Experimental Study on the Swelling Behavior of Expansive Soil at Different Depths Under Unidirectional Seepage

applied sciences

Article Experimental Study on the Swelling Behavior of Expansive Soil at Different Depths under Unidirectional Seepage

Jianbo She 1,2, Zheng Lu 1,3,*, Hailin Yao 1,3, Ran Fang 1,2 and Shaohua Xian 1,2 1 State Key Laboratory of Geomechanics and Geotechnical Engineering, Institute of Rock and Soil Mechanics, Chinese Academy of Sciences, Wuhan 430071, China; [email protected] (J.S.); [email protected] (H.Y.); [email protected] (R.F.); [email protected] (S.X.) 2 University of Chinese Academy of Sciences, Beijing 100049, China 3 Hubei Key Laboratory of Geo-Environmental Engineering, Wuhan 430071, China * Correspondence: [email protected]

Received: 21 February 2019; Accepted: 20 March 2019; Published: 24 March 2019

Abstract: The swelling properties of expansive soil at different depths were investigated by the laboratory model test. It was experimentally found that the swelling deformation of different expansive soil layers was obviously different under unidirectional seepage. Pore solution collection boxes were designed to collect the pore solution of different soil layer interfaces during the saturation process. Cation concentration tests were then used to analyze the migration regulation of cations in these soil layers. Results showed that the divalent cations in the surface soil layer migrated downward along with water molecules and aggregated around clay particles to form the electrical environment in expansive soil. Additionally, swelling tests were performed to compare with their initial values to research the variation of swelling behavior of expansive soil at different depths after completion of the model test. It was observed that the swelling potential of the surface expansive soil layer at a depth of 0–30 mm was released while that of the middle and bottom ones at depths of 90–120 mm and 180–210 mm was inhibited. An analysis on the above results was used to clarify the mechanism controlling the swelling difference of different expansive soil layers. This swelling difference may be explained by the joint effect of two factors that can be treated as an electric charge effect, namely (i) the modiﬁcation effect, (ii) the electrical environment.

Keywords: expansive soil; swelling behavior; soil depth; unidirectional seepage; electric charge effect

1. Introduction Expansive soil is one typical special soil that is very sensitive to the change in moisture content, which is widely distributed around the world [1]. Such soil swells and shows the considerable volumetric deformation in response to changes in moisture content [2]. Consequently, expansive soil causes serious damage and distortion to structures, particularly for light buildings and pavements [3,4]. As a result, the swelling behavior of expansive soil is closely related to the safety and stability of engineering structures [5,6]. Free swelling ratio, swelling pressure, and swelling percent are usually identiﬁed as three important swelling characteristics which are required for the assessment of safe and economical designs of engineering structures resting on expansive soils. However, in the process of engineering design and construction, it is conventionally considered that expansive soil is a homogeneous material that the swelling behavior is the inherent property and consistent along the depth of the soil layer. In fact, the parameters such as moisture content, void ratio and swelling indexes of different soil layers in the active zone are not the same under actual ﬁeld conditions. Hence, their swelling behavior is different even under saturated state. In this respect, the results of numerous

Appl. Sci. 2019, 9, 1233; doi:10.3390/app9061233 www.mdpi.com/journal/applsci Appl. Sci. 2019, 9, 1233 2 of 13 laboratory and field tests on black cotton soil from MRBC 76 reported by Katti and Katti [7] are noteworthy. It was found that the lower black cotton soil layer will not produce excessive swelling deformation though the swelling pressure (224 KPa) is far greater than the weight stress of the upper soil layer under saturated conditions. In particular, when the thickness of the upper black cotton soil layer reaches 1.2 m, the swelling of the lower black cotton soil can be completely inhibited, and its moisture content, void ratio, dry density and shear strength remain unchanged. This finding indicates that the black cotton soil at different depths has an obvious swelling difference during the saturated process [7,8]. Previous studies revealed that the types and concentrations of cations in soil solution have significant influence on the potential of soil surface and its electrochemical properties [9,10]. In the soil–water–electrolyze system, as a kind of particular clay soil that contains a certain amount of montmorillonite and carries negative charges, the surface of clay particles is absorbed by the cations, so that a very high ion concentration is induced surrounding it [11]. Consequently, it can be speculated that the swelling difference of expansive soil at different depths is closely related to its electrochemical properties and may depend largely on the type and concentration of electrolyte in pore solution. Several scholars have pointed out that different kinds and concentrations of electrolyte can cause the expansion of swelling clays to varying degrees [12–15]. It was demonstrated that different pore solutions have a significant effect on the swelling characteristics of bentonite [16–21]. Lee et al. [17] demonstrated that the swelling pressure of Ca-bentonite is distinct from that of Na-bentonite, and this difference depends on the concentration of electrolyte, which can be ascribed to changes to the thickness of diffuse double-layer (DDL) or cation exchange [22]. According to DDL theory, the DDL thickness decreases as the concentration of solutions increases, resulting in a decrease in the swelling pressure. Additionally, one cation with lower valence can be replaced by another with higher valence. So, when Na–bentonite is infiltrated with calcium solutions, sodium will be replaced by calcium [23,24], resulting in a reduction in the swelling potential. Therefore, when the cation concentration of bulk liquid in expansive soil changes, the electrochemical properties of expansive soil will also change, which finally affects its macroscopic swelling characteristics. Apparently, the difference of moisture content in the upper and lower soil layers will affect the magnitude of concentration of electrolyte in pore solution, resulting in the different swelling deformation. Based on the above analyses, this paper studied the variation regularity of swelling behavior of expansive soil at different depths by laboratory model test under saturated condition of water (de-ionized water) injection from top to bottom. Given the variation of cation concentration in pore solution at different soil layer interfaces in the saturated process, before and after the model test, as well as the variation of swelling characteristics of expansive soil, it is expedient to clarify the mechanism of swelling difference of expansive soil at different depths.

