Effects of Soil Conditioning on Characteristics of a Clay-Sand

applied sciences

Article Eﬀects of Soil Conditioning on Characteristics of a Clay-Sand-Gravel Mixed Soil Based on Laboratory Test

Yingjie Wei 1,2,3,* , Duli Wang 2, Jianguang Li 2 and Yuxin Jie 1 1 State Key Laboratory of Hydroscience and Engineering, Tsinghua University, Beijing 100084, China; [email protected] 2 AVIC Institute of Geotechnical Engineering Co., LTD., Beijing 100098, China; [email protected] (D.W.); [email protected] (J.L.) 3 School of Engineering and Technology, China University of Geosciences (Beijing), Beijing 100083, China * Correspondence: [email protected]

Received: 22 April 2020; Accepted: 7 May 2020; Published: 9 May 2020

Abstract: Soil conditioning is of great significance for tunneling in soft ground with earth pressure balance shield (EPBS) machines, which leads to safe, highly efficient, and high-quality tunneling. To study the effects of soil conditioning on properties of a clay-sand-gravel mixed soil encountered in an EPBS tunneling project in Beijing, a series of geotechnical test methods was carried out based on laboratory test in this paper. The decay behaviors of foam particles generated by the Waring-Blender method were studied first using the image particle analysis system, and then the feasibility of soil conditioning on the mixed soil was qualitatively and quantitatively assessed through the mixing test, slump test, and friction coefficient test. The preliminary test results indicate that drainage of water in liquid film plays an important role in the decay of foam microstructure. The viscosity and flowability of the conditioned soil were modified dramatically by using various amount of water and foam, and a suitable state meeting the requirement of EPBS machines was obtained. The net power and mixing time, which reflects the interaction between the blending rods and tested soils, as well as the slump value, providing the overall indication for liquidity of the conditioned soils, and friction coefficient, reflecting the friction between steel and tested soils, were used to provide insight into the variation in viscosity and flowability of the tested soils.

Keywords: soil conditioning; feasibility; mixed soil; earth pressure balance shield

1. Introduction For the sustainable development of the big city, developing more underground space is effective, and the development of intensive tunnels, including metro tunnels, road tunnels, and utility tunnels, is carried out throughout the world. Earth pressure balance shield (EPBS) machines, which were originally developed and used in a close mode for fine grained soils or mixed grained soils with at least 30 wt.% fines (d < 0.06 mm) [1,2], have been extensively used in various tunnels because of their high security in driving, high efficiency in tunneling, and high feasibility in stratums. EPBS machines achieve stability of tunnel face by filling the chamber and applying chamber pressure at the back of cutterhead [3], and they have been used in many kinds of excavation soils with complicated characteristics, such as caking property of clayed soils [4], looseness of sandy soils [5], and abrasivity of gravelly soils [6]. Many studies on the application of soil conditioning in various given soils have been carried out in recent years. Today, EPBS tunneling is possible through almost all geological conditions, and more and more EPBS machines work in mixed ground due to their large excavation diameter [7] thanks to the application of the soil conditioning system and soil conditioning agents.

Appl. Sci. 2020, 10, 3300; doi:10.3390/app10093300 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 3300 2 of 16

Soil conditioning, which is often done by injecting water, foam, bentonite slurry, or polymer in the excavation face, soil chamber, and screw conveyor during EPBS tunneling [8], is used to improve the stability of tunnel face and then minimize the deformation of ground surface, reduce cutting tools wear, and discharge the excavated soils smoothly [9,10]. The torque of cutterhead and the thrust of EPBS machines increases even if the screw conveyor does not work when the soil chamber and the screw conveyor are filled with excavated soils [11]. The workability of the excavated soils is improved dramatically after the mixing caused by cutters and cutterhead in tunnel face and stirring caused by rods in soil chamber. For soil conditioners, water is a common conditioner which is very easy to access. Foam agent is a widely used soil conditioner, and bentonite slurry are usually used to modify the grain size distribution of the excavated soils with unfavorable particle composition for EPBS tunneling [12]. For challenging geological conditions, in which the use of water is insufficient, foam and slurry, the structuring and water-binding polymers, are warranted to increase the muck cohesion and decrease permeability, respectively [7]. Foam produced by chemical products, named surfactants or surface-active agents, is usually used in two fields of underground works, including cellular concrete and drilling operations, although the industrial applications of foam are various [13]. In most cases, it is necessary to mix the excavated soils with some soil conditioners to modify the original characteristics of the natural soils to meet the requirement of EPBS machines. In order to obtain and evaluate the state and properties of the conditioned soils for various purposes, a series of evaluation methods were proposed, such as a mixing test, slump test, friction coefficient test, adhesive resistance test, penetration test, and compressibility test. Among these tests, the mixing test is used to imitate the real mixing process in the soil chamber [14], and it is necessary to mix the soil conditioner with the tested soils in laboratory tests [15]. The variation of the electric motor power, the mixing time necessary to obtain a homogeneous mixture, and the quality and behavior of the mixture could be observed and evaluated during the evolution of the foam injection in the tested soil [13]. Some mixing tests have been carried out in previous studies [14–16]. The slump test [17], which is originally used for concrete materials, has been intensively used to evaluate the characteristics of conditioned soils in EPBS tunneling [3,5,12–14,16,18–22]. For the flowability of conditioned soils, slump values in the range of 100–200 mm are accepted and considered reasonable in soft ground tunneling [2,3,18,19], which means that the overall state of conditioned soils meets the requirement of EPBS machines during tunneling, and the EPBS machines could advance smoothly. The friction coefficient test, imitating the friction process of soil and steel, is used to obtain the frictional coefficient by measuring the friction angle between soil and steel or angle of external friction [14], which also reflects the effect of soil conditioning on the tested soils. The mixing test, slump test, and friction coefficient test are widely used in evaluating of properties of conditioned soils in laboratory-scale tests because of their simplicity, speed, and low cost. EPBS machines, which were originally used for fine-grained or mixed -rained soils with at least 30 wt.% fines (d < 0.06 mm), have been proved to be applicable for coarse-grained and cohesionless soils by the utilization of soil conditioning, such as sand [19], rock masses [20], tar sand [12], weathered granite soil [3], and so forth. The current research mainly focused on the soil conditioning in single soils, while the feasibility of excavation soils from mix ground composed of cohesive soil and cohesionless soil have rarely been investigated to date, which is more difficult to resolve in EPBS tunneling. The characteristics of the mixed soil, which is abundant in large-diameter EPBS tunneling, are different from those of either clay or sand, as the conditioning agents applicable to single soils cannot be directly used in mixed soils. There are still some knowledge gaps in the application of soil conditioning in mixed soils. This article presents an experimental investigation of effects of common foam and water on the characteristics of a mixed soil consisting of silty clay, fine-medium sand, and sandy gravel, from an EPBS tunneling project in Beijing. The microstructure of the foam particle was first investigated to explore the decay mechanism, followed by the description of the evaluation methods, testing procedures, Appl. Sci. 2020, 10, x FOR PEER REVIEW 3 of 17

