sustainability

Article Experimental Investigation of Consolidation Properties of Nano-Bentonite Mixed Clayey

Gang Cheng 1,2, Hong-Hu Zhu 2,* , Ya-Nan Wen 2,3, Bin Shi 2 and Lei Gao 4

1 School of Computer Science, North China Institute of Science and Technology, Beijing 101601, China; [email protected] 2 School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China; [email protected] (Y.-N.W.); [email protected] (B.S.) 3 Design Institute, China Design Group Co., Ltd., Nanjing 210014, China 4 Key Laboratory of Ministry of Education for Geomechanics and Engineering, Hohai University, Nanjing 210098, China; [email protected] * Correspondence: [email protected]; Tel.: +86-025-89681137

 Received: 11 July 2019; Accepted: 26 November 2019; Published: 7 January 2020 

Abstract: As a new soil improvement method, adding nano-bentonite can enhance the engineering properties of soil. To study the stabilization effect of nano-bentonite on properties, a series of one-dimensional odometer tests were conducted on a clayey soil with different nano-bentonite mixing contents (i.e., 0.5%, 1%, 1.5%, and 2%). The effects of nano-bentonite on the coefficient of consolidation and permeability of the test soil were analyzed. The results show that adding a certain amount of nano-bentonite does not significantly affect the original consolidation characteristics of soil samples, but displays a notable effect on accelerating water drainage. Among all the soil samples, when the nano-bentonite mixing content is 0.5%, the final compression amount is the largest and the final is the smallest. The coefficients of consolidation and permeability increase with increasing nano-bentonite mixing content under high state. The test results indicate that nano-bentonite can facilitate internal cementation of soil particles, which effectively reduces the compressibility of clayey soil.

Keywords: nano-bentonite; clayey soil; consolidation; compressibility; permeability

1. Introduction Natural clayey rarely meet the requirements of modern projects in terms of of their foundations. As more and more high-rise buildings and massive structures are being built in developing countries such as China, effectively improving the strength and stiffness of a natural is urgently required in geotechnical engineering practices. Vidal first proposed the concept of soil modification in the 1960s [1]. This technology has been developed rapidly and is now widely used in roadbeds, retaining walls, anti-seismic infrastructures, and other fields [2–4]. In general, a variety of reinforcing or treatment materials can be added to foundation soils to modify or improve their strength and deformation properties [5–9]. Such materials are divided into three types: inorganic binders, ionic soil stabilizing agents, and composite curing agents. Among various solidified materials, inorganic binders (e.g., cement, lime, fly ash, and their mixtures) are widely used for chemical modification of soils [10]. When inorganic binders are mixed in the soil, hydrolysis, hydration, and ion exchange occur. For instance, the addition of cement will increase the between soil particles, and greatly reduce the void ratio, improving the structural strength. However, the use of cement to reinforce soil is limited by the soil type. Because the organic matter

Sustainability 2020, 12, 459; doi:10.3390/su12020459 www.mdpi.com/journal/sustainability Sustainability 2020, 12, 459 2 of 15 has an adverse effect on the cementation process of soil, it is not appropriate for soils rich in organic matter [11]. In addition, the production of cement is an intensive carbon-emission process that has negative environmental impacts [12,13]. The strength of lime treated soil develops slowly and its water stability is relatively weak. Another major disadvantage of this method is that it increases soil brittleness [14]. The strength of soil modified with lime-fly ash is poor at the early stage, which directly affects the progress and quality of construction. A relatively high strength can be achieved in at least 28 days, and the longer the time available, the higher the strength becomes. It can be concluded that the traditional soil modification materials have certain disadvantages and cannot fully meet engineering needs. Hollister et al. found that changes in the surface area and volume of material lead to some changes in its physical properties when the particle size of the material is reduced from micron to nanometer [15]. Therefore, more and more researchers began to study nanomaterials. These materials have been popularized and applied in metal smelting, chemical industry, microelectronics, medicine, and biology, which are known as “the most promising materials in the 21st century” [16]. Nanomaterials are a kind of ultra-fine material with a large specific surface area and a 1–100 nm particle size. They have some special structural characteristics, with four major effects: the size effect, quantum effect, surface effect, and interface effect [17]. With the wide application of nanomaterials, their cost has sharply decreased, providing the possibility for large-scale application of nanomaterials in geotechnical engineering. Norazlan et al. carried out a series of experiments to study the geotechnical properties of nano-kaolin mixed soil (most of the nano-kaolin sizes are between 39 d.nm and 70 d.nm), and found that the basic properties and engineering characteristics of soils are significantly affected by the addition of a small amount of nanoparticles [18]. Recently, researchers in Malaysia [19–21], India [22], Egypt [23], and other countries conducted a series of experimental studies in this field. These studies mainly focused on the influence of nanomaterials on the compressive, tensile, and shear strengths of soil. The results show that the soil mixed with nano-clay displayed a higher strength than the original one. With the increase in nanomaterial content, the strength of soil does not increase continually but rather shows as a curve with a peak inflection point. To explain the micro-mechanism of nanomaterials modified soil, Mostafa et al. conducted an experimental study [24]. In this study, a different percent of nano-clay montmorillonite was used in order to check out the created changes in soil characteristics with an increasing percentage of nano-clay. Based on the study, they found that the cohesion of soil increased gradually with the filling of soil voids after the incorporation of nanoparticles, according to the results of scanning electron microscopy (SEM). Gao et al. investigated the unconfined compressive strength of clay mixed with nano-MgO (the soil moisture content was 10% and soil dry density was 1.5 g cm 3, the different dosages of nano-MgO, i.e., 0%, 2%, 4%, and 6% were put into the soil · − samples) and nano-Al2O3, respectively [25,26]. They were found to have a profound physical and chemical influence on the soil samples. The above research results mainly focus on the contribution of nanomaterials to the improvement of soil strength, whereas studies of the consolidation, permeability, compression, and other characteristics of soil mixed with nanomaterials are lacking, and their action mechanisms are not clear. Wang et al. completed one-dimensional (1D) compression tests of nano-MgO modified cement-soil with different mixing ratios, and demonstrated that the compressive deformation of modified cement-soil gradually decreased with the increase in nano-MgO mixing ratio. Ng et al. studied the change in the permeability of clay after adding different amounts of nano-oxides (two different nanomaterials namely gamma-aluminum oxide powder (γ-Al2O3) and nano-copper oxide (CuO) were selected and mixed with clay at different percentages (i.e., 2%, 4%, and 6%)) [27]. They found that with the increase in nano-oxides content, the void of clay was filled with nanomaterials and the permeability coefficient decreased. Neethu et al. studied the effect of nano-clay on the consolidation and permeability of laterite and kaolinite [22]. They found that when the content of nano-clay was 1%, the consolidation and permeability coefficients of laterite and kaolinite decreased significantly. To compensate for the long duration of the conventional drainage consolidation method, Li used the electro-osmosis method to study the consolidation characteristics and studied the influence SustainabilitySustainability 20202020,, 1212,, 459x FOR PEER REVIEW 33 of 1516

but has no effect on the electro-osmosis drainage of soil. As an anti-seepage cushion material, under ofa larger nano-montmorillonite engineering load, on nanomaterial-modified the electro-osmosis and clay mechanical inevitably properties produces of certain soil [28 ].compression It is found thatdeformation, nano-montmorillonite which affects can the eff ectivelyanti-seepage improve and the anti-fouling performance. of soil, but has In no addition, effect on thethe electro-osmosisreplacement of drainagecement by of clay soil. offers As an anti-seepagelower life-cyc cushionle carbon material, footprint under as clay a larger is a engineeringnatural resource. load, nanomaterial-modified clay inevitably produces certain compression deformation, which affects the Therefore, this can result in reduced emissions of CO2 and is a more sustainable approach compared anti-seepagewith soil-cement and anti-foulingsolidification performance. [29–32]. In addition, the replacement of cement by clay offers lower life-cycleIn this carbon paper, footprint a series asof clayone-dimensional is a natural resource. (1D) consolidation Therefore, thistests can were result conducted in reduced on clayey emissions soil ofwith CO 2differentand is a morenano-bentonite sustainable approachmixing contents. compared Based with soil-cementon the test solidification results, the [29 compression,–32]. consolidation,In this paper, and apermeability series of one-dimensional characteristics of (1D) the consolidationsoil samples after tests adding were conducted nano-bentonite on clayey were soilsystematically with different investigated. nano-bentonite mixing contents. Based on the test results, the compression, consolidation, and permeability characteristics of the soil samples after adding nano-bentonite were systematically2. Materials and investigated. Methods

