A New Soil Conditioner for Highly Permeable Sandy Gravel Stratum in Epbs

applied sciences

Article A New Soil Conditioner for Highly Permeable Sandy Gravel Stratum in EPBs

Xin Lin 1, Xiong Zhou 2,* and Yuyou Yang 1

1 School of Engineering and Technology, China University of Geosciences (Beijing), Beijing 100083, China; [email protected] (X.L.); [email protected] (Y.Y.) 2 School of Geophysics and Information Technology, China University of Geosciences (Beijing), Beijing 100083, China * Correspondence: [email protected]; Tel.: +86-10-8232-2628

Abstract: Full-face water-rich gravel stratum is a large challenge during tunnel excavation with earth pressure balance shields (EPBs) because of accidents such as water spewing from the screw conveyor and ground collapse. Slurry and polymer have been used as conditioning agents to avoid such problems and thus ensure a successful tunneling. However, limited improvement of sandy gravel was achieved when traditional soil conditioner were applied. This study proposes a new conditioner (modified slurry) consisting of bentonite slurry, viscosity modifier, sodium silicate and polymer, which will enhance the properties of sand gravel stratum. Low reaction time, high apparent viscosity, good plastic behavior and low permeability were employed for investigating the optimum ratio of the ingredients. The proposed modified slurry has a good performance in conditioning sandy gravel soils and can be the reference for EPBs’ excavation in highly permeable, non-adhesive coarse-grained soil stratum. Keywords: modified slurry; EPBs; sandy gravel; plasticity; permeability

Citation: Lin, X.; Zhou, X.; Yang, Y. A New Soil Conditioner for Highly Permeable Sandy Gravel Stratum in EPBs. Appl. Sci. 2021, 11, 2109. 1. Introduction https://doi.org/10.3390/app11052109 In recent years, with the increasing demand for underground space, underground tunnel excavation technology has transitioned from open excavation to underground Academic Editors: Daniel Dias and excavation and has become a research hotspot [1–3]. Among all excavation technologies, Neftali Nuñez the earth pressure balance shield (EPBs) is the most commonly used [4]. Under different stratum conditions, specific accidents may occur in the application of EPBs, which makes it Received: 13 January 2021 difficult to apply this method. For example, different from tunneling in clayey soil, in a Accepted: 23 February 2021 sandy gravel stratum accidents, such as groundwater spew and ground collapse, may occur Published: 27 February 2021 when EPBs is used [5–7]. Furthermore, since Beijing, Chengdu and other cities that are vigorously developing the metro are rich in sandy pebble stratum, a new soil conditioner Publisher’s Note: MDPI stays neutral for highly permeable sandy gravel to prevent water spewing has become a key technology with regard to jurisdictional claims in problem to be solved in underground tunnel construction and in the application of EPBs in published maps and institutional affil- iations. China [8]. To ensure the successful excavation of a tunnel in the process of EPBs, an improvement technology to obtain the “ideal soil” (low permeability, moderate compressibility, small shear strength and certain fluidity) by adding soil conditioners was proposed [9]. Large particle size, low clay content and low cohesion lead to poor plastic paste and the Copyright: © 2021 by the authors. high permeability of sandy gravel soils. Soil conditioners can decrease the permeabil- Licensee MDPI, Basel, Switzerland. ity and improve the plastic paste of sandy gravel soils, and many successful cases were This article is an open access article obtained [10]. distributed under the terms and conditions of the Creative Commons Considerable studies on the improvement of sandy gravel soils have been conducted Attribution (CC BY) license (https:// by researchers. A foam agent was added to enhance the plastic paste of the soil. Peila et al. creativecommons.org/licenses/by/ (2009) [11] studied the effect of different ratios of foam injection on the soil slump of various 4.0/). non-viscous coarse-grained soils. According to the foam loss, slump and water loss, the

Appl. Sci. 2021, 11, 2109. https://doi.org/10.3390/app11052109 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, 2109 2 of 17

improvement results were divided into the non-suitable, the suitable, and the borderline. Mori et al. (2018) [12] concluded that a foam agent can effectively fill the granular soil void, reduce the friction and shear strength between soil particles and improve the plasticity of soil. In the conditioned soil, qualitative evaluation standard considers the slump to be mainly related to the foam injection ratio and water content [13]. However, enhancing the plastic and reducing the permeability are required to prevent water spewing in water-rich sandy gravel stratum [14,15]. When excavating in a coarse- grained stratum, it is insufficient to obtain a satisfactory conditioning by simply adding a foam agent. Some researchers proposed polymer or bentonite slurry for reducing soil permeability [15]. Adding polymer material to the slurry not only increases the viscosity of the liquid, but also the polymer’s long-chain structure can enhance the adhesion between the fine particles of the slurry, so that the conditioned slurry has a better sealing effect on the large-grit sand-gravel soil [16]. During the construction of the earth pressure balance shield tunnel in the high permeability stratum, some methods have been proposed to improve its permeability by adding water-insoluble fine particle solids (such as fine sand, calcium carbonate powder, etc.) or polymer materials. Fritz (2007) [17] added fine sand (<1 mm), vermiculite (0.7–4 mm) and a small amount of high-polymer material to the soil of high permeability, which significantly reduced the permeability. In addition, filtration problems always occur in the formation of slurry. Bosshard (2011) [18] used slurry and high polymer to improve the high-permeability (k > 3 × 10−3 m/s) of gravel stratum in the process of Weinberg tunnel construction. Carrieri (2006) [19] injected a turbid liquid of calcium carbonate powder as a filler into the excavation chamber, which improved high-permeability coarse sand formation in Turin Metro Line 1. Zhao (2018) [20] improved the viscosity of sodium bentonite by sodium carbonate and carboxymethyl cellulose sodium (CMC) and obtained S1 bentonite slurry (the ratio of bentonite, water, Na2CO3 and CMC is 6:1:0.042:0.056), which was successfully applied to the waterless sand-pebble stratum of Urumqi Metro Line 1. Previous studies concluded that the polymer can increase the cohesion by filling in the voids between the gravel particles, while the bentonite slurry or filler mainly filled in voids between the fine-grained particles and forms a slurry film, consequently reducing the permeability of the stratum. To ensure a successful tunneling by EPBs in a water-rich sandy gravel stratum, this study proposed a new soil conditioner that is capable of effectively improving the plasticity and impermeability of sand and gravel. The main objects were: (1) to study the effect of ammonium chloride as a viscosity modifier on the apparent viscosity and rheology of sodium bentonite slurry; (2) to study the improvement effect of modified slurry on the plasticity of non-cohesive sand and gravel; and (3) to study the impermeability improvement of sandy gravel conditioned by sodium silicate solution, high molecular polymers and modified slurry.

2. Materials and Methods 2.1. The Modified Slurry Modifier Components The traditional slurry is commonly the aqueous solution of sodium bentonite with 10–18 wt%. The preparation method added the weighted sodium bentonite powder to distilled water several times. It should be noted that the incorporation quality of sodium bentonite was less than 5 g to avoid an uneven blend and the solution was stirred at 1000 r/min. In this study, ammonium chloride, sodium silicate and high molecular polymers were mixed into the traditional slurry to modify and improve it, in which ammonium chloride was the viscosity modifier to adjust the reaction time. In order to distinguish the traditional slurry, the mixed solution of modifier and slurry was called modified slurry. In addition, high molecular polymers were helpful for enhancing the impermeability of the mixed system. In this study, the modified slurry preparation process mainly included two mixed solution systems, namely, the mixed solution of bentonite and sodium silicate as well as Appl. Sci. 2021, 11, 2109 3 of 17

the mixed solution of ammonium chloride and high polymers. The sodium bentonite (Henan Zhongyuan Mining Company, Henan Province, China) chemical components and physical characteristics are shown in Tables1 and2. Ammonium chloride (Beijing Chemical Reagent Factory, Beijing, China) formed the viscosity modiﬁer for modiﬁed materials. The ammonium chloride was analytically pure (>99%), the sodium silicate solution (Shandong Dongyue Group, Shandong Province, China) had a Baume Degree of 40, and the content was 34 wt%. There were 6 kinds of polymers. They are carbomer (97%, Sumitomo Chemical Co., Ltd., Tokyo, Japan), guar gum (99%, Guangzhou Feirui Chemical Co., Ltd. Company, Guangzhou, China), xanthan gum (99%, Fufeng Group, Jinan, China), carboxymethyl cellulose sodium (99%, Yixing Tongda Chemical Co., Ltd., Yixing, China), hydroxyethyl cellulose (99%, Shanghai Huiguang Fine Chemical Co., Ltd., Shanghai, China) and polyacrylamide (99%, Shandong Baomo Biochemical Co., Ltd., Dongying, China). Details of the components are shown in Table3. Code B represents a solution of 13 wt% bentonite, other codes have similar meanings, and the solvent was water. In the process of polymer solution preparation, it should be noted that the weighed reagent was slowly added into the 50–60 ◦C distilled water and then the solution was stirred at 2000 r/min for more than 30 min until the solution was clear.

