Volcanic ash stratum large-diameter large-gradient ultra-shallow-buried shield launching construction method

Abstract

The invention discloses a volcanic ash stratum large-diameter large-gradient ultra-shallow buried shield launching construction method which comprises the following steps: S1, construction preparation: checking tunnel buried depth H, underground water buried depth and gradient parameters; s2, building envelope construction is conducted, and plain concrete water stop piles and plain underground diaphragm walls are sequentially constructed; s3, stratum reinforcement is conducted, specifically, the stratum around the hole is reinforced through high-pressure jet grouting piles; s4, back pressure construction is conducted, and solidified soil is backfilled at the top of the shield; s5, tunnel portal sealing is conducted, specifically, double curtain cloth and a tunnel portal sealing device are installed, and grouting is conducted on a sealing cavity; and S6, shield launching is conducted, specifically, a shield is started, and ground surface settlement is monitored in real time. According to the technical scheme, the reliability of the volcanic ash stratum component when the large-diameter, large-gradient and shallow-buried tunnel is constructed is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of shield tunnel engineering, and in particular to a method for starting construction of a large-diameter, large-slope, ultra-shallow buried shield in a volcanic ash stratum. Background Art

[0002] With the rapid development of urban underground space, the demand for large-diameter shield tunneling in complex strata is growing. Volcanic ash strata, a typical example of unfavorable geological conditions, are characterized by loose structure, high porosity, and strong permeability. The initial construction of large-diameter (diameter > 10 m), steep slopes (slope > 5%), and ultra-shallow (burial depth < 1 tunnel diameter) shield tunnels in these strata presents numerous technical challenges. First, the poor self-stability of the stratum makes face collapse highly likely. Second, controlling surface subsidence in ultra-shallow conditions is challenging. Furthermore, controlling the shield's attitude is difficult in steep slope conditions.

[0003] In addition, the high permeability of volcanic ash strata makes conventional water-stopping measures ineffective. Among related technologies, conventional shield starting methods are difficult to solve the above problems at the same time. Especially under high-pressure groundwater conditions, traditional tunnel sealing devices are prone to leakage, and the stratum reinforcement effect is difficult to guarantee.

[0004] Therefore, it is urgent to develop a method for starting construction of large-diameter, large-slope, and ultra-shallow shield tunnels in volcanic ash strata to ensure construction safety. Summary of the Invention

[0005] The main purpose of this invention is to propose a method for starting construction of a large-diameter, large-slope, and ultra-shallow shield tunnel in volcanic ash strata, aiming to improve the reliability of large-diameter, large-slope, and shallow-buried tunnels in volcanic ash strata.

[0006] To achieve the above-mentioned object, the present invention proposes a method for starting construction of a large-diameter, large-slope, ultra-shallow shield machine in a volcanic ash stratum. The method comprises the following steps: Step S1: Construction preparation, verifying tunnel depth H, groundwater depth and slope parameters; Step S2: Construction of the enclosure structure, including sequential construction of plain concrete water stop piles and plain underground continuous walls; Step S3: Ground reinforcement: Use high pressure jet grouting piles to reinforce the ground around the hole. The jet pressure P jet Satisfies the following formula: Among them, K s is the soil system number of the volcanic ash formation, γ is the unit weight of the formation, and C is the basic pressure constant; Step S4: back-pressure construction, backfilling solidified soil on the top of the shield; Step S5: Tunnel door sealing, install double curtains and tunnel door sealing device, and inject grout into the sealing cavity, the grouting pressure P seal Satisfies the following formula: Among them, P mud is the mud water pressure in the shield excavation chamber, ΔP is the pressure margin; Step S6: Shield machine starts, starts the shield machine and monitors the ground settlement in real time.

[0007] In some embodiments, in formation reinforcement, K s The value is determined according to the type of volcanic ash formation, where the value is 1.2-1.5 for sandy volcanic ash, 1.0-1.2 for clayey volcanic ash, and 1.5-1.8 for gravelly volcanic ash.

[0008] In some embodiments, in the tunnel door sealing, grouting uses bentonite slurry with a viscosity of 120S; ΔP is dynamically adjusted according to the formation permeability, and 0.05MPa≤ΔP≤0.10MPa.

[0009] In some embodiments, when the permeability of the formation is greater than 10-4 m / s, the value of ΔP is constant at a maximum value of 0.10 PMa.

[0010] In some embodiments, the portal sealing device includes four pre-buried portal steel rings, a double-layer steel section is provided inside the portal steel ring for support, and a plurality of grouting pipes are evenly arranged on the upper part.

