Drag reduction construction method for shield construction of deeply-buried sand layer

Abstract

The invention relates to a resistance reduction construction method for shield construction of a deeply-buried sand layer. The construction method comprises the following construction steps: preparing before construction; preparing thixotropic slurry; grouting parameter control is conducted, specifically, the grouting amount is calculated according to the sand layer porosity, the shield parameters and the advancing speed, and the grouting pressure is determined in combination with the soil pressure and the underground water pressure; performing staged grouting construction: before propelling, injecting thixotropic slurry through the radial grouting holes to form a pre-lubricating layer; thixotropic slurry is dynamically supplemented in the tunneling process to compensate loss; and when advancing to 0.3-0.5 m away from the end point, final-process reinforced grouting is carried out. Through a novel thixotropic slurry formula and staged thixotropic slurry grouting construction, a thixotropic slurry lubricating layer is formed around a shield body, the holding and wrapping effect of a sand layer on a shield tunneling machine is reduced, and the frictional resistance in the shield tunneling construction process in the deeply-buried water-rich sand layer is greatly reduced. And an efficient and safe solution is provided for the urban underground traffic development technology facing deep-buried sand layer construction such as subsequent urban underground deep-buried tunnel construction and cross-sea tunnel crossing.

Description

Technical Field

[0001] The present invention belongs to the field of underground tunnel shield construction, and specifically relates to a shield construction drag reduction method suitable for deeply buried sand layers (burial depth ≥ 30m). In particular, the invention aims to address the problem of sharp increase in frictional resistance and susceptibility of shield machines to getting trapped in water-rich deeply buried sand layers. Efficient drag reduction is achieved by optimizing the thixotropic mud formula and the staged grouting process. Background Art

[0002] With the continuous development of urban construction, urban underground space has gradually shifted from breadth to depth. For example, the shield tunnels required for underground urban rail transit have generally exceeded 30 meters in recent years. However, the construction of deep underground tunnels still faces many great risks, such as high confining pressure in deep sand layers, low self-stability of soil, high friction resistance and other problems. At present, there are few relevant technical literature and engineering examples for shield construction in deep sand layers at home and abroad, especially how to solve the high friction resistance and escape problems of shield machines during construction. At present, the construction of deep sand layer tunnels mostly directly follows the experience of shallow layer construction, which leads to many construction risks when constructing in the complex stress environment of deep sand layers.

[0003] Existing technologies, such as the bentonite grouting method disclosed in patent document (CN102434174A), only compensate for pressure loss in the soil bin and maintain stable pressure to control settlement. It lacks drag reduction design, and the grouting holes are located only along the upper centerline of the shield. This fails to form a uniform lubrication layer within the sand layer, making it difficult to meet the comprehensive drag reduction requirements of deep sand layers. For example, the thixotropic slurry drag reduction method described in patent document (CN104534167B), suitable for pipe jacking construction, has grouting holes located at the rear of the pipe jacking machine and at the pipe sections, making it incompatible with the shield structure of the machine. Furthermore, the material contains ingredients such as carboxymethyl cellulose and sodium carbonate, which are easily squeezed out under the high pressure of deep sand layers, preventing the formation of a stable lubrication layer.

[0004] Therefore, in this context, developing a set of shield construction drag reduction technologies for use with deeply buried sand layers is a key innovative direction for breaking through the development of underground space. Summary of the Invention

[0005] The present invention proposes a drag reduction method suitable for shield construction in deep-buried sand layers, aiming to solve construction difficulties such as a sharp increase in frictional resistance and easy entrapment of the shield machine during advancement under conditions of water-rich deep-buried sand layers. Through the innovative design of thixotropic mud formula and staged grouting process, a stable lubricating layer is formed between the shield body and the sand layer, thereby achieving a significant reduction in frictional resistance and improving the efficiency and safety of shield construction in deep-buried sand layers.

