Colloidal Silica and Geotechnical Properties of Liquefiable Soils: History
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Subjects: Engineering, Civil
Contributor:
Giovanni Ciardi

Colloidal silica (CS) is a kind of nanomaterial used in soil/rock grouting techniques in different branches of civil engineering. CS grout is chemically and biologically inert and, when injected into a subsoil, it can form a silica gel and stabilize the desired soil layer, thus representing an attractive, environmentally friendly alternative to standard chemical grouting techniques.

  • colloidal silica
  • liquefaction
  • nanomaterials
  • chemical grouting
  • soil stabilization

1. Introduction

In the present era of demographic expansion, the need for buildings and infrastructures is growing exponentially and, as a result, the demand for land is increasing very quickly. However, especially in some areas of the world, the scarcity or inadequacy of available soils for urban development often pose serious challenges on how to use them in a sustainable way to meet the needs required by society.
Consequently, ground improvement measures are increasingly being carried out to make the soil suitable for construction or to increase the safety of existing structures and other constructed facilities. A variety of soil improvement techniques have been developed over the past decades [1,2], and most of them are still being currently applied [3]. Among the existing techniques, permeation grouting is frequently used in geotechnical engineering practice to fill the soil voids: one of the most established chemical grouting techniques, it involves the injection under low pressure of a low-viscosity liquid by means of grouting pipes specially installed into the subsoil. The liquid permeates the soil, and it hardens over time, increasing the soil strength and creating a stiffer and less permeable material. Cohesionless soils, like clean sands or sand with low fines content, are best suited for this method. In chemical grouting, cement, silicates, acrylate, methacrylate, polyurethane, epoxy are the most traditional and commonly used stabilizing agents. The uncontrolled homogeneity of the grouting treatment, as well as any disturbance induced by the injection under pressure nearby existing structures, pipes and other constructed facilities, may affect the suitability of the use of some traditional types of grouts in urbanized areas. According to DeJong et al. [4], the poor-quality control resulting from common grouting procedures forces to an overestimated design, with consequences regarding environmental impact and economic costs. Therefore, sustainable alternatives to the use of traditional types of grouts are currently being investigated by researchers all over the world.
The recent steep growth of nanotechnology has led to a significant development of nanomaterials that could be successfully used in grouting practice with minimal drawbacks. According to the European Commission [5], a nanomaterial can be defined as “a natural, incidental or manufactured material containing particles, in an unbound state or as an aggregate or as an agglomerate and where, for 50% or more of the particles in the number size distribution, one or more external dimensions is in the size range 1–100 nm”. According to Huang and Wang [6], the ratio of cost over performance of nanomaterials for soil improvement is better than that of traditional grouts, such as cement, sodium silicate, epoxy or acrylate. When this ratio was not too different, the environmental friendliness and the disturbance induced in the surrounding ground is in favor of nanomaterials.
In the field of geotechnical engineering, colloidal silica, bentonite, laponite and carbon nanotube are nanomaterials that have been recently applied to soil improvement [6]. Nanomaterials can be properly added to different kinds of soil to modify and improve some physical/mechanical properties of the natural material [6,7,8,9]. Significant changes in plasticity characteristics and hydraulic conductivity are induced in cohesive soils by the addition of nano-clay [10,11,12], and a significant decrease in the hydraulic conductivity can be achieved in sandy soils by grouting them with nano-silica solutions [13,14]. Nano-silica grout treatments can be used for soil remediation, e.g., to remove contaminants [15] or to form impervious barriers [13,16,17], or in tunneling industry to prevent piping and to prevent water leaking through cracks [18,19]. Recently, the use of innovative nanomaterials has also been extensively investigated for the purpose of earthquake risk reduction. In view of possible applications for mitigation of seismic liquefaction hazard, a great number of studies has focused on analyzing the performance, under cyclic loading conditions, of sandy soils treated by nanomaterials such as bentonite, laponite and colloidal silica. The use of bentonite and laponite has proved effective in improving the behavior of sandy soils under cyclic loading [20,21,22] and a significant improvement of the cyclic liquefaction resistance has been observed in sandy soils treated with nano-silica solutions (e.g., [23,24,25]). This type of treatments can be listed among the recent innovative remediation methods against liquefaction [26,27], which also include the adoption of specific microbial-driven processes [28,29,30], the inducing of partial saturation [31,32,33], the recycling of waste materials, such as tire scraps [34,35]. Due to their environmental and economic sustainability, these innovative techniques may represent an alternative to other more conventional soil improvement methods that are commonly used to mitigate seismic liquefaction hazards (e.g., vibro-compaction, blasting, drainage) [36].
Among the above-mentioned innovative techniques for the improvement of liquefiable soils, this paper deals with colloidal silica (CS) grouting, which involves injecting a low-viscosity, time-hardening, non-pollutant, chemically and biologically inert solution into the soil voids [6,23]. Although the CS grouting technique had already been used for many years in different fields of civil engineering, the earliest studies on the use of CS grout for liquefaction risk mitigation date back to the beginning of this century (e.g., [23,37,38,39]). Since then, a huge number of works dealing with this topic have been published. Most of them refer to experimental studies on small-size soil elements, while others refer to physical models and field tests. These studies have allowed rapid development with non-stop updates of knowledge on CS grouting, as recently highlighted in [40,41].