2. Materials and Methods 2.1. Test Materials 2.1. TestThe Materials soil samples used in this experiment were collected at the depth of 2 m along the Yangtze RiverThe in Qixia soil samples District, used Nanjing, in this China. experiment The basic were collectedphysical and at the mechanical depth of 2 mparameters along the Yangtzeof the soil River are inlisted Qixia in District, Table 1. Nanjing, The grain China. size The distribution basic physical was and determined mechanical by parameters of the and soil are listed in Tableanalysis,1. The which grain is size shown distribution in Figure was 1. The determined natural moisture by sieve analysis content andwas hydrometer obtained by analysis, using the which drying is shownmethod. in The Figure liquid1. The and natural plastic moisture limit tests content were conducted was obtained in accordance by using the with drying ASTM method. standard The D4318. liquid andBy performing plastic limit standard tests were Proctor conducted in accordance tests according with ASTM to ASTM standard standard D4318. D698, By the performing optimum standardmoisture Proctorcontent compaction and maximum tests accordingdry unit toweight ASTM of standard the soil D698, were the obtained. optimum According moisture content to the andClassification maximum and dry codes unit weight for Chinese of the soil were(GB/T obtained. 17296-2009 According) [33], the to test the soil Classification is classified and as codessilty clay for Chinesewith medium soil (GB plasticity./T 17296-2009) In order [ 33to], study the test the soilmineral is classified composition as silty in claythe soil, with X-ray medium Diffraction plasticity. (XRD) In orderanalysis to studyof the thesoil mineralsample compositionwere carried inout. the As soil, shown X-ray in Di Figureffraction 2, the (XRD) mineral analysis components of the soil in samplethe test weresoil are carried mainly out. composed As shown of in quartz, Figure2 ,kaolin, the mineral montmori componentsllonite, inmuscovite, the test soil illite, are mainlychlorite, composed and calcite. of quartz,Some clay kaolin, minerals montmorillonite, will absorb muscovite, water during illite, chlorite, the test, and which calcite. leads Some to clay a decrease minerals willin the absorb soil waterpermeability. during the test, which leads to a decrease in the soil permeability.

TableTable 1.1. BasicBasic parametersparameters ofof thethe testtest soil.soil. Soil Particle Natural Moisture Natural Moisture SpecificSpecific FractionSoil Particle (%) FractionContent (%) Liquid LiquidPlastic PlasticMaximumMaximum Dry Unit Dry OptimumOptimum Moisture Content Limit Limit Unit Weight Moisture GravityGravity Limit (%) Limit (%) Weight (g·cm–3) Content (%) Clay (%) (%) (%) (g cm 3) Content (%) Clay Silt Sand (%) · − 2.73 34 59 7 3.4 43.6 28.1 1.52 26.0 2.73 34 59 7 3.4 43.6 28.1 1.52 26.0

Figure 1. Grain size distribution of the test soil. Blue dots represent the content of soil particles less Figure 1. Grain size distribution of the test soil. Blue dots represent the content of soil particles less than a particle size (%). than a particle size (%). Sustainability 2020, 12, 459 4 of 15 Sustainability 2020, 12, x FOR PEER REVIEW 4 of 16

Figure 2. Figure 2. X-rayX-ray Diffraction Diffraction results of the test soil. The nanomaterial used in the experiments was high-purity nano-bentonite. It is a kind of fine The nanomaterial used in the experiments was high-purity nano-bentonite. It is a kind of fine white powder with high hydrophilicity and a large specific surface area (SSA). This nano-bentonite white powder with high hydrophilicity and a large specific surface area (SSA). This nano-bentonite is produced by Zhejiang Fenghong New Material Co. Ltd. (Huzhou, China). The properties of the is produced by Zhejiang Fenghong New Material Co. Ltd. (Huzhou, China). The properties of the nano-bentonite are as follows: the specific gravity is 1.8; over 97% of soil particles are smaller than nano-bentonite are as follows: the specific gravity is 1.8; over 97% of soil particles are smaller than 74 74 µm; the is less than 0.3 g/cm3; the SSA is 750 m2/g; the Cation Exchange Capacity (CEC) µm; the bulk density is less than 0.3 g/cm3; the SSA is 750 m2/g; the Cation Exchange Capacity (CEC) is 90 mol/100g; the interlayer spacing d001 determined using XRD is 2.2 nm. is 90 mol/100g; the interlayer spacing d001 determined using XRD is 2.2 nm. 2.2. Test Method 2.2. Test Method Four nano-bentonite mixing contents (0.5%, 1%, 1.5%, and 2%, w/w) were selected in this study. TheseFour four nano-bentonite percentages were mixing determined contents referring(0.5%, 1%, to 1. the5%, literatureand 2%, w/w) on soil were stabilizing selected methodsin this study. [34]. TheThese preparation four percentages method were of the determined soil sample referring was as follows: to the literature the air-dried on soil soil stabilizing sample was methods crushed [34]. and The preparation method of the soil sample was as follows: the air-dried soil sample was crushed and sieved through a 2-mm sieve. Then, the nano-bentonite, as set above, was evenly mixed with the sieved through a 2-mm sieve. Then, the nano-bentonite, as set above, was evenly mixed with the soil. soil. After the nano-bentonite was dispersed, de-aired water was added to the soil sample, which After the nano-bentonite was dispersed, de-aired water was added to the soil sample, which was was packed in a plastic bag and placed in the humidifier for 24 h to distribute the water evenly in the packed in a plastic bag and placed in the humidifier for 24 h to distribute the water evenly in the soil soil sample. The static compaction method was used for preparing the soil samples at the maximum sample. The static compaction method was used for preparing the soil samples at the maximum dry dry density. The soil samples were then fully saturated in a vacuum chamber. Afterward, the 1D density. The soil samples were then fully saturated in a vacuum chamber. Afterward, the 1D consolidation test was conducted according to ASTM standard D2435. consolidation test was conducted according to ASTM standard D2435. The instrument used in the tests was the WG-1C triple high-pressure oedometer, produced by The instrument used in the tests was the WG-1C triple high-pressure oedometer, produced by Nanjing Ningxi Soil Instrument Co. Ltd. (Nanjing, China). The cross-sectional area of the sample was Nanjing Ningxi Soil Instrument Co. Ltd. (Nanjing, China). The cross-sectional area of the sample was 30 cm2 and the height was 2 cm. The multi-stage loading method was adopted with double drainage 30 cm2 and the height was 2 cm. The multi-stage loading method was adopted with double drainage condition. The load sequence of the consolidation test was 25 kPa, 50 kPa, 100 kPa, 200 kPa, 400 kPa, condition. The load sequence of the consolidation test was 25 kPa, 50 kPa, 100 kPa, 200 kPa, 400 kPa, 800 kPa, 1200 kPa, 1600 kPa, 2400 kPa, and 3200 kPa. In accordance with the Standard of Geotechnical 800 kPa, 1200 kPa, 1600 kPa, 2400 kPa, and 3200 kPa. In accordance with the Standard of Geotechnical Test Method [35], the samples were loaded step by step and each loading lasted for 24 h until the test Test Method [35], the samples were loaded step by step and each loading lasted for 24 h until the test was completed. The soil compression varying with time under each load were automatically collected was completed. The soil compression varying with time under each load were automatically collected by the displacement sensor. Foreign scholars had compared the strength of the soil samples at curing by the displacement sensor. Foreign scholars had compared the strength of the soil samples at curing period of 1, 7, 14, 28 days, and found that the soil samples with different nanomaterial contents showed period of 1, 7, 14, 28 days, and found that the soil samples with different nanomaterial contents a similar trend of strength change: the greater the curing time, the higher the strength [36]. The main showed a similar trend of strength change: the greater the curing time, the higher the strength [36]. purpose of this study was to investigate the consolidation characteristics of clayey soil mixed with The main purpose of this study was to investigate the consolidation characteristics of clayey soil nano-bentonite. Relevant results are expected to be applied to the rapid in-situ modification of clayey mixed with nano-bentonite. Relevant results are expected to be applied to the rapid in-situ soil. Therefore, the test samples were designed with one day curing time. In the next step study, the modification of clayey soil. Therefore, the test samples were designed with one day curing time. In influence of curing time on the strength of soil samples will be systematically analyzed. the next step study, the influence of curing time on the strength of soil samples will be systematically analyzed.