[0006] To achieve the above-mentioned purpose, the technical solution of the present invention is as follows: A drag reduction method for shield construction in a deep buried sand layer, comprising the following construction steps: (1) Preparation before construction: Set radial grouting holes with one-way valves at the shield position of the shield machine; prepare thixotropic slurry consisting of sodium bentonite, Baosheng 70 type admixture and water; (2) Preparation of thixotropic mud: prepare thixotropic mud according to the mass volume ratio, including 250kg / m³ sodium bentonite, 4.75L / m³ Baosheng 70 type admixture, and the balance is water; (3) Grouting parameter control: Calculate the grouting volume based on the porosity of the sand layer, shield parameters, and advancement speed, and determine the grouting pressure based on soil pressure and groundwater pressure; (4) Grouting construction in stages: before advancing, thixotropic mud is injected through the radial grouting holes to form a pre-lubricating layer; during the excavation process, thixotropic mud is dynamically replenished to compensate for the loss; when advancing to 0.3-0.5m from the end point, the final reinforcement grouting is carried out.

[0007] Furthermore, in step (1), the number of radial grouting holes is 6, and the circular array is distributed around the circumference of the middle shield.

[0008] Furthermore, in step (1), the one-way valve is a sand-blocking one-way valve, which can prevent the sand layer from entering the grouting channel due to pressure difference.

[0009] Furthermore, in step (1), a thixotropic mud grouting channel is reserved in the grouting hole, and grouting is performed from the middle shield to the outside to expand the scope of action of the thixotropic mud; each radial grouting hole is equipped with a one-way valve.

[0010] Furthermore, in step (2), the thixotropic slurry is prepared during the segment assembly to ensure that the shield advancement rhythm is not affected.

[0011] Furthermore, add and mix in the order of “water → sodium bentonite → Baosheng 70 type admixture” and continue stirring for not less than 5 minutes after mixing.

[0012] Furthermore, in step (1), the grouting equipment is arranged on a supporting trolley behind the shield machine, the grouting pipeline is arranged in a ring along the outer side of the shield body, and a warning sign is set in the area where the pipeline crosses.

[0013] Furthermore, in step (3), the grouting volume is calculated according to the formula Q = k × [π / 4 × (D 2 −d 2 )]×L, where D is the outer diameter of the shield machine, d is the outer diameter of the middle shield, L is the advancement distance of one ring, and k is the correction coefficient, which is taken as 1.5~2.

[0014] Furthermore, in step (3), the grouting pressure is calculated according to the formula P = k p ×[ c × h +( c − cw )×( h − h w )× K 0] calculation, where γ is the weight of the covering soil, h is the buried depth of the shield machine center, c w is the groundwater density, h w is the depth of groundwater level, K 0 is the lateral pressure coefficient of soil, kp is the correction coefficient, which is taken as 1.3~1.6.

[0015] Furthermore, in step (4), the amount of pre-lubrication grouting before advancement accounts for 30% of the total grouting amount, the amount of compensating grouting during excavation accounts for 40%, and the amount of final strengthening grouting accounts for 30%.

[0016] The technical solution of the present invention has the following advantages and beneficial effects: 1. The grouting volume is calculated accurately to adapt to the construction and avoid waste; 2. Dynamic pressure control reduces friction and improves efficiency; 3. Optimized grouting hole design ensures uniformity, stabilizes soil, and protects safety. The overall process is simple and efficient, with obvious advantages in full-section sand layer deep buried shield construction and has promotional value.

[0017] In summary, the present invention's method for reducing drag during shield tunneling in deep sand layers is developed to address the construction difficulties encountered in working conditions in water-rich deep sand layers, such as a dramatic increase in shield friction and the entrapment of the shield machine itself. Through a novel thixotropic mud formulation and phased thixotropic mud grouting, the frictional resistance during shield tunneling in deep, water-rich sand layers is significantly reduced. This provides an efficient and safe solution for the development of urban underground transportation technologies facing the construction of deep sand layers, such as subsequent urban underground tunnel construction and cross-sea tunnels. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 This is a schematic diagram of radial grouting holes for thixotropic slurry in shield construction; Figure 2 This is a schematic diagram of thixotropic slurry grouting construction; Figure 3 Schematic diagram of the change of friction resistance under the action of thixotropic mud; In the figure, 1- radial grouting hole, 2- radial grouting hole one-way valve, 3- radial grouting hole, 4- thixotropic mud filling layer, 5- shield machine. DETAILED DESCRIPTION

[0019] The following examples of the technical solution of the present invention will be described in conjunction with the accompanying drawings and engineering examples. The following embodiments are only used to more clearly illustrate the technical solution of the present invention and are therefore only examples and cannot be used to limit the scope of protection of the present invention.