2. Overview of Literature on the Use of Colloidal Silica Grouting

Since the beginning of this century, engineers have become increasingly familiar with the best practical use of treatments by nanomaterials, and among them numerous studies have been published highlighting benefits and drawbacks of colloidal silica (CS) grouting. Although CS grouting could potentially be applied in several fields of civil and environmental engineering (e.g., in tunneling construction [19], to prevent piping and to prevent water leaking through cracks, or in environmental interventions [17] to remove contaminants or to form impervious barriers; in the petroleum industry [42], to reduce the rock permeability to water and gases), most of the published studies analyze the behavior of grouted soil under cyclic loading, with the main purpose of investigating the effectiveness of the treatment for seismic liquefaction mitigation. The interest in this specific field of application has known a great impulse starting from the concept of passive site remediation [43], according to which a soil layer can be stabilized by permeating CS grout under low injection pressure, or by exploiting ground-water flow, without causing disturbance to the overlying existing structures.
A colloidal silica grout is a low-viscosity and harmless solution which increases its viscosity over time forming a colloidal silica gel [44,45,46]. The resulting colloidal silica gel act as a filler/binder among soil particles, stabilizing the material [14,24,47,48,49,50]. Initially and for some time, depending on a rheological behavior that can be controlled and designed, the CS grout can be moved through highly permeable media, like sands, just by natural or groundwater flow supplied without the need of high injection pressures, allowing minimization of negative side-effects on the surrounding environment [23,51,52]. Therefore, the use of colloidal silica grouting would be particularly favorable where other standard ground improvement techniques (e.g., densification, drains installation, etc.) could not be safely applied [37,53,54].
To date, most of the different aspects concerning the colloidal silica grout properties and the effects of grouting on the behavior of cohesionless and sandy soils, as well as the entire grouting process, have been investigated. The published literature includes studies on the grout properties and on the management of its rheology, on the grout delivery mechanism, and on the effects of the colloidal silica grouting on physical soil properties and soil mechanical behavior. Excluding studies which only analyzed the behavior of pure colloidal silica grout [46,55,56,57,58,59], most of the published studies on colloidal silica grouting deal with laboratory investigations on grouted cohesionless materials. These studies differ in the materials used (kind of grout and soil), in the specimens’ formation methods and in the kind of performed laboratory tests, so that it is often not immediately possible to compare their results. As this review comes from a critical evaluation of existing published data, some themes are discussed more in-depth than others, reflecting the same tendency of scientific literature.
The response of colloidal silica grouted liquefiable material to dynamic and cyclic loading has been widely investigated by many authors. Published data generally agree that the liquefaction resistance of grouted soil improves due to soil grouting, and that the level of improvement is directly related: (i) to the amount of silica particles diluted in the stabilizing grout; (ii) to the time between the end of the gelation process and the beginning of testing (known as curing time), increasing as both these factors increase [14,23,24,25,37,38,39,47,48,49,50,60,61,62,63,64,65,66,67,68,69,70]. The same findings apply for grouted material subjected to static loading conditions [14,37,48,49,50,60,61,63,64,65,67,69,70,71,72,73,74,75,76]. The behavior of grouted soil under dynamic loading at low–medium strain levels has been much less analyzed [77,78,79]. According to published data, the shear modulus of grouted sand is expected to be greater than that of untreated material; however, there is still some disagreement on how much the damping ratio is affected by grouting.
On the contrary, it is well established that grouting leads to a dramatic reduction in the soil hydraulic conductivity. This decrease can be of several magnitudes, and the more the silica concentration in the grout, the more the expected permeability decreases, up to values typical of clay [13,14,17,66,72]. Finally, the effects of grouting on the soil compressibility have been scarcely investigated, even though this knowledge is fundamental for practical applications (the compressibility of soil would influence the settlements of structures and infrastructures in the area of intervention) [14,37,67,70,71,72]. Most of the results show that the treated soil is more compressible than the untreated one, even though a strict comparison among these data is not possible due to the differences in testing conditions.
The mechanism of grout-delivery has been investigated in several studies by large-scale grouting laboratory tests [51,52,80,81,82]. These studies agree that the most important factors affecting the grout transportation are the increase in grout viscosity, the injection pressure, the grout sinking. Field-test and full-scale applications of colloidal silica grouting are documented, representing, however, a very small part of the published literature [83,84]. These studies also contain limited information about in situ tests (e.g., the cone penetration test, CPT) used to evaluate the performance of colloidal silica grouting. Lastly, few studies discuss the constitutive modelling approach to represent the behavior of cohesionless soils stabilized by colloidal silica grouting [60,85]; in these studies, an effort in modeling this behavior was made by adapting existing constitutive models to reproduce some experimental data from grouted soil.