3. Results and Discussion

3.1. Compression Characteristics SustainabilitySustainability2020, 122019, 459, 11, x FOR PEER REVIEW 5 of 165 of 15

The compression curve reflects the deformation characteristics of soil under loading, which is 3. Resultsimportant and Discussionfor settlement prediction. Figure 3 shows the relationship between the compression and consolidation pressure of the soil samples. It is found that the curve shapes of the soil compression 3.1. Compressionwith variations Characteristics in consolidation pressure are similar, irrespective of whether nano‐bentonite is added or not. The relationship between final compression and nano‐bentonite mixing content is plotted in The compression curve reflects the deformation characteristics of soil under loading, which is Figure 4. The final compression of samples without nano‐bentonite is the smallest, which is 5.2 mm. importantWhen for the settlement nano‐bentonite prediction. mixing content Figure 3is shows about 0.5%, the relationship the final compression between theof the compression soil sample and consolidationreaches a pressure peak value of the of soil 5.6 samples. mm. When It is found the nano that‐ thebentonite curve shapesmixing ofcontent the soil is compression higher, the with variationscompressibility in consolidation decreases, pressure but the are final similar, compression irrespective of the of whether four groups nano-bentonite of samples iswith added nano or‐ not. The relationshipbentonite is larger between than final that of compression samples without and nano-bentonitenano‐bentonite. For mixing all the content nano‐bentonite is plotted mixed in Figure soil 4. The finalsamples, compression the final ofcompression samples without is the smallest nano-bentonite when the is thenano smallest,‐bentonite which mixing is 5.2content mm. is When 2%. the nano-bentoniteHowever, this mixing value content is still 5.2% is about higher 0.5%, than thethat finalof the compression samples without of the nano soil‐bentonite. sample This reaches shows a peak valuethat of 5.6 the mm. final Whencompression the nano-bentonite of the clayey soil mixing is significantly content enhanced is higher, by the adding compressibility a small amount decreases, of nano‐bentonite. The strength of the test sample increases with the addition of nano‐bentonite, but at but the final compression of the four groups of samples with nano-bentonite is larger than that of the same time, the settlement becomes larger. Thus, when this method was used to improve soil samples without nano-bentonite. For all the nano-bentonite mixed soil samples, the final compression properties, its adverse effect on settlement need to be carefully considered. Meanwhile, the addition is theof smallest nano‐bentonite when the may nano-bentonite cause a variation mixing in permeability content is of 2%. the However,test samples, this which value will is be still discussed 5.2% higher than thatin the of following the samples part. without nano-bentonite. This shows that the final compression of the clayey soil is significantlyFrom Figure enhanced 4, it can be by seen adding that the a small influence amount of the ofnano nano-bentonite.‐bentonite mixing The content strength on the of final the test samplevoid increases ratio of clayey with the soil additionis significant. of nano-bentonite, When the nano‐bentonite but at the mixing same content time, theis 0.5%, settlement the final becomesvoid larger.ratio Thus, of the when samples this methodwith nano was‐bentonite used to reaches improve its lowest soil properties, value, and its the adverse final void eff ectratio on of settlement nano‐ need tobentonite be carefully mixed considered. clayey soil is Meanwhile, smaller than the pure addition soil. This of nano-bentoniteindicates that the may addition cause of a variationa small in permeabilityamount of nano the test‐bentonite samples, can whichfill the voids will be between discussed clay inparticles, the following therefore, part. the void ratio of clayey soil decreases.

Sustainability 2020, 12, x FOR PEER REVIEW 6 of 16 Figure 3. Curves of compressive deformation s versus consolidation pressure p. Figure 3. Curves of compressive deformation s versus consolidation pressure p.

y = -0.0252x3+0.1024x2-0.118x+0.8729, R2 = 0.9054 y = 0.198x3-0.806x2+0.9293x+5.2528, R2 = 0.9054 6 0.80 Final void ratio

0.82 f (mm) 5.8 e s 0.84 5.6 0.86 5.4

Final compression 0.88 Final void ratio

Final compression 5.2 0.90 0.0 0.5 1.0 1.5 2.0 Nano-bentonite content (%) FigureFigure 4. Relationship 4. Relationship of of di ffdifferenterent nano-bentonite nano-bentonite mixi mixingng contents contents versus versus compressive compressive deformation deformation s s and diandfferent different nano-bentonite nano-bentonite mixing mixing contents contents versus versus void ratio ratio efe. f.

The e–p (e represents the soil void ratio, and p represents the effective pressure) and e–lgp (e represents the soil void ratio, lgp represents the logarithm of effective pressure) curves in Figures 5 and 6, respectively, show that the relationship between the void ratio and consolidation pressure is basically the same irrespective of the nano-bentonite mixing content addition. The compression index is calculated to be 0.404. With the increase in pressure, the soil void ratio decreases gradually. When the consolidation pressure is high, the decreasing trend of the soil void ratio slows down. Figure 6 shows that the initial stage of the curve drops in a gentle parabola tendency, and the back part exhibits a linear correlation with a certain slope. Curve fitting is conducted on the four groups of data, where the fitting curve of soil without nano-bentonite added is y=−0.165lnx + 2.21. When the nano-bentonite mixing content is 0%, 0.5%, 1%, 1.5%, and 2%, the corresponding slope of fitting curve is 0.165, 0.169, 0.173, 0.185, and 0.177, respectively. When the nano-bentonite mixing content is 1.5%, the value of the curve slope reaches the maximum, which shows that the compressibility of soil is high. The pre- consolidation pressure calculated from the e–lgp curve is 99.75 kPa [37].

1.6 Nano-bentonite content 0% 1.4 0.5% e 1% 1.2 1.5% 2% Voidratio 1

0.8 0 1000 2000 3000 4000 Pressure p (kPa)

Figure 5. e–p curves for clayey soil with different nano-bentonite mixing contents added. e represents the void ratio of the soil, while p represents the effective pressure.

Sustainability 2020, 12, x FOR PEER REVIEW 6 of 16

y = -0.0252x3+0.1024x2-0.118x+0.8729, R2 = 0.9054 y = 0.198x3-0.806x2+0.9293x+5.2528, R2 = 0.9054 6 0.80 Final void ratio

