceeded the radial strain; however, the radial
6. Conclusions A series of isotropic compression tests and drained and undrained triaxial compression tests were implemented on CDG soil to investigate its behavior and the transitional mode. ICLs were found to merge into a unique line for the naturally graded soil. However, the reconstituted soils prepared by increasing the sand content by 5% and 10% revealed non-unique compression lines within the stress levels used. Soil 247
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Fig. 7. Axial and radial strains with confining stress via isotropic compression; a) natural soil B0, b) reconstituted soil B5, and c) reconstituted soil B10.
Fig. 8. Axial and radial strains versus the confining stress for reconstituted soil B10; a) compaction ratio 85%, and b) compaction ratio 87.5%.
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Fig. 9. Axial and radial strains during the isotropic compression path.
Fig. 11. Critical states in the deviator and effective mean stresses space q-p'.
mineralogy and morphology had significant effects on the compressive response. Compression lines depend greatly on the initial state for the reconstituted soils. The degree of convergence of the ICLs that was quantified by parameter m did not equal 1, and the ICLs were not quite parallel and tended to merge into a unique line at high stress levels. When the sand content in the reconstituted soils was increased, the
degree of ICLs convergence decreased, and the value of m parameter increased. The stress level required to reach a unique compression line increased as the additional amount of sand increased. The fabric induced by increasing the sand content could persist against the compression path used in this study, but it was not sufficiently robust to withstand the large strains of the shearing stage and was completely destroyed. The reconstituted soils thus revealed unique fabrics at the
Fig. 10. Critical states and compression lines in the e-log p' space; a) natural soil B0, b) reconstituted soil B5, and c) reconstituted soil B10. 249
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critical state that had unique CSLs regardless of the initial state. Natural and reconstituted CDG soils revealed isotropic behavior during the loading and unloading compression paths. The volumetric change could not be recovered in either the reconstituted or naturally graded soils. The stiff response increased as the initial density increased according to the compaction effort for reconstituted soils. However, the initial density affected only the initial compression response and did not extend to the normal compression line for naturally graded soil. In the case of reconstituted soils, the fabrics that resulted from varying degrees of compaction effort had a significant effect on compression behavior and resulted in non-unique compression lines. Increasing the sand content leads to quantitative changes in the soil's mineral composition, so it can be a cause of transitional behavior in addition to the induced fabric. It can be concluded that transitional behavior is no longer limited to gap-graded soils; grading is no longer the only governing factor in this mode, and the transitional mode may occur due to relatively inconsiderable changes in the soil minerals, grading, and fabric. Further research is necessary to identify the role of the CDG soils' fabric during both the isotropic consolidation and shearing paths via the industrial X-ray CT scanning to monitor the fabric of the soil sample step by step to clarify the fabric's impact on the lack of convergence of the ICLs and CSLs. A new applicable framework should be developed to describe soils that exhibit transitional behavior because the critical state framework is not applicable to these soils. New vocabulary is urgently needed, as idioms such as “normal compression line” no longer have a clear meaning and no longer represent a state boundary surface, and the “critical state” term no longer has a clear meaning for soils that exhibit non-unique CSL. The “state parameter” concept also no longer has a clear meaning.
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Acknowledgments This study was supported by funding from the National Natural Science Foundation of China (NSFC) (Nos. 416772267 and 51508216). The authors gratefully acknowledge the editor and reviewers for their interest in the paper. Their detailed and constructive comments helped in the improvement of the paper. References 1:200,000 Geological Map F4915 (Guangdong Province) Data. Retrieved from. http:// www.geodata.ngac.cn. Alavi Nezhad Khalil Abad, S.V., Tugrul, A., Gokceoglu, C., Jahed Armaghani, D., 2016. Characteristics of weathering zones of granitic rocks in Malaysia for geotechnical engineering design. Eng. Geol. 200, 94–103. Altuhafi, F.N., Coop, M.R., 2011. Changes to particle characteristics associated with the compression of sands. Géotechnique 61 (6), 459–471. Altuhafi, F., Baudet, B.A., Sammonds, P., 2010. The mechanics of subglacial sediment: an example of new “transitional” behaviour. Can. Geotech. J. 47 (7), 775–790. Baudet, B., Stallebrass, S., 2004. A constitutive model for structured clays. Géotechnique 54 (4), 269–278. Been, K., Jefferies, M., 1985. A state parameter for sands. Geotechnique 35 (2), 99–112. Chiu, C.F., Ng, C.W.W., 2014. Relationships between chemical weathering indices and
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Transitional behavior in well-graded soils: An example of completely decomposed granite Elsayed Elkamhawya,b, Bo Zhoua, Huabin Wanga, a b
T
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School of Civil Engineering and Mechanics, Huazhong University of Science and Technology, Wuhan 430074, PR China Faculty of Engineering, Zagazig University, Zagazig 44519, Egypt
A R T I C LE I N FO
A B S T R A C T
Keywords: Completely decomposed granite Transitional behavior Uniqueness Convergence
This study presents the results of a series of isotropic compression tests and drained and undrained triaxial shearing tests on naturally graded and reconstituted completely decomposed granite soils to investigate their compressive response and transitional behavior. Naturally graded soils exhibited unique isotropic compression and critical state lines within the stress levels studied. Soils reconstituted by increasing the sand content by 5% and 10% revealed non-unique isotropic compression lines; however, unique critical state lines were present in the e-log p' space. The degree of non-convergence increased as the sand content increased. The value of the m parameter, which is used to quantify the transitional mode and is defined as the ratio between the changes in the final and initial specific volumes (i.e., m = Δv2/Δv1; see Fig. 1), increased from 0 in the case of naturally graded soil to 0.3 and 0.73 for reconstituted soils with increased sand content of 5% and 10%, respectively. For naturally graded soils, the initial void ratio affects only the initial compression response, and its influence does not extend to the normal compression line. In the case of reconstituted soils, the fabric that results from various compaction efforts exerts a significant effect on the soil′s compressive response. However, this fabric cannot withstand large strain during the shearing path and is completely destroyed, producing a unique fabric in the critical state. Both natural and reconstituted soils revealed isotropic responses during the loading and unloading compression paths with unrecoverable volume change. The transitional behavior is no longer limited to gap-graded soils, and grading is no longer the only dominant factor in this behavior. The transitional mode occurs due to relatively inconsiderable changes in soil mineralogy, grading, and microstructure.
