applied sciences

Review State-of-the-Art of Colloidal Silica-Based Soil Liquefaction Mitigation: An Emerging Technique for Ground Improvement

Mingzhi Zhao 1,2, Gang Liu 1,2,*, Chong Zhang 1,2, Wenbo Guo 1,2 and Qiang Luo 3 1 School of Civil Engineering, Architecture and Environment, Xihua University, Chengdu 610039, China; [email protected] (M.Z.); [email protected] (C.Z.); [email protected] (W.G.) 2 Institute of Geotechnical Engineering, Xihua University, Chengdu 610039, China 3 MOE Key Laboratory of High-speed Railway Engineering, Southwest Jiaotong University, Chengdu 610031, China; [email protected] * Correspondence: [email protected]



Received: 21 November 2019; Accepted: 29 November 2019; Published: 18 December 2019


Abstract: In the booming field of nanotechnology, colloidal silica (CS) has been introduced for ground improvement and liquefaction mitigation. It possesses a great ability to restrain pore pressure generation during seismic events by using an innovative stabilization technique, with the advantages of being a cost-effective, low disturbance, and environmentally friendly method. This paper firstly introduces molecular structures and some physical properties of CS, which are of great importance in the practical application of CS. Then, evidence that can justify the feasibility of CS transport in loose sand layers is demonstrated, summarizing the crucial factors that determine the rate of CS delivery. Thereafter, four chemical and physical methods that can examine the grouting quality are summed and appraised. Silica content and chloride ion concentration are two effective indicators recommended in this paper to judge CS converge. Finally, the evidence from the elemental tests, model tests, and field tests is reviewed in order to demonstrate CS’s ability to inhibit pore water pressure and lower liquefaction risk. Based on the conclusions drawn in previous literature, this paper refines the concept of CS concentration and curing time being the two dominant factors that determine the strengthening effect. The objective of this work is to review CS treatment methodologies and emphasize the critical factors that influence both CS delivery and the ground improving effect. Besides, it also aims to provide references for optimizing the approaches of CS transport and promoting its responsible use in mitigating liquefaction.

Keywords: liquefaction mitigation; colloidal silica; transport mechanism; grouting material

1. Introduction In the past few decades, large destructive earthquakes, such as the 1964 Niigata earthquake in Japan, the 1964 Alaskan earthquake, the 1985 Michoacán earthquake in Mexico, the 1995 Hyogoken-Nambu earthquake in Kobe, Japan, the 2008 Wenchuan earthquake in China, the 2010 Chile earthquake, and the 2011 Great East Japan earthquake, have resulted in various geological hazards in which liquefaction was particularly remarkable [1–6]. The 1976 Tangshan earthquake in China induced serious liquefaction in a vast area of 24,000 square meters. In the liquefaction area, the ground suffered a rapid and dramatic loss of soil strength, and the bearing capacity suddenly disappeared due to the motion of the earthquake, leading to the significant settlement or collapse of massive buildings. Liquefaction is a kind of natural hazard that usually occurs in loose, saturated sand during an earthquake. Under the effect of a vibrating load, sand particles deviate from their original location, separating from each other and becoming suspended in the pore water. Both dead seismic loads can only act on the pore water under

Appl. Sci. 2020, 10, 15; doi:10.3390/app10010015 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 15 2 of 31 this circumstance, and the lateral movement of intact soil blocks over shallow liquefied deposits is typically induced. Some sloping areas in mountainous regions and those along the waterfront are more susceptible to lateral movement compared with the flat terrain. Therefore, the infrastructure in these areas is presented with a great risk from lateral spreading due to liquefaction [7]. Liquefaction-induced lateral spreading and ground failure will bring great damage to the upper parts of structures, which is a costly natural phenomenon. For example, a major part of the billion-dollar damage in the 1964 Niigata earthquake can be attributed to the lateral movement triggered by liquefaction. Besides, liquefaction also caused billions of dollars of damage to port facilities during the 1995 Kobe event [8,9]. To increase liquefaction resistance and limit the potential deformations of ground soil, one of the most popular soil improvement techniques is densification. Dynamic compaction, vibro-compaction, and explosive compaction have been extensively adopted to densify soil to mitigate liquefaction risk. Pioneering works, conducted by Dappolonia et al. (1954), Menard and Broise (1975), Mayne et al. (1984), and Welsh (1986), have facilitated the development and implementation of these densification techniques [10–13]. However, these reinforcement measures can only be adopted in undeveloped sites, given the fact that the buildings and structures are sensitive to vibration and dynamic compaction. At constrained and developed sites, underpinning and grouting seems to be the most feasible technique [7]. With underpinning, foundation is extended in depth or breadth so that it can either rest on a more supportive soil stratum or distribute its load across a broader area. The stress states of the existing structural elements are significantly improved in order to mitigate liquefaction. Nevertheless, this method targets specific structures rather than the entire ground. In the case of grouting, permeation grout can be classified into two categories, namely, cement suspension and chemically based solutions [14]. Both grouting materials are of high viscosity, so that they are often adopted to form grout columns rather than a uniform distribution across the entire ground beneath the structure [7,15]. In addition, most typical chemical solutions, such as acrylate and epoxy, are toxic and likely to create pollution to the local underground water. Especially in the case of cement, where the mean particle size of Portland cement ranges from 10–30 µm, allowing it to permeate through voids with a size greater than 0.2–0.3 mm. As a consequence, only on the condition that the soil stratum is mainly composed of medium to coarse sand can Portland cement be adopted as a grouting material. This is even the case with ultra-fine cement, where the mean grain size decreases to 3–5 µm, which has been proposed to be used in pressure grouting in recent years. Even this cement is only appropriate for use in fine to medium sand. However, liquefaction is more likely to occur when the stratum is mainly composed of silty sand or fine sand [16,17], so there are some limitations when using these traditional materials to mitigate liquefaction risk. With the development of nanotechnology, nanomaterials have been proposed by many researchers to be introduced into ground treatment to improve mechanical properties and mitigate liquefaction risk [18–25]. The concepts and inspirations of nanotechnology originate from a famous talk hold by Richard Feynman entitled “There’s Plenty of Room at the Bottom” at Caltech, in Pasadena, California, in December of 1959 [26]. Since then, a new field, exploring and manipulating nanoscale microscopic particle has begun to develop at a breathtaking pace. At present, the economic benefit created by nanomaterials and relevant products annually increases by 20%. Meanwhile, the cost of nanoproduction decreases significantly with the promoting effect of booming market development [15]. Under these circumstances, nanomaterials, such as carbon nanotubes, nanoalumina, colloidal silica, nanobentonite, laponite, and so on, have an expansive application space in civil engineering, especially as kinds of additives to improve the properties of cement, concrete and soils [27–29]. Among these advanced nanomaterials, the cheapest and most widely used one in soil treatment is colloidal silica (CS) [30,31]. CS is a kind of powerful material that can be used for passive site stabilization, which is a relatively new technique that has been proposed by Gallagher (2002) as a nondisruptive treatment to mitigate liquefaction risk [32]. The passive site stabilization technique involves three critical procedures, namely, the injection of grouting materials at the upgradient edge of the site, the delivery of the stabilizing materials to the targeted location, and gelation into a rigid gel Appl. Sci. 2020, 10, 15 3 of 31 Appl. Sci. 2019, 9, x FOR PEER REVIEW 3 of 31

(Figuregrouting1). Thematerials most, critical with the point condition in passive of low site- stabilizationgradient terrain. is the Here, smooth it is delivery require ofd that the grouting the given materials,material in with use thehas conditiona low initial of low-gradientviscosity and excellent terrain. Here, permeability it is required characteristics, that the given which material it needs in to usesustain has a for low sufficient initial viscosity time such and that excellent the modifier permeability could be characteristics, transported to which the desired it needs location to sustain [32,33 for]. suSincefficient CS time has suchan extremely that the modifierlow initial could viscosity be transported of around to 2 the cP desired(where, location as a comparison, [32,33]. Since pure CS water has anhas extremely an initial lowviscosity initial of viscosity 1 cP), after of around mixing 2with cP (where, a salt solution, as a comparison, it can easily pure flow water through has an the initial voids viscosityof liquefiable of 1 cP), soil after and mixing permeate with across a salt solution, the soil stratum it can easily, along flow with through the underground the voids of liquefiable water flow soilinduced and permeate by the extraction across the wells soil stratum,[32]. Once along CS reaches with the the underground target location, water extraction flow induced is eliminated, by the extractionwaiting for wells the CS [32 grouting]. Once CS gel reaches to change the into target a firm location, solid. CS extraction has been is investigated eliminated, by waiting many scholars for the CSas grouting a kind of gel grouting to change material into a firmfor soil solid. improvement CS has been and investigated liquefactio byn manymitigation. scholars The as mechanical a kind of groutingbehavior material of the soil for stabilized soil improvement with CS has and been liquefaction evaluated mitigation. in conventional The mechanical geotechnical behavior tests, ofbench the - soilscale stabilized model test withs and CS field has been tests evaluated [17–19,32– in37 conventional]. It has been prove geotechnicaln that CS tests, can bench-scale decrease the model axial strain tests andand field restrain tests [ pore17–19 pr,32essure–37]. It development has been proven in the that soil CS can subjected decrease to the cyclic axial loading. strain and Therefore, restrain pore it is pressurepotential development material to inbe the adopted soil subjected in ground to cyclic treatment, loading. especially Therefore, for it strata is potential mainly material composed to be of adoptedliquefiable in ground soils. treatment, especially for strata mainly composed of liquefiable soils.

Stabilizer Injection Wells

Extraction Wells Building

ow ter fl by ndwa orted rou ansp G rs Tr ilize ow Stab ter fl ndwa Grou

Figure 1. Concept for passive site stabilization [7,32]. Figure 1. Concept for passive site stabilization [7,32]. The paper reviews the main experimental studies and scientific findings in the field of the applied The paper reviews the main experimental studies and scientific findings in the field of the research of colloidal silica for soil improvement and liquefaction mitigation. The paper is organized applied research of colloidal silica for soil improvement and liquefaction mitigation. The paper is as follows: The first section describes the physical, gelling, curing, and mechanical properties of CS. organized as follows: The first section describes the physical, gelling, curing, and mechanical The effect of ionic strength and pH on the gelation process are both discussed in detail. The paper properties of CS. The effect of ionic strength and pH on the gelation process are both discussed in then focuses on the mechanism of CS transport. Here, soil type, CS viscosity, and injection rate detail. The paper then focuses on the mechanism of CS transport. Here, soil type, CS viscosity, and are summarized as three main factors that determine grouting quality and CS coverage. Thereafter, injection rate are summarized as three main factors that determine grouting quality and CS coverage. evidence from laboratory tests and field tests is presented to demonstrate the potential ability of CS in Thereafter, evidence from laboratory tests and field tests is presented to demonstrate the potential inhibiting pore water generation and decreasing dynamic strain. Finally, in the last section, the paper ability of CS in inhibiting pore water generation and decreasing dynamic strain. Finally, in the last reviews further CS applications in other branches of civil engineering and sums some significant section, the paper reviews further CS applications in other branches of civil engineering and sums advantages of this innovative technique compared with traditional methods. some significant advantages of this innovative technique compared with traditional methods. 2. Characteristics of Colloidal Silica (CS) 2. Characteristics of Colloidal Silica (CS) 2.1. Physical Properties of CS 2.1. Physical Properties of CS Colloidal silica (CS) is an aqueous suspension of microscopic nanosilica, extracted from saturated solutionsColloidal of silicic silica acid. (CS) In general, is an aqueous the particle suspension size of nanosilica of microscopic is uniform, nanosilica ranging, extracted from 7–22 from nm. CSsaturated is nontoxic, solutions chemically of silicic and acid. biologically In general, inert, the and particle environmentally size of nanosilica friendly is unifor [38,39].m, When ranging diluted from to7– a22 5% nm. concentration CS is nontoxic, by weight, chemically the viscosity and biologically of CS solution inert, is and only environmentally 1.5–2 cP, and is quite friendly similar [38,39 to]. When diluted to a 5% concentration by weight, the viscosity of CS solution is only 1.5–2 cP, and is Appl. Sci. 2020, 10, 15 4 of 31 Appl. Sci. 2019, 9, x FOR PEER REVIEW 4 of 31 Appl. Sci. 2019, 9, x FOR PEER REVIEW 4 of 31 quite similar to pure water. With the addition of an electrolyte, a CS solution can be destabilized in quitepure water.similar With to pure the water. addition With of anthe electrolyte, addition of a CSan electrolyte, solution can a beCS destabilized solution can in be order destabilized to start thein order to start the gelation process. ordergelation to start process. the gelation process. CS nanoparticles are formed as a result of the interaction between H4SiO4 and other molecules. CSCS nanoparticles nanoparticles are are formed formed as as a a result of the interaction between H 4SiOSiO44 andand other other molecules. molecules. During the manufacturing process, alkaline solutions are added into the CS to keep the silica particles DuringDuring the the manufacturing manufacturing process, process, alkaline solutions are added into the CS CS to to keep keep the the silica silica particles particles ionized and repelled from each other, so that gelation can be inhibited, and the CS solution can stay ionizedionized and and repelled repelled from from each each other, other, so so that that gelati gelationon can can be be inhibited, inhibited, and and the the CS CS solution solution can can stay stay steady [7]. The surfaces of the CS particles are initially distributed by uncombined silanol (Si-O-H) steadysteady [7]. [7]. The The surfaces surfaces of of the the CS CS particles particles are are in initiallyitially distributed distributed by by uncombined uncombined silanol silanol (Si-O-H) (Si-O-H) groups. When exposed to an alkaline environment, the alkali materials react with the surfaces of groups.groups. When When exposed exposed to to an an alkalin alkalinee environment, environment, the the alkali alkali materi materialsals react react with with the the surfaces surfaces of of nanoparticles, leading to negative charges (O−) on the particles and resulting in repulsive forces nanoparticles,nanoparticles, leading leading to to negative negative charges charges (O (O−−) )on on the the particles particles and and resulting resulting in in repulsive repulsive forces forces (Figure 2a). As the amount of charge on particle surfaces is proportional to the amount of hydroxyl (Figure(Figure 22a).a). As the amount of charge on particleparticle surfacessurfaces is proportionalproportional to thethe amountamount ofof hydroxylhydroxyl ions, interparticle repulsive force weakens and particles start to interact with each other when the pH ions,ions, interparticle interparticle repulsive repulsive force force weakens weakens and and particle particless start start to interact to interact with with each each other other when when the pH the drops to a relatively low value. Under this circumstance, some of the silica particles still have negative dropspH drops to a relatively to a relatively low value. low value. Under Underthis circumstan this circumstance,ce, some of some the silica of the particles silica particlesstill have stillnegative have charges (O−), while the others become uncharged (–OH). Here, siloxane bonds (Si-O-Si) start to chargesnegative (O charges−), while (O −the), while others the become others become uncharged uncharged (–OH). (–OH). Here, Here,siloxane siloxane bonds bonds (Si-O-Si) (Si-O-Si) start start to develop (Figure 2b) and the gelation of the CS suspension is motivated (Figure 3). As the pH drops developto develop (Figure (Figure 2b)2b) and and the the gelation gelation of of the the CS CS susp suspensionension is is motivated motivated (Figure (Figure 33).). AsAs thethe pHpH dropsdrops continuously below 5, positive charges attach to the silica particles, and repulsive forces dominate continuouslycontinuously below 5,5, positivepositive charges charges attach attach to to the the silica silica particles, particles, and and repulsive repulsive forces forces dominate dominate once once again (Figure 2c). Therefore, the gelation time increases with a decreasing formation rate of onceagain again (Figure (Figure2c). Therefore, 2c). Therefore, the gelation the gelation time increases time increases with a decreasingwith a decreasing formation formation rate of siloxane rate of siloxane bonds. It has been investigated and determined that the minimum gelation time can be siloxanebonds. It bonds. has been It investigatedhas been investigated and determined and determined that the minimum that the gelation minimum time gelation can be obtained time can when be obtained when the pH ranges from 5 to 7 [40-42].