Table 1. Main chemical constituents of sodium bentonite.

Ingredient SiO2 Al2O3 Fe2O3 Na2O MgO K2O TiO2 Content (%) 63.43 17.88 2.06 0.98 3.05 0.9 0.13

Table 2. Sodium bentonite properties.

Expansion Gum Price Wet-Heat Tensile Steady State Tensile Blue Absorption 200 Mesh Capacity (mL/g) (mL/15 g) Strength (KPa) Strength (KPa) Power (mmol/100 g) Throughput (%) 30 >100 1.4 45 25 95

Table 3. Experimental materials.

Material Category/Use Name Concentration (wt%) Density (g/mL) Code Slurry Sodium bentonite 13 1.11 B Water shutoff agents Sodium silicate 34 1.37 S reaction material Viscosity modiﬁer Ammonium chloride 20 1.04 A Carbomer 0.5 1.06 Carbom Guar gum 0.5 1.02 Guar Polymers / Water Xanthan gum 0.5 1.04 Xanthan shutoff agents Carboxymethyl 0.5 1.04 CMC cellulose Sodium Hydroxyethyl cellulose 0.5 1.08 HEC/H Polyacrylamide 0.5 1.04 PAM

Four different grades of sandy gravel soil samples were prepared in the laboratory, in which the coarse sand was selected from standard sand, the main mineral component was quartz, and the gravel was a natural breccia-like cobblestone whose main mineral composition was quartz and carbonate minerals. The gradation curve and dry density information are shown in Figure1 and Table4. Appl. Sci. 2021,, 11,, 2109x FOR PEER REVIEW 4 of 17

128 129 Figure 1. Experimental sandy gravel particle grading curve.

130 TableTable 4. Sand 4. gravelSand gravel material material physical ph characteristics.ysical characteristics.

Coarse Sand Gravel Sand Gravel Size Sand Gravel Coarse Sand Gravel 0–2 mm 2–4 mm 4–6 mm 6–9 mm 10–20 mm Size 0–2 mm 2–4 mm 4–6 mm 6–9 mm 10–20 mm Dry Density (g/mL) 1.77 1.77 1.78 1.71 1.66 Dry Density 1.77 1.77 1.78 1.71 1.66 131 2.2. Slurries(g/mL) Conditioner Preparation 132 Modified slurry is a mixed material prepared by adding ammonium chloride, so- 2.2. Slurries Conditioner Preparation 133 dium silicate and high molecular polymer into traditional slurry. In the preparation pro- 134 cess ofModified the modified slurry isslurry, a mixed the materialtraditional prepared slurry byand adding each modifier ammonium solution chloride, were sodium sepa- 135 ratelysilicate configured. and high molecular Before the polymer relevant into tests, traditional the solutions slurry. were In mixed the preparation according process to the ex- of 136 perimentalthe modified design slurry, ratio, the traditionaland then stirred slurry evenly and each with modifier a glass solutionrod. were separately con- 137 figured.In engineering Before the relevant applications, tests, all the fluid solutions materials were (such mixed as according slurry, foam, to the polymers experimental solu- 138 tion,design etc.) ratio, must and be then pumped stirred into evenly the excavati with a glasson chamber, rod. which consequently requires the 139 materialsIn engineering to have good applications, fluidity. On all the fluid on materialse hand, the (such high as apparent slurry, foam,viscosity polymers and non- so- 140 flowinglution, etc.) state must of the be pumpedmodified intoslurry the are excavation the key chamber,to improving which the consequently plasticity and requires water 141 blockingthe materials of sand to haveand gravel good fluidity.soil. On the On other the one hand, hand, the thefluidity high of apparent the slurry viscosity is a prereq- and 142 uisitenon-flowing for actual state engineering of the modified use. Based slurry on are that, the keythe totraditional improving slurry the plasticity and modifier and watersolu- 143 tionblocking involved of sand in this and article gravel need soil. fluid On thecompos otherition hand, with the good fluidity fluidity of the to ensure slurry isthat a pre-the 144 materialrequisite can for be actual easily engineering pumped into use. the Based excavation on that, cabin the to traditional mix and react slurry to andform modifier a modi- solution involved in this article need fluid composition with good fluidity to ensure that 145 fied slurry. the material can be easily pumped into the excavation cabin to mix and react to form a 146 In this study, the main objective was to obtain the traditional slurry with moderate modified slurry. 147 apparent viscosity, good time stability and fluidity. The apparent viscosity of traditional In this study, the main objective was to obtain the traditional slurry with moderate 148 slurry samples with four bentonite concentrations (i.e., 10 wt%, 13 wt%, 15 wt% and 18 apparent viscosity, good time stability and fluidity. The apparent viscosity of traditional 149 wt%) over time (0–50 h) are measured. The dumping method was used to compare with slurry samples with four bentonite concentrations (i.e., 10 wt%, 13 wt%, 15 wt% and 150 and analyze the slurry flow corresponding to different apparent viscosity values, and then 18 wt%) over time (0–50 h) are measured. The dumping method was used to compare 151 the fluidity of each experimental group was evaluated. Afterwards, the optimal bentonite with and analyze the slurry flow corresponding to different apparent viscosity values, 152 concentration was obtained. In order to study the apparent viscosity and fluidity of the and then the fluidity of each experimental group was evaluated. Afterwards, the optimal 153 mixed system after adding ammonium chloride solution to the traditional slurry, the ex- bentonite concentration was obtained. In order to study the apparent viscosity and fluidity 154 perimental goals were to obtain the modified slurry with high apparent viscosity and low of the mixed system after adding ammonium chloride solution to the traditional slurry, the 155 fluidity. In this study, five ratios of bentonite slurry and ammonium chloride solution experimental goals were to obtain the modified slurry with high apparent viscosity and 156 were selected (B:A = 20:1, 10:1, 5: 1, 10: 3 and 5:2). low fluidity. In this study, five ratios of bentonite slurry and ammonium chloride solution 157 To study the effect of ammonium chloride on the water absorption time of bentonite were selected (B:A = 20:1, 10:1, 5: 1, 10: 3 and 5:2). 158 slurry, six modifiers were prepared with different ammonium chloride concentrations, Appl. Sci. 2021, 11, 2109 5 of 17

To study the effect of ammonium chloride on the water absorption time of bentonite slurry, six modifiers were prepared with different ammonium chloride concentrations, one modifier was used (i.e., Ammonium Chloride) but in six different ratios, one containing no ammonium chloride as a control. At this stage, no further additive was included in the mixture. The details are shown in Table5. The preparation process was as follows: a certain amount of water was added to the beaker, then bentonite with 13 wt% was added slowly to the beaker and the solution was stirred for another 30 min until there were no lumps on the surface and no sediment at the bottom of the solution (note that, through the process, the stir rate of the magnetic stirrer was 1500 r/m)). According to the solution dissolution efficiency and the experiment results, the optimum dissolved concentration of ammonium chloride was 20 wt%. In this study, the slurry solution was mainly composed of bentonite (B) and ammonium chloride (A), which is called B/A modified slurry. B/A refers to the mixture mainly formed by B and A.

Table 5. B/A modiﬁed slurries modiﬁer samples.

Ammonium Chloride Sample No. Bentonite (mL) Remarks (mL)/Concentration (wt%) B-A-0 100 0/0 Control sample B-A-10 100 10/10 Study the effect of B-A-15 100 10/15 ammonium chloride B-A-20 100 10/20 concentration on the B-A-25 100 10/25 degree of expansion B-A-30 100 10/30

In addition, to study the inﬂuence of sodium silicate concentration and polymers types on the impermeability improvement degree of soil, the conditioners containing different concentrations of sodium silicate and different types of polymers were prepared.

2.3. Analysis Method 2.3.1. Apparent Viscosity Test The concentration and hydration time of bentonite play the key roles in affecting the fluidity of traditional slurry. Traditional slurry is the basic composition of modified slurry. The main targets for optimizing the composition of the traditional slurry are to be equipped with higher viscosity value, better viscosity stability and fluidity. Apparent viscosity (i.e., effective viscosity) is an important parameter that character- izes fluid flow performance and can be used as an evaluation index for fluid pumping performance. The apparent viscosities of the conditioners prepared were tested by a ZNN- D6 six-speed rotational viscometer [21]. In this study, the apparent viscosity values of the slurry samples were measured after the samples prepared 5–20 min. The test procedure was as follows: (1) the cup of the sample (350 mL) was placed on a tray that subsequently was elevated until the scale line outside the rotary sleeve was completely immersed in the sample; (2) stable readings at 600 r/m, 300 r/m, 200 r/m, 100 r/m, 6 r/m and 3 r/m were recorded. Steps (1) and (2) were repeated twice. The interval time of each experiment should not exceed 2 min. Half of the average value of three readings at a rate of 600 r/m is the apparent viscosity.