1. Introduction Completely decomposed granite (CDG) soils are abundant in southeastern China, and they are used extensively as fill materials in various engineering applications, such as back-filling materials for retaining structures, levees, roads, and embankments. As China develops rapidly, it is urgent to study and understand the behavior of these soils. Many studies have been conducted on CDG, including coarse and finegraded soils, well-graded soils that are classified as well-graded sand according to the unified soil classification system (Lee and Coop, 1995), and the well-graded soils SC-CL (Yan and Li, 2012) and SC-CH (Ng et al., 2004) (SC is clayey sand, and CL and CH are low and high plasticity clay, respectively). The main feature of the previously mentioned CDG soils is that the isotropic compression lines (ICLs) and critical state lines (CSLs) merge into unique lines. This means that a unique void ratio–effective stress state relationship exists with normally consolidated soils and that the soils obey Rendulic's principle, in which
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the state boundary surfaces of various initial void ratios (i.e., various densities) could be unique. Soil should be elastic when its state is below the state boundary surface and plastic after reaching the yield once the state boundary surface has been reached. A critical state framework is applicable in cases in which the soil has a unique CSL. However, the intact CDG soils studied by Wang and Yan (2006) exhibited non-unique compression lines, implying that compression lines depend robustly on the initial densities; this behavior is termed “transitional behavior.” Been and Jefferies (1985) found that the behavior of clean sands can be described in a manner similar to that of clayey soils within the critical state framework because both compression lines and CSLs merge into unique lines. Because clays and clean sands define unique compression lines and are considered the extremes of the whole range of soils, a transitional mode in the soils' compression behavior must exist between these extremes (Martins et al., 2001). Nocilla et al. (2006) defined transitional behavior as the behavior between clays and clean sands. The transitional mode is particularly characterized by the non-
Corresponding author. E-mail address: [email protected] (H. Wang).
https://doi.org/10.1016/j.enggeo.2019.02.027 Received 17 September 2018; Received in revised form 22 February 2019; Accepted 26 February 2019 Available online 28 February 2019 0013-7952/ © 2019 Elsevier B.V. All rights reserved.
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m parameter is calculated from the initial and final specific volumes v1 and v2, respectively. The parameter m can be defined as the ratio of the changes of the final and initial specific volumes (i.e., m = Δv2/Δv1), as shown in Fig. 1b. The parameter P can be also calculated from the initial specific volumes and the intersect of the CSLs with the specific volume axis at log P′ = 1 (Γ), as indicated in Fig. 1d (i.e., P = ΔΓ/Δv1). The soil behavior is considered to be completely transitional when m and/or P equal 1, as the ICLs are in parallel and the CSLs also exhibit parallelism. In the case of unique ICLs and CSLs, the values of m and P equal 0. Values between 0 and 1 represent various degrees of convergence from uniqueness to parallelism. As mentioned above, the transitional mode was observed in soils with fine to coarse grading, and several factors dominate this mode. Plastic fine content was found to be a paramount factor (Nocilla et al., 2006; Shipton and Coop, 2015), and grading and mineralogy are also key factors (Shipton and Coop, 2012). The sample reconstitution method, over-consolidation ratio, and stress levels have no influence on this mode (Martins et al., 2001; Shipton and Coop, 2015). Xu and Coop (2017) showed that the transitional mode was more pronounced in intact soils than in reconstituted soils. For CDG saprolite soils, disturbance and compression paths were considered to be substantial factors, as Wang and Yan (2006) found nonconvergence of ICLs in intact samples. Yan and Li (2012) found unique ICLs for recompacted soil samples. They also conducted conducted anisotropic compression paths with various stress ratios and found that the compression lines merged into a unique line in samples compressed at the same stress ratio, whereas the samples that were compressed to different stress ratios showed nearly parallel compression lines. In contrast, CDG soils exhibited unique CSLs regardless of whether they were disturbed or intact, which indicates that the initial fabric was completely destroyed throughout large strains during shearing, producing a unique fabric at the critical state. Because of the abundance of CDG soils in China and their applications, there is an urgent need to survey and understand their