obtainedthe pH ranges when fromthe pH 5 to rang 7 [40es– from42]. 5 to 7 [40-42].

- OH - O OH O - - - O

O - Si O Si

Si O Si O Si Si O

O

O O O

Si Si O O

O O O O O

O Si

O - - Si O O

Si O O O Si

O - - Si Si

Si O O Si O OO OO O

OO Si Si OO O Si Si Si - Si Si - OH O O O- Si OH O- (a) - - (b) (a) O O (b)

+ + H2O H2O

H O+ H O+

Si 2 2 Si

O

O Si Si O

O O O

O + +

Si H O H O Si O

O 2 + 2 +

Si H2O H2O Si O

OO O OO O Si Si Si + + H2O H O Si (c) + 2 + (c) H2O H2O

Figure 2. Surface charge characteristics of silica particles on the condition of different pH values: (a) FigureFigure 2. 2. SurfaceSurface charge charge characteristics characteristics of ofsilica silica particles particles on onthe the condition condition of different of different pH pHvalues: values: (a) pH > 8, O− on the particle surface and repulsive forces demonstrated; (b) 5 ≤ pH ≤ 8, some particles pH(a) pH> 8,> O8,− on O− theon theparticle particle surface surface and and repulsive repulsive forces forces demonstrated; demonstrated; (b (b) )5 5≤ pHpH ≤ 8,8, some some particles particles − + still with O and others become uncharged; (c) pH > 8, H2O+ on the particle≤ surface≤ and repulsive stillstill with with O − andand others others become become uncharged; uncharged; ( (cc)) pH > 8, H 2OO+ onon the the particle particle surface surface and and repulsive repulsive forces indicated [7]. forcesforces indicated indicated [7]. [7].

Figure 3. FormationFormation of of siloxane siloxane bonds bonds and and a a schematic schematic of the colloidal silica (CS) particle [40, [40, 4343,,44 44].]. Figure 3. Formation of siloxane bonds and a schematic of the colloidal silica (CS) particle [40, 43, 44]. InIn addition to to pH, pH, the the critical critical factors factors that that could could influence influence gelation gelation process process also alsoinclude include the ionic the In addition to pH, the critical factors that could influence gelation process also include the ionic ionicstrength, strength, silica silica solids solids concentration, concentration, and and temper temperatureature [7,36,45,46]. [7,36,45,46 ].Since Since silica silica concentration concentration andand strength, silica solids concentration, and temperature [7,36,45,46]. Since silica concentration and temperaturetemperature areare relativelyrelatively constant,constant, gelationgelation timetime cancan bebe controlledcontrolled byby adjustingadjusting thethe ionicionic strengthstrength temperature are relatively constant, gelation time can be controlled by adjusting the ionic strength and pH value. The evolvement characteristics of viscosity in the gelation process for solutions with 5 and pH value. The evolvement characteristics of viscosity in the gelation process for solutions with 5 Appl. Sci. 2020, 10, 15 5 of 31 Appl. Sci. 2019, 9, x FOR PEER REVIEW 5 of 31 Appl. Sci. 2019, 9, x FOR PEER REVIEW 5 of 31 andwt % pH colloidal value. silica The evolvement have been measured characteristics and depicted of viscosity quantitatively in the gelation [7,40,45], process as shown for solutions in Figure with 4. wt % colloidal silica have been measured and depicted quantitatively [7,40,45], as shown in Figure 4. 5Itwt is obvious % colloidal that silica the shape have of been gelation measured curve and is si depictedmilar regardless quantitatively of gel time. [7,40, 45Moreover,], as shown silica in Figureparticles4. It is obvious that the shape of gelation curve is similar regardless of gel time. Moreover, silica particles Ithave is obvious an induction that the period shape in of the gelation gelation curve process, is similar during regardless which of viscosity gel time. stays Moreover, fairly silicalow and particles silica have an induction period in the gelation process, during which viscosity stays fairly low and silica havesuspension an induction has a flow period rate inclose the to gelation pure water. process, With during an appropriate which viscosity amount stays of time fairly elapsed, low and viscosity silica suspension has a flow rate close to pure water. With an appropriate amount of time elapsed, viscosity suspensionstarts to increase has a flowdramatically, rate close and to pure a firm water. gel is With finally an appropriate obtained [47,48]. amount In ofdetail, time gel elapsed, time increases viscosity starts to increase dramatically, and a firm gel is finally obtained [47,48]. In detail, gel time increases startswith pH to increasevalue on dramatically, the condition and of pH a firm > 7.5, gel while is finally it decreases obtained as [ 47normality,48]. In detail,(i.e., ionic gel timestrength) increases rises with pH value on the condition of pH > 7.5, while it decreases as normality (i.e., ionic strength) rises withfrompH 0.225 value N to on 0.3 the N. condition As a consequence, of pH > 7.5, the while time from it decreases mixing asto normalitythe point where (i.e., ionic viscosity strength) increases rises from 0.225 N to 0.3 N. As a consequence, the time from mixing to the point where viscosity increases fromhas a 0.225significantly N to 0.3 wide N. As scope, a consequence, ranging from the a time few fromminutes mixing to several to the pointmonths. where In other viscosity words, increases CS has has a significantly wide scope, ranging from a few minutes to several months. In other words, CS has hasa widely a significantly ranged controllable wide scope, gel ranging time, which from a is few favorable minutes for to the several transportation months. In of other grouting words, materials CS has a widely ranged controllable gel time, which is favorable for the transportation of grouting materials ato widely the desired ranged location. controllable gel time, which is favorable for the transportation of grouting materials toto the the desired desired location. location.

(a) (b) (a) (b) Figure 4. Viscosity increasing law of CS suspension in the gelation process at different (a) pH values FigureFigure 4. ViscosityViscosity increasing increasing law law of of CS CS suspension suspension in in the the gelation gelation process process at at different different (aa)) pH pH values values and (b) ionic normalities [40,45]. andand ( (b)) ionic ionic normalities normalities [40,45]. [40,45]. 2.2. Gelling and Curing Process for CS 2.2.2.2. Gelling Gelling and and Curing Curing Process Process for for CS CS Persoff (1999) divided the gelation process into 11 gel states to describe the course during which PersoffPersoff (1999)(1999) divided divided the the gelation gelation process process into into 11 gel 11 states gel states to describe to describe the course the courseduring duringwhich a CS solution evolves into a rigid gel [49], as shown in Figure 5. Gel state 1 is such a process, where awhich CS solution a CS solution evolves evolves into a rigid into agel rigid [49], gel as [shown49], as shownin Figure in 5. Figure Gel state5. Gel 1 stateis such 1 isa suchprocess, a process, where an original CS solution has a viscosity quite close to pure water and no detectable formed gel. During anwhere original an original CS solution CS solutionhas a viscosity has a viscosityquite close quite to pure close water to pure and no water detectable and no formed detectable gel. During formed this stage, CS acts like a Newtonian fluid. When the pH or salt concentration is adjusted, the gelation thisgel. stage, During CS this acts stage, like a CS Newtonian acts like a fluid. Newtonian When th fluid.e pH When or salt the concentration pH or salt concentration is adjusted, the is adjusted,gelation process is triggered, and CS enters into gel state 2. Here, viscosity increasing gradually, and the gel processthe gelation is triggered, process is and triggered, CS enters and into CS entersgel state into 2. gel Here, state viscosity 2. Here, viscosityincreasing increasing gradually, gradually, and the andgel becomes a non-Newtonian fluid. Since then, CS transforms from a flowing gel into a nonflowing gel, becomesthe gel becomes a non-Newtonian a non-Newtonian fluid. Since fluid. then, Since CS then, transf CSorms transforms from a from flowing a flowing gel into gel a into nonflowing a nonflowing gel, and finally into a rigid gel. When arriving at gel state 10, CS particles gel into chainlike structures and andgel, finally and finally into intoa rigid a rigid gel. When gel. When arriving arriving at gel at state gel state10, CS 10, particles CS particles gel into gel chainlike into chainlike structures structures and then into uniform three dimensional networks. As a result, a CS resonating, rigid gel can be obtained. thenand theninto uniform into uniform three dimensional three dimensional networks. networks. As a result, Asa a result,CS resonating, a CS resonating, rigid gel can rigid be gel obtained. can be Therefore, gel time can be defined as the time needed for a CS solution to evolve from gel state 1 to Therefore,obtained. Therefore,gel time can gel be time defined can be as definedthe time as needed the time for needed a CS solution for a CS to solution evolve from to evolve gel state from 1gel to gel state 10 [40,45]. gelstate state 1 to 10 gel [40,45]. state 10 [40,45].

Figure 5. Gel state classification and description [49]. Figure 5. Gel state classification and description [49]. Figure 5. Gel state classification and description [49]. Appl. Sci. 2020, 10, 15 6 of 31 Appl. Sci. 2019, 9, x FOR PEER REVIEW 6 of 31

SinceSince the the relationship relationship of of gel time withwith pHpH valuevalue isis not not as as clear clear as as that that with with ionic ionic strength, strength, the the salt saltconcentration concentration has has always always been been adjusted adjusted to to control control the the gel gel time time in in previous previous experimental experimental studies studies in in the literature [14,17,20,32,50]. Sodium chloride (NaCl) or calcium chloride (CaCl2) has mostly been the literature [14,17,20,32,50]. Sodium chloride (NaCl) or calcium chloride (CaCl2) has mostly been adoptedadopted as as the the electrolyte electrolyte solution solution of of choice choice to to adju adjustst salt salt concentration concentration in in order order to to start start the the gelation gelation process.process. The The addition addition of of salt salt will will shrink shrink the the double double layer layer of of CS CS particles, particles, which which will will increase increase the the probabilityprobability of interparticleinterparticle collisions collisions [45 [45].]. These Th collisionsese collisions will undoubtedlywill undoubtedly the motivate the motivate gel formation gel formationprocess and process facilitate and thefacilitate CS suspension the CS suspension to transform to transform into a rigid, into firm a rigid, gel. With firm NaClgel. With addition NaCl to additionadjust the to adjust salt concentration, the salt concentration, the CS gelling the CS process gelling was process monitored was monitored by Wong by et al.Wong (2018), et al. as (2018), shown asin shown Figure in6. Figure 6.

(a) (b) (c)

FigureFigure 6. 6. CSCS gelation gelation process: process: (a ()a before) before gelling; gelling; (b ()b close) close to to gelling; gelling; (c ()c after) after gelling gelling [50]. [50].

