Keywords

1 Introduction

Rapid growth of urban rail transportation projects, the subway shield construction has become a popular choice due to its distinct advantages [1]. During the construction of Earth Pressure Balance (EPB) shield tunnels in Beijing, the team regularly runs into difficulties due to the sandy gravel soil [2, 3]. The natural state of the excavated soil often lacks the required plastic fluidization properties [4, 5], which can lead to various issues such as excavation surface instability, pressure chamber closure [6, 7], pressure chamber caking [8], and water ingress [9]. In order to optimize the functionality of shield tunneling, the soil must possess suitable plasticity and a low permeability [10, 11]. Muck improvement technology, a key aspect of EPB shield construction, has gained increasing attention in shield research [12]. Currently, foam agents, dispersants, clay minerals, and flocculants are the most common types of improvement additives. Clay and bentonite are frequently utilized minerals [13], which are typically used to supplement fine particles and lower the soil's internal friction angle within the pressure chamber [14]. Foam agents, made up of particular foam agents with compressed air, enhance the fluidity and impermeability of the excavated soil. Dispersants, on the other hand, dispersing material in water to generate colloidal solutions, thereby diminishing particle adhesion [15]. Many experts have studied into the types and amounts of soil improvers that affect the construction of EPB shields, primarily through indoor tests. For example, Wang [16] conducted field experiments to assess the efficacy of nano bentonite CMC as a muck improver in a tunnel project in Fuzhou. Zhu and others [17,18,19,20] assessed the earth pressure shields’ stability in layers of sand and gravel, proposing measures to enhance fluidity and wear resistance using DEM modelling. Chao [21] carried out laboratory studies to find out how modified bentonite grouting affected the shield driving in layers of sand and gravel. Xu and others [22, 23] added air bubbles to soil with different moisture contents to minimize power consumption during mixing. Regardless of these insightful observations, there are still not enough theoretical methods and guidelines for analysing improver properties, matching improvers with improved soil, evaluating improver effectiveness, and understanding the connection between shield construction parameters and improvers. This lack of comprehensive guidance has led to the haphazard use of improvers in shield tunneling, potentially resulting in issues like excessive environmental pollution and material consumption.

Hence, this research conducted to develop a new kind of mud improver and studies the soil improvement techniques specific to the characteristics of Shenzhen stratum focusing on the sandy gravel stratum, drawing on lessons learned from the Shenzhen Metro Line shield tunneling project. The research involves laboratory test that uses bentonite as the mud base with several additives, including xanthan gum, guar gum, CMC, and medium particles of clay, to analyse their impact on mud performance. The research aims to identify the best-performing material for enhancing mud properties. Using two distinct varieties of mud, subsequent indoor simulated muck improvement tests are conducted, which include sliding plate tests, mixing tests, adhesion resistance tests and slump tests. The research investigates the comparative efficacy of novel and conventional mud amendments in pebble/round gravel and granular soil strata.