uniqueness of normal compression lines and/or CSLs (Altuhafi et al., 2010; Ferreira and Bica, 2006; Nocilla et al., 2006; Ponzoni et al., 2014). This mode of soil behavior cannot be described within a critical state framework, and as such, these soils do not follow Rendulic's principle. Many studies have shown that transitional behavior is more common than previously thought; it had been limited to gap-graded soils (Ferreira and Bica, 2006; Martins et al., 2001). Transitional behavior has also been observed in well-graded silty clayey soils (Nocilla et al., 2006; Ponzoni et al., 2014; Xu and Coop, 2017), well-graded sands (Altuhafi et al., 2010; Altuhafi and Coop, 2011), well-graded coarse granular soils (Xiao et al., 2016), and gap-graded soils (Ferreira and Bica, 2006; Martins et al., 2001; Shipton and Coop, 2015). Non-unique compression lines have been observed in both odometer and isotropic compression tests (Altuhafi et al., 2010; Altuhafi and Coop, 2011; Ferreira and Bica, 2006; Martins et al., 2001; Nocilla et al., 2006; Ponzoni et al., 2014; Shipton and Coop, 2012; Shipton and Coop, 2015; Wang and Yan, 2006; Xiao et al., 2016; Xu and Coop, 2017). Although the compression lines do not merge into a unique line, Altuhafi et al. (2010) clearly observed a unique CSL; Wang and Yan (2006) found the same response in the case of intact CDG, whereas nonunique CSLs were observed in other studies (Ferreira and Bica, 2006; Nocilla et al., 2006; Ponzoni et al., 2014; Shipton and Coop, 2015). Two explanations have been given for both the uniqueness and non-uniqueness of CSLs. First, for unique CSL, the fabric responsible for nonunique ICLs is weak and is completely destroyed in the shearing stage, producing a unique fabric at the critical state. However, in many cases, the fabric responsible for non-unique ICLs is extremely sturdy and resists damage to an extent that allows its effects to persist even at the critical state, which results in the non-uniqueness of CSLs (Baudet and Stallebrass, 2004; Ferreira and Bica, 2006; Shipton and Coop, 2015). Ponzoni et al. (2014) proposed two parameters m and P to quantify the transitional mode via the quantification of the degree of non-convergence for normal compression lines and CSLs. Fig. 1 shows how the
Fig. 1. Quantification of the convergence of the ICLs and CSLs: a) schematic illustration of ICLs; b) calculation of parameter m; c) schematic illustration of CSLs; d) calculation of parameter P (Ponzoni et al., 2014). 241
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Fig. 2. Site of the soil used in this study.
Guangdong province, in southeastern China. CDG soils are reddish brown with some white spots, as shown in Fig. 2. Core stones are not found at the soil site, which implies that the parent rock has been completely decomposed (Alavi Nezhad Khalil Abad et al., 2016). The parent granitic rock of the soils is monzonite granite (retrieved from www.geodata.ngac.cn). The decomposed granite soil is classified as grade V based on the Geotechnical Engineering Office classification system of decomposed rocks (GEO, 2017). The x-ray diffraction (XRD) was used to analyze the soil composition and showed large quantities of clay minerals (e.g., kaolinite, montmorillonite, and illite) and quartz. Table 1 shows a statistical summary of the percentages of soil minerals. The mean value of quartz is obviously the maximum value, probably because quartz is the most stable mineral and is inert to chemical weathering; thus, it was observed extensively in all soil samples. Kaolinite was the most abundant clay mineral, at ratios of 33% to 43%; this can be attributed to the intensive weathering environment and the unimpeded drainage conditions, such as well-drained hill slopes, joints, and fractures in the parent rock. Other clay minerals such as montmorillonite and illite were observed in small amounts, possibly because of the removal of limited amounts of cations in a stagnant condition near the bottom of the slope (Irfan, 1996; Ng et al., 2001). XRD analysis also showed no trace of feldspar in the CDG soil samples, as feldspar is considered to be the most vulnerable mineral to chemical weathering and eventually forms clay minerals (Irfan, 1996; Zauyah et al., 2010).
