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Article

Consolidation Properties of Soil/Modified Bentonite Backfill in Salt Solution

by
Aiguo Fang
1,
Yuling Yang
2,
Zhongjun Yang
1,
Sudong Hua
3,
Jinchao Wang
1 and
Feng Zhou
2,*
1
China Construction Industrial & Energy Engineering Group Co., Ltd., Nanjing 210023, China
2
College of Transportation Engineering, Nanjing Tech University, Nanjing 210009, China
3
College of Materials Science and Engineering, Nanjing Tech University, Nanjing 210009, China
*
Author to whom correspondence should be addressed.
Water 2022, 14(12), 1967; https://doi.org/10.3390/w14121967
Submission received: 18 May 2022 / Revised: 10 June 2022 / Accepted: 16 June 2022 / Published: 20 June 2022
(This article belongs to the Section Soil and Water)
Browse Figures
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Abstract

:
The consolidation properties of soil-bentonite (SB) backfills containing 20% of the weight of sodium-hexametaphosphate-modified calcium bentonite (SHMP-20CaB) and prepared with 0 mM to 1000 mM calcium chloride (CaCl2) solution were evaluated. The results indicated that both consolidation stress and CaCl2 concentration had significant effects on the consolidation behaviors of the SHMP-20CaB backfill. In general, an increase in consolidation stress resulted in a decrease in the void ratio (e) and in the coefficient of volume change (mv), alongside an increase in the coefficient of consolidation (cv). The increased CaCl2 concentration yielded a dropped void ratio, and a nonlinear decreased compression index (Cc) and rebound index (Cs), respectively, from 0.18 to 0.13 and from 0.022 to 0.010, and a nonlinear increase cv. A threshold CaCl2 concentration of 100 mM was observed at the inflection points of the Cc, Cs, and cv. In contrast, the mv was insensitive to the CaCl2 solutions. The deterioration in e, Cc, and Cs and the increase in cv were the result of a compressed diffuse double layer of the bentonite by the CaCl2 solution, which thus exerted certain negative effects on the consolidation behaviors of the SHMP-20CaB backfill.

1. Introduction

Soil–bentonite (SB) slurry walls are widely used in North America to control the lateral migration of contaminated groundwater in geoenvironmental applications [1]. This type of engineering barrier is constructed by the slurry trench method. First, a trench is excavated in highly permeable soils and is simultaneously backfilled by a bentonite slurry that is composed of water and from 4% to 6% by weight of sodium bentonite to support the stability of the trench walls. The excavated trench soils are then mixed with the slurry and additional dry sodium bentonite (if needed), placed back into the trench, and displaced by the in-trench slurry to create a slurry wall with hydraulic conductivity lower than 10−9 m/s [2,3].
Sodium bentonite is the key functional material which endows the SB slurry walls with a low hydraulic conductivity through its excellent swelling capability in water. However, in China, India and other countries where high-quality sodium bentonite is scarce and calcium bentonite (Ca-bentonite) is relatively abundant; the latter can be used as an alternative material in SB slurry wall construction after proper modification. Recently, some novel modified Ca-bentonites, such as polymer-modified bentonite [4,5], sodium-hexamatephosphate (SHMP)-modified bentonite [6] have been developed as alternative barrier materials in contaminant control. Especially, a sand-modified Ca-bentonite backfill with an optimum SHMP-modified Ca-bentonite content of 20% by weight was manifested to exhibit comparable hydraulic performance to the typical sand–sodium-bentonite backfill commonly with a bentonite content of 1% to 5% by weight [7,8].
Alongside hydraulic conductivity, the compressibility and strength of the backfill are equally important considerations for the design and construction of the SB slurry wall as well as other engineering applications [3,9,10,11,12,13,14,15,16]. The consolidation behavior of the SB backfill is important for several reasons: (1) The hydraulic conductivity is strongly dependent on consolidation pressure [17]. (2) The chemical resistance and hydraulic conductivity compatibility of the bentonite materials in polluted solution are closely related to its pressure status [18]. (3) The consolidation of the SB backfill in the trench due to the lateral squeezing mechanism has a significant influence on the deformation of the ground adjacent to the SB slurry walls [19]. (4) Increasing the amount of fine in the SB backfill not only decreases its hydraulic conductivity but also increases its compressibility [3,20]. On the basis of these considerations, understanding the basic consolidation properties of the SB backfills is necessary to further clarify the state of stress within and adjacent to the SB slurry walls, especially when the fine content of backfill is high.
Although the consolidation performance for some SB backfills have been reported [3,13,17,19,20], they are mainly concerned on SB backfill with sodium bentonite content ranged from 1% to 7.1%, and the test liquid is commonly clean water or tap water. However, there are limited numbers of systematic assessments conducted on the consolidation of SB backfills with bentonite content higher than 10% or attacked by solutions with a wide range of chemical concentration [21]. As a result, this study aims to evaluate the consolidation properties of SB backfill with 20% by weight of SHMP-modified Ca-bentonite. Moreover, the influences of calcium chloride (CaCl2) solution with a concentration of 0 mM to 1000 mM on the consolidation behaviors of the backfill are explored. The concentration window of CaCl2 adopted in this study is based on salt concentration ranges reported by previous studies that investigated influences of contaminants on the containment performance of SB backfills [22,23]. The obtained results are useful for the safe design and construction of SB slurry wall containing modified calcium bentonite in geoenvironmental applications.

