Development of polymer-modified slurry for chamber opening under pressure in slurry shield tunneling

Abstract

In shield tunnel projects, the shield machine has to stop and open the slurry chamber under pressure to replace the cutterhead tools, clear the obstacles ahead or repair the pipeline gates. During this period, the airtightness capacity requirement of filter cake is much higher than tunnelling period. The polymer-modified slurry composed of hydrolytic polyacrylamide (HPAM), natrium carbonicum, aluminum sulfate and Na-bentonite was developed for chamber opening under pressure period. A high-pressure infiltration test device was assembled to simulate the pressure-maintaining process of the filter cake. Both the limit airtightness pressure and the limit airtightness duration in the stratum with different grain composition was tested. The test results show that the unique “bridging network” and “adsorption” characteristics of the HPAM molecule can reduce the additive amount of bentonite by 40% to 85%, shorten the formation time of filter cake by 20% to 50%, and increase the limit airtightness pressure by 10% to 40%. In high permeability sandy pebble stratum, conventional bentonite slurry cannot form filter cake, while the limit airtightness duration of the filter cake formed by HPAM slurry reached to 1096 s and the airtightness duration under normal air pressure surpassed the minimum requirement in practice engineering by 115%. The research results could provide references for the optimization pathway of slurry preparation during chamber opening under pressure.

Graphical abstract

Keywords

slurry shield / chamber opening under pressure / airtightness / filter cake / polymer

1 Introduction

As an inhomogeneous dispersion system of soil and water mixing, slurry was first used in oil drilling [1]. Similar to the mud cake formed on the wall protection, slurry can infiltrate into the soil and form a dense filter cake on the tunnel face, which can balance the slurry pressure in chamber and the sum of the earth and hydrostatic pressure [2–4]. In slurry shield tunneling, bentonite is commonly used as the base material owing the excellent adsorption properties [5]. In addition, some additives such as sodium carboxymethyl cellulose (CMC) was commonly used to enhance the viscosity of the slurry in highly permeable stratum (such as sandy gravel and sandy pebble stratum). However, the reserve of bentonite is limited. The disposal and reuse also require to establish a complex separation and treatment system [6]. Bentonite slurry also adsorbs a large number of soil particles, which is difficult to deal with and separate. In some regions, bentonite has been banned in some sensitive areas owing to the environmental problems caused by mixed waste [7]. However, the molecular chain of CMC is shorter, resulting in the weaker capacity in enhancing soil suspensibility [8]. Therefore, how to minimize the content of bentonite and maintain the excellent properties in complex geological environment is an untapped area to explore.

With the promotion of low-carbon concepts, some scholars have tried to find efficient, biodegradable and environmentally friendly slurry substitutes in polymers field due to the wide stratigraphic applicability and unique properties [9–12]. Cui et al. [13] added pregelatinized starch (PGS) into the conventional bentonite slurry. According to the results of slurry mixing ratio tests and infiltration tests, it was concluded that in the slurry with the mass mixing ratio between bentonite and water is 0.0833, in 0.3% PGS content, the slurry has the great ability of water loss, and the dense external filter cake can also form. In addition, the other natural polymers, such as guar gum, xanthan gum, white dextrin have also been explored as the potential additives, and the viscosity and the suspension of bentonite slurry is improved to some extent [14,15]. However, the applied properties of these natural polymers are easily affected by producing state, season or extraction process of the plant-based raw materials. In addition, the natural polymers exhibit biologically active, leading to the demanding requirements for storage. The usage of preservatives and paraformaldehyde are inevitable, which inevitably lead to water and soil pollution. In contrast, hydrolytic polyacrylamide (HPAM) is formed by free radical polymerization of aqueous solution, the modification method is easy to control. The molecular chain of HPAM possesses stable viscosity enhancement properties and can be kept for as long in the dry and ventilated environment [16].