3. Colloidal Silica: An Overview

3.1. Main Characteristics and Gelation Process

A colloidal silica (CS) sol can be defined as a stable dispersion of silica (SiO2) solids of colloidal size (ranging from 1 up to 100 nm) into a liquid phase [44]. It is obtained from saturated solutions of silicic acid and it is a clear, harmless, and low viscosity fluid that is commercially available with different (given) silica contents. Colloidal silica is not pollutant, it is chemically and biologically inert [6,44,45,86], and it has excellent durability properties [50,87]. If diluted to 5% CS particles by weight, the material cost could be comparable to that of microfine cement, with a viscosity (1.5–2 cP) comparable to that of water (1 cP at 20 °C) [25].
The colloidal silica gel originates from the hardening process of a given CS dispersion. Silica particles agglomerate, forming a network of silica clusters connected by siloxane (Si-O-Si) bonds. The gelation process depends on the different factors affecting the interaction among silica nanoparticles, namely the silica content, the ion concentration (ionic strength), the temperature, the particles’ size and the pH in the CS dispersion [23,46,57,58,59]. The time between the start and the end of the hardening process (known as gel time) decreases as silica content, ionic strength and temperature increase, while it increases as particles’ size increases; the minimum gel time is achieved for pH in the range 5–6 [13,43,57,80,88,89,90]. 
 
The gel time can be varied from minutes to days [13,46,52,57,63,80]. The control and prediction of the gelation process are crucial issues for practical application of CS grouting, because the gel time should be long enough to avoid early gelation, so that the grout can be delivered where designed [83], but it should also be as short as possible to avoid grout dilution and sedimentation [81,84]. Visual observation of the formed gel [13,72] or viscosity measurements [25,46,52] are usually performed in the laboratory to define the gel time for a given CS grout, the latter often indicating the mixture of a CS dispersion with an activator, whose job is to promote the gelation process. Using an electrolyte (i.e., a salt-based solution, such as a NaCl, KCl or CaCl2 solution) as an activator is considered the simplest method for the gel time control [81]. It should be emphasized that, since the commercial colloidal silica solutions have generally different characteristics (i.e., particles’ mean size), the same value of CS content may not mean the same thing in different research studies and this should be considered when comparing their results.