2. Materials and Methods

2.1. Backfill Constituents and Test Liquids

The sand–bentonite backfill tested in this study composed of sand and SHMP-modified Ca-bentonite. The sand used was obtained from a quarry in Beloit, WI, USA, and had a coefficient of uniformity (cu) of 2.44, a coefficient of curvature (cc) of 1.00, and was classified as poorly graded sand (SP) as per ASTM D2487 [24]. The SHMP-modified Ca-bentonite was prepared by procedures provided in [7,8,25]. In brief, the procedures included creating a slurry with a SHMP-to-bentonite ratio of 2%, curing the slurry in room temperature for 24 h, oven drying for 24 h, grinding the dried soil, and passing it through a No. 200 sieve.
Two types of test liquids, tap water and CaCl2 solutions with a solute concentration of 5 mM to 1000 mM, were used to prepare the consolidation specimen. The CaCl2 solutions were prepared by dissolving the precalculated amount of CaCl2 into distilled water to create a stock solution with a solute concentration of 1000 mM. The stock solution was then diluted to a target solute concentration of 5 mM to 500 mM, as shown in Table 1.

2.2. Consolidation Tests

The sand and SHMP-modified Ca-bentonite backfills (denoted as SHMP-20CaB) prepared with either tap water or 5 mM to 1000 mM CaCl2 solutions were subjected to one-dimensional consolidation tests. These backfills were created by mixing 80% by weight of sand, 20% by weight of SHMP-modified Ca-bentonite, and one of the test liquids to achieve a desired moisture content of 30% that corresponded to the target slump of 125 mm as suggested by [8,26]. The moist backfills were then sealed in plastic bags and cured at 20 °C room temperature for approximately 15 days to allow chemical equilibrium [27]. The total mass of each backfill and the plastic bag was monitored at the beginning and end of the curing period, and the appropriate amount of distilled water was added to the backfill to compensate for evaporation loss.
The consolidation tests were performed in accordance with ASTM D2435 [28]. The stepwise loading sequence began at 24 kPa and was doubled after each stage (24 h), until it achieved a maximum vertical effective stress of 1532 kPa, as recommended by [29]. Subsequently, each specimen was successively unloaded by reducing the applied load with a factor of four relative to the previous load. The initial properties of the prepared specimens are shown in Table 1.