Table 1 Statistical analysis of minerals for CDG soil. Soil minerals Montmorillonite Illite Kaolinite Quartz
Mean (%) 1 2.5 38.25 58.25
Minimum (%) 2 3 33 50
Maximum (%) 2 7 43 67
Range 0 4 10 17
mechanical characteristics and investigate the transitional mode. The presence of a transitional mode in soil behavior requires a reconsideration of terms such as the unique compression line, the unique critical state line, and the state parameter proposed by Been and Jefferies (1985). Thus, a new framework is needed to describe soils that show a transitional mode. The essential objective of this study is to illuminate the compressive response and transitional mode of CDG soils that originate from the decomposition of monzonitic granite rock via a series of isotropic compression tests and drained and undrained shearing tests —in particular, to investigate the ICL patterns and quantify the degree of transitional behavior with the parameter m proposed by Ponzoni et al. (2014). 2. Test materials 2.1. Mineralogical features The CDG soils used in this study were obtained from a site in Bozhi, 242
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additional amount of sand fraction was added from the sand retained on each sieve, as shown in Fig. 4. The reconstituted soil B10 was prepared in the same manner. According to the unified soil classification system, the soil can be classified as high plasticity clayey sand (SC-CH). Table 4 summarizes the main physical properties of both the natural and reconstituted soils. The specific gravity (G.s) was found to be 2.67 for naturally graded soil. The liquid and plastic limits for naturally graded soil were 55.84% and 27.85%, respectively, with (1.51 g/cm3) maximum dry density corresponding to the optimum moisture content (25.25%). The maximum dry densities of the reconstituted soils were 1.572 g/cm3 and 1.564 g/ cm3 after the sand content was increased by 5% and 10%, respectively. The maximum dry density increased when the sand content was increased by 5% and then decreased when the sand content was increased by 10%; this can be explained by the binary-mixed soil overall void ratio model proposed by Lade et al. (1998). Where, the overall void ratio decreased as the sand content increased by 5%, and an increase by 10% led to an increase in the overall void ratio; thus, the maximum dry density increased and then decreased again. The trend of the maximum dry density shows that the total sand content obtained by adding 5% to the sand content represents a threshold. Ham et al. (2010) observed the same trend between the maximum dry density and gravel contents. As expected, the optimum moisture content decreased as the sand content increased.
Fig. 3. Gradation curves.
2.2. Physical characteristics Fig. 3 shows the gradation curves of the CDG soils and other decomposed granite soils from Hong Kong and South Korea, and Table 2 summarizes the main features of these soils. Compared with decomposed granite soils from Hong Kong (Ng et al., 2004; Wang and Yan, 2006; Yan and Li, 2012) and South Korea (Lee and Coop, 1995), the studied soils are well graded, and the CDG soils are typically characterized as well graded (Chiu and Ng, 2014; Rocchi and Coop, 2016). The soils in this study are predominantly fine well graded, like those that studied by Wang and Yan (2006). High fines content can be imputed to the severe circumstances of the weathering environment and/ or to the relatively greater vulnerability of the soil to chemical weathering (Lee and Coop, 1995; Ng et al., 2004; Yan and Li, 2012). Although all of the soils reported in Fig. 3 are CDG soils, the grading varies substantially, perhaps due to differences in mineral composition resulting from different weathering environments and the structure of the parent rock. The reconstituted soils were prepared by increasing the sand content by 5% (B5) and 10% (B10); thus, the whole sand content reached 36.9% and 41.9%, respectively. Due to increasing sand content, the gravel and fines content decreased, and their new ratios were calculated based on the predetermined ratio of sand (i.e., 36.9% and 41.9%). For example, for the reconstituted samples B5, the natural grading of sand fractions were 16.2%, 4.7%, and 11% retained by 0.5-mm, 0.25-mm, and 0.075-mm sieves, respectively; the reconstituted grading of sand fractions were then calculated as 18.74%, 5.44%, and 12.72%. Then, 1 kg of the oven-dried natural soil was weighted, and the required additional weight of sand to reach the 36.9% was then calculated and found to be 79.23 g. This additional 79.23 g was then added with the pre-calculated new ratios (i.e., 40.24, 11.67, and 27.32 g were added from the sand retained by the 0.5-mm, 0.25-mm, and 0.075-mm sieves, respectively), as indicated in Table 3. It is worth noting that the
3. Specimen preparation and test program The tests in this study were performed with an automated stress path triaxial testing system. The triaxial device was manufactured by Global Digital Systems (GDS) Ltd. in the UK. The GDS triaxial testing system (GDSTTS) is a fully automated triaxial testing system designed principally for stress path testing. The system is based on the classic Bishop and Wesley stress path triaxial cell, which controls stress on the sample directly. The triaxial apparatus was equipped with three controllers (cell, back pressure, and axial stress and axial displacement controllers) in addition to an external linear variable displacement transducer to measure the axial strain. Back pressure was applied from the top cap, the pore water pressure was measured from the base of the specimen, and the volume change was measured via the back-pressure controller. The triaxial specimens were prepared with the moist tamping method. The specimens were compacted inside a split mold at each soil's optimum moisture content in three layers, with scratching between the adjacent layers to limit the effects of layering. Various densities were obtained by controlling the weight of the soil in each layer and the applied compaction effort. After compaction, the mold was dismantled, and the specimen was encapsulated with a rubber membrane. The soil samples have to be saturated to avoid the complication of pore air pressure that differs from the pore water pressure and the accompanying measurement difficulties. To improve the degree of saturation, the soil specimens were submerged in a water tank subjected to vacuum pressure for 24 h. The sample was installed in the GDS device and the back and cell pressures were then increased simultaneously to maintain constant effective mean stress throughout the saturation process. A saturation check was then conducted to inspect the degree of saturation using Skempton's B-value (Skempton, 1954), and in all tests