0.82 f (mm) 5.8 e s 0.84 5.6 0.86 Sustainability 2020, 12, 459 5.4 6 of 15

Final compression 0.88 Final void ratio

Final compression 5.2 0.90 From Figure4, it can be seen that the influence of the nano-bentonite mixing content on the final 0.0 0.5 1.0 1.5 2.0 void ratio of clayey soil is significant. When the nano-bentonite mixing content is 0.5%, the final Nano-bentonite content (%) void ratio of the samples with nano-bentonite reaches its lowest value, and the final void ratio of nano-bentoniteFigure 4. Relationship mixed clayey of different soil is smallernano-bentonite than pure mixi soil.ng contents This indicates versus compressive that the addition deformation of a s small amountand ofdifferent nano-bentonite nano-bentonite can fillmixi theng voidscontents between versus void clay ratio particles, ef. therefore, the void ratio of clayey soil decreases. The e–p (e represents the soil void ratio, and p represents the eeffectiveffective pressure) and ee–lgp ((e represents the soil void ratio, lgp represents the logarithm of eeffectffectiveive pressure) curves in Figures5 5 and6 6,, respectively,respectively, show show that that the the relationship relationship between between the the void void ratio ratio and and consolidation consolidation pressurepressure isis basically the same irrespective of the nano-bentoni nano-bentonitete mixing content addition. The compression index is calculatedcalculated toto be be 0.404. 0.404. With With the the increase increase in in pressure, pressure, the the soil soil void void ratio ratio decreases decreases gradually. gradually. When When the consolidationthe consolidation pressure pressure is high, is high, the decreasing the decreasing trend trend of the of soil the void soil ratio void slows ratio down.slows down. Figure 6Figure shows 6 thatshows the that initial the stage initial of stage the curve of the drops curve in drops a gentle in a parabolagentle parabola tendency, tendency, and the an backd the part back exhibits part exhibits a linear correlationa linear correlation with a certain with a slope. certain Curve slope. fitting Curve is conductedfitting is conducted on the four on groups the four of groups data, where of data, the where fitting curvethe fitting of soil curve without of soil nano-bentonite without nano-bentonite added is y added= 0.165ln is y=x−+0.165ln2.21.x When + 2.21. the When nano-bentonite the nano-bentonite mixing − contentmixing content is 0%, 0.5%, is 0%, 1%, 0.5%, 1.5%, 1%, and 1.5%, 2%, and the 2%, corresponding the corresponding slope of slope fitting of fitting curve iscurve 0.165, is 0.169,0.165, 0.169, 0.173, 0.185,0.173, and0.185, 0.177, and respectively.0.177, respectively. When the When nano-bentonite the nano-bentonite mixingcontent mixing iscontent 1.5%, the is 1.5%, value the of the value curve of slopethe curve reaches slope the reaches maximum, the maximum, which shows which that theshows compressibility that the compressibility of soil is high. of The soil pre-consolidation is high. The pre- pressureconsolidation calculated pressure from calculated the e–lgp fromcurve the is 99.75e–lgp kPacurve [37 is]. 99.75 kPa [37].

1.6 Nano-bentonite content 0% 1.4 0.5% e 1% 1.2 1.5% 2% Voidratio 1

0.8 0 1000 2000 3000 4000 Pressure p (kPa)

Figure 5. e–p curves for clayey soil with different nano-bentonite mixing contents added. e represents SustainabilityFigure 2020 5. e–, 12p curves, x FOR forPEER clayey REVIEW soil with different nano-bentonite mixing contents added. e represents7 of 16 the void ratio of the soil, while p represents the effective pressure. the void ratio of the soil, while p represents the effective pressure. 1.6

1.4 e

1.2 Nano-bentonite content 0% 0.5% Voidratio 1 1% 1.5% 2% 0.8 1 10 100 1000 10000 Pressure p (kPa)

Figure 6.6. e–lge–lgppcurves curves for for clayey clayey soil withsoil diwithfferent different nano-bentonite nano-bentonite mixing contentsmixing added.contentse representsadded. e representsthe void ratio the ofvoid the ratio soil, whileof the lgsoil,p represents while lgp therepresents logarithm the oflogarithm effective of pressure. effective pressure.

The compression coefficient is one of the critical parameter used to characterize soil compressibility. Compression coefficients of the test soil in various pressure ranges were calculated according to the e–p curves. Table 2 shows that the compression coefficient varies from 0.053 to 0.972. When the applied consolidation pressure was relatively small, both the void ratio and the compression coefficient varied considerably with the nano-bentonite mixing content. With the increase in pressure, the void ratio of the test soil decreased and the compression coefficient also reduced accordingly; however, the influence of nano-bentonite mixing content on the compression coefficient is relatively small. The main reason is that the soil voids at low pressures are relatively large, and therefore the addition of nano-bentonite has a significant influence on the result. While at high pressures, the soil samples were compacted tightly, and effect of the nano-bentonite on compression coefficient is significantly weakened.

Table 2. Coefficient of compressibility changes with the consolidation pressure. The unit of the compression coefficient is MPa−1.

Nano- Consolidation Pressure (kPa) Bentonite Mixing 25–50 50–100 100–200 200–400 400–800 800–1200 1200–2400 2400–3200 Content (%) 0 0.972 0.778 0.950 0.590 0.290 0.161 0.099 0.062 0.5 0.813 0.851 1.199 0.632 0.317 0.166 0.097 0.053 1.0 0.843 0.709 1.107 0.661 0.314 0.164 0.095 0.058 1.5 0.843 0.650 0.855 0.661 0.335 0.180 0.103 0.063 2.0 0.880 0.660 0.905 0.629 0.317 0.171 0.105 0.063

It can be seen from Figure 4 that the effect of nano-bentonite mixing content on the change of the final void ratio of clayey soil is basically the same as that on the final compression amount, that is, the final void ratio of clayey soil with nano-bentonite is smaller than that of the samples without nano-bentonite, and the minimum void ratio of the soil samples without nano-bentonite is 0.87. When the nano-bentonite mixing content is 0.5%, the change of void ratio reaches the peak value, which is 0.83, 4.6% lower than that of the soil samples without nano-bentonite. And in the clayey soil with nano-bentonite, the maximum void ratio is 0.85, it is still 2.3% lower than that without nano- bentonite. At the same time, the addition of a small amount of nano-bentonite will fill the voids between the soil particles, thus reducing the clayey soil void ratio. Figure 4 shows that after the addition of nano-bentonite, as there is an increase of consolidation pressure, the final compression of the test soil increases, and the final void ratio decreases. This is because the addition of nano- bentonite changes the cohesion between clay particles, reduces the number and size of voids between soil particles, alters the soil microstructure, and thus improves the ability of clay to resist compressive Sustainability 2020, 12, 459 7 of 15

The compression coefficient is one of the critical parameter used to characterize soil compressibility. Compression coefficients of the test soil in various pressure ranges were calculated according to the e–p curves. Table2 shows that the compression coe fficient varies from 0.053 to 0.972. When the applied consolidation pressure was relatively small, both the void ratio and the compression coefficient varied considerably with the nano-bentonite mixing content. With the increase in pressure, the void ratio of the test soil decreased and the compression coefficient also reduced accordingly; however, the influence of nano-bentonite mixing content on the compression coefficient is relatively small. The main reason is that the soil voids at low pressures are relatively large, and therefore the addition of nano-bentonite has a significant influence on the result. While at high pressures, the soil samples were compacted tightly, and effect of the nano-bentonite on compression coefficient is significantly weakened.

Table 2. Coefficient of compressibility changes with the consolidation pressure. The unit of the 1 compression coefficient is MPa− .

Nano-Bentonite Consolidation Pressure (kPa) Mixing Content (%) 25–50 50–100 100–200 200–400 400–800 800–1200 1200–2400 2400–3200 0 0.972 0.778 0.950 0.590 0.290 0.161 0.099 0.062 0.5 0.813 0.851 1.199 0.632 0.317 0.166 0.097 0.053 1.0 0.843 0.709 1.107 0.661 0.314 0.164 0.095 0.058 1.5 0.843 0.650 0.855 0.661 0.335 0.180 0.103 0.063 2.0 0.880 0.660 0.905 0.629 0.317 0.171 0.105 0.063

It can be seen from Figure4 that the e ffect of nano-bentonite mixing content on the change of the final void ratio of clayey soil is basically the same as that on the final compression amount, that is, the final void ratio of clayey soil with nano-bentonite is smaller than that of the samples without nano-bentonite, and the minimum void ratio of the soil samples without nano-bentonite is 0.87. When the nano-bentonite mixing content is 0.5%, the change of void ratio reaches the peak value, which is 0.83, 4.6% lower than that of the soil samples without nano-bentonite. And in the clayey soil with nano-bentonite, the maximum void ratio is 0.85, it is still 2.3% lower than that without nano-bentonite. At the same time, the addition of a small amount of nano-bentonite will fill the voids between the soil particles, thus reducing the clayey soil void ratio. Figure4 shows that after the addition of nano-bentonite, as there is an increase of consolidation pressure, the final compression of the test soil increases, and the final void ratio decreases. This is because the addition of nano-bentonite changes the cohesion between clay particles, reduces the number and size of voids between soil particles, alters the soil microstructure, and thus improves the ability of clay to resist compressive deformation. The specific reason for this effect is that the nano-bentonite particles are very small, and the surface lattice structure is different from conventional materials. When it is mixed into the soil, the soil particles are combined and voids are filled, it will change the adhesion characteristics of soil particles, and form new cementation between soil particles, as shown in Figure7. The improvement of the microstructure can effectively improve the strength of the soil.