2.3.2. Slump Test Slump test is a commonly used method to evaluate the effect of soil flow plasticity improvement during the construction of the earth pressure balance shield. Based on the results of the traditional slurry and the ammonium chloride optimization experiments, in order to compare with and analyze the plasticity improvement effects of traditional slurry and modified slurry on soil flow, the effects of the traditional slurry and ammonium chloride modified on the slump of six soil samples were studied. The flow plasticity of the improved soil is classified according to its slump value. Combining with the improvement Appl. Sci. 2021, 11, 2109 6 of 17

of the goal of sand and gravel soil in actual engineering, the reasonable flow plasticity state of sand and gravel are discussed in this study. In this study, the slump test was adopted to evaluate the flow plasticity of conditioned soil by modified slurry. Before the slump test, the soil samples (6 L) and the modified slurry were mixed with different experimental ratio (5%, 10%, 15%, 20%, 25% and 30%) and stirred with a mixer for 3 min. According to the slump test standard [22], the test procedure was as follows: (1) the conditioned soil samples were loaded into the slumping cylinder three times until the cylinder was full, with 25 instances of tamping at each load, so that the thickness of each layer was approximately one third of the cylinder height; (2) the cylinder was elevated vertically and smoothly (within 5–20 s). The height difference s1 (before and after the test) was recorded, and the test was repeated two more times to obtain s2 and s3, respectively. The average value of the three slump tests is the result. In addition, the slump experiment was completed after the slurry samples had been prepared for no more than 20 min.

2.3.3. Permeability Test Based on the apparent viscosity and slump test, decreasing the permeability of soil sample plays the key role in preventing the spewing problem of the screw machine in the excavation process of the earth pressure balance shield on the sand and gravel ground. In order to find a slurry additive material and a better ratio decrease to reduce the permeability of the slurry-soil mixture, a large number of experiments show that the permeability of mixing system is decreased with the sodium silicate solution or polymers solution is added into the traditional slurry-ammonium chloride mixing system. It is mainly caused by the fact that the silicate ions ionized by the sodium silicate solution can not only react with the calcium and magnesium ions in the slurry to form calcium silicate, magnesium silicate and other water-insoluble fine particles, but also react with chlorination the ammonium ion in the ammonium solution to produce water-insoluble silicic acid. The formation of insoluble matter, such as silicic acid, magnesium silicate and calcium silicate can increase the water- insoluble fine particulate matter in the slurry mixing system to reduce the permeability of the mixing system. Due to the special long-chain molecular structure of the polymer, the integrity of the bentonite particles was increased at the microscopic level, thereby improving the water blocking ability of the slurry. Based on the experimental phenomena, the types and ratios of sodium silicate solution and polymer solution were optimized. This study investigated the change curve of the penetration rate and time of the slurry-soil mixed system when sodium silicate solution and polymer materials are used as slurry additives. Taking the minimum permeation amount of the mixing system per unit time as the preferred index, the ratio of sodium and sodium silicate solution and the type and ratio of polymer materials are optimized, respectively. A permeability test device was developed, which mainly consists of a sealing cover, rubber pad, pressure-maintaining vessel, metal filter, high-pressure gas pipe, pressure regulator valve, air compressor, metering container, and electronic weighing platform (see Figure2). The pressure maintaining vessel is made of pressure-resistant acrylic material, and the cylinder is 260 mm high, 70 mm in diameter, 5 mm in wall thickness and 60 mm in inner diameter. The pressure vessel was designed to withstand a pressure of 5 bar. The test procedure was as follows: (1) a gravel (2–4 mm) permeable layer was placed at the bottom of the pressure-maintaining vessel, and subsequently the conditioned soil sample was packed into the vessel until it was 15 cm in height. The sample was loaded every 2–3 cm of height with compaction; (2) 200 mL of water was poured into the vessel, which was subsequently sealed up; (3) The pressure was adjusted to 2 bar; (4) The pressure regulator valve was opened and timed at the same time; and (5) the water seepage volume was recorded every 1 min. When the water seepage volume was greater than 200 mL or the test time was greater than 30 min, the experiment was stopped and the pressure was relieved. Appl.Appl. Sci.Sci. 20212021,, 1111,, 2109x FOR PEER REVIEW 77 ofof 1717

252 253 FigureFigure 2.2. PermeabilityPermeability testtest device.device. 3. Results and Discussion 254 3. Results and Discussion 3.1. Apparent Viscosity Test 255 3.1. Apparent Viscosity Test 3.1.1. Difference between Traditional Slurry and B/A Modified Slurry 256 3.1.1.Viscosity Difference tests between of traditional Traditional bentonite Slurry slurryand B/A at Modified different concentrations,Slurry including 257 10 wt%,Viscosity 13 wt%, tests 15 of wt%,traditional and 18bentonite wt% were slurry performed. at different As concentrations, shown in Figure including3a, the 10 258 viscositywt%, 13 wt%, is closely 15 wt%, related and to 18 time wt% regardless were perfor ofmed. which As slurry shown concentration in Figure 3a, is the used. viscosity More 259 specifically,is closely related the viscosity to time ofregardless traditional of slurrywhich increasesslurry concentration first and then is remainsused. More stable specifi- over 260 time.cally, Forthe example,viscosity theof traditional viscosity of slurry traditional increases slurry first at and a concentration then remains of stable 15 wt% over sharply time. 261 increasesFor example, from the 24 viscosity mPa·s to of 41 traditional mPa·s within slurry 0–10 at h.a concentration Subsequently, of its 15 viscosity wt% sharply is stable in- 262 atcreases approximately from 24 mPa·s 43 mPa to ·41s after mPa·s 10 within h. This 0–10 is because h. Subsequently, the main component its viscosity of is bentonite stable at 263 isapproximately clay mineral 43 (i.e., mPa·s montmorillonite), after 10 h. This which is because has a the multilayer main component crystal structure. of bentonite When is 264 waterclay mineral was added (i.e., montmorillonite), into bentonite, the which water has molecules a multilayer easily crystal entered structure. the smectite When layerswater 265 andwas thusadded lead into to bentonite, the lattice the expanding. water molecules As hydration easily timeentered increases, the smectite the water layers molecules and thus 266 werelead to transformed the lattice expanding. into a hexagonal As hydration ring structure time increases, through the hydrogen water molecules bonding, were and trans- thus 267 theformed interaction into a hexagonal force montmorillonite ring structure crystal through was hydrogen enhanced [bonding,23]. In addition, and thus it canthe beinterac- seen 268 fromtion force Figure montmorillonite3b that the fluidity crystal of slurrywas enhanced is great when[23]. In its addition, viscosity it is can less be than seen 35 from mPa Fig-·s, 269 whileure 3b the that fluidity the fluidity deteriorates of slurry as is its great viscosity when increasesits viscosity to largeris less thanthan 4035 mPamPa·s,·s, andwhile then the 270 thefluidity slurry deteriorates exhibits a slowas its flow viscosity state increases when the to viscosity larger than exceeds 40 mPa·s, 50 mPa and·s. then the slurry 271 exhibitsAccording a slow flow to the state construction when the experience viscosity exceeds [24], proper 50 mPa·s. pumpability of bentonite slurry 272 is necessaryAccording to ensure to the smooth construction tunneling. experience Since bentonite [24], proper slurry pumpability at 13 wt% concentration of bentonite 273 exhibitedslurry is necessary proper viscosity to ensure and smooth sufficient tunneling. hydration Since degree, bentonite the concentration slurry at 13 wt% was concen- used to 274 preparetration exhibited the B/A modified proper viscosity slurry sample. and sufficient Figure3 chydration is the schematic degree, diagram the concentration of the mixture was 275 ofused 13 to wt% prepare bentonite the B/A slurry modified and 20 slurry wt% sample ammonium. Figure chloride 3c is the solution. schematic Compared diagram of with the 276 themixture traditional of 13 wt% slurry bentonite (Figure 3slurryb), the and viscosity 20 wt% of ammonium bentonite slurry chloride rapidly solution. increases Compared after 277 addingwith the ammonium traditional chlorideslurry (Figure solution 3b), and the the viscosity B/A modified of bentonite slurry slurry exhibits rapidly a nonflowing increases 278 state.after adding Therefore, ammonium the ammonium chloride chloride solution solution and the can B/A be modified used as a slurry viscosity exhibits modifier a non- for 279 traditionalflowing state. slurry. Therefore, the ammonium chloride solution can be used as a viscosity mod- Figure3d illustrated that the ammonium ions can exchange with calcium, magnesium 280 ifier for traditional slurry. and sodium in bentonite and are not easily occluded by other ions, which is caused by 281 Figure 3d illustrated that the ammonium ions can exchange with calcium, magne- ammonium ions that have a small hydration radius, low hydration energy, similar ionic 282 sium and sodium in bentonite and are not easily occluded by other ions, which is caused radius with a six-membered ring radius in a clay crystal lattice [25]. Therefore, ammonium 283 by ammonium ions that have a small hydration radius, low hydration energy, similar chloride solution can act as a viscosity modifier of traditional slurry. The apparent viscosity 284 ionic radius with a six-membered ring radius in a clay crystal lattice [25]. Therefore, am- of traditional slurry increased and the fluid loss of sand gravel samples reduced after 285 monium chloride solution can act as a viscosity modifier of traditional slurry. The appar- adding solution containing ammonium ions. Subsequently, the solution added with excess 286 ent viscosity of traditional slurry increased and the fluid loss of sand gravel samples re- ammonium ions would lead to aggregation of montmorillonite particles and then the 287 duced after adding solution containing ammonium ions. Subsequently, the solution aggregates were surrounded by water molecules to form high consistency and viscosity 288 added with excess ammonium ions would lead to aggregation of montmorillonite parti- copolymer slurry. 289 cles and then the aggregates were surrounded by water molecules to form high con- 290 sistency and viscosity copolymer slurry. Appl. Sci. 2021, 11, 2109 8 of 17 Appl. Sci. 2021, 11, x FOR PEER REVIEW 8 of 17

