Reusing Fine Silty Sand Excavated from Slurry Shield Tunnels as a Sustainable Raw Material for Synchronous Grouting

Abstract

Using the Nanjing Dinghuaimen Yangtze River Tunnel project as a case study, we proposed a method to reuse the excavated silty-fine sand by adjusting the proportion of the waste sand to replace the commercial sand. This would address the issue of recycling the significant amount of waste sand generated when the slurry shield passes through the silty-fine sand stratum. Moreover, we have evaluated grout indicators such as density, fluidity, consistency, bleeding rate, volumetric shrinkage, setting time, and unconfined compressive strength and examined how the particle size and distribution of the sand affected the grout’s performance. The findings show that as the replacement ratio increases, the grout’s density, fluidity, consistency, and bleeding rate gradually increase; meanwhile, the volumetric shrinkage increases initially before decreasing; the setting time decreases gradually; the unconfined compressive strength initially decreases before increasing. The key factor altering the grout’s performance when the replacement ratio is less than 50% is the weakening of the adsorption effect of fine sand particles on water due to the increase in the sand’s fineness modulus. When it is greater than 50%, the particle size of the sand tends to be distributed nonuniformly and fine particles fill the voids between larger particles, thus contributing to the changes in grout properties.

1. Introduction

In recent years, the slurry shield tunnelling method has been widely used in the construction of river-crossing tunnels and submarine tunnels under regions with complex hydrogeology [1,2,3]. Examples include the Tokyo Bay Tunnel [4], Nanjing Yangtze River Tunnel [3,5], Tuen Mun-Chek Lap Kok Tunnel [4,6] and other projects [7,8,9]. With the slurry shield method, slurry pressure is applied to effectively balance the stratum soil pressure and water pressure and maintain the stability of the excavation surface [10,11,12]. A large volume of soil is cut by the shield cutterhead, mixed with the slurry, and then discharged to ground slurry treatment equipment via the slurry system. After treatment, the discharged soil and slurry are separated; the slurry is recycled and the excess soil is discarded and stacked. In addition to consuming a lot of land resources, this also readily leads to secondary pollution, which is bad for the environment. The waste residue itself is a form of resource, and the methods of transportation and landfill have resulted in significant resource waste. Therefore, one of the project’s main concerns is how to implement the environmentally friendly treatment and resource usage of a substantial amount of waste residue.

Generally, the abovementioned discharged soil is disposed of by stacking it in a yard at relatively high cost, which depletes land resources [13,14,15]. In addition, the properties of the slurry have a great impact on shield tunnelling. To obtain a high-quality slurry, polymer materials are added. Despite the slurry treatment system, residues remain in the discharged soil, which adversely affect the surrounding environment. The fine particles (mean diameter < 0.075 mm) in the discharged soil after slurry separation and treatment can be used as slurry materials, while larger particles (such as fine silty sand) can be used as a sustainable raw material for synchronous grouting materials [13,16,17]. Such recycling has economic and environmental significance. Therefore, it is important to study the engineering properties of recycled discharged soil and its application in synchronous grouting for tunnel construction.

Most research on shield synchronous grouting materials has focused on its engineering properties and performance during the processes of (1) injection of synchronous grouting material into a shield tail void and (2) volumetric deformation of the grout itself after injection. Traditional shield synchronous grouting materials [18,19] have been improved by adding polymers or other materials. To understand the deformation induced by consolidation after the grout is injected into a shield tail void, the variations in grout pressure and volume in the shield tail void have been studied by field testing/monitoring [20,21] and laboratory tests [22,23]. The consolidation equations of grouts have been derived thusly. These studies have promoted our understanding of synchronous grouting techniques and their functions and provide a theoretical basis for optimizing grout for shield tunnel construction. With the extensive engineering application of shield tunnelling, the disposal of excavated soil is becoming an even more urgent issue. Some studies [24,25,26] have focused on the recycling of such discharged soil, and corresponding recycling schemes have been proposed.

