Stacking type receiving method for deeply-buried water-rich composite stratum tunnel shield tunneling machine

Abstract

The invention provides a stacking type receiving method of a deeply-buried water-rich stratum tunnel shield tunneling machine. The stacking type receiving method comprises the following steps that according to the soil texture of a receiving end soil body, different sleeve valve pipe grouting technologies are adopted for reinforcing the receiving end soil body; an emergency pressure reduction and precipitation well is arranged at the position away from the receiving tunnel portal by a certain distance, and underground water is lowered to a reasonable position; a sealing water stop device is installed on the periphery of the receiving tunnel portal; a steel sleeve lower segment is installed, and after a mortar foundation is constructed at the bottom of the steel sleeve, a steel sleeve upper segment is installed; earthwork at the position of the receiving tunnel portal is chiseled away; filling the steel sleeve with a filler, and pressurizing to check the leakproofness of the steel sleeve; the shield tunneling machine sequentially enters the receiving end and the steel sleeve; the receiving tunnel portal is subjected to grouting plugging; and the steel sleeve and the shield tunneling machine are disassembled, and the shield tunneling machine is recycled. According to the stacking type receiving method suitable for the tunnel shield tunneling machine, efficient reinforcement can be conducted on the deeply-buried water-rich composite stratum, and the sand gushing and water gushing risks are reduced.

Description

Technical Field

[0001] The present application relates to a related technical field of shield machine reception, and in particular to a stacking receiving method for a shield machine in a deep-buried water-rich composite stratum tunnel. Background Art

[0002] Shield tunnel construction is a commonly used construction method in urban subway tunnel construction. Among them, ensuring that the shield arrives smoothly in the correct posture and preventing accidents such as water gushing and collapse are the key points of construction. However, for interval tunnel projects with deep burial depths, water-rich composite strata at the end area, overlapping receiving sections, and complex surrounding environments, the shield receiving technology is often difficult because of the small spacing between the two lines, obvious mutual influence, and relatively complex soil structure with strong water-richness and permeability. The traditional receiving method used in existing technologies is highly risky and difficult to construct.

[0003] On the one hand, deeply buried water-rich composite formations have the risk of sand inrush, and the use of traditional receiving methods such as direct receiving methods in end wells carries a high safety risk.

[0004] On the other hand, since the overlapping tunnels cross different soil layers, it is difficult to ensure the quality of piles when using conventional receiving-end soil reinforcement methods such as high-pressure jet grouting and deep mixing piles, and it is difficult to meet the requirements of soil strength and stability, which will have a certain impact on the subsequent shield machine reception. If the freezing method is used for reinforcement, there are problems such as long construction period, greater impact on the environment caused by frost heave and thaw settlement during construction, easy dead corners in the reinforcement range, more equipment and relatively high cost.

[0005] Therefore, in this context, how to provide a stacking receiving method suitable for tunnel shield machines that can efficiently reinforce deeply buried water-rich composite strata and reduce the risk of sand and water gushing is a technical problem to be solved. Summary of the invention

[0006] In view of the above problems in the prior art, the present application provides a stacking receiving method for a tunnel boring machine in a deep-buried water-rich composite stratum, which can efficiently reinforce the deep-buried water-rich composite stratum and reduce the risk of sand and water gushing.

[0007] To achieve the above-mentioned purpose, the first aspect of the present application provides a stacking receiving method for a deep buried water-rich composite stratum tunnel boring machine, comprising:

[0008] S1: Reinforce the receiving end soil by using a corresponding sleeve valve pipe grouting process according to the soil quality of the receiving end soil; wherein the receiving end soil at least includes the soil located outside the receiving tunnel door of the shield machine receiving shaft;

[0009] S2: An emergency decompression well is set up at a certain distance from the receiving tunnel gate to reduce the groundwater to a reasonable level;

[0010] S3: Install a sealing water-stopping device around the receiving tunnel door;

[0011] S4: installing the lower segment of the steel sleeve in the receiving shaft of the shield machine, and installing the upper segment of the steel sleeve after constructing a mortar foundation at the bottom of the steel sleeve;

[0012] S5: Chiseling away the earth at the receiving tunnel gate position;

[0013] S6: Filling the steel sleeve with filler and applying pressure to check its airtightness;

[0014] S7: The shield machine enters the receiving end soil and the steel sleeve in sequence;

[0015] S8: performing grouting to seal the receiving tunnel;

[0016] S9: dismantling the steel sleeve and the shield machine, and recovering the shield machine.

[0017] From the above, the sleeve valve tube grouting process is used to reinforce the soil layer, and different processes are used for different soil layers to strengthen the soil quality, which has better applicability and pertinence; the reinforcement effect is particularly obvious for sandy soil, silt soil, silty soil, etc., and because the construction machinery is light, simple, economical and reasonable, the overall reinforcement effect for deep buried water-rich composite strata balances quality and effect. At the same time, by using a steel sleeve as a receiving method, the airtightness during receiving can be effectively controlled, thereby reducing the risk of sand and water gushing.

[0018] As a possible implementation of the first aspect, the following also includes:

[0019] Receiving the shield machine of the stacked lower tunnel according to the steps S1-S8;

[0020] Carry out station underground slab construction;

[0021] According to the steps S1-S8, the shield machine of the stacked upper tunnel shield machine is received.

[0022] From above, the shield machine of the lower tunnel is first received, and then the shield machine of the upper tunnel is received. The construction is more convenient and safer. It also reduces the additional pressure, deformation and collapse risks brought to the lower soil by the upper tunnel construction first.

[0023] As a possible implementation manner of the first aspect, step S1 includes:

[0024] If the soil of the receiving end is a silty clay layer, the following steps are used for grouting: a sleeve valve pipe is installed at the receiving end soil position, and the sleeve material is injected; grouting is started until the grouting index reaches the technical requirements, and then the grouting is stopped; the grouting plug is retreated for a period of time, and the next stage of grouting is started; the grouting and the grouting plug retreat steps are repeated until the grouting is completed;

[0025] For the soil at the receiving end that is a water-rich sand and gravel formation, the following steps are used for grouting: installing an anti-surge orifice pipe and a drainage steel pipe, and constructing a grouting stop wall; drilling a segmented length at the soil position at the receiving end, constructing a sleeve valve pipe, and inserting a grouting plug; starting grouting through the grouting plug, and stopping grouting when the grouting index meets the technical requirements; removing the grouting plug, and continuing to drill the segmented length at the original drilling position; inserting the grouting plug and carrying out the next stage of grouting; and repeating the steps of grouting, removing the grouting plug, continuing drilling, and inserting the grouting plug until the grouting is completed.

[0026] From the above, different construction techniques are adopted for silty clay strata and water-rich sand and gravel strata; among them, the silty clay strata mainly adopt the backward construction method, which can evenly reinforce the silty clay and has better diffusibility; while the water-rich sand and gravel strata adopt the forward construction method to effectively prevent the sudden surge phenomenon.

[0027] As a possible implementation of the first aspect, grouting is first performed on the soil above the receiving end, and then grouting is performed on the soil below the receiving end; the grouting diffusion radius is 80 cm, and the sleeve valve pipe adopts a flexible φ50 PVC sleeve valve pipe.

[0028] From the top, the stability of the whole soil is enhanced by grouting from top to bottom, reducing the diffusion effect of the lower soil on the upper soil. The appropriate diffusion radius and sleeve valve pipe are selected to ensure the smooth delivery of slurry.

[0029] As a possible implementation of the first aspect, the grouting material includes at least one of the following: ordinary Portland cement single-liquid slurry plus HPC admixture, ultrafine cement single-liquid slurry, and ordinary cement-water glass double-liquid slurry.

[0030] From the above, different grouting materials are more suitable and targeted for different soil types. Among them, ordinary Portland cement single-liquid slurry plus HPC admixture has a good reinforcement effect on silty clay strata and general sandy soil; ultra-fine cement mortar is targeted at fine sand and silt strata with smaller particles; ordinary cement-water glass double-liquid slurry has a better effect on water-rich sand and gravel strata due to its rapid coagulation ability.

[0031] As a possible implementation of the first aspect, after the step S1 is completed, the soil at the receiving end is inspected, and the unconfined compressive strength of the soil at the receiving end satisfies that it is greater than 0.7 MPa, and the permeability coefficient satisfies that it is not greater than 10-6 cm / s.

[0032] From the above, when the relevant parameters of soil reinforcement meet the above requirements, it means that the strength and airtightness of the soil meet the conditions, and then the next step of construction of precipitation wells, sealing and water stopping can be carried out. Otherwise, the precipitation effect cannot be effectively guaranteed.

[0033] As a possible implementation of the first aspect, the steel sleeve includes: a cylinder body, a transition ring, a rear end cover plate and a reaction force support frame; wherein the cylinder body includes at least two segmented parts, and each segmented part has two upper and lower pipe segments.

[0034] From the above, the use of a multi-section steel sleeve makes it easy to adjust the position of the steel sleeve according to the position and posture of the shield machine, thereby reducing safety risks.

[0035] As a possible implementation manner of the first aspect, step S4 includes:

[0036] Determine the axis of the receiving portal, hoist in a transition ring so that the center of the transition ring coincides with the center of the receiving portal; weld the receiving portal and the transition ring; hoist in the lower pipe segment of the first section of the cylinder so that the center position of the cylinder coincides with the center of the receiving portal; hoist in the lower pipe segment of the second section of the cylinder, and adjust the lower pipe segment of the first section by placing debris inside the pipe segment of the first section or applying external force to the outside of the lower pipe segment of the first section in at least one of the two ways; connect the lower pipe segment of the second section with the lower pipe segment of the first section; after all the lower pipe segments are installed, install the upper pipe segment of the steel sleeve after constructing the mortar foundation at the bottom of the steel sleeve.

[0037] From above, the position of the steel sleeve is fine-tuned according to the forward position (posture) of the shield machine.

[0038] As a possible implementation of the first aspect, the pressurized inspection of airtightness in step S6 includes: applying 0.2 MPa air pressure to the sealed steel sleeve, and maintaining the air pressure in the steel sleeve above 0.18 MPa within 12 hours.

[0039] Therefore, it is ensured that the steel sleeve meets the airtightness requirements, thereby reducing the risk of sand gushing during reception.

[0040] As a possible implementation manner of the first aspect, step S7 includes the following steps:

[0041] When the shield machine penetrates the first 50m, correct the deviation and posture of the shield machine;

[0042] The shield machine stops excavating and overhauls the shield machine from the time when the remaining 20 rings are penetrated to the time when the distance to the reinforced receiving end soil is 1m;

[0043] When the shield machine excavates to the soil at the reinforced receiving end, secondary grouting is performed, and the advancing speed of the shield machine is controlled to be less than 10 mm / min, the thrust is less than 10000 kN, and the soil bin pressure is between 0.9-1.8 bar;

[0044] When the shield machine enters the steel sleeve, the tunneling speed of the shield machine is 5mm / min, the thrust is less than 4000kN, and the deviation of the shield machine's forward direction on the axis of the steel sleeve is controlled within ±20mm. From the above, the shield machine's travel parameters are changed according to the position of the shield machine, thereby ensuring that the shield machine safely enters the steel sleeve in the correct direction and posture, and reducing the risk caused by impact. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 It is a flow chart of the stacking receiving method of the deep-buried water-rich composite stratum tunnel boring machine provided by the first embodiment of the present application;

[0046] Figure 2a It is a flow chart of a stacking receiving method of a deep-buried water-rich composite stratum tunnel boring machine provided in the second embodiment of the present application;

[0047] Figure 2b It is a flow chart of a stacking receiving method of a deep-buried water-rich composite stratum tunnel boring machine provided in the second embodiment of the present application;

[0048] Figure 2c This is a process flow chart of horizontal sleeve valve pipe drilling and grouting for silty clay strata provided in the second embodiment of the present application;

[0049] Figure 2d This is a process flow chart of horizontal sleeve valve pipe drilling and grouting for sand and gravel formations provided by the second embodiment of the present application;

[0050] Figure 2e It is a schematic diagram of the steel sleeve structure provided in the second embodiment of the present application;

[0051] Figure 2f This is a schematic diagram of a steel sleeve packing provided in the second embodiment of the present application;

[0052] Figure 2gThis is a schematic diagram of the division of the construction stages of the tunneling section of the shield machine provided in the second embodiment of the present application: (a) is the first stage when the shield machine starts to advance from the remaining 20 rings to when the cutterhead is 1 m away from the reinforcement body; (b) is the second stage when the shield machine excavates to the reinforcement body; and (c) is the third stage when the shield machine cutterhead enters the steel sleeve.

[0053] It should be understood that the size and shape of each block diagram in the above structural diagram are for reference only and should not constitute an exclusive interpretation of the embodiment of the present invention. The relative position and inclusion relationship between the blocks presented in the structural diagram are only schematic representations of the structural association between the blocks, and do not limit the physical connection method of the embodiment of the present invention. DETAILED DESCRIPTION

[0054] The technical solution provided by the present application is further described below with reference to the accompanying drawings and examples. It should be understood that the system structure and business scenarios provided in the examples of the present application are mainly to illustrate the possible implementation methods of the technical solution of the present application and should not be interpreted as the only limitation on the technical solution of the present application. It is known to those of ordinary skill in the art that with the evolution of the system structure and the emergence of new business scenarios, the technical solution provided by the present application is also applicable to similar technical problems.

[0055] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by technicians in the technical field of this application. In the event of any inconsistency, the meaning described in this specification or the meaning derived from the contents recorded in this specification shall prevail. In addition, the terms used herein are only for the purpose of describing the embodiments of the present application and are not intended to limit the present application. In order to accurately describe the technical content in this application and to accurately understand the present invention, the following explanations or definitions are given to the terms used in this specification before describing the specific embodiments:

[0056] 1) Earth Pressure Balance Principle of Tunnel Boring Machine (EPB-TBM): During the excavation process, the soil in front is cut by the cutter head, and the cut soil residue enters the soil bin. The speed and excavation volume of the screw conveyor are controlled to balance the soil pressure in the soil bin with the soil pressure and water pressure of the excavation surface, thereby preventing the collapse or bulge of the excavation surface, maintaining the stability of the excavation surface, and ensuring safe and efficient tunnel construction. At the same time, the performance of the residue can be improved by injecting additives into the soil bin, making it easier to transport and maintain pressure balance.

[0057] 2) Left line and right line: In the tunnel engineering of underground urban rail transit projects, the left line and the right line usually refer to two tunnels excavated for two-way traffic.

[0058] 3) Overlap reception: In this application, it refers to a method of receiving shield machines in a specific order when there are two tunnels (usually referred to as the left line and the right line) that are very close to each other in the upper and lower layers during the shield tunnel construction process.

[0059] The stacking receiving method of the shield machine for a tunnel in a deep-buried water-rich composite stratum provided in the embodiment of the present application can be achieved by using the sleeve valve tube grouting process to reinforce the soil at the receiving end, setting up an emergency decompression well for dewatering, installing a tunnel door sealing and water-stopping device, installing a steel sleeve and a construction mortar foundation, chiseling out the tunnel door, filling the filler, receiving the shield machine, grouting to seal the tunnel door, and recovering the steel sleeve and the shield machine. This provides a stacking receiving method suitable for a tunnel shield machine, which can be used for efficient reinforcement of deeply buried water-rich composite strata and reduce the risk of sand and water gushing. The embodiment of the present application can be applied to various shield machine receiving scenarios in underground tunnel projects, especially subway projects. The embodiments of the present application are described in detail below with reference to the accompanying drawings.

[0060] The first embodiment of the present application provides a stacking receiving method for a deep buried water-rich composite stratum tunnel boring machine. Figure 1 , the implementation of each step of the method is specifically described, including steps S10-S90.

[0061] S10: Reinforce the receiving end soil body by using a corresponding sleeve valve pipe grouting process according to the soil quality of the receiving end soil body, wherein the receiving end soil body at least includes the soil body located outside the receiving tunnel door of the shield machine receiving shaft.

[0062] In some embodiments, if the soil at the receiving end is a silty clay layer, the following steps are used for grouting: a sleeve valve pipe is constructed at the soil position at the receiving end, and a casing material is injected; grouting is started, and during the grouting process, the grouting pressure, grouting flow, grouting time and other indicators are continuously monitored until the grouting indicators meet the technical requirements, and the grouting is stopped; the grouting plug is retreated for a period of time, and the next stage of grouting is started; the grouting and the grouting plug retreat steps are repeated until the grouting is completed. For the soil at the receiving end that is a water-rich sand and gravel formation, the following steps are used for grouting: installing an anti-surge orifice pipe and a drainage steel pipe, and constructing a grouting stop wall; drilling a segmented length at the soil position at the receiving end, constructing a sleeve valve pipe, and inserting a grouting plug; starting grouting through the grouting plug, and stopping grouting when the grouting index meets the technical requirements; removing the grouting plug, and continuing to drill the segmented length at the original drilling position; inserting the grouting plug and carrying out the next stage of grouting; and repeating the steps of grouting, removing the grouting plug, continuing drilling, and inserting the grouting plug until the grouting is completed.

[0063] In some embodiments, grouting is first performed on the soil above the receiving end, and then grouting is performed on the soil below the receiving end; the grouting diffusion radius is 80 cm, and the sleeve valve pipe adopts a flexible φ50 PVC sleeve valve pipe.

[0064] In some embodiments, the grouting material is selected based on the actual water gushing conditions and geological conditions during on-site construction.

[0065] The grouting material includes at least one of the following: ordinary Portland cement single-liquid slurry plus HPC admixture, ultrafine cement single-liquid slurry, and ordinary cement-water glass double-liquid slurry.

[0066] In some embodiments, after the grouting reinforcement is completed, an on-site coring experiment is performed to inspect the receiving end soil. The unconfined compressive strength of the receiving end soil meets the requirement of being greater than 0.7 MPa and the permeability coefficient meets the requirement of being less than 10 -6 cm / s.

[0067] S20: An emergency decompression well is set up at a certain distance from the receiving tunnel portal to lower the groundwater to a reasonable level.

[0068] In some embodiments, a water pump is installed in the emergency decompression well to lower the groundwater level. Multiple dewatering wells can be set at the receiving end, and the power of the water pump can be adjusted in time according to the pumping situation.

[0069] S30: Install a sealing water-stopping device around the receiving tunnel door.

[0070] In some embodiments, the sealing and water-stopping device of the receiving tunnel portal includes a circular plate, a rubber curtain, a rubber folding plate, a fan-shaped pressure plate, and bolts. When the shield machine passes through the tunnel portal sealing device, the rubber curtain shrinks and presses the fan-shaped pressure plate to prevent water from flowing into the receiving well along the outer diameter of the segment, and also prevents the synchronous grouting slurry from overflowing. The sealing and water-stopping device can be removed after the shield machine body passes through the receiving tunnel portal. Before the shield machine enters the receiving tunnel portal, lubricating oil is applied to the outer side of the shield machine's outer cutterhead and the curtain rubber plate to prevent the outer cutterhead of the shield machine from hanging and damaging the curtain rubber plate and affecting the sealing effect.

[0071] S40: installing the lower segment of the steel sleeve in the receiving shaft of the shield machine, and installing the upper segment of the steel sleeve after constructing a mortar foundation at the bottom of the steel sleeve.

[0072] In some embodiments, the steel sleeve includes: a cylinder body, a transition ring, a rear end cover plate, a front end cover plate and a reaction force support frame, etc.; wherein, the cylinder body includes at least two segmented parts, and each segmented part has two upper and lower pipe segments.

[0073] In some embodiments, the steel sleeve cylinder is divided into four sections, A1, A2, A3, and A4, and each section is divided into upper and lower semicircles. The cylinder material is made of 20 mm thick steel plate, and longitudinal and circumferential ribs are welded on the periphery of each section of the cylinder to ensure the rigidity of the cylinder. The end of each section of the cylinder and the upper and lower semicircular joint surfaces are welded with circular flanges, and the flanges are made of 40 mm thick steel plates. The upper and lower semicircles and the two sections of the cylinder are connected by M30 high-strength bolts, and a 10 mm thick rubber pad is added in the middle. A bottom bracket is made at the bottom of the cylinder, and the bracket and the lower part of the cylinder are welded together.

[0074] In some embodiments, a transition ring is provided between the steel sleeve and the steel ring of the receiving tunnel portal. The length of the transition ring can be adjusted according to the length of the shield receiving shaft. The transition ring and the rubber folding plate are welded to ensure the airtightness between the steel sleeve and the folding plate. One end of the rear end cover plate of the steel sleeve is placed on the reaction support frame, and the other end is connected to the cylinder. The reaction support frame is formed by welding of steel sections and installed close to the rear end cover plate. The front end cover plate of the steel sleeve does not contact the reaction support frame. The reaction support frame should be reliably connected or tightened with the rear (station), and pre-pressing should be carried out before receiving. It can only be officially received after there are no problems.

[0075] In some embodiments, the central axis of the steel sleeve must be consistent with the design axis of the tunnel, and the elevation is controlled by ±10mm between the center height of the shield and the actual installation elevation of the portal.

[0076] In some embodiments, the transition ring is hoisted into the receiving well according to the measured and laid-out reference line, and the center line of the transition ring coincides with the center of the portal first. After the transition ring is positioned, two arc-shaped plates are welded on the left and right sides of the transition ring and the receiving portal steel ring to prevent the transition ring from moving. After that, the lower part of the A1 segment is hoisted into the measured and positioned position in the working well and connected to the transition ring. Then, the lower segments of the A2, A3, and A4 segments are installed in the same way, and the lower half of the segment and the transition ring are connected with M30 high-strength bolts. A welding support is added in the middle to fix it to prevent the steel sleeve from deforming.

[0077] In some embodiments, the axis of the receiving portal is determined, and a transition ring is hoisted in so that the center of the transition ring coincides with the center of the receiving portal; the receiving portal and the transition ring are welded; the lower segments of the first section of the cylinder are hoisted in so that the center position of the cylinder coincides with the center of the receiving portal; the axis of the receiving portal is redetermined according to the latest position of the shield machine; the receiving portal and the transition ring are connected; the lower segments of the second section of the cylinder are hoisted in, and the lower segments of the first section are adjusted by placing debris inside the segments of the first section or applying external force to the outside of the lower segments of the first section; the second section is connected to the first section; after all the lower segments are installed, the upper segments of the steel sleeve are installed after constructing a mortar foundation at the bottom of the steel sleeve.

[0078] In some embodiments, after the cylinder of the steel sleeve is installed, the rear cover plate and the lower part of the A4 segment are bolted together, wherein a sealing gasket and a sealant are added to the connection surface. Then, the rear cover plate is tightened against the station floor through a reaction support frame, and the diagonal brace of the reaction support frame is welded firmly to the embedded steel plate of the floor.

[0079] In some embodiments, after the steel sleeve is fully installed, all connections, welds and other locations of the steel sleeve are inspected as a whole.

[0080] In some embodiments, the mortar foundation is poured within a range of 60° from the bottom of the steel sleeve, and the pouring thickness is 27 cm.

[0081] S50: removing the earth at the receiving tunnel gate position.

[0082] In some embodiments, the tunnel portal is manually chiseled out. After the lower part of the steel sleeve is installed, that is, when the shield machine cutter head is pushed forward to the front of the ground-connected wall, construction begins. When chiseling, work is carried out in order of numbering, first from top to bottom, and then from the middle to the sides.

[0083] In some embodiments, the ground surface around the ground-connected wall is inspected during the removal process, and if obvious wet marks are found, the operation is stopped immediately.

[0084] S60: Fill the steel sleeve with filler and apply pressure to check its airtightness.

[0085] In some embodiments, the filler is mainly made of slag and bentonite excavated by the shield machine and water, and the steel sleeve filling is completed in three times. The last part of the top is backfilled with inert slurry.

[0086] In some embodiments, the pressurized inspection includes: applying 0.2MPa air pressure to the sealed steel sleeve, and maintaining the air pressure in the steel sleeve above 0.18MPa for 12 hours. Strain gauges can also be used to observe the displacement between the components of the steel sleeve, such as installing strain gauges in different areas where the steel sleeve is connected to the annular plate or the rubber folding plate.

[0087] S70: The shield machine enters the receiving end soil and the steel sleeve in sequence.

[0088] In some embodiments, the tunneling construction of the shield machine entry section is divided into the following three stages: the shield machine starts from penetrating the remaining 20 rings, the shield machine excavates to the reinforced receiving end soil, and the shield machine cutter head enters the steel sleeve.

[0089] In some embodiments, when the shield machine penetrates the first 50m, the deviation and posture of the shield machine are corrected; when the shield machine penetrates the remaining 20 rings to 1m away from the reinforced receiving end soil, the shield machine stops excavating and is repaired; when the shield machine excavates to the reinforced receiving end soil, secondary grouting is performed, and the advancing speed of the shield machine is controlled to be less than 10mm / min, the thrust is less than 10000kN, and the soil bin pressure is between 0.9-1.8bar; when the shield machine enters the steel sleeve, the excavation speed of the shield machine is 5mm / min, the thrust is less than 4000kN, and the deviation of the shield machine's forward direction from the axis of the steel sleeve is controlled within ±20mm.

[0090] In some embodiments, pressure sensors, displacement sensors, attitude sensors and geological radars are used to monitor soil bin pressure, shield machine attitude, and geological conditions ahead. According to parameters such as geological radar, soil bin pressure, and shield machine position, tunneling parameters such as thrust and propulsion speed are automatically controlled to prevent ground subsidence. Among them, corresponding displacement and pressure sensors can be set in the working well of the receiving end, the soil of the adjacent tunnel, and other locations, and parameters such as ground and surrounding building settlement are sent to the shield machine to jointly set safety thresholds. When a safety warning occurs, emergency braking is performed to ensure the safety of the tunnel and staff.

[0091] S80: performing grouting and sealing on the receiving tunnel door.

[0092] In some embodiments, double liquid slurry is used as the grouting material; the double liquid slurry ratio is: water glass: cement slurry = 1:1.15, and the cement slurry ratio is: 1:1; the grouting pressure is 0.2-0.3Mpa, and the grouting amount is 140-200% of the theoretically calculated grouting amount.

[0093] S90: dismantling the steel sleeve and the shield machine, and recovering the shield machine.

[0094] In some embodiments, the shield machine of the stacked lower tunnel (left line) is first received according to the steps S10-S80; after the left line shield machine and the steel sleeve are dismantled, the underground floor construction of the station is carried out; and the underground floor template is used as a structural support, and the shield machine of the stacked upper tunnel (right line) is received according to the steps S10-S80.

[0095] The second embodiment of the present application provides a stacking receiving method for a shield machine in a deep-buried water-rich composite stratum tunnel. The embodiment of the present application is a new shield machine receiving method developed based on the EPB-TBM balance principle. Specifically, it refers to the entire construction process of receiving the shield machine using a closed receiving device steel sleeve from 100m before the shield machine arrives at the next station receiving well to the stage when the shield machine passes through the interval tunnel and enters the station or receiving well. The shield machine located in the bottom tunnel is first received, and after the dismantling and hoisting are completed, the shield machine in the top tunnel is received. The following will refer to Figure 2a-2b The method provided by the second embodiment includes the following steps S200-S310.

[0096] S200: Use corresponding sleeve valve pipe grouting technology to reinforce the soil according to the soil quality of the receiving end.

[0097] First, according to the soil quality of the receiving end, drilling and grouting construction are carried out for the silty clay layer and the sandy gravel layer respectively. The order of grouting reinforcement is from top to bottom, that is, the upper soil layer is reinforced first, and then the lower soil layer is reinforced.

[0098] like Figure 2c As shown in the figure, for silty clay layers, the specific steps are as follows:

[0099] Determine the number and layout of grouting reinforcement holes according to the specifications.

[0100] Measure and lay out the drilling positions according to the design on the drawings, and then start drilling; after drilling is completed, clean the holes.

[0101] Insert the sleeve valve tube into the hole and fix it at the designated position. The sleeve valve tube adopts a flexible φ50PVC sleeve valve tube (core tube).

[0102] Inject sleeve valve sleeve shell material, wait for it to solidify, then lower the grouting plug and start grouting. The slurry penetrates the valve sleeve through the grouting pipe and enters the soil that needs to be reinforced. Among them, the grouting reinforcement materials for silty clay strata and water-rich sand and gravel strata are ordinary silicate cement single liquid slurry plus HPC admixture, ultrafine cement single liquid slurry, and ordinary cement-water glass double liquid slurry. During the construction process, they are selected according to the actual water gushing situation and geological conditions, and adjusted and optimized according to the on-site grouting conditions.

[0103] During the grouting process, continuously monitor indicators such as grouting pressure, grouting flow, and grouting time. When the grouting indicators meet the technical requirements, stop grouting.

[0104] Move the grouting plug back a certain distance and proceed to the next stage of grouting.

[0105] Among them, after completing a stage of grouting, the valve sleeve will shrink after the pressure disappears, tightly hugging the sleeve, forming a one-way valve, making it difficult for the slurry in the soil to pass through the valve sleeve into the sleeve valve pipe, thereby preventing the slurry from flowing back.

[0106] When the reinforcement effect of a certain area is not ideal or a new part that needs reinforcement is found, the grouting plug can be re-inserted into the appropriate position of the casing and grouting can be carried out again.

[0107] When all the grouting of a borehole is completed, the reinforcement effect of the soil is checked. If the effect is not good, drilling and grouting are carried out again near the hole; if the effect is good, the construction of the next hole is carried out according to the drawing.

[0108] like Figure 2d As shown, for the water-rich sand and gravel formation, the specific steps are as follows:

[0109] Determine the number and layout of grouting reinforcement holes according to the specifications.

[0110] Install the anti-surge orifice pipe according to the drilling position designed in the drawing, drive in the drainage steel pipe, and carry out the slurry stop wall construction;

[0111] Use a multi-functional drilling rig to drill a hole of 1-1.5m in length, construct the sleeve valve pipe and casing in the same way as the construction method for silty clay strata, put in the grouting plug, and start grouting;

[0112] When the grouting index reaches the technical requirements, stop grouting;

[0113] Remove the grouting plug and continue drilling the same length at the original drilling location;

[0114] Putting in the grouting plug and carrying out the next stage of grouting;

[0115] The steps of grouting, removing the grouting plug, continuing drilling, and inserting the grouting plug are repeated until the grouting is completed.

[0116] Among them, the grouting effect of each hole and the whole is checked. If the grouting effect of a single hole is not good or the overall grouting effect is not good, it is necessary to add holes and continue grouting.

[0117] After the reinforcement is completed, a field core test is carried out to test the uniformity and self-support of the soil. If the unconfined compressive strength of the soil is >0.7MPa and the permeability coefficient is ≤10 -6 cm / s, it means that the soil reinforcement effect is good and can be received by the subsequent shield machine.

[0118] S210: Set up emergency pressure relief wells to lower the groundwater level to a reasonable level.

[0119] After the soil reinforcement construction at the end was completed, an emergency decompression and dewatering well was constructed when the shield machine reached the end. A total of 9 dewatering wells were drilled with a diameter of 600mm, a spacing of 6 meters, and a depth of 39 meters.

[0120] Place a water pump 175QJ32-48 / 4 in the well and adjust the size of the water pump in time according to the pumping situation.

[0121] S220: Install a sealing water-stopping device around the receiving tunnel door.

[0122] An annular sealing rubber sheet water-stopping device is installed around the shield machine receiving hole ring on the lining wall to prevent the water in the hole and the backfill grouting from flowing out along the shield machine shell toward the hole opening.

[0123] Among them, the water-stopping device includes a circular ring plate, a rubber curtain, a fan-shaped pressure plate and bolts.

[0124] Before the shield machine enters the reserved tunnel door, apply lubricating oil to the outer cutter head and the outer side of the rubber curtain sheet to prevent the shield machine cutter head from damaging the rubber curtain sheet and affecting the sealing effect.

[0125] When the shield machine passes through the tunnel sealing device, the rubber curtain shrinks and presses the fan-shaped pressure plate, thereby preventing water from flowing into the receiving well along the outer radial direction of the segment, and also preventing the synchronous grouting slurry from overflowing.

[0126] The sealing and water-stopping device can be removed after the shield machine exits the tunnel.

[0127] S230: Install the lower segment of the steel sleeve in the receiving shaft of the shield machine.

[0128] like Figure 2e As shown, the steel sleeve provided in the embodiment of the present application is composed of a cylinder, a transition ring, a rear end cover plate and a reaction force support frame. Among them, the cylinder part of the steel sleeve is divided into four sections A1, A2, A3 and A4, and each section is divided into two upper and lower semicircles.

[0129] The cylinder is made of 20mm thick steel plate, and longitudinal and circumferential ribs are welded on the periphery of each section of the cylinder to ensure the rigidity of the cylinder. A bottom bracket is made at the bottom of the cylinder, and the bracket and the lower cylinder are welded together.

[0130] The specific installation steps are as follows:

[0131] Before installing the steel sleeve, first determine the center line of the steel sleeve and the axis of the receiving tunnel in the receiving working well. Considering the posture of the shield machine entering the tunnel, ensure that the center axis of the steel sleeve is consistent with the design axis of the receiving tunnel and the tunnel. The elevation of the center of the steel sleeve and the center point of the receiving tunnel are controlled within ±10mm.

[0132] According to the drawings, the layout is carried out and the transition ring is hoisted in so that the center of the transition ring coincides with the center of the receiving tunnel. The length of the transition ring can be adjusted according to the length of the receiving working shaft of the shield machine.

[0133] After the transition ring is positioned, two arc plates are welded on the left and right sides of the transition ring and the circular ring of the tunnel gate to prevent the transition ring from moving. The transition ring is welded to the fan-shaped pressure plate (folding plate) of the water stop device to ensure the airtightness between the two.

[0134] After that, the lower half of the steel sleeve A1 is hoisted into the working pit for positioning and installation, so that the center of the steel sleeve coincides with the center of the receiving tunnel. Then the lower half of the A2, A3, and A4 cylinder segments are hoisted in turn.

[0135] M30 high-strength bolts are used to connect A1 and the lower part of the transition ring, and welded supports are used in the middle to prevent deformation of the steel sleeve.

[0136] The ends of each section of the cylinder are welded with round flanges, and the flanges are made of 40mm thick steel plates. The upper and lower semicircles and the two sections of the cylinder are connected with M30 high-strength bolts, and a 10mm thick rubber pad is added in the middle. Before each connection, check the center position of the steel sleeve again, use a jack to apply pressure from the outside, or put slag in the front and rear positions inside the cylinder to make slight adjustments to the position and direction of the steel sleeve.

[0137] S240: Construct mortar foundation at the bottom of the steel sleeve.

[0138] Pour a 27cm thick mortar foundation within the 60° range of the bottom of the steel sleeve, and ensure that the mortar foundation extends into the tunnel gate and connects with the reinforced soil to prevent the cutter head from getting stuck when it comes out of the reinforced body.

[0139] S250: Install the upper segments of the steel sleeve, and install the steel supports and rear end cover.

[0140] Lift in the upper segments of the steel sleeves A1, A2, A3, and A4 in sequence, and use welded round flanges to connect the upper and lower semicircular surfaces. After installation, tighten the bolts, check the connections of each part, and inspect each connection installation location to ensure the integrity of the connection.

[0141] Bolt the rear cover plate to the lower part of A4, and add sealing gasket and sealant to the connection surface. One end of the rear cover plate is placed on the reaction support frame, and the other end is connected to the cylinder.

[0142] The reaction force support frame is formed by welding of steel sections and is installed closely against the rear end cover plate, and the crown ball cover part of the rear end cover plate does not contact with the reaction force support frame.

[0143] The reaction support frame should be reliably connected or tightened with the rear station floor or the working pit ground, the supporting diagonal brace should be firmly welded to the embedded steel plate of the floor, and the weld position should be checked to ensure that there are no hidden dangers such as slag inclusion and false welding. Pre-pressing should be carried out before receiving, and formal acceptance can only be carried out after there are no problems.

[0144] Finally, the steel sleeve support is positioned. After all the above installation is completed, the joints of each part are checked to ensure the integrity of the connection.

[0145] S260: Chisel out the receiving tunnel door.

[0146] In the embodiment of the present application, since the underground continuous wall at the receiving end uses threaded steel bars, the cutter head of the shield machine cannot directly grind the wall and advance, and the underground continuous wall must be chiseled out before entering the tunnel.

[0147] The tunnel portal is chiseled out manually after the lower part of the steel sleeve is installed, that is, when the shield machine cutter head is pushed forward to the front of the ground-connected wall.

[0148] Before chiseling construction, draw the outline of the tunnel entrance on the receiving end wall according to the designed dimensions, and divide the tunnel entrance into 9 parts. When chiseling, work in the order of numbers, first from top to bottom, first from the middle to the sides.

[0149] In addition, during the excavation period, the surface monitoring work around the ground-connected wall was strengthened, and special personnel were arranged to conduct patrols and pay attention to the condition of the tunnel entrance wall. Once obvious wet marks appeared, the work was stopped immediately and a treatment plan was studied.

[0150] S270: Fill the steel sleeve with filler and apply pressure to check its airtightness.

[0151] The steel sleeve filling is carried out in three times. The filling is mainly made of slag and bentonite excavated by the shield machine and water.

[0152] First, after the installation of the lower half of the steel sleeves A1, A2, A3, and A4, the first filling and tunnel chiseling are carried out;

[0153] After the installation of the upper half of the segments A1, A2, A3, and A4 is completed, the second filling is carried out (such as Figure 2f shown);

[0154] The last section at the top is backfilled with an inert slurry.

[0155] During construction, a funnel is first set up on the ground to hold the filler, and then a conveying pipeline is led to transport the filler directly from the funnel to the steel sleeve, filling the entire steel sleeve densely.

[0156] S280: Shield machine enters tunnel section for excavation control.

[0157] After the steel sleeve is installed and filled, pressurize the steel sleeve to check its tightness.

[0158] Before the pressure test, strain gauges are installed in different areas at the connection between the steel sleeve and the hinged plate ring plate, and a dial indicator is installed on the surface of the steel sleeve with a measuring range of 3-5mm, which can control the deformation or displacement accuracy to about 0.5mm.

[0159] Apply 0.2MPa air pressure to the steel sleeve. If the pressure is maintained at 0.18MPa within 12 hours, the acceptance requirements of the steel sleeve can be met. If it is less than 0.18MPa, find out the leaking part, check and repair its sealing quality, and then conduct pressure test again until the pressure test requirements are met.

[0160] If pressure leakage is found during the pressure test, the pressure should be relieved immediately and handled in a timely manner. The bolts should be tightened or re-welded, and the pressure test should be carried out again until the pressure stabilizes at 0.18MPa and no leakage is found. Only then can it be confirmed that the steel sleeve has good sealing and can be put into use.

[0161] S290: Shield machine enters the tunnel section and steel sleeve.

[0162] Before the TBM arrives, the TBM positioning and receiving portal recheck are first carried out. All the measurement control points at the receiving end are re-measured as a whole and systematically, and the coordinates of all control points are precisely and accurately adjusted. At the same time, 50m before the breakthrough, the TBM posture and segment measurement are further strengthened, and the deviation is corrected in time according to the re-measurement results. In combination with the measured receiving portal position, the TBM posture is appropriately adjusted when the tunnel is broken through to ensure that the TBM arrives according to the designed route.

[0163] like Figure 2g As shown, the embodiment of the present application divides the tunneling construction of the shield machine into the following three stages, and dynamically adjusts the tunneling parameters and performs grouting control in each stage:

[0164] (1) The first stage is when the shield machine starts to penetrate the remaining 20 rings and advances until the cutterhead is 1m away from the reinforcement body. At this time, the shield machine stops excavating and is overhauled to put it in the best rotation state.

[0165] (2) The second stage is the tunneling of the shield machine to the reinforced receiving end soil.

[0166] When the filling in the receiving steel sleeve is completed and the shield machine has advanced to the reinforced body, the secondary grouting is carried out at the beginning of the 20th ring from the end. During the secondary grouting, the position of the grouting holes, the grouting pressure and the grouting amount are strictly controlled.

[0167] At the same time, the advancement speed and total thrust of the shield machine are strictly controlled to avoid the cutterhead being stuck due to excessive penetration of the cutter. The advancement speed is controlled within 10mm / min, the thrust is less than 10000kN, the soil bin pressure is between 0.9-1.8bar, and mud and foam are added to the soil bin and in front of the cutterhead during the rotation of the cutterhead to improve the soil.

[0168] (3) The third stage is when the shield machine cutter head enters the steel sleeve.

[0169] At this time, the shield machine's propulsion speed is guaranteed to be less than 5mm / min, the thrust is less than 4000KN, and it is adjusted dynamically in time. In addition, the shield machine's posture is controlled based on the actual measured steel sleeve installation center line, the center line deviation is controlled within ±20mm, and the settlement deformation monitoring work around the ground, surrounding buildings, steel sleeves and tunnel gates is done well.

[0170] When the shield machine is excavating inside the steel sleeve, ensure unimpeded communication with the outside world and closely observe the situation on the top of the steel sleeve. Once excessive deformation or leakage is found, stop excavation immediately and take remedial measures in time.

[0171] S300: Grouting and sealing of receiving tunnel gate.

[0172] After the shield machine completely enters the steel sleeve, double-liquid slurry is injected immediately. The mixing ratio of the double-liquid slurry is: water glass: cement slurry = 1:1.15, the mixing ratio of cement slurry is: 1:1, and the grouting pressure is 0.2~0.3Mpa.

[0173] At the connection position between the cylinder barrel and the receiving tunnel, the grouting amount is 150-200% of the theoretical grouting amount to ensure that the receiving tunnel structure is sealed. After the double-liquid grouting solidifies, the grouting effect is confirmed through the exploration hole on the back of the steel sleeve segment. If there is no sand leakage or water leakage, the steel sleeve can be disassembled.

[0174] S310: Dismantle the steel sleeve and shield machine, and recover the lower tunnel shield machine.

[0175] Remove each part of the steel sleeve in turn, clean the shield machine, and dismantle and recycle the shield machine.

[0176] After the shield machine recovery of the lower tunnel (left line in the embodiment of the present application) is completed, the construction of the underground floor of the station can be carried out. When the floor reaches the design strength, the disc-type scaffolding is not removed first, and the underground floor template support is used to support the upper tunnel receiving work plane (structural plate). The structure can be reinforced or adjusted as necessary to ensure that it can meet all technical requirements in the receiving process. Then the shield machine of the upper tunnel (right line) is received. Its construction and other steps are the same as those of the lower tunnel.

[0177] In addition, the words "first, second, third, etc." or module A, module B, module C and other similar terms in the specification and claims are only used to distinguish similar objects and do not represent a specific ordering of the objects. It can be understood that the specific order or sequence can be interchanged where permitted so that the embodiments of the present application described herein can be implemented in an order other than that illustrated or described herein.

[0178] In the above description, the numbers representing the steps, such as S110, S120, etc., do not necessarily mean that the steps will be executed accordingly. If permitted, the order of the steps can be interchanged or they can be executed simultaneously.

[0179] The term "comprising" as used in the description and claims should not be interpreted as being limited to what is listed thereafter; it does not exclude other elements or steps. Therefore, it should be interpreted as specifying the presence of the features, integers, steps or components mentioned, but does not exclude the presence or addition of one or more other features, integers, steps or components and groups thereof. Therefore, the expression "a device comprising means A and B" should not be limited to a device consisting of components A and B only.

[0180] References to "one embodiment" or "an embodiment" in this specification mean that a particular feature, structure, or characteristic described in conjunction with the embodiment is included in at least one embodiment of the present application. Therefore, the phrases "in one embodiment" or "in an embodiment" appearing in various places in this specification do not necessarily refer to the same embodiment, but may refer to the same embodiment. In addition, in one or more embodiments, the particular features, structures, or characteristics can be combined in any appropriate manner, as would be apparent to one of ordinary skill in the art from this disclosure.

[0181] Note that the above are only preferred embodiments of the present application and the technical principles used. Those skilled in the art will understand that the present application is not limited to the specific embodiments described herein, and that various obvious changes, readjustments and substitutions can be made by those skilled in the art without departing from the scope of protection of the present application. Therefore, although the present application is described in more detail through the above embodiments, the present application is not limited to the above embodiments, and may also include more other equivalent embodiments without departing from the concept of the present application, all of which belong to the scope of protection of the present application.

Claims

1. A stacking receiving method for a shield machine of a deep buried water-rich composite stratum tunnel, characterized in that: The steps include: S1: Reinforce the receiving end soil by using a corresponding sleeve valve pipe grouting process according to the soil quality of the receiving end soil; wherein the receiving end soil at least includes the soil located outside the receiving tunnel door of the shield machine receiving shaft; S2: An emergency decompression well is set up at a certain distance from the receiving tunnel gate to reduce the groundwater to a reasonable level; S3: Install a sealing water-stopping device around the receiving tunnel door; S4: installing the lower segment of the steel sleeve in the receiving shaft of the shield machine, and installing the upper segment of the steel sleeve after constructing a mortar foundation at the bottom of the steel sleeve; S5: Chiseling away the earth at the receiving tunnel gate position; S6: Filling the steel sleeve with filler and applying pressure to check its airtightness; S7: The shield machine enters the receiving end soil and the steel sleeve in sequence; S8: performing grouting to seal the receiving tunnel door; S9: dismantling the steel sleeve and the shield machine, and recovering the shield machine.

2. The method according to claim 1, characterized in that Also includes: Receiving the shield machine of the stacked lower tunnel according to the steps S1-S8; Carry out station underground slab construction; According to the steps S1-S8, the shield machine of the stacked upper tunnel shield machine is received.

3. The method according to claim 1, characterized in that The step S1 comprises: If the soil at the receiving end is a silty clay layer, the following steps are used for grouting: A sleeve valve pipe is constructed at the soil position of the receiving end, and a casing material is injected; Start grouting and stop grouting when the grouting index reaches the technical requirements; Move the grouting plug back a certain distance and start the next stage of grouting; The grouting and grouting plug retreating steps are repeated until the grouting is completed; If the soil at the receiving end is a water-rich sandy gravel stratum, the following steps are used for grouting: Install anti-surge orifice pipes and drainage steel pipes, and carry out slurry-stop wall construction; Drill a segmented length of hole at the receiving end soil position, construct a sleeve valve pipe, and insert a grouting plug; Starting grouting through the grouting plug, and stopping grouting when the grouting index reaches the technical requirements; Remove the grouting plug and continue drilling the segmented length at the original drilling position; Putting in the grouting plug and carrying out the next stage of grouting; The steps of grouting, removing the grouting plug, continuing drilling, and inserting the grouting plug are repeated until the grouting is completed.

4. The method according to claim 3, characterized in that The soil above the receiving end is firstly grouting-injected, and then the soil below the receiving end is grouting-injected; the grouting diffusion radius is 80 cm, and the sleeve valve pipe adopts a flexible φ50 PVC sleeve valve pipe.

5. The method according to claim 3, characterized in that: The grouting material includes at least one of the following: ordinary Portland cement single-liquid slurry plus HPC admixture, ultrafine cement single-liquid slurry, and ordinary cement-water glass double-liquid slurry.

6. The method according to any one of claims 1 to 5, characterized in that: After the step S1 is completed, the soil at the receiving end is inspected, and the unconfined compressive strength of the soil at the receiving end meets the requirement of being greater than 0.7 MPa, and the permeability coefficient meets the requirement of being less than 10-6 cm / s.

7. The method according to claim 1, characterized in that The steel sleeve comprises: a cylinder body, a transition ring, a rear end cover plate and a reaction force support frame; wherein the cylinder body comprises at least two segmented parts, and each segmented part has two upper and lower pipe segments.

8. The method according to claim 7, characterized in that The step S4 comprises: Determine the axis of the receiving tunnel door, and hoist in a transition ring so that the center of the transition ring coincides with the center of the receiving tunnel door; Welding the receiving hole door and the transition ring; Hanging in the lower segment of the first section of the cylinder so that the center position of the cylinder coincides with the center of the receiving tunnel; Re-determining the axis of the receiving tunnel portal according to the latest position of the shield machine; Connecting the receiving tunnel door and the transition ring; The lower pipe segment of the second segment is hoisted into the cylinder, and the lower pipe segment of the first segment is adjusted by placing debris inside the pipe segment of the first segment or applying external force outside the lower pipe segment of the first segment; Connecting the lower segment of the second segment to the lower segment of the first segment; After all the lower pipe segments are installed, the upper pipe segments of the steel sleeve are installed after constructing the mortar foundation at the bottom of the steel sleeve.

9. The method according to claim 8, characterized in that The pressurized inspection of airtightness in step S6 includes: applying 0.2 MPa air pressure to the sealed steel sleeve, and maintaining the air pressure in the steel sleeve above 0.18 MPa within 12 hours.

10. The method according to claim 1, characterized in that The step S7 comprises the following steps: When the shield machine penetrates the first 50m, correct the deviation and posture of the shield machine; The shield machine stops excavating and overhauls the shield machine from the time when the remaining 20 rings are penetrated to the time when the distance to the reinforced receiving end soil is 1m; When the shield machine excavates to the soil at the reinforced receiving end, secondary grouting is performed, and the advancing speed of the shield machine is controlled to be less than 10 mm / min, the thrust is less than 10000 kN, and the soil bin pressure is between 0.9-1.8 bar; When the shield machine enters the steel sleeve, the excavation speed of the shield machine is 5mm / min, the thrust is less than 4000kN, and the axial deviation of the shield machine's forward direction in the steel sleeve is controlled within ±20mm.

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