2. Materials and Methods

2.1. Materials The basic physical and chemical properties of expansive soil selected for this study are listed in Table1. An X-ray diffraction (XRD) test was conducted with D8 Advance X-ray diffractometer at the tube voltage of 40 KV and current of 40 mA, using a focused and monochromatized Cu Kα source. In the 20 ◦C and 60% relative humidity conditions, a homogeneous powder with a particle size of <20 µm was ground with an agate mortar. According to the method in Test Methods of Soils for Highway Engineering (JTG E40-2007) [25], the soil samples were dried to a constant weight and then were ground until they all passed through a 1 mm standard sieve, the above cation concentration could be obtained by a HACH DR1900 portable spectrophotometer and a HACH HQD portable analyzer to measure the solution through Appl. Sci. 2019, 9, 1233 3 of 13 centrifugation from the soil samples added with de-ionized water. The measured method is based on the Water Analysis Handbook published by the Hach Company.

Table 1. Basic physical and chemical parameters of materials used in experimental work.

Property Expansive Soil Grain size distribution Sand (%) 3.90 Silt (%) 31.70 Clay (%) 64.40 Specific gravity 2.70 Atterberg limits (%) Liquid limit 63.00 Plastic limit 24.94 Unified soil classification CH Compaction properties (standard Proctor) Optimum moisture content (%) 16.20 Maximum dry density (g/cm3) 1.80 Swell properties Free swelling ratio (%) 65.50 Swelling pressure (KPa) 182.72 Swelling percent (%) 19.00 Main minerals Montmorillonite (%) 23.17 Kaolinite (%) 5.93 Quartz (%) 59.65 Calcite (%) 11.25 Main cations of pore solution (mg/L) Na+ 12.50 K+ 10.00 Mg2+ 35.00 Ca2+ 405.50

2.2. Test Apparatus The experimental setup for the laboratory model test is shown in Figure1. It is composed of three parts: pore solution collection box, polymethyl methacrylate (PMMA) strip, and PMMA box. As cation concentration in soil bulk solution cannot be accurately and conveniently measured, but it can be reﬂected by the change of that in soil pore solution according to the Boltzmann distribution equation. Therefore, pore solution collection boxes were designed to collect the pore solution of different expansive soil layers in the saturation process for analyzing the migration regulation of cations. It contains a rectangle case, a water outlet pipe, and an intake pipe. These two pipes are respectively connected to both sides of the collection box, the former bottom being connected to the pump water device and the latter being used for air admission. The pump water device with the maximum negative pressure of 100 KPa can ensure that the pore solution in soil layer is extracted into separate container. The size of collection box is 5 cm × 5 cm × 2cm, its upper cover has multiple holes of 1 mm diameter, and the whole box body is wrapped with a layer of geotextile. The size of PMMA box is 30 cm × 30 cm × 30 cm, its outer wall is marked with a scale on four sides, and the minimum scale is 1 mm. Four PMMA strips with dimensions 10 cm × 2 cm × 0.5 cm are placed at the four sides of different soil layers every 30 mm. When expansive soil produces swelling deformation, the PMMA strip will move along with it. This value can be observed by the scale in outer wall of the PMMA box, which is the swelling deformation of expansive soil. Appl. Sci. 2019, 9, 1233 4 of 13 Appl. Sci. 2019, 9, x FOR PEER REVIEW 4 of 13

Figure 1. 1.Model Model test test apparatus: apparatus: 1 Pore① Pore solution solution collection collection box; 2 polymethyl box; ② polymethyl methacrylate methacrylate (PMMA) (strip;PMMA 3 )PMMA strip; ③ box. PMMA box.

AAss shownshown inin FigureFigure1 ,1, the the depth depth of expansiveof expansive soil soil is 210 is 210 mm, mm, and theand thickness the thickness of each of soileach layer soil layeris 30 mm.is 30 Themm. surface The surface expansive expansive soil refers soil refers to the to soil the layersoil layer at a depthat a depth of 0–30 of 0 mm,–30 mm, the middlethe middle and andbottom bottom expansive expansive soil aresoil the are soil the layerssoil layers at depths at depth of 90–120s of 90– mm120 andmm 180–210and 180– mm,210 mm, respectively. respectively.