291 292 FigureFigure 3. (a(a) )The The apparent apparent viscosity viscosity of diffe of differentrent concentrations concentrations of bentonite of bentonite slurry slurry changes changes with time, with ( time,b) Display (b) Display of various of 293 variousapparent apparent viscosity, viscosity, (c) B/A ( cmodified) B/A modiﬁed slurry slurryreaction reaction process process and apparent and apparent viscosity viscosity change, change, and ( andd) Ion (d) exchange Ion exchange of am- of 294 monium in bentonite slurry. ammonium in bentonite slurry.

295 3.1.2. Optimization of Viscosity ModifierModifier ConcentrationConcentration 296 As mentioned above, the incorporation ofof ammoniumammonium chloridechloride solutionsolution cancan increaseincrease 297 thethe apparent viscosity viscosity of of traditional traditional slurry slurry instantaneously. instantaneously. In In order order to toinvestigate investigate the the ef- 298 effectfect of of ammonium ammonium chloride chloride so solutionlution volume volume on on the the viscosit viscosityy of B/A modified modified slurry, thethe 299 blend of 1313 wt%wt% bentonitebentonite slurryslurry andand 2020 wt%wt% chlorinechlorine withwith differentdifferent volumevolume ratiosratios waswas 300 prepared, includingincluding 20:1,20:1, 10:1,10:1, 5:1,5:1, 10:310:3 andand 5:2.5:2. TheThe viscosityviscosity ofof thethe B/AB/A modifiedmodified slurryslurry 301 waswas measuredmeasured andand thethe resultsresults werewere shownshown inin FigureFigure4 4aa and and Table Table6. 6. It It can can be be observed observed 302 that,that, asas thethe volumevolume fractionfraction ofof ammoniumammonium chloridechloride solutionsolution increases,increases, thethe consistencyconsistency ofof 303 thethe B/AB/A modified modified slurry slurry first first increases and then decreases. The depth changechange ofof thethe conecone 304 (tested(tested byby mortarmortar consistencyconsistency meter)meter) isis consistentconsistent withwith thethe apparentapparent viscosity.viscosity. WhenWhen thethe 305 ratio isis 10:1,10:1, thethe initialinitial consistencyconsistency of thethe slurryslurry isis thethe largestlargest andand showsshows aa non-flowingnon-flowing 306 state compared with other other samples. samples. In In additi addition,on, Figure Figure 4b4b shows shows the the viscosity viscosity changes changes of 307 ofvarious various B/A B/A modified modified slurry slurry samples samples over over time time.. It can It canbe seen be seen that thatthe viscosity the viscosity of B/A of Appl. Sci. 2021, 11, 2109 9 of 17 Appl. Sci. 2021, 11, x FOR PEER REVIEW 9 of 17

308 B/Amodified modified slurry slurry slightly slightly increases increases and then and sh thenows showsa stable a trend stable over trend time, over but time, the increas- but the · 309 increasinging degree degree does not does exceed not exceed 10 mPa·s 10 mPa durings during the first the 30 first min. 30 Among min. Among them, them, the B/A the mod- B/A 310 modifiedified slurry slurry with with10:1 volume 10:1 volume ratio has ratio the has greate the greatestst plasticity plasticity and its and viscosity its viscosity is the largest. is the largest. According to the experiment’s results, the B-A-20 sample (B/A = 10:1, 20 wt% 311 According to the experiment’s results, the B-A-20 sample (B/A = 10:1, 20 wt% NH4Cl) was NH Cl) was selected as the basic sample because of its great effectiveness. 312 selected4 as the basic sample because of its great effectiveness.

313

314 FigureFigure 4.4. ((aa)) EffectEffect ofof thethe B/AB/A modified modiﬁed slurry dumping state under different ratios,ratios, andand ((bb)) ChangeChange ofof apparentapparent viscosityviscosity 315 with time. with time.

316 TableTable 6.6. ApparentApparent viscosityviscosity andand conecone penetrationpenetration ofof differentdifferent samplesamplegroups. groups.

Apparent Viscosity MatchMatch Ratio Ratio Apparent Viscosity (mPa·s) StatusStatus DescriptionDescription (mPa·s) 13 wt% bentonite (B) 18 Good liquidity. 13 wt% bentonite (B) 18 Good liquidity. 20 wt% NH4Cl (A) 0.5 Very fluid, equivalent to water. 4 0.5 Very fluid, equivalent to water. 13 wt% bentonite:20 20 wt% wt% NH NHCl4Cl (A) = 20:1 127 Moderate consistency, inverted microflow. 13 wt%13 bentonite: wt% bentonite: 20 wt% 20 NH wt%4Cl NH = 10:14Cl = 20:1 146127 Consistency Moderate consistency, is high, inverted inverted does microflow. not flow. 13 wt% bentonite: 20 wt% NH Cl = 5:1 118 Moderate consistency, inverted microflow. 13 wt% bentonite: 20 wt%4 NH4Cl = 10:1 146 Consistency is high, inverted does not flow. 13 wt% bentonite: 20 wt% NH4Cl = 10:3 79 Less consistency, inverted flow. 13 wt% bentonite: 20 wt% NH4Cl = 5:1 118 Moderate consistency, inverted microflow. 13 wt% bentonite: 20 wt% NH4Cl = 5:2 56 Small consistency, flowing after dumping. 13 wt% bentonite: 20 wt% NH4Cl = 10:3 79 Less consistency, inverted flow.

13 wt% bentonite: 20 wt% NH4Cl = 5:2 56 Small consistency, flowing after dumping. 3.2. Slump Test 3.2.1. Comparison of Traditional Slurry and B/A Modified Slurry 317 3.2. Slump Test A slump test, a common method for conditioned soil, was used in this study to evalu- 318 3.2.1. Comparison of Traditional Slurry and B/A Modified Slurry ate the modification effectiveness of traditional and B/A modified slurry to sandy gravel 319 samplesA slump with test, different a common diameters, method including for conditioned 0–2 mm, soil, 2–4 was mm, used 6–9 in mm this and study10–20 to eval- mm. 320 Theuate results the modification of the slump effectiveness tests on the of mixturestraditional of and various B/A modified sandy gravel slurry and to slurrysandy ofgravel 5%, 321 10%samples and with 20% (volumedifferent ratio)diameters, are shown includ ining Table 0–2 7mm, and 2–4 Figure mm,5. 6–9 The mm following and 10–20 results mm. 322 wereThe results observed: of the slump tests on the mixtures of various sandy gravel and slurry of 5%, 323 10% and 20% (volume ratio) are shown in Table 7 and Figure 5. The following results were 324 Tableobserved: 7. Comparison of slump between slurry and B/A modified slurry. 325 (1) As shown in Table 7, B/A modified slurry shows a positive effect on improving Injection Ratio 5% 10% 20% 326 the plasticitySample Group of sandy gravel sample, while traditional slurry has the opposite effect. Fur- 327 thermore, as the traditional slurryAdditive injection ratio increases, Average the slump Slump of (mm)sand gravel sam- 328 ple gradually increases, becauseTraditional traditional slurry slurry 158is fluid and thus 170 its incorporation 178 in- 2–4 mm Gravel 329 creases the flow ability ofB/A sand modified gravel. slurry The addition 155 of B/A modified 136 slurry reduces 0 the Traditional slurry 177 180 196 330 slump6–9 value mm Gravel because the voids of sand gravel are filled by B/A modified slurry and the B/A Modified slurry 172 162 0 331 cohesion between sand gravel particles is enhanced by B/A modified slurry. The slump Traditional slurry 162 168 174 10–20 mm Grave 332 value of the mixture of 20%B/A B/A Modified modified slurry slurry and 150 sand gravel 146 samples with 135 2–4 mm 333 and 6–9 mm diameter decreased to 0. 334 (2) It can be seen from Figure 5 that slurry is prone to flow away from sand gravel 335 pores and the amount of slurry loss increased with its injection ratio increasing, due to the 336 large fluidity of traditional slurry and large voids of sand gravel sample, such as for the Appl. Sci. 2021, 11, x FOR PEER REVIEW 10 of 17