95 EPBS tunneling project in Beijing. The microstructure of the foam particle was first investigated to Appl. Sci. 2020, 10, 3300 3 of 16 96 explore the decay mechanism, followed by the description of the evaluation methods, testing 97 procedures, and tested soil used in this study. After that, the detailed results and discussion of the 98 andlaboratory tested soiltests used, which in this demonstrate study. Afters the that, effect the of detailed the soil results conditioning and discussion, are presented of the laboratory. tests, which demonstrates the effect of the soil conditioning, are presented. 99 2. Methods and Materials 2. Methods and Materials 100 2.1. Image Particle Analysis System 2.1. Image Particle Analysis System 101 Foam was defined as a physical state in which a gaseous phase is dispersed in bubble form in a 102 secondFoam liquid was phase defined [13 as]. In a physicalorder to stateobtain in the which decay a gaseous behaviors phase of the isdispersed used foam, in the bubble BT-1600 form-type in a 103 secondimage particle liquid phaseanalysis [13 system]. In order, which to obtain is the theproduction decay behaviors of combing of thetraditional used foam, microscopic the BT-1600-type methods 104 imagewith modern particle image analysis processing system, whichtechniques, is the was production used to observe of combing the vari traditionalation in microscopic foam microstructure methods 105 with modern time. The image BT-1600 processing-type image techniques, particle was analysis used to system observe is the a multifunctional variation in foam particle microstructure analysis 106 withsystem time. consist The BT-1600-typeing of optical image microscope, particle analysis digital system charge is a coupled multifunctional device particle(CCD) analysiscamera, system image 107 consistingprocessing ofand optical analysis microscope, software, digital and so charge on. The coupled test procedure device (CCD) is as follows: camera, (1) image The generated processing foam and 108 analysiswas dripped software, on the and glass so on.slide The; (2) test the procedure dripped foam is as follows:was covered (1) The with generated a coverslip foam immediately was dripped to 109 onobtain the glassa thin slide; foam (2) layer; the dripped(3) videos foam and wasphotos covered were withtaken a coverslipunder the immediatelycondition of tothe obtain microscope a thin 110 foamwith 10 layer;-times (3) eyepiece videos and and photos 4-times were objective taken lens under magnification the condition for of 60 the mi microscopen; (4) the particle with 10-times images 111 eyepiecewere transferred and 4-times to a objectivecomputer lens using magnification USB data transmission for 60 min; (4) and the the particle image imagess were were analyzed transferred using 112 tospecial a computer particle using image USB analysis data transmission software; (5) and the theresults images were were exported analyzed using using the special display particle and printer. image analysis software; (5) the results were exported using the display and printer. 113 2.2. Mixing Test 2.2. Mixing Test 114 To mix foam with tested soils and preliminarily obtain the flowability state of conditioned soils, To mix foam with tested soils and preliminarily obtain the flowability state of conditioned soils, 115 a mixing tester with electric power meter was used in this study. The device consists of a vertically a mixing tester with electric power meter was used in this study. The device consists of a vertically 116 oriented steel cylindrical chamber with 385 mm in inner diameter and 255 mm in height, as shown in oriented steel cylindrical chamber with 385 mm in inner diameter and 255 mm in height, as shown 117 Figure 1. A rod with eight steel sheets driven by an electric motor can rotate inside the vertical in Figure1. A rod with eight steel sheets driven by an electric motor can rotate inside the vertical 118 container. The mixer has the capability of mixing water and foam with tested soils. The tested soils container. The mixer has the capability of mixing water and foam with tested soils. The tested soils 119 were mixed with soil conditioning agents according to the following procedure: (1) A quantity of 25 were mixed with soil conditioning agents according to the following procedure: (1) A quantity of 25 kg 120 kg of mixed soils was first placed in the cylinder container; (2) water with the designed injection of mixed soils was first placed in the cylinder container; (2) water with the designed injection ratio was 121 ratio was added into the soil and mixed thoroughly; (3) the generated foam with the required added into the soil and mixed thoroughly; (3) the generated foam with the required concentration 122 concentration of foam agent (Cf), foam expansion ratio (FER), and foam injection ratio (FIR) was of foam agent (C ), foam expansion ratio (FER), and foam injection ratio (FIR) was added to soil 123 added to soil samplesf ; (4) the mixture was blended for 2 min and the mixing time necessary to obtain samples; (4) the mixture was blended for 2 min and the mixing time necessary to obtain a homogeneous 124 a homogeneous mixture without apparent foam. The electric motor power used for the mixing mixture without apparent foam. The electric motor power used for the mixing process and pictures of 125 process and pictures of conditioned state after pulling up of the stirring rod, were recorded, conditioned state after pulling up of the stirring rod, were recorded, monitored, and taken for each test. 126 monitored, and taken for each test.

127

128 Figure 1. Mixer with power meter (after [[14]14]).). Appl. Sci. 2020, 10, 3300 4 of 16 Appl. Sci. 2020, 10, x FOR PEER REVIEW 4 of 17

129 2.3. Slump T Testest 130 After the mechanical mixing tests,tests, slump teststests werewere carried out to provide an overall indication 131 of the flowabilityflowability of the fully conditioned soil. A standard slump cone with 100 mm in diameter of 132 top opening, 200 mm in diameter of bottom opening,opening, and 300 mm in height was used,used, as shown in 133 Figure 22.. TheThe procedureprocedure ofof thethe slumpslump testtest isis asas follows:follows: (1)(a) TheThe slumpslump conecone waswas filledfilled threethree times,times, 134 that is,is, toto thethe heightheight of of 1 1/3,/3, 2 2/3,/3, and and 3 /3/33, respectively;, respectively (2); (2) the the filled filled soil soil was was tamped tamped after after each each fill; fill; (3) the(3) 135 soilthe soil was was flatted flatted at the at topthe openingtop opening and theand cone the cone was liftedwas lifted slowly; slowly; (4) the (4) dropping the dropping height height of the testedof the 136 materialstested materials was measured. was measured.

137 138 Figure 2. SSlumplump test device device..

139 2.4. Friction C Coeoefficientfficient TestTest 140 ToTo studystudy the the eff effectect of soil of soil conditioning conditioning on the on viscosity the viscosity of the conditioned of the conditioned soil, the friction soil, the coe frictionfficient 141 betweencoefficient soil between and steel soil was and tested steel by was measuring tested by the measur frictioning angle the usingfriction the angle friction using coe thefficient friction test. 142 Thecoefficient procedure test. of The the procedure friction coe offfi thecient friction test is coefficient as follows: test (1) Ais plasticas follows: cylinder (a) A with plastic an inner cylinder diameter with 143 ofan 100inner mm diameter and height of 100 of 100 mm mm and was height filled of with 100 soilmm samples was filled and with then soil it was samples placed and on thethen flat it steelwas 144 plate;placed (2) on onethe flat end steel of the plate plate; (2) was one liftedend of up the slowly plate was and lifted the angle up slowly on the and angle the dialangle was on the recorded angle 145 whendial was the recorded cylinder startswhen tothe move, cylinder as shown starts into Figuremove,3 as; (3) shown the friction in Figure coe 3ffi; cient(3) the was friction obtained coefficient by the 146 followingwas obtained equation: by the following equation: f = f  tantanδ (1) (1) where f is the friction coefficient, and δ is the friction angle between soil and steel (◦) 147 where f is the friction coefficient, and  is the friction angle between soil and steel (°) Appl. Sci. 2020, 10, 3300 5 of 16

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148 149 FigureFigure 3.3. Friction coecoefficientﬃcient testtest device.device.