3. Results and Analysis

3.1. Void Ratio–Stress Relationships

Figure 1 shows the relationship between the void ratio (e) and the logarithm of effective consolidation stress [log(σ’)] of the backfill specimens prepared with a CaCl2 concentration of 0 mM to 1000 mM. For all the specimens, noticeable inverse S-shape compression curve can be observed at the loading segment of each e-log(σ’) curve. In contrast, a straight rebound line occurs at the unloading segment of each consolidation curve. This finding is consistent with those reported by previous studies conducted on remolded natural clayey soils [30], sodium bentonite slurry [31], and clayey SB backfills [21,23,32]. Considering that there is an approximately equal initial moisture content and liquid limit in the SHMP-20CaB backfill, e.g., with an initial moisture content of 30% to 33% as shown in Table 1 versus a liquid limit of 30% as reported by [8], the presence of the inverse S-shape compression curve can be attributed to the existence of the shear strength of the backfill provides sufficient resistance to low external compression stress [21,30].
The void ratios of the SHMP-20CaB backfill are found varied from 0.9 to 0.5 and decreased with an increase in CaCl2 concentration throughout the whole-loading and unloading stages. The sequential order of these void ratios is consistent with that of initial void ratios as shown in Table 1. For example, the lower the CaCl2 concentration in solutions used to prepare the SHMP-20CaB backfill, the higher the initial void ratio that is measured. This phenomenon is the result of the shrinkage in the diffuse double layer of the bentonite induced by the CaCl2 solution, and can be described by the Gouy–Chapman theory model expressed as Equation (1) [33]:
1 κ = ( ε 0 D k B T 2 n 0 E 2 v 2 ) 1 / 2
where the 1/κ is the thickness of the diffuse double layer; ε0 is the permittivity of vacuum; D is the dielectric constant of the medium; kB is the Boltzmann constant; T is absolute temperature; n0 is the cation concentration; E is the electronic charge; and v is the cation valence.
It is shown in Equation (1) that the thickness of the diffuse double layer varies inversely with the square root of the pore cation concentration. An increase in cation concentration results in a thinner diffuse double layer on the bentonite, and decreased interparticle repulsive forces among the clay particles as a consequence [33]. One practical consequence of this is deteriorated swelling of the bentonite, which directly generates a decrease in the void ratio of the sand–bentonite backfill specimen.
In addition, the void ratios of the SHMP-20CaB backfill prepared with tap water (i.e., a 0 mM CaCl2 solution) in this study are significantly lower than those of typical SB backfills containing sodium bentonite, as reported by previous studies [13,20]. This may be the result of two reasons: (1) a lower swelling capacity of the SHMP-modified Ca-bentonite as compared to the high-quality sodium bentonite, and the former occupying less void space within the backfill than the latter; (2) the SHMP-20CaB possesses higher bentonite content relative to its sodium bentonite backfill counterpart, i.e., 20% in the former versus 2% to 5% in the latter.

3.2. The Influence of CaCl2 Concentration on Cc and Cs

Figure 2 presents variations of two characteristic parameters, compression index, Cc, and rebound index, Cs, with CaCl2 concentration in a solution used to prepare the SHMP-20CaB backfill. The values of Cc and Cs are, respectively, calculated from the slopes of the linear loading and unloading portion of the e-log(σ’) curves. The Cc of the backfill first sharply decreases from 0.18 to 0.13 and then maintains stability as the CaCl2 concentration increases from 0 mM to 1000 mM. A Cc inflection point is obtained at the threshold CaCl2 concentration of 100 mM. The same change trend is also found in the Cs value of the backfill. For example, the Cs of the backfill first decreases dramatically from 0.022 to 0.010 and then maintains stability as the CaCl2 concentration rises up from 0 mM to 1000 mM. The Cs inflection point is also observed at the threshold CaCl2 concentration of 100 mM. Such a variation in Cc and Cs with solute concentration is not unprecedented. For instance, [21] investigated compression behaviors of the clayey SB backfill containing 5% to 15% by weight of bentonite in lead solution. They indicated that the Cc of the backfill changes significantly at low lead concentration, while it tends to be stable at a relatively high lead concentration. The Cc inflection point occurs at a solute concentration of 120 mM in their experiments, which is slightly higher than that showed in this study (i.e., 100 mM CaCl2).
The relatively low Cc value in a high level of CaCl2 concentration reflects a relatively incompressible SHMP-20CaB backfill in salt solution. This is likely the result of shrinkage in the diffuse double layer of the bentonite induced by the CaCl2 solution [34,35,36], which endows the backfill a lower initial void ratio and relatively dense skeleton to sustain the compressive stress. Similarly, relatively low Cs value in high levels of CaCl2 concentration indicates less swelling potential for this backfill in salt solution, which is the response of the compressed diffuse double layer of the bentonite. In summary, the decrease in Cc and Cs values with CaCl2 concentration implies an adverse influence of the salt solution on the compression and swelling capacities of the SHMP-20CaB backfill.