2.3. Test Procedures 2.3. Test Procedures 2.3.1. Sample Preparation 2.3.1. Sample Preparation In this study, a 210 mm thick expansive soil was used for the laboratory model test. According In this study, a 210 mm thick expansive soil was used for the laboratory model test. According to the method of soil sample preparation in Test Methods of Soils for Highway Engineering (JTG to the method of soil sample preparation in Test Methods of Soils for Highway Engineering (JTG E40-2007) [25], the naturally dried soil samples were ground to through a 2 mm standard sieve, E40-2007) [25], the naturally dried soil samples were ground to through a 2 mm standard sieve, and and then be added to the appropriate amount of water and mixed thoroughly to prepare the soil then be added to the appropriate amount of water and mixed thoroughly to prepare the soil samples samples with the optimal moisture content. with the optimal moisture content. 2.3.2. Model Elaboration 2.3.2. Model Elaboration The layer-by-layer ﬁll construction method was used to control the uniform density of the soil samples.The layer The degree-by-layer of fill compaction construction and method initial moisture was used content to control of each the soiluniform layer density was controlled of the soil at samples. The degree of compaction and initial moisture content of each soil layer was controlled at 90 % and optimal moisture content of 16.2 %, respectively. The initial stress (σi) of each soil layer is 90equivalent % and o toptim theal self-weight moisture content stressof of the 16.2 overlying %, respectively. soil layer, The can initial be expressed stress ( i as:) of each soil layer is equivalent to the self-weight stress of the overlying soil layer, can be expressed as: σi = 0.9ρghi(1 + 0.01w) (1) ii0.9(10.01ghw ) (1) where, ρ is the maximum dry density, g represents gravity acceleration, hi denotes the thickness of the where,  is the maximum dry density, g represents gravity acceleration, hi denotes the thickness of overlying soil layer, and w is the optimum moisture content. the overlying soil layer, and w is the optimum moisture content. Silicon grease was applied on the inner side of PMMA box to reduce the side friction. Moreover, Silicon grease was applied on the inner side of PMMA box to reduce the side friction. Moreover, three pore solution collection boxes were respectively buried in the surface, middle and bottom three pore solution collection boxes were respectively buried in the surface, middle and bottom expansive soil layers to collect the pore solution in these three interfaces, while four PMMA strips expansive soil layers to collect the pore solution in these three interfaces, while four PMMA strips were placed at the four sides of different soil layers every 30 mm to observe the swelling deformation were placed at the four sides of different soil layers every 30 mm to observe the swelling of expansive soil. After the model was ﬁlled completely, a layer of geotextile was placed over the deformation of expansive soil. After the model was filled completely, a layer of geotextile was model surface to avoid the impact of water injection on the soil surface. In this study, all the tests were placed over the model surface to avoid the impact of water injection on the soil surface. In this study, carried out indoors at a constant temperature of 20 ◦C, and the effect of temperature was not taken all the tests were carried out indoors at a constant temperature of 20 °C , and the effect of into account. temperature was not taken into account. Appl. Sci. 2019, 9, 1233 5 of 13

2.3.3. Cation Concentration Tests The cation concentration was measured by HACH DR1900 portable spectrophotometer and HACH HQD portable analyzer. The initial cation concentration of pore solution in expansive soil is shown in Table1. After completion of model test, the cation concentration of soil samples taken from the surface, middle and bottom expansive soil layers was obtained by the same method. On the second day after de-ionized water had been injected into the model, the pore solution accumulated by the collection boxes buried in soil layer was extracted to measure its ion concentration. The collection frequency was every 3–4 days until the model test was completed.

2.3.4. Swelling Tests In compliance with Test Methods of Soils for Highway Engineering (JTG E40-2007) [25], the dried sample ground to through a 0.5 mm standard sieve was prepared for free swelling ratio test. Free swelling ratio is deﬁned as the ratio of the difference in volumes of soil sample in water and air, to the volume of soil in air. Moreover, the cylindrical specimens with 61.8 mm diameter and 20 mm height were compacted at a target dry density of 1.62 g/cm3 and an initial moisture content of 16.2%, then were inserted into a traditional oedometer along with the stainless steel ring for the swelling pressure, swelling percent and loaded expansion ratio tests. Swelling pressure is determined as the vertical pressure required to keep the original height upon the imbibition of water. The swelling percent and loaded expansion ratio are deﬁned as the ratio of the increase in the height to the original thickness of the specimen. Specimens for swell percent test were allowed to swell freely, but those for loaded expansion ratio test were performed under a certain pressure until swelling was complete. Additionally, the initial values of above each test were obtained by the four parallel experiments. Then, these parameters of expansive soil were measured again by the same method after completion of model test.

3. Results and Discussions

3.1. Swelling Deformation of Expansive Soil Layer at Different Depths In this study, the approach of unidirectional water injection from top to bottom was adopted to make the expansive soil saturated. The water height was controlled at 1 cm in the total testing process. When the swelling deformation was stable for 48 h, the model test was considered to be completed. After completion of model test, four cylindrical samples with 61.8 mm diameter and 20 mm height were respectively taken from the three soil layers at depths of 0–30 mm, 90–120 mm and 180–210 mm by stainless steel rings. Then, these soil samples were put into separate containers for moisture content determination by oven drying method. At the same time, the dry density could also be obtained. After that, the void ratio was determined according to the speciﬁc gravity of 2.7, then based on this to calculate the saturation. The test results obtained are presented in Figure2. The moisture content, dry density, void ratio and saturation are the average of the measured values of the four samples taken from each soil layer. As shown in Figure2, the values of saturation of the soil samples show that all soil layers achieved a saturated state during the unidirectional seepage. Unfortunately, because there is no humidity sensor embedded in the soil, the moisture content of each soil layer in the saturated process is unknown and uncontrollable. Additionally, it can be observed that the dry density of surface expansive soil layer is the minimal while the moisture content and void ratio are the maximal. As the depth of soil layer increases, the dry density increases while the moisture content and void ratio are dropping down. Furthermore, these three parameters of the lower soil layer change slightly, while those of the upper soil layer vary greatly. This phenomenon indicates that the swelling deformation of expansive soil is nonlinear with the depth of soil layer and has the apparent difference between the upper and lower soil layers. Appl. Sci. 2019, 9, 1233 6 of 13 Appl. Sci. 2019, 9, x FOR PEER REVIEW 6 of 13