3.2. Consolidation Characteristics The coefficient of consolidation is a parameter reflecting the drainage rate of soil during consolidation. According to the relationship between soil compression and time that acquired in the test, the coefficient of consolidation of samples with different nano-bentonite mixing contents under various loads can be obtained by using the time square root method [38], as shown in Figure8. It is found that the coefficient of consolidation of the test soil varies in the range of 6.9 10 3 to × − 1.8 10 1 cm2 s 1. Under lower pressure, the coefficient of consolidation of the test soil decreases × − · − remarkably with the increase in pressure, range from about 1.4 10 1 to 2.8 10 2 cm2 s 1. As the × − × − · − pressure continues to increase, the coefficient of consolidation of the test sample decreases slightly. The relationship between coefficient of consolidation and void ratio is investigated, as shown in Figure9. The results show that the coe fficient of consolidation of the soil sample decreases sharply Sustainability 2019, 11, x FOR PEER REVIEW 8 of 16 deformation.Sustainability 2020The, 12specific, 459 reason for this effect is that the nano‐bentonite particles are very small, and8 of 15 the surface lattice structure is different from conventional materials. When it is mixed into the soil, thewith soil theparticles decrease are combined in void ratio. and Atvoids the are void filled, ratio it of will 1.5~1.3, change the the reduction adhesion rate characteristics of the coeffi ofcient soil of particles,consolidation and form is very new significant. cementation between soil particles, as shown in Figure 7. The improvement of the microstructure can effectively improve the strength of the soil.

(a)

(b) Sustainability 2020, 12, x FOR PEER REVIEW 9 of 16 FigureFigureSustainability 7. Filling 7. Filling 2020 effect, 12 e,ff xect FORand and PEERinteraction interaction REVIEW between between soil soil particles: particles: (a) (withouta) without nano nano-bentonite‐bentonite added, added, and9 of and 16 (b)( bwith) with nano nano-bentonite‐bentonite added. added. 250 Nano-bentonite content 3.2. Consolidation Characteristics 250 200 Nano-bentonite0% content The coefficient of consolidation200 is a parameter reflecting0%0.5% the drainage rate of soil during )

-1 150 consolidation. According to the relationship between soil compression0.5%1% and time that acquired in the )

-1 150 test, the coefficient of consolidation of samples with different nano1%1.5%‐bentonite mixing contents under cm²·s 100 various loads can be obtained by-3 using the time square root method1.5%2% [38], as shown in Figure 8. It is cm²·s 100 (10 -3 2% found that the coefficient of consolidation50 of the test soil varies in the range of 6.9 × 10–3 to 1.8 × 10–1 (10 cm2∙s–1. Under lower pressure, the 50coefficient of consolidation of the test soil decreases remarkably Consolidation coefficient with the increase in pressure, range from0 about 1.4 × 10–1 to 2.8 × 10–2 cm2∙s–1. As the pressure continues Consolidation coefficient 0 500 1000 1500 2000 2500 3000 3500 to increase, the coefficient of consolidation0 of the test sample decreases slightly. The relationship 0 500 1000Pressure 1500 2000p (kPa) 2500 3000 3500 between coefficient of consolidation and void ratioPressure is investigated, p (kPa) as shown in Figure 9. The results show that the coefficientFigure of8. Curvesconsolidation of the coefficient of the of soil consolidation sample decreasesversus consolidation sharply pressure. with the decrease in void ratio. At theFigure voidFigure 8. ratioCurves 8. Curves of of1.5~1.3, theof the coe coefficient thefficient reduction of of consolidation consolidation rate of versusthe versus coefficient consolidation consolidation of pressure.consolidation pressure. is very significant. 250 250 Nano-bentonite content 200 Nano-bentonite content 0% 200 ) 0%0.5% -1 150

) 0.5%1% -1 150 cm²·s 100 1%1.5% -3

cm²·s 1.5%2% (10 100 -3 50 2% (10 50 Consolidation coefficient 0 Consolidation coefficient 01.5 1.4 1.3 1.2 1.1 1.0 0.9 0.8 1.5 1.4 1.3 Void1.2 ratio1.1 e 1.0 0.9 0.8

Void ratio e FigureFigure 9. Curves 9. Curves of of the the coe coefficientfficient ofof consolidationconsolidation versus versus the the void void ratio. ratio. Figure 9. Curves of the coefficient of consolidation versus the void ratio. Taking the consolidation pressure of 1800 kPa as an example, the influence of nano-bentonite mixingTaking content the onconsolidation the coefficient pressure of consolidation of 1800 kPa of as the an testexample, soil was the further influence studied, of nano-bentonite as shown in mixingFigure 10.content For other on the pressures, coefficient the of phenomenon consolidation is ofnot the as testobvious soil wasas the further 1800 kPastudied, pressure, as shown but the in Figurepattern 10. is similar.For other The pressures, results show the phenomenon that when th ise notnano-bentonite as obvious as mixing the 1800 content kPa pressure,is between but 0.5% the patternand 2%, is the similar. curve Theshows results a non-linear show that increasing when th etr nano-bentoniteend with the increase mixing in content nano-bentonite is between mixing 0.5% andcontent. 2%, Whenthe curve the showsnano-bentonite a non-linear mixing increasing content tr isend 2%, with the theconsolidation increase in rate nano-bentonite of clayey soil mixing is the content.largest, and When the thecoefficient nano-bentonite of consolidation mixing iscontent about is2.14 2%, times the largerconsolidation than the rate sample of clayey with 0.5% soil nano-is the largest,bentonite and mixing the coefficient content. ofIn consolidation the consolidation is about test 2.14, the times coefficient larger than of cons the olidationsample with of the0.5% sample nano- bentonitewithout nano-bentonite mixing content. was In 8.7 the × 10consolidation−3 cm2·s−1, which test ,is the 6.6 coefficient× 10−3 cm2·s of−1 lowerconsolidation than that of of thethe sample withoutwith 2% nano-bentonite wasmixing 8.7 content.× 10−3 cm It2·s seems−1, which that is the 6.6 consolidation × 10−3 cm2·s−1 lowerrate is thanlower that than of that the sampleof pure withsoil, 2%except nano-bentonite for the soil mixing with content.2% nano-bentonite It seems that mixingthe consolidation content. Thisrate isshows lower thanthat thatadding of pure an soil,appropriate except fornano-bentonite the soil with mixing 2% nano-bentonite content to th emixing soil can content. accelerate This theshows consolidation that adding rate. an appropriateCompared withnano-bentonite pure soil, whenmixing the content nano-bentonite to the soilmixing can contentaccelerate is lessthe thanconsolidation 1.5%, the rate.soil Comparedpermeability with is smaller. pure soil, If the when nano-bentonite the nano-bentonite mixing content mixing is 2%,content the permeabilityis less than coefficient1.5%, the willsoil permeabilitybe larger. is smaller. If the nano-bentonite mixing content is 2%, the permeability coefficient will be larger.

Sustainability 2020, 12, 459 9 of 15

Taking the consolidation pressure of 1800 kPa as an example, the influence of nano-bentonite mixing content on the coefficient of consolidation of the test soil was further studied, as shown in Figure 10. For other pressures, the phenomenon is not as obvious as the 1800 kPa pressure, but the pattern is similar. The results show that when the nano-bentonite mixing content is between 0.5% and 2%, the curve shows a non-linear increasing trend with the increase in nano-bentonite mixing content. When the nano-bentonite mixing content is 2%, the consolidation rate of clayey soil is the largest, and the coefficient of consolidation is about 2.14 times larger than the sample with 0.5% nano-bentonite mixing content. In the consolidation test, the coefficient of consolidation of the sample without nano-bentonite was 8.7 10 3 cm2 s 1, which is 6.6 10 3 cm2 s 1 lower than that of the × − · − × − · − sample with 2% nano-bentonite mixing content. It seems that the consolidation rate is lower than that of pure soil, except for the soil with 2% nano-bentonite mixing content. This shows that adding an

Sustainabilityappropriate 2020 nano-bentonite, 12, x FOR PEER mixing REVIEW content to the soil can accelerate the consolidation rate. Compared 10 of 16 with pure soil, when the nano-bentonite mixing content is less than 1.5%, the soil permeability is smaller. If the nano-bentonite mixing content is 2%, the permeability coefficient will be larger.