337 6–9 mm (particle size) and 10–20 mm sand gravel samples mixed with 20% traditional 338 slurry. In addition, the larger the size of sand gravel, the greater the loss degree of slurry 339 occurs. This is because the increasing gravel diameter increases the porosity of the gravel. 340 (3) As shown in Figure 5, the addition of B/A modified slurry significantly improved 341 the cohesiveness of sand gravel particles. As the B/A modified slurry injection ratio in- 342 creases the plasticity of the sand gravel samples gradually increases and its fluidity de- 343 creases.

344 Table 7. Comparison of slump between slurry and B/A modified slurry.

Injection Ratio 5% 10% 20% Sample Group Additive Average Slump (mm) Traditional slurry 158 170 178 2–4 mm Gravel B/A modified slurry 155 136 0 Traditional slurry 177 180 196 6–9 mm Gravel Appl. Sci. 2021, 11, 2109 B/A Modified slurry 172 162 0 10 of 17 Traditional slurry 162 168 174 10–20 mm Grave B/A Modified slurry 150 146 135

345 346 FigureFigure 5.5. ComparisonComparison ofof slumpslump betweenbetween traditionaltraditional slurryslurry andand B/AB/A modiﬁedmodified slurry at variousvarious injectioninjection ratios.ratios.

347 3.2.2.(1) Effect As shownof the Injection in Table7 Ratio, B/A and modified Particle slurry Size of shows the Gravel a positive on Slump effect on improving the plasticity of sandy gravel sample, while traditional slurry has the opposite effect. Furthermore, as the traditional slurry injection ratio increases, the slump of sand gravel sample gradually increases, because traditional slurry is fluid and thus its incorporation increases the flow ability of sand gravel. The addition of B/A modified slurry reduces the slump value because the voids of sand gravel are filled by B/A modified slurry and the cohesion between sand gravel particles is enhanced by B/A modified slurry. The slump value of the mixture of 20% B/A modified slurry and sand gravel samples with 2–4 mm and 6–9 mm diameter decreased to 0. (2) It can be seen from Figure5 that slurry is prone to flow away from sand gravel pores and the amount of slurry loss increased with its injection ratio increasing, due to the large fluidity of traditional slurry and large voids of sand gravel sample, such as for the 6–9 mm (particle size) and 10–20 mm sand gravel samples mixed with 20% traditional slurry. In addition, the larger the size of sand gravel, the greater the loss degree of slurry occurs. This is because the increasing gravel diameter increases the porosity of the gravel. (3) As shown in Figure5, the addition of B/A modified slurry significantly improved the cohesiveness of sand gravel particles. As the B/A modified slurry injection ratio increases the plasticity of the sand gravel samples gradually increases and its fluidity decreases. Appl. Sci. 2021, 11, x FOR PEER REVIEW 11 of 17

348 The gravel slump is not only related to the type of bentonite slurry, but is also affected 349 by the B/A modified slurry concentration and the size of the sand gravel sample. There- 350 fore, the slump experiments of different sand gravel samples mixed with different injec- 351 tion ratios of B/A modified slurry were conducted and the results are shown in Figure 6. 352 Peila et al. (2013) proposed that the modification state of sand gravel can be classified into 353 three categories according to the flow, suitable and stiff states, as shown in Table 8. 354 As seen from Figure 6, the slump value of differently sized sand gravel samples with- 355 out the addition of modifier is between 170 mm and 180 mm. When the injection ratio of 356 B/A modified slurry is 5%, the slump value of sand gravel with a diameter of 0–2 mm 357 sharply decreases to 132.5 mm, while the flow plasticity state almost does not change for Appl. Sci. 2021, 11, 2109 11 of 17 358 the sand gravel samples of 2–20 mm in size. As the slurry injection ratio increases to 10%, 359 the slump value of the sand gravel a size of 0–2 mm decreases to zero, and the flow plas- 360 ticity state reaches a plastic non-flow state. For the sand gravel sample with a diameter of 361 3.2.2.2–6 mm, Effect the offlow the plasticity Injection state Ratio is transforme and Particled from Size flow of theplasticity Gravel to onplastic Slump flow, while 362 the state of the sand gravel of 6–20 mm in size is still the flow plasticity state. When the 363 slurryThe concentration gravel slump continually is not only increases related to to15%, the the type 2–6 ofmm bentonite sand gravel slurry, sample but changes is also affected 364 byfrom the plastic B/A modified flow state slurry to medium concentration plastic state, and and the the size mixture of the sandof 6–20 gravel mm sand sample. gravel Therefore, 365 theand slump B/A modified experiments slurry changes of different from sandflow plasticity gravel samples state to plastic mixed flow with state. different Further- injection 366 ratiosmore, ofthe B/Aslump modified value of the slurry sandy were gravel conducted sample with and a diameter the results of 0–10 are mm shown decreases in Figure 6. 367 Peilato nearly et al. zero, (2013) andproposed the plasticity that paste the modification state is the plastic state state of sand with gravel the addition can be of classified 20% into 368 threeB/A modified categories slurry. according to the flow, suitable and stiff states, as shown in Table8.

Appl. Sci. 2021, 11, x FOR PEER REVIEW 12 of 17

Appl. Sci. 2021, 11, x FOR PEER REVIEW 12 of 17 369 Appl. Sci. 2021, 11Figure, x FOR 6.PEERChanges REVIEW of slump with various injection ratio on different sandy gravel particle sizes.12 of 17 370 Figure 6. Changes of slump with various injection ratio on different sandy gravel particle sizes. Appl. Sci. 2021, 11, x FOR PEER REVIEW 12 of 17

Dumping flow state 200–250 Yogurt, cream, etc. 371 Table 8.TableConditioned 8. Conditioned sandy sandy gravel gravel state state classification. classification. Dumping flow state 200–250 Yogurt, cream, etc. Major ClassDumping flow state Class Slump 200–250 Range (mm) Yogurt, Illustration cream, etc. Major Class Class Dumping flow state Slump Range (mm) 200–250 Illustration Yogurt, cream, etc. Self-flowing state 250–300 Water, milk, etc. flow state Flow plasticityDumping state flow state 150–200 200–250 Yogurt, cream, etc. flow state Self-flowing stateFlow plasticity state 250–300 150–200 Water, milk, etc. Flow plasticity state 150–200 suitable state Flow plasticityFlow state plasticity state 150–200 150–200 suitable state suitable state suitable state suitable state Plastic flow state 100–150 Plastic flow state 100–150 Plastic flowPlastic state flow state 100–150 100–150 Plastic flow state 100–150

Moderate plastic state 50–100 Moderate Moderateplastic state plastic state 50–10050–100 stiff state Moderate plastic state 50–100 Moderate plastic state 50–100 stiff state stiff state stiff state stiff state Plastic statePlastic state 0–50 0–50 Plastic state 0–50