[0020] like Figure 1 to Figure 3 As shown, a deep buried sand layer shield construction drag reduction method of the present invention comprises the following steps: (1) Preparation before construction 1) Radial grouting holes are set at the middle shield of the shield machine, and a circular array layout is adopted to optimize grouting efficiency and anti-clogging performance. Thixotropic mud channels are reserved in the grouting holes and equipped with one-way valves to prevent sand from entering the channels and causing blockage due to pressure differences. Grouting is carried out outward through the middle shield to ensure that the thixotropic mud can effectively wrap the shield body and expand the range of action.

[0021] 2) Thixotropic slurry materials are configured according to the following mass-to-volume ratio: sodium bentonite 250kg / m³, Baosheng 70 admixture 4.75L / m³, and the balance is water. A space for grouting equipment is reserved on the supporting trolley behind the shield machine. The pipeline layout is optimized and warning signs are installed. Regular inspections are carried out to prevent pipeline damage or gushing risks.

[0022] (2) Thixotropic slurry grouting parameter control 1) The thixotropic slurry injection volume used in deep sand shield construction is calculated according to the following formula: Taking into account the porosity of the sand layer, deep buried ground stress, shield advancement speed and other factors, a dedicated calculation formula is established: Q=k×[π / 4×(D 2 −d 2 )]×L Where, the outer diameter of the shield machine is D, the outer diameter of the middle shield is d, the advancing distance of one shield construction ring is L, and k is a correction coefficient obtained by comprehensively considering various complex construction factors, such as thixotropic mud loss during advancement and the porosity ratio of the surrounding soil (taken between 1.5 and 2): 2) Thixotropic slurry pressure control: For shield construction in deep sand layers, the grouting pressure is determined by taking into account multiple factors such as vertical earth pressure, lateral earth pressure, and groundwater pressure. The earth pressure is calculated using relevant methods such as the Rankine earth pressure theory.

[0023] Assume that the weight of the covering soil at the location of the shield machine is c (unit: kN / m 3 ), the shield machine center burial depth is h (unit: m ), the lateral pressure coefficient of soil is K 0, the groundwater density is c w (unit: kN / m3 ), the groundwater depth is h w (unit: m ), the correction factor for grouting pressure control is kp , then the thixotropic mud grouting pressure P The calculation formula can be expressed as: P = k p ×[ c × h +( c − c w )×( h − h w )× K 0] Where, c × h Represents the vertical self-weight stress part (i.e. the vertical earth pressure generated by the self-weight of the overlying soil layer), ( c − yw )×( h − hw ) represents the lateral earth pressure generated by the effective weight of the soil layer under the influence of groundwater level (multiplied by the lateral pressure coefficient K 0 and then converted into the corresponding lateral earth pressure). kp As the correction coefficient, a value of 1.3 to 1.6 is taken into comprehensive consideration to avoid the problem of insufficient grouting range due to insufficient pressure.

[0024] (3) Thixotropic slurry grouting The grouting operation of thixotropic slurry is a key step to ensure the drag reduction effect and smooth progress of construction. The specific operations are as follows: 1) Preparation of thixotropic slurry Considering the characteristics of shield construction in sandy layers, the thixotropic slurry preparation was performed during segment assembly. Construction operations are relatively stable during this period, allowing for the thixotropic slurry preparation to be fully utilized, ensuring that subsequent grouting can proceed in a timely and orderly manner without impacting the overall progress of the shield tunneling. The required materials were added to the preparation equipment in the order of water, bentonite, and Baosheng 70 admixture. Baosheng 70 should only be added after the water and bentonite are continuously stirred until uniform. After mixing, the stirring time should be maintained for at least 5 minutes (or extended depending on the mixing quality).