2 2 Cv= 8.8007x -14.561x+8.8218, R =0.9829 k = 3.0069x2-4.5179x+3.5591, R2=0.9994 )

7 )

-1 18 -1 ·s 2 16 cm 6 cm·s -3 -6 14 (10

v 5 12 p=1800 kPa V0.5 C -

10 0.5 4 V2 k k 2- C 8 k 3 6 Cv 2 4

2 1 Permeability (10coefficient k Consolidation coefficient coefficient C Consolidation 0.0 0.5 1.0 1.5 2.0 Nano-bentonite content (%)

Figure 10. RelationshipsRelationships of the coefficient coefficient of consolidation Cv andand the the permeability permeability coefficients coefficients k with differentdifferent nano-bentonite mixing contents.contents. Cv is thethe coecoefficientfficient ofof consolidationconsolidation ofof soil,soil, C Cv0.5v0.5 (or CCv2v2)) means the coefficient coefficient of consolidation of soil with th thee nano-bentonite nano-bentonite mixing mixing content content of of 0.5% 0.5% (or (or 2.0%). 2.0%).

k is the permeability coecoefficientfficient ofof clay,clay, kk0.50.5 (or k 2)) means means the the permeability permeability coefficient coefficient of soil with the nano-bentonite mixing content of 0.5% (or 2.0%). P is the consolidation pressure. 3.3. Secondary Consolidation Characteristics 3.3. Secondary Consolidation Characteristics For clayey soil, secondary consolidation is significant, which will lead to large post-construction For clayey soil, secondary consolidation is significant, which will lead to large post-construction settlement in engineering practices. The long-term foundation settlement due to secondary settlement in engineering practices. The long-term foundation settlement due to secondary consolidation cannot be ignored in most cases. In terms of mechanism, the secondary consolidation of consolidation cannot be ignored in most cases. In terms of mechanism, the secondary consolidation clayey soil is a slow deformation process caused by creeping of bound water film on the soil particle of clayey soil is a slow deformation process caused by creeping of bound water film on the soil surface and the rearrangement of soil particles. This phenomenon usually occurs when the excess pore particle surface and the rearrangement of soil particles. This phenomenon usually occurs when the water pressure is fully dissipated, and the effective pressure is substantially stable. excess is fully dissipated, and the effective pressure is substantially stable. The coefficient of secondary consolidation of soil can be calculated from the slope of the back The coefficient of secondary consolidation of soil can be calculated from the slope of the back section of the curve of void ratio versus log time. Figure 11 shows the coefficient of secondary section of the curve of void ratio versus log time. Figure 11 shows the coefficient of secondary consolidation of the test soil with different nano-bentonite mixing contents. It is shown that the consolidation of the test soil with different nano-bentonite mixing contents. It is shown that the coefficient of secondary consolidation varies from 0.3 10 3 to 8.9 10 3, and its law of change coefficient of secondary consolidation varies from 0.3 × ×10−3 −to 8.9 × 10×−3, and− its law of change with with consolidation pressure is as follows: when the consolidation pressure is less than 500 kPa, the consolidation pressure is as follows: when the consolidation pressure is less than 500 kPa, the coefficient of secondary consolidation increases significantly with the increase in pressure; when the coefficient of secondary consolidation increases significantly with the increase in pressure; when the consolidation pressure is greater than 500 kPa, the coefficient of secondary consolidation varies within a small range with the increase of consolidation pressure. This is because when the consolidation pressure is low, as the pressure increases, the consolidation deformation of the clayey soil becomes less obvious, whereas the secondary consolidation effect gradually dominates the deformation process. However, as the pressure continues to increase, the clayey soil is tightly compacted, and therefore the secondary consolidation is weakened. Sustainability 2020, 12, 459 10 of 15 consolidation pressure is greater than 500 kPa, the coefficient of secondary consolidation varies within a small range with the increase of consolidation pressure. This is because when the consolidation pressure is low, as the pressure increases, the consolidation deformation of the clayey soil becomes less obvious,Sustainability whereas 2020, 12, thex FOR secondary PEER REVIEW consolidation effect gradually dominates the deformation process. 11 of 16 However, as the pressure continues to increase, the clayey soil is tightly compacted, and therefore the secondarySustainability consolidation2020, 12, x FOR PEER is weakened.10 REVIEW 11 of 16

108 ) -3 86 ) -3 Nano-bentonite content 6 4 0% Nano-bentonite0.5% content 4 1% consolidation (10 consolidation 2 0%

Coefficient of secondary of secondary Coefficient 0.5%1.5% 1%2% consolidation (10 consolidation 20 Coefficient of secondary of secondary Coefficient 1.5% 0 500 1000 1500 2000 25002% 3000 3500 0 Pressure p (kPa) 0 500 1000 1500 2000 2500 3000 3500 Figure 11. Curves of the coefficient ofPressure secondary p (kPa) consolidation versus the void ratio.

Figure 11. Curves of the coefficient of secondary consolidation versus the void ratio. The relationshipFigure 11. Curvesbetween of thethe coefficient nano-bentonite of secondary mixing consolidation content and versus the the coefficient void ratio. of secondary consolidation is shown in Figure 12. Under high pressure condition, the addition of nano-bentonite The relationship between the nano-bentonite mixing content and the coefficient of secondary in clayeyThe relationshipsoil has a significant between effectthe nano-bentonite on the coefficient mixing of secondary content and consolidation. the coefficient With of the secondary increase consolidation is shown in Figure 12. Under high pressure condition, the addition of nano-bentonite in consolidationin the nano-bentonite is shown in mixing Figure 12.content, Under thehigh co prefficientessure condition, of secondary the addi consolidationtion of nano-bentonite increases clayey soil has a significant effect on the coefficient of secondary consolidation. With the increase in the incontinuously clayey soil hasand a reachessignificant its effectpeak onvalue the coeffiwhencient the nano-bentoniteof secondary consolidation. mixing content With is the 1.5%. increase This nano-bentonite mixing content, the coefficient of secondary consolidation increases continuously and inindicates the nano-bentonite that the size and mixing shape content,of voids inthe the co clayeyefficient soil of may secondary change dueconsolidation to the physical increases filling reaches its peak value when the nano-bentonite mixing content is 1.5%. This indicates that the size and continuouslyeffect of nano-bentonite and reaches particles. its peak The value rearrangem when thente ofnano-bentonite the soil structure mixing and contentcreeping is of 1.5%. the bound This shape of voids in the clayey soil may change due to the physical filling effect of nano-bentonite particles. indicateswater film that in the the size nano-bentonite and shape of voidsmixed in soil the arclayeye slower soil maythat changethe pure due soil to thein physicalthe secondary filling The rearrangement of the soil structure and creeping of the bound water film in the nano-bentonite effectconsolidation of nano-bentonite stage. Indeed, particles. this effect The rearrangem is not obviousent atof the pressuresoil structure of 500 and kPa creeping or lower, of whichthe bound may mixed soil are slower that the pure soil in the secondary consolidation stage. Indeed, this effect is not waterbe due film to the in fact the that nano-bentonite under low pressures, mixed soilthe nano-bentoniteare slower that particles the pure have soil little in effect the secondaryon the soil obvious at the pressure of 500 kPa or lower, which may be due to the fact that under low pressures, the consolidationskeleton. stage. Indeed, this effect is not obvious at the pressure of 500 kPa or lower, which may nano-bentonite particles have little effect on the soil skeleton. be due to the fact that under low pressures, the nano-bentonite particles have little effect on the soil skeleton. 9