Table 2 Main features of the natural CDG studied soil and other reported soils. Study Soil in this study Yan and Li (2012) Wang and Yan (2006) Ng et al. (2004) Lee and Coop (1995)
Mean dia. (D50), mm 0.025 0.30 0.01 0.70 2.0
Fine %
silt%
Clay%
Soil classification based on USCS
61.1 35 65 25 5
36.3 25% 40% 10% _
24.8 10% 25% 15% _
SC-CH SC-CL SC-CH SC-CH SW
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Max. dry density (g/ cm3) 1.51 1.71 _ 1.65 1.794
Slope of ICL(ξ)
Slope of CSL (λ) in e-log p'
0.1 0.091 0.076 0.13 0.087
0.083 0.066 0. 072 0.093 0.087
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Table 3 Soil content of the natural and the reconstituted soils and the additional amounts of sand. Soil ID B0
B5
B10
Dia. (mm) Soil fraction % of Pass % of soil contents B0 Sand sizes (mm) % of sand % of Pass % of soil contents B5 Sand sizes (mm) % of sand Amount of additional sand/1 kg = 79.23 g % of pass % of soil contents B10 Sand sizes (mm) % of sand Amount of additional sand/1 kg = 135 g
10.0 Gravel 100 7% – – 100 6.5% – – – 100 5.97% – – –
5.0
2.0 Sand 93.0 31.9% 2–0.5 16.2 93.5 36.9% 2–0.5 18.74 40.24 94.03 41.9% 2–0.5 21.28 68.55
99.5
99.5
99.5
0.5
0.25
76.8
72.1
0.5–0.25 4.7 74.75
0.25–0.075 11 69.32
0.5–0.25 5.44 11.67 72.75
0.25–0.075 12.72 27.32 66.58
0.5–0.25 6.17 19.90
0.25–0.075 14.45 46.55
0.075 Fines 61.1 61.% – – 56.6 56.6% – – – 52.13 52.13% – – –
0.005
0.002
33.2
24.8
30.75
22.97
28.32
21.16
Note: B refers to site name Bozhi; 0, 5 and 10 after B refer to the additional amount of sands.
Fig. 4. Separated soil content. Table 4 Physical characteristics of the studied CDG soils. Soil ID B0 B5 B10
G.S 2.67 2.65 2.64
P.L (%) 27.85 – –
L.L (%) 55.84 – –
γdry max (g/cm3) 1.51 1.572 1.564
P·I (%) 27.99 – –
Optimum W.C (%) 25.25 23.5 22.4
Soil classification SC-CH – –
Note: B refers to the site name Bozhi; 0, 5 and 10 after B refers to the additional amount of sand.
the B-values were higher than 0.95. The loading and unloading rates during isotropic compression were 10 kPa/h and 20 kPa/h, respectively. No noticeable excess pore water pressure was triggered by these rates. The drained and undrained shearing tests were conducted in a strain-controlled mode using axial strain rates of 0.02%/min and 0.05%/min, respectively, following Head (1992). The isotropic compression tests and the drained and undrained shearing tests were performed on normally consolidated specimens, as summarized in Tables 5, 6 and 7. Many different methods are used to calculate the void ratios of soil samples, and the two methods discussed in detail by Verdugo and Ishihara (1996) are the most common. In the first method, the specimen's initial dimensions are measured before the test begins to calculate the specimen's initial void ratio, and the void
Table 5 Experimental details for the isotropic compression test. Test ID
Compaction ratio
Load cycle, P′ kPa
B0
85% 87.5% 90% 85% 87.5% 90% 85% 87.5% 90%
20–600 20–250–150–450 20–200–50–400–50 20–600 20–250–150–450 20–200–50–400–50 20–400 20–250–150–450 20–200–50–400–75
B5
B10
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Initial void ratio 0.982 0.935 0.916 0.915 0.855 0.768 0.877 0.848 0.817
Final void ratio 0.686 0.709 0.741 0.641 0.644 0.647 0.665 0.634 0.653
Slope of ICL(ξ) 0.1
–
–
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determined using a scanning electron microscope (SEM). The SEM photos shown in Fig. 6 indicate that kaolinite and illite minerals were present in flake and leaf shapes. These shapes are characterized by high deformability and significantly affect soil compressibility even if existed in small quantities (Lee et al., 2007; Madhusudhan and Baudet, 2014). However, the volumetric change during the unloading path was almost insignificant, which indicates that the volume change was unrecoverable. The slope of the ICL (ξ) of the naturally graded soil was 0.1, and those of other CDG soils were ξ = 0.091 in Yan and Li (2012) and ξ = 0.087 in Lee and Coop (1995). The high compressibility of the naturally graded soil can be clearly seen compared with other CDG soils. Although the fines content in the studied soils was high, the slope of the compression line was lower than that in the CDG soils reported by Ng et al. (2004), with ξ = 0.13. This difference can be explained by the presence of crushable feldspar in the soil studied by Ng et al. (2004). Wang and Yan (2006) studied intact CDG soil, which showed parallel compression lines with ξ = 0.076 even though its grading was finer than those of the soils in this study; this can be illustrated by the fabric and microstructure of the parent rock in the intact soil samples. Particle rearrangement and particle crushing are the two mechanisms responsible for volume changes. Yamamuro and Lade (1997) concluded that the mechanism of volume change is due to particle rearrangement under low consolidation stress levels and that particle crushing is the dominant mechanism during high consolidation stress levels. However, both mechanisms can occur simultaneously under low stress levels for CDG soil in case of