MollamahmutogluMollamahmutoglu and and Yilmaz Yilmaz (2010) (2010) performed performed a aseries series of of gel gel experiments experiments with with different different salt salt concentrationsconcentrations and and salt salt solution solution quantities, quantities, and and th thee gel gel time time curves curves for for three three magnitudes magnitudes of of salt salt concentrationsconcentrations were were plotted plotted against against the the addition addition quan quantitiestities of of the the salt salt solution. solution. It It was was concluded concluded that that gelgel time time decreases withwith anan increasing increasing salt salt concentration concentration and and an increasingan increasing ratio ofratio salt of solution salt solution volume volumeto CS volume to CS volume [14]. Wong [14]. proposed Wong proposed that a 1.7 that mol a/ L1.7 NaCl mol/L solution NaCl shouldsolution be should hand mixed be hand with mixed CS in witha 1:5 CS ratio in by a volume1:5 ratio to obtainby volume a 1 h gelto formationobtain a 1 time h gel [50 ].formation The required time salt [50]. concentrations The required to obtain salt concentrationsthe targeted gel to time obtain can the be determinedtargeted gel with time the can analytical be determined model with proposed the analytical by Pedrotti model et al. proposed (2017) [51 ]. by PedrottiUpon et rigid al. (2017) gel formation, [51]. the gelation process is accomplished, and the curing period begins. TheUpon curing rigid time gel is theformation, time from the wheregelation gel process formation is accomplished, has finished toand strength the curing testing period starts. begins. It has Thebeen curing verified time that is the curing time time from has where a significant gel formation effect on has the finished strength to properties strength testing of a CS-soil starts. mixture. It has beenWong verified et al. (2018)that curing investigated time has the a significant influence effe of curingct on the time strength and curing properties conditions of a CS-soil on the mixture. strength Wongproperties et al. of(2018) CS specimens. investigated Four the CSinfluence samples of werecuring placed time underand curing de-aired conditions water toon cure the forstrength 1, 2, 4, propertiesand 8 weeks of CS after specimens. gelation wasFour accomplished. CS samples were At theplaced same under time, de-aired another water CS sample to cure was for cured 1, 2, for4, and1 week 8 weeks at 90% after relative gelation humidity was accomplished. (RH). Oedometer At the same tests weretime, performedanother CS onsample the five was samples, cured for and 1 weekthe corresponding at 90% relative compression humidity (RH). curves Oedometer are shown tests in were Figure performed7. It can beon seenthe five in Figuresamples,7a thatand thethe correspondingsamples cured compression for 1 and 2 weeks curves had are ashown single in compression Figure 7. It can curve, be seen while in theFigure samples 7a that cured the samples for 4 and cured8 weeks for have1 and another 2 weeks single had a (but single diff compressionerent) compression curve, while curve. the The samples samples cured cured for for 4 4and and 8 8weeks weeks havetended another to have single a sti (butffer different) behavior compression at low vertical curve. stress. The Figure samples7b depictscured for that 4 and the sample8 weeks cured tended at to90% have relative a stiffer humidity behavior su atff lowered ve somertical waterstress. loss Figure during 7b depicts the curing that the period, sample leading cured to at a 90% lower relative initial humidityvoid ratio suffered compared some with water the specimens loss during immersed the curing in de-airedperiod, leading water. Moreover, to a lower the initial 90% void RH sample ratio comparedshowed a with stiffer the response specimens than immersed the immersed in de-aired samples. water. Moreover, the 90% RH sample showed a stiffer Persoresponseff et al.than (1999) the immersed measured samples. the strength properties of CS-sand mixtures for a year and found that there is a continuously increasing strength gain during that period [49]. However, Gallagher and Lin (2005) found that strength gain only occurs during curing periods approximately four times as long as the gel time [45]. In any event, although a rigid gel is formed during the gelation process, the mechanical properties continuously develop during the curing process [52,53]. Therefore, both the curing time and curing condition have a significant impact on the strength behavior of a CS sample. Appl. Sci. 2020, 10, 15 7 of 31 Appl. Sci. 2019, 9, x FOR PEER REVIEW 7 of 31

σ ′

(a) e Void ratio,

σ ′

(b)

FigureFigure 7. 7.One-dimensional One-dimensional compression compression curves curves for CS for samples: CS samples: (a) Those (a) cured Those with cured different with durations; different (bdurations;) those cured (b) those in diff curederent evaporationin different evaporation conditions [ 50conditions]. [50]. 2.3. Mechanical Properties of CS-Soil Mixtures Persoff et al. (1999) measured the strength properties of CS-sand mixtures for a year and found that Oncethere theis a gellingcontinuously and curing increasing process strength have been gain accomplished, during that period some experiments [49]. However, can Gallagher be performed and onLin CS-soil (2005) samples found that to assess strength their gain static only mechanical occurs during properties. curing Seyedi periods et al. approximately (2013) studied the four properties times as oflong a clay-lime as the gel mixture time [45]. by In performing any event, CBRalthough (California a rigid Bearinggel is formed Ratio) during tests and the foundgelation that process, strength the ofmechanical the mixture properties increased continuously by 2, 7.5, and develop 8 times during whenadding the curing 1%, process 3%, and [52,53]. 5% CS [Therefore,54]. Yonekora both and the Mikacuring (1993) time observedand curing a significant condition unconfinedhave a significant compressive impact strength on the strength gain for behavior CS-treated of sand a CS samplessample. compared to the original ones. They declared that the unconfined compressive strength reached 3352.3. kPaMechanical when 32 Properties wt % CS of was CS-Soil grouted Mixtures [18]. Moreover, the unconfined compressive strength of the treatedOnce sample the wasgelling increased and curing with the process increased have concentration been accomplished, of the CS some additive experiments [19,55], as showncan be inperformed Figure8. Theon CS-soil increase samples in CS concentrationto assess their willstatic aid mechanical in intensifying properties. the bonds Seyedi between et al. (2013) CS and studied soil the properties of a clay-lime mixture by performing CBR (California Bearing Ratio) tests and found Appl. Sci. 2019, 9, x FOR PEER REVIEW 8 of 31 that strength of the mixture increased by 2, 7.5, and 8 times when adding 1%, 3%, and 5% CS [54]. Yonekora and Mika (1993) observed a significant unconfined compressive strength gain for CS- treated sand samples compared to the original ones. They declared that the unconfined compressive strength reached 335 kPa when 32 wt % CS was grouted [18]. Moreover, the unconfined compressive strengthAppl. Sci.of 2020the, 10treated, 15 sample was increased with the increased concentration of the CS8 additive of 31 [19,55], as shown in Figure 8. The increase in CS concentration will aid in intensifying the bonds between CS and soil particles, thus developing a higher compressive strength. They further reported that particles,the logarithm thus developing of hydraulic a higher conductivity compressive of strength. the mixtures They further decreases reported almost that the linearly logarithm with of the hydraulic conductivity of the mixtures decreases almost linearly with the increase of CS concentration. increase of CS concentration. The relationship between CS concentration and unconfined The relationship between CS concentration and unconfined compressive strength, as well as hydraulic compressive strength, as well as hydraulic conductivity, has been recognized and summarized by conductivity, has been recognized and summarized by Ghasabkolaei et al. (2017) [56]. Noll (1992) Ghasabkolaeialso reported et al. the (2017) reduced [56]. hydraulic Noll (1992) conductivity also reported of loose the sand reduced when mixedhydrauli withc conductivity 5 wt % CS. It wasof loose sandinteresting when mixed to findwith that 5 wt the % hydraulic CS. It was conductivity interesting reduced to find that to 9 the10 hydraulic9 cm/s for conductivity the grouted sandreduced × − to 9 ×sample 10−9 cm/s [57]. for the grouted sand sample [57].

Figure 8. Relationship between unconfined compressive strength and CS concentration (data collected Figure 8. Relationship between unconfined compressive strength and CS concentration (data from Persoff et al.) [19]. collected from Persoff et al.) [19]. Since soil samples grouted with CS will improve compressive strength, CS-soil mixtures should alsoSince obtain soil samples a higher grouted shear strength with CS compared will improv with puree compressive soil samples. strength, Changizi CS-soil and Haddad mixtures (2015) should also addedobtain 0.5%,a higher 0.7% shear and 1% strength CS to soil compared to discover with the mechanical pure soil enhancingsamples. eChangiziffect. They and found Haddad that both (2015) addedthe 0.5%, friction 0.7% angle and and 1% cohesion CS to weresoil enhancedto discover for groutedthe mechanical soils [58]. enhancing Besides, the effect. relationship They between found that both thethe shear friction stress angle ratio and (the cohesion ratio of shear were stress enhanced to effective for verticalgrouted stress) soils and[58]. shear Besides, displacement the relationship was betweeninvestigated the shear for astress pure sandratio sample, (the ratio a 1-week-cured of shear CS-sandstress to sample, effective and avertical 4-week-cured stress) CS-sand and shear displacementsample, as was shown investigated in Figure9[ 50for]. Ina pure the conditions sand sample, where thea 1-week-cured effective vertical CS-sand stress equaled sample, 100 and kPa, a 4- 200 kPa, and 300 kPa, all of the grouted sand samples had a higher peak stress when compared with week-cured CS-sand sample, as shown in Figure 9 [50]. In the conditions where the effective vertical the untreated samples. Furthermore, the 4-week-cured CS-sand sample had a greater peak shear stress stress equaled 100 kPa, 200 kPa, and 300 kPa, all of the grouted sand samples had a higher peak stress than the 1-week-cured sample when the vertical stress was 200 kPa. At an effective vertical stress whenof compared 100 kPa and with 300 the kPa, untreated although thesamples. shear stressesFurthermore, of the grouted the 4-week-cured samples seem CS-sand to share sample a similar had a greaterpeak peak value shear regardless stress ofthan curing the time,1-week-cured the sample samp curedle for when 4 weeks the showed vertical an stress initial was stiff er200 response kPa. At an effectivethan vertical the sample stress cured of 100 for 1kPa week. and In300 summary, kPa, although CS grouting the shear can remarkably stresses of improvethe grouted the static samples seemmechanical to share a propertiessimilar peak of sand value samples. regardless In this of process, curing curingtime, the time sample is a main cured factor for that 4 weeks determines showed an initialthe strengthening stiffer response effect. than the sample cured for 1 week. In summary, CS grouting can remarkably improve the static mechanical properties of sand samples. In this process, curing time is a main factor that determines the strengthening effect. Appl. Sci. 2020, 10, 15 9 of 31 Appl. Sci. 2019, 9, x FOR PEER REVIEW 9 of 31

σ ′ = v 100kPa ′ v /

τσ

(a)

σ ′ = v 200kPa ′ v /

τσ

(b) σ ′ = v 300kPa ′ v /

τσ

(c)

FigureFigure 9. Horizontal 9. Horizontal displacement displacement versus versus thethe ratio of of shear shear stress stress to vertical to vertical stress stress for a forpure a sand pure sand samplesample and a and sand a sand sample sample grouted grouted with with CS CS under under effective effective vertical vertical stresses stresses of (a of) 100 (a) kPa, 100 ( kPa,b) 200 ( bkPa,) 200 kPa, and (c) 300 kPa [50]. and (c) 300 kPa [50].

3. Colloidal3. Colloidal Silica Silica Transport Transport Through Through Soil Soil Stratum Stratum 3.1. Feasibility of CS Transport Through Porous Media 3.1. Feasibility of CS Transport Through Porous Media The transport issue of CS is the most critical aspect, determining the feasibility of CS when Theadopted transport for ground issue of improvement CS is the most and critical liquefaction aspect, mitigation. determining Laboratory the feasibility and field of CS tests when have adopted for groundindicated improvement that colloids and can liquefactionpotentially mobilize mitigation. in aquifers Laboratory [45], which and provides field tests direct have evidence indicated for that colloidsCS can transport potentially in liquefiable mobilize soil layers. in aquifers Ryan et [45 al.], (1999) which investigated provides the direct permeability evidence characteristics for CS transport in liquefiable soil layers. Ryan et al. (1999) investigated the permeability characteristics of a mixture of bacteriophage PRD1 (62 nm diameter) and CS (107 nm diameter) in sewage-contaminated and uncontaminated areas of an iron oxide-coated sand layer. Although attachment of the mixture to the porous media appeared initially, release occurred by adjusting the pH, adding an anionic surfactant to change the surface charge, and adding a reductant to dissolve the iron oxide coating on the particles. Appl. Sci. 2020, 10, 15 10 of 31