Plastic state 0–50 Plastic state 0–50

372 It can be concluded that, as the slurry concentration increases, the slump value of

372373 variousIt can sand be gravelconcluded samples that, gradually as the slurry decreases concentration and finally increases, the flow the plasticity slump value reaches of 373372374 variousthe plasticIt can sand bestate. gravelconcluded However, samples that, the gradually as injection the slurry decreases ratio co ncentrationof B/Aand modifiedfinally increases, the slurry flow the plasticitythat slump makes value reaches sand of 374372373375 thevariousgravel plasticIt samplecan sand bestate. gravel concludedwith However, differentsamples that, the sizegradually as injection thereach slurry decreasesplastic ratio co ncentrationofstate B/Aand is modifieddifferent.finally increases, the slurrySpecifically, flow the plasticitythat slump makes the value reachessmaller sand of 375373374376 gravelvariousthe sandplastic sample sand gravel state. gravel with diameter However, differentsamples is, thethe sizegradually lowerinjection reach the plasticdecreases slurratiory stateof concentration B/Aand is modifieddifferent.finally theis. Specifically,slurryAs flow for plasticitysandthat makesgravelthe reachessmaller sam-sand 376374375377 thegravelple sandwithplastic sample 0–2gravel state. mm with diameter size,However, different the is,plastic thethe size lower injectionstate reach isthe reachedplastic slurratiory ofstate concentrationwhen B/A is B/Amodifieddifferent. modified is. Specifically,slurryAs forslurry sandthat injection makes gravelthe smaller sam-sandratio 377375376378 plegraveltheis between withsand sample 0–2gravel 5% mm with anddiameter size, 10%,different the is,whileplastic the size lowerthe state reach injection isthe reachedplastic slur ratiory state concentrationwhen is is15%–20% B/Adifferent. modified is. for Specifically,As 2–10 forslurry sandmm injection sandgravelthe smaller gravel sam-ratio 378376377379 istheplesamples. between withsand 0–2 gravelSand 5% mm gravelanddiameter size, 10%, thewith is,whileplastic athe diameter lowerthe state injection isthe largerreached slur ratio rythan concentrationwhen is 10 15%–20% B/Amm modifiedreaches is. for As 2–10 a forslurry plastic sandmm injection sandstategravel withgravel sam-ratio a 379377378380 samples.pleismixture between with 0–2ofSand 20%–30%5% mm graveland size, 10%, B/Athewith whileplastic modified a diameter the state injectionslurry. is largerreached In ratio thanaddition, when is 10 15%–20% B/Amm the modifiedreaches voids for 2–10ofa slurry plasticthe mm 0–20 injection statesand mm withgravel sandratio a 380378379381 mixtureissamples.gravel between sample of Sand 20%–30%5% cangraveland be 10%, completely B/Awith while modified a diameter the filled injectionslurry. when larger In theratio thanaddition, addition is 10 15%–20% mm the of reachesB/A voids for modified 2–10 ofa plasticthe mm 0–20slurry statesand mm injection withgravel sand a 381379380382 gravelsamples.mixtureratio is sample 30%. ofSand 20%–30% For can gravel the be earth completely B/Awith pressuremodified a diameter filled balance slurry. when larger shield,In the thanaddition, addition some 10 mm scholars theof reachesB/A voids modified believe ofa plasticthe that slurry0–20 statethe mm injection suitable with sand a 382380381383 ratiomixturegravelslope is of sample30%. theof 20%–30% soilFor can shouldthe be earth completelyB/A be pressure modifiedbetween filled balance 100slurry. when mm shield,In theand addition, addition 200 some mm. scholarsthe of But B/A voids for modified believe the of full-facethe that slurry0–20 the water-richmm injection suitable sand 383381382384 slopegravelratiogravel, is of sample 30%.how the soiltoFor canmaintain shouldthe be earth completely bethe pressure between stability filled balance 100 of when the mm excavationshield, theand addition 200 some mm. surface scholars of But B/A andfor modified believe thereduce full-face that theslurry thepenetration water-rich injection suitable 384382383385 gravel,ratioslopeis the is ofmain 30%.how the soiltoconsideration.For maintain shouldthe earth bethe pressure betweenTherefore, stability balance 100 of by the mm en excavationshield, hancingand 200 some themm. surface scholarscohesion But andfor believe thereduceof sandfull-face that the and penetrationthe water-rich gravel suitable to 385383384386 isslopegravel,achieve the ofmain how athe plastic soilconsideration.to maintain should(flow) state bethe betweenTherefore, stabilityis an effective 100 of by the mm en waexcavation hancingandy of 200 improving themm. surface cohesion But the andfor self-stability thereduceof sandfull-face the and penetrationof water-rich gravelthe exca- to 386384385387 achievegravel,isvation the main facehow a plastic and consideration.to maintain preventing (flow) state the Therefore,the stabilityis an instability effective of by the ofen waexcavation hancingthey of formation improving the surface cohesion. the and self-stability reduceof sand the and penetrationof gravelthe exca- to 387385386 vationisachieve the mainface a plastic and consideration. preventing (flow) state Therefore,the is aninstability effective by ofen wa hancingthey offormation improving the cohesion. the self-stability of sand and of gravelthe exca- to 386387388 achievevation3.3. Permeability face a plastic and preventingTest (flow) state the is aninstability effective of wa they of formation improving. the self-stability of the exca- 388387389 3.3.vation PermeabilityAs face mentioned and preventingTest above, the the 0–20 instability mm void of the of saformationnd gravel. can be filled by B/A modified 389388390 3.3.slurry. PermeabilityAs Therefore, mentioned Test the above, addition the 0–20 of B/A mm modi voidfied of sa slurrynd gravel (13 wt% can bentonite:be filled by 20 B/A wt% modified NH4Cl 390388389391 slurry.3.3.= 10:1) PermeabilityAs hasTherefore, mentioned a positive Test the above, effectaddition the on 0–20 improvingof B/A mm modi void plasticityfied of saslurrynd of gravel sand(13 wt% cangravel bentonite:be filledto reach by 20 B/Aa wt%plastic modified NH non-4Cl 391389390392 =slurry.flow 10:1) Asstate. hasTherefore, mentioned However,a positive the above, whether effectaddition the on the 0–20 improvingof impermeabilityB/A mm modi void plasticityfied of saslurry ofnd ofsand gravel sand(13 gravel wt% cangravel bentonite:becan filledto be reach enhanced by 20 B/Aa wt%plastic modified with NH non- the4Cl 392390391 flowslurry.= 10:1) state. hasTherefore, However,a positive the whether effectaddition on the improvingof impermeabilityB/A modi plasticityfied slurry of ofsand sand(13 gravel wt% gravel bentonite:can to be reach enhanced 20 a wt%plastic with NH non- the4Cl 391392 =flow 10:1) state. has However,a positive whethereffect on the improving impermeability plasticity of ofsand sand gravel gravel can to be reach enhanced a plastic with non- the 392 flow state. However, whether the impermeability of sand gravel can be enhanced with the Appl. Sci. 2021, 11, 2109 12 of 17

As seen from Figure6, the slump value of differently sized sand gravel samples without the addition of modifier is between 170 mm and 180 mm. When the injection ratio of B/A modified slurry is 5%, the slump value of sand gravel with a diameter of 0–2 mm sharply decreases to 132.5 mm, while the flow plasticity state almost does not change for the sand gravel samples of 2–20 mm in size. As the slurry injection ratio increases to 10%, the slump value of the sand gravel a size of 0–2 mm decreases to zero, and the flow plasticity state reaches a plastic non-flow state. For the sand gravel sample with a diameter of 2–6 mm, the flow plasticity state is transformed from flow plasticity to plastic flow, while the state of the sand gravel of 6–20 mm in size is still the flow plasticity state. When the slurry concentration continually increases to 15%, the 2–6 mm sand gravel sample changes from plastic flow state to medium plastic state, and the mixture of 6–20 mm sand gravel and B/A modified slurry changes from flow plasticity state to plastic flow state. Furthermore, the slump value of the sandy gravel sample with a diameter of 0–10 mm decreases to nearly zero, and the plasticity paste state is the plastic state with the addition of 20% B/A modified slurry. It can be concluded that, as the slurry concentration increases, the slump value of various sand gravel samples gradually decreases and finally the flow plasticity reaches the plastic state. However, the injection ratio of B/A modified slurry that makes sand gravel sample with different size reach plastic state is different. Specifically, the smaller the sand gravel diameter is, the lower the slurry concentration is. As for sand gravel sample with 0–2 mm size, the plastic state is reached when B/A modified slurry injection ratio is between 5% and 10%, while the injection ratio is 15–20% for 2–10 mm sand gravel samples. Sand gravel with a diameter larger than 10 mm reaches a plastic state with a mixture of 20–30% B/A modified slurry. In addition, the voids of the 0–20 mm sand gravel sample can be completely filled when the addition of B/A modified slurry injection ratio is 30%. For the earth pressure balance shield, some scholars believe that the suitable slope of the soil should be between 100 mm and 200 mm. But for the full-face water-rich gravel, how to maintain the stability of the excavation surface and reduce the penetration is the main consideration. Therefore, by enhancing the cohesion of sand and gravel to achieve a plastic (flow) state is an effective way of improving the self-stability of the excavation face and preventing the instability of the formation.