9 8 ) -3 8 )

-3 7 800 kPa 7 6 1200 kPa

consolidation (10 8002400 kPakPa Coefficient of secondary secondary Coefficient of 6 12003200 kPakPa

consolidation (10 5 2400 kPa Coefficient of secondary secondary Coefficient of 0 0.5 1 1.53200 kPa 2 5 Nano-bentonite content (%)

0 0.5 1 1.5 2 Figure 12. Curves of the coefficient of secondaryNano-bentonite consolidation content versus (%) nano-bentonite mixing content. Figure 12. Curves of the coefficient of secondary consolidation versus nano-bentonite mixing content. Figure 12. Curves of the coefficient of secondary consolidation versus nano-bentonite mixing 3.4. Permeabilitycontent. Soil permeability has an essential influence on the consolidation and strength of soil, which will 3.4. Permeability seriously affect the safety of geotechnical infrastructures. Therefore, it is necessary to study the permeabilitySoil permeability of nano-bentonite has an essential mixed influence soil. According on the consolidationto Terzaghi’s andone-dimensional strength of soil, consolidation which will seriouslytheory [39], affect the relationshipthe safety of between geotechnical the permeability infrastructures. coefficient Therefore, and coefficient it is necessary of consolidation to study theis permeability of nano-bentonite mixed soil. According to Terzaghi’s one-dimensional consolidation theory [39], the relationship between the permeability coefficient and coefficient of consolidation is Sustainability 2020, 12, 459 11 of 15

3.4. Permeability Soil permeability has an essential influence on the consolidation and strength of soil, which will seriously affect the safety of geotechnical infrastructures. Therefore, it is necessary to study the permeability of nano-bentonite mixed soil. According to Terzaghi’s one-dimensional consolidation Sustainabilitytheory [39], 2020 the, 12 relationship, x FOR PEER betweenREVIEW the permeability coefficient and coefficient of consolidation 12 of is 16

Cvγwav k = 𝐶𝛾𝑎 (1) 𝑘= 1 + e0 (1) 1+𝑒 where k is the permeability coefficient of the soil, av is the compressibility coefficient, e0 is the initial void where k is the permeability coefficient of the soil, 𝑎 is the compressibility coefficient, 𝑒 is the initial ratio of soil, and γw is the specific weight of water. All the samples show similar behavior, independent 𝛾 voidof the ratio additive of soil, content. and The is permeability the specific coeweightfficient of obtainedwater. All from the Equation samples (1)show was similar used to behavior, study the independentpermeability withof the nano-bentonite additive content. mixed The soil. permeability coefficient obtained from Equation (1) was used Figureto study 13 theshows permeability the variation with of nano-bentonite permeability coe mixedfficient soil. with the change in consolidation pressure. Figure 13 shows the variation of permeability coefficient with6 the change4 in1 consolidation It is found that the permeability coefficient varies in the order of 10− to 10− cm s− , and is greatly −6 · −4 −1 pressure.influenced It byis consolidationfound that the pressure. permeabilit In they coefficient whole test varies process, in the order permeability of 10 to coe 10fficientcm·s decreases, and is greatlywith the influenced increase in by consolidation consolidation pressure. pressure. ItIn is th alsoe whole found test that process, the permeability the permeability coefficient coefficient of the decreasesclayey soil with samples the increase decreases in consolidation significantly pressure. when the It consolidationis also found that pressure the permeability is low, ranging coefficient from of the clayey4 soil samples5 decreases1 significantly when the consolidation pressure is low, ranging 4.5 10− to 6 10− cm s− . As the pressure continues to increase, the permeability coefficient tends × −4× −·5 −1 fromto stabilize. 4.5 × 10 to 6 × 10 cm·s . As the pressure continues to increase, the permeability coefficient tends to stabilize. 6 Nano-bentonite content 0% )

-1 5 0.5% 1% cm·s

-4 1.5% 4 2% 0.5 0.4 3 0.3 0.2 2 0.1 0.0 0 500 1000 1500 2000 2500 3000 1 Permeability Permeability coefficient (10 0 0 500 1000 1500 2000 2500 3000 Pressure p (kPa) Figure 13. Curves ofof thethe permeabilitypermeability coe coefficientfficient versus versus consolidation consolidation pressure. pressure. The The small small graph graph in thein thefigure figure is an is enlargementan enlargement of the of Y-axisthe X-axis in the in rangethe range of 0–0.5. of 0–0.5.

The soil permeability is closely related to its void ratio. As As shown shown in in Figure Figure 14,14, the the permeability permeability coefficientcoefficient significantlysignificantly reduces reduces with with the the decrease decrease of theof the void void ratio. ratio. By fittingBy fitting the e –lgthek ecurve,–lgk curve, it is found it is foundthat the that relationship the relationship between between the void ratiothe voide and ratio logarithm e and of logarithm permeability of coepermeabilityfficient is approximately coefficient is approximatelylinear, which indicates linear, which that the indicates void ratio that plays the void a dominant ratio plays role ina dominant affecting soilrole permeability in affecting [soil40]. permeabilityAt the nano-bentonite [40]. At mixingthe nano-bentonite content of 2%, mixing the linear content relationship of 2%, can the be linear expressed relationship as follows: can ln kbe= 8.7275e 4.27. expressed− as follows: lnk = 8.7275e−4.27. Sustainability 2020, 12, 459 12 of 15 Sustainability 2020, 12, x FOR PEER REVIEW 13 of 16 Sustainability 2020, 12, x FOR PEER REVIEW 13 of 16 1000 )

-1 1000 Nano-bentonite content )

-1 Nano-bentonite0% content cm·s

-6 0%0.5% cm·s -6 100 0.5%1% 100 1%1.5% 1.5%2% 2% 10 10 Permeability Permeability coefficient (10 1 Permeability Permeability coefficient (10 1 1.6 1.4 1.2 1 0.8 1.6 1.4 Void1.2 ratio e 1 0.8 Figure 14. Curves of the permeabilityVoid ratiocoefficient e versus void ratio. Figure 14. Curves of the permeability coefficient versus void ratio. Figure 14. Curves of the permeability coefficient versus void ratio. When the consolidation pressure is 1800 kPa, the influence of nano-bentonite on the permeability When the consolidation pressure is 1800 kPa, the influence of nano-bentonite on the permeability coefficientWhen theof theconsolidation soil samples pressure is shown is 1800 in kPa,Figure the 10. influence When ofthe nano-bentonite nano-bentonite on mixing the permeability content is coefficient of the soil samples is shown in Figure 10. When the nano-bentonite mixing content is coefficientbetween 0.5% of the and soil 2%, samples the influence is shown of in the Figure nano-b 10.entonite When the mixing nano-bentonite content on mixing the permeability content is between 0.5% and 2%, the influence of the nano-bentonite mixing content on the permeability coefficient betweencoefficient 0.5% is similar and 2%, to thatthe influenceof the coefficient of the nano-b of consolidation.entonite mixing That is,content with theon increasethe permeability in nano- is similar to that of the coefficient of consolidation. That is, with the increase in nano-bentonite mixing coefficientbentonite mixingis similar content, to that the of curve the coefficient of permeability of consolidation. coefficients showsThat is, a non-linearwith the increase increasing in nano-trend. content, the curve of permeability coefficients shows a non-linear increasing trend. The permeability bentoniteThe permeability mixing content, coefficient the of curve clayey of soilpermeability with 2% nano-bentonite coefficients shows mixing a non-linear content is increasing 3.15 times trend.higher coefficient of clayey soil with 2% nano-bentonite mixing content is 3.15 times higher than that with 0.5% Thethan permeability that with 0.5% coefficient nano-bentonite of clayey soil mixing with 2%content. nano-bentonite To better mixing compare content the isinfluence 3.15 times of higher nano- nano-bentonite mixing content. To better compare the influence of nano-bentonite on the permeability thanbentonite that onwith the 0.5% permeability nano-bentonite coefficient, mixing the permeabilitycontent. To coefficientbetter compare results the of soilinfluence with 0% of and nano- 2% coefficient, the permeability coefficient results of soil with 0% and 2% nano-bentonite addition under bentonitenano-bentonite on the addition permeability under coefficient, low and high the permeabilitypressure ranges coefficient were presented results of in soil Figure with 15.0% It and is seen 2% low and high pressure ranges were presented in Figure 15. It is seen in Figure 15a that the permeability nano-bentonitein Figure 15a that addition the permeability under low andcoefficient high pressure decreases ranges with were the addition presented of innano-bentonite Figure 15. It is at seen low coefficient decreases with the addition of nano-bentonite at low consolidation pressures, while the inconsolidation Figure 15a that pressures, the permeability while the difference coefficient becomes decreases less with obvious the addition with the of increase nano-bentonite of pressure. at low The difference becomes less obvious with the increase of pressure. The reason is because nano-bentonite consolidationreason is because pressures, nano-bentonite while the particlesdifference have becomes little lesseffect obvious on the withsoil skeletonthe increase under of pressure.low pressure. The particles have little effect on the soil skeleton under low pressure. At a higher pressure (more than reasonAt a higher is because pressure nano-bentonite (more than 500particles kPa), havethe permeability little effect on coefficient the soil skeleton of nano-bentonite under low mixed pressure. soil 500 kPa), the permeability coefficient of nano-bentonite mixed soil is higher, which indicates that Atis higher,a higher which pressure indicates (more that than nano-bentonite 500 kPa), the permeabilitycontributes to coefficient permeability of nano-bentonite modification of mixed soil. soil isnano-bentonite higher, which contributes indicates that to permeabilitynano-bentonite modification contributes of to soil. permeability modification of soil. )