presence weak minerals in a soil composition, such as crushable feldspar (Ng et al., 2004). Yan and Li (2012) concluded that the volumetric change was due to particle rearrangement because the consolidation stress level was not high (400 kPa) and the soil grading did not change noticeably after the isotropic compression test. Lee and Coop (1995) included crushable feldspar in their soil and used a high stress level (10,000 kPa), and they attributed the mechanism of volume change to particle breakage. The XRD analysis of the studied soil showed no crushable feldspar in the soil minerals, the stress levels were not high (450 kPa), and the fines content of the studied soil was much greater than that in the soil reported by Yan and Li (2012), and clay particles serve as a pillow for sand grains, cushioning them from stresses (Nocilla et al., 2006). Consequently, the mechanism of volume change can be attributed to particle rearrangement rather than particle breakage. As mentioned before, the high fines content, particularly of plastic fines, led to a steepening of the initial compression path without a distinct yield stress. This also emphasizes that particle rearrangement is the dominant mechanism governing the volume change. The compression curves in Fig. 5 reveal that transitional behavior is present in the CDG soils. Although the ICLs of naturally graded soil merged in a unique compression line, the reconstituted soils prepared by increasing the sand content exhibited non-unique compression lines under the stress levels used in this study. An identical response was reported in Italian silt by Nocilla et al. (2006): the reconstituted soil with 45% clay content revealed a unique compression line, and as the clay content decreased, the compression lines gradually diverged to become perfectly parallel at a clay content of 3.5%. The presence of a transitional mode in CDG soils, which was characterized by its wellgraded nature, confirmed that transitional behavior was much more prevalent and not limited to gap-graded soils (Ferreira and Bica, 2006; Martins et al., 2001). Ponzoni et al. (2014) proposed the m parameter to quantify the transitional mode during the one-dimensional compression test, as indicated in Fig. 1b. The changes in specific volume and void ratio are equal, Δv = Δe, for the same range of mean effective stress; thus, in this study, the m parameter was adopted in the isotropic compression tests in terms of the void ratio rather than the specific volume. Values of the parameter m range from 0 to 1: when m equals 1, soils exhibit a full transitional mode and the compression lines are parallel, whereas when m equals 0, soils exhibit a unique compression
Table 6 Experimental details for the drained triaxial compression shearing test. Test ID B0
B5
B10
Compaction ratio, % 89.58 89.74 87.31 88.43 86.93 87.20 85.76 89 89.10 86.07 87.56 86.85
Consolidated stress P′(kPa) 100 200 300 400 100 200 300 400 100 200 300 400
Void ratio before shearing 0.814 0.781 0.745 0.706 0.787 0.732 0.687 0.666 0.766 0.712 0.664 0.655
Final void ratio 0.757 0.698 0.663 0.627 0.684 0.641 0.602 0.592 0.665 0.627 0.583 0.567
Table 7 Experimental details for the undrained triaxial compression shearing test. Test ID B0
B5
B10
Compaction ratio, % 87.57 89.04 88.28 89.16 86.96 85.87 88.18 89.36 88.34 88.41 86.25 86.31
Consolidated stress P′(kPa) 100 200 300 400 100 200 300 400 100 200 300 400
Final void ratio 0.816 0.780 0.744 0.706 0.788 0.730 0.695 0.660 0.768 0.693 0.670 0.657
ratio during the test path is then calculated from the volume change measurements. The roughness of the specimen's surface may produce errors in the initial dimension measurements, so the initial void ratio calculation is not accurate. In the second method, the specimen's water content is measured after the test is finished, and by assuming that the sample is fully saturated, the void ratio is back-calculated from the final void ratio and volume change. This second method was used in this study because of its reliability and accuracy. 4. Compression behavior 4.1. Isotropic compression Table 5 shows the details of the isotropic compression loading and unloading tests. To produce specimens with various initial densities, three different compaction efforts and different amounts of soil were used in each layer. Fig. 5 depicts the isotropic compression responses of both the loading and unloading paths. The number after the soil type represents the compaction ratio. For example, in the sample symbolized by (B0-85), B0 means that the soil was natural soil and 85 denotes that the sample was compacted to 85% from the maximum dry density. The ICLs of naturally graded soil (B0) merge in a unique compression line, which represents a state boundary of the soil regardless of the initial density and void ratio. However, the other two reconstituted soils prepared by increasing the sand content exhibited non-unique compression lines. The soils showed high compressibility via isotropic compression with an indistinct yield point, as the high plastic fines content steepened the initial compression lines. The high compressibility during the loading path can be attributed to the well-graded nature and high fines content (Yan and Li, 2012). The total fines content was 61.1%, including 24.8% clays (see Table 2), and the XRD analysis showed that the soil content of the clay minerals was high, 41.75% (see Table 1). These results can also be attributed to the loose structure induced by the moist tamping technique used for sample preparation (Ng et al., 2004). The microstructure of the natural soil was 245
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Fig. 5. Isotropic compression loading and unloading paths; a) Natural soil B0, b) reconstituted soil B5, and c) reconstituted soil B10.
Fig. 6. SEM photos for clay minerals of the natural soil; a) irregular flake and page shapes of kaolinite, and b) leaf-shape of illite.