The resultsAppl. Sci. demonstrate 2019, 9, x FOR PEER that REVIEW the physicochemical properties of the porous media could be10 of modified 31 to facilitate the permeability of colloids [59]. Therefore, the transport of colloids in porous media is of a mixture of bacteriophage PRD1 (62 nm diameter) and CS (107 nm diameter) in sewage- feasible with or without treatment of the soil stratum. contaminated and uncontaminated areas of an iron oxide-coated sand layer. Although attachment of Higgothe mixture et al. (1993)to the porous monitored media the appeared permeable initially, properties release ofoccurred CS in a by kind adjusting of glacial the sandpH, adding by conducting an a laboratoryanionic columnsurfactant test to andchange field the test. surface In the charge, laboratory and adding test, a almost reductant 100% to ofdissolve the silica the iron particles oxide could be transportedcoating on in the the particles. porous The media. results However, demonstrate only th 70–80%at the physicochemical CS particles mobilizedproperties of over the aporous distance of 1.6 m.media The di couldfferences be modified in these to two facilitate tests could the permeabili be due toty theof colloids heterogeneity [59]. Therefore, of the sand the transport layer in theof field comparedcolloids with in thatporous in media the column, is feasible but with both or testswithout provide treatment evidence of the soil that stratum. CS particles can be delivered in a liquefiableHiggo sand et al. layer (1993) to monitored the targeted the locationpermeable with properties sufficient of CS concentrations in a kind of glacial in order sand to stabilizeby conducting a laboratory column test and field test. In the laboratory test, almost 100% of the silica the soil [60]. Saiers et al. (1994) proposed that a one-dimensional advection-dispersion equation could particles could be transported in the porous media. However, only 70–80% CS particles mobilized be adoptedover a todistance model of CS 1.6 transportm. The diff througherences in nonreactive these two tests porous could media,be due to on the the heterogeneity condition thatof the a small amountsand of layer CS deposition in the field occurredcompared [with61]. that In engineeringin the column, practice, but both CStests will provide suff erevidence various that degrees CS of physicalparticles and chemicalcan be delivered deposition in a due liquefiable to the geochemical sand layer to properties the targeted of thelocation underground with sufficient water and porousconcentrations media. The in deposition order to stabilize process the cansoil [60]. significantly Saiers et al. delay (1994) the propos transported that a of one-dimensional CS [62]. However, Johnsonadvection-dispersion et al. (1996) performed equation a seriescould ofbe shortadopted column to model tests CS to transport study the through influence nonreactive of ionic porous strength on the transportationmedia, on the ofcondition CS. He pointedthat a small out amount that CS of deposition CS deposition took occurred place for [61]. chemically In engineering active practice, sand media CS will suffer various degrees of physical and chemical deposition due to the geochemical properties and was more obvious at a low ionic strength. Despite deposition occurring, full colloid concentration of the underground water and porous media. The deposition process can significantly delay the couldtransport be achieved of CS at [62]. the However, desired location Johnson et [63 al.]. (1996) performed a series of short column tests to study Athe box influence model of was ionic adopted strength by on Gallagher the transportation et al. (2007) of CS. toHe study pointed the out feasibility that CS deposition of CS transport took in liquefiableplace sand.for chemically The box active was dividedsand media into and three was compartments, more obvious namely,at a low aionic central strength. chamber Despite used for sand placementdeposition andoccurring, two water full colloid reservoirs, concentration located could at both be sides,achieved for at groundwater the desired location control [63]. [7]. The model was filled withA box Nevada model was sand, adopted with relative by Gallagher density et al. of (2007) 40%, toto astudy height the of feasibility 200 mm, of as CS shown transport in Figure in 10. Five injectionliquefiable wells sand. constructed The box was from divided 19-mm into PVCthree pipescompartments, were arranged namely, bya central pushing chamber them used into for the sand sand placement and two water reservoirs, located at both sides, for groundwater control [7]. The deposit. Two extraction wells were set at the other side of the box model. The CS solution was colored model was filled with Nevada sand, with relative density of 40%, to a height of 200 mm, as shown in with redFigure food 10. dye, Five so injection that the wells advancement constructed of groutfrom 19-mm could bePVC visually pipes were observed arranged on theby toppushing and sidesthem of the model.into At the sand same deposit. time, the Two chloride extraction concentration, wells were set which at the is aother good side indicator of the box of the model. CS concentration, The CS was measuredsolution was in order colored to with reflect red the food transport dye, so that of CS the grout. advancement After CS of transportgrout could was be visually completed, observed the model was curedon the for top 14 and days. sides Then, of the sixmodel. block At samplesthe same weretime, the excavated chloride andconcentration, carved intowhich smaller is a good samples to performindicator unconfined of the CS concentration, compression was tests measured with. Itin wasorder indicated to reflect the that transport the unconfined of CS grout. compressive After strengthCS rangedtransport from was 16completed, kPa to 61 the kPa, model which was is quitecured closefor 14 todays. the resultsThen, six reported block samples by Gallagher were and excavated and carved into smaller samples to perform unconfined compression tests with. It was Mitchell (2002) [32]. Here, visual observation, chloride concentration measurement, and the unconfined indicated that the unconfined compressive strength ranged from 16 kPa to 61 kPa, which is quite compressive strength clarify that fairly uniform CS-sand mixtures were developed, which further close to the results reported by Gallagher and Mitchell (2002) [32]. Here, visual observation, chloride demonstrateconcentration that CS measurement, could transported and the unconfined by a low-gradient compressive stabilizer strength delivery. clarify that These fairly findingsuniform CS- indicate that thesand targeted mixtures place were could developed, be filled which with CSfurther at su demonstratefficient concentrations that CS could to stabilizetransported the by soil, a low- even with physicalgradient and chemical stabilizer deposition,delivery. These which findings was indicate unavoidable that the during targeted the place transport could be process, filled with which CS at would typicallysufficient restrain concentrations the delivery to ofstabilize CS. the soil, even with physical and chemical deposition, which was unavoidable during the transport process, which would typically restrain the delivery of CS.

Figure 10. Photo of box model test [7]. Appl. Sci. 2019, 9, x FOR PEER REVIEW 11 of 31

Figure 10. Photo of box model test [7].

3.2. Transport Mechanisms of CS in Liquefiable Sand Appl. Sci. 2020, 10, 15 11 of 31 CS transport, which is a complicated physical process, is determined by several internal and external3.2. Transportfactors. Since Mechanisms CS has of already CS in Liquefiable been mixed Sand with an electrolyte prior to injection, the transport of CS is significantly influenced by the time-dependent gelation process and is a more complicated course thanCS transport,a simple colloid which istransport. a complicated Previous physical to gelation, process, isCS determined behaves like by severala Newtonian internal fluid. and CS external factors. Since CS has already been mixed with an electrolyte prior to injection, the transport of transport is quite similar to a simple colloid delivery. When gelation is motivated, CS becomes a non- CS is significantly influenced by the time-dependent gelation process and is a more complicated course Newtonian fluid. Viscosity increases continuously and gradually become the dominating factor that than a simple colloid transport. Previous to gelation, CS behaves like a Newtonian fluid. CS transport controlsis quite the similar transport to a simple of CS colloid [49,64,65]. delivery. On When the gelationother hand, is motivated, porous CS media becomes with a non-Newtonian higher hydraulic conductivityfluid. Viscosity presents increases larger continuouslyporosity, which and is gradually more convenient become the for dominating CS delivery factor than that that controls with lower porositythe transport (hydraulic of CS conductivity). [49,64,65]. On Therefore, the other hand, viscosity porous and media soil with type higher are the hydraulic fundamental conductivity internal factorpresents that could larger influence porosity, CS which transport. is more Moreover, convenient some for CS external delivery factors, than that such with as lowerthe injection porosity rate, could(hydraulic also have conductivity). a significant Therefore, effect on viscositythe transp andort soil of typeCS. The are theinfluencing fundamental mechanisms internal factor of soil that type, CS viscosity,could influence and injection CS transport. rate on Moreover, CS deliv someery are external reviewed factors, and such summarized as the injection as follows. rate, could also have a significant effect on the transport of CS. The influencing mechanisms of soil type, CS viscosity, 3.2.1 andSoil injectionType rate on CS delivery are reviewed and summarized as follows.

3.2.1.Gallagher Soil Type and Lin (2009) designed a series of column tests to investigate the factors that could influence CS transport. The column used was made of a PVC pipe, with an internal diameter of 10 cm and aGallagher total length and of Lin 90 (2009) cm. Nevada designed and a series Ottawa of column sand, testswith to hydraulic investigate conductivities the factors that of could 8.9 × 103 influence CS transport. The column used was made of a PVC pipe, with an internal diameter of 10 cm and 220 × 103 cm/s, respectively, were adopted as the porous media to fill in the column. The inlet and a total length of 90 cm. Nevada and Ottawa sand, with hydraulic conductivities of 8.9 103 and water chamber was connected to the bottom end cap to supply steady water flow, as shown× in Figure 220 103 cm/s, respectively, were adopted as the porous media to fill in the column. The inlet water × 11. CSchamber grout was(pH connected= 6.8, ionic to strength the bottom = 0.1 end N) cap was to supply delivered steady through water flow,the valve as shown located in Figure 5 cm 11 above. the bottomCS grout cap. (pH During= 6.8, ionic testing, strength water= 0.1 and N) wasCS were delivered transported through thethrough valve locatedthe entire 5 cm column above the to the outletbottom chamber. cap. DuringNine sampling testing, water ports and were CS wereset to transported obtain samples through in the order entire to column study tothe the influencing outlet factors.chamber. The results Nine samplingshow that ports the traveling were set to time obtain of the samples CS grout in order through to study Ottawa the influencing sand was factors.two orders of magnitudeThe results lower show than that through the traveling the Nevada time of thesand. CS It grout is worth through noting Ottawa that sandthe hydraulic was two ordersconductivity of of Ottawamagnitude sand lower is two than orders through of the magnitude Nevada sand. higher It is worth than notingNevada that sand, the hydraulic which conductivityindicates that of the hydraulicOttawa conductivity sand is two orders of the of porous magnitude media higher used than is significantly Nevada sand, whichcorrelated indicates with that the the delivery hydraulic rate of CS grout.conductivity of the porous media used is significantly correlated with the delivery rate of CS grout.

Figure 11. Schematic and photo of the column test [45]. Figure 11. Schematic and photo of the column test [45].

3.2.2 CS Viscosity Appl. Sci. 2020, 10, 15 12 of 31

Appl. Sci. 2019, 9, x FOR PEER REVIEW 12 of 31 3.2.2. CS Viscosity With the increase of viscosity of the used colloid, the hydraulic conductivity of the porous media With the increase of viscosity of the used colloid, the hydraulic conductivity of the porous media decreases synchronously. According to the Kozeny–Carman equation, the hydraulic conductivity of decreases synchronously. According to the Kozeny–Carman equation, the hydraulic conductivity of the the porous media is inversely proportional to the viscosity of the permeant, as expressed by the porous media is inversely proportional to the viscosity of the permeant, as expressed by the following following equation: equation: κγκγ k =k = (1) µμ (1) where k is the hydraulic conductivity, κ is the intrinsic permeability, γ is the permeant density and where k is the hydraulic conductivity, κ is the intrinsic permeability, γ is the permeant density and μ µ is the permeant viscosity [45]. With a double increase of permeant viscosity during the transport, is the permeant viscosity [45]. With a double increase of permeant viscosity during the transport, there is a double decrease in the hydraulic conductivity of the porous media, and there is a double there is a double decrease in the hydraulic conductivity of the porous media, and there is a double decrease of flow rate under the same hydraulic gradient when substituting Equation (1) into Darcy’s decrease of flow rate under the same hydraulic gradient when substituting Equation (1) into Darcy’s law. In summary, although CS can be transported though porous media (loose sand) with deposition law. In summary, although CS can be transported though porous media (loose sand) with deposition and release, the viscosity of the permeant must be controlled to be as low as possible to facilitate the and release, the viscosity of the permeant must be controlled to be as low as possible to facilitate the transportation of colloids. transportation of colloids. Several permeation tests with different pH and ionic strengths have been performed using the Several permeation tests with different pH and ionic strengths have been performed using the column shown in Figure 11 to study CS transport mechanisms [45]. In these tests, the column was column shown in Figure 11 to study CS transport mechanisms [45]. In these tests, the column was filled with Nevada sand by the pluviation method. It was indicated that the smoothest CS flow could filled with Nevada sand by the pluviation method. It was indicated that the smoothest CS flow could be obtained when the pH was 6.5 and the ionic strength was 0.1 N. When pH was lowered to 6.2 or be obtained when the pH was 6.5 and the ionic strength was 0.1 N. When pH was lowered to 6.2 or ionic strength was increased to 0.15 N, the CS gelled prematurely, and it was difficult to reach 100% ionic strength was increased to 0.15 N, the CS gelled prematurely, and it was difficult to reach 100% coverage of the entire column. It should be noted that pH and ionic strength influences the viscosity, coverage of the entire column. It should be noted that pH and ionic strength influences the viscosity, thus influencing the transport of CS. The flow rate of CS remarkably decreases as a result of the increase thus influencing the transport of CS. The flow rate of CS remarkably decreases as a result of the of viscosity in the source CS, which is quantitatively shown in Figure12. Therefore, viscosity is the increase of viscosity in the source CS, which is quantitatively shown in Figure 12. Therefore, viscosity dominating factor that controls the flow rate and scope of CS in the column. is the dominating factor that controls the flow rate and scope of CS in the column.

FigureFigure 12.12.Relationship Relationship betweenbetween flowflow raterate ofof CSCS andand viscosityviscosity [45[45].]. 3.2.3. Injection Rate 3.2.3 Injection Rate Conlee (2010) reported that CS has a tendency to sink downwards in a 9 m2 field injection when 2 the injectionConlee rate(2010) is less reported than 3700 that mLCS/ hasmin /awell tendency [66]. Hamderi to sink downwards and Gallagher in a (2013) 9 m field verified injection the sinking when behaviorthe injection of CS rate using is anumericalless than 3700 method. mL/min/well They believe [66]. thatHamderi the maximum and Gallagher transport (2013) distance verified for CSthe rangessinking from behavior 2.5 m of to CS 4 m using due toanumerical the higher method. density They of CS believe solutions that compared the maximum with pore transport water distance [67,68]. Tofor further CS ranges investigate from 2.5 the m sinking to 4 m behaviordue to the and higher horizontal density movement of CS solutions of CS grouting,compared they with developed pore water a 10[67,68]. m3 pilot-scale To further box investigate model [69 the], as sinking shown behavior in Figure and13. Thehorizontal loose sand movement layer was of placedCS grouting, in the they box 3 modeldeveloped by freely a 10 m pouring pilot-scale sand box onto model water. [69], Four as injection shown in wells Figure and 13. 4 The extraction loose sand wells layer were was set placed at the upstreamin the box and model downstream by freely sides,pouring respectively. sand onto Awater. constant Four head injection water wells flow and was 4 established, extraction wells lasting were for set at the upstream and downstream sides, respectively. A constant head water flow was established, lasting for 7 days, saturating the sand layers prior to grouting. In low injection rate tests, injection rates of 65 and 130 mL/min/well were adopted to transport CS into the sand layers. However, CS was Appl. Sci. 2020, 10, 15 13 of 31

7 days, saturating the sand layers prior to grouting. In low injection rate tests, injection rates of 65 and 130Appl. mL Sci./ min2019,/ well9, x FOR were PEER adopted REVIEW to transport CS into the sand layers. However, CS was mainly present13 of 31 at the bottom of the model upon excavation, indicating that the CS sinking behavior is significant. Whenmainly the present injection at the rate bottom increases of the above model 1900 upon mL/min excavation,/well, suffi indicatingcient horizontal that the pressure CS sinking forces behavior the CS groutingis significant. to horizontally When the move injection to the ratetargeted increases location. above Therefore, 1900 mL/ themin/well, transport of sufficient CS is more horizontal likely to bepressure successful forces with the aCS high grouting injection to horizontally rate. Since it move is diffi tocult the fortargeted CS to location. permeate Therefore, through porous the transport media whenof CS theis more viscosity likely increases to be successful up to 4 cPwith [45 a], high the gel injection time is rate. expected Since to it beis difficult appropriately for CS controlled to permeate to meetthrough the conditionsporous media where when viscosity the viscosity can increase increases beyond up 4 to cP 4 to cP prevent [45], the gravitational gel time is sinking expected once to thebe targetappropriately location controlled is reached. to meet the conditions where viscosity can increase beyond 4 cP to prevent gravitational sinking once the target location is reached.