150 3.3. Soil SamplesSamples and Foam AgentAgent UsedUsed inin ExperimentsExperiments 151 AA mixedmixed soilsoil consisting consisting of of silty silty clay, clay, fine-medium fine-medium sand, sand, andand sandy sandy gravel, gravel, withwith aa volumevolume 152 proportionproportion of 2 2:2:1:2:1 encountered encountered in in an an EPBS EPBS tunneling tunneling projects projects in Beijing in Beijing,, was was used used in this in thisstudy. study. The 153 Theselected selected properties properties of the of soil the were soil tested were testedbased basedon GB/T on 50123 GB/T [2 501233] and [23 are] andlisted are in listedTable in1. For Table silty1. 154 Forclay, silty X-ray clay, diffraction X-ray diff testsraction were tests performed were performed to determine to determine the mineral the mineralconstituent, constituent, as shown as in shown Table 155 in2. Figure Table2 .4 Figureshows 4the shows grain the size grain distribution size distribution of the fine of-medium the fine-medium sand and sandy sand gravel. and sandy Based gravel. on the 156 Basedliquid- onplastic the liquid-plastic limit combined limit method combined [23],method the plastic [23 ],limit the plastic(WP) and limit liquid (WP limit) and ( liquidWL) of limitthe silty (WL clay) of 157 thewere silty 30% clay and were 44%, 30% respectively. and 44%, respectively.For the natural For silty the clay natural with silty a water clay withcontent a water of 25%, content the plasticity of 25%, 158 theindex plasticity (IP) and index liquidity (IP) andindex liquidity (IL) were index 14% (I andL) were -0.36, 14% respectively. and -0.36, respectively.The gravel particles The gravel wrapped particles by 159 wrappedsilty clay byand silty sand clay particles and sand cohere particles the cutting cohere tools the cuttingon the cutterhead tools on the and cutterhead it is hard andto discharge it is hard the to 160 dischargemixed soils the via mixed screw soils conveyor via screw. The conveyor. mechanical The mechanicalbehavior of behavior the mixed of thesoil mixedis not soilsuitable is not for suitable EPBS 161 fortunneling EPBS tunneling.. Soil conditioning Soil conditioning is needed is needed to modify to modify characteristic characteristicss of the of the mixed mixed soil soil to to meet meet the the 162 requirementrequirement ofof EPBSEPBS machine.machine. The natural water content of the mixedmixed soil was approximately 25%,25%, 163 andand thethe originaloriginal waterwater contentcontent waswas setset toto 25%25% forfor aa groupgroup ofof soilsoil samplesample testtestin inthis thisstudy. study.

164 TableTable 1.1. GeotechnicalGeotechnical propertiesproperties ofof testedtested soil.soil.

NaturalNatural Bulk bulk Density density VoidVoid Ratio ratio CohesionCohesion Internal Internal Frictionfriction Angle angle Soils Soils ρ ρ ee c c ϕφ 3 (g/cm(g/cm) 3) (kPa)(kPa) ((°)◦) SiltySilty clay clay 1.961.96 0.770.77 2525 1010 Fine to medium sand 2.00 0.60 0 30 FineSandy to medium gravel sand 2.052.00 0.450.60 00 4530 Sandy gravel 2.05 0.45 0 45

For foam agents, there are two performance indicators which are often used to characterize foaming behaviors and select a foam agent: Foamability and foam stability. Foamability means how many volumes of bubbles with stable structure and moderate size can be produced by a given amount of a foaming agent, which is quantiﬁed by the ratio between the resulting volume of the foam and the volume of foaming liquid (foaming agent + water), that is, the foam expansion ratio (FER). Foam stability (or drainage behavior) means how long the foam can remain and how long foamed soils can sustain the desirable properties or behaviors. The half-life period (T1/2), representing the time for half volume of the liquid matrix ﬂowing out of the foam, can be used to characterize the foam Appl. Sci. 2020, 10, 3300 6 of 16

stability. There are several important conditioning parameters for foam agent, which are deﬁned as follows [2,15]: V f C f = 100(%) (2) VL × V FER = F (3) VL V FIR = FIR 100(%) (4) VS ×

where Cf is the concentration of the foaming agent (%), FER is the foam expansion ratio (dimensionless), FIR is the foaming injection ratio (%), Vf is the volume of foaming agent (mL), VF is the volume of foam (mL), VL is the volume of foaming agent and water (mL), and VS is the volume of soil (mL).

Table 2. Mineral constituent of silty clay.

Soil Mineral Type Percent by Mass (%) Quartz 39 Plagioclase 23 Microcline 7 Calcite 7 Silty clay hematite 2 mica 9 montmorillonite 9 Clinochlore 4 Appl. Sci. 2020, 10, x FOR PEER REVIEW 6 of 17

165

166 Figure 4. Grain size distribution curves of tested soil sample.

167 In order to obtain the foamTable during 2. Mineral laboratory constituent test, of silty a commonly clay. used method called the Waring-Blendermethod was used in this study. The procedure of foam production in the Waring-Blender soil Mineral type Percent by mass (%) test is as follows: (1) 100 mL foaming liquid (foaming agent + water) with diﬀerent concentration of Quartz 39 foaming agent (C , vol.%) was added into a mixer; (2) the foaming liquid was thoroughly mixed for 1 min f Plagioclase 23 at a rotational speed of 3000 rpm (revolutions per minute), as shown in Figure5a. Then, the foamability Microcline 7 (i.e., FER) and foam stability (i.e., T ) of the generated foam were tested, as shown in Figure5b. 1/2 Calcite 7 Finally, the foam agent withSiltyC clay= 4%, FER = 7 and T = 30 min under the room temperature of 25 °C f hematite 1/2 2 was used in this study. The generated foam was used for the observation of the microstructure and mica 9 subsequent evaluation tests of the conditioned soil. The generated foam was added after thoroughly montmorillonite 9 mixing of tested soils and water (see Figure6a). The foam injection ratios (FIR) were set as 0%, 10%, Clinochlore 4

168 For foam agents, there are two performance indicators which are often used to characterize 169 foaming behaviors and select a foam agent: Foamability and foam stability. Foamability means how 170 many volumes of bubbles with stable structure and moderate size can be produced by a given 171 amount of a foaming agent, which is quantified by the ratio between the resulting volume of the 172 foam and the volume of foaming liquid (foaming agent + water), that is, the foam expansion ratio 173 (FER). Foam stability (or drainage behavior) means how long the foam can remain and how long 174 foamed soils can sustain the desirable properties or behaviors. The half-life period (T1/2), 175 representing the time for half volume of the liquid matrix flowing out of the foam, can be used to 176 characterize the foam stability. There are several important conditioning parameters for foam agent, 177 which are defined as follows [2,15]:

Vf C =×100(%) (2) f V L VF FER= (3) VL

VFIR FIR =×100(%) (4) VS

178 where Cf is the concentration of the foaming agent (%), FER is the foam expansion ratio 179 (dimensionless), FIR is the foaming injection ratio (%), Vf is the volume of foaming agent (ml), VF is 180 the volume of foam (ml), VL is the volume of foaming agent and water (ml), and VS is the volume of 181 soil (ml). 182 In order to obtain the foam during laboratory test, a commonly used method called the 183 Waring-Blender method was used in this study. The procedure of foam production in the Appl. Sci. 2020, 10, x FOR PEER REVIEW 7 of 17 Appl. Sci. 2020, 10, x FOR PEER REVIEW 7 of 17 184 Waring-Blender test is as follows: (1) 100 ml foaming liquid (foaming agent + water) with different 184 Waring-Blender test is as follows: (1) 100 ml foaming liquid (foaming agent + water) with different 185 concentration of foaming agent (Cf, vol.%) was added into a mixer; (2) the foaming liquid was 185 concentration of foaming agent (Cf, vol.%) was added into a mixer; (2) the foaming liquid was 186 thoroughly mixed for 1 min at a rotational speed of 3000 rpm (revolutions per minute), as shown in 186 thoroughly mixed for 1 min at a rotational speed of 3000 rpm (revolutions per minute), as shown in 187 Figure 5(a). Then, the foamability (i.e., FER) and foam stability (i.e., T1/2) of the generated foam were 187 Figure 5(a). Then, the foamability (i.e., FER) and foam stability (i.e., T1/2) of the generated foam were 188 tested, as shown in Figure5(b). Finally, the foam agent with Cf = 4%, FER = 7 and T1/2 = 30 min under 188 tested, as shown in Figure5(b). Finally, the foam agent with Cf = 4%, FER = 7 and T1/2 = 30 min under 189 Appl.the roomSci. 2020 temperature, 10, 3300 of 25 ℃ was used in this study. The generated foam was used for7 ofthe 16 189 the room temperature of 25 ℃ was used in this study. The generated foam was used for the 190 observation of the microstructure and subsequent evaluation tests of the conditioned soil. The 190 observation of the microstructure and subsequent evaluation tests of the conditioned soil. The 191 generated foam was added after thoroughly mixing of tested soils and water (see Figure 6(a)). The 191 20%,generated and 30%foam for was various addedsoil after samples. thoroughly After mixing that, of the tested injected soil foams and andwater soil (see were Figure stirred 6(a)) into. The a 192 foam injection ratios (FIR) were set as 0%, 10%, 20%, and 30% for various soil samples. After that, the 192 homogeneousfoam injection mixtureratios (FIR) (see were Figure set6 b).as 0%, 10%, 20%, and 30% for various soil samples. After that, the 193 injected foam and soil were stirred into a homogeneous mixture (see Figure 6(b)). 193 injected foam and soil were stirred into a homogeneous mixture (see Figure 6(b)).

194 194 195 Figure 5. Warning-Blender method: (a) Waring-Blender tester, (b) generated foam. 195 Figure 5. WarningWarning-Blender-Blender method method:: (a) (a) Waring Waring-Blender-Blender tester,tester, ((b)b) generatedgenerated foam.foam.

196 196 197 Figure 6. Addition of foam agent :(a) Injection of foam before mixing, (b) homogenous mixtures after 197 Figure 6. Addition of foam agent :(a) Injection of foam before mixing, (b) homogenous mixtures after 198 Figuremixing. 6. Addition of foam agent: (a) Injection of foam before mixing, (b) homogenous mixtures 198 mixing. after mixing. 199 4. Results and Discussions 199 4. Results and D Discussionsiscussions 200 4.1. Analysis of Decay Behavior of Foam Microstructure 200 4.1. Analysis of Decay Behavior of F Foamoam M Microstructureicrostructure 201 Foam is a gas-liquid two-phase conditioner. The picture of foam microstructure before 201 Foam is is a gas-liquida gas-liquid two-phase two-phase conditioner. conditioner The. picture The picture of foam of microstructure foam microstructure before treatment before 202 treatment under 40-times magnification is clearly shown in Figure 7, and the pictures of 202 undertreatment 40-times under magnification 40-times magnification is clearly shown is in clearly Figure shown7, and the in picturesFigure 7 of, microstructureand the pictures under of 203 microstructure under different decay times are shown in Figure 8. The inside nucleus was filled with 203 dimicrostructurefferent decay under times aredifferent shown decay in Figure time8s. are The shown inside in nucleus Figure was8. The filled inside with nucleus air and was the filled surface with is 204 air and the surface is covered with liquid film. After 0.5 min of decay, the generated bubbles were 204 coveredair and the with surface liquid is film. covered After with 0.5 minliquid of decay,film. After the generated 0.5 min of bubbles decay, the were generated separated bubbles by the thickwere 205 separated by the thick liquid film full of free water and microbubbles. The shape of bubble particle 205 liquidseparated film by full the of thick free water liquid and film microbubbles. full of free water The shapeand microbubbles. of bubble particle The wasshape approximately of bubble particle circle 206 was approximately circle or oval regardless of bubble size, as shown in Figure 8(a). After 5 min of 206 orwas oval approximately regardless of circle bubble or size,oval as regardless shown in of Figure bubble8a. size, After as 5 minshown of decay,in Figure the relatively8(a). After large 5 min size of bubbles became bigger, while the relatively small bubbles become smaller gradually until vanishment. The shape of bubbles started to be polygon, especially for large-size bubbles, as shown in Figure8b. After 10 min of decay, the bubble size further became larger, and the bubbles were firmly attached to each other because of thinner liquid film induced by the loss of free water. The number of bubble particles decreased further on account of the vanishment of microbubbles and small bubbles because Appl. Sci. 2020, 10, x FOR PEER REVIEW 8 of 17 Appl. Sci. 2020, 10, x FOR PEER REVIEW 8 of 17 207 decay, the relatively large size bubbles became bigger, while the relatively small bubbles become 207 decay, the relatively large size bubbles became bigger, while the relatively small bubbles become 208 smaller gradually until vanishment. The shape of bubbles started to be polygon, especially for 208 smaller gradually until vanishment. The shape of bubbles started to be polygon, especially for 209 large-size bubbles, as shown in Figure 8(b). After 10min of decay, the bubble size further became 209 large-size bubbles, as shown in Figure 8(b). After 10min of decay, the bubble size further became 210 largerAppl. Sci., and2020 ,the10, 3300bubbles were firmly attached to each other because of thinner liquid film induced8 of by 16 210 larger, and the bubbles were firmly attached to each other because of thinner liquid film induced by 211 the loss of free water. The number of bubble particles decreased further on account of the 211 the loss of free water. The number of bubble particles decreased further on account of the 212 vanishment of microbubbles and small bubbles because of the merger. The shape of the remaining 212 vanishmentof the merger. of Themicro shapebubbles of the and remaining small bubbles bubbles because became of polygon, the merger while. The the shape pentagon of the and remaining hexagon 213 bubbles became polygon, while the pentagon and hexagon were the most common, as shown in 213 werebubbles the became most common, polygon as, while shown the in pentagon Figure8c. and After hexagon 15 min, were 20 min, the 25 most min, common and 30, min as shown of decay, in 214 Figure 8(c). After 15min, 20min, 25min, and 30min of decay, it can be seen that the bubble size 214 Figureit can be 8(c) seen. After that 15min the bubble, 20min, size 25min, further and became 30min larger of decay, and the it liquid can be film seen further that the became bubble thinner. size 215 further became larger and the liquid film further became thinner. The liquidity of the bubbles dried 215 furtherThe liquidity became of larger the bubbles and the dried liquid up graduallyfilm further and became the bubbles thinner. no The longer liquidity had obvious of the bubbles deformations, dried 216 up gradually and the bubbles no longer had obvious deformations, as shown in Figure 8(d–g). After 216 upas showngradually in Figure and the8d–g. bubbles After no 60 longer min of had decay, obvious the film deformation was adsorbeds, as shown and bent in Figure by the 8 micro(d–g). solidAfter 217 60 min of decay, the film was adsorbed and bent by the micro solid body and the bubble structure 217 60body min and of thedecay, bubble the structurefilm was adsorbed had all but and disappeared bent by the (see micro Figure solid8h). body and the bubble structure 218 had all but disappeared (see Figure 8(h)). 218 had all but disappeared (see Figure 8(h)).