Research on the recycling of excavated soil from shield tunnelling and its applications in shield synchronous grouting material has mainly focused on the factors affecting the engineering properties and composition of grout (such as its water–binder and soil-sand ratios) when excavated soils are used as a raw material. Sand is used as a fine aggregate in shield synchronous grouting material and comprises >50% of the material’s volume. Hence, the characteristics of sand have a significant impact on the engineering performance of the grout. The main sand characteristics are the particle size and its distribution; however, there is little research on the influence of such characteristics on shield synchronous grouting material properties. Given the similarities between shield synchronous grouting material and cement mortar, research on the effects of sand properties—particularly the fineness modulus and particle size distribution—on the performance of cement mortar was used as a reference. Previous studies on cement mortar have shown that the fineness modulus of sand has a significant impact on the water retention capability and strength of the mortar [27]. Sand with different fineness moduli and relative specific surface areas are correlated with the water consumption of the mortar [28,29]; meanwhile, the fineness modulus of sand also affects the rheological properties of concrete [30]. Despite the similarities between the shield synchronous grouting material and the cement mortar [31] and the grout used in jet grouting methods [32], the construction technology of shield synchronous grouting material requires higher workability and pumpability; and the main function is to limit the ground settlement to an acceptable level, which requires it to have good filling property.Therefore, the influence of sand characteristics on the engineering properties of shield synchronous grouting material and the volumetric change after its injection into a shield tail void require further study.

This study investigates the feasibility and technical aspects of recycling excavated silty sand from the silty sand stratum of the Nanjing Dinghuaimen Yangtze River Tunnel. This study proposed a specific method to reuse the waste sand as the raw material for synchronous grouting and analyzed the influence of sand properties on the grout’s performance. The findings may serve as a foundation for the design of synchronous grouting and serve as a guide for similar projects.

2. Project Overview

2.1. Project Site

Nanjing is located in the Yangtze River Delta, China. The Yangtze River divides the city into two parts. Figure 1 shows the location of the Nanjing Dinghuaimen Yangtze River Tunnel project. Located 5.0 km downstream of the Nanjing Yangtze River Tunnel and 4.5 km upstream of the Nanjing Yangtze River Bridge, the tunnel connects the main urban area of Nanjing and the planned new urban centre of Pukou. The tunnel adopts an eight-lane X-shaped scheme and was designed as a double-layer bidirectional eight-lane tunnel. The tunnel is divided into two lines: a northern line with a total shield section length of 3537 m and a southern line with a total shield section length of 4135 m. Two slurry shields with diameter of 14.96 m were used to conduct tunnel excavation in the same direction from an originating well to the north of the Yangtze River. The total construction cost is RMB 5.2 billion. It is one of most challenging large-diameter slurry shield tunnelling projects crossing the Yangtze River.

2.2. Geological Conditions

The tunnel runs through the Yangtze River’s alluvial plain, which mostly consists of the river’s water area and central bar as well as its waterside beach, high landside floodplain, and low landside floodplain. Except for the levees on both sides of the Yangtze River, the proposed tunnel does not pass through any major roadways; all of the crossing areas are farmland and levees. The Nanjing Yangtze River levee, a high-level levee, is the main water conservancy facility that the tunnel passes through. The stability of the bank slope and levee is ensured by the employment of dry masonry slope protection and mortar masonry toe protection close to the waterside of the levee. In the passage area of the tunnel site, there are no fractures or fracture zones.

The upper part of the stratum through which the shield tunnel passes is filled with soil and quaternary Holocene alluvial soil consisting of muddy-silty clay, silty clay, silt, and silty sand. The middle stratum is composed of medium-to-dense silty-fine sand of the Quaternary Holocene. The lower stratum is dense gravel sand and round gravel of the Upper Pleistocene. The rock stratum is mainly Cretaceous mudstones and silty mudstone, as shown in Figure 2. The geological profile of the northern line is shown in Figure 2; the specific length of the southern line shield passing through each stratum is shown in

Table 1. Lengths of the southern line shield passing through each stratum (m).

Soil Stratum	Muddy-Silty Clay	Muddy-Silty Clay and Silty-Fine Sand	Silty-Fine Sand	Silty-Fine Sand, Gravel Sand, and Pebbles	Gravel Sand and Pebbles	Gravelly Sand, Pebbles, and Moderately Weathered Sandstone
Length	870	345	1670	330	380	540

. It can be seen that the shield section passing through the silty-fine sand stratum comprises about 45% of the total length. The grain size distribution of the sand in the silty-fine sand stratum is shown in Figure 3; 85% of particles are <1.25 mm, while the content of clay particles is only 2%. The particle size distribution is suitable for use as shield synchronous grouting material.