[0025] 2) Grouting volume distribution and grouting timing Based on the geological characteristics of the full-section sand layer, especially the key factor of the strong fluidity of the sand layer, the preset thixotropic mud grouting volume is divided into three grouting operations. The grouting operations and corresponding purposes of each stage are as follows: S1. Pre-lubrication before propulsion: Before the shield machine begins advancing, an initial injection of thixotropic slurry is performed. By injecting an appropriate amount of thixotropic slurry around the shield through radial grouting holes before advancing, a lubricating and drag-reducing protective film is pre-formed between the shield and the sand layer, reducing the friction coefficient between the two and minimizing propulsion resistance, creating favorable conditions for the smooth start of the shield machine. The initial injection volume accounts for 30% of the planned grouting volume (the specific proportion here can be re-allocated based on actual engineering experience and preliminary testing).

[0026] S2. Compensation grouting during advancement: During the shield machine's advancement, thixotropic slurry injection continues. As the shield machine advances through the sand, the constant friction between the shield and the sand, as well as the inherent flow of the sand, can cause a certain degree of loss of the injected thixotropic slurry. Without timely replenishment, the drag reduction effect will be difficult to maintain. The grouting volume during this stage accounts for 40% of the planned grouting volume (the specific percentage also needs to be determined based on actual conditions). The grouting rate is dynamically adjusted in real time based on parameters such as the shield machine's advancement speed and formation feedback pressure to ensure timely and appropriate slurry replenishment.

[0027] S3. Promote terminal strengthening grouting: When the shield machine reaches the final distance of 0.3-0.5m, the third injection of thixotropic slurry is carried out. This further strengthens the lubrication and drag reduction between the shield and the sand layer, preventing the sand from re-wrapping the shield due to pauses in construction (segment assembly), which would increase propulsion resistance. This also lays a good foundation for the smooth progress of subsequent segment assembly and other processes. The grouting volume during this stage accounts for 30% of the planned grouting volume. The grouting operation should be accurately controlled in conjunction with the real-time position and propulsion status of the shield machine to ensure that the slurry is evenly and sufficiently injected into the designated area.

[0028] The method described in this invention is suitable for shield tunneling and drag reduction operations in deep, water-rich sand layers. The following describes its implementation in detail using a specific example from a subway underground tunnel project. The tunnel's maximum depth is 50.34 meters, with total lengths of 2,598.296 meters and 2,597.154 meters, respectively. It required traversing a 1,900-meter-long, full-section, confined water-rich sand layer. Construction was performed using a shield machine with an excavation radius of 9.29 meters and a shield diameter of 9.28 meters, with a per-ring advance distance of 1.8 meters.

[0029] 1. Preparation before construction 1. Radial Grouting Hole Layout: Referring to Figure 1, six radial grouting holes 1 are arranged in a circular array at the center shield of the TBM. This layout is optimized to balance grouting efficiency and anti-clogging requirements. Grouting holes 1 provide channels for thixotropic slurry injection. Grouting from the center shield outward effectively expands the thixotropic slurry's range of application. Each radial grouting hole 1 is equipped with a one-way valve 2 to prevent mobile sand in the deep sand layer from entering the grouting channel due to internal and external pressure differences, thus ensuring smooth grouting.

[0030] 2. Thixotropic mud material preparation: Thixotropic mud is prepared with sodium-based bentonite, Baosheng 70-type additive, and water in a specific ratio. The specific mass-volume ratio is: sodium-based bentonite 250 kg / m³, Baosheng 70-type additive 4.75 L / m³, and the rest is water (1 m³). This ratio is designed for the characteristics of deep sand layers. Sodium-based bentonite can provide good plasticity and water retention, and Baosheng 70-type additive can enhance the thixotropic properties of the mud, ensuring the formation of a stable lubricating layer in a high confining pressure environment.