However, it would be interesting to investigate the behavior of the reconstituted soils under high stress levels to check whether the soils would have a unique void ratio–effective stress relation and reach unique ICL. The stiff response increased as the initial density increased based on the compaction effort. However, the initial density affected only the initial compression response, and its influence did not extend to the
line. Increasing the sand content significantly decreased the degree of convergence. The value of m increased from 0 in the case of naturally graded soil (B0) to 0.3 for reconstituted soil when the sand content increased by 5% (B5) and to 0.73 for reconstituted soil when the sand content increased by 10% (B10). The consolidation stresses were selected to simulate the loading conditions of the fill slopes in southeast China, and the available triaxial device is limited to low stress levels. 246
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strain was greater than the axial strain for a compaction ratio of 85% (i.e., B10-85), and the axial and radial strains were almost identical for a compaction ratio of 87.5% (i.e., B10-87.5). For reconstituted soils prepared by increasing the sand content by 10%, the axial strain increased gradually as the compaction effort increased to exceed the radial strain, as depicted in Fig. 8. The stress-strain curves show that the axial and radial strains were parallel during the loading and unloading paths of the isotropic compression tests, which implies that the soil behavior was isotropic. The relationship between axial and radial strains in this context was plotted in Fig. 9 to shed more light on the soils' volumetric response. The gradient of the axial and radial strain relationship for the CDG soils used in this study emphasizes the soils' isotropic behavior, as the slope of the axial–radial strain relationship almost equals 1:1.
normal compression line in the natural soil B0, as indicated in Fig. 5. XRD analysis showed that quartz was the dominant mineral (58.25%) in the natural soil (B0) (see Table 1). Because the sand is present in a form with high quartz content, any increase in sand content changes the composition of soil minerals, and the induced fabric differs from the fabric of the natural soil regardless of a slight change in the soil grading (see grading curves). Because the soil grading changes slightly but is still classified as a well-graded soil, the transitional mode is not limited to gap-graded soil, which indicates that not only is grading the responsible for the transitional mode but also that the soil fabric is the paramount factor governing this behavior. Although the re-compacted CDG soils reported by Ng et al. (2004) and Yan and Li (2012) exhibited unique ICLs under low stress levels, the intact CDG soil studied by Wang and Yan (2006) showed parallel ICLs in the same stress levels. These observations closely match the conclusions of Xu et al. (2018), because the transitional mode is more pronounced in intact soils than in reconstituted soils, which indicates that fabric is a powerful factor in transitional behavior (Baudet and Stallebrass, 2004). Nocilla et al. (2006) attributed the lack of convergence of the compression lines of soil samples in different initial states to the inability of the compression path to erase the differences in their initial fabrics to reach a unique relationship between the void ratio and effective stress. Xiao et al. (2016) also attributed the non-uniqueness of CSLs to the robust fabric, where the shearing path was not sufficient to destroy it. Under the same conditions as the compression test, the compression lines merged into a unique line in naturally graded soil, whereas the reconstituted soils exhibited non-unique compression lines, and as the sand content increased, the degree of non-convergence increased. In other words, the transitional mode became more noticeable as the sand content increased. It can therefore be concluded that the fabric induced by increasing the sand content of the reconstituted soils was more robust than that in naturally graded soil and that the compression path was not sufficient to erase the difference in the initial induced fabric, hence the lack of convergence of the compression lines. Nocilla et al. (2006) showed that it is not possible to identify visually the differences via SEM photos due to the complexity of the soil fabric. Thus, it would be interesting to use an industrial computed tomographic (CT) scanner coupled with a triaxial device to scan soil samples horizontally and vertically during the isotropic compression at different mean effective stresses to monitor the fabric of the soil sample step by step.
5. Critical state The transitional mode is generally characterized by the non-uniqueness of ICLs and/or CSLs in the e-log p' space. The two conditions can be achieved together; nevertheless, despite the non-uniqueness of ICLs, a unique CSL can be also found. Wang and Yan (2006) found nonunique ICLs for intact CDG soils; however, a unique CSL has been achieved regardless of the initial states (i.e., initial density and void ratio). A unique CSL in the e-log p' space was robustly present in both the natural and reconstituted soils regardless of the drainage conditions and initial states, as indicated in Fig. 10. The uniqueness of CSLs is considered a common feature of CDG soils, as CSLs are considered to be an intrinsic feature independent of the initial density, drainage condition, and stress path (Fung, 2001; Lee and Coop, 1995; Ng and Chiu, 2003; Ng et al., 2004; Wang and Yan, 2006; Yan, 2003; Yan and Li, 2012). Although the reconstituted soil prepared by increasing sand content revealed non-unique ICLs, a unique CSL was present. The same response was also reported by Altuhafi et al. (2010), as non-unique ICLs were identified; however, a unique CSL was found in both the e-log p' and q-p' spaces. Altuhafi et al. (2010) suggested that the fabric of the soil that caused the lack of convergence of ICLs could be completely destroyed during shearing. Consequently, the unique CSL was attributed to a complete destruction of the induced fabric from the isotropic compression path during large strain-controlled shearing paths, which eventually led to a unique fabric at the critical state. In other words, the fabric responsible for non-unique ICLs was not sufficiently robust to withstand large strain-controlled shearing and was completely destroyed to reach a unique fabric in the critical state. It would be interesting to conduct a shearing path coupled with a CT scanning device to scan the soil sample vertically and horizontally at different axial strains to monitor the fabric evolution until the critical state is reached. Fig. 11 depicts the critical states in the deviator and the effective mean stress q-p' space. The critical states obviously lie on a unique line regardless of the initial state and drainage conditions. As expected, the reconstituted soil prepared by increasing the sand content by 5% (B5) exhibited the highest CSL gradient, followed by B10 and then the naturally graded soil (B0), where the gradients of CSL M equal 1.26, 1.21, and 1.03, respectively. The corresponding friction angles were 31.4°, 30.2°, and 26.1°, respectively. The trend of CSL gradients can be illustrated by the trend of the maximum dry density, as the maximum dry density obtained from the standard Proctor compaction test is considered an indicator of soil strength parameters.