The pressure difference between slurry chamber and soil−water pressure is about 20 kPa and the airtightness duration is approximately 100 min [13–16], which is suitable for the tunneling period. In contrast, this study focuses on the extreme static conditions of high-pressure pressurized chamber opening. For saturated or loose soil, opening chamber under pressure is the only choice to ensure the safety of constructors. Under the sealing effect of filter cake, the air pressure keeps balance with the soil and water pressure in the stratum, to maintain the stability of the tunnel face. In addition, the demand for the airtightness capacity of filter cake during chamber opening period is much higher than tunneling period. Based on the successful cases of chamber opening under pressure in actual engineering, the difference between the air chamber pressure and the earth and hydrostatic pressure ranges from 150 to 350 kPa, the single working duration for the constructors ranges from 4 to 8 h [17].

In this study, HPAM polymer was combined with conventional bentonite slurry to be applied in the chamber opening under pressure with soil in different particle grades. A high-pressure airtightness test rig was developed to compare the airtightness properties between HPAM polymer slurry and conventional bentonite slurry. The variation of water filtration in slurry infiltration process was recorded to investigate the airtightness properties of the filter cake. The research results could provide references for the preparation of high performance filter cake used in chamber opening under pressure.

2 Test materials

In the slurry shield projects, the constituent materials of the slurry are generally classified into three categories: water, soil materials, and additives. The content of water usually exceeds 80%, ensuring the fluidity and lubricity. Due to the excellent water absorption and expansion properties of bentonite, slurry plays the role of wall protection and lubrication during the advancement of the shield machine. Additives were selected based on the applied environment. In this study, the polymer slurry is composed of HPAM, natrium carbonicum (Na₂CO₃), aluminum sulfate (Al₂(SO₄)₃), and bentonite. HPAM and bentonite are the basic component materials, and other additives are mainly used as binders to adjust the viscosity and water filtration.

2.1 Basic materials

1) Bentonite

Bentonite, also known as porphyry, was discovered by geologist W.C. Knight in 1888. Compared to malleable clay, bentonite has a much better ability to absorb water. Upon contact with water, bentonite will expand significantly to 15 times its dry volume and form a gel-like gray-green substance [18]. The reason for this is that bentonite is rich in montmorillonite, with a structure consisting of silica-oxygen tetrahedra interspersed with aluminum-oxygen octahedra, which conferred excellent adsorption properties to montmorillonite. Based on the structure and charge distribution properties of montmorillonite, bentonite can also be subdivided into sodium-based bentonite (Na-bentonite), calcium-based bentonite (Ca-bentonite), and natural bleaching clay. In this study, Na-bentonite, which is commonly used in the slurry shield tunneling, was selected.

2) HPAM

HPAM is a general term for homopolymers and copolymers of acrylamide [19]. Polymers containing more than 50% of the structural units of the acrylamide monomer are called polyacrylamides (PAMs). The structure is shown in Fig. 1 and the chemical formula is (C₃H₅NO)_n, HPAM is a glassy solid at room temperature, easily soluble in water. PAM is a series of molecular mass of different polymers. In chemistry, HPAM is categorized into non-ionic, anionic, cationic, and amphoteric types. From the physical form point of view, HPAM can appear solid (powder, bead), colloidal and liquid (reverse-phase emulsion, aqueous solution), and other forms.

In the existed researches, HPAM has been widely used in quick-acting wall protection and leakage plugging projects as the polymerized polymeric viscosifier [19]. The amino group (-NH₂) imparts excellent hydrophilicity to HPAM, while the amide group (-CONH₂) contributes to its superior adsorption capabilities. And the molecular chain can connect with multiple groups and forms the network structure, which significantly enhances the strength of the plugging material. However, although the wall protection speed of HPAM is very high, the strength of the flocculation formed is relatively weak. To enhance the clogging effect, inorganic substances were introduced as crosslinking agents to promote the cross-linking reaction of HPAM to form solid and insoluble salts in this study. It should be noted that HPAM is a biodegradable polymer under both aerobic and anaerobic conditions. The degradation process involves enzymatic hydrolysis of the amide side-chains and subsequent cleavage of the main carbon backbone into smaller, non-toxic organic molecules. Studies utilizing combined anaerobic-aerobic systems have demonstrated high HPAM removal efficiencies of up to 89.8% [20]. Similarly, aerobic systems like sequencing batch biofilm reactors have achieved significant degradation. The total organic carbon removal reached 70.1%, while anaerobic systems have also proven effective [21,22].