3. Materials and Experimental Methods

3.1. Materials

The test materials included pulverized limestone, fly ash, sand, water, bentonite, and a water reducer. The content of calcium hydroxide in the hydrated lime was 89% by dry weight. The fly ash was grade-III fly ash from Nanjing Xiaguan Power Plant. The bentonite was grade-I sodium bentonite from Nanjing Tangshan Bentonite Co., Ltd. (Nanjing, China). The water reducer was HLC-NAF high-efficiency water reducer from Nanjing R&D Hi-tech Co., Ltd. (Nanjing, China). The main chemical compositions of the cement, fly ash and bentonite, are shown in

Table 2. Main chemical compositions of the cement, fly ash, and bentonite (%).

Chemical	SiO2	Al2O3	Fe2O3	CaO	MgO	SO3
Cement	21.50	5.80	2.70	63.00	2.09	3.55
Fly ash	52.70	30.50	4.32	6.38	1.46	0.72
Bentonite	70.76	14.93	1.89	0.96	1.77	-

, and the characteristics of the cement and fly ash are shown in

Table 3. Characteristics of the cement.

Requirement of Normal Consistency (%)	Setting Time (h:min)		Flexural Strength (28 Days; MPa)	Compressive Strength (28 Days; MPa)
	Initial Setting Time	Final Setting Time
25.0	2:35	3:50	8.1	38.4

and

Table 4. Characteristics of the fly ash.

Fineness	45 μm Sieve Residue	Loss on Ignition (%)	Water Demand Ratio (%)
25.6	14.6	1.53	101

. The grain size distribution of the original sand used in the project, the excavated silty-fine sand, and the excavated silty-fine sand passing through a 1.25 mm sieve are shown in Figure 4.

The shield synchronous grouting material used in the northern line was selected as the test material. Its composition is shown in

Table 5. Composition of shield tunnel grout (kg/m³).

Pulverized Limestone	Fly Ash	Sand	Bentonite	Water Reducer	Water
60.0	390.0	950.0	120.0	4.0	417.0

. This grout is inert because it does not contain cement.

3.2. Test Program

The excavated sand was first screened through a 1.25 mm sieve and then dried in an oven for 24 h for standby. The excavated sand was mixed with the original sand used in the northern line at different ratios by weight, and the water–binder ratio was adjusted by adding different amounts of tap water. Then, the mixture was stirred uniformly with a stirrer to form the grouts. All the steps in this process were performed at room temperature (25 ± 2 °C). Fifteen groups of shield synchronous grouting materials were prepared according to the test plan presented in

Table 6. Test plan.

No.	Replacement Ratio	Water–Binder Ratio	Fineness Modulus	Test Item
A1 A2 A3	0	0.92	0.781	Density; Consistency (0 h, 3 h); Setting time; Fluidity (0 h, 3 h); Bleeding rate; Volume shrinkage (under 0.3 MPa); Unconfined compressive strength (days 7, 28).
		0.85
		0.80
B1 B2 B3	0.25	0.92	1.046
		0.85
		0.80
C1 C2 C3	0.50	0.92	1.210
		0.85
		0.80
D1 D2 D3	0.75	0.92	1.372
		0.85
		0.80
E1 E2 E3	1.00	0.92	1.652
		0.85
		0.80

. The water–binder ratio is defined as the mass ratio of water to cementitious material (the sum of pulverized limestone and fly ash). The density, consistency (0 h, 3 h), setting time, fluidity (0 h, 3 h), bleeding rate, volume shrinkage (under 0.3 MPa), and unconfined compressive strength (days 7, 28) of the grouts were tested.

3.3. Test Methods

The fluidity of the shield synchronous grouting material was measured following Chinese Standard GB/T2419—2005 [33]. The consistency, unconfined compressive strength, and setting time of the grouts were measured according to Chinese Standard JGJ/T70—2009 [34]. To investigate the variation in the engineering properties of different grouts, their consistency and fluidity were tested at ages of 0 h and 3 h. The bleeding rate was determined following Chinese Standard GB/T 50080—2002 [35]. The volume shrinkage of the grout was measured by one-dimensional consolidation tests, as shown in Figure 5. The consolidation tests were carried out on specimens with a diameter of 75 mm under a pressure of 300 kPa. The maximum consolidation volume compression was measured, and its ratio in relation to the total volume was taken as the volumetric shrinkage of the shield synchronous grouting material.

4. Results

4.1. Changes in Grout Properties

The density of grout is an important influence on the stability of a shield tunnel at the early stage. Figure 6 shows that the densities of the 15 groups of grouts in the test were basically within the range of 1.90–2.00 g/cm³. At a given water–binder ratio (w/b), the density increased with the replacement ratio. If the proportion of excavated silty-fine sand was <50%, the density remained almost constant, while a slight increase was observed at replacement ratios >50%. As the water–binder ratios used in this study only varied slightly, the ratio had little influence on grout density.