3.2. Pure Silica Gel Properties

The properties of the pure CS gel (i.e., not combined with soil) for engineering purposes have been scarcely presented and discussed in scientific literature. The strength of the gel structure is strongly related to the amount and strength of silica agglomerates developed during the gelation process. The gel strength increases over time because of the progressive formation of siloxane bonds, while the electrolyte concentration in the grout affects the gel time, but not the strength of the resulting gel pattern [59], which is in turn affected by the amount of silica solids and by the type of the activator [58].
Experimental evidence proves that the strength of the pure gel is generally low, (typically few kilopascals after few days from specimen preparation, up to a few dozen as aging increases); in addition to the silica content, strength is affected by the relative humidity and increases, for a certain test type, with samples’ age. Liao et al. [38] measured a gel strength of 6 kPa after 7 d, increasing up to 18 kPa after 28 d, in vane shear tests. Axelsson [55] performed fall cone and compressive strength tests on rectangular samples 40 × 40 × 160 mm (height × width × length). The CS silica sol (35% CS by weight) was mixed with accelerator at the 8:1 weight ratio. Tests were performed at 8 °C with relative humidity 75%, 95%, 100%. It was found that: (i) the strength of silica gel increased over time without having reached an apparent ultimate value after 6 months; (ii) the strength increase was faster at low humidity; (iii) the friction angle, in the Mohr–Coulomb failure model, increased from ≈20° to ≈50° after 6 months.
The CS liquid solution, apparently irrespective of CS content, is more compressible than water [66], and the gelled material is compressible [91]; this may explain why the pore pressure build-up is delayed in treated specimens subjected to cyclic loading (Section 5.1.1). In detail, Towhata [91] showed results from an unconfined compressive strength test performed on a 6.5% CS content gel specimen (47/70 mm diameter/height, tested after 30 d curing in water), measuring both axial and radial strains. It was shown that the sample collapsed showing brittle behavior and strain softening, with peak deviatoric stress slightly > 5 kPa, and that the calculated Poisson’s ratio was ≈0.3 for gelled material, thus implying a volume compressibility of the gelled mixtures greater than that of water. Vranna and Tika [66] evaluated the compressibility of CS solutions before the gelation (CS contents 6%, 10%) measured using a pressure and volume controller, in the isotropic stress range 100–300 kPa. They showed that the compressibility of CS was 14–33 (under a stress of 100 kPa) and 12–18 (under a stress of 300 kPa) times greater than that of distilled and deaired water, and that the CS content did not have effects on the compressibility of CS. Similar results have also been recently shown in Vranna and Tika [68].
When subjected to confined 1D-compression tests, gelled samples exhibit significant strain depending on samples’ age, gel time and curing environment. Butrón et al. [56] performed unconsolidated and consolidated undrained triaxial, unconfined compressive strength and oedometer tests on samples cured in different storage environment (temperature, relative humidity, chemical surrounding) and tested over a period of 5 months; 35% silica sol was mixed with accelerator at 8:1 weight ratio. They found that, given a combination of relative humidity and temperature, the shear stress in unconfined compressive strength tests increased with samples’ age. Furthermore, specimens tested within 13 days after mixing exhibited a strain-hardening behavior, while samples tested after 29 d and 69 d showed a peak shear stress (strain-softening) in the range 27–33 kPa from consolidated undrained triaxial tests (effective consolidation stresses around 10 kPa and 20 kPa for radial and axial stress, respectively). Up to ≈30% strain was measured 1 d after mixing from oedometer tests in the stress range 0–100 kPa, and strain generally decreased, given a stress level, with samples’ age. From oedometer tests, they obtained hydraulic conductivity values in the range 10−10 to 10−11 m/s. These values, typical of clays, result from water flowing through small pore spaces randomly formed by particles aggregates [58]. Wong et al. [72] performed oedometer tests on pure CS gel samples (75/20 mm diameter/height, 1 h or 5 h gel times; cured under water or at 90% relative humidity (1–4 weeks); CS content 34%, vertical stress up to 1000 kPa). All samples, irrespective of curing conditions, gel and curing time, exhibited significant void ratio decrease during loading. Samples cured for 4 and 8 weeks showed a stiffer response at low vertical stress levels than samples cured for 1 and 2 weeks while, at effective vertical stresses greater than about 25 kPa, the slope of the compression curve was similar for all samples.
In conclusion, the experimental results from existing research on the behavior of CS solution and pure gel point out some fundamental aspects: (i) CS solution and gel are more compressible than fresh water; (ii) the strength of CS gel increases with time, and it is affected by environmental conditions such as relative humidity and temperature; (iii) CS gel has hydraulic conductivity values typical of clays.

4. Transport of Colloidal Silica Grout through Porous Media

A key aspect for practical application of CS grouting is the permeation mechanism of the stabilizing mixtures into targeted soil layers. The groutability of materials depends on the grout viscosity and on the hydraulic conductivity and grain size distribution of the permeated medium. Soils with hydraulic conductivity in the range 10−3–10−5 m/s and particle size in the range 0.05–2 mm are easily groutable by all commonly used chemical grouts [2]; liquefiable soils usually match both these criteria. Previous research dealing with colloids showed that they can move through sand aquifers [92,93,94]. Fujita and Kobayashi [95] evaluated the effects of the soil matrix, of the charge amounts of colloidal silica particles, and of the air–water interface on the transport of the colloid in saturated and unsaturated conditions, showing that the liquid transport significantly depends on pH (high mobility at higher pH). Noll et al. [89] formed a permeability barrier (0.9 m wide) with CS in a 3.6 m × 1.8 m × 1.2 m sand box. The grout was delivered by means of three injection wells placed in the center of the container and three extraction wells located on two sides of the model. Noll et al. [96] used CS to stabilize a contaminated area, disposing one injection well surrounded by six extraction wells on a 6 m radius.
Gallagher and Finsterle [51] used delivered CS grout into a box model by means of injection and extraction wells under low hydraulic gradient. The model consisted of a 20 cm thick layer of loose sand (relative density, Dr = 40%) pluviated into a box (76 × 30 cm, 26.5 cm height). Five PVC pipes were used to deliver the colloidal silica solution into the soil layer for ≈10 h (16.5 L of CS solution). Two additional pipes were used as extraction wells. Curing time was set as 14 d once the grout delivery was completed. Specimens collected for unconfined compressive strength (UCS) tests after the box was dismantled showed UCS values in the range 16–61 kPa. The authors concluded that low-gradient injection and extraction wells could be successfully used for colloidal silica delivery.
 