2 New Mud Modifier

In EPB shield construction, the excavation surface stability depends on the pressure within the enclosed cabin. Infiltration of mud into the soil results in the formation of a thin mud layer that has a low permeability. Through this mud film, the mud pressure efficiently works on the excavation surface, maintaining stability and avoiding deformation and collapse. A significant challenge in EPB shield construction is selecting an appropriate muck modifier tailored to the engineering geological parameters. To address this, the research investigated the construction site's mud configuration plan [24]. Pure bentonite is the on-site mud with a bentonite concentration of 9%. One cubic meter of water is specifically mixed with 90–120 kg of bentonite as presented in Fig. 1 Schematic diagram of experimental steps. However, this mud exhibits high consistency and poor flowability. Even after standing, it remains highly viscous and resistant to flow [25]. Additional additives are considered in order to decrease the quantity of bentonite utilized, thereby enhancing the overall efficacy of the mud and lowering cost. The purpose of this study is the development of a unique soil amendment by combining water-soluble polymer improvement materials with mineral improvement materials. Bentonite is the major raw material that is used in the production of the mineral modifier. More precisely, red sodium bentonite is manufactured by the Shamaying Sodium Clay Factory in Weifang City, Shandong Province. This particular bentonite is identical to the material that is used on the construction site. Assemble the novel mud solution by incorporating sodium bentonite and additional components into a container containing water, while agitating at a medium speed with a mixer. If the mixture becomes thick, the mixer is stopped periodically to stir the bentonite that may have settled at the bottom, aiding in its dissolution. This stirring process continues at medium speed for approximately ten minutes. When utilizing only bentonite to prepare the slurry, a significant amount of bentonite is required, resulting in high slurry costs and difficulties in achieving the desired slurry performance for EPB shield slag improvement. Therefore, additives are introduced to optimize mud performance. The addition of soda ash significantly improves mud performance. Hence, a suitable amount of soda ash is included in the formulation to adjust the mud's PH value. In line with the objective of reducing the bentonite dosage to lower costs and enhance mud performance for EPB shield use, the preliminary bentonite dosage is set between 3 and 5%. Key parameters include plastic viscosity (PV), apparent viscosity (AV), initial shear (Gelin), dynamic shear (YP), final shear (Gel10), filtration (FL), consistency index (K), fluidity coefficient (n), and dynamic plastic ratio (YP/PV). The guar gum-augmented slurry has a low overall viscosity and a colloid rate of around 80%, suggesting a considerable quantity of water precipitation. This implies that the guar gum addition does not significantly improve the mud's properties, and there is a notable separation of water from the mixture. When CMC is added to the mud, there is a noticeable improvement in mud performance. Viscosity increases, and the filtration rate falls within the acceptable range of 20 mL/30 min, fulfilling the requirements of filtration. Furthermore, the colloid rate significantly improves, indicating a better overall performance of the mud. A significant number of insoluble materials precipitates during the production of schemes F10–F12, which contain polyacrylamide. The majority of these materials appear as white blocky colloids. The results of the API filtration test reveal that the liquid maintains a downward flow, undergoing an estimated filtration loss of 45 mL/30 min. This substantially surpasses the predetermined criteria. Consequently, the application of polyacrylamide in the mud is associated with high insolubility, and it does not produce the desired viscosity increase in the mud. Therefore, polyacrylamide is not considered suitable for future testing. In summary, guar gum leads to lower viscosity and significant water separation, while CMC notably enhances mud performance, meeting filtration requirements and improving overall colloid properties. In contrast, polyacrylamide introduces insoluble matter and fails to achieve the desired viscosity increase, making it unsuitable for further testing.

Fig. 1
A schematic diagram of the mud modifier experiment setup. It consists of a shield shell, soil spewing, screw conveyor, jack, earth pressure, and water pressure. Two photographs of the mud modifier setups are at the bottom.

Schematic diagram of experimental steps

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3 Comparative Study on Mud Performance After Adding Different Additives

3.1 Viscosity Contrast

Viscosity is vital in understanding the performance of different mud formulations. Here's a breakdown of the relevant viscosity parameters: When the slurry flows, this parameter represents the friction that occurs between the solid particles, within the solid particles and the liquid phase, and as well as between liquid molecules. It quantifies the resistance to flow within the mud due to these various interactions. The ratio of shear stress to shear rate at a certain shear rate is equal to the apparent viscosity of the material. It's essentially the sum of structural and plastic viscosity. Structural viscosity is related to the overall structure and behavior of the mud when it flows. Now, refer to Fig. 2 which displays a comparison of the viscosity of several mud schemes. With the exception of F1 and F4, the apparent viscosity of every scheme is greater than that of the building site mud (F0). However, these two schemes (F1 and F4) were both rejected because they did not meet the specifications needed for the Beijing Metro Line 10 sand gravel layer, which favors a greater slurry viscosity. One of the formulations includes the inclusion of xanthan gum, guar gum and CMC which generates the maximum apparent viscosity. This formulation has a bentonite dose of 5%. Scheme F9 stands out among them because it has the greatest apparent viscosity. This suggests that xanthan gum is preferable to CMC and guar gum when it comes to increasing the apparent viscosity of the mud. The trend in the plastic viscosity is similar to that of apparent viscosity. The addition of a viscosity enhancer improves plastic viscosity, with xanthan gum showing better performance compared to guar gum and CMC. Taking all factors into account, xanthan gum emerges as the most effective additive for improving mud viscosity, surpassing guar gum and CMC in this regard.