3.3. Permeability Test As mentioned above, the 0–20 mm void of sand gravel can be filled by B/A modified slurry. Therefore, the addition of B/A modified slurry (13 wt% bentonite: 20 wt% NH4Cl = 10:1) has a positive effect on improving plasticity of sand gravel to reach a plastic non-flow state. However, whether the impermeability of sand gravel can be enhanced with the incorporation of B/A modified slurry is not certain (Figure7). Sodium silicate or high molecular polymer, as additives of traditional slurry, were often used to improve the impermeability of soil [26,27]. As a result of the chemical reaction between sodium silicate and ammonium, calcium, magnesium ions of B/A modified slurry, silicic acid, calcium silicate and magnesium silicate can be formed to fill the voids and then to enhance the impermeability of sandy gravel. Therefore, the influences of the concentration of sodium silicate and the type of polymer on the impermeability of sand gravel were investigated, which aims to provide a proper mix ratio of B/A modified slurry. Appl. Sci. 2021, 11, x FOR PEER REVIEW 13 of 17

393 incorporation of B/A modified slurry is not certain (Figure 7). Sodium silicate or high mo- 394 lecular polymer, as additives of traditional slurry, were often used to improve the imper- 395 meability of soil [26,27]. As a result of the chemical reaction between sodium silicate and 396 ammonium, calcium, magnesium ions of B/A modified slurry, silicic acid, calcium silicate 397 and magnesium silicate can be formed to fill the voids and then to enhance the imperme- 398 ability of sandy gravel. Therefore, the influences of the concentration of sodium silicate 399 and the type of polymer on the impermeability of sand gravel were investigated, which 400 aims to provide a proper mix ratio of B/A modified slurry.

401 3.3.1. Effect of the B/A Modified Slurry Concentration on Permeability 402 To study the effect of B/A modified slurry injection ratios on the impermeability re- 403 sistance of sand gravel, the impermeability of coarse sand with 0–2 mm diameter condi- 404 tioned by different injection ratio B/A modified slurry (i.e., 0%–50% with 10% interval) 405 was obtained by impermeability tests under 2 bar air pressure. The results are shown in 406 Figure 7a. It is observed that, with the addition of B/A modified slurry into a coarse sand 407 sample, its impermeability was significantly enhanced compared with the sample without 408 modifier. With the injection ratio of B/A modified slurry increasing from 10% to 40%, the 409 permeation value reduced from 46.6 mL/min to 16.2 mL/min. With the combination of 410 impermeability of conditioned coarse sand and economic effectiveness, the proper B/A 411 modified slurry injection ratio is 30%. In the following experiments, the injection ratio of 412 the B/A modified slurry is 30% if not specified. 413 The blends of B/A modified slurry and different mix ratios of sodium silicate aqueous 414 solution (i.e., B:A:S = 10:1:0–3 with 0.5 interval) were used as coarse sand conditioner to 415 improve its permeability. The permeability tests of conditioned coarse sand samples were 416 conducted under 2 bar air pressure to investigate the effect of sodium silicate concentra- 417 tion on impermeability. Results were shown in Figure 7b. It is observed that the addition 418 of sodium silicate solution can significantly improve the impermeability of coarse sand. 419 For example, the permeability test result of the B:A = 10:1 (volume ratio) sample is 18.5 420 mL/min, while the result of the B:A:S = 10:1:0.5 sample is 9.4 mL/min. In addition, as the 421 sodium silicate concentration increases, the permeation of the modified coarse sand sam- 422 ple first decreases and then increases. More specifically, as the concentration of sodium 423 silicate solution increases from 10:1:0.5 to 10:1:2, the permeation value decreases from 9.4 Appl. Sci. 2021, 11, 2109 13 of 17 424 to 3.6 mL/min, and when the concentration continually increases to 10:1:3, the permeation 425 value increases to 5.3 mL/min.

426 427 Figure 7.Figure (a) Effect 7. (a) Effectof B/A of B/Ainjection injection ratio ratio on on seepage seepage flow. ﬂow. ( b(b) Effect) Effect of sodiumof sodium silicate silicate content content on B/A on seepage B/A seepage ﬂow. flow.

3.3.1. Effect of the B/A Modified Slurry Concentration on Permeability 428 3.3.2. Effect of Polymer Type and Concentration on Permeability of Modifier To study the effect of B/A modified slurry injection ratios on the impermeability resistance of sand gravel, the impermeability of coarse sand with 0–2 mm diameter conditioned by different injection ratio B/A modified slurry (i.e., 0–50% with 10% interval) was obtained by impermeability tests under 2 bar air pressure. The results are shown in Figure7a. It is observed that, with the addition of B/A modified slurry into a coarse sand sample, its impermeability was significantly enhanced compared with the sample without modifier. With the injection ratio of B/A modified slurry increasing from 10% to 40%, the permeation value reduced from 46.6 mL/min to 16.2 mL/min. With the combination of impermeability of conditioned coarse sand and economic effectiveness, the proper B/A modified slurry injection ratio is 30%. In the following experiments, the injection ratio of the B/A modified slurry is 30% if not specified. The blends of B/A modified slurry and different mix ratios of sodium silicate aqueous solution (i.e., B:A:S = 10:1:0–3 with 0.5 interval) were used as coarse sand conditioner to improve its permeability. The permeability tests of conditioned coarse sand samples were conducted under 2 bar air pressure to investigate the effect of sodium silicate concentration on impermeability. Results were shown in Figure7b. It is observed that the addition of sodium silicate solution can significantly improve the impermeability of coarse sand. For example, the permeability test result of the B:A = 10:1 (volume ratio) sample is 18.5 mL/min, while the result of the B:A:S = 10:1:0.5 sample is 9.4 mL/min. In addition, as the sodium silicate concentration increases, the permeation of the modified coarse sand sample first decreases and then increases. More specifically, as the concentration of sodium silicate solution increases from 10:1:0.5 to 10:1:2, the permeation value decreases from 9.4 to 3.6 mL/min, and when the concentration continually increases to 10:1:3, the permeation value increases to 5.3 mL/min.

3.3.2. Effect of Polymer Type and Concentration on Permeability of Modiﬁer In this study, the effect of high molecular polymer type, including Kapom, Guar, Xanthan, Carboxymethyl cellulose Sodium (CMC), Hydroxyethyl cellulose (HEC) and Polyacrylamide (PAM), on the impermeability of coarse sand was investigated. Among them, the solubility of Xanthan is lowest, 0.5 wt%, which was used as the concentration of various polymers to study the effect of polymer type on the impermeability property. Therefore, coarse sand is conditioned by 30% modiﬁed slurry consisting of B, A, S and X (i.e., 0.5 wt% polymer) and their volume ratio is 10:1:2:2. The permeability tests under 2 bar Appl. Sci. 2021, 11, x FOR PEER REVIEW 14 of 17

429 In this study, the effect of high molecular polymer type, including Kapom, Guar, 430 Xanthan, Carboxymethyl cellulose Sodium (CMC), Hydroxyethyl cellulose (HEC) and 431 Polyacrylamide (PAM), on the impermeability of coarse sand was investigated. Among 432 them, the solubility of Xanthan is lowest, 0.5 wt%, which was used as the concentration of 433 various polymers to study the effect of polymer type on the impermeability property. 434 Therefore, coarse sand is conditioned by 30% modified slurry consisting of B, A, S and X 435 (i.e., 0.5 wt% polymer) and their volume ratio is 10:1:2:2. The permeability tests under 2 436 bar air pressure of conditioned coarse sand were performed and the results were shown 437 in Figure 8a. It is observed that the improvement degree of impermeability of coarse sand 438 with addition of HEC is the greatest compared to that of the sand sample with the addition 439 of other polymers. The order of improvement performance degree of polymer from high 440 to low is HEC, CMC, Kapom, PAM, Guar, Xanthan. 441 In order to study the effect of HEC concentration on the impermeability of coarse 442 Appl. Sci. 2021, 11, 2109 sand, coarse sand samples were prepared by adding different concentrations14 ofof 17 HEC (i.e., 443 0–4 volume concentration with 0.5 interval). Permeability tests were conducted under 2 444 bar air pressure. The results were shown in Figure 8b. It is observed that the higher the 445 HECair concentration, pressure of conditioned the greater coarse the sand obtained were performed impermeability and the resultsof conditioned were shown coarse in sand. Figure8a. It is observed that the improvement degree of impermeability of coarse sand 446 When the HEC proportion is 2, the volume of fluid per unit time is the lowest (<1 mL/min). with addition of HEC is the greatest compared to that of the sand sample with the addition 447 However,of other with polymers. the HEC The order proportion of improvement is 2–4, performance the impermeability degree of polymer of coarse from high sand to shows a 448 reversedlow is trend. HEC, CMC, Kapom, PAM, Guar, Xanthan.