[0031] 3. Equipment and pipeline layout: A trolley is provided behind the shield machine to reserve a thixotropic mud grouting equipment site, and the grouting pipeline is arranged according to the optimal route. Warning signs are installed at the pipeline crossing position and the radial grouting hole 1 grouting area. Special personnel are arranged to regularly inspect the pipeline, focusing on checking pipeline aging, damage, and abnormal grouting pressure to prevent thixotropic mud gushing risks and ensure construction safety.

[0032] II. Thixotropic mud preparation Thixotropic mud is prepared during segment assembly to fully utilize the construction gap and avoid affecting the shield advancing rhythm. The preparation strictly follows the "water → bentonite → Baosheng 70 additive" pouring sequence: first, add a certain amount of water to the mixing equipment, then add sodium-based bentonite, and continue to mix until uniform to form the base mud; then add Baosheng 70-type additive, continue to mix for not less than 5 minutes (can be appropriately extended according to the material mixing condition), to ensure that the components are fully integrated, improving the lubricity and stability of the mud.

[0033] III. Thixotropic mud grouting construction 1. Grouting parameter calculation and control Grouting quantity calculation: According to the actual parameters of the project, the formula Q = k ×[ π / 4 ×( D 2 − d 2 )]× L is used to calculate the grouting quantity. Among them, the shield machine cutterhead excavation outer diameter D = 9.29 m, the middle shield outer diameter d = 9.28 m, the one-ring advancing distance L = 1.8 m, and the correction coefficient k is taken as 1.5~2 considering factors such as sand layer porosity, deep buried stress, and shield advancing speed. The theoretical grouting quantity is 0.26 m³ per ring. Combined with the thixotropic mud loss and equipment preparation efficiency in construction, the actual grouting quantity per ring is determined as 0.4 m³.

[0034] Grouting pressure control: Based on Rankine's earth pressure theory, the grouting pressure is determined by combining vertical earth pressure, lateral earth pressure and groundwater pressure. The formula is: P = k p ×[ c × h +( c − c w )×( h − h w )× K 0] Among them, the cover soil density γ, the shield machine center burial depth h, the soil lateral pressure coefficient K0, the groundwater density γw and the groundwater level burial depth hw are determined according to the engineering geological survey data, and the grouting pressure correction coefficient kp is taken as 1.3~1.6 to ensure that the thixotropic mud can effectively fill the pores of the sand layer and form a stable lubrication layer.

[0035] 2. Grouting operation in stages S1. Pre-lubrication Grouting Before the TBM begins advancing: Before the TBM begins advancing, thixotropic slurry, accounting for 30% of the planned grouting volume, is injected around the shield through radial grouting holes 1. Referring to Figure 2, the thixotropic slurry forms a lubricating, drag-reducing protective film (i.e., thixotropic slurry filling layer 4) between the shield and the sand layer, reducing the sand's grip on the shield and creating favorable conditions for the smooth start of the TBM 5.

[0036] S2. Compensatory Grouting During Tunneling: During the TBM's advancement, thixotropic slurry, representing 40% of the planned grouting volume, is continuously injected. Due to the constant friction between the shield and the sand layer and the influence of sand layer fluidity, the thixotropic slurry will be depleted. At this time, the grouting rate must be adjusted in real time based on parameters such as the TBM's advancement speed and formation feedback pressure. Mud is replenished promptly to maintain the integrity of the thixotropic slurry filling layer 4 and ensure a continuous and stable drag reduction effect.

[0037] S3. Final reinforcement grouting: When the shield machine advances to 0.3-0.5 m from the end of the first segment, the remaining 30% of the thixotropic slurry is injected for reinforcement grouting. This stage further enhances the lubrication of the thixotropic slurry fill layer 4, preventing sandy soil from re-wrapping the shield body and increasing propulsion resistance due to pauses in subsequent segment assembly, thus laying a good foundation for the construction of the next segment.