219 219 220 Figure 7. Microstructure of liquid film before treatment. 220 Figure 7. MicrostructureMicrostructure of liquid film ﬁlm before treatment treatment..

221 221 222 Figure 8. Variation of foam microstructure under 40-times magnification after treatment (a) 0.5min; 222 Figure 8. Variation of foam microstructure under 40-times magnification after treatment (a) 0.5min; 223 (b)Figure 5min; 8. (c)Variation 10min; of(d) foam 15min; microstructure (e) 20min; (f) under25min; 40-times (g) 30min; magnification (h) 60min. after treatment (a) 0.5 min; 223 (b)(b) 5min; 5 min; (c) (c) 10min; 10 min; (d) (d )15min; 15 min; (e) (e 20min;) 20 min; (f) (25min;f) 25 min; (g) (30min;g) 30 min; (h) 60min (h) 60. min. 224 The above phenomena were caused by the drainage of water in liquid film and air fusion 224 The above phenomena were caused by the drainage of water in liquid film and air fusion 225 amongThe bubbles above phenomena[16]. The drainage were caused of water by the in drainagethe liquid of film water was in caused liquid film by gravity and air fusionand pressure among 225 among bubbles [16]. The drainage of water in the liquid film was caused by gravity and pressure 226 difference.bubbles [16 The]. The free drainage water flow of watered under in the the liquid influence film wasof gravity caused when by gravity the liquid and film pressure was full diff erence.of free 226 difference. The free water flowed under the influence of gravity when the liquid film was full of free 227 water.The free Meanwhile, water flowed a plateau under the boundary influence was of formed gravity at when the theintersection liquid film of was adjacent full of bubbles free water. (see 227 water. Meanwhile, a plateau boundary was formed at the intersection of adjacent bubbles (see 228 FigureMeanwhile, 9(a)). aT plateauhe movement boundary of free was water formed caused at the by intersection pressure difference of adjacent can bubbles be expressed (see Figure by 9 tha).e 228 Figure 9(a)). The movement of free water caused by pressure difference can be expressed by the 229 LaplaceThe movement equation of as free follows water: caused by pressure difference can be expressed by the Laplace equation 229 Laplaceas follows: equation as follows: σ P PB = (5) A − R

where PA, PB are the pressure at A point and B point, respectively; σ is surface tension of liquid ﬁlm; and R is radius of constant volume circle. Free water ﬂows from A point to B point when PA > PB, Appl. Sci. 2020, 10, 3300 9 of 16

as shown in Figure9a. The existence of microbubbles in the liquid ﬁlm hinders the movement of the free water and contributes to the stability of the bubble microstructure. When the angle of plateau boundary is 120◦, the pressure at A point and B point is the minimum, and the bubble is stable. Therefore, the shape of bubble in two-dimension plane is hexagon, which expresses the phenomena of

existenceAppl. Sci. of 20 hexagon20, 10, x FOR in PEER Figure REVIEW8c–e. 10 of 17

253 254 Figure 9. Diagram of foam microstructure: (a) Plateau junction; (b) surface tension on a Figure 9. Diagram of foam microstructure: (a) Plateau junction; (b) surface tension on a 255 three-dimensional surface. three-dimensional surface. 256 4.2. Results and Discussion of Mixing Test Figure9b shows the sketch of curved surface in three dimensional. The pressure di fference can be 257expressed Various as follows: amounts of water content (25 w.t.% and 40 w.t.%) and foam injection ratios (0 vol.%, 10 ! 258 vol.%, 20 vol.%, and 30 vol.%) were added to the tested1 soil1 to achieve different conditioned soils. 259 Figure 10 shows the pictures of tested soils∆P with= variousγ water content and FIR after 2 min of mixing. (6) R1 − R2 260 For the soil sample with w = 25% &FIR = 0%, the soil composed of loose soil clumps, which was 261where caused∆P is by pressure the gravel di particlefference, wrappedγ is coe byffi cientsilty clay of surface and fine tension,-medium and sand,R1 showand edR2 aare dry, curvature loose, and radius 262at orthogonalrigid state, plane.as shown in Figure 10(a). For w = 25% &FIR = 10%, w = 25% &FIR = 20%, and w = 25% 263 &IfFIR the = bubble30%, the is tested the standard soils show spherical,ed a sticky, then stiffR,1 and= waterR2 = shortageR, and state, then as the shown Equation in Figure (6) can be 264changed 10(b– to:d). The foam could not work in a water shortage soil because the water in liquid film of foam 265 was easily absorbed by the unsaturated soil, which2 γ was found in the foam microstructure test. ∆P = (7) 266 Therefore, it was necessary to improve the water contentR in the tested soils before the addition of 267 foam, and then the other group soil samples with w = 40% were adopted. For tested soil with w = 40% The pressure difference between large size bubble and the adjacent small size bubble can be 268 &FIR = 0%, the state was consistent and the flowability increased, as shown in Figure 10(e). For w = expressed as: 269 40% &FIR = 10%, w = 40% &FIR = 20%, and w = 40% &FIR = 30%, the! tested soils were turned into a 270 loose and fluffy state, where the soil volume left on the rod1 decrease1 d as FIR increased, as shown in ∆P = Ps Pb = 2γ (8) 271 Figure 10(f–h). This indicates that the flowability− increaseRds and− R viscosityb decreased because of the 272 lubrication and dispersion induced by foam air bubbles. where Ps and Pb are the pressure in small size bubbles and adjacent large size bubbles, respectively; 273 and Rs and Rb are the radius of small size bubbles and adjacent large size bubbles, respectively. According to Equation (8), the air in small-size bubbles diffuses into adjacent large-size bubbles, and then the size of small bubbles becomes smaller until vanishment while adjacent large size bubbles become larger. The large size bubbles become larger as the micro and small bubbles are continuously captured by the neighboring large size bubbles with higher adsorption energy. To make the foam stable and maximum its effect, the water should be sufficient to prevent from the decay of foam microstructure induced by drainage of water. Appl. Sci. 2020, 10, 3300 10 of 16