3.3. The Coefficients of Volume Change and Consolidation

Figure 3 illustrates the relationship between the coefficient of volume change, mv, and average effective stress, σ’, where the average effective stress represents as the average value of initial and final effective stress of each loading increment. The mv of all the backfills tested in this study has a similar variation trend and exhibits two segments of change as the average effective stress increases. For instance, the value of mv increases with increasing average effective stress before the average effective stress is over 100 kPa. Then, as the average effective stress further increases to 1532 kPa, the value of mv shows a linear decrease trend in the log(mv)-log(σ’) plot. In general, the SHMP-20CaB backfills prepared with 0 mM to 1000 mM CaCl2 possess a similar mv value, and the variation in CaCl2 concentration did not yield an obvious discrepancy in the mv of the SHMP-20CaB backfill. Therefore, the CaCl2 solution is concluded to have a negligible effect on the volume change of the backfill tested in this study.
The coefficients of consolidation, cv, were determined using the square-root-of time (Taylor) method in loading segments of the consolidation tests. The cv values are plotted against average effective stress on the dual-logarithmic scale, as shown in Figure 4. In general, the cv of the all the tested specimens increases across the range of 10−7 to 10−4 m2/s as it increases in the average effective stress. This agrees well with the cv trends reported for typical SB backfills and clayey SB backfills [13,20,21]. Figure 5 shows variation of cv with increasing CaCl2 concentration in solutions used to prepare the SHMP-20CaB backfill. It can be seen from Figure 5 that the higher the CaCl2 concentration in solution used to prepare the backfill, the higher the cv value that is obtained. As compared to the specimen prepared with 0 mM CaCl2, a maximum cv increase of two orders of magnitude is obtained on specimens prepared with 1000 mM CaCl2.
The increased cv with increasing average effective stress is attributed to a greater decrease rate in mv relative to the decrease rate in the hydraulic conductivity of the backfill as the average effective stress increases [20]. In addition, the cv value depends on the dissipation rate of excess pore pressure. A relatively lower cv is the result of the slower dissipation of the excessive pore water [21], which corresponds to a more swollen diffuse double layer of the bentonite. The variation trend again manifests the adverse influence of the salt solution on consolidation behaviors of the tested backfill.

4. Conclusions

Sodium hexametaphosphate (SHMP)-modified Ca-bentonite is an acceptable alternative material to the soil–bentonite slurry walls when used based on the availability of local materials. This study investigated consolidation properties of sand/SHMP-modified Ca-bentonite backfill containing 20% by weight of bentonite (SHMP-20CaB) when exposed to groundwater with different salt concentrations. Based on the obtained results, the following conclusions can be drawn.
The void ratio–logarithm of effective consolidation stress, i.e., e-log(σ’), relationship of the SHMP-20CaB backfill displayed an inverse S-shape compression curve, which is a result of the existence shear strength, provided sufficient support to low compression stress. The solution with an increase in CaCl2 concentration from 0 mM to 1000 mM yielded a decrease in the void ratio of the backfill due to shrinkage in the diffuse double layer of the bentonite.
The values of the compression index Cc and the rebound index Cs of the backfill varied from 0.18 to 0.13 and from 0.022 to 0.010, respectively, and exhibited a nonlinear decrease with an increase in CaCl2 concentration due to deterioration in the diffuse double layer of the bentonite when exposed to salt solution. The inflection points of both the Cc and Cs occurred at the threshold of the CaCl2 concentration of 100 mM.
The values of the coefficient of volume change mv first increased and then decreased, with an increase in average effective stress, and were unresponsive to the CaCl2 concentration. In contrast, the coefficients of consolidation cv possessed an approximately linear increase from 10−7 to 10−4 m2/s with an increasing average effective stress and exhibited a nonlinear increase as CaCl2 concentration rose from 0 mM to 1000 mM. A maximum cv increase of two orders of magnitude occurred at the backfill prepared with 1000 mM CaCl2 solution.
Overall, the CaCl2 solution yielded an adverse influence on the consolidation behaviors of the SHMP-20CaB backfill. This influence should be carefully considered during the design and construction of the soil–bentonite slurry walls.