2.2 Additive materials

2.2.1 Na₂CO₃

Na₂CO₃ is mainly used in food processing, and is harmless to the human body and water quality. After hydrolysis, Na₂CO₃ will be hydrolyzed in water and generate hydroxide, so that the slurry presents an alkaline environment, which is more conducive to the polyacrylamide solubility and viscosity. In addition, as the dispersant, Na₂CO₃ can improve the activity of the slurry [16]. Sodium ions can replace the original high-valent ions on the surface layer of the bentonite particles to increase the dispersion of the slurry, which can reduce the water filtration and improve the viscosity of the slurry.

2.2.2 Al₂(SO₄)₃

Al₂(SO₄)₃ is a white crystalline powder, which can dissolve in water and form aluminum ions and aluminum hydroxide colloids. It can react with the small suspended particles with anisotropic charge in the water to promote the aggregation and sedimentation of suspended matter. In addition, through the colloid adsorption bridging and precipitation reaction, Al₂(SO₄)₃ can further enhance the flocculation effect [16]. Consequently, Al₂(SO₄)₃ is often used as a drinking water treatment agent, which not only can effectively purify the water quality but also has the function of disinfection and sterilization, decolorization, and deodorization.

3 Microscopic mechanism

Under the air pressure of the slurry chamber, the conventional bentonite slurry particles infiltrate into the pores of the soil skeleton. As the pores are filled, the infiltration rate slows down, and a slightly permeable filter cake forms on the surface of the stratum. For HPAM polymer slurry, -NH₂ groups and amide groups are fused to the long chain structure of the HPAM molecule. -NH₂ provides HPAM with excellent hydrophilic ability, while amide groups provide strong adsorption ability, enabling the flocculation and bridging effects. During the hydration process, the hydrophilic group can adsorb a large number of polar water molecules. Due to the isoelectric repulsion, the long chains of HPAM molecules can be stretched in water and thus can be dissolved. In addition, the adsorption groups can be tightly bound to the surface of the bentonite particles. In this study, scanning electron microscopy (SEM) made in HITACHI(SU3500), Japan and refrigeration transmission preparation system made in Quorum (PP3013), Britain are adopted. The magnification is uniformly set to 600 and the scale bar is 50 μm. Figure 2(a) shows the SEM electron scanning micrograph of HPAM solution, the mass concentration of HPAM, Na₂CO₃ and Al₂(SO₄)₃ is 0.15%, 0.18%, 0.03%, respectively. The microstructure of the hydrolytic HPAM shows the irregular spatial network, which not only prevents the dispersion of bentonite and stabilizes the structure of sandy soil, but also wraps the free water. The addition of Na₂CO₃ resulted in an alkaline environment of the solution, and the hydrolysis degree of HPAM also subsequently increased, which in turn increased the viscosity of the HPAM solution. As shown in Fig. 2(b), the spatial network of the HPAM solution containing Na₂CO₃ is more uniform and compact. Al₂(SO₄)₃ undergoes double hydrolysis reaction with Na₂CO₃ and produces flocculent precipitate, which can fully wrap the laminar bentonite structure, as shown in Figs. 2(c) and 2(d).

