1 Introduction

In recent years, mechanized excavation with Tunnel Boring Machines (TBMs) is recognized as a popular way to establish underground spaces for different purposes specially to provide the possibility to control the traffic density. The effect of underground excavations such as tunnels must be kept minimum on the surrounding media and nearby surface and sub-surface structures and shield mechanized tunneling is, therefore, an appropriate method for this purpose. During shield tunneling using a TBM, the face is excavated via a spinning cutterhead which is composed of rotating disc-cutters excavating the front face of the tunnel while the machine advances forward. The advancement of the machine is carried out using hydraulic jacks which are placed behind the TBM, moving the machine forward applying a specific load on the newly installed segmental linings. Precast segmental linings are carried in the tail shield of the TBM and are erected and placed into the location (behind the TBM) as the machine advances to act as a supporting media for the tunnel. Due to the difference in the diameter of the excavation area and the extrados of the segmental lining induced by the overcutting procedure and shield conicity, an annular gap is created between the linings and surrounding soil resulting in significant ground deformation around the tunnel and subsequent surface settlement. As a common practice, this gap must be continuously filled with the grouting material injected from the pipelines in the tail shield of the TBM to compensate the potential ground deformations, optimizing the stresses induced in the segmental lining as well as posture maintenance of the segments, and in some situations, to act as a sealing agent for the tunnels excavated in water-rich media (Anagnostou and Kovári 1994; Bezuijen 2007; Zhao et al. 2017; Pinto and Whittle 2014; Tang et al. 2016; Wang, et al. 2021). The grout is initially injected into the tail void via a certain pressure (i.e., grouting pressure) filling a thickness of around 16–37 cm which accounts for approximately 3–16% of the whole excavated volume of the tunnel (Wang, et al. 2021). Depending on the type and the injection procedure, the tail void grout consolidates over a certain period and gains a specific strength value (Wang, et al. 2021).

Various studies have been conducted to evaluate different aspects of the grouting material in mechanized tunneling application. These studies mainly focus on the usage of different grout types and mixtures (Peila, et al. 2015; Pelizza, et al. 2010; Todaro et al. 2022; Thewes and Budach 2009; Shirlaw, et al. 2004; Li et al. 2022), different experimental tests to investigate various properties of the grout (Youn and Breitenbücher 2014; Oreste et al. 2021; Oggeri et al. 2022; Zhang et al. 2018; Todaro et al. 2021; Luo 2022; Rahmati et al. 2022; Wang 2023; Bezuijen and Zon 2007), and finally, numerical simulation of the tunneling procedure incorporating the role of grouting material (Mohammadzamani et al. 2023; Kasper and Meschke 2006; Qiao, et al. 2018; Zhao, et al. 2018; Oh and Ziegler 2014; Lavasan and Schanz 2017; Lavasan et al. 2018; Shah et al. 2018; Yang et al. 2022; Wu et al. 2020; Do et al. 2014) or formulation of constitutive models accounting for various characteristics of the grout or similar cementitious materials (e.g., shotcrete in tunnels) such as phase transition, diffusion law, time-dependent behavior and so on (Eekelen, et al. 2002; Liu et al. 2021; Schütz et al. 2011; Schädlich and Schweiger 2014; Neuner et al. 2017).

The application of a certain type of grouting material in various mechanized tunneling projects has proven to be complicated and not feasible when encountering different geological conditions. Meeting performance requirements in terms of fresh properties, mechanical properties of the hardened body, scour resistance, and durability of the grouts is a challenging task. Moreover, in certain scenarios, properties of grouts may contradict each other, thereby impeding the attainment of optimal ratios of constituent materials. As a result, researchers tend to focus on specific aspects of grouting material performance during the design process. For example, in water-rich strata and karst ground, scour resistance of grouts is crucial. In sulfate-rich and marine environments, as well as squeezing ground, durability of grouts is paramount. Meanwhile, for soft soil strata, loose ground, and expansive soil ground, mechanical properties of the hardened body are the most critical factors (He et al. 2020). On the other hand, various characteristics of the grouts such as permeability or hydraulic conductivity, strength, shrinkage, early-age creep and so on, could be of high importance when it comes to hydro-mechanical interactions around the tunnel. As an instance, the generation and dissipation of excess pore water pressure around the tunnel during and after the excavation could highly impact the ground deformations and axial forces as well as bending moments in the segmental linings. This is, therefore, one of the most important applications of grouting material and grouting pressure which could be designed in a way to control the mechanical and hydraulic regime around the tunnel to mitigate the possible tunnel convergence and subsequent ground deformations. Having this in mind, a vast knowledge is, thus, required in terms of tail void grouting material such as different types, various characteristics (i.e., fresh- or hardened-state properties in short- and long-term), and adaptability in different geological conditions during the mechanized tunneling procedure using various numerical-experimental approaches.

According to the abovementioned facts, a detailed overview regarding tail void grouting material has been provided in this paper, focusing on different types of grouts, rheological and geotechnical engineering characteristics, various additives which could be added to the grout mixture, different experimental tests to assess the properties of the grout, and numerical investigations focusing on the role of grouting material in mechanized tunnelling using TBMs.

2 Different Types of Grouting Material in Mechanized Tunnelling

As mentioned earlier, it is necessary to develop grouting materials with specific characteristics to address various geological challenges which may be faced during the tunneling process. Generally, the grouting material are categorized into three main groups namely cement-based grouts (CBG), lime-based grouts (LBG), and bi-component grouts (BCG). The former two are also referred to as single-component grouts (SCG) (He et al. 2020). In the cement-based and lime-based grouts, the main additives are cement and lime which are mixed with water and other materials to gain rigidity after a certain period of time upon the injection into the annular gap. Bi-component (also known as double-liquid) grouts, however, are composed of single-component materials, including ordinary Portland cement or sulphoaluminate cement, industrial residue (IR), soil additives (SA), water, and chemical additives (CA) as A liquid, and sodium silicate or water–glass (WG) and extra amount of water as B liquid (He et al. 2020). Generally, tail void grouts can also be categorized into ordinary and modified grouts based on the mentioned constituent ingredients used in their mixture (Liang et al. 2022). Performance of bi-component grouts changes dramatically after mixing A liquid and B liquid, and they can quickly set afterwards. These grouts are utilized in various geological conditions such as soft soil, water-rich strata, swelling ground and so on. Generally, LBGs have the deficiency of low strength and therefore, are not normally used in unstable strata. CBGs have low fluidity and may cause pipe blockage during the injection, and BCGs have low durability which must be improved using different additives before the injection (He et al. 2020; Liang et al. 2022). Cement-based and bi-component grouts could be used in various ground conditions such as stable, soft (unstable), and water-rich conditions. The categorization of ordinary single- and bi-component grouts as well as common additives in their mixture is illustrated in Fig. 1.

Fig. 1
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General categorization of ordinary grouts in mechanized tunneling (modified from He et al. 2020))

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It is worth mentioning that quick-lime and hydrated-lime cannot be used together in LBGs, while in CBGs, ordinary Portland cement (OPC) or sulphoaluminate cement can be mixed together in the mixture (He et al. 2020). According to Fig. 1, various additives used in the grout mixture could have a certain effect on their engineering property which will be discussed in more detail in the following section.

In another category, single-component grouts are also categorized into active, semi-active, and inert (inactive) grouts depending on the amount of cement used in their mixture. In active grouts, the cement content is usually over 200 kg/m3 while in semi-active grouts, its content varies between 50 and 200 kg/m3. No cement is used in the mixture of inert grouting material (e.g., lime-based grouts and fly-ash grouts) to decrease the risk of cement hydration or clogging (Thewes and Budach 2009). When TBMs are employed in hard rock formations without pressure at the working face, a grout injection is often used to fill the annular gap. However, this may result in the penetration of grout into the excavation chamber, causing damage. Furthermore, it is challenging to fill the crown area of the annular gap, resulting in incomplete embedment of the rings. Therefore, in hard rock formations, pea gravel is frequently used as a cover layer for segmental linings, with washed gravel of diameter between 8 and 12 mm (broken or rounded) required as grouting material. It is important to use gravel without fines to avoid clogging. A combination of mortar and gravel is also commonly used as grouting material (Thewes and Budach 2009). In other especial cases such as high-pressure condition (e.g., deep tunnels with swelling or squeezing behavior of host rock), using deformable grout is another alternative instead of using common grouts. These grouts can be compressed up to 50% and allow for more deformation of the surrounding ground in order to decrease squeezing/swelling pressure acting on the segmental linings and therefore, reducing the induced forces at the segments (Thewes and Budach 2009; Vu et al. 2020).

3 Additives Used in the Grout Mixture for Mechanized Tunneling Application

As mentioned earlier, each type of grout has a certain shortcoming which could negativly impact the tunnel behavior in short- and long-term. Therefore, to address this issue, different materials are added to the grout to enhance its performance and . Various additives are used for this purpose such as industrial residues namely fly ash, ground granulated steel slag, and silica fume; different types of soil additives such as sand, bentonite, gypsum, clay and metakaolin; as well as different chemical additives such as superplasticizers, dispersion resistance agents, setting accelerators, retarder agents, early strength agents, water-retaining agents, micro-expansive agents, and viscosity modifying admixtures (He et al. 2020). A more detailed illustration of the additives used in the modified grouts is shown in Fig. 2. It should be noted that the modified grouts are composed of ordinary grout material and selected additives used in the mixture to meet the project-specific technical requirements.

Fig. 2
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Additives used in modified grouts in mechanized tunneling (modified from He et al. 2020; Liang et al. 2022)

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To investigate the mass content of various additives and mixing ratios in different grouts, a statistical analysis was performed on different literature dealing with tail void grouting material and the resulting ratios and percentage of additives (upper and lower limits) are presented in Tables 1 and 2, respectively. It should be noted that the presented values are in evaluated in a general manner without considering the geological condition or the presence of groundwater.

Table 1 Ratios used by researchers in fabrication of tail void grouting material
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Table 2 Percentages of additives used by researchers in fabrication of tail void grouting material (values in wt %)
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Using the values obtained from the above statistical analysis, the common limit range for the constituent additives of the grouting material could be specified and used in the future experimental and field projects. Concerning the impact of the grout mixing ratio and the additives on the mechanical and hydraulic interactions around the tunnel, the optimal design of the grout should be interrelated with the geological and hydrogelogical conditions in the field. In a study done by Liang et al. (2022), typical ratios for single-component grouts were obtained based on various tunneling projects in China in terms of different geological condition (Liang et al. 2022). The upper and lower ranges obtained for grout ratios in this study is listed in Table 3.

Table 3 Grout ratios based on various ground types (Liang et al. 2022)
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In terms of ratios for bi-component grouts, on the other hand, Todaro et al. (2022), evaluated the content of the additives for various tunneling applications around the world and concluded that the most important ratio in such grouts which determines the behavior of the grout is water to cement ratio (W/C) the value of which was reported 2.62 based on various data provided from different projects. The optimum amount of accelerator (sodium silicate) was also reported equal to 7% of the total mass of the mixture (Todaro et al. 2022).

Figure 3 represents the content value of various additives used in tail void grouting material according to another statistical analysis performed on different tunneling projects as well as data provided by previous researchers in terms of ground type and presence of groundwater.

Fig. 3
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Content value of the additives in tail void grouting material for SCGs in a clay with groundwater b clay without groundwater c sand with groundwater d sand without groundwater e rock with groundwater f rock without groundwater and for g BCGs in various ground conditions

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As mentioned earlier, the addition of each ingredient illustrated in Fig. 2 could result in a different behavior in the final mixture of the grouting material. Various experimental tests have been conducted in order to determine additive-induced behavior of the grout which will be discussed in the following sections. In terms of common effects that each additive could have on the properties of the grout, it is necessary to introduce various fresh- and hardened-state properties of the tail void grouts which could be evaluated in short- or long-term.

Regarding the mixing design as well as required additive ratios in various cases, myriads of studies have focused on the additive selection and percentage of different additives in the grout mix along with field requirements of grout properties, among which, some extensive research including (Wang, et al. 2021; Peila, et al. 2015; Peila, et al. 2011; Pelizza, et al. 2010; Todaro et al. 2022; Thewes and Budach 2009; Shirlaw, et al. 2004; He et al. 2020; Liang et al. 2022) could be referred to.

4 Properties of the Grouting Material in Mechanized Tunneling

Typical properties of grouts for shield tunnelling are divided into fresh properties of grouts, mechanical properties of hardened body, scour resistance and durability of grouts. Fluidity, bleeding rate, setting time, consistency, and hydration heat are considered as fresh properties. Mechanical properties of the grout (usually evaluated in the hardened state) include compressive strength, flexural and shear strength, volume shrinkage and micro-expansibility. Scour resistance of grouts is evaluated based on pH and water-land strength ratio (WLSR). Durability of grouts include impermeability, creep behavior, and water corrosion resistance (WCR) which are usually evaluated in a long period (He et al. 2020). The fresh-state properties are generally connected with proper pumpability, flowability and filling properties of the grout which could lead to prevention of segment uplift if adjusted properly. The ratios of water to cement (W/C), binder to sand (B/S), and water to binder (W/B) could significantly impact the fresh properties of the grouting material. The addition of soil additives (SA), chemical additives (CA), and water–glass (WG) content could change the fresh-state properties of the grout to a noticeable content. These same ratios as for the fresh-state properties along with cement to fly ash (C/FA) ratio could also be influential for the mechanical properties leading to a significant change in the strength characteristics of the grout. The dosage of various additives could also impact the mechanical properties which should be evaluated based on the geological condition and tunneling application of a specific case.

As far as fresh-state properties of the grouts are concerned, fluidity is measured to evaluate the plasticity and flowability of grouts and mitigation of pipe blocking risks. It is connected with pumpability and filling property of the grout and its high values lead to segregation and segment uplift. Setting time is divided into initial setting time which is defined as the time required for the grout to start gel formation and final setting time which is the time required for the grout to form a rigid body and undergoes certain loads. Bleeding rate is used to evaluate stability (good cohesion and water locking behavior) of grouts. High values of bleeding lead to segregation. Viscosity is evaluated to measure the friction force of grouts and their rheological behavior (e.g., fluidity and stability). Consistency, on the other hand, evaluates the degree of thinness and pumpability of grouting material as well as controlling the diffusion radius of grout. High values of consistency lead to segment uplift. As for the mechanical properties of the grouting material, compressive strength, which is divided into early and late strength, is measured to evaluate the strength characteristics of the grout (usually with UCS test). Flexural strength is also divided into early and late strength and is measured to evaluate the capacity of hardened body to withstand bending moments. The ratio of compressive to flexural strength is usually in range of 0.25–0.55. Volume shrinkage is considered to evaluate volume change of hardened body. By increasing the speed of cement hydration, the volume shrinkage has an increasing trend, and its high values lead to poor filling property. With regards to the scour resistance and durability of the grouts, pH is usually measured to evaluate the stability of hardened body in water-rich media. Dissolving and strength reduction of hard body occurs if grout’s pH does not match the pH of the surrounding environment. Water-land strength ratio evaluates the scour resistance of grouts especially in water-rich strata. Different types and combinations of chemical additives are considered as a dominating factor in adjusting the WLSR. Impermeability of the grouts is measured to evaluate resistance to water pressure and water penetration after a long period, and water corrosion resistance evaluates the ability of hard body to resist ion erosion in groundwater for a long time (He et al. 2020).

Generally, LBGs have low cost, very good filling property, high fluidity, low strength and long setting time due to the lack of cement (usually higher than 20 h), low bleeding rate due to the fineness of lime particles, high volume shrinkage and high pumpability, while CBGs show a weak alkaline behavior, have shorter setting time (usually 5–15 h) high impermeability, high strength (relatively high early strength and high late strength), low fluidity, low pumpability and high WCR. Bi-component grouts, however, show highly alkaline behavior, have higher cost and indicate very short setting time due to the presence of accelerator (within seconds up to 30 min), middle to high strength, high segregation, and high filling behavior. As stated earlier, while fresh-state properties of the grouts are commonly investigated to check the workability of the grout (i.e., the mixture is transferrable and pumpable and shows a good filling behavior), the hydraulic and mechanical properties are evaluated in order to investigate the engineering application of the grout and grouting-induced interactions around the tunnel during and after the excavation procedure.

Fresh-state, hardened-state, and durability properties of tail void grouting material along with their optimum ratios and additives in each grout type are introduced in Tables 4, 5, 6, respectively.

Table 4 Fresh-state properties of the tail void grouting material
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Table 5 Hardened-state (mechanical) properties of the tail void grouting material
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Table 6 Scour resistance and durability of the tail void grouting material
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A more detailed review on the effect of additive-induced hydro-mechanical behavior through conducting various experimental tests by previous researchers is also listed in Table 13 in the appendix section. To assess the general effect of each individual additive, Table 7 summarizes Cross-effects of common materials on grout properties.

Table 7 Cross-effects of common additives on properties of the grouting material for mechanized tunneling
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5 Experimental Assessments on Grouting Material for Mechanized Tunnelling Application

As mentioned earlier, various experimental studies have been conducted in order to determine their rheological or hydro-mechanical behavior of different grout types for tunnelling purposes. The most common tests to determine the fresh-state behavior of grout are density assessment, bleeding and marsh funnel test to investigate pumpability and homogeneity of the grout, and setting time test to evaluate the hydration time of grout. These tests are commonly performed on the fresh samples prior to the hardened-state evaluation. On the other hand, various experiments are performed to evaluate the hardened-state behavior of tail void grouting material such as permeability assessment, unconfined compressive strength (UCS) test to evaluate the mechanical characteristics of the grout, triaxial or direct shear tests to determine shear strength properties of grout, small- or large-scale oedometer or dewatering tests to estimate the dewatering (consolidation) characteristic of the grout under a certain confined pressure (i.e., injection pressure at TBM tail or earth pressure acting on the injected grout) and also to investigate of consolidation-induced volume shrinkage.

In terms of sample preparation for the testing purposes, usually for the single-component grouts, bentonite is hydrated for a certain period (24–48 h) and then, it is added to the dry mixed ingredients (cement, soil additives, and so on). The final mixture is prepared after a proper mixing of the combined materials. For bi-component grouts, however, the A component (hydrated bentonite, cement, retarder, and so on) is mixed with the B component and mixed properly to obtain the final mixture. The mixing of the two components usually takes places very close to the testing time for the experimental approaches, or the injection time during the mechanized tunneling procedure. Depending on the aim of investigation, samples are sealed or cured in wet or dry conditions for a certain time before conducting the tests (see Fig. 4).

Fig. 4
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Sample preparation prior to the experimental assessment of the tail void grouting material

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Regarding the mixing of the material, manual mixing or mixing through a propeller (stirring machine) is carried out. However, for mixing the component A and B in bi-component grouts, Di Giulio et al. (2020), designed a new mixing system to simulate a more realistic injection procedure of the grout behind the TBM. The system comprises of a pair of cylindrical containers that are connected to two pipes at their lower end. These pipes converge at the end of a brief section, which includes a static mixer of approximately 15 cm in length. Each component, A and B, remains separate within their respective containers and pipes until they blend in the final segment before injection directly into the molds. The system incorporates valves to regulate the dosing of components and can apply adjustable pressure to the outlet flow. TBM injection systems in actuality, with variations dependent on the supplier, include two lines that converge in a final section of the pipe, where the blending of the components occurs without the static mixer just before the injection towards the end of the lining segments. The laboratory system differs from the real one in various ways; however, in comparison to manual mixing, it provides a more realistic means of sample preparation (Giulio et al. 2020). The schematic view of the system is shown in Fig. 5.

Fig. 5
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New mixing system for bi-component sample preparation (Giulio et al. 2020)

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A summary of some experimental tests performed on the grouting material to determine their hydraulic and mechanical characteristics (which is of great importance for mechanized tunneling applications) by previous researchers and the representing results is listed in Table 8.

Table 8 Experimental tests for hydro-mechanical evaluation of the grouting material
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As far as the property requirements of the tail void grouting material for mechanized tunneling application is concerned, various researchers have investigated the performance requirements based on experimental tests or data analysis on various tunneling projects. Liang et al. (2022), performed a series of experimental tests on cement-based grouts (CBG) to evaluate their performance requirements in terms of fresh- and hardened-state properties under various geological conditions (Liang et al. 2022). The representing results of their work are presented in Table 9. It should be noted that based on authors recommendation, this table could only be considered for modified grouts under non-ordinary conditions (e.g., water-rich media).

Table 9 Performance requirement of CBGs in various ground types for mechanized tunneling application (Liang et al. 2022)
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As mentioned before, Todaro et al. (2022) on the other hand, represented a detailed investigation regarding the mix composition and requirements of bi-component grouts through investigating various tunneling projects (Todaro et al. 2022).

6 Numerical Simulation of Mechanized Tunneling Considering the Role of Tail Void Grout

Concerning the crucial role of grouting material in controlling the hydro-mechanical regime around the tunnel in short- and long-term, numerical modeling could also be adopted to investigate the grouting-induced interactions in terms of deformations, pore pressures and lining responses in mechanized tunneling applications. Various researchers have conducted 2D or 3D numerical investigation of mechanized tunneling with an emphasize on the grouting material to evaluate its specific impact in tunneling procedure. A summary of numerical packages carried out by some researchers have been presented in Table 10.

Table 10 Summary of numerical simulation procedures to evaluate the role of grouting material in mechanized tunneling
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According to Table 10, the method of simulating the grout pressure and grouting material behavior significantly impacts the model results in terms of deformations and hydraulic regime around the tunnel as well as lining responses. This justifies the great attention to simulate a detail-oriented mechanized tunneling procedure with especial attention to the role of grouting material and grouting-induced interactions around the tunnel. Some insights that should always be noticed during the simulation procedure is that during the advancement of TBM, the grouting material usually migrates along the TBM shield where it pushes the bentonite slurry towards the face. The space between shield and ground (i.e., shield annulus gap) is, therefore, filled with flowing pressurized material which generates a pressure regime around the shield known as shield annulus pressure (Bezuijen 2007). This is the reason why most of the researchers do not consider the TBM shield in their model and instead, apply a radial pressure along the tunnel circumference usually smaller than face pressure and/or with a stepwise increase both vertically (as a result of slurry density) and to the opposite direction of TBM advancement (as a result of grout migration along the shield) (Bezuijen 2007; Epel et al. 2021). The grouting pressure is applied to the tunnel circumference as well as extrados of linings behind the TBM. However, this pressure is not limited only to the section behind the TBM where the fresh grout is injected. According to the studies, the grouting pressure is still active in the previously installed segments where the grouting material is under the hydration process and thus, this pressure is under a stepwise decrease as TBM moves further away until it fully dissipates as the grout reaches its full hardened phase. The effective length for this pressure could vary depending on the type of grout and its speed of hardening (Bezuijen 2007; Epel et al. 2021; Talmon and Bezuijen 2009; Bezuijen et al. 2009).

7 Conclusive Remarks

The choice of grouting material is crucial in tunnel excavation using TBMs due to its significant role in tunnel support and controlling temporal and spatial deformations and excess pore pressures. A variety of grout types have been developed and used in different geological conditions. Depending on the ground behavior and tunneling application, either ordinary or modified grouts may be used to act as a sealing and/or stabilizing cover for the tunnel. Several researchers have conducted review studies and experimental tests on different grout types, their fresh- and hardened-state properties, and their constituent materials. A summary of various grout types, along with important ratios and percentages of various additives required for grout fabrication under different geological conditions, was presented based on a statistical analysis. Furthermore, a summary for the sample preparation, experimental tests performed on different grout types to evaluate their hydro-mechanical properties (which is of high importance in engineering aspects of tunneling), and grout performance requirements was presented and at the end, a literature review on the numerical simulation techniques adapted by different researchers was introduced.

Since various additives could impose different simultaneous effects on the grout behavior (also see Table A1), it is of great significance to evaluate the coupled effects caused by different additives in the grout mixture to better understand the grouting-induced interactions around a mechanized tunnel in short- and long-term. For this purpose, Table 11, indicating the increasing and decreasing coupled effect of common additives in the grout mix is achieved as a result of extensive literature review performed on previously conducted experiments and field applications.

Table 11 Coupled effect of various additives on hydro-mechanical behavior of the grouting material
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Taking into account the coupled effects of different additives as presented in the above table, one could suggest various grout mixtures depending on the tunnel application, geological and engineering property of the excavating area and other technical factors to mitigate the potential risks and damages that could emerge during or after tunnel construction. As an instance, Table 12 is presented suggesting different grout compositions with respect to the ground situation in mechanized tunneling. Table 12 indicates that the selection of grout mix materials may vary depending on the geological conditions. For instance, when dealing with soft ground, where controlling subsequent surface settlements during and after tunnel excavation is of utmost importance, specific additives should be employed to improve the early strength of grout and decrease its setting time. It is also crucial to prevent volume shrinkage of the grout after hydration to minimize surface settlements. Various materials could be used for this purpose, including sand to increase the strength of hardened body and reduce the setting time, water–glass as an accelerator to further reduce the setting time (more effective when mixed with metakaolin, steel slag, and silica fume), early strength agent such as Triethanolamine to improve the early strength, and expansive agents to prevent volume shrinkage. Bentonite is useful in reducing the setting time, and superplasticizers can be used in case of high cement content to enhance the flowability and workability of the grout and prevent pipe blockage. Other crucial factors related to grout fabrication based on the geological conditions and the common grout type for each medium as well as the impact of various additives are already discussed in the previous sections.

Table 12 Material suggestion for grout mixtures in mechanized tunneling based on ground conditions
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