Figure 2. The variation of saturation, moisture content, dry density, void ratio of expansive soil layer with depth. Appl. Sci.with 2019 depth., 9, x FOR PEER REVIEW 7 of 13

Additionally, it can be observed that the dry density of surface expansive soil layer is the ratio Duringand the thethickness saturated of soil process layer. of Based unidirectional on this, the water inhibition injection, effect the of swelling self-weight deformation of overlying of each soil minimal while the moisture content and void ratio are the maximal. As the depth of soil layer layersoil layer on expansive is the average soil can value also of be the evaluated. four-side expansion. The results obtained are depicted in Figure3. increases, the dry density increases while the moisture content and void ratio are dropping down. Furthermore, these three parameters of the lower soil layer change slightly, while those of the upper soil layer vary greatly. This phenomenon indicates that the swelling deformation of expansive soil is nonlinear with the depth of soil layer and has the apparent difference between the upper and lower soil layers. During the saturated process of unidirectional water injection, the swelling deformation of each soil layer is the average value of the four-side expansion. The results obtained are depicted in Figure 3. It is further known from Figure 3 that the swelling deformation of expansive soil non-linearly increases with the depth of soil layer. The swelling deformation of upper soil layer is higher than that of lower soil layer, which is consistent with the conclusion of Figure 2. It is illustrated that there is the inhibition effect on the swelling of lower soil layer under saturated process. Figure 4 shows the relationship of the measured swelling deformation of different soil layers and those calculated via a loaded expansion ratio. In this section, the saturated density of each soil layer could be calculated by its average void ratio (Figure 2) and specific gravity (Table 1), so that its saturated self-weightFigureFigure 3. 3. was TheThe determined.swelling swelling deformation deformation The applied of of expansive expansive load ofsoil soil loaded with with depth depth expansion of of soil soil layer. layer. ratio is considered equivalent to the saturated self-weight of overlying soil layer. It is indicated that the loaded expansionIt is further ratio of known different from soil Figure layers3 thatis different the swelling and controlled deformation by the of overburden expansive soil load. non-linearly Therefore, increasesif the error with of the the estimation depth of soil method layer. Theis neglected, swelling deformationthe calculated of swelling upper soil deformation layer is higher of expansive than that soil at different depths can be determined by the product of the corresponding loaded expansion

Figure 4. The relationship of the measured and calculated swelling deformation of expansive soil at different depths.

According to the Figs. 2 and 3, there is the inhibition effect on the swelling of expansive soil due to the self-weight of overlying soil layer. However, from Figure 4, the calculated swelling deformation of expansive soil at different depths just decreases slowly with the self-weight of overlying soil layer. Even though the thickness of overlying soil layer reaches 180 mm, the swelling deformation is reduced only 0.62 mm. This means that the self-weight of overlying soil layer only has a weak effect on the swelling of expansive soil. However, it can be observed that the measured swelling deformation of expansive soil is sharply reduced with the increase of soil depth. The swelling deformation of the surface expansive soil layer is 7 mm, and that of the bottom one is only 2 mm. This difference illustrates that these alterations are brought about not only by the self-weight but are mainly attributed to other factors. Previous attempts indicated that this effect is most likely to be the electrical charge effect which caused by the aggregated cations under the saturation process [7,8].

3.2. Cation Concentration of Pore Solution In the soil–water–electrolyze system, it is difficult to accurately measure the cation concentration of soil bulk solution. According to the Boltzmann distribution equation, the cation Appl. Sci. 2019, 9, x FOR PEER REVIEW 7 of 13 ratio and the thickness of soil layer. Based on this, the inhibition effect of self-weight of overlying soil layer on expansive soil can also be evaluated.

Appl. Sci. 2019, 9, 1233 7 of 13 of lower soil layer, which is consistent with the conclusion of Figure2. It is illustrated that there is the inhibition effect on the swelling of lower soil layer under saturated process. Figure4 shows the relationship of the measured swelling deformation of different soil layers and those calculated via a loaded expansion ratio. In this section, the saturated density of each soil layer could be calculated by its average void ratio (Figure2) and speciﬁc gravity (Table1), so that its saturated self-weight was determined. The applied load of loaded expansion ratio is considered equivalent to the saturated self-weight of overlying soil layer. It is indicated that the loaded expansion ratio of different soil layers is different and controlled by the overburden load. Therefore, if the error of the estimation method is neglected, the calculated swelling deformation of expansive soil at different depths can be determined by the product of the corresponding loaded expansion ratio and the thickness of soil layer. Based on this, the inhibition effect of self-weight of overlying soil layer on expansive soil canFigure also 3. beThe evaluated. swelling deformation of expansive soil with depth of soil layer.