4 Test of limit airtightness pressure

4.1 Test apparatus

A high-pressure resistant slurry infiltration test device was designed to simulate the infiltration and the formation process of filter cake during the shutdown period of the slurry machine. As shown in Fig. 3, the device includes a pressurized air infiltration column, a slurry storage drum, an air pressure control device, a water filtration collector, and a steel bracket. The pressurized air infiltration column consists of a hollow column and a base made of plexiglass. The outside of the column is covered with a stainless steel shell. To observe the morphological characteristics and the thickness of the filter cake, a glass viewing window with a scale has been placed on the stainless steel shell. The air pressure was managed and monitored through dedicated air pressure control system connected to the air compressor. The pressure adjustment precision and monitoring precision is 0.1 kPa. The top cover is equipped with a pressure relief valve to facilitate real-time monitoring of pressure variation during the test. In addition, a slurry inlet hole was also provided for connection to the slurry storage drum. The mass of water filtration was measured in real time by an electronic scale with a 200 ms collection interval, which can directly reflect the formation process of the filter cake.

4.2 Test materials

4.2.1 Stratum materials

In this study, the base sand was international organization for standardization (ISO) standard sand produced by Xiamen Aisio Standard Sand Co. The coarse sand, medium coarse sand and fine sand required for the test were sifted from the standard sand by motorized sand sifter. In addition, gravel and pebbles were also prepared to simulate a sandy and gravelly stratum and a sandy and pebble stratum. Four stratums with different particle grades and permeability coefficients were prepared, and their basic physical properties of are shown in Table 1. The size distribution curve of the test stratums is shown in Fig. 4.

4.2.2 Slurry materials

In the formation mechanism of the filter cake, the properties have to adapt the grain size of the stratum. Meanwhile, the viscosity of slurry is directly correlated to the form type of filter cake, which is commonly divided in to three types: external filter cake, external filter cake + internal filter cake and internal filter cake [23]. The plastic viscosity of the bentonite slurry for chamber opening ranges from 12–20 cP for the gravel-cobble stratum (k = 3.7 × 10⁻² cm/s). For sandy soil (k = 9.2 × 10⁻²–2.8 × 10⁻¹ cm/s), the plastic viscosity is deployed to 32 cP. In this study, the plastic viscosity of slurry was used as the control index. Based on the experimental data presented above, the plastic viscosity was designed as the same value for both conventional bentonite slurry and HPAM slurry to avoid the influence of slurry viscosity difference. For stratum S1 and S2, only bentonite and water were added to prepare the convential bentonite slurry B1 and B2 due to the low permeability coefficient. The plastic viscosity was set at 9 and 12 cP, respectively. For stratum S3 and S4, the permeability coefficient increased by nearly 10 to 50 times compared to S1 and S2. 0.1% CMC was added to prepare bentonite slurry B3 and B4, and the plastic viscosity was set at 26 and 44 cP, respectively.

According to the existed studies [13,14], the additive content of bentonite in polymer slurry was attempted to set as 15%–60% of the conventional bentonite slurry. In this study, the content of bentonite in HPAM slurry (B5–B8) was 15%, 30%, 45%, 60% compared to slurry B1–B4. To determine the optimum content of the HPAM, Na₂CO₃ and Al₂(SO₄)₃, several sets of slurry plastic viscosity tests were conducted with different additive contents. It should be noted that the plastic viscosity of the slurry was measured by 12-speed rotational viscometer and the accuracy is 1 cP.

The content of HPAM as pipeline fluid additive should not exceed 0.02% to avoid pipeline blockage [24]. As shown in Fig. 5(a), the content of HPAM is positively related to the slurry viscosity, and the content of HPAM was selected to be 0.015%, 0.06%, 0.06%, and 0.09% for slurry proporation, respectively. After the addition of Na₂CO₃, the plastic viscosity of HPAM slurry first increased and then decreased, indicating that the dispersant could effectively promote the extension of HPAM molecules, and then reduce the slurry viscosity. The optimum Na₂CO₃ additive content was taken at the lowest plastic content, which is 0.015%, 0.06%, 0.06%, and 0.09% for stratum S1–S4. Figure 5(c) shows the influence of additive content of Al₂(SO₄)₃ on HPAM + Na₂CO₃ slurry, which is also positively related with the slurry viscosity. Taking the plastic viscosity of the conventional bentonite slurry as the control value, the additive of the Al₂(SO₄)₃ is 0.015%,0.06%,0.09%, and 0.09% for slurries B5, B6, B7, B8, respectively.