6 )

-1 0.3 -1 ) 6 Nano-bentonite content ) -1 0.3 Nano-bentonite content -1

cm·s 5 cm·s -4 Nano-bentonite0% content -4 Nano-bentonite content

cm·s 5 0% cm·s -4

4 -4 0%2% 0.2 0%2% 4 3 2% 0.2 2% 3 2 0.1 2 1 0.1 1 0 0.0 Permeability coefficient (10 coefficient Permeability 0 100 200 300 400 500 (10 coefficient Permeability 0 0.0 500 1000 1500 2000 2500 3000 Permeability coefficient (10 coefficient Permeability 0 100Pressure 200 p (kPa) 300 400 500 (10 coefficient Permeability Pressure p (kPa) 500 1000 1500 2000 2500 3000 Pressure p (kPa) Pressure p (kPa) (a) (b) (a) (b) Figure 15. Curves of the permeability coefficient versus consolidation pressure: (a) permeability Figurecoefficient 15. CurvesofCurves soil with of of the a consolid permeabilityation pressure coecoefficientfficient of 0–500 versus kPa, consolidation and (b) permeability pressure: coefficient( a)) permeability of soil coefficientcoewithffi cienta consolidation of soil with pressure a consolidationconsolid of ation500–3000 pressure kPa. of 0–500 kPa, and ( b) permeability coefficient coefficient of soil with a consolidation pressure of 500–3000 kPa. Based on the above findings, it is concluded that, with the increase of consolidation pressure, the permeabilityBased on the coefficientabove findings, findings, of the it clayey is concluded soil shows that that, a, decreasingwith the increase trend. ofThis consolidation is because under pressure, pressure, low theconsolidation permeability pressure, coefficient coefficient the of porous the clayey flow in soil the showssh owsclayey a decreasingsoil is mainly trend. dominated This is because by free under water low and consolidationbound water. pressure,pressure, With the the the dissipated porous porous flow flowof inpore thein the clayeywater clayey soilpressure, soil is mainly is mainlythe dominated void dominated ratio by changes free by water free significantly, andwater bound and boundwater.resulting Withwater. in thea substantialWith dissipated the dissipated change of pore in water theof porek pressure,value. water Under the pressure, void high ratio consolidation the changes void ratio significantly, pressure, changes the resultingsignificantly, porous flow in a resultingof clayey in soil a substantial is mainly dominatedchange in the by kbound value. water.Under Athigh this consolidation time, the pore pressure, water isthe hard porous to drain, flow ofshowing clayey asoil characteristic is mainly dominated that the permeability by bound water.change At is thissmall time, and thetends pore to bewater stable. is hard The toinfluence drain, showing a characteristic that the permeability change is small and tends to be stable. The influence Sustainability 2020, 12, 459 13 of 15 substantial change in the k value. Under high consolidation pressure, the porous flow of clayey soil is mainly dominated by bound water. At this time, the pore water is hard to drain, showing a characteristic that the permeability change is small and tends to be stable. The influence of nano-bentonite on the permeability coefficient is as follows: when the nano-bentonite mixing content is 0.5%–2%, the curve shows a non-linear increasing trend with the increase in the nano-bentonite mixing content. The permeability coefficient of the test soil with nano-bentonite is higher than the pure soil under high pressure. This is because adding nano-bentonite changes the bonding interaction between clay particles, forming new cementation between clay particles, playing a good bridge role, providing a new channel for water discharge, and promoting drainage consolidation [41]. Under high pressure, nano-bentonite bears, transfers, and disperses part of the pressure, and sufficient hydration and a hardening reaction occur between the clayey soil particles, which improves the hydration speed of the clay particles and is conducive to the drainage consolidation of the clay. Under low pressure, the results are different from high pressure. The effect of the nano-bentonite mixing content on the soil structure is only obvious under high pressure. When the pressure is low, the effect is not obvious.

4. Conclusions In this paper, the one-dimensional consolidation characteristics of nano-bentonite mixed clayey soil were investigated through laboratory experiments. The influence of the nano-bentonite mixing content on the compression coefficient, coefficient of consolidation, coefficient of secondary consolidation, and permeability coefficient was analyzed. The following conclusions are drawn: (1) Mixing 0.5% to 2% nano-bentonite into the clayey soil has little effect on the original consolidation characteristics. However, when the consolidation pressure increases gradually, the final settlement, coefficient of secondary consolidation, and coefficient of consolidation of the clayey soil with the addition of nano-bentonite all show an increasing trend, and the test soil has a larger permeability coefficient under high pressure, which indicates that the addition of nano-bentonite accelerates the drainage consolidation of the clayey soil. (2) After adding a small amount of nano-bentonite, the clayey soil exhibits different consolidation characteristics. When the nano-bentonite mixing content is 0.5%, the final settlement reaches a maximum, and the final void ratio is the smallest. Under high pressure conditions, the consolidation and permeability coefficients increase with the increase in nano-bentonite mixing content, which indicates that nano-bentonite has modified the consolidation and permeability characteristics of clay. (3) The mechanism of consolidation and permeability of nano-bentonite mixed soil is mainly manifested in the cementation filling effect of nano-bentonite. The addition of nano-bentonite changes the adhesion and bonding between clay particles. At the same time, the number of voids between clay particles reduces and the size of the voids become smaller. The new cementation between clay particles rearranges the microstructure of the clayey soil. As such, the ability of the clayey soil to resist compression deformation is improved.

Author Contributions: H.-H.Z. designed the overall framework and conceived the idea of this paper, G.C., Y.-N.W. and H.-H.Z. designed the experiments and analyzed the data, B.S. and L.G. provided some suggestions on the structure of the paper, and G.C., Y.-N.W. and H.-H.Z. wrote the paper. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Key Research and Development Program of China (No. 2018YFC1505104), National Natural Science Foundation of China (No. 41702347), Natural Science Foundation of Hebei Province, China (No. D2018508107), Key Laboratory of Ministry of Education for Geomechanics and Embankment Engineering, Hohai University (No. 201709), China Postdoctoral Science Foundation (No. 2019M651786), Hebei IoT Monitoring Engineering Technology Research Center (No. 3142018055), and Key Research and Development Program of Hebei Province, China (No. 19270318D). Acknowledgments: The authors would like to thank Dingfeng Cao, Qing Cheng and Jinghong Wu for their assistance in revising the manuscript. Conflicts of Interest: The authors declare no conflict of interest. Sustainability 2020, 12, 459 14 of 15

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