FigureFigure 4. The relationship of the measured and calculated swelling deformation of of expansive expansive soil soil at at differentdifferent depths.

AccordingAccording to the Figs. 2 2 and and 3, 3, there there is is the the inhibition inhibition effect effect on on the the swelling swelling of of expansive expansive soil soil due due toto thethe self-weight self-weight of overlyingof overlying soil layer. soil layer. However, However, from Figure from4 , theFigure calculated 4, the swelling calculated deformation swelling deformationof expansive soilof expansive at different soil depths at different just decreases depths slowly just decreases with the self-weight slowly with of overlying the self-weight soil layer. of overlyingEven though soil the layer. thickness Even though of overlying the thickness soil layer of reaches overlying 180 mm,soil layer the swelling reaches deformation 180 mm, the is swelling reduced defoonlyrmation 0.62 mm. is Thisreduced means only that 0.62 the mm. self-weight This means of overlying that the self soil-weight layer only of overlying has a weak soil effect layer on only the hasswelling a weak of effect expansive on the soil. swelling However, of expansive it can be observedsoil. However, that the it can measured be observed swelling that deformation the measured of swellingexpansive deformation soil is sharply of reducedexpansive with soil the is increase sharply reducedof soil depth. with Thethe swelling increase deformationof soil depth. of T thehe swellingsurface expansive deformation soil of layer the is surface 7 mm, expansive and that of soil the layer bottom is one7 mm, is onlyand 2that mm. of Thisthe bottom difference one illustrates is only 2 mm.that theseThis difference alterations illustrates are brought that about these not alterations only by the are self-weight brought about but are not mainly only by attributed the self- toweight other butfactors. are mainly Previous attri attemptsbuted to indicated other factors. that this Previous effect is attempts most likely indicated to be the that electrical this effect charge is effectmost whichlikely tocaused be the by electrical the aggregated charge effect cations which under caused the saturation by the aggregated process [7 cations,8]. under the saturation process [7,8]. 3.2. Cation Concentration of Pore Solution 3.2. CationIn the Concentration soil–water–electrolyze of Pore Solution system, it is difﬁcult to accurately measure the cation concentration of soilIn bulk the solution. soil–water According–electrolyze to the system, Boltzmann it is distribution difficult to equation, accurately the measure cation concentration the cation concentrationof soil pore solution of soil can bulk reﬂect solution. the changeAccording of that to the in soil Boltzmann bulk solution. distribution Therefore, equation, in this the section, cation the cation concentration of pore solution in expansive soil at different depths was measured to analyze the migration regulation of cations. Appl. Sci. 2019, 9, x FOR PEER REVIEW 8 of 13

concentration of soil pore solution can reflect the change of that in soil bulk solution. Therefore, in Appl.this Sci. section,2019, 9 , the 1233 cation concentration of pore solution in expansive soil at different depths8 ofwas 13 measured to analyze the migration regulation of cations.

3.2.1.3.2.1. In the Saturated Process ToTo researchresearch whetherwhether thethe overlyingoverlying soilsoil layerlayer cancan induceinduce thethe cationscations toto migratemigrate downwarddownward andand aggregate,aggregate, andand then then form form the the electrical electrical environment, environment on, theon secondthe second day afterday injectingafter injecting de-ionized de-ionized water intowater the into model, the model, the pore the solution pore solution was extracted was extracted to measure to measure its ion concentration.its ion concentration. 2+ 2+ + + InIn soso farfar asas cationscations in in expansive expansive soils soils in in China China are are mainly mainly Ca Ca,2+ Mg, Mg2+and and Na Na+,, whilewhile K K+ accountsaccounts forfor only only about about 2–3% 2–3% of of the the total total cations cations [26 ]. [2 Moreover,6]. Moreover, the expansive the expansive soil selected soil selected in this instudy this mostly study 2+ 2+ 2+ 2+ containedmostly contained Ca and Ca Mg2+ and(Table Mg1).2+ Therefore,(Table 1). only Therefore, Ca and only Mg Ca2+concentrations and Mg2+ concentrations were analyzed were and theanalyzed results and are the presented results inare Figure presented5. in Figure 5.

FigureFigure 5.5. TheThe cationcation concentrationconcentration of porepore solutionsolution atat threethree soilsoil layerlayer interfacesinterfaces underunder thethe saturationsaturation process:process: ((aa)) MgMg2+2+;;( (bb)) Ca Ca2+.2+.