[0011] In some embodiments, the double-layer steel sections are removed before the cord rubber is installed.

[0012] In some embodiments, during the construction of the retaining structure, when the plain water-stop piles are being constructed, the verticality of the holes is monitored in real time, and when the hole wall collapses, quick-setting concrete is immediately filled in.

[0013] In some embodiments, during counter-pressure construction, the anti-floating balance is controlled to satisfy the following formula: Among them, F buoy is the segment buoyancy, γ cover is the weight of the solidified soil, V cover is the volume of compacted and solidified soil, and ∑W is the sum of the weight of the segment, box culvert and track layer.

[0014] In some embodiments, when backfilling the solidified soil at the top of the shield, the compaction degree of the solidified soil backfill is controlled to be ≥95%, and the error of the covering soil thickness is ≤5cm.

[0015] In some embodiments, the shield is started and the surface settlement is monitored in real time. When the surface settlement rate exceeds 2 mm / h, the grouting pressure P is controlled. sealIncreased from 0.02MPa to 0.05MPa.

[0016] The technical solution of the present invention can improve the injection pressure P during stratum reinforcement by means of enclosure structure construction, stratum reinforcement parameter optimization, back pressure balance control and dynamic grouting pressure regulation. jet And the grouting pressure P when the tunnel is sealed seal It can effectively solve the problems of collapse, settlement and leakage when the shield is started in the volcanic ash stratum, and has the advantages of effectively preventing the collapse of the face, accurately controlling the surface settlement, ensuring the stability of the shield posture and improving the water-stopping and sealing effect. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the structures shown in these drawings without paying any creative work.

[0018] Figure 1 A flowchart of a method for starting construction of a large-diameter, large-slope, ultra-shallow shield tunnel in a volcanic ash stratum provided by one embodiment of the present invention.

[0019] The purpose, features and advantages of the present invention will be further described with reference to the accompanying drawings and in conjunction with the embodiments. DETAILED DESCRIPTION

[0020] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts shall fall within the scope of protection of the present invention.

[0021] It should be noted that if the embodiments of the present invention involve directional indications (such as up, down, left, right, front, back, etc.), the directional indications are only used to explain the relative position relationship, movement status, etc. between the various components under a certain specific posture. If the specific posture changes, the directional indications will also change accordingly.

[0022] In addition, if there are descriptions involving "first", "second", etc. in the embodiments of the present invention, the descriptions of "first", "second", etc. are only for descriptive purposes and cannot be understood as indicating or suggesting their relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features limited to "first" and "second" may explicitly or implicitly include at least one of such features. In addition, if "and / or" or "and / or" appears in the full text, its meaning includes three parallel solutions. Taking "A and / or B" as an example, it includes solution A, solution B, or solutions that satisfy both A and B. In addition, the technical solutions between the various embodiments can be combined with each other, but it must be based on the ability of ordinary technicians in this field to implement. When the combination of technical solutions is mutually contradictory or cannot be implemented, it should be deemed that such a combination of technical solutions does not exist and is not within the scope of protection required by the present invention.

[0023] With the growing demand for urban underground space development, the construction of large-diameter shield tunnels in volcanic ash strata presents significant challenges. Due to the loose structure, high porosity, and strong permeability of this stratum, traditional construction methods struggle to effectively control face collapse and surface subsidence. Conventional shield tunneling techniques have shortcomings in retaining structure stability, calculation of stratum reinforcement pressure, and reliability of sealing devices. These shortcomings increase the risks of construction in extremely shallow buried conditions, particularly when steep slopes and an insufficient diameter-to-depth ratio are prone to instability accidents.

[0024] The main risks are as follows: The risk of portal instability and face collapse. After the retaining structure is removed, the volcanic ash layer softens and disintegrates easily when exposed to water, causing soil instability around the portal. Furthermore, traditional single-layer sealing devices struggle to effectively seal under high pressure differentials, preventing the buildup of mud-water pressure. Face collapses are a frequent occurrence, and statistics show that such incidents account for over 35% of shield failures in volcanic ash layers. Insufficient anti-buoyancy can cause ground buoyancy. Under ultra-shallow buried conditions, groundwater levels fluctuate significantly, and the buoyancy forces on the segments can reach more than twice their own weight. In existing backpressure construction, insufficient cover compaction or volumetric miscalculation can cause ground buoyancy and localized surface collapse. Project examples show a buoyancy rate as high as 20%. Surface settlement is difficult to control, and volcanic ash strata are highly sensitive to deformation. Conventional high-pressure jet jet pile reinforcement pressure parameters do not take into account the coupling effect of soil characteristic coefficient and burial depth. As a result, when the pressure is too low, the size of the consolidated body is insufficient, and the permeability of the reinforced area is only reduced by 10% to 30%. When the pressure is too high, the stratum disturbance clamps and the surface settlement rate exceeds 3mm / h.

[0025] Seal aging causes slurry pressure fluctuations, and there's no mechanism for dynamically matching the portal seal grouting pressure with the shield excavation chamber slurry pressure. When the grouting pressure is less than or equal to the shield excavation chamber slurry pressure, slurry backflow can easily cause seal failure, resulting in a sudden drop in slurry pressure of up to 40%, and even direct instability of the tunnel face.

[0026] In summary, there is an urgent need to provide a new shield starting control method that can adapt to the large-diameter and large-slope construction environment of volcanic ash strata and can carry out ultra-shallow buried construction to solve the above problems, and systematically solve the problems of tunnel portal stability, sealing reliability, anti-floating accuracy and settlement controllability.

[0027] See also Figure 1 The present invention proposes a method for starting construction of a large-diameter, large-slope, ultra-shallow shield machine in a volcanic ash stratum. The method comprises the following steps: Step S1: Construction preparation, verifying the tunnel depth H, groundwater depth and slope parameters.

[0028] This step is a preparatory step before construction. The tunnel depth H, the vertical distance from the track surface to the ground surface, can be directly verified from the tunnel construction drawings. Simultaneously, a field survey of the construction site can be conducted for verification. This verification can include, but is not limited to, measuring the ground elevation of the tunnel axis using a total station or GPS-RTK (Real Time Kinematic), and then verifying the actual elevation of the tunnel roof through drilling. Tunnel depth H = ground elevation - tunnel roof elevation.

[0029] Groundwater depth can be determined through hydrogeological surveys. For example, monitoring wells can be deployed in the construction area, with spacing no greater than 100 meters between adjacent wells. Water level gauges or pressure sensors can be used to measure the static water level in real time. Groundwater depth is calculated by subtracting the static water level from the ground surface elevation.

[0030] Slope parameters can be directly obtained from the design file.

[0031] Step S2: Construction of the retaining structure, sequentially constructing plain concrete water stop piles and plain underground continuous walls.

[0032] Plain concrete waterstop piles are vertical water-blocking structures made of unreinforced concrete. Through continuous pouring, they form a continuous underground water-blocking curtain, blocking the path of groundwater infiltration. The composite structure of plain concrete waterstop piles and underground diaphragm walls enhances the integrity of the enclosure system.

[0033] Step S3: Ground reinforcement: Use high pressure jet grouting piles to reinforce the ground around the hole. The jet pressure P jet Satisfies the following formula: Among them, Ks is the soil system parameter of the volcanic ash formation, which can be adjusted according to the looseness of the formation, and the value range can be but is not limited to 1.0 to 1.8; γ is the unit weight of the formation, which can be measured by collecting soil samples; C is the basic pressure constant, which is set to ensure the initial penetration of the slurry and avoid the situation where the pressure is too low at a shallow burial depth. The value is usually 0.1MPa to 0.3MPa.

[0034] High-pressure jet jet pile reinforcement refers to cutting the soil with high-pressure fluid and adding a curing agent to form a composite pile body. For example, cement slurry is used as a curing medium to enhance the integrity of the stratum around the hole.

[0035] In this step, the injection pressure P is provided for the high-pressure jet grouting pile to reinforce the surrounding stratum. jet The calculation formula is based on the coupling of geological parameters (volcanic ash stratum soil system number K s , formation unit weight γ, and foundation pressure constant C), achieves dynamic and precise control of high-pressure jet grouting pile pressure, directly addressing the issues of insufficient strength, uncontrolled settlement, and material waste in volcanic ash formation reinforcement. Compared to related technologies, this method can provide differentiated reinforcement for volcanic ash formations with complex geological conditions, resolving issues such as uniform jetting pressure, which results in insufficient pressure in sandy layers leading to a smaller consolidated body diameter, and excessive pressure in clay layers, which leads to soil splitting and increased permeability.

[0036] In the formula, the injection pressure P jet It is positively correlated with the tunnel burial depth H, so that the pressure can be automatically reduced in the shallow buried area (H < 5m) to reduce the disturbance to the soil. The soil system number K of the volcanic ash stratum s Differentiated pressure control is carried out according to the characteristics of each soil body to further improve the reinforcement effect of high-pressure jet grouting piles in each stratum.

[0037] Step S4: Back pressure construction, backfilling solidified soil on the top of the shield.

[0038] In this step, the improved soil can be compacted and covered on the top of the shield. For example, lime or cement can be added to improve the mechanical properties of the soil and provide anti-floating reaction force through volume compaction.

[0039] Step S5: Tunnel door sealing, install double curtains and tunnel door sealing device, and inject grout into the sealing cavity, the grouting pressure P seal Satisfies the following formula: Among them, P mud is the mud water pressure in the shield excavation chamber, and ΔP is the pressure margin.

[0040] The double curtain and tunnel door sealing device refers to an annular sealing structure composed of two layers of flexible materials, such as an inner and outer layer of rubber curtain and a grouting cavity, which compensates for the pressure loss caused by formation penetration through the grouting pressure.

[0041] In this step, the grouting pressure P seal Dynamically coupled shield excavation chamber mud water pressure P mud With the pressure margin ΔP, the sealing grouting pressure P seal Accurately adapting to the characteristics of volcanic ash strata can effectively improve the problems of face collapse, tunnel instability and uncontrolled surface subsidence, while significantly improving construction economy.

[0042] In these embodiments of the present application, the shield excavation chamber mud water pressure P mud The earth pressure sensor at the front end of the cutterhead can be used to collect and obtain the pressure in real time during shield tunneling. The pressure margin ΔP can be determined based on the permeability of different strata in the volcanic ash stratum. For example, it can be determined by drilling sampling. In dense clay layers, the permeability coefficient is usually ≤10 -5 m / s, the pressure margin ΔP can be set to 0.05MPa, while in loose sandy layers, the permeability coefficient is usually >10 -5 m / s. At this time, the pressure margin ΔP can be set to 0.08 MPa to 0.10 MPa to compensate for the seepage loss of the volcanic ash formation.

[0043] In these embodiments of the present application, the grouting pressure P seal Real-time matching of shield excavation chamber mud water pressure P mud By setting the pressure margin ΔP, the grouting pressure P seal The mud water pressure P in the shield excavation chamber mud There is always a pressure difference of pressure margin ΔP between the two, which can effectively reduce the accident rate of tunnel face collapse and further improve the reliability of construction.

[0044] Step S6: Shield machine starts, starts the shield machine and monitors the ground settlement in real time.

[0045] During the initial stage of shield tunneling, real-time settlement monitoring data is combined to dynamically adjust the grouting pressure to maintain stratum stability, so as to achieve millimeter-level settlement control (allowable value ≤15 mm) under the ultra-shallow buried conditions of volcanic ash strata, and prevent ground grouting and structural damage.

[0046] In these embodiments of the present application, real-time monitoring of surface subsidence can be performed by, but is not limited to, automated total station network monitoring, liquid level connecting pipe sensor system, distributed optical fiber sensing (BOTDR), three-dimensional laser scanning, and the like.

[0047] Through the above technical solution, this application effectively suppresses the risk of face collapse at the start of shield tunneling in volcanic ash strata, and improves the stability of the excavation face through the synergistic effect of the retaining structure and stratum reinforcement; two-stage sealing and dynamic grouting pressure control significantly reduce the probability of groundwater leakage; quantitative back pressure measures and real-time settlement monitoring form a closed-loop control system, which controls surface settlement within a safe threshold, providing a reliable solution for large-slope ultra-shallow buried shield construction.

[0048] In some embodiments, in formation reinforcement, K s The value is determined according to the type of volcanic ash formation, where the value is 1.2-1.5 for sandy volcanic ash, 1.0-1.2 for clayey volcanic ash, and 1.5-1.8 for gravelly volcanic ash.

[0049] Sandy volcanic ash refers to a loose sediment layer in which volcanic ash particles are mainly sand particles. It can be identified by screening or microscopic observation. Its porosity is relatively high, and the reinforcement pressure needs to be increased by increasing the coefficient value.

[0050] Clayey volcanic ash refers to strata in which the clay mineral content in the volcanic ash exceeds 30%. It can be specifically determined by X-ray diffraction analysis. Its permeability is low, and a relatively small coefficient value needs to be used to avoid over-grouting.

[0051] Gravelly volcanic ash refers to a coarse-grained formation consisting of a mixture of volcanic ash and gravel, which can be identified through on-site drilling sampling. It has poor structural stability and requires a higher coefficient value to enhance the reinforcement effect.

[0052] Before shield tunneling begins, the type of volcanic ash formation must be determined based on the geological survey report. For example, if the formation is sandy volcanic ash, the coefficient can be set in the range of 1.2-1.5, and the corresponding high-pressure jet grouting pressure can be calculated using a formula. If it is clayey volcanic ash, the coefficient is adjusted to 1.0-1.2 to match its lower permeability. If it is gravelly volcanic ash, the coefficient is further increased to 1.5-1.8 to account for the grouting diffusion resistance caused by the gravel skeleton. This allows the formation reinforcement pressure to dynamically adapt to the physical properties of different volcanic ash formations, avoiding collapse caused by insufficient pressure or waste of slurry due to excessive pressure.

[0053] The large-diameter, large-slope, and ultra-shallow shield tunneling method proposed in this application addresses the problem of unstable reinforcement and low resource utilization in conventional methods, which typically employs a single coefficient value for ground reinforcement, failing to account for variations in ground types. This approach, by classifying coefficient values, allows the injection pressure of high-pressure jet jet piles to precisely match the permeability and structural characteristics of sandy, clayey, and gravelly volcanic ash.

[0054] In some embodiments, during portal sealing, bentonite slurry with a viscosity of 120S is used for grouting; ΔP is dynamically adjusted according to the permeability of the formation, and 0.05MPa≤ΔP≤0.10MPa.

[0055] Among them, the bentonite slurry with a viscosity of 120S is achieved by adding a viscosity enhancer to adjust the rheological properties of the sodium-based bentonite suspension. Its shear-thinning properties can enhance the penetration and filling ability of the slurry in porous formations.

[0056] Dynamic adjustment of the pressure margin ΔP refers to correcting the difference between the grouting pressure and the mud and water pressure in the excavation chamber within a preset range based on the real-time monitored changes in the formation permeability coefficient. This can be achieved through the linkage between the pressure sensor and the PLC control system. For example, when the formation permeability increases, the pressure margin is increased to compensate for slurry loss.

[0057] During the portal sealing phase, bentonite slurry with a viscosity of 120S is injected into the sealing cavity through the grouting pipe. Its thixotropic properties cause the slurry to form a gel structure in a static state, effectively sealing the gap between the curtain and the portal steel ring. Dynamic adjustment of the pressure margin ΔP is achieved through a closed-loop control system. For example, when the monitored permeability coefficient exceeds a set threshold, the system automatically increases the pressure margin to the upper limit, ensuring that the pressure in the sealing cavity remains higher than the excavation chamber slurry pressure, preventing groundwater carrying fine particles into the shield machine. This pressure control logic avoids seal failure due to insufficient pressure and prevents excessive pressure from causing stratum fracture.

[0058] Through the above-mentioned technical solution, this application can effectively maintain the pressure stability of the tunnel portal sealing cavity and prevent the exacerbation of surface subsidence caused by mud and water leakage. The high viscosity of the bentonite slurry enhances the filling and sealing effect of loose volcanic ash formations. The dynamic adjustment mechanism of the pressure margin ΔP ensures pressure balance under conditions of different permeability formations, thereby reducing the risk of face instability during the initial stage of shield tunneling.

[0059] For example, in these embodiments of the present application, the value of ΔP can be set to, but is not limited to, 0.06 MPa, 0.07 MPa, 0.08 MPa or 0.09 MPa. In some embodiments, when the permeability of the formation is greater than 10-4 m / s, the value of ΔP is constant at a maximum value of 0.10 PMa.

[0060] The permeability coefficient refers to the ability of a formation to transmit water per unit time. It can be measured through on-site pumping tests or indoor permeability tests and is used to characterize the degree of pore connectivity in volcanic ash formations. When the permeability coefficient exceeds 10-4 m / s, it indicates a significant increase in formation porosity and the presence of rapid seepage channels. Continuing to dynamically adjust the pressure margin ΔP at this point may result in the grouting pressure failing to effectively seal the leakage path. By fixing the pressure margin ΔP at 0.10 MPa, a stable pressure gradient can be formed within the sealing cavity, allowing the bentonite slurry to fully fill the formation pores. During implementation, when the permeability coefficient test value exceeds the critical threshold, the control system automatically locks the pressure margin ΔP adjustment function to maintain a constant grouting pressure output.

[0061] In some embodiments, the portal sealing device includes four pre-buried portal steel rings, a double-layer steel section is provided inside the portal steel ring for support, and a plurality of grouting pipes are evenly arranged on the upper part.

[0062] The portal steel ring is a circular steel structure embedded in the portal. It can be constructed by welding Q345B steel plates assembled in sections, forming a rigid support base for the sealing device. Double-layer steel sections are upper and lower support frames composed of H-shaped steel. They can be constructed by cross-welding H300×300×10×15 steel sections to maintain the geometric accuracy of the steel ring during construction. Grouting pipes are pipe structures embedded in the top of the steel ring. They can be constructed by evenly spacing DN50 seamless steel pipes at 800mm intervals. Grouting pipes are used to inject grout into the sealing cavity to fill the gaps.

[0063] Four pre-buried portal steel rings are bolted together to form a complete annular structure. During construction, a double layer of H-shaped steel was welded to the inner surface of the rings to form a temporary support system. The upper layer of H-shaped steel was removed to free up space before the rubber curtain was installed. Grouting pipes pre-buried at the top of the rings were evenly distributed along the circumference. During construction, grouting was simultaneously injected through multiple pipes to ensure uniform grout pressure distribution within the sealing cavity. The rigid support of the double layer of steel offsets the lateral pressure on the rings from the loose soil of the volcanic ash formation. The multiple grouting pipes ensure full circumferential filling of the sealing cavity.

[0064] Through the above technical solution, this application solves the problem of sealing failure of the portal steel ring caused by the squeezing of loose soil in the volcanic ash stratum. The double-layer steel support effectively controls the installation accuracy deviation of the steel ring. The uniform arrangement of multiple grouting pipes enables the sealing slurry to form a continuous annular waterstop, thereby preventing groundwater carrying fine particles from rushing into the shield chamber in the initial stage.

[0065] In some embodiments, the double-layer steel sections are removed before the cord rubber is installed.

[0066] The removal of double-layer steel sections can be achieved by, but is not limited to, cutting or segmented lifting to make room for the subsequent installation of the rubber cord.

[0067] In volcanic ash formations, after the portal ring is installed, the loose and highly permeable ground structure makes it susceptible to displacement due to water and soil pressure around the portal. In this case, installing a double layer of steel sections to internally support the portal ring effectively improves its deformation resistance. Once the portal ring is stabilized and the surrounding structure is complete, the double layer of steel sections is removed to prevent it from occupying the space required for the installation of the rubber cord. This process not only ensures the stability of the portal structure during construction, but also creates conditions for the subsequent complete installation of the sealing device.

[0068] This application realizes the connection between the installation process of the portal steel ring support and the sealing device, preventing the insufficient pressure of the curtain rubber or the blockage of the grouting pipe due to the residual support structure, thereby improving the reliability of the portal sealing and reducing the risk of water and soil leakage in the initial stage.

[0069] In some embodiments, during the construction of the retaining structure, when the plain water-stop piles are being constructed, the verticality of the holes is monitored in real time, and when the hole wall collapses, quick-setting concrete is immediately filled in.

[0070] Real-time monitoring of borehole verticality involves dynamic measurement of the drilling trajectory using an inclinometer or laser plummet. This can be achieved by measuring once every meter of drilling. This measure can promptly detect borehole deviation and prevent pile failure caused by vertical deviation. Infilling with rapid-setting concrete refers to using a concrete material with an initial setting time of less than 10 minutes. Specifically, this can be achieved using a mixture of sulfoaluminate cement and an early strength agent. This material can quickly solidify to form a support structure when the hole wall collapses, preventing the collapse from spreading.

[0071] Under the loose geological conditions of the volcanic ash formation, localized spalling of the borehole wall is prone to occur during drilling due to groundwater infiltration and the deadweight of the formation. By monitoring verticality data in real time, if the borehole axis deflection exceeds the allowable threshold or abnormal deformation of the borehole wall is detected, rapid-setting concrete is immediately pumped into the collapsed area. This concrete sets within 30 seconds and reaches a compressive strength of 0.5 MPa within 60 seconds, effectively sealing the collapsed area and restoring the stability of the borehole wall, ensuring the continuity of subsequent drilling operations.

[0072] Through the above-mentioned technical solution, this application effectively solves the problem of pile verticality loss caused by hole collapse during drilling in volcanic ash formations, ensuring the continuous construction of water-stop piles and creating stable working conditions for subsequent underground continuous wall construction. The immediate filling effect of rapid-setting concrete blocks the path of hole collapse, maintains the original stress state of the formation, and reduces the risk of overall instability of the retaining structure.

[0073] In some embodiments, during counter-pressure construction, the anti-floating balance is controlled to satisfy the following formula: Among them, F buoy is the segment buoyancy, γcover is the weight of the solidified soil, V cover is the volume of compacted and solidified soil, and ∑W is the sum of the weight of the segment, box culvert and track layer.

[0074] Segment buoyancy F buoy It refers to the buoyancy of groundwater on the shield segments during construction. This can be achieved by calculating the product of the hydrostatic pressure of groundwater on the segments and the volume of water displaced by the segments. This parameter is used to quantify the anti-buoyancy stability requirements.

[0075] Solidified soil density γ cover It refers to the unit volume weight of the solidified soil, which can be obtained by laboratory measurement of the product of the wet density of the backfill material and the acceleration of gravity. This parameter directly affects the ability of the backfill soil to offset the buoyancy of the pipe segment.

[0076] Compacted soil volume V cover It refers to the actual volume of the backfill solidified soil after compaction, which can be calculated through 3D laser scanning or cross-sectional measurement. This parameter is used to evaluate the coverage effect of the backfill soil on the pipe segments.

[0077] ∑W refers to the total weight of the segments, box culverts, and track layers, which can be calculated by superimposing the structural design parameters. This parameter is used to provide static load support to prevent floating.

[0078] During the shield top backfill process, the balance between groundwater buoyancy, backfill soil, and the deadweight of the structure is dynamically monitored to adjust the backfill material's compaction and cover thickness in real time. When the segment buoyancy exceeds the anti-buoyancy provided by the backfill soil and the deadweight of the structure, the amount of backfill soil or the compaction level can be increased to ensure the anti-buoyancy safety factor always meets the design requirements.

[0079] Through the above technical solution, this application effectively solves the problem of segment displacement caused by the sudden increase in groundwater buoyancy due to the high permeability of volcanic ash strata. By quantifying the anti-buoyancy balance relationship, the segment posture is ensured to be stable at the initial stage of shield tunneling, avoiding the risk of structural cracking or axis deviation caused by buoyancy imbalance.

[0080] In some embodiments, when backfilling the solidified soil at the top of the shield, the compaction degree of the solidified soil backfill is controlled to be ≥95%, and the error of the covering soil thickness is ≤5cm.

[0081] The compaction degree of solidified soil backfill is a quantitative indicator of the density of solidified soil particles per unit volume. It can be tested on-site using the ring knife method or sand injection method. This indicator enhances anti-floating stability by improving soil density.

[0082] The soil cover thickness error refers to the deviation between the actual soil cover thickness and the designed value. This can be monitored in real time using a laser rangefinder or ultrasonic sensor. This indicator avoids local stress concentration by controlling the uniformity of the soil cover.

[0083] During the initial phase of shield tunneling in volcanic ash formations, the loose strata are susceptible to groundwater buoyancy, leading to segmental displacement. By backfilling the soil in layers and controlling the compaction of each layer, a continuous, dense structure is formed, effectively resisting the upward force of groundwater. When using a vibratory roller for compaction, each layer is kept to a thickness of no more than 30 cm, and compaction is monitored in real time using a nuclear density meter. Cover thickness is measured using a multi-point method, with measurement points set every 5 meters along the shield axis to ensure thickness tolerances are within acceptable limits.

[0084] In some embodiments, if it is detected that the compaction degree of a local area does not meet the standard, additional rolling or local replacement of solidified soil can be taken. For areas with excessive cover thickness, correction can be made by manual paving or mechanical leveling.

[0085] Through the above technical solution, this application effectively suppresses the floating tendency of the shield segments caused by groundwater in the initial stage, avoids local settlement caused by uneven cover thickness, and ensures the structural stability of the ultra-shallow buried shield in loose volcanic ash strata.

[0086] In some embodiments, the shield is started and the surface settlement is monitored in real time. When the surface settlement rate exceeds 2 mm / h, the grouting pressure P is controlled. seal Increased from 0.02MPa to 0.05MPa.

[0087] The surface subsidence rate refers to the rate of change in ground subsidence per unit time during shield tunneling. This can be monitored in real time using a laser rangefinder or subsidence sensor, with thresholds set to trigger the grouting pressure adjustment mechanism. Dynamic grouting pressure adjustment involves adjusting the pressure within the sealed cavity in real time based on the formation's response. This can be achieved by using a variable frequency pump or proportional valve to achieve pressure gradient control to balance formation pressure fluctuations.

[0088] During shield tunneling, the ground settlement rate, a key indicator of ground stability, is collected in real time. When the monitoring data exceeds a preset threshold, indicating an acceleration in ground deformation, the grouting pressure is increased to increase the sealing cavity's support for the surrounding soil. The magnitude of the pressure increase is determined by the correlation between the settlement rate and ground permeability. For example, when the settlement rate reaches a critical value, the lower limit of the pressure increase, 0.02 MPa, is used to compensate for minor ground losses, while the upper limit, 0.05 MPa, is used to address sudden soil loss.

[0089] Through the above technical solution, this application can quickly respond to abnormal deformation of the stratum during the starting stage of the shield machine, timely block the soil loss channel through pressure compensation, inhibit the development speed of settlement, reduce the risk of face instability caused by continuous soil loss, and ensure the safe starting of large-diameter shield machines under ultra-shallow burial conditions.

[0090] The above description is only a preferred embodiment of the present invention and does not limit the patent scope of the present invention. All equivalent structural transformations made by using the contents of the present invention description and drawings under the inventive concept of the present invention, or direct / indirect application in other related technical fields are included in the patent protection scope of the present invention.

Claims

1. A method for starting construction of a large-diameter, large-slope, ultra-shallow shield tunnel in a volcanic ash stratum, characterized in that: The steps include: Construction preparation, verification of tunnel depth H, groundwater depth and slope parameters; The retaining structure is constructed by sequentially constructing plain concrete water stop piles and plain underground continuous walls; Ground reinforcement: high pressure jet grouting piles are used to reinforce the ground around the tunnel. The jet pressure P jet Satisfies the following formula: Among them, K s is the soil system number of the volcanic ash formation, γ is the unit weight of the formation, and C is the basic pressure constant; Back pressure construction, backfilling solidified soil on top of the shield; Tunnel door sealing, install double curtain and tunnel door sealing device, and inject grout into the sealing cavity, the grouting pressure P seal Satisfies the following formula: Among them, P mud is the mud water pressure in the shield excavation chamber, ΔP is the pressure margin; The shield machine starts, starts the shield machine and monitors the surface settlement in real time.

2. The method for starting construction of a large-diameter, large-slope, ultra-shallow shield tunnel in volcanic ash strata according to claim 1 is characterized in that: In the ground reinforcement, the K s The value is determined according to the type of volcanic ash formation, where the value is 1.2-1.5 for sandy volcanic ash, 1.0-1.2 for clayey volcanic ash, and 1.5-1.8 for gravelly volcanic ash.

3. The method for starting construction of a large-diameter, large-slope, ultra-shallow shield tunnel in volcanic ash strata according to claim 1 is characterized in that: In the tunnel door sealing, bentonite slurry with a viscosity of 120S is used for grouting; ΔP is dynamically adjusted according to the formation permeability, and 0.05MPa≤ΔP≤0.10MPa.

4. The method for starting construction of a large-diameter, large-slope, ultra-shallow shield tunnel in volcanic ash strata according to claim 3 is characterized in that: When the permeability of the local layer is greater than 10-4m / s, the value of ΔP is constant at the maximum value of 0.10PMa.

5. The method for starting construction of a large-diameter, large-slope, ultra-shallow shield tunnel in volcanic ash strata according to claim 1 is characterized in that: The tunnel portal sealing device comprises four pre-buried tunnel portal steel rings, the interior of the tunnel portal steel rings is provided with double-layer steel sections for support, and a plurality of grouting pipes are evenly arranged on the upper part.

6. The method for starting construction of a large-diameter, large-slope, ultra-shallow shield tunnel in volcanic ash strata according to claim 5, characterized in that: The double-layer steel section is removed before the cord rubber is installed.

7. The method for starting construction of a large-diameter, large-slope, ultra-shallow shield tunnel in volcanic ash strata according to claim 1, characterized in that: During the construction of the retaining structure, when the plain water-stop piles are being constructed, the verticality of the holes is monitored in real time, and when the hole wall collapses, quick-setting concrete is immediately filled in.

8. The method for starting construction of a large-diameter, large-slope, ultra-shallow shield tunnel in volcanic ash strata according to claim 1, characterized in that: In the counter-pressure construction, the anti-floating balance is controlled to satisfy the following formula: Among them, F buoy is the segment buoyancy, γ cover is the weight of the solidified soil, V cover is the volume of compacted and solidified soil, and ∑W is the sum of the weight of the segment, box culvert and track layer.

9. The method for starting construction of a large-diameter, large-slope, ultra-shallow shield tunnel in volcanic ash strata according to claim 8, characterized in that: When backfilling the solidified soil at the top of the shield, the compaction degree of the solidified soil backfill is controlled to be ≥95%, and the error of the covering soil thickness is ≤5cm.

10. The method for starting construction of a large-diameter, large-slope, ultra-shallow shield tunnel in volcanic ash strata according to claim 1, characterized in that: When the shield is started and the surface settlement is monitored in real time, the grouting pressure P is controlled when the surface settlement rate exceeds 2 mm / h. seal Increased from 0.02MPa to 0.05MPa.

Images