Author Contributions

Conceptualization, Y.Y.; methodology, Y.Y. and F.Z.; writing—original draft preparation, A.F. and Y.Y.; writing—review and editing, Z.Y., S.H., J.W. and F.Z.; funding acquisition, Y.Y. and F.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by National Natural Science Foundation of China (grant number 41907248), National Primary Research & Development Plan of China (grant number 2019YFC1806000), and Postgraduate Education Reform Project of Jiangsu Province (grant number JGKT22_C029).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

Authors thank Krishna R. Reddy at University of Illinois at Chicago for providing the experiment conditions.

Conflicts of Interest

The authors declare no conflict of interest.

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Water 14 01967 g001 550
Figure 1. Effective consolidation stress versus void ratio for the SHMP-20CaB backfill prepared with 0 mM to 1000 mM CaCl2 solution.
Figure 1. Effective consolidation stress versus void ratio for the SHMP-20CaB backfill prepared with 0 mM to 1000 mM CaCl2 solution.
Water 14 01967 g001
Water 14 01967 g002 550
Figure 2. Compression index and rebound index of the SHMP-20CaB backfill versus CaCl2 concentration of solution used to prepare the backfill.
Figure 2. Compression index and rebound index of the SHMP-20CaB backfill versus CaCl2 concentration of solution used to prepare the backfill.
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Water 14 01967 g003 550
Figure 3. Coefficient of volume change as a function of average effective stress during loading of the SHMP-20CaB backfills prepared with 0 mM to 1000 mM CaCl2 solution.
Figure 3. Coefficient of volume change as a function of average effective stress during loading of the SHMP-20CaB backfills prepared with 0 mM to 1000 mM CaCl2 solution.
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Water 14 01967 g004 550
Figure 4. Coefficient of consolidation as a function of average effective stress during loading of the SHMP-20CaB backfills prepared with 0 mM to 1000 mM CaCl2 solution.
Figure 4. Coefficient of consolidation as a function of average effective stress during loading of the SHMP-20CaB backfills prepared with 0 mM to 1000 mM CaCl2 solution.
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Water 14 01967 g005 550
Figure 5. Coefficient of consolidation as a function of CaCl2 concentration in solution used to prepare the SHMP-20CaB backfill.
Figure 5. Coefficient of consolidation as a function of CaCl2 concentration in solution used to prepare the SHMP-20CaB backfill.
Water 14 01967 g005
Table
Table 1. Program and initial specimen state for one-dimensional consolidation tests.
Table 1. Program and initial specimen state for one-dimensional consolidation tests.
Test SolutionTarget Moisture Content/%Actual Moisture Content/%Initial Void RatioCaCl2 Concentration/mM
Tap water30.033.10.880
CaCl2 solution30.029.90.845
29.60.8110
29.70.7920
30.30.8050
31.20.77100
30.40.77500
30.00.731000
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Fang, A.; Yang, Y.; Yang, Z.; Hua, S.; Wang, J.; Zhou, F. Consolidation Properties of Soil/Modified Bentonite Backfill in Salt Solution. Water 2022, 14, 1967. https://doi.org/10.3390/w14121967

AMA Style

Fang A, Yang Y, Yang Z, Hua S, Wang J, Zhou F. Consolidation Properties of Soil/Modified Bentonite Backfill in Salt Solution. Water. 2022; 14(12):1967. https://doi.org/10.3390/w14121967

Chicago/Turabian Style

Fang, Aiguo, Yuling Yang, Zhongjun Yang, Sudong Hua, Jinchao Wang, and Feng Zhou. 2022. "Consolidation Properties of Soil/Modified Bentonite Backfill in Salt Solution" Water 14, no. 12: 1967. https://doi.org/10.3390/w14121967

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