4.3 Test flow

1) Preparation of slurry

The slurry was prepared by mixing bentonite and tap water according to the proportions shown in Table 2. The mixture was stirred with an electric mixer to obtain uniform dispersion and then expanded for 24 h. For the HPAM polymer slurry, the mixed solution of HPAM and Na₂CO₃ was left to stand for 6–8 h in advance, and Al₂(SO₄)₃ solution was prepared for 5 min previously [19]. Before the test, the additive solution was poured into the expanded bentonite slurry, and stirred for another 10 min to ensure that the slurry particles were uniformly distributed.

2) Preparation of stratum

The sand and gravel were weighed and mixed in different grain sizes at the ratio. A screen was laid at the bottom of the infiltration column to prevent the sandy soil from blocking the water outlet. The soil was poured into the infiltration column in rainfall method. The stratum was prepared by layered hammering method, and the height of each layer after hammering was calculated according to the specified porosity, which was controlled to be 3 cm. Once the stratum was prepared to the specified height and porosity, the column was saturated by bottom-up percolation method with water.

3) Device connection and test

Connect the air pressure control system to the slurry storage unit and the permeation column by gas pipelines. The outlet at the bottom of the infiltration column is connected to the water filtration bucket, and the mass of the water filtration is transferred to the water filtration collector through the data line and the time interval is 200 ms.

3) Slurry infiltration test

Place the infiltration column horizontally on the test bench, start the acquisition system and open the water outlet. Set the air pressure as 140 kPa [25,26]. Record the variation of the water filtration mass. When the filtrate infiltration rate tends to stabilize, it is considered that the filter cake with airtightness ability has been formed

4) Limit airtightness pressure test

By the end of the infiltration and filter cake formation, horizontally place the infiltration column, the un-infiltrated slurry on the soil surface settled under gravity, thereby exposing the most of the filter cake directly contact with compressed air. Then, increase the air pressure 360 kPa at 20 kPa/min [4,26]. And the hydraulic pressure in the infiltration column is neglected. Air leakage of the filter cake is a sudden process. The rapid growth of the water filtration can be judged as the sign of the failure airtightness ability.

4.4 Test results

Figure 6 shows the mass variation of water filtration with infiltration time of slurry B1 and B5 in fine sand stratum S1. It is obvious that the water filtration mass shows a clear stage change with time. In the first stage, the slurry particles infiltrated into the soil pore, leading to the rapid discharge of the pore water. When the filtration water mass showed an obvious inflection point, the filter cake with airtightness capacity was initially formed [26]. The formation time of filter cake for conventional bentonite slurry B1 was 63 s, while reduced to 33 s for HPAM slurry B5. The filtration mass of slurry B1 rose to 85% of the total value before the failure of filter cake in shorter order, while slurry B5 only rose by 60%. The second stage is the airtightness stage of the filter cake. In this stage, the filter cake grew continuously and the densification also gradually increased. The water contented in slurry passed through the filter cake and discharged the original pore water in stratum, and the growth of water filtration mass tended to stabilize. The third stage is the failure stage of the filter cake. The air broke through the filter cake, the mass of water filtration increased steeply. As shown in Fig. 6, when the air pressure increased to 240 kPa, the conventional bentonite filter cake was failed. Consequently, the limit airtightness pressure is 220 kPa, and increases to 260 kPa for HPAM polymer slurry B5.

Figure 7 shows the mass variation of water filtration for slurry B2 and B6 in the standard sand stratum S2. Compared to stratum S1, the increasing rate of water filtration mass is significantly lower. The formation time of filter cake for conventional bentonite slurry B2 is 332 s, while for HPAM slurry B6, it is 23.8% earlier at 253 s. And the limit airtightness pressure is 200 kPa and 240 kPa respectively in stratum S3. Figure 8 shows the variation in stratum S3. The formation time of filter cake is 66 s for slurry B7 and 125 s for slurry B3. In comparison, the formation time of slurry B7 is advanced by 47.2%. The airtightness time of filter cake of slurry B3 and B7 is 160 and 220 kPa, respectively, which improves by 37.5%. As shown in Fig. 9, the variation of water filtration mass also appears periodical change in stratum S4, the formation duration of filter cake is 90 s and the limit airtightness pressure is 160 kPa. However, the conventional bentonite slurry B4 has completely leaked at the beginning of the test and failed to form a dense filter cake, which indicated that the long chain molecule of HPAM polymer enhanced the adhesive force among bentonite particles.