ItIt isis seenseen in in Figure Figure5 that5 that the the cation cation concentration concentration of pore of pore solution solution in the in surface the surface expansive expansive soil layer soil islayer the minimal is the minimal while that while of middle that andof middle bottom and ones bottom is the maximal. ones is Moreover, the maximal. the concentration Moreover, the in theconcentration surface expansive in the surface soil layer expansive decreases soil whereas layer decreases those of middlewhereas and those bottom of middle ones increase and bottom when ones the soilincrease gradually when reaches the soil the gradually saturated reaches state, whichthe saturated indicates state, the Cawhich2+ and indicates Mg2+ will themigrate Ca2+ and downward Mg2+ will alongmigrate with downward water molecules along with and aggregate water molecules around andclay aggregateparticles to around form the clay electrical particles environment. to form the electrical environment. 3.2.2. After Completion of Model Test 3.2.2.The After next Completion goal of this of study Model was Test to verify whether the divalent cations of surface expansive soil layerThe migrate next downward goal of this and study aggregate was to aroundverify whether the clay particles.the divalent Therefore, cations afterof surface completion expansive of model soil test,layer the migrate soil samples downward taken and from aggregate the surface, around middle the and clay bottom particles. expansive Therefore, soil layersafter completion (soil layers atof depthsmodel test, of 0–30 the mm,soil 90–120samples mm taken and from 180–210 the surface, mm, respectively) middle and were bottom dried expansive to a constant soil weightlayers (soil and thenlayers were at depth grounds of through 0–30 mm, a 1 mm90–120 standard mm and sieve. 180 Then,–210 mm, the cation respectively) concentration were dried in pore to solutiona constant of theweight above and soil then samples were ground was measured. through Thea 1 mm results standard obtained sieve. are Then plotted, the in cation Figure concentration6. in pore 2+ 2+ solutionAsseen of the in above Figure soil6, the samples concentration was measured. of Ca The, Mg resultsof poreobtained solution, are plotted in contrast in Figure to the 6. initial values (Table1), drops down in the surface expansive soil layer while increases in the middle and bottom ones. Therefore, this phenomenon proves that the Ca2+ and Mg2+ cations in the surface soil layer migrate downward along with water molecules and aggregate around clay particles in the saturated process, and eventually form the electrical environment. Appl. Sci. 2019, 9, x FOR PEER REVIEW 9 of 13

Appl. Sci. 2019, 9, 1233 9 of 13 Appl. Sci. 2019, 9, x FOR PEER REVIEW 9 of 13

Figure 6. The cation concentration of pore solution at three soil layer interfaces after completion of the model test: (a) Mg2+; (b) Ca2+.

As seen in Figure 6, the concentration of Ca2+, Mg2+ of pore solution, in contrast to the initial values (Table 1), drops down in the surface expansive soil layer while increases in the middle and bottom ones. Therefore, this phenomenon proves that the Ca2+ and Mg2+ cations in the surface soil

layer migrate downward along with water molecules and aggregate around clay particles in the saturatedFigureFigure process, 6. 6.The The cation andcation eventually concentration concentration form of poreof the pore solution electrical solution at three environment.at three soil soil layer layer interfaces interfaces after after completion completion of the of 2+ 2+. modelthe model test: (test:a) Mg (a2+) Mg;(b); Ca (b2+.) Ca 3.3. Swelling Behavior of Expansive Soil at Different Depths 3.3. SwellingAs seen Behavior in Figure of Expansive 6, the concentration Soil at Different of DepthsCa2+, Mg2+ of pore solution, in contrast to the initial Since the above analyses confirmed the assumption on the migration and aggregation of Ca2+ values (Table 1), drops down in the surface expansive soil layer while increases in the middle2+ and Since2+ the above analyses conﬁrmed the assumption on the migration and aggregation of Ca and andbottom Mg ones, the. Theref next stepore, wasthis phenomenonto check whether proves these that aggregatedthe Ca2+ and cations Mg2+ cations participate in the in surface the ionic soil Mg2+, the next step was to check whether these aggregated cations participate in the ionic exchange exchangelayer migrate with expansive downward so alongil and withassess water the modification molecules and effect aggregate produced around by the claylatter particles exchange in on the with expansive soil and assess the modiﬁcation effect produced by the latter exchange on expansive expansivesaturated soil.process, After and completion eventually of form the modelthe electrical test, the environment. surface, middle and bottom expansive soil soil.layers After (soil completion layers at depth of thes of model 0–30 test,mm, the90– surface,120 mm middleand 180 and–210 bottom mm, respectively) expansive soil was layers removed (soil layers at depths of 0–30 mm, 90–120 mm and 180–210 mm, respectively) was removed from the test from3.3. Swellingthe test apparatusBehavior of andExpansive was put Soil into at Different separate Depths containers, then was naturally dried and ground apparatusto prepare andthe waspowder put intoand separatecylindrical containers, samples thenfor the was free naturally swelling dried ratio, and swelling ground topressure, prepare and the Since the above analyses confirmed the assumption on the migration and aggregation of Ca2+ powderswelling and percent cylindrical tests samplesby the same for the method free swelling in Section ratio, 2 swelling.3.4. Noteworthy pressure, andis that swelling the each percent swelling tests and Mg2+, the next step was to check whether these aggregated cations participate in the ionic byparameter the same was method determined in Section by 2.3.4the four. Noteworthy parallel experiments. is that the each The swelling test results parameter obtained was are determined presented exchange with expansive soil and assess the modification effect produced by the latter exchange on byin Figure the four 7. parallel experiments. The test results obtained are presented in Figure7. expansive soil. After completion of the model test, the surface, middle and bottom expansive soil layers (soil layers at depths of 0–30 mm, 90–120 mm and 180–210 mm, respectively) was removed from the test apparatus and was put into separate containers, then was naturally dried and ground to prepare the powder and cylindrical samples for the free swelling ratio, swelling pressure, and swelling percent tests by the same method in Section 2.3.4. Noteworthy is that the each swelling parameter was determined by the four parallel experiments. The test results obtained are presented in Figure 7.