Electrical Conductivity Constant Head Logging System Inlet Reservoir Ups tream side City Injec wate tion r wells

Inlet

Extr Constant Head actio n Outlet Reservoir wells

Aggregate layer O utlet

Ci Steel ty wa ter Container

Dow nstr eam side

Figure 13.13. Setup of the pilot-scalepilot-scale model test to investigateinvestigate the influenceinfluence ofof injectioninjection raterate (modified(modified after Hamderi andand Gallagher,Gallagher, 2015)2015) [[6969].]. 3.3. Advancement for CS Transport 3.3. Advancement for CS Transport As stated above, CS grouting is conventionally implemented by injecting a stabilizer through a verticalAs stated injection above, well CS and grouting consequent is conventionally horizontal transport, implemented where by theinjecting underground a stabilizer water through flow isa motivatedvertical injection by the extractionwell and consequent well (Figure horizonta1). The gell transport, time of CS where should the be controlledunderground to be water su ffi flowciently is long,motivated such by that the the extraction CS viscosity well stays(Figure below 1). The 4 cP gel during time of delivery. CS should A lowbe controlled viscosity statusto be sufficiently should be sustainedlong, such tothat facilitate the CS CSviscosity horizontal stays transport.below 4 cP However, during delivery. once CS A arrives low viscosity at the targeted status should location, be sustained to facilitate CS horizontal transport. However, once CS arrives at the targeted location, sinking and deposition will occur on the condition of low viscosity [66–70]. Therefore, gel time is sinking and deposition will occur on the condition of low viscosity [66–70]. Therefore, gel time is difficult to control in the conventional CS grouting method. difficult to control in the conventional CS grouting method. To settle this problem, Rasouli et al. (2016) reported a method where the delivering injection To settle this problem, Rasouli et al. (2016) reported a method where the delivering injection nozzle to the designated area uses a controlled curved drilling (CCD) machine [37], as shown in nozzle to the designated area uses a controlled curved drilling (CCD) machine [37], as shown in Figure 14. The new CCD machine used for CS grouting originates from the curved drilling machine Figure 14. The new CCD machine used for CS grouting originates from the curved drilling machine that has been adopted in the oil industry [71,72]. The critical technology of a CCD machine is the that has been adopted in the oil industry [71,72]. The critical technology of a CCD machine is the curved drilling rod. To make such a curvature, the drilling head and water jet should firstly advance curved drilling rod. To make such a curvature, the drilling head and water jet should firstly advance slantwise. At the location where the bore needs to be curved, the drilling head is inclined downwards slantwise. At the location where the bore needs to be curved, the drilling head is inclined downwards and water is injected from the nozzle under a high pressure to push the drilling rod and the earth. and water is injected from the nozzle under a high pressure to push the drilling rod and the earth. As As a consequence, an upward pressure is produced and applied on the inclined surface of drill head, a consequence, an upward pressure is produced and applied on the inclined surface of drill head, finally creating a curvature along the boring path, as shown in Figure 15. Here, a CCD machine has finally creating a curvature along the boring path, as shown in Figure 15. Here, a CCD machine has been adopted to transport CS to reduce the liquefaction potential of runways at Fukuoka International Airport in a seismic-retrofitting project. By using the CCD method, a CS grout with short gelation time is required. Post-treatment measurements show that the CCD method could be applied to inject CS to the loose sand layer to mitigate the liquefaction risk without disturbing the Appl. Sci. 2020, 10, 15 14 of 31

Appl.been Sci. adopted 2019, 9, x to FOR transport PEER REVIEW CS to reduce the liquefaction potential of runways at Fukuoka International14 of 31 Airport in a seismic-retrofitting project. By using the CCD method, a CS grout with short gelation time surfaceis required. of the Post-treatment runways. In measurementsaddition, the new show grouting that the CCDmethod method could could transport be applied CS to to the inject desired CS to Appl.locationthe looseSci. 2019 directly, sand, 9, x FOR layer avoiding PEER to mitigate REVIEW unnecessary the liquefaction attachment risk and without deposition disturbing during the the surface delivery of process. the runways.14 of 31 In addition, the new grouting method could transport CS to the desired location directly, avoiding surface of the runways. In addition, the new grouting method could transport CS to the desired unnecessary attachment and deposition during the delivery process. location directly, avoiding unnecessary attachment and deposition during the delivery process.

Figure 14. Controlled curved drilling (CCD) machine with the permeation grouting method (PGM) [37]. Figure 14. Controlled curved drilling (CCD) machine with the permeation grouting method (PGM) [37]. Figure 14. Controlled curved drilling (CCD) machine with the permeation grouting method (PGM) [37].

FigureFigure 15. 15. MechanismMechanism for for making making a a curvature curvature [37]. [37].

3.4.3.4. Evaluation Evaluation Method Method for for CS CS Grouting Grouting Quality Quality Figure 15. Mechanism for making a curvature [37]. AfterAfter the the injection injection and and transport transport of of CS CS in in the the li liquefiablequefiable layer, layer, the the stabilizer stabilizer should should reach reach the the desired location where improvement is needed. However, since the transport of CS is not always 3.4.desired Evaluation location Method where for improvement CS Grouting Quality is needed. However, since the transport of CS is not always controllable,controllable, it it is is necessary necessary to to conduct conduct a a post-operation post-operation investigation investigation to to check check the the grouting grouting quality. quality. After the injection and transport of CS in the liquefiable layer, the stabilizer should reach the SeveralSeveral investigation investigation methods methods have have been been reported reported in in previous previous literature literature to to evaluate evaluate the the grouting grouting desired location where improvement is needed. However, since the transport of CS is not always quality,quality, and these are detail detaileded in the sections below. controllable, it is necessary to conduct a post-operation investigation to check the grouting quality. Several3.4.13.4.1. Colored Colored investigation Tracer Tracer Chemical Chemicalmethods have been reported in previous literature to evaluate the grouting quality, and these are detailed in the sections below. ColoredColored tracer tracer chemicals chemicals can can be be grouted grouted and and transp transportedorted with with CS CS to to provide provide a a visual visual observation observation 3.4.1methodmethod Colored to to indicate Tracer Chemical the locationlocation wherewhere the the stabilizer stabilizer has has arrived arrived at. at. Generally, Generally, red red food food coloring coloring and and the themethyl methyl red red indicator indicator are selectedare selected as the as tracer the tracer materials materials here [7here,37, 40[7,37,40,45].,45]. Red food Red coloring food coloring is mostly is Colored tracer chemicals can be grouted and transported with CS to provide a visual observation mostlyadopted adopted in the columnin the column test and test centrifuge and centrifuge model test model to reflect test to the reflect grouting the grouting quality. The quality. emerging The method to indicate the location where the stabilizer has arrived at. Generally, red food coloring and emerginggrout in the grout model in the is verymodel light is very in color light when in color initially when compared initially compared with the source with the grout, source due grout, to dilution due the methyl red indicator are selected as the tracer materials here [7,37,40,45]. Red food coloring is tovia dilution pore water. via pore As the water. grouting As the process grouting continues, process new continues, CS grout new arrives CS grout at the arrives location, at andthe location, the grout mostly adopted in the column test and centrifuge model test to reflect the grouting quality. The andbecomes the grout darker. becomes At last, darker. the grout At islast, observed the grout to presentis observed the same to present color asthe the same source color grout, as the indicating source emerging grout in the model is very light in color when initially compared with the source grout, due grout,that the indicating observed that point the is observ filled withed point CS at is afilled full concentration. with CS at a full The concentration. methyl red indicator The methyl was usedred to dilution via pore water. As the grouting process continues, new CS grout arrives at the location, indicatorin the grouting was used project in the at grouting Fukuoka project International at Fukuoka Airport. International The working Airport. mechanism The working of the mechanism methyl red and the grout becomes darker. At last, the grout is observed to present the same color as the source ofindicator the methyl is that red itindicator could present is that asit could red in presen color ift as the red soil in was color treated if the withsoil was CS treated on the conditionwith CS on of grout, indicating that the observed point is filled with CS at a full concentration. The methyl red theacidic condition ground of water. acidic Theground CSgrout water. used The inCS this grout project usedhad in this a pH project of 3.5, had leading a pH of to 3.5, the leading soil becoming to the indicator was used in the grouting project at Fukuoka International Airport. The working mechanism soil becoming weakly acidic. Therefore, the methyl indicator will become red at the location where of the methyl red indicator is that it could present as red in color if the soil was treated with CS on CS treatment has occurred. Soon afterwards, samples can be extracted from the treated layer for the condition of acidic ground water. The CS grout used in this project had a pH of 3.5, leading to the visual observation, as shown in Figure 16. soil becoming weakly acidic. Therefore, the methyl indicator will become red at the location where CS treatment has occurred. Soon afterwards, samples can be extracted from the treated layer for visual observation, as shown in Figure 16. Appl. Sci. 2020, 10, 15 15 of 31 weakly acidic. Therefore, the methyl indicator will become red at the location where CS treatment has occurred. Soon afterwards, samples can be extracted from the treated layer for visual observation, asAppl. shown Sci. 2019 in,Figure 9, x FOR 16 PEER. REVIEW 15 of 31

FigureFigure 16. 16.Checking Checking groutgrout qualityquality usingusing the the methyl methyl red red indicator indicator [ 37[37].].

3.4.2.3.4.2 Silica Content SilicaSilica content content is is a a directdirect indicator indicator of of CS. CS. GallagherGallagher and and Lin Lin(2009) (2009)proposed proposed that that the the silica silica content content ofof thethe porepore fluidfluid can be be measured measured by by burning burning a agiven given volume volume of offluid fluid at 200 at 200 °C ◦forC for2–3 2–3days days [45]. [ 45The]. Theweight weight of the of material the material remaining remaining after afterburning burning is considered is considered as silica-containing as silica-containing material material and can and be canused be to used calculate to calculate the silica the content silica content in the pore in the solution. pore solution. However, However, some electrolyte some electrolyte solutions, solutions, such as such as NaCl or CaCl , may added in the solution to motivate the gelation process. Thus, the remaining NaCl or CaCl2, may2 added in the solution to motivate the gelation process. Thus, the remaining materialmaterial after after burning burning also also contains contains some some other othe chemicals.r chemicals. Although Although Gallagher Gallagher and Lin (2009)and Lin believed (2009) thatbelieved the weights that the of weights the dissolved of the dissolved solids in the solids solution in the could solution be neglectedcould be neglected when compared when compared with the weightwith the of weight the silica, of the the silica, burning the burning method method may overestimate may overestimate the silica the content silica content in the porein the fluid, pore fluid, thus overestimatingthus overestimating the grouting the grouting quality. quality. ToTo avoid avoid overestimation, overestimation, Rasouli Rasouli et et al. al. (2016)(2016) suggestedsuggested thatthat thethe silicasilica contentcontent cancan be be measured measured byby atomic atomic absorption absorption spectrometry spectrometry (ABS) (ABS) [ 37[37].]. InIn thisthis method,method, 100100 mL mL of of potassium potassium hydroxide hydroxide with with aa concentration concentration ofof 20% 20% is is added added for for every every 50 50 g g of of improved improved soil soil sample. sample.Then, Then,the themixture mixture is is heated heated inin an an oven oven at at 120 120◦ °CC for for 30 30 min. min. Afterwards,Afterwards, 2020mL mLof ofsupernatant supernatantliquid liquidis iscollected collectedto tomeasure measure the the silicasilica contentcontent viavia thethe ABSABS method. Here, Here, it it was was shown shown that that the the unconfined unconfined compression compression strength strength of ofthe the improved improved soil soil almost almost linearly linearly increased increased with with the the silica silica content content measured measured by bythe the ABS ABS method method for forthe the soils soils at the at thesame same sampling sampling site, site, indicating indicating that thatABS ABS is an is appropriate an appropriate approach approach to evaluate to evaluate silica silicacontent. content. 3.4.3.3.4.3 Chloride Ion Ion Concentration Concentration ChlorideChloride ion ion concentration concentration is muchis much easier easier to obtain to obtain than CS than concentration. CS concentration. An ion meter An equippedion meter withequipped chloride with probes chloride can beprobes used can to detectbe used the to chloride detect concentration.the chloride concentration. Meanwhile, theMeanwhile, coverage the of CScoverage grout canof CS be grout evaluated can be by evaluated measuring by chloridemeasuring ion chloride concentrations ion concentrations in the pore in fluid. the pore It has fluid. been It verifiedhas been that verified the normalized that the normalized CS concentration CS concentr is proportionalation is toproportional the normalized to the chloride normalized concentration, chloride asconcentration, shown in Figure as shown 17[ 45 in]. TheFigure normalized 17 [45]. The concentration normalized refersconcentration to the concentration refers to the calculatedconcentration by dividingcalculated the by sample dividing concentration the sample toconcentration the source solution to the source concentration. solution Thisconcentration. result indicates This result that − chlorideindicates concentration that chloride is concentration a precise indicator is a precise for CS indicator concentration. for CS Clconcentration.− concentration Cl hasconcentration been adopted has tobeen monitor adopted CS transportto monitor and CS coverage transport in and other coverage previous in worksother previous [7]. works [7].

