To understand the advanced support of tunnel pipe shed at one t

• Posted by William William August 25, 2022 - Filed in Arts & Culture - 367 views

  Original title: Understand the advanced support of tunnel pipe shed at one time. There are too many detailed words. You need to collect them first! Source: Zhulong Forum Copyright belongs to the original author. As an auxiliary method of tunnel excavation, pipe-roof advanced support technology plays an important role in preventing tunnel landslide, inhibiting stratum displacement and controlling surface subsidence. Today, let's learn in detail what should be paid attention to in this tunnel construction technology! Advance support method for underground engineering Advance support technology refers to the technology that before tunnel excavation, an arched continuum is formed on the cross section of the tunnel by grouting, freezing, driving steel pipes, steel plates, anchor rods and other technical measures into the stratum in front of the tunnel face to reinforce the stratum in front of the excavation face, and at the same time, its supporting force is used to maintain the stability of the soil in front and reduce the surface settlement. The research shows that the grouting reinforcement can improve the strength and deformation modulus of the surrounding rock, and fundamentally improve the deformation law of the surrounding rock. The data show that the strength of sandstone can be increased by 50% -70% after grouting, siltstone and argillaceous rock can be increased by 2-4 times, and the increase of rock strength can reduce the support load by 2/3-4/5. The practice shows that the advanced support system can effectively limit the ground subsidence and fully maintain the natural strata in a stable state during tunnel excavation. The Research shows that 30% -40% of the total subsidence of the ground and 40% -50% of the total subsidence of the underground stratum occur before the general support begins to work, and the advance support has a 30-35% inhibition effect on the ground subsidence and a 40% inhibition effect on the tunnel top stratum (vault) subsidence. Reinforcing the stratum in front of the tunnel face plays a very important role in inhibiting land subsidence. Advance support technology has been widely used in underground engineering construction as an auxiliary construction method to reinforce the stratum, stabilize the vault and tunnel face, and reduce the surface settlement. According to the influence of reinforcement measures on the characteristics of surrounding strata and stress distribution, the advance support can be divided into stratum improvement method and pre-support method. Stratum improvement method is a method to improve the characteristics of the stratum soil around the excavation face, which includes grouting, soil reinforcement, drainage and stratum freezing, etc. Pre-support method is a method to reinforce the surrounding rock in advance before tunnel excavation, so as to increase the self-stability of the surrounding rock and minimize the stress interference around the excavation face. Advanced support methods mainly include: pipe shed method, mechanical pre-cutting method, pre-lining method, horizontal jet grouting method, advanced small conduit method, advanced anchor method, freezing method and so on. Among them, pipe roof method, horizontal jet grouting method, small conduit method and other support methods also improve and reinforce the stratum. Pipe shed grouting support Expand the full text A group of steel pipes are driven into the stratum along the drilled holes outside the excavation contour, and are combined with the steel arch to form a strong scaffolding pre-supporting and reinforcing system to support the load from the upper part of the pipe shed, and grouting is pressurized into the stratum through the quincunx grouting holes of the steel pipes to reinforce the weak and broken stratum and improve the self-stability of the stratum. Pipe roof grouting is a long-distance advanced support method with long advanced distance and large rigidity, which is suitable for the stratum where the tunnel face can not be self-stabilized and contains water, and has good effect of controlling surface settlement and anti-seepage and water sealing, and has high requirements for construction technology. If the pipe roof grouting is combined with the small pipe supplementary grouting method, it can prevent the collapse of the triangular soil below the pipe roof in addition to the characteristics of the large pipe roof, and the effect of this combination of long and short pre-support is more ideal. Main functions and advantages of pipe shed support (1) Beam-arch effect: The pipe shed constructed in advance forms a beam structure with the tunnel face and the rear support as the fulcrum. The two form a shell-like structure surrounding the tunnel outline, which can effectively inhibit the loosening and collapse of surrounding rock. (2) Reinforcement effect: The grouting slurry is pressed into the surrounding rock cracks through the pipe wall holes, so that the loose rock mass is cemented and consolidated, thus improving the physical and mechanical properties of the weak surrounding rock, enhancing the self-supporting capacity of the surrounding rock, and