Figure 7. The variation of swelling properties of expansive soil after completion of the modelmodel test: ( a) Free swelling ratio; ((bb)) swellingswelling pressurepressure andand swellingswelling percent.percent.

It can can be be observed observed from from Figure Figure7 that 7 all that swelling all swelling parameters parameters of the middle of the and middle bottom and expansive bottom soilexpansive layers aresoil lower layers than are theirlower initial than values,their initial while values, these parameterswhile these ofparameters surface expansive of surface soil expansive layer are slightlysoil layer higher are slightly than its higher initial values.than its Thisinitial phenomenon values. This indicates phenomenon that the indicates swelling that of lower the swelli expansiveng of soil layer is inhibited in the saturated process. The most reasonable explanation is that the lower expansiveFigure soil 7. layerThe variation has been of modiﬁedswelling properties due to the of cation expansive exchange soil after with completion the migrating of the mod andel aggregating test: (a) cationsFree where swelling the ratio; divalent (b) swelling cations pressure (Ca2+, Mgand2+ swelling) enter percent. into the diffuse double layer (DDL) of clay particles and interlayer of montmorillonite crystals, but the swelling potential of the surface expansive It can be observed from Figure 7 that all swelling parameters of the middle and bottom expansive soil layers are lower than their initial values, while these parameters of surface expansive soil layer are slightly higher than its initial values. This phenomenon indicates that the swelling of Appl. Sci. 2019, 9, 1233 10 of 13 soil layer is released because of the downward migration of bivalent cations. However, the swelling pressure of the middle and bottom expansive soil layers is over 120 KPa that is still higher than the self-weight of overlying soil layer. This implies that the inhibition effect produced by cation exchange cannot completely balance the swelling pressure. A tentative explanation is that the impact of electrical environment formed by the migrating and aggregating cations counteracts this part of the swelling pressure (described in the next section). But due to the above soil samples soaked in de-ionized water, the electrical environment was destroyed so that the swelling potential of expansive soil cannot be completely inhibited.

3.4. Discussions Based on the above analyses, the mechanism of swelling difference of different expansive soil layers can be explained as follows. As shown in Figure8, during the saturation process of water (de-ionized water) injection from top to bottom, the divalent cations (Ca2+, Mg2+) in the surface soil layer migrate downward along with water molecules and aggregate around clay particles, eventually form the electrical environment. In this process, the cation exchange with expansive soil occurs where the bivalent cations (Ca2+, Mg2+) enter into the DDL of clay particles and interlayer of montmorillonite crystals, resulting in the modification effect to restrain the swelling potential of the lower expansive soil layer. On the other hand, the hydrated exchangeable cations adhere to soil particles and form the adsorbed water film surrounding the aggregates [11], which produces the crystalline swelling and the diffuse double-layer swelling [15,27,28]. Followed by the swelling of aggregates, this induces the collapse and rebuilt of soil skeleton at the expense of porosity which decreases in volume under the confined condition [21,29]. After the cations further migrate downward and aggregate, the aggregates continue to swell and fill the micropores [29]. As high concentration of solution will cause the reduction in the DDL thickness and consequently a decrease of the diffuse double-layer swelling based on the DDL theory [15,30–32]. Thus water molecules tend to adsorb in the interlayer space rather than in the micropores in the case of the confined condition [33]. Moreover, with the increase of concentration of the migrating and aggregating cations, the adsorbed water, particularly the inner water molecules layer surrounding the surface of aggregates, may be in the state of solid water without movement and exchange under the influence of Coulumbian electrical charges [8,34]. Therefore, the water molecules are hard to penetrate the adsorbed water film surrounding the aggregates into the interlayer space, resulting in the inhibition effect on the swelling of the lower expansive soil layer. On the contrary, the swelling potential of surface expansive soil layer is released because of the downward migration of bivalent cations. Consequently, the swelling difference between the upper and lower expansive soil layers can be observed during the saturated process. Such a difference is mainly attributed to the joint effect of two factors that can be treated as an electric charge effect, namely (i) the modification effect, (ii) the electrical environment. In addition, the concentration of the migrating and aggregating cations increases with the thickness of overlying expansive soil layer, and so the swelling potential of the middle expansive soil layer is higher than that of the bottom expansive soil layer. In the literature reported by Katti and Katti [7,8], the reason why the 1.2 m thick soil layer can completely restrain the swelling of the lower black cotton soil is that the electric charge effect can completely balance its swelling pressure, the water molecules only accumulate in micropores rather than entering into the soil particles and montmorillonite interlayer, and thus it will not lead to swelling deformation. Based on the above analyses, the electric charge effect on expansive soil provides helpful insights into the swelling behavior of expansive soil under seepage. For the subgrade, foundation and cross-drainage structures on expansive soil areas, the damage to structures caused due to volumetric changes occurring through the active zone of expansive soil has always been of concern to civil engineers [35]. Conservative design of a structure on expansive soil must consider the maximum amount of heave that can occur in the lifetime of the structure [36]. In order to predict the maximum heave of expansive soil, it is necessary to define the largest active zone of expansive soil that can be Appl. Sci. 2019, 9, 1233 11 of 13 wetted and determine the properties of expansive soil in that zone [36]. In this process, traditional methods usually neglect the effect of electric charge on the swelling properties of expansive soil in the active zone. In fact, the electric charge effect produced by the migration and aggregation of cations in the soil can significantly eliminate the swelling potential of expansive soil, which is the reason why the depth of the active zone obtained by dividing the swelling pressure by the self-weight of the overlying soil layer is usually greater than the actual value. Therefore, the results obtained in this study will provide a good basis for the rational design, construction and maintenance of engineering Appl.constructions Sci. 2019, 9, x on FOR expansive PEER REVIEW soils. 11 of 13

FigureFigure 8. 8. TheThe mechanism mechanism of of electric electric charge charge effect effect on on the the swelling swelling of expansive soil.