4.2. Results and Discussion of Mixing Test Various amounts of water content (25 w.t.% and 40 w.t.%) and foam injection ratios (0 vol.%, 10 vol.%, 20 vol.%, and 30 vol.%) were added to the tested soil to achieve different conditioned soils. Figure 10 shows the pictures of tested soils with various water content and FIR after 2 min of mixing. For the soil sample with w = 25% &FIR = 0%, the soil composed of loose soil clumps, which was caused by the gravel particle wrapped by silty clay and fine-medium sand, showed a dry, loose, and rigid state, as shown in Figure 10a. For w = 25% &FIR = 10%, w = 25% &FIR = 20%, and w = 25% &FIR = 30%, the tested soils showed a sticky, stiff, and water shortage state, as shown in Figure 10b–d. The foam could not work in a water shortage soil because the water in liquid film of foam was easily absorbed by the unsaturated soil, which was found in the foam microstructure test. Therefore, it was necessary to improve the water content in the tested soils before the addition of foam, and then the other group soil samples with w = 40% were adopted. For tested soil with w = 40% &FIR = 0%, the state was consistent and the flowability increased, as shown in Figure 10e. For w = 40% &FIR = 10%, w = 40% &FIR = 20%, and w = 40% &FIR = 30%, the tested soils were turned into a loose and fluffy state, where the soil volume left on the rod decreased as FIR increased, as shown in Figure 10f–h. This indicates that the flowability increased and viscosity decreased because of the lubrication and dispersion induced by foam air bubbles. Appl. Sci. 2020, 10, x FOR PEER REVIEW 11 of 17

274 275 Figure 10. Photographs of tested soils with various water content and foam injection ratio (FIR) after Figure 10. Photographs of tested soils with various water content and foam injection ratio (FIR) after 276 mixing: (a) w = 25%, FIR = 0%; (b) w = 25%, FIR = 10%; (c) w = 25%, FIR = 20%; (d) w = 25%, FIR = 30%; mixing: (a) w = 25%, FIR = 0%; (b) w = 25%, FIR = 10%; (c) w = 25%, FIR = 20%; (d) w = 25%, FIR = 30%; 277 (e) w = 40%, FIR = 0%; (f) w = 40%, FIR = 10%; (g) w = 40%, FIR = 20%; (h) w = 40%, FIR = 30%. (e) w = 40%, FIR = 0%; (f) w = 40%, FIR = 10%; (g) w = 40%, FIR = 20%; (h) w = 40%, FIR = 30%.

278 Each mixing mixing test test was was conducted conducted for 2 minfor 2 and min the andmixing the time mixing necessary time to necessary obtain a homogeneous to obtain a 279 mixturehomogeneous without mixture apparent without foam apparent was recorded foam bywas a recorded timer, as shownby a timer, in Figure as shown 11. The in Figure time used 11. The for 280 foamedtime used soils for with foamedw = soils25% with &FIR w= =10% 25% was &FIR less = than10% was the timeless than used the for foamedtime used soils forwith foamedw = soils25% 281 &withFIR w= =20%. 25% This&FIR might = 20%. have This been might caused have bybeen the caused smaller by amount the smaller and faster amount defoaming and faster in testeddefoaming soils 282 within testedw = 25%soils &withFIR =w 10%.= 25% For &FIR tested = 10%. soils For with tested w = 40%,soils thewith general w = 40%, mixing the general time was mixing less than time tested was 283 soilsless than with testedw = 25% soils because with w the = 25% soils because already the had soils a good already ﬂowability had a good at w =flowability40%. The at general w = 40%. mixing The 284 timegeneral for soilsmixing with timew = for40% soils decreased with w as =FIR 40%increased, decrease whichd as FIR might increas haveed been, which caused might by the have decrease been 285 ofcaused viscosity by the induced decrease by foamof viscosity dispersion induced and by increase foam ofdispersion ﬂowability and induced increase by of foam flowability lubrication. induced 286 by foam lubrication. 287 Meanwhile, the net power of the electric mixer motor with a resolution of 1 W was monitored 288 during the mixing test, as shown in Figure 12. The net power value and fluctuation value induced by 289 the soils with w = 25% were higher than tested soils with w = 40%, which might have been caused by 290 the poor liquidity and high viscosity. The average net power and standard deviation, as well as the 291 change rate of the net power for various tested soils, are further plotted in Figure 13. Figure 13(a) 292 indicates that the fluctuation of net power for tested soils with w = 25% did not change significantly 293 with increase of FIR, while the fluctuation of net power for tested soils with w = 40% decreased 294 drastically as the FIR increased from 0% to 10%. Figure 13(b) shows the change rate of net power 295 with FIR. For tested soils with w = 25%, the change rate of net power increased as FIR increased from 296 0% to 10% and then decreased with the further injection of foam. This might have been caused by the 297 increase of viscosity in the mixed soils because of the defoaming of foam at w = 25% &FIR = 10% 298 caused by insufficient water in soil. However, the net power of tested soils with w = 40% decreased 299 by 50% as FIR increased from 0% to 30%, which might have been caused by the decrease of viscosity 300 and increase of flowability in the mixed soils because of the dispersion and lubrication induced by 301 enough air bubbles. The decrease of net power value and fluctuation value in the mixing test means 302 that the cutter head torque, as well as electric energy, could be decreased during EPBS tunneling 303 after the reasonable application of soil conditioning in this mixed soil. Appl. Sci. 2020, 10, 3300 11 of 16 Appl. Sci. 2020, 10, x FOR PEER REVIEW 12 of 17

304

305 FigureFigure 11. 11.Mixing Mixing timetime forfor variousvarious foamed foamed soils. soils.

Meanwhile, the net power of the electric mixer motor with a resolution of 1 W was monitored during the mixing test, as shown in Figure 12. The net power value and fluctuation value induced by the soils with w = 25% were higher than tested soils with w = 40%, which might have been caused by the poor liquidity and high viscosity. The average net power and standard deviation, as well as the change rate of the net power for various tested soils, are further plotted in Figure 13. Figure 13a indicates that the fluctuation of net power for tested soils with w = 25% did not change significantly with increase of FIR, while the fluctuation of net power for tested soils with w = 40% decreased drastically as the FIR increased from 0% to 10%. Figure 13b shows the change rate of net power with FIR. For tested soils with w = 25%, the change rate of net power increased as FIR increased from 0% to 10% and then decreased with the further injection of foam. This might have been caused by the increase of viscosity in the mixed soils because of the defoaming of foam at w = 25% &FIR = 10% caused by insufficient water in soil. However, the net power of tested soils with w = 40% decreased by 50% as FIR increased from 0% to 30%, which might have been caused by the decrease of viscosity and increase of flowability in the mixed soils because of the dispersion and lubrication induced by enough air bubbles. The decrease of net power value and fluctuation value in the mixing test means that the cutter head torque, as well as electric energy, could be decreased during EPBS tunneling after the reasonable application of soil conditioning in this mixed soil.