449

450 Figure 8.Figure (a) Effect 8. (a) Effectof high of highpolymer polymer types types on on B/A/S B/A/S seepage seepage ﬂow.flow. (b ()b Effect) Effect of HEC of HEC content content on B/A/S on B/A/S seepage seepage ﬂow. flow.

In order to study the effect of HEC concentration on the impermeability of coarse 451 3.3.3. Polymer Compounding and Verification of Conditioned Gravel Permeability sand, coarse sand samples were prepared by adding different concentrations of HEC (i.e., 452 0–4Further, volume in concentration order to verify with whether 0.5 interval). differ Permeabilityent types testsof high were poly conductedmer blends under will pro- 453 duce2 better bar air impermeability, pressure. The results based were shownon the inabov Figuree experimental8b. It is observed results, that the the higher impermeability the 454 of HECHEC samples concentration, mixed the with greater Kapom, the obtained CMC impermeability and Xanthan of conditionedwas tested. coarse The sand.results were When the HEC proportion is 2, the volume of fluid per unit time is the lowest (<1 mL/min). 455 shown in Figure 9a. It can be seen from the experimental results that the permeation of However, with the HEC proportion is 2–4, the impermeability of coarse sand shows a 456 the threereversed groups trend. of compound experiments were larger than that of the pure HEC sample 457 group. The impermeability improvement performance in decreasing order is CMC, 458 Kapom3.3.3. and Polymer Xanthan. Compounding The experi andmental Verification results of Conditioned were consistent Gravel Permeabilitywith the preferred exper- 459 iment resultsFurther, shown in order in toFigure verify 8a. whether Therefore, different it types can ofbe high concluded polymer blendsthat the will single produce component better impermeability, based on the above experimental results, the impermeability of HEC 460 of the sealing water effect of HEC on bentonite is better than the compounding compo- samples mixed with Kapom, CMC and Xanthan was tested. The results were shown in 461 nent,Figure and 9therea. It canis no be seensignificant from the difference experimental in resultsthe performance that the permeation of compounded of the three polymers 462 withgroups better ofperformance compound experiments were larger than that of the pure HEC sample group. 463 TheAccording impermeability to the improvement permeability performance tests, the inoptimal decreasing ratio order of isthe CMC, proposed Kapom and conditioner 464 was:Xanthan. B:A:S:H The = 10:1:2:2. experimental To veri resultsfy the were impermeability consistent with the effect preferred of the experiment modified results slurry on the shown in Figure8a. Therefore, it can be concluded that the single component of the 465 gravel under the ratio, experiments were carried out on 2–4 mm, 4–6 mm, 6–9 mm, 9–12 sealing water effect of HEC on bentonite is better than the compounding component, 466 mm andand there 10–20 is nomm significant gravel, differencerespectively, in the and performance the results of compounded were compar polymersed with with the coarse better performance Appl. Sci. 2021, 11, x FOR PEER REVIEW 15 of 17

467 sand group. The experimental results were shown in Figure 9b, it can be seen that when 468 the gravel particle size is within 12 mm, the impermeability improvement effect of the 469 modified slurry is gradually weakened, and when the particle size ranges from 10 to 20 470 mm, the volume of fluid is close to that of coarse sand. The average permeation value of 471 Appl. Sci. 2021, 11, 2109 each samples was less than 1 mL/min, indicating that the modified slurry15 showed of 17 good 472 impermeability on the gravel under different particle sizes.

473 474 Figure 9.Figure (a) Effect 9. (a) of Effect two of polymers two polymers mixing mixing on seepageseepage flow. flow. (b) Effect (b) Effect of B/A/S/H of B/A/S/H at the optimal at the ratio optimal on gravel ratio impermeability on gravel .imperme- 475 ability. According to the permeability tests, the optimal ratio of the proposed conditioner was: B:A:S:H = 10:1:2:2. To verify the impermeability effect of the modified slurry on the gravel 476 4. Conclusionsunder the ratio, experiments were carried out on 2–4 mm, 4–6 mm, 6–9 mm, 9–12 mm and 10–20 mm gravel, respectively, and the results were compared with the coarse sand group. 477 When EPB is used in the excavation of full-face water-rich sandy pebble stratum with The experimental results were shown in Figure9b, it can be seen that when the gravel 478 highparticle permeability, size is within applying 12 mm, thetraditional impermeability conditioner improvement may effect be insufficient, of the modified which slurry will lead 479 to accidentsis gradually such weakened, as the spewing and when theof the particle screw size machine ranges from and 10 toground 20 mm, collapse. the volume In of view of it, 480 a newfluid conditioner, is close to that modified of coarse slurry, sand. The was average propos permeationed in this valuepaper. of eachModified samples slurry was was pre- 481 paredless by than using 1 mL/min, ammonium indicating chloride that the as modified a viscosity slurry modifier showed good and impermeability sodium silicate on solution the gravel under different particle sizes. 482 and polymer as water shutoff agents. Two main aspects were studied: (1) viscosity modi- 483 fier effect4. Conclusions of modified slurry conditioner’s performance on the plasticity of sandy gravel 484 soils withWhen different EPB is usedparticle in the sizes; excavation (2) the of effect full-face of water-rich concentration sandy pebble and type stratum of water with shutoff 485 agenthigh on permeability,the improvement applying of traditional conditioned conditioner soil impermeability. may be insufficient, which will lead 486 toAmmonium accidents such chloride as the spewing can act of as the the screw viscosity machine modifier and ground of collapse. traditional In view slurry of and in- it, a new conditioner, modified slurry, was proposed in this paper. Modified slurry was 487 stantaneously increased the apparent viscosity of traditional slurry. The developed new prepared by using ammonium chloride as a viscosity modifier and sodium silicate solution 488 modifiedand polymer slurry as has water an shutoff effect agents. on improving Two main aspects the plasticity were studied: of sandy (1) viscosity gravel modifier soils. Sodium 489 silicateeffect and of modifiedhigh-molecular slurry conditioner’s polymer performanceas the main on water the plasticity shutoff of agents sandy gravelfor modified soils slurry 490 greatlywith enhanced different particle the impermeability sizes; (2) the effect of of conditioned concentration samples. and type ofThe water optimum shutoff agent slurry is com- 491 posedon of the B:A:S:H improvement = 10:1:2:2 of conditioned and its suggested soil impermeability. injection ratio is 30%. Ammonium chloride can act as the viscosity modifier of traditional slurry and in- 492 In addition, the suggested field application method is, separately, the injection of B, stantaneously increased the apparent viscosity of traditional slurry. The developed new 493 A, S modifiedand H (A slurry and hasH should an effect better on improving be premixed) the plasticity at the of ratio sandy of gravel10:1:2:2 soils. into Sodium the excavation 494 chambersilicate through and high-molecular the slurry polymer pipelines. as the After main stirring water shutoff through agents the for cutterhead, modified slurry these com- 495 ponentsgreatly react enhanced rapidly the to impermeability form a modified of conditioned slurry, samples.which will The consequently optimum slurry increase is the 496 plasticitycomposed and of impermeability B:A:S:H = 10:1:2:2 of and the its conditioned suggested injection soils. ratio is 30%. In addition, the suggested field application method is, separately, the injection of B, A, S and H (A and H should better be premixed) at the ratio of 10:1:2:2 into the excava- 497 Authortion Contributions: chamber through Conceptualization, the slurry pipelines. X.L., After X.Z. stirring and Y. throughY.; methodology, the cutterhead, X.L and these X.Z.; formal 498 analysis,components X.Z. and react Y.Y.; rapidly investigation, to form a X.Z.; modified writin slurry,g—original which will draft consequently preparation, increase X.L. and the X.Z.; writ- 499 ing—reviewplasticity and and editing, impermeability X.L, X.Z. of theand conditioned Y.Y.; fund soils.ing acquisition, Y.Y. All authors have read and 500 agreed to the published version of the manuscript.

501 Funding: This research was funded by the National Key Research and Development Program of 502 China, grant number 2017YFC0805008. Appl. Sci. 2021, 11, 2109 16 of 17

Author Contributions: Conceptualization, X.L., X.Z. and Y.Y.; methodology, X.L and X.Z.; formal analysis, X.Z. and Y.Y.; investigation, X.Z.; writing—original draft preparation, X.L. and X.Z.; writing— review and editing, X.L, X.Z. and Y.Y.; funding acquisition, Y.Y. 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, grant number 2017YFC0805008. Institutional Review Board Statement: “Not applicable” for studies not involving humans or animals. Informed Consent Statement: “Not applicable” for studies not involving humans. Data Availability Statement: Data available in a publicly accessible repository. Acknowledgments: Special thanks are due to Li Fan for her suggestion and assistance in the experimental work. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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