4. ConclusionsBased on the above analyses, the electric charge effect on expansive soil provides helpful insightIns thisinto study, the swelling the swelling behavior behavior of expansive of expansive soil under soil atseepage. different For depths the subgrade, was experimentally foundation andinvestigated cross-drainage through structures laboratory on model expansive test under soil saturated areas, the condition damage ofto water structures injection caused from due top to to volumetricbottom. Two changes different occurring factors controlling through the the active swelling zone difference of expansive of different soil has expansive always been soil layersof concern were todiscussed. civil engineers The following [35]. Conservative conclusions can design be drawn of a structure from the test on results: expansive soil must consider the maximum(1) It couldamount be observedof heave thatthat therecan occur was the in distinctthe lifetime swelling of the difference structure between [36]. In the order expansive to predict soil thelayers maximum at different heave depths of expansive under the soil, saturation it is necessary process. to The define swelling the potentiallargest active of the zone surface of expansive expansive soilsoil layerthat can at a be depth wetted of 0–30 and mm det wasermine released the properties while that of of expansive the middle soil and in bottom that zone ones [36] at depths. In this of process,90–120 mm traditional and 180–210 methods mm wasusually inhibited. neglect the effect of electric charge on the swelling properties of expansive(2) It was soil experimentally in the active zone. proved In thatfact, thethe divalentelectric charge cations effect (Ca2+ produced, Mg2+) in by the the surface migration soil layer and aggregationmigrated downward of cations along in the with soil can water significantly molecules eliminate and aggregated the swelling around potential clay particles of expansive to form soil, the whichelectrical is the environment reason why under the depth unidirectional of the active seepage. zone obtained Moreover, by thedividing concentration the swelling of migrated pressure and by theaggregated self-weight cations of the was overlying positively so correlatedil layer is with usually the thicknessgreater than of the the overlying actual value soil layer.. Therefore, the results(3) obtained The swelling in this difference study will of different provide expansivea good basis soil for layers the rationalwas mainly design, caused construction by the electric and maintenancecharge effect of including engineering two factors,constructions namely on (i) expansive the modiﬁcation soils. effect, (ii) the electrical environment. Firstly, the cation exchange between the aggregated bivalent cations and the expansive soil could 4.reduce Conclusions the swelling potential of expansive soil. Additionally, the adsorbed water ﬁlm surrounding the aggregatesIn this obstructedstudy, the theswelling further behavior adsorption of expansive of water molecules soil at different into the depths montmorillonite was experimentally interlayer, investigatedwhich restrained through the swellinglaboratory of model the expansive test under soil. saturated condition of water injection from top to bottom. Two different factors controlling the swelling difference of different expansive soil layers Author Contributions: The paper was written by J.S. under the guidance of H.Y. and Z.L. The formal analysis wereand pre-literature discussed. researchThe following were carried conclusions out by R.F. can and be S.X.drawn The experimentfrom the test was results performed: and research method was proposed(1) It could by J.S.be underobserved the help that of there H.Y. andwas Z.L. the distinct swelling difference between the expansive soilFunding: layersThis at work different was supported depths under by the the Youth saturat Innovationion process. Promotion The Association swelling CAS, potential the outstanding of the surface youth expansivefund of Hubei soil Province layer at (No. a depth 2017CFA056), of 0–30 mm the Heilongjiangwas released applied while technologythat of the research middle and and development bottom ones plan at depthguidances of project 90–120 (No. mm GZ16B007), and 180– the210 Science mm was and inhibited. Technology Service Network Initiative (No. KFJ-STS-ZDTP-037) and the Natural Science Foundation of China (Nos. 41472286, 41472290 and 41672312). (2) It was experimentally proved that the divalent cations (Ca2+, Mg2+) in the surface soil layer migrated downward along with water molecules and aggregated around clay particles to form the electrical environment under unidirectional seepage. Moreover, the concentration of migrated and aggregated cations was positively correlated with the thickness of the overlying soil layer. (3) The swelling difference of different expansive soil layers was mainly caused by the electric charge effect including two factors, namely (i) the modification effect, (ii) the electrical environment. Firstly, the cation exchange between the aggregated bivalent cations and the expansive soil could reduce the swelling potential of expansive soil. Additionally, the adsorbed water film surrounding the aggregates obstructed the further adsorption of water molecules into the montmorillonite interlayer, which restrained the swelling of the expansive soil. Appl. Sci. 2019, 9, 1233 12 of 13

Conﬂicts of Interest: The authors declare that there are no conﬂict of interest.

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