4.3. Result and Discussion of Slump Test After the mechanical mixing test, a series of slump tests were carried out in triplicate on each soil sample to evaluate the workability and ﬂowability of the conditioned soils quantitatively. Figure 14 shows the selected pictures of the slump test, and the test results are summarized in Table3. For conditioned soils with w = 25% regardless of foam injection ratio, the soil sample was dry, rigid, and stiﬀ, such that they either stood straight with very small drop value (see Figure 14a–c) or collapsed (see Figure 14d) once the cones were lifted. For tested soils with w = 25% in Table3, the slump 306 values show that the foamed soil had a small increase in liquidity as FIR increased, although a collapse still occurred after the lifting of the cone. It was hard to obtain a desirable state when w = 25%. 307 Figure 12. The measured net power during the mixing tests in various tested soils with w = 25% and Proper slump values of 100–200 mm were obtained when the water content was improved to 40% 308 40% and: (a) FIR = 0%; (b) FIR = 10%; (c) FIR = 20%; (d) FIR = 30%. regardless of foam injection ratio. The mean slump values of w = 40% &FIR = 0%, w = 40% &FIR = 10%, 309 w = 40% &FIR = 20%, and w = 40% &FIR = 30% were 154.3 mm, 174.7 mm, 179.0 mm, and 182.3 mm, respectively, as shown in Table3. The slump value increased as the foam injection ratio increased. Appl. Sci. 2020, 10, x FOR PEER REVIEW 12 of 17

Appl. Sci. 2020, 10, 3300 12 of 16

Figure 14e–h shows that the foamed soils with w = 40% had an increased liquidity and no collapse occurred. It indicates that the flowability of tested soils with w = 40% was improved dramatically after the improvement of water content, and the flowability increases with increase of FIR. Tested soils 304 with w = 40% regardless of FIR meet the requirement of EPBS tunneling according to the slump value. 305 It also indicates that if the waterFigure content11. Mixing is time enough, for various even foamed low FIR soils.can cause it to be too fluid-like [3].

306 307 FigureFigure 12. TheThe measured measured net net power power during during the mixing the mixing tests in tests various in various tested soils tested with soils w = with 25% wand= 25% and 308 Appl.40%40% Sci. and:and: 2020 (a) (, a10) FIR,FIR x FOR = =0%; PEER0%; (b)( REVIEWbFIR) FIR = 10%;= 10%; (c) FIR (c) =FIR 20%;= (d)20%; FIR ( d= )30%.FIR = 30%. 13 of 17

309

310

311 FigureFigure 13. 13.Variation Variation ofof netnet power during during mixing mixing test: test: (a) Net (a) power Net power value, (b) value, the change (b) the rate change of net rate of 312 power. net power. 313 4.3. Result and Discussion of Slump Test 314 After the mechanical mixing test, a series of slump tests were carried out in triplicate on each 315 soil sample to evaluate the workability and flowability of the conditioned soils quantitatively. Figure 316 14 shows the selected pictures of the slump test, and the test results are summarized in Table 3. For 317 conditioned soils with w = 25% regardless of foam injection ratio, the soil sample was dry, rigid, and 318 stiff, such that they either stood straight with very small drop value (see Figure 14(a–c)) or collapsed 319 (see Figure 14(d)) once the cones were lifted. For tested soils with w = 25% in Table 3, the slump 320 values show that the foamed soil had a small increase in liquidity as FIR increased, although a 321 collapse still occurred after the lifting of the cone. It was hard to obtain a desirable state when w = 322 25%. Proper slump values of 100–200 mm were obtained when the water content was improved to 323 40% regardless of foam injection ratio. The mean slump values of w = 40% &FIR = 0%, w = 40% &FIR 324 = 10%, w = 40% &FIR = 20%, and w = 40% &FIR = 30% were 154.3 mm, 174.7 mm, 179.0 mm, and 182.3 325 mm, respectively, as shown in Table 3. The slump value increased as the foam injection ratio 326 increased. Figure 14(e–h) shows that the foamed soils with w = 40% had an increased liquidity and 327 no collapse occurred. It indicates that the flowability of tested soils with w = 40% was improved 328 dramatically after the improvement of water content, and the flowability increases with increase of 329 FIR. Tested soils with w = 40% regardless of FIR meet the requirement of EPBS tunneling according 330 to the slump value. It also indicates that if the water content is enough, even low FIR can cause it to 331 be too fluid-like [3].

332 Table 3. Test results of slump test of conditioned soil.

w C FER FIR Slump (mm) No. f (%) (%) (-) (%) Test 1 Test 2 Test 3 Mean Sample 1 25 4 7 0 0 - - - Sample 2 25 4 7 10 - 4 2 - Sample 3 25 4 7 20 7 5 - - Sample 4 25 4 7 30 - - 7 - Sample 5 40 4 7 0 160 154 149 154.3 Sample 6 40 4 7 10 173 180 171 174.7 Sample 7 40 4 7 20 179 183 175 179.0 Sample 8 40 4 7 30 187 182 178 182.3 333 Appl. Sci. 2020, 10, 3300 13 of 16

Appl. Sci. 2020, 10, x FOR PEER REVIEW 14 of 17

334

335 Figure 14. Photographs of slump test with water content contentss of 25% and 40% 40%,, as well as foam injection 336 ratioratioss of 0%, 10%,10%, 20%,20%, andand 30%:30%: ((a)a) w w= = 25%,25%, FIRFIR= = 0%;0%; ((b)b) w = 25%25%,, FIR = 110%;0%; (c) (c) w = 25%25%,, FIR = 20%20%;; 337 (d)(d) w == 25%25%,, FIRFIR = =30%30%;; (e) ( ew) w= 40= %40%,, FIRFIR = 0%=;0%; (f) w (f =) w40%=,40%, FIR =FIR 10%=; (g)10%; w = (g 40) w%,= FIR40%, = 2FIR0%; =(h)20%; w = 338 40(h)%w, FIR= 40%, = 30%FIR. = 30%.

339 4.4. Results and Discussion ofTable Friction 3. Test Coefficient results of T slumpest test of conditioned soil.