[0038] After applying this method to this project, frictional resistance data from continuous advancement of loops 459 to 485 of a single line were analyzed (see Figure 3). Without thixotropic grouting, the maximum construction frictional resistance reached 24,000 kN. With this method, the maximum advancement frictional resistance was reduced by 14,300 kN, a 60% decrease. Furthermore, the frictional resistance remained stable during continuous grouting, significantly improving the efficiency and safety of shield construction in deep sand layers and validating the applicability and superiority of this method.

Claims

1. A method for reducing resistance in deep buried sand layer shield construction, characterized in that: The construction steps include: (1) Preparation before construction: Set radial grouting holes with one-way valves at the shield position of the shield machine; prepare thixotropic slurry consisting of sodium bentonite, Baosheng 70 type admixture and water; (2) Preparation of thixotropic mud: prepare thixotropic mud according to the mass volume ratio, including 250 kg / m³ sodium bentonite, 4.75 L / m³ Baosheng 70 type admixture, and the balance is water; (3) Grouting parameter control: Calculate the grouting volume based on the porosity of the sand layer, shield parameters, and advancement speed, and determine the grouting pressure based on soil pressure and groundwater pressure; (4) Grouting construction in stages: before advancing, thixotropic mud is injected through the radial grouting holes to form a pre-lubricating layer; during the excavation process, thixotropic mud is dynamically replenished to compensate for the loss; when advancing to 0.3-0.5m from the end point, the final reinforcement grouting is carried out.

2. The drag reduction method for deep buried sand layer shield construction according to claim 1, characterized in that: In step (1), the number of radial grouting holes is 6, and the circular array is distributed around the circumference of the middle shield.

3. The drag reduction method for deep buried sand layer shield construction according to claim 1 is characterized by: In step (1), the one-way valve is a sand-blocking one-way valve that can prevent the sand layer from entering the grouting channel due to pressure difference.

4. The drag reduction method for deep buried sand layer shield construction according to claim 1, characterized in that: In step (1), a thixotropic mud grouting channel is reserved in the grouting hole, and grouting is performed from the middle shield to the outside to expand the scope of action of the thixotropic mud; each radial grouting hole is equipped with a one-way valve.

5. The drag reduction method for deep buried sand layer shield construction according to claim 1 is characterized by: In step (2), the thixotropic slurry is prepared during the segment assembly to ensure that the shield advancement rhythm is not affected.

6. The drag reduction method for deep buried sand layer shield construction according to claim 5 is characterized by: Add and mix in the order of "water → sodium bentonite → Baosheng 70 type admixture", and continue stirring for no less than 5 minutes after mixing.

7. The drag reduction method for deep buried sand layer shield construction according to claim 1 is characterized by: In step (1), the grouting equipment is arranged on the supporting trolley behind the shield machine, the grouting pipeline is arranged in a ring along the outer side of the shield body, and a warning sign is set in the area where the pipeline crosses.

8. The drag reduction method for deep buried sand layer shield construction according to claim 1, characterized in that: In step (3), the grouting volume is calculated according to the formula Q = k × [π / 4 × (D 2 −d 2 )]×L, where D is the outer diameter of the shield machine, d is the outer diameter of the middle shield, L is the advancement distance of one ring, and k is the correction coefficient, which is taken as 1.5~2.

9. The drag reduction method for deep buried sand layer shield construction according to claim 1, characterized in that: In step (3), the grouting pressure is calculated according to the formula P = k p ×[ γ × h +( γ − γ w )×( h − h w )× K 0] calculation, where γ is the weight of the covering soil, h is the buried depth of the shield machine center, γ w is the groundwater density, h w is the depth of groundwater level, K 0 is the lateral pressure coefficient of soil, kp is the correction coefficient, which is taken as 1.3~1.

6.

10. The drag reduction method for deep buried sand layer shield construction according to claim 1, characterized in that: In step (4), the pre-lubrication grouting volume before advancement accounts for 30% of the total grouting volume, the compensating grouting volume during excavation accounts for 40%, and the final strengthening grouting volume accounts for 30%.

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