When dealing with CS grouting, together with the different density and viscosity of the grout and the water it displaces, the time-dependent gelation process must be considered. The gelation process leads to a progressive decrease in the hydraulic conductivity of the permeated media [17,83,96], thus decreasing the rate of grout delivery. Gallagher and Lin [80] evaluated the permeation of colloidal silica grout by means of four soil columns (Dr = 40%) (Nevada 120 silica sand, mean particle diameter D50 = 0.15 mm). They pointed out that the mixture’s viscosity had the greatest influence on the grout travel time through the columns: once this viscosity approached 3.6 cP the flow rate was greatly reduced. In a subsequent study, Gallagher and Lin [52] performed several column tests (0.9 m-length) on loose sand or graded silty sand specimens (Dr = 40%) permeated by CS grout under low hydraulic gradients (0.04, 0.09, 0.18). The authors showed that the CS solution could successfully permeate the soil columns. For a viscosity value of ≈4 cP, the grout flow inside the columns essentially stopped. It was concluded that hydraulic gradients (i.e., injection pressures) must be appropriately chosen in accordance with the distance the grout has to cover, and by considering that the initial hydraulic conductivity of the medium decreased as the grout started gelling in the pores. Bolisetti et al. [82] analyzed the grout injection process (40% CS grout mixed with NaCl solution (10% by weight) at a 4:1 weight ratio) and the grout flow in sand columns (fine sand with D50 = 0.2 mm and hydraulic conductivity in the range 2.7–3.3·10−4 m/s), confirming that the increase in the grout viscosity during gelation significantly affected its permeation; sinking of silica due to the grout density should also be considered because the CS particles of the grout may sediment where not required.
Hamderi and Gallagher [97] evaluated injection and extraction rates, and optimum wells disposal, for in-field colloidal silica treatment by means of numerical analysis. The authors pointed out the importance of the extraction flow rate for horizontal colloidal silica delivery. They also found that a distance from the inlet to the outlet well between 2 and 4 m could provide the soil with enough improvement for the maximum simulated injection-extraction rates available. Conversely, when low injection/extraction rates were adopted, the problem of grout sinking may occur, suggesting properly estimation of the minimum flow rate when designing the treatment for a specific site. Hamderi and Gallagher [81] developed a pilot-scale facility to investigate the delivery mechanism of colloidal silica grout. Liquefiable sand (Dr = 22%, hydraulic conductivity 1.8 × 10−3 m/s) was put into a large box facility (243 × 366 cm, 122 cm deep) and treated by a colloidal silica grout transported by means of injection and extraction wells. Both low-rate (2) and high-rate (6) injection tests were performed. For low-rate tests, 6% CS solution was used, and it was made to move at a flow rate of 65 mL/min/well (total flow rate equal to 260 mL/min) for the first test; due to sinking problems during the grout delivery, the flow rate was doubled for the second test and the relative density of the sand was increased to 48%. For high-rate tests, a 9% CS by weight solution was used for just one test, while for the remaining part a tracer NaCl solution was used. At the end of the high-rate test with CS grout, the treated sand was left 25 days to cure before it was excavated. The injection rate for this test was set at 6800 mL/min/well (approximately 50 kPa/well). A 0.3 m deep horizontal surface was used for testing with a pocket penetrometer. Finally, 24 cylindrical samples were trimmed and subjected to unconfined compressive strength tests. The CS concentration resulting after the grout permeation was estimated from electrical conductivity measurements, and by relating pocket penetrometer and UCS measures on treated sand samples.
In addition to the viscosity increase, which may stop the delivery process and prevent the target location being reached, the grout sinking is a crucial issue in the CS grout treatment. It is governed by the difference between the density of the grout and that of the water it displaces [81,97,98,99] and, as a consequence, the grout could not be delivered to the target location with the required design properties. Hamderi and Gallagher [97] pointed out that the maximum practical injection distance for colloidal silica is in the range 2.5–4 m, to avoid sinking. Hamderi and Gallagher [81] observed on a physical model that sinking was much more significant when low injection rates were used instead of high injection rates for grout delivery. As stated by Weggel et al. [98], while the viscosity increase reduces the grout flow due to a decrease in hydraulic conductivity, the density of CS, greater than that of the water it displaces, tends to increase its flow-rate. In numerical simulation of grout injection, Gallagher and Finsterle [51] took into account the different densities of water and stabilizer assuming that their volumes were additive. Moreover, they also used a relationship to describe the stabilizer viscosity increase and a mathematical power-law expression to calculate the time-dependent viscosity of the liquid phase, which they assumed consisted of water, stabilizer and dissolved air. Agapoulaki et al. [99], after performing 1D-flow tests on soil columns permeated by 10% CS solution, proposed an analytical tool based on a modified Darcy’s law in order to predict the grout flow rate considering different densities of water and CS, and viscosity changes over time. Weggel et al. [98,100] developed an analytical model to evaluate the rate at which the CS solution displaces the water while moving through a porous medium column (1D flow). The combined effects of density and viscosity difference between CS dispersion and water were taken into account, and the proposed model was validated by comparison with data on one-dimensional CS flow described in a previous work by Lin [101].