Figure 10 shows the filter cake samples from the stratum surface of the tests. Figures 10(a) and 10(b) are the external filter cake formed by conventional bentonite slurry B1 and HPAM polymer slurry B5 in the fine sand stratum S1. And the thickness of the external filter cake formed by slurry B5 is 5 mm, while it is decreased to 2 mm for slurry B1, which indicates that HPAM polymer can shorten the infiltration distance and promote the accumulation of the bentonite particles in the pore of stratum. For the standard sand stratum S2, external filter cake + internal filter cake formed by conventional bentonite slurry B2 and HPAM polymer slurry B6 on the stratum surface. Both the thickness of the external filter cake is about 2 mm. The thickness of the internal filter cake is 35 mm for slurry B6 and increased to 45mm for slurry B2. For sandy gravel stratum S3, the type is external filter cake + internal filter cake for HPAM polymer slurry B2. While only internal filter cake formed by conventional bentonite slurry S3, and the airtightness capacity is poor. For sandy pebble stratum, the type is external filter cake + internal filter cake for HPAM polymer slurry B8. For the case S4-B4, the gravel has disclosed and direct contact with compressed air.

Take the cases of S1-B1 and S1-B5 as an example, Figure 11 shows the SEM microstructures of the filter cake. In conventional bentonite slurry, the extended bentonite is in the form of an incompact lamellar structure. The pores between the lamellar structures make it easier for water to infiltrate through the filter cake, and the filter cake is more likely to deform under air pressure. After the addition of HPAM, the mesh structure fills the pores in stratum and reduces the infiltration path, improving the airtightness capacity. To a certain extent, the dense mesh structure also enhances the mechanical strength of the filter cake.

5 Test of limit airtightness duration

Airtightness duration is commonly thought as the stable duration of filter under with some air pressure, which is meaningful to estimate the continuously working duration for constructors [17]. In this study, the limit airtightness duration is defined as the duration for filter cake remains dense under the air pressure value, which is a key parameter to determine the evacuation time for constructors to cope the air pressure fluctuations caused by the stuck of air valve or the unreasonable setting of discharge flow. In Section 4, the limit airtightness pressure of HPAM polymer slurry and conventional bentonite slurry have been investigated with different grain size stratum. In this section, the influence of slurry type, stratum type on both limit airtightness duration and airtightness duration are explored. The test device and materials are shown in Tables 1, 2 and Fig. 3. According to the test results in Fig. 9, the filter cake at the case S4-B4 is failed. Consequently, only the other seven sets of filter cake were measured.

The air pressure was also pressurized to the limit airtightness pressure at 20 kPa/min. Once reached, the target pressure was maintained within a stable fluctuation range of ±0.1 kPa, and the duration was timed until a sudden water filtration improvement occurred, signifying the structural failure of the filter cake. Figures 12–15 shows the mass change curves of water filtration with different slurry in stratum S1 under limit airtightness pressure. For stratum S1, both the HPAM polymer and bentonite filter cake can maintain stable state more than 2 h without air breakdown phenomenon. The filter cake formed by the conventional slurry B5 is failed after 3492 s of limit airtightness pressure. In the case of S3 stratum, both the filter cake formed by HPAM and bentonite were failed. The airtightness duration is 2280 and 2040 s, respectively, and decreases to 1096 s for HPAM slurry in stratum S4. As shown in Figs. 16(a)–16(f), the water filtration of external filter cake and external filter cake + internal filter cake contains a small amount of sand, but the solution is clear. However, the water filtration of external filter cake in stratum S3 becomes cloudy. For the case S4-B4, almost no bentonite particles are deposited in the soil pores.