Fig. 2
A fitted-line graph plots the apparent and plastic viscosities versus the mud schemes F 0 to F 9. The apparent viscosity begins at (F 0, 15), fluctuates, and ends at (F 9, 35). The plastic viscosity begins at (F 0, 3), fluctuates, and ends at (F 9, 7). All values are approximated.

Viscosity of different mud scheme

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3.2 Dynamic Shear Force Comparison Microscopically

The response of the spatial grid structure forces between clay particles while the slurry is in motion is known as dynamic shear force. This parameter gives information on the slurry's partial capacity to move gravel and sand. Figure 3 shows the comparison of the dynamic shear force for each scheme.

Fig. 3
A fitted-line graph plots the dynamic shear force versus the mud schemes F 0 to F 9. The curve begins at (F 0, 10), fluctuates, rises, and ends at (F 9, 18). The values are approximated.

Dynamic shear force of various scheme

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When compared to the reference scheme F0, only schemes F8 and F9 show larger dynamic shear force values in the mud. This shows that xanthan gum is the only way to considerably increase mud dynamic shear force. Now, turning to the comparison of static shear force, which represents the strength of the internal gel network while the drilling fluid is stationary, is presented in Fig. 4.

Fig. 4
A fitted-line graph plots the initial and final static shear forces versus the mud schemes F 0 to F 9. Both have similar rises and falls while the final static shear force has a slightly higher range that begins at (F 0, 11) with a sharp fall to (F 1, 2) and then gradually rises to (F 9, 10) approximately.

Static shear force variation for several muds

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The measurement of static shear force provides information on the drilling fluid's internal gel network structural strength at rest. The magnitude of the initial shear force plays a crucial role in lowering the shield machine's initial power consumption, so it is beneficial to keep the initial shear force at an appropriate low level. On the other hand, while working with bigger sand and pebble deposits, having a higher final shear force is advantageous for effectively suspending and transporting mud to the muck disposal area. Therefore, adjustments to the initial and final shear forces are essential based on the geological conditions.

With the exception of schemes F8 and F9, the results in Fig. 4 show that the static shear forces in all the schemes are not very high. This might not be good for dealing with the sandy cobble layer. For schemes F8 and F9, which involve the addition of xanthan gum, exhibit static shear forces that closely resemble those of the construction site mud. Notably, scheme F8 demonstrates adjustments in both the initial and final shear forces that are more conducive to the challenges posed by the sandy cobble stratum.

3.3 Dynamic Plastic Ratio Comparison

The dynamic plastic ratio (τd/ηp) is a key measure for figuring out how shear dilution works in mud. In the context of shield soil improvement materials, it is essential to maintain an optimum dynamic plastic ratio in order to prevent the accumulation and settling of excavated soil at the bottom of the earth ballast tank. To ensure that the mud has an adequate capacity to carry solid particles, a recommended dynamic plastic ratio range of 0.5–1.5 Pa/MPa ⋅ s has been established for this test. Analyzing Fig. 5, several observations can be made: The dynamic-plastic ratio (τd/ηp) of the mud scheme (F0) used on the building site is excessively high. This high dynamic-plastic ratio is not conducive to effective mud pumping. However, schemes F1–F6 have excessively low mud dynamic-plastic ratios. This lower dynamic-plastic ratio is not favorable for the efficient transport of cuttings and solid particles. In summary, maintaining an appropriate dynamic plastic ratio is essential to ensure effective mud pumping and the transport of cuttings and solid particles during shield soil improvement. On the construction site, the mud scheme used had a dynamic-plastic ratio that was too high, while schemes F1–F6 had dynamic-plastic ratios that were too low, indicating the need for adjustments to achieve the desired balance.