5. Influence of Colloidal Silica Treatment on Soil Behavior—Laboratory Investigations

Most of previous research studies dealing with CS grouting consist of laboratory studies. This section reviews the most significant results obtained by researchers from laboratory experimental activities on CS grouted soil. The behavior of treated material under dynamic and cyclic loading is discussed distinguishing between liquefaction-related problems and soil response up to medium-strain levels, followed by a discussion on the soil response under static loading conditions.
When performing laboratory tests on CS-treated soil, a crucial issue is represented by the specimen preparation method. Basically, two different methods have been extensively used in the literature. The first one consists of pluviating the dry sand directly into the liquid grout to achieve a desired initial relative density, waiting for the gel time and curing time to expire, and then moving the specimen into the testing apparatus. The second one consists of grout permeation through the specimen, which was previously prepared in standard ways (e.g., dry tamping, moist tamping, dry deposition, etc.) and possibly pre-saturated with deaired water. While the first method ensures good homogeneity, full gel saturation and reproducible laboratory results, it is not representative of field conditions (i.e., grout injection). On the other hand, despite being much more representative of field injection, the second one does not guarantee a homogeneous grout distribution.
Before going through details of tests and results collected from the literature, the role of back-pressure saturation in cyclic as well as monotonic triaxial tests needs to be discussed. The application of a back-pressure is usually performed as a standard procedure to dissolve air bubbles and to promote water saturation of the sample before the consolidation and shearing phases. For treated soil, most of the tests reported in older studies and in some recent ones were performed without prior water saturation [14,23,63,66,68,69] to avoid disturbance to the gelled material. On the contrary, in other studies soil specimens were previously saturated [48,64,70,71,74], in some cases using very high values of the back-pressure (up to 700 kPa). When back-pressure saturation is not performed, pore pressure response is not measured during the shearing phase and test results are processed with reference to total stress, or assuming that mean total and effective stresses are practically identical [68]. Vranna et al. [68] showed the results from monotonic undrained triaxial tests on treated and untreated loose sand specimens subjected to different back-pressure values, pointing out that the magnitude of back-pressure had a strong influence on the stress–strain response of the material. Georgiannou et al. [71] showed that the results of drained monotonic triaxial tests were independent from the back-pressure magnitude, which was in the range 300–700 kPa. The same range of values was adopted by Triantafyllos et al. [74], who stated that increasing the back-pressure within this range does not produce damage to the gel since they observed that drained loading on similar stabilized samples gave reproducible results. Pavlopoulou and Georgiannou [70] obtained repeatable results from specimens that were back-pressure saturated in a standard way; they also stated that increasing the pore water pressure does not damage the gel, and that the treated sand response is controlled by the effective stresses; thus, measuring the pore water pressure response is required in laboratory tests on CS-treated sand.
It has been demonstrated that CS grouting increases the soil response under both static and cyclic loading conditions; however, some key issues for practical applications of this stabilization technique (e.g. the effects of grouting on soil compressibility and hydraulic conductivity, or the effects on damping) have been less investigated. 