4.2. Volumetric change The triaxial GDS apparatus was equipped with an external linear variable displacement transducer to measure the axial displacement, and the samples' volume change was monitored using the back-pressure transducer. To identify whether the soil's behavior is isotropic or anisotropic, the gradient of the relationship between the axial and volumetric strains must be 3:1 or 1:1 in the axial and radial strain relationship. In this context, the deformation of soil samples was accurately assessed during the isotropic compression loading and unloading paths. The axial and radial strains were plotted against the effective mean stress, as indicated in Fig. 7, in which a 90% compaction ratio was used to represent other compaction ratios. The radial strain was clearly higher than the axial strain for naturally graded soil (B0) and reconstituted soil (B5). This difference could be explained by the preparation methods; the samples were prepared with the moist tamping method, and when the soil sample was compacted in one direction, that direction became stiffer. Thus, its contractive response became very low, whereas the behavior became softer and more contractive when the sample was loaded in parallel to the compaction planes (Ham et al., 2012). The same behavior was observed in both intact and re-compacted CDG soils (Wang and Yan, 2006; Yan and Li, 2012). The reconstituted soil sample B10-90 revealed a reverse trend in which the axial strain exceeded the radial strain; however, the radial
6. Conclusions A series of isotropic compression tests and drained and undrained triaxial compression tests were implemented on CDG soil to investigate its behavior and the transitional mode. ICLs were found to merge into a unique line for the naturally graded soil. However, the reconstituted soils prepared by increasing the sand content by 5% and 10% revealed non-unique compression lines within the stress levels used. Soil 247
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Fig. 7. Axial and radial strains with confining stress via isotropic compression; a) natural soil B0, b) reconstituted soil B5, and c) reconstituted soil B10.
Fig. 8. Axial and radial strains versus the confining stress for reconstituted soil B10; a) compaction ratio 85%, and b) compaction ratio 87.5%.
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Fig. 9. Axial and radial strains during the isotropic compression path.
Fig. 11. Critical states in the deviator and effective mean stresses space q-p'.
mineralogy and morphology had significant effects on the compressive response. Compression lines depend greatly on the initial state for the reconstituted soils. The degree of convergence of the ICLs that was quantified by parameter m did not equal 1, and the ICLs were not quite parallel and tended to merge into a unique line at high stress levels. When the sand content in the reconstituted soils was increased, the
degree of ICLs convergence decreased, and the value of m parameter increased. The stress level required to reach a unique compression line increased as the additional amount of sand increased. The fabric induced by increasing the sand content could persist against the compression path used in this study, but it was not sufficiently robust to withstand the large strains of the shearing stage and was completely destroyed. The reconstituted soils thus revealed unique fabrics at the
Fig. 10. Critical states and compression lines in the e-log p' space; a) natural soil B0, b) reconstituted soil B5, and c) reconstituted soil B10. 249
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critical state that had unique CSLs regardless of the initial state. Natural and reconstituted CDG soils revealed isotropic behavior during the loading and unloading compression paths. The volumetric change could not be recovered in either the reconstituted or naturally graded soils. The stiff response increased as the initial density increased according to the compaction effort for reconstituted soils. However, the initial density affected only the initial compression response and did not extend to the normal compression line for naturally graded soil. In the case of reconstituted soils, the fabrics that resulted from varying degrees of compaction effort had a significant effect on compression behavior and resulted in non-unique compression lines. Increasing the sand content leads to quantitative changes in the soil's mineral composition, so it can be a cause of transitional behavior in addition to the induced fabric. It can be concluded that transitional behavior is no longer limited to gap-graded soils; grading is no longer the only governing factor in this mode, and the transitional mode may occur due to relatively inconsiderable changes in the soil minerals, grading, and fabric. Further research is necessary to identify the role of the CDG soils' fabric during both the isotropic consolidation and shearing paths via the industrial X-ray CT scanning to monitor the fabric of the soil sample step by step to clarify the fabric's impact on the lack of convergence of the ICLs and CSLs. A new applicable framework should be developed to describe soils that exhibit transitional behavior because the critical state framework is not applicable to these soils. New vocabulary is urgently needed, as idioms such as “normal compression line” no longer have a clear meaning and no longer represent a state boundary surface, and the “critical state” term no longer has a clear meaning for soils that exhibit non-unique CSL. The “state parameter” concept also no longer has a clear meaning.
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