Fig. 5
A fitted-line graph depicts the dynamic plastic ratio versus the mud schemes F 0 to F 9. It includes a plot that begins at (F 0, 3.5), sharply falls to ( F 1, 0.1), and gradually increases to (F 9, 2.0).

Variation in the dynamic-plastic ratio across various schemes

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3.4 Flowability Index and Consistency

The fluidity index, denoted as “n,” reflects a fluid's non-Newtonian behavior within a certain range of flow velocities and serves as an indicator of the fluid's overall flow characteristics. An ideal popularity index “n” in the context of drilling fluids is approximately 0.5 and is typically less than 1. A smaller value of “n” indicates higher fluidity.

On the other hand, the consistency coefficient, denoted as “k,” represents the fluid’s viscosity under a 1 s−1 flow velocity gradient. A fluid with a greater consistency coefficient is thicker and more viscous. The best range for the consistency coefficient “k” of the EPB shield, while dealing with bigger sand and pebble formations, is 1.5–2.

Examining Fig. 6, which illustrates the consistency coefficient and fluidity index for each mud scheme:

Fig. 6
A fitted-line graph compares the liquidity and consistency indexes versus the mud schemes F 0 to F 9. The consistency index has a maximum range that begins at (F 0, 4.0), sharply falls to (F 2, 0.1), and gradually rises to (F 9, 3.0).

Fluidity index and consistency changes for various schemes

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The construction site (F0) mud scheme has a fluidity index of 0.17, which is exceptionally low. Such a low value can hinder the effectiveness of mud in transporting gravel. Conversely, if the consistency coefficient is excessively high, it can negatively impact the pumpability of the slurry.

In schemes F2–F7, the consistency coefficients are all less than 1. These values are more suitable for the desired flow characteristics of the EPB shield in the presence of larger and pebble and sand formations. Taking both the consistency coefficient and fluidity index into account, schemes F2–F7 exhibit more favorable properties for the intended application.

3.5 API Filtration Loss

It is essential to compare the mud filtration in each scheme in order to assess the stability of the mud. The amount of filtration loss is a measure of the mud stability. A minimal quantity of water filtration by the sediment contributes to the maintenance of a consistent pore water pressure close to the excavation face. The production of the mud film is vital for maintaining the stability of the excavation surface, and this stability guarantees that the excavation face's effective mud-water pressure stays constant following the formation of the mud film.

In essence, a smaller water loss during filtration is indicative of better mud performance. Please refer to Fig. 7 for a visual representation of the comparison of each scheme's mud filtration.

Fig. 7
A fitted-line graph depicts the net stirring power versus mud addition amount for New Mud and Mud for the Construction Site. The mud for the construction site has a slight maximum range that begins at (0.20, 0.35) and gradually falls to (0.32, 0.010). The values are approximate.

Two kinds of mud comparison using net power of pebble/round gravel

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4 Conclusions

Laboratory tests evaluated the impact of different additives, including xanthan gum, guar gum, CMC and polyacrylamide, on slurry performance using bentonite as the base slurry. Tests included viscosity comparisons, dynamic shear force comparisons, dynamic plasticity ratio comparisons, flowability index comparisons and API filtration losses. The findings revealed that best material for slurry performance is xanthan gum. The optimal slurry composition is determined to be 1% clay particles, 0.2% xanthan gum, 4% bentonite, and 0.04% soda ash. This slurry ratio has better performance and is more appropriate for enhancing the soil characteristics of the water-rich sand layer.