Fluidity and consistency can be used to evaluate the potential to pump shield synchronous grouting material. In this study, the initial fluidity, fluidity after 3 h, initial consistency, and consistency after 3 h of the shield synchronous grouting were measured, as shown in Figure 7 and Figure 8. The initial fluidity of the grout was relatively high, being >23.0 cm for all 15 groups of test materials. As the replacement ratio increased, the fluidity tended to increase. The growth rate in fluidity is faster at replacement ratios >50% than at ratios <50%. The grout fluidity was measured 3 h after preparation. As shown in Figure 7, the grout fluidity at this time was lower than the initial fluidity. Moreover, the variation in fluidity with replacement ratio exhibited a similar pattern to that of the initial fluidity. As shown in Figure 8, the variation in grout consistency with increases in the replacement ratio was similar to that of fluidity.

The bleeding rate is an index reflecting the solid–liquid separating property of a shield synchronous grouting material. The smaller the bleeding rate, the better the stability of the grout. As shown in Figure 9, the bleeding rates of the prepared grout were <5%. At a given water–binder ratio, the bleeding rate of the grout increased linearly with the replacement ratio of excavated fine-silty sand. At a given replacement ratio, the grout bleeding rate increased with the water–binder ratio.

Volumetric shrinkage is an index for evaluating grout shrinkage under pressure. A lower grout volume shrinkage also reflects lower ground subsidence. Figure 10 shows the shrinkage deformation rate of the shield synchronous grouting material. The volumetric shrinkage of the prepared grouts was <20%, which is low. At a given water–binder ratio, the grout volumetric shrinkage increased with excavated silty-fine sand replacement ratios of <50% but decreased with ratios >50%.

The setting time is an index of the time it takes for the grout to become solid. It should be determined according to the stratum, for example, for a high-permeability stratum, it should be short. If the setting time is too short, pipe blocking or incomplete filling might occur, while a long setting time is not conducive to the stability of a shield tunnel at the early stage. The variations in the setting times of the prepared grouts are shown in Figure 11. Due to the grout being inert (no cement component), the setting times are 45–65 h. The replacement ratio of the excavated silty-fine sand influences the setting time of the shield synchronous grouting material. At a given water–binder ratio, the setting time decreases with the replacement ratio.

The strength soon after injection into a shield tail void is an important property of shield synchronous grouting material and is of great significance to the stability and antiseepage properties of a shield tunnel. Therefore, shield synchronous grouting material of a certain strength is required at the early stage. Generally, it is advisable to use shield synchronous grouting material with an unconfined compressive strength (UCS) close to that of the surrounding rock–soil mass. The UCS of the grout was tested on days 7 and 28, as shown in Figure 12. The UCSs of the prepared grouts were low, being lower on days 7 than on days 28, while they did not differ much for grouts with different water–binder ratios of the same age. The UCSs of grouts with the same water–binder ratio decreased first and then increased with increases in the replacement ratio of excavated silty-fine sand.

4.2. Selection of Grout Formula

According to the test results, when the proportion of excavated silty-fine sand accounted for 25% of the total sand (B1, B2, B3), the UCS (days 7, 28) was less than that of the original-formula grout (A1, A2, A3). The volume shrinkage was greater than that of the original, while the other engineering properties were similar. With 50% excavated silty-fine sand (C1, C2, C3), properties such as fluidity and consistency were higher than those of the original formula, while the volume shrinkage, bleeding rate, and strength were lower. At 75% (D1, D2, D3), the engineering properties other than bleeding rate were close to or slightly higher than those of the original formula. When all the original sand was replaced by excavated silty-fine sand (100%; E1, E2, E3), the bleeding rate of the grout was high, the setting time was short, and the other engineering properties were close to or slightly higher than those of the original formula. Therefore, grout D was selected as the basic grout and one of the three grouts was selected as the new grout formula.

The engineering performance of grouts A1, D1, D2, and D3 are shown in

Table 7. Engineering performance of grouts A1, D1, D2, and D3.