6. In-Field Colloidal Silica Grouting for Liquefaction Mitigation

Most of the earliest studies on field applications of colloidal silica grouting (e.g., [86,89,96]) aimed to prevent the flow of contaminants, while CS applications for liquefaction mitigation have been rarely discussed in the literature. Among these past studies, Noll et al. [96] described the treatment of a 5 m diameter zone (≈3 m thick) with a 5% CS solution by means of a central injection well and radially disposed extraction wells, to demonstrate the ability of CS grout to prevent pollutants flow. Moridis et al. [17] injected a CS solution into a heterogeneous unsaturated gravel-sand-silt deposit by means of tube-à-manchette technology at depths of 3.0, 3.6 and 4.2 m, with the aim of waste containment. A soil column (≈3 m height) uniformly grouted could be excavated after treatment.
Gallagher et al. [83] performed a field test on a 2 m thick sand layer (Dr = 40–45% based on cone penetration test, CPT, results) located ≈6.5 m below the ground surface and treated with silica solution. CS grouting (injection by tube-a-manchette, two stage bottom-up process) was followed by a blast-test to evaluate the level of improvement at the target site by comparing the pore pressure response and settlements measured in the treated area with those recorded in an adjacent untreated site. Eight injection wells, disposed radially on a 4.5 m radius zone, were used to pump the grout into the subsoil, and one extraction well was equipped with a submersible pump in the center of the improved area to direct the grout flow toward the inner of the circle. Low grouting pressures (75–150 kPa, 175 kPa occasionally) were used for grout injection. After grouting, the injection wells were equipped with explosive charges at depth (6.4 m and 8.5 m below the ground surface) to induce liquefaction. Based on the pore pressure analysis, the treated area liquefied (the calculated pore pressure ratio ranged from 0.93 to 1.04 immediately after blasting). However, no large deformations were recorded in the treated site, suggesting that the CS treatment had significantly improved the deformation resistance of the sand layer. Typical deformations connected to flow liquefaction (e.g., sand boils) were not observed. In an adjacent untreated test pile area, 0.5 m maximum settlements were recorded, while the maximum values in the treated site were ≈0.3 m. The authors concluded that, even if liquefaction occurred in the treated layer according to the pore water pressure criterion, the minor settlements were clear indicators of the increased stiffness of the grouted site.
Rasouli et al. [84] presented a detailed report on the application of a controlled permeation CS grouting technique used to improve liquefiable areas at the Fukuoka Airport (Japan). A controlled curve drilling machine was developed and used to inject the stabilizing grout point-by-point by an injection hose inserted in a borehole. Two different areas of the Fukuoka Airport, named A and B in the study, needed to be improved. Zone A corresponded to the runway; therefore, to maintain its full operation during the treatment, vertical boring was not possible. The controlled curved drilling machine was used in zone A. Both zones A and B consisted of shallow liquefiable loose sand layers with 10.5% (zone A) and 9.9% (zone B) fines contents. A colloidal silica grout (8% silica content) was used for the improvement of both areas. Grouting pressures ranged between 300 kPa and 500 kPa. The full operation of the runway in zone A was maintained while the point-by-point improvement was successfully performed. Controlled permeation grouting was proven to be effective to deliver the silica grout in the vicinity of existing structures.
To date, the lack of data from field experience does not allow an a priori identification of the best injection technique for a given site. Furthermore, since not only the subsoil characteristics, but also the equipment and procedures can strongly influence the effectiveness of the subsoil stabilization, it would be of great advantage to produce test sections (e.g., before injection), allowing a proper evaluation of the effectiveness of the improvement treatment [36].

7. Mechanism of Soil Improvement

As shown in the previous sections, CS grouting improves the soil response under monotonic and cyclic loading, increasing the liquefaction resistance of cohesionless soil. However, the mechanism of improvement is still unclear: it has not yet been well clarified if the soil improvement entirely/truly depends on the grains’ bonding. From the results of drained torsional shear tests, Kodaka et al. [61] stated that “the intercept of failure line occurs due to the cohesive properties of treated sand”; some studies agree with this interpretation (e.g., [14,48]); in some others, however, a curved, stress-dependent failure envelope was adopted for the treated soil [70,71], assuming that the stabilized sand behaves substantially different from cemented or lightly cemented materials. According to Gallagher et al. [83], “the improvement mechanism of colloidal silica is bonding between the gel and the individual sand particles”.
Bonding reduces the contractive tendency of loose soil, thus producing a delay in the development of extra pore pressure. Some experimental results, however, show that the extra pore water pressure build up is faster in treated than untreated sand subjected to cyclic loading [64,70]; in addition, the initial stiffness of treated soil is lower than for the untreated one under monotonic and cyclic loading [70,107], at least for cost-effective CS contents [14,70,71,74]. Furthermore, the bonding of grains justifies the increase in shear strength under monotonic loading, but not the behavior observed by some authors under 1D—normal compression. Additional complexity arises from the nature of the pore gel: being more compressible than water, it follows that “the significance of ru for CS sand does not represent the same physical meaning” as for untreated material [24]. The experimental results prove that the failure mechanism in treated sand is not intimately related to the extra pore water pressure accumulation, like for the untreated sand [64,70,83].
Concerning Monterey sand, Persoff et al. [13] stated that “any unconfined compressive strength results from the cementing effect of the grout […]. This suggests that the colloidal particles bond not only to each other, but also to the silica surface of the sand”. Wong et al. [72], supported by microscopic analysis, stated that the silica particles filling the pores of grouted sand are responsible for the improved shear strength and stiffness under monotonic loading. The improved dilative behavior of treated soil agrees with this interpretation. Similarly, according to Salvatore et al. [49], the gel filling the pore space of grouted sand prevents the mobility of sand particles, thus improving its response under both static and cyclic loading conditions. Bonding would justify the increase in the shear modulus at small strain levels, and a decrease in the damping ratio due to artificial cementation should be expected. However, experimental results showed that the damping ratio of grouted sand was higher than for untreated material, ranging from negligible up to significant increase. It is probably due to the nature of the pore gel that the damping ratio is expected to increase, since the energy dissipation would be enhanced by the viscous nature of the gel. Moreover, the shear velocity of grouted material increased when measured in laboratory [78], but a remarkable increase in the shear wave velocity was not measured in field [83]. In conclusion, the mechanism of soil improvement has not been fully clarified yet.