340 w Cf FER FIR Slump (mm) The frictionNo. coefficient test device, developed by the China University of Geosciences in Beijing, 341 China, was adopted(%) to conduct (%) the friction (-) coefficient (%) test Test in 1 triplicate Test 2 on each Test 3tested Mean soil after the 342 mechanicalSample mixing 1 test. 25 The angle 4 of external 7 friction 0 and friction 0 coefficient - are - shown in - Figure 15. 343 For soilSample group 2 with 25w = 25%, 4 it can be 7 seen that 10 the friction - angle 4 and friction 2 coefficient - first 344 increaseSampled to an 3 apparent 25 maximum 4 as the 7 FIR increases 20 from 7 FIR = 0% 5 (without - foam) to - FIR = 10%. 345 As the SampleFIR continue 4 25d to increase 4 beyond 7 10% 30 until FIR - = 30%, the- friction 7 angle and - friction Sample 5 40 4 7 0 160 154 149 154.3 346 coefficient decreased. The trend is consistent with the trend of net motor power in Figure 13, which Sample 6 40 4 7 10 173 180 171 174.7 347 might haveSample been 7 caused 40 by the 4 increase 7 of viscosity 20 because 179 of dissolution 183 of 175 foam. For 179.0 the tested 348 soils withSample w = 840% regardless 40 4of FIR, the 7 friction 30 angle and 187 friction 182 coefficient 178 were 182.3less than the 349 tested soils with w = 25% and they decreased as FIR increased from 0% to 30%, as shown in Figure 350 14.4.5. This Results also and indicates Discussion that of the Friction increase Coeffi ofcient water Test dramatically changes the characteristics of tested 351 soils firstly, and then the foam reduces the viscosity and improve the flowability. The increase of 352 flowabilityThe friction and decrease coefficient of testviscosity device, are developed good for the by thedischarging China University of the mixed of Geosciences soil in soil in chamber Beijing, 353 throughChina, was screw adopted conveyor to conduct during EPBS the friction operation coeffi incient soft ground. test in triplicate on each tested soil after the 354 mechanical mixing test. The angle of external friction and friction coefficient are shown in Figure 15. For soil group with w = 25%, it can be seen that the friction angle and friction coefficient first increased to an apparent maximum as the FIR increases from FIR = 0% (without foam) to FIR = 10%. As the FIR continued to increase beyond 10% until FIR = 30%, the friction angle and friction coefficient decreased. The trend is consistent with the trend of net motor power in Figure 13, which might have been caused by the increase of viscosity because of dissolution of foam. For the tested soils with w = 40% regardless of FIR, the friction angle and friction coefficient were less than the tested soils with w = 25% and they Appl. Sci. 2020, 10, 3300 14 of 16

decreased as FIR increased from 0% to 30%, as shown in Figure 15. This also indicates that the increase of water dramatically changes the characteristics of tested soils firstly, and then the foam reduces the viscosity and improve the flowability. The increase of flowability and decrease of viscosity are good for the discharging of the mixed soil in soil chamber through screw conveyor during EPBS operation in soft ground. Appl. Sci. 2020, 10, x FOR PEER REVIEW 15 of 17

355

356 FigureFigure 15.15. Variation ofof externalexternal frictionfriction angleangle andand frictionfriction coecoefficient.ﬃcient.

357 5.5. Conclusions 358 ThisThis articlearticle presentspresents the the results results of of an an experimental experimental study, study, which which assessed assessed the the influence influence of foamof foam on 359 aon mixed a mixed soil consisting soil consisting of silty of clay, silty fine-medium clay, fine-medium sand, and sand, sandy and gravel sandy utilizing gravel several utilizing geotechnical several 360 methodsgeotechnical to provide methods a reference to provide for soila reference conditioning for soil on mixedconditioning soils often on encounteredmixed soils often in EPBS encountered tunneling. 361 Basedin EPBS on tunneling. the preliminary Based on tests the conducted preliminary in tests this conducted study, the mainin this findings study, the specific main findings to the soils specific and 362 conditionersto the soils and tested conditioners in this study tested can in be this summarized study can asbe follows:summarized as follows: 363 FoamFoam microstructuremicrostructure changeschanges withwith thethe decaydecay ofof thethe liquidliquid film.film. TheThe microstructuremicrostructure testtest carriedcarried 364 outout inin thisthis study study showed showed that that the the bubbles bubbles decayed decaye asd theas the time time went went by andby and the decaythe decay mechanism mechanism was 365 thewas drainage the drainage of water of water in the in liquid the liquid film causedfilm caus byed the by gravity the gravity and pressure and pressure difference, difference, and the and foam the 366 fusionfoam fusion among among various various size bubbled size bubbled due to due the pressureto the pressure difference. difference. 367 WaterWater playsplays an an important important role role in modifyingin modifying the characteristicsthe characteristics of the of mixed the mixed soil. Whensoil. When the water the 368 waswater at was the lowat the level low (w level= 25%), (w = the 25%), foam the failed foam to failed modify to themodify properties the properties of the mixed of the soil mixed because soil 369 ofbecause its failure of its behavior failure behavior of foam particleof foam inducedparticle induced by the tested by the soil tested with soil insu withfficient insufficient water content. water 370 However,content. However, when w = when40%, w the = 40%, flowability the flowability increased increased and viscosity and viscosity decreased decreased dramatically. dramatically. 371 FoamFoam playsplays anan importantimportant role role in in increasing increasing flowability flowability and and decreasing decreasing viscosity viscosity when when the the water water is 372 suis ffisufficientcient in thein the mixed mixed soil. soil. The The mixed mixed soil soil in this in this study study could could be modified be modified into ainto proper a proper state tostate meet to 373 themeet slump the slump requirement requirement of 100–200 of 100–200 mm by mixingmm by the mi foamxing withthe foam the water with content the water of 40%. content Foam of could 40%. 374 maximumFoam could its maximum effect when its the effect mixed when soil the with mixed suffi socientil with water sufficient content water (i.e., 40%).content (i.e., 40%). 375 WhenWhen thethe waterwater contentcontent waswas 40%,40%, thethe mixingmixing time,time, netnet powerpower valuevalue andand fluctuationfluctuation value, 376 slumpslump value,value, externalexternal frictionfriction angleangle andand frictionfriction coecoefficientfficient werewere betterbetter thanthan soilsoil samplessamples withwith waterwater 377 contentcontent ofof 25%.25%. TheThe netnet powerpower causedcaused byby mixedmixed soilsoil withwithw w= = 40%40% andand FIRFIR == 30% could decreasedecrease byby 378 approximatelyapproximately 50%. The The foam foam could could improve improve the the prop propertieserties of of mixed mixed soil soil further further when when the the water water is 379 issufficient. sufficient.

380 Author Contribution: Conceptualization, Y.W; methodology, Y.W; software, Y.W, J.L; validation, 381 J.L; formal analysis, Y.W and D.W; investigation, Y.W and D.W; resources, Y.W; writing—original 382 draft preparation, Y.W; supervision, D.W and Y.J; project administration, Y.W; funding acquisition, 383 Y.W. All authors have read and agreed to the published version of the manuscript.

384 Funding: This research was funded by the National Natural Science Foundation of China (NSFC) ， 385 Grant number41472278, and the National Key Research and Development Program of China, Grant 386 number2017YFC0805008. Appl. Sci. 2020, 10, 3300 15 of 16

Author Contributions: Conceptualization, Y.W.; methodology, Y.W.; software, Y.W., J.L.; validation, J.L.; formal analysis, Y.W. and D.W.; investigation, Y.W. and D.W.; resources, Y.W.; writing—original draft preparation, Y.W.; supervision, D.W. and Y.J.; project administration, Y.W.; funding acquisition, Y.W. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Natural Science Foundation of China (NSFC), Grant number41472278, and the National Key Research and Development Program of China, Grant number2017YFC0805008. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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