The limit airtightness pressure P_lim represents the theoretical maximum air pressure which could be applied on the tunnel face during chamber open period. However, the stratum is not entirely homogeneous, and seepage-induced failure at localized weak zones may lead to the overall failure of the filter cake. Consequently, the set air pressure is typically lower than P_lim in practical engineering. As shown in Fig. 17, this study evaluates airtightness durations under 0.75P_lim and 0.5P_lim, with test durations capped at 10 h based on typical 4–8 h operational requirements [17]. The experimental results demonstrate that the airtightness duration is negatively related with the air pressure. In the low-permeability stratum (S1 and S2), both bentonite and HPAM polymer slurry at 0.75P_lim exceeded the minimum 4 h operational threshold. In stratum S3 at 0.75P_lim, the airtightness duration increased by 298% and 906% compared to limit airtightness duration for bentonite slurry and HPAM slurry, respectively. Remarkably, HPAM slurry at 0.5P_lim maintained airproof integrity beyond 10 h. In stratum S4, the airtightness duration at 0.5P_lim achieved 8.6 h, surpassing the minimum requirement by 115%.

6 Conclusions

During the chamber opening under pressure in slurry shield tunnelling, the airtightness capacity of the filter cake is directly related to the safety of the constructors. In this study, the HPAM polymer slurry with low bentonite was developed and tested by the self-developed high-pressure resistant slurry infiltration test device. The limit airtightness pressure and airtightness duration for both HPAM polymer slurry and conventional bentonite slurry were measured with stratums in different grain sizes. Some of the main conclusions are summarized below.

1) The main materials of the new polymer slurry are HPAM, Na₂CO₃, Al₂(SO₄)₃ and bentonite. Under the hydrolysis activation of Na₂CO₃, and the electrical neutralization of Al₂(SO₄)₃, the HPAM polymer appears “bridging network” and “adsorption” characteristics. Compared with the conventional bentonite slurry, HPAM polymer slurry can reduce the addition content of bentonite by 60% to 85%. HPAM and bentonite are the leading factors of the water filtration mass, and Na₂CO₃ and Al₂(SO₄)₃ are the leading factors the slurry plastic viscosity.

2) In the sandy stratum, the airtightness capacity of the HPAM polymer slurry is much better than that of conventional bentonite slurry of the same plastic viscosity. And the improvement is positively related with the permeability coefficient of the stratum. Compared with the conventional bentonite slurry, the formation duration of the filter cake for HPAM polymer slurry is reduced by about 20%–50%, and the limit airtightness pressure is increased by about 10%–40%. In terms of the filter cake type, the “external filter cake” is superior to “external filter cake + internal filter cake”, and superior to “internal filter cake.”

3) Both the HPAM polymer and conventional bentonite filter cake can maintain stable state for more than 1 h under the pressure at limit airtightness pressure, while the mass of water filtration for HPAM polymer slurry is less. In sandy gravel stratum, the limit airtightness duration of HPAM polymer slurry is 2280 s, while it is decreased to 2040 s for conventional bentonite slurry. In sandy pebble stratum, the conventional bentonite slurry is almost completely failed. However, the limit airtightness duration of the HPAM polymer filter cake reached to 1096 s and the airtightness duration under 0.5 times the limit airtightness pressure reached 8.6 h, which surpassed the minimum requirement in practice engineering by 115%.

This study still has some deficiencies. The test only focused on the airtightness capacity of HPAM slurry within short duration (≤ 10 h), while not considered the deterioration of filter cake in long-term continuous shutdown condition. In actual engineering, pressure fluctuation is closely related to the stability of the filter cake. In addition, there are commonly multiple layers of stratum distributed in three-dimensional stacking state on the tunnel face in the practice engineering, which could influence the infiltration path and result in uneven distribution of airtightness capacity on the whole filter cake. There limitations are the focus of our subsequent research.

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