3.4.4. Unconfined Compression Strength A rudimentary unconfined compression strength was adopted by many researchers to evaluate the grouting quality and degree of stabilization achieved [7,14,37,45]. It has been widely acknowledged that the unconfined compression strength is positively correlated with the cyclic strength for both liquefiable and improved soils. Since the unconfined compression test is easier to operate and handle, it has been adopted as an index indicator test to reflect the cyclic strength of the targeted soil specimen and further evaluate the coverage of CS grout. In addition, when larger particles exist in soil Appl. Sci. 2020, 10, 15 16 of 31 samples, the unconfined compression strength is difficult to obtain with an unconfined test. In this instance, an unconsolidated undrained (UU) triaxial test can be used to obtain the Mohr’s stress circle. Then, the shear strength can be deduced from the failure line and finally the unconfined compression Appl. Sci. 2019, 9, x FOR PEER REVIEW 16 of 31 strength can be calculated by multiplying the shear strength by 2 [37].

R2 = 0.99

Figure 17. Relationship between chloride ions and silica concentration [45]. Figure 17. Relationship between chloride ions and silica concentration [45]. In summary, four methods, including the colored tracer chemical method, the silica content method,3.4.4 Unconfined the chloride Compression concentration Strength method, and the unconfined compression strength method, were proposedA rudimentary in the previous unconfined literature compression to evaluate strength the coverage was adopted of CS by grout many and researchers to make judgements to evaluate concerningthe grouting grouting quality quality. and The degree colored of tracer stabilizatio chemicaln methodachieved is the[7,14,45,37] most simple. It andhas direct been method, widely byacknowledged which visual judgementthat the unconfined can be conducted compression by color strength variation is positively in the grouting correlated process. with However, the cyclic CSstrength concentration for both and liquefiable grouting and quality improved at a given soils. place Since cannot the unconfined be evaluated compression quantitatively test is with easier this to method.operate The and unconfined handle, it has compression been adopted strength as an method index in candicator be applied test to to reflect evaluate the thecyclic grouting strength quality of the properlytargeted and soil quantitatively, specimen and however, further the evaluate mechanical the test,coverage which of is CS more grout. complicated In addition, than thewhen physical larger indexparticles test, exist needs in to soil be performedsamples, the here. unconfined Silica content comp andression chloride strength concentration is difficult canto obtain be measured with an quantitativelyunconfined test. by a In simple this instance, physical an index unconsolidated test, and they undrained are perfect (UU) indicators triaxial of test CS coverage.can be used Therefore, to obtain thethe silica Mohr’s content stress method circle. and Then, chloride the shear concentration strength ca methodn be deduced have been from mostly the failure adopted line in and experiments finally the andunconfined practices compression to judge grouting strength quality. can be calculated by multiplying the shear strength by 2 [37]. 4. LiquefactionIn summary, Mitigation four methods, Using Colloidal including Silica the colo Groutred tracer chemical method, the silica content method, the chloride concentration method, and the unconfined compression strength method, were 4.1.proposed Dynamic in Properties the previous of CS-Soil literature Mixture to evaluate the coverage of CS grout and to make judgements concerning grouting quality. The colored tracer chemical method is the most simple and direct CS is an excellent additive for liquefiable loose sand to improve its liquefaction mitigation ability. method, by which visual judgement can be conducted by color variation in the grouting process. When CS is properly grouted, cementation occurs between sand particles due to the presence of CS, However, CS concentration and grouting quality at a given place cannot be evaluated quantitatively and thus, loose sand layers tend to have a stronger cyclic resistance, avoiding liquefaction during with this method. The unconfined compression strength method can be applied to evaluate the an earthquake event. Clough et al. (1989) investigated the influence of cementation on the dynamic grouting quality properly and quantitatively, however, the mechanical test, which is more properties of liquefiable sands. They believed that loose, cemented sand tend to have a similar behavior complicated than the physical index test, needs to be performed here. Silica content and chloride to dense, uncemented sands. Moreover, liquefaction resistance increases as the degree of cementation concentration can be measured quantitatively by a simple physical index test, and they are perfect increases [73]. Towhata and Kabashima (2001) performed a triaxial test on a 5 wt % CS-grouted sand indicators of CS coverage. Therefore, the silica content method and chloride concentration method sample and found that the non-deformability of a loose sand sample at 40% relative density is similar have been mostly adopted in experiments and practices to judge grouting quality. to that of the same kind of sand sample at 75% relative density after being mixed with 5% CS [74]. Rodriguez4. Liquefaction and Izarraras Mitigation studied Using the Colloidal reduction Silica of liquefaction Grout risk for loose sand treated with CS. They pointed out that grouting with 15 wt % CS would significantly improve the dynamic resistance against4.1. Dynamic liquefaction Properties [75]. of CS-Soil Mixture CS is an excellent additive for liquefiable loose sand to improve its liquefaction mitigation ability. When CS is properly grouted, cementation occurs between sand particles due to the presence of CS, and thus, loose sand layers tend to have a stronger cyclic resistance, avoiding liquefaction during an earthquake event. Clough et al. (1989) investigated the influence of cementation on the dynamic properties of liquefiable sands. They believed that loose, cemented sand tend to have a similar behavior to dense, uncemented sands. Moreover, liquefaction resistance increases as the Appl. Sci. 2019, 9, x FOR PEER REVIEW 17 of 31

degree of cementation increases [73]. Towhata and Kabashima (2001) performed a triaxial test on a 5 wt % CS-grouted sand sample and found that the non-deformability of a loose sand sample at 40% relative density is similar to that of the same kind of sand sample at 75% relative density after being mixed with 5% CS [74]. Rodriguez and Izarraras studied the reduction of liquefaction risk for loose Appl. Sci. 2020, 10, 15 17 of 31 sand treated with CS. They pointed out that grouting with 15 wt % CS would significantly improve the dynamic resistance against liquefaction [75]. ToTo evaluate evaluate the the liquefaction liquefaction resistance resistance quantitatively, quantitatively, Gallagher Gallagher and and Mitchell Mitchell (2002) (2002) performed performed triaxialtriaxial tests tests on on pure pure Monterey Monterey sand sand and and Monterey Monterey sand sand treated treated with with CS CS [ 32[32].]. The The axial axial strain strain during during thethe tests tests was was recorded recorded and and is is shown shown in in Figure Figure 18 18.. Although Although very very little little strain strain was was observed observed prior prior to to liquefactionliquefaction for for the the untreated untreated sample, sample, large large strains strains occurred occurred rapidly rapidly onceonce liquefactionliquefaction waswas motivated.motivated. TheThe sample, sample, tested tested at at a a cyclic cyclic stress stress ratio ratio CSR CSR of of 0.27, 0.27, could could only only su sufferffer 13 13 cycles cycles before before it it thoroughly thoroughly collapsed.collapsed. Furthermore, 5% 5% double double amplitude amplitude axial axial strain strain was was produced produced in in12 12cycles. cycles. However, However, the thesample sample treated treated with with 10 10 wt wt % % CS CS experienced experienced much much less less strain duringduring cycliccyclic loading.loading. TheThe treatedtreated samplesample requiredrequired 276 276 cycles cycles to to reach reach 5% 5% strain. strain. SinceSince aa magnitude magnitude 7.57.5 earthquake earthquake was was deduced deduced to to generategenerate 15 15 uniform uniform stress stress cycles cycles [76], [76], the 10%the CS-treated10% CS-treated sample sample could be could expected be expected to have a suto ffihavecient a abilitysufficient to resist ability liquefaction to resist liquefaction in such an earthquakein such an earthquake event. event.

(a) Untreated sample

(b) 10% CS-treated sample

FigureFigure 18. 18.Axial Axial strain strain during during cyclic cyclic loading loading for for both both the the untreated untreated and and treated treated samples samples [ 32[32].].

ApartApart from from the the triaxial triaxial test, test, other other laboratory laboratory element element tests, tests, such such as as the the cyclic cyclic torsional torsional shear shear test,test, resonantresonant column column test, test, and and cyclic cyclic simple simple shear shear test, test, were were also also performed performed to to investigate investigate the the dynamic dynamic propertiesproperties of of the the CS-soil CS-soil mixture. mixture. Ann Ann undrained undrained hollow hollow cylinder cylinder torsional torsional shear shear test test was was performed performed byby Kodaka Kodaka et et al. al. (2005)(2005) onon bothboth CS-treatedCS-treated and and untreated untreated soil soil samples samples [ 77[77].]. ForFor thethe untreateduntreated sample,sample, the cyclic stress ratio was set to be 0.23, and large strains occurred rapidly with a few additional cycles after arriving at phase transformation line. However, the CS-treated sample had a cyclic stress ratio of 0.60 and presented much less shear strain during the cyclic loading process (Figure 19). In fact, collapse and liquefaction did not occur in the whole cyclic loading process for the treated sample. Appl. Sci. 2019, 9, x FOR PEER REVIEW 18 of 31 the cyclic stress ratio was set to be 0.23, and large strains occurred rapidly with a few additional cycles Appl.after Sci. arriving2020, 10 ,at 15 phase transformation line. However, the CS-treated sample had a cyclic stress18 ratio of 31 of 0.60 and presented much less shear strain during the cyclic loading process (Figure 19). In fact, collapse and liquefaction did not occur in the whole cyclic loading process for the treated sample. SpencerSpencer etet al. (2008) conducted resonant columncolumn teststests onon pure Nevada sandsand and Nevada sand treated withwith twotwo didifferentfferent concentrationsconcentrations ofof CSCS [[41].41]. TheyThey foundfound thatthat thethe CS-sandCS-sand mixturesmixtures hadhad aa slightlyslightly higherhigher shearshear modulusmodulus thanthan thethe untreateduntreated sample.sample. Besides, anan increaseincrease ofof 66 MPaMPa in the shear modulus waswas observedobserved forfor thethe samplesample treated with a 5% concentration of CS after a curing period of 28 days. However,However, thethe groutinggrouting ofof CSCS hadhad littlelittle eeffectffect onon thethe damping damping ratio ratio for for Nevada Nevada sand. sand.

= Dr 40% σ ′ = m0 50kPa τσ′ = /m0 0.600

= Dr 40% σ ′ = m0 50kPa τσ′ = /m0 0.230

(a) Untreated sample (b) Treated sample

FigureFigure 19. ShearShear strain strain of of the the untreated untreated and and treated treated soil soil samples samples in an in undrained an undrained cyclic cyclic torsional torsional shear sheartest [77]. test [77].

InIn addition,addition, DDíaz-Rodríguezíaz-Rodríguez etet al.al. (2008) investigatedinvestigated the dynamicdynamic behaviorsbehaviors ofof sandsand samplessamples groutedgrouted withwith CS in moremore detaildetail through a simple shear test [[17].17]. The required cycles to reach a 5% doubledouble amplitudeamplitude (DA)(DA) shear strain were recorded forfor bothboth thethe untreateduntreated andand CS-treatedCS-treated samplessamples atat 40%40% andand 60% 60% relative relative density density (Figure (Figure 20 ).20). It is It remarkable is remarkable that that the CS-treated the CS-treated samples samples need moreneed cyclesmore tocycles reach to 5% reach DA 5% strain DA than strain the than untreated the untreated samples undersamples diff undererent edifferentffective vertical effective stresses, vertical which stresses, was thewhich case was for boththe case of the for samples both of withthe samples 40% and with 60% relative40% and densities. 60% relative Furthermore, densities. the Furthermore, required cycles the torequired reach liquefaction cycles to reach increased liquefaction significantly increased with significantly the increase with of CS the concentration. increase of CS Taking concentration. a 5% DA shearTaking strain a 5% as DA the shear label strain of onset as ofthe liquefaction, label of onset when of liquefaction, the CS concentration when the rangesCS concentration from 0% to ranges 14.5%, thefrom required 0% to 14.5%, cycles the to initiate required liquefaction cycles to initiate remarkably liquefaction increase remarkably from 2 to 17 increase at a cyclic from stress 2 to ratio 17 at of a 0.41,cyclic as stress shown ratio in Figureof 0.41, 21 as. shown in Figure 21. SeveralSeveral kindskinds ofof laboratory element tests have indicated that grouting CS can obviously improve thethe liquefactionliquefaction resistance of of loose loose sandy sandy samples. samples. CS CS gel gel forms forms in in the the gelation gelation process process and and tends tends to tobond bond sand sand particles particles together, together, which which improve improve the the strength strength of of the the soil soil structure and inhibits thethe generationgeneration ofof excessexcess porepore waterwater pressurepressure duringduring thethe cycliccyclic loadingloading process.process. In the CS-treated ground improvementimprovement technique,technique, CS CS concentration concentration is ais dominant a dominant factor factor that that influences influences the strengthening the strengthening effect. Theeffect. higher The higher the CS the concentration CS concentration is, the is, stronger the stronger the liquefaction the liquefaction resistance resistance of the of treated the treated layers layers will be.will In be. general, In general, the investigatedthe investigated CS concentrationCS concentration in liquefaction in liquefaction mitigation mitigation varies varies from from 5% 5% to 20%to 20% in practicalin practical scenarios. scenarios. Appl. Sci. 2020, 10, 15 19 of 31 Appl. Sci. 2019, 9, x FOR PEER REVIEW 19 of 31