No.	Density (g/cm3)	Consistency (cm)		Fluidity (cm)		Bleeding Rate (%)	Setting Time (h)	Volume Shrinkage (%)	UCS (MPa)
		Initial	3 h	Initial	3 h				7 Days	28 Days
A1	1.92	11.0	9.4	21.2	18.7	1.7	62.5	16.5	0.21	0.35
D1	1.96	13.1	11.7	26.3	21.7	3.6	52.0	17.2	0.25	0.36
D2	1.96	12.6	10.7	25.2	20.0	3.6	49.0	16.1	0.25	0.38
D3	1.97	12.2	10.1	23.8	18.9	3.0	45.5	14.8	0.26	0.38

. Although the consistency and fluidity of grout D3 were the lowest among the four grouts, the differences were small, and the engineering requirements are still satisfied. The bleeding rate and volume shrinkage of grout D3 were also the lowest among the three grouts, which is very important for shield synchronous grouting materials. Therefore, grout D3 (75% excavated silty-fine sand) was selected as the optimum grout formula.

5. Discussion

This study found the main influences on the engineering properties of shield synchronous grouting material to include the sand’s fineness and particle size distribution and the water–binder ratio. The former two are varied by adjusting the proportion of excavated silty-fine sand, while the latter is varied by adjusting the water consumption in the original formula.

5.1. Influence of Fineness Modulus

The mixed sand with a different proportion of excavated silty-fine sand, which screened through a 1.25 mm sieve, was screened to calculate the fineness modulus. Figure 13 shows the relationship between the fineness modulus of the mixed sand and the replacement ratios. It is indicated that the fineness modulus of mixed sand increases with the replacement ratio.

The replacement ratio (x-axis in Figure 13) could be directly replaced by the fineness modulus for discussion in Figure 6, Figure 7, Figure 8, Figure 9, Figure 10, Figure 11 and Figure 12. As the fineness modulus of the sand increases, the density of the grout tended to increase, while the range of variation in the density of the grout is not high. This indicates that the fineness modulus of the sand has little influence on the density of the shield synchronous grouting material.

The fluidity, consistency, and bleeding rate of the grout increase with the proportion of excavated silty-fine sand in the total sand. However, the growth rates in fluidity and consistency are different in different regimes (replacement ratios < or >50%). This is because, with increases in the proportion of excavated silty-fine sand, the sand particle size increases, the fineness modulus of the sand increases, the total surface area of particles reduces, the adsorption of water by particles decreases, and the free water around particles increases so that the fluidity and consistency of the grout tend to increase. With <50% excavated silty-fine sand, the sand grain size is relatively uniform so the increases in the grout’s fluidity and consistency are mainly affected by the sand’s fineness. At >50%, the sand grain size tends to be nonuniform and fine particles fill the voids between larger particles. This reduces friction between them, thus further increasing the fluidity and consistency of the shield synchronous grouting material. Due to the range of variation in the grout bleeding rate being small, only the fineness modulus has a significant influence on this variation.

With increases in the proportion of sand that is excavated silty-fine sand, the volumetric shrinkage of grout (under a pressure of 0.3 MPa) first increases and then decreases, while the UCS first decreases and then increases. At an excavated silty-fine sand ratio of <50%, with increases in fineness modulus, the grain size of sand particles increases, which leads to increases in void spaces between larger particles. The fine particles cannot effectively fill these pores, so the volumetric shrinkage of grout increases and the UCS decreases. When the ratio reaches 50%, more fine particles fill the voids between larger particles. The increased contact points between larger particles form a skeleton so that the volumetric shrinkage of grout gradually decreases and the UCS gradually increases. The grouts prepared in this study did not contain cement or other materials that could induce a rapid hydration reaction. The fly ash contents were the same in different shield synchronous grouting materials, so the influence of the grout’s chemical reactions on setting time can be ignored. With increasing sand particle size, the liquid resistance of particles per unit volume decreases, so the contacts between particles are increased, resulting in a shorter setting time for the shield synchronous grouting material.

5.2. Influence of Grain Size Distribution

Based on the above analyses, the fineness of the sand in the shield synchronous grouting material (fineness modulus) has an obvious influence on the grout’s engineering properties. The sand used in the original formula of shield synchronous grouting material in the project was fine sand with a relatively uniform particle size. The silty sand excavated from the stratum has a certain size distribution.

Mixed sand samples with different amounts of excavated silty-fine sand from the silty-fine sand stratum were screened to obtain their grain size distributions (Figure 14). The figure shows that the grain size distribution of the sand in the grout changed gradually as the amount of added excavated silty-fine sand increased. The nonuniformity coefficient C_u was calculated for five mixed sand samples (

Table 8. Indexes of fine sand samples.