8. Numerical Modeling of CS Grouted Materials

All the aspects shown in the previous sections pose specific questions that need to be answered before dedicated constitutive models can be developed for CS grouted material. Specifically, the validity of the Effective Stress Principle for water-saturated soils is questionable when the treated soil is saturated with compressible gel. Moreover, due to the absence of a unique framework able to match all the different aspects of the behavior of grouted materials, dedicated constitutive models have not been developed yet. To date, only few studies discuss the adaption of pre-existing constitutive models to grouted soils. Kodaka et al. [60] modified an existing elasto–plastic constitutive model [113] for sand to account for the reduced excess pore pressure development due to the soil stabilization. Andrianopoulos et al. [85] adopted an existing bounding surface critical-state plasticity model for sands (NTUA-sands proposed by Andrianopoulos et al. [114]) without changing its mathematical formulation. The simulation of passively stabilized sand was undertaken by following two approaches, the first one being a recalibration of some of the model constants, to consider a stiffer and more dilative material. This approach essentially reflects the modifications induced by grouting to the soil skeleton. Following the second procedure, the authors assumed the same model parameters for both treated and untreated sand, while the more seemingly compressible pore gel was modeled by reducing the bulk modulus of pore fluid in coupled analysis, to simulate the slower pore pressure build-up in treated sand. Element tests on stabilized sand were successfully simulated, while the model was not fully capable of simulating the system response in a boundary value problem (i.e., a centrifuge test). The authors stated that the most promising approach to simulate boundary value problems consisted in the decrease in the pore fluid bulk modulus. This phenomenological approach used to simulate the effects of CS grouting was also used in Papadimitriou et al. [115], to numerically investigate the effectiveness of this grouting method to mitigate the liquefaction-induced buried pipe uplift.

9. Summary and conclusions

The current state of knowledge on the main aspects concerning the soil treatment by colloidal silica grouting, a non-standard soil stabilization technique, was presented and discussed. While the behavior of small-sized soil elements has been widely described in the literature, research studies on physical and on-site models are still limited. The main conclusions which emerged from a critical review of the existing studies can be summarized as follows:

  • CS grout can be successfully transported through porous media; however, the effects related to grout viscosity increase over time and grout sinking need to be considered in design;
  • CS grouting improves the liquefaction resistance of liquefiable soil;
  • CS grouting enhances the soil strength under monotonic loading conditions;
  • The strength of the treated material increases with the increase in both CS content and gel aging;
  • The treated soil shows enhanced dilative behavior under monotonic loading conditions;
  • The mechanism of soil improvement has not been fully clarified yet;
  • The effects of CS grouting at low-medium strain levels have not been fully clarified yet;
  • The compressibility of the treated material seems to be greater than that of the untreated one, at least for the CS contents that would be cost-effective for liquefaction mitigation;
  • The CS treatment produces a significant decrease in soil hydraulic conductivity;
  • Field experiences of CS grouting demonstrated the feasibility of this improvement method;
  • Appropriate constitutive models able to simulate the behavior of the CS-treated material have not been developed yet;

In conclusion, from the current state of knowledge on the subject, it is clear that the behavior of soil treated with CS grout is very complex. To shed light on controversial or not yet fully discussed aspects, future research on colloidal silica grouting should further investigate the following aspects:

  • The effects of CS grout on soil compressibility;
  • The effects of CS grout on damping ratio;
  • The sinking phenomenon in the grout delivery process;
  • The development of adequate constitutive laws to describe the behavior of the stabilized soil;

Finally, before the application of the CS grouting stabilization technique in engineering practice, the use of test sections is strongly recommended, for both identifying the most appropriate site-specific design parameters and evaluating the performance and effectiveness of the CS treatment.

This entry is adapted from the peer-reviewed paper 10.3390/geotechnics1020022

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