σ ′ = v0 88.2kPa

σ ′ = v0 58.8kPa

σ ′ = v0 29.4kPa

(a) Sample at 40% relative density

σ ′ = v0 88.2kPa

σ ′ = v0 58.8kPa

σ ′ = v0 29.4kPa

(b) Sample at 60% relative density

FigureFigure 20. 20. ComparisonComparison of of cyclic cyclic resistance resistance for for untreated untreated and and CS-treated CS-treated sand sand [17]. [17]. Appl. Sci. 2020, 10, 15 20 of 31 Appl. Sci. 2019, 9, x FOR PEER REVIEW 20 of 31

σ ′ v0 =29.4kPa

FigureFigure 21.21. ShearShear strainsstrains forfor treatedtreated samplessamples atat variousvarious CSCS concentrationsconcentrations [[17].17]. 4.2. Centrifuge Model Behaviors of CS-Grouted Soil 4.2. Centrifuge Model Behaviors of CS-Grouted Soil Laboratory element experiments have indicated that CS-grouted soil has a sufficient ability to Laboratory element experiments have indicated that CS-grouted soil has a sufficient ability to restrain DA strain and inhibit pore pressure generation, and, as such, it can mitigate liquefaction risk. restrain DA strain and inhibit pore pressure generation, and, as such, it can mitigate liquefaction risk. However, the element test can only indicate the material properties of CS-soil mixtures. The structural However, the element test can only indicate the material properties of CS-soil mixtures. The behavior still needs to be investigated through model tests. structural behavior still needs to be investigated through model tests. Taboada (1995) reported that significantly lower strains (~0.5–1%) were observed for the treated Taboada (1995) reported that significantly lower strains (~0.5–1%) were observed for the treated centrifuge models compared with the strains (~3–6%) of untreated model in the same kind of test [78]. centrifuge models compared with the strains (~3–6%) of untreated model in the same kind of test Another centrifuge experiment was performed to verify the enhancing effect of CS grout to mitigate [78]. Another centrifuge experiment was performed to verify the enhancing effect of CS grout to liquefaction risk for loose sand [7]. A laminar box composed of a stack of aluminum rings was used to mitigate liquefaction risk for loose sand [7]. A laminar box composed of a stack of aluminum rings provide a flexible-wall container. Grout supply ports were set at the bottom of the container to provide was used to provide a flexible-wall container. Grout supply ports were set at the bottom of the CS under a vacuum. The sand drain was pluviated around the grouting ports to permit the grouting container to provide CS under a vacuum. The sand drain was pluviated around the grouting ports to material to permeate evenly into the sand, as shown in Figure 22. The CS was delivered through the permit the grouting material to permeate evenly into the sand, as shown in Figure 22. The CS was soil with a saturation period of 13 h. Then, a firm, resonating gel formed after 56 h of gelling and a delivered through the soil with a saturation period of 13 h. Then, a firm, resonating gel formed after 240 h curing time. The prepared model was motivated by two shaking events, with 20 cycles of a 2 Hz 56 h of gelling and a 240 h curing time. The prepared model was motivated by two shaking events, sinusoidal input for each. The uniform peak accelerations were 0.2 g and 0.25 g, respectively. Lateral with 20 cycles of a 2 Hz sinusoidal input for each. The uniform peak accelerations were 0.2 g and 0.25 displacement and vertical settlement were measured by the linear variable differential transformers g, respectively. Lateral displacement and vertical settlement were measured by the linear variable (LVDTs) mounted on the container rings at various depths. It was found that shear strains of 0.5% and differential transformers (LVDTs) mounted on the container rings at various depths. It was found 1% were observed during the first and second shake. Furthermore, only 30 mm settlement (0.3% strain) that shear strains of 0.5% and 1% were observed during the first and second shake. Furthermore, only and 10 mm settlement (0.1% strain) were measured during the first and second episodes of shaking, 30 mm settlement (0.3% strain) and 10 mm settlement (0.1% strain) were measured during the first respectively. The results demonstrate that the treated soil layer did not liquify during both shaking and second episodes of shaking, respectively. The results demonstrate that the treated soil layer did events, which provides evidence of the contribution of CS in mitigating liquefaction risk. not liquify during both shaking events, which provides evidence of the contribution of CS in mitigating liquefaction risk. Appl.Appl. Sci.Sci. 20202019,, 109, ,x 15 FOR PEER REVIEW 2121 of 3131

(a)

(b)

FigureFigure 22.22. Model set for grouting: (a) Schematic forfor CSCS groutinggrouting inin laminarlaminar box;box; ((bb)) photographphotograph ofof CSCS groutinggrouting [[7].7].

ConleeConlee etet al.al. (2012) (2012) performed performed a a well-designed centrifuge test to discuss the eeffectffect ofof CSCS concentrationconcentration on on the the magnitude magnitude of cyclicof cyclic shear shear strain stra forin CS-treated for CS-treated models models [40]. In the[40]. test, In thethe container test, the wascontainer mainly was constructed mainly constructed with a 4.8 with m-thick a 4.8 liquefiablem-thick liquefiable loose sand loose layer sand and layer two and slopes two slopes made upmade of loamup of thatloam sloped that sloped 3◦ towards 3° towards a 3 m-wide a 3 m-wide central cent channel.ral channel. In test In 1, onetest slope1, one was slope grouted was grouted with 9% with by weight9% by weight CS (labelled CS (labelled CS-9), whileCS-9), the while other the was other left wa untreated.s left untreated. In test 2,In the test two 2, the slopes two were slopes treated were withtreated 4% with and 4% 5% and CS, labelled5% CS, labelled CS-4 and CS-4 CS-5, and respectively. CS-5, respectively. Eight shaking Eight shaking events events were applied were applied to the model,to the model, with a centrifugalwith a centrifugal acceleration acceleration of 15 g. Immediatelyof 15 g. Immediately after test after 1, large test deformations 1, large deformations occurred atoccurred the untreated at the untreated side due toside the due liquefaction to the liquefaction of the loose of sandthe loose layer, sand and thelayer, upper and loamthe upper slope loam was submergedslope was submerged in water. However, in water. theHowever, treated the side treated remain side intact, remain as can intact, be seen as can in Figure be seen 23 in. Figure 23. Appl. Sci. 2020, 10, 15 22 of 31 Appl. Sci. 2019, 9, x FOR PEER REVIEW 22 of 31 Appl. Sci. 2019, 9, x FOR PEER REVIEW 22 of 31

FigureFigure 23. Model 23. Model surface surface for for test test 1 (1a )(a prior) prior to to the the shakingshaking events events and and (b ()b after) after the the shaking shaking events events [40]. [40]. Figure 23. Model surface for test 1 (a) prior to the shaking events and (b) after the shaking events [40]. In addition,In addition, they they also also calculated calculated the the shear shear strainsstrains at at different different depths depths by by double double integration integration accelerationaccelerationIn addition, in timein time they to to also obtainobtain calculated the the transient transient the shear displa displacement, stracement,ins at and different then and differentia depths then dibytingff erentiatingdouble in space. integration It in was space. It wasaccelerationobserved observed that in that thetime maximum the to obtain maximum shear the transientstrain shear γmax strain displa in theγcement, untreatedmax in and the side then untreated reached differentia up side to ting1% reached in in shake space. up 3, whileIt to was 1% in γmax was only 0.3% and 0.6% for CS-9 and CS-5, respectively. Similar reductions can be observed in shakeobserved 3, while thatγmax thewas maximum only 0.3% shear and strain 0.6% γmax for in the CS-9 untreated and CS-5, side respectively. reached up toSimilar 1% in shake reductions 3, while can shakes 4 and 5 (Figure 24). This experiment demonstrates that surface settlement and lateral be observedγmax was only in shakes 0.3% and 4 and 0.6% 5 for (Figure CS-9 and24). CS-5, This respectively. experiment Similar demonstrates reductions that can surface be observed settlement in spreading can be effectively controlled with CS treatment. CS grouting offers significant advantages andshakes lateral spreading4 and 5 (Figure can be 24). effectively This experiment controlled demonstrates with CS treatment. that surface CS grouting settlement off ersand significantlateral for liquefiable sand layers overlain by existing structures that may be otherwise be difficult to treat advantagesspreading for can liquefiable be effectively sand controlled layers overlain with CS by treatment. existing structures CS grouting that offers may significant be otherwise advantages be difficult by other methods. The results of the centrifuge model test are in agreement with those of the for liquefiable sand layers overlain by existing structures that may be otherwise be difficult to treat to treatlaboratory by other element methods. test Thein demonstrating results of the the centrifuge mitigation modeleffect of test CS-grouted are in agreement soil in terms with of reducing those of the by other methods. The results of the centrifuge model test are in agreement with those of the laboratoryliquefaction element risk. test in demonstrating the mitigation effect of CS-grouted soil in terms of reducing laboratory element test in demonstrating the mitigation effect of CS-grouted soil in terms of reducing liquefaction risk. liquefaction risk.

Figure 24. Cyclic stress ratio CSR against shear strain at the midpoint of liquefiable layer for the untreated, CS-9, and CS-5 samples [40]. Appl. Sci. 2019, 9, x FOR PEER REVIEW 23 of 31

Figure 24. Cyclic stress ratio CSR against shear strain at the midpoint of liquefiable layer for the untreated, CS-9, and CS-5 samples [40].

Appl. Sci. 2020, 10, 15 23 of 31 4.3. Field Tests for Soils Grouted with CS Since the performance of CS in soil improvement and liquefaction mitigation has been verified 4.3. Field Tests for Soils Grouted with CS by a series of element and model tests, it was also justified by a field test conducted by Gallagher et al. (2007)Since [33]. the The performance test site ofhere CS inwas soil within improvement a Canadian and liquefaction liquefaction mitigation experiment has been verifiedsite located by in Richmond,a series ofBritish element Columbia. and model The tests, soil itstratigraphic was also justified sequence by a field at the test test conducted site is characterized by Gallagher by et al.a sand (2007) [33]. The test site here was within a Canadian liquefaction experiment site located in Richmond, and silty sand layer at the surface, overlain over a layer of silt and sandy silt, following a 10 m British Columbia. The soil stratigraphic sequence at the test site is characterized by a sand and silty liquefiable layer of sand to silty sand, buried at a depth of 5–15 m [79]. At the treated area, eight sand layer at the surface, overlain over a layer of silt and sandy silt, following a 10 m liquefiable layer injectionof sand wells to silty were sand, spaced buried equally at a depth around of 5–15 the mperimeter [79]. At the of treateda 9 m-diameter area, eight circle, injection with wells an extraction were wellspaced at the equallycenter. aroundTwo injection the perimeter stages of were a 9 m-diameter used for the circle, injection with anwell extraction to permeate well at CS the into center. the soil layersTwo buried injection at depths stages were of 6.5–7.5 used for m the and injection 7.5–8.5 well m, torespectively. permeate CS An into injection the soil layers well buriedis designed at depths to inject moreof than 6.5–7.5 7000 m andL of 7.5–8.5 CS at a m, concentration respectively. An of injection7%. As a well matter is designed of fact, since to inject some more malfunctions than 7000 L of occurred CS duringat a the concentration injection process, of 7%. As only a matter 45,000 of L fact, of diluted since some CS malfunctionssuspensions occurredwere finally during injected the injection in the field test.process, After the only grouting 45,000 L of injection diluted CS was suspensions completed, were finallytwo decks injected of in thePentolite field test. 50/50 After explosives the grouting were installedinjection at each was completed,injection well two to decks create of Pentolite blast-indu 50/ced50 explosives liquefaction. were At installed the untreated at each injection area adjacent well to the totreated create one, blast-induced eight similar liquefaction. wells were At the excavate untreatedd areaas blast adjacent holes to to the accommodate treated one, eight explosives, similar as wells were excavated as blast holes to accommodate explosives, as shown in Figure 25. Six survey shown in Figure 25. Six survey lines extended from the center, and a total of 14 settlement lines extended from the center, and a total of 14 settlement measurements were taken along each ray. measurements were taken along each ray.

Figure 25. Site layout for full-scale test (modified after Gallagher et al., 2007) [33]. Figure 25. Site layout for full-scale test (modified after Gallagher et al., 2007) [33].

Excess pore pressure was monitored in the treated area. The excess pore pressure ratio Ru was u adoptedExcess pore to indicate pressure the was evolutive monitored law of in pore the watertreated pressure. area. The It excess was shown pore thatpressureRu reached ratio R 1, was adoptedeven into theindicate treated the area, evolutive indicating law the of grouted pore water layer can pressure. be considered It was to shown have liquified that Ru here.reached However, 1, even in the treatedthe settlement area, measurementsindicating the indicate grouted that layer a maximum can be settlementconsidered of to 0.5 have m was liquified recorded here. in the untreatedHowever, the settlementarea, while measurements the maximum indicate settlement that reduced a maximum to 0.3 m insettlement the treated of area 0.5 (Figure m was 26 recorded). Moreover, in Gallagherthe untreated area,et while al. (2007) the deducedmaximum that settlement the 0.3 m deformationreduced to mainly0.3 m in occurred the treated in the area untreated (Figure layer, 26). and Moreover, the Gallagherreduction et inal. settlement (2007) deduced should bethat attributed the 0.3 to m the deformation grouted layer mainly (at depths occurred of 6.5–8.5 in m). the They untreated believed layer, and thatthe thereduction majority in of settlement settlement would should be be prevented attribut ifed the to entire the grouted layer was layer grouted. (at depths of 6.5–8.5 m). They believed that the majority of settlement would be prevented if the entire layer was grouted. Appl. Sci. 2020, 10, 15 24 of 31 Appl. Sci. 2019, 9, x FOR PEER REVIEW 24 of 31

Figure 26. Elevation profile profile prior to and after blasting in full-scale test [[33].33]. Conlee (2010) then performed another field test to investigate the mitigation effect of CS grouting on Conlee (2010) then performed another field test to investigate the mitigation effect of CS grouting liquefaction damage [66]. In this field experiment, CS was grouted and transported to treat a 1.5 m-thick, on liquefaction damage [66]. In this field experiment, CS was grouted and transported to treat a 1.5 poorly graded sand layer, with an area of 9.3 m2. In the preliminary test layout, two kinds of grouting m-thick, poorly graded sand layer, with an area of 9.3 m2. In the preliminary test layout, two kinds techniques, including mandrel injection and the well/packer injection method, were planned to be of grouting techniques, including mandrel injection and the well/packer injection method, were adopted to permeate CS to the targeted area. Furthermore, a TRex mobile shaker was intended to planned to be adopted to permeate CS to the targeted area. Furthermore, a TRex mobile shaker was be used to induce liquefaction, and the targeted area was set hexagonally to match the shaker pad intended to be used to induce liquefaction, and the targeted area was set hexagonally to match the of the TRex shaker [66,80]. However, two dominant difficulties were presented in the field. First of shaker pad of the TRex shaker [66,80]. However, two dominant difficulties were presented in the all, the mandrel injection method proved to be unsuccessful in practice. Secondly, the measured pore field. First of all, the mandrel injection method proved to be unsuccessful in practice. Secondly, the pressure response indicated no significant increases in pore water pressure in vibro-seismic shaking measured pore pressure response indicated no significant increases in pore water pressure in vibro- when using TRex shaker. Thus, it is uncertain whether the TRex shaker will produce remarkable shear seismic shaking when using TRex shaker. Thus, it is uncertain whether the TRex shaker will produce strains and initiate liquefaction at greater depths. Therefore, the packer injection method was finally remarkable shear strains and initiate liquefaction at greater depths. Therefore, the packer injection adopted, and mandrel replaced TRex mobile shaker to induce liquefaction. The mandrel was equipped method was finally adopted, and mandrel replaced TRex mobile shaker to induce liquefaction. The with a vibratory hammer to supply the excitation. For each ground motion, the vibratory hammer was mandrel was equipped with a vibratory hammer to supply the excitation. For each ground motion, employed as the steel mandrel and was driven into the ground to a maximum depth of 6 m. As the the vibratory hammer was employed as the steel mandrel and was driven into the ground to a mandrel is driven to produce a combination of compression waves and shear waves, complex shearing maximum depth of 6 m. As the mandrel is driven to produce a combination of compression waves in deeper soil deposits is initiated, and liquefaction can be achieved in larger depths. and shear waves, complex shearing in deeper soil deposits is initiated, and liquefaction can be Shear strain was measured by liquefaction sensors in both the horizontal (X-Y) and vertical achieved in larger depths. (Y-Z and X-Z) planes. Each liquefaction sensor consisted of 3 accelerometers that formed a triangular Shear strain was measured by liquefaction sensors in both the horizontal (X-Y) and vertical (Y- array. For each accelerometer, motions were recorded in the X, Y, and Z axes. In this way, shear strains Z and X-Z) planes. Each liquefaction sensor consisted of 3 accelerometers that formed a triangular were obtained in X-Y, Y-Z, and X-Z directions. The shear strains, which predominantly occurred in array. For each accelerometer, motions were recorded in the X, Y, and Z axes. In this way, shear the X-Y and Y-Z axes in the untreated area, reached 0.05% in both directions. Even the shear strain strains were obtained in X-Y, Y-Z, and X-Z directions. The shear strains, which predominantly in the X-Z direction increased beyond 0.01% in the original area. However, after being grouted with occurred in the X-Y and Y-Z axes in the untreated area, reached 0.05% in both directions. Even the CS, the shear strains measured in the treated area reduced significantly in all three of these directions. shear strain in the X-Z direction increased beyond 0.01% in the original area. However, after being Except for the shear strain in the X-Y direction at 12 s, which arrived at 0.025%, the others remained grouted with CS, the shear strains measured in the treated area reduced significantly in all three of below 0.01% throughout the whole experiment process. The results indicate that permeation of CS these directions. Except for the shear strain in the X-Y direction at 12 s, which arrived at 0.025%, the others remained below 0.01% throughout the whole experiment process. The results indicate that Appl. Sci. 2020, 10, 15 25 of 31 gel will aid in decreasing the shear strain occurred during cyclic loading and improving liquefaction resistance for liquefiable ground. The experimental element tests, centrifuge tests, and field tests performed in the literature have shown that CS has potential application in improving ground conditions and mitigating liquefaction in an earthquake event. By grouting CS, both the settlement and shear strain during the cyclic loading process decrease, and the generation of pore water pressure is inhibited. However, some problems still exist in the grouting process. The field test conducted by Gallagher et al. (2007) [33], for example, suffered some grouting failures in three injection holes. These failures lead to a significant reduction in the injected CS volume compared to the other holes. Post-treatment analysis showed that the failure could be attributed to the gelation of CS grouted in other boreholes and the return of grout around the annulus. Nevertheless, the treated area still produced less settlement in a blast-induced liquefaction, indicating the remarkable effect of CS in mitigating liquefaction.

5. Prospect of Colloidal Silica as a Grouting Material

5.1. Other Applications for CS in Civil Engineering CS is a kind of promising nanomaterial and has various potential applications in other branches of civil engineering. Apart from ground improvement and liquefaction mitigation, CS has been investigated by some researchers to be used for controlling water production and fluid flow in the petroleum industry, reducing permeability and fixing contaminants in environmental engineering, and preventing water ingress in underground and tunneling construction. Jurinak and Summers (1991) used CS in a practical reservoir fluid-flow control system for workovers at an oilfield [81]. Eleven wells were treated with CS for water injection profile modification, water production control, and remedial casing repair. The results show that CS treatment works well in water production control and remedial casing repair. Zones that have been previously hydraulically fractured can be perfectly treated using CS. Moreover, CS has also been verified to be suitable as a permeation grout for barrier systems in contaminated sites [82–85]. Moridis et al. (1995) used CS when injecting in a heterogeneous unsaturated sand layer at 10–14 feet in depth. It was shown that CS could effectively permeate through this porous media and form a fairly uniform plume. A post-treatment test showed a four order of magnitude permeability reduction for the treated layers. Thus, CS is a potential material that can remarkably decrease hydraulic conductivity and, therefore, can be adopted for use in contaminant barrier systems [82]. Manchester et al. (2001) injected CS gel into large sand tanks to determine in situ hydraulic conductivity. Consequently, a permeation test indicated that by grouting CS, a core approximately 60 cm in diameter was produced, with a remarkable reduction in saturated hydraulic conductivity [85]. Besides, the soil-water characteristic curves of grouted soils further verified the use of CS as a grouting material to isolate activated soils in the vadose zone. In addition, CS can also be adopted as grouting agent to permeate through hard rocks in tunneling and underground engineering. Geotechnical procedures were adopted to analyze silica soil’s mechanical behaviors over time and in different storage conditions, representing different environments encountered in tunnels [86,87]. It was shown that the initial strength of silica soil increases faster at high temperatures or low humidity, indicating sufficient strength can be obtained in this instance to withstand most grouting conditions. Moreover, when sufficiently confined, silica soil is ductile. These properties indicate that it can suffer loading and unloading cycles in fairly large 10 11 deformations. Meanwhile, a hydraulic conductivity between 10− and 10− m/s was observed for silica soil, which implies that silica soil is a kind of low permeation material and is a suitable material to be grouted in hard rocks in tunneling practice. Appl. Sci. 2020, 10, 15 26 of 31

5.2. Advantages of CS as a Grouting Material

5.2.1. Low Disturbance Among ground improvement techniques, dynamic compaction, vibro-compaction and explosive compaction have been widely adopted to densify soil layers. However, all of these compaction methods produce great disturbances to the surrounding areas and are not applicable to solely the constrained or developed area. Except for the compaction methods, the grouting method is another common practice to improve ground liquefaction resistance. In all grouting materials, a cement-based material is mostly used. Nevertheless, the particle size of cement averages at 20 microns, which greatly restricts its permeation properties. Cement particles can only transport through voids three to five times larger than their own particle size [15]. Besides, cement has a relatively high initial viscosity, so cement is usually injected under pressure or is adopted to form grout columns, rather than distribution through the entire ground. In addition, grouting cement has a remarkable impact to the surrounding environment. Since conventional methods show great disturbance, grouting CS is an effective method and presents significant advantages over cement. CS has a very low initial viscosity which is very close to pure water once mixed with an electrolyte. The low initial viscosity provides excellent flowability to the grouting materials. Additionally, the particle size of CS ranges from 7–20 nm in general, which is much finer than cement particles. Thus, CS can achieve transport through the entire targeted scope in the porous media under a low pressure. Therefore, grouting CS for ground improvement is feasible for both undeveloped and constrained areas, as it only has low levels of disturbance to the surrounding area.

5.2.2. Environmentally Friendly The production of cement is an energy-intensive process which consumes massive amounts of coal, discharging many harmful gases in the production process [88]. The chemical grouting materials, such as acrylamide, polyurethane, and epoxy, have better penetrability when compared to cement-based grouting materials, but they are expensive and may cause pollution to underground waterways if handled improperly [89]. The poisoning incident of Fukuoka in 1974, for example, was caused by acrylamide grouting materials for ground improvement. Since then, Japan’s Ministry of Construction has set rules that allow only soluble silica to be adopted as a chemical grouting material to improve liquefaction resistance [90]. CS is a kind of nanomaterial and a derivative of soluble silica. It is nontoxic, chemically and biologically inert, and environmentally friendly. When added to soil as a grouting material, CS will not cause pollution, which is known on a basis of learned knowledge. Environmental scientists have also noted that great attention should be paid to the behaviors and application of nanomaterials, with CS included. Thus, CS is a practically potential grouting material due to its environmental-friendly feature.

5.2.3. Low Cost The price/performance ratios of CS and some other chemical solutions have been analyzed in detail. [15,44]. Although the unit price of CS is relatively high, only a spot of CS is effective for liquefaction mitigation. Each cubic meter of treated soil consumes CS that costs 59 USD when CS concentration is controlled to be 5% [15]. However, the treatment costs are about 180, 325, and 500 USD per cubic meter of grouted soil for sodium silica, acrylamide, and epoxy respectively. Thus, CS is such an economical material that its treatment cost is less than one third of the cost of the traditional chemical solutions. Meanwhile, with the booming advancement of nanotechnology practices worldwide, the price of nanomaterials is expected to largely decrease [91]. For instance, the price of carbon nanotubes has significantly fallen from 150 USD per gram in 2000 to 50 USD per gram in 2010. Thus, it can be deduced that CS, as a kind of nanomaterial, will become much cheaper than its current price in the future. As noted by Huang and Wang (2016), although the application of CS for ground improvement and Appl. Sci. 2020, 10, 15 27 of 31 liquefaction mitigation is still in experimental study stage, it will finally be successful for commercial use, due to the advantages of CS in terms of both the environmental and cost aspects [15].

6. Conclusions With the astonishing development of nanotechnology, nanomaterials are beginning to extensively be adopted in civil engineering, due to their apparent advantages of being cost-effective, with low disturbance, and being environmentally friendly. Colloidal silica (CS), which has been shown to have the ability of mitigating liquefaction, is a kind of such nanomaterial with high cost performance. This paper has reviewed this innovative technique by using CS for ground improvement and liquefaction mitigation. Specially, the critical factors that can significantly influence CS transport and the CS strengthening effect were discussed in detail. Based on the conclusions drawn from the previous literature, it was summarized in this paper that the CS concentration and curing time are the two dominant factors that determine the ground improvement effect. Moreover, the fact that CS can greatly mitigate liquefaction for loose sand layers is widely recognized. However, this CS-based soil liquefaction mitigation technique still faces certain challenges associated with control of the gelling time, the CS injection success rate, CS delivery with minimum deposition, and so on. It is of great concern to further develop the approaches to transport CS to the targeted location as precisely as possible with minimum deposition. To settle this issue, ongoing studies should focus on both the gelation properties and the permeation ability of CS in porous media. Although a CS-based soil stabilization technique was put forward after 2000 and is still in the preliminary study and application process, we are pleased to find that this innovative technique was adopted to treat the runways at Fukuoka International Airport in 2016. It is promising that by settling the current challenges, the CS-based liquefaction mitigation technique will ripen gradually and can be expected to be widely adopted in future anti-seismic projects.

Author Contributions: Conceptualization, G.L. and M.Z.; methodology, M.Z. and G.L.; formal analysis, M.Z. and C.Z.; investigation, M.Z., W.G. and C.Z.; writing-original manuscript preparation, M.Z. and G.L.; writing-review and editing, M.Z., G.L., W.G. and Q.L. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by National Science Foundation of China, grant number 51408491 and 51878560. Conflicts of Interest: The authors declare no conflict of interest.

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