Sample	Fineness Modulus	Cu	Cc
A	0.781	1.817	0.90
B	1.046	1.961	0.91
C	1.210	2.352	0.85
D	1.372	3.263	0.73
E	1.652	3.853	0.86

). The C_u value of the sand used in original grout formula was 1.817, and that of the excavated silty-fine sand with particles >1.25 mm removed was 3.853. As the amount of added excavated silty-fine sand increased, the C_u value of the mixed sand gradually increased within this range. Based on the calculation of the curvature coefficient C_c, the C_c values of all sand samples were <1.0, which indicates that the grain size distributions of the sand became increasingly uneven. However, since the C_u values are <5.0, the grain sizes of the mixed sands still have poor grading and lack particles of intermediate sizes.

In summary, the sand used in the original grout formula was fine with a relatively low fineness modulus and uniform particle size. When a small amount of excavated silty-fine sand was added, the fineness modulus of the mixed sand increased, and the engineering properties of the grout changed accordingly. The grain size distribution of the sand samples did not differ much from that of the original sand. Therefore, changes in the grout’s engineering performance were mainly influenced by increases in the sand particle size. As the amount of excavated silty-fine sand increased, the grain size distribution of the mixed sand gradually became uneven. The number of large particles in the mixed sand gradually increased and formed voids, while small particles in the sand filled the voids between large particles and reduced rolling friction, thus intensifying the changes in the engineering performance of the shield synchronous grouting material.

5.3. Economic Analysis and Environmental Benefits

Generally, the cost of the synchronous grouting material has become a concern for the construction unit. The calculation parameters for Equation (1): outer diameter of shield D₁ = 14.96 m; outer diameter of segment D₂ = 14.5 m; ring width B = 2 m; injection rate α = 150%; and injection volume of a single ring = 31.9 m³. In other words, the grouted weights per ring of pulverized limestone, fly flash, bentonite, river sand, water reducer, and water were approximately 1.914 t, 12.444 t, 3.829 t.302 t, 30.314 t, 0.128 t, and 13.306 t, respectively, according to the local market price of the raw material (river sand about RMB 250 per ton). Thus, this method decreases the cost of raw materials by RMB 5683.9. Moreover, taking into account the collection and treatment costs of the excavated silty-fine sand (about RMB 40 per ton), this method decreases the collection and treatment cost by RMB 909.42. As a result, the total costs are decreased by 6593.32 RMB/ring by taking the recycling option for the excavated silty-fine sand.

$$V=\frac{\pi }{4}\left(D_1^2-D_2^2\right)B\alpha$$

(1)

At the time of the study, there were about 1200 rings left, and the total injection volume was about 38,280 m³. Considering the collection and treatment costs of the excavated silty-fine sand, the replacement according to the D3 scheme was expected to save about RMB 7.911 million.

Furthermore, the recycling of the excavated silty-fine sand from the construction site enhances a portion of the environmental benefits, and it includes the following three aspects: (1) the accumulated excavated silty-fine sand will occupy the construction site, which will influence the smell of the surrounding environment and blockages of the drainage system; (2) the transport of excavated silty-fine sand to spoil area will causes adverse an environmental impact to the surroundings, such as CO₂ emissions, soil splash, and raised dust; and (3) the accumulated excavated silty-fine sand in the spoil area will result in a potential source of landslides and debris flows [36].

Therefore, it is significant to reuse the excavated silty-fine sand from the silty-fine sand stratum as the raw material for shield synchronous grouting material.

6. Conclusions

The following conclusions can be drawn:

(1)

The engineering performance of the shield synchronous grouting material in which 75% of the sand in the original formula was replaced by excavated fine-silty sand screened through a 1.25 mm sieve, and in which the water/binder ratio was adjusted to 0.8, is close to or slightly higher than that of the original formula. Hence, it can be used in the recycling scheme.

(2)

As the replacement ratio increases, the grout’s density, fluidity, consistency, and bleeding rate gradually increase; meanwhile, the volumetric shrinkage increases initially before decreasing; the setting time decreases gradually

(3)

The sand used in the original formula was fine sand with a small fineness modulus and uniform particle size. At a mixing amount of <50%, a small change in the grain size distribution does not affect the engineering performance of the shield synchronous grouting material. However, the fineness modulus is the main influencing factor. At mixing amounts >50%, the grain size distribution changes greatly and has a leading influence on the shield synchronous grouting material’s performance.

(4)

Recycling the excavated silty-fine sand as shield synchronous grouting material can not only greatly reduce engineering costs but also significantly reduce environmental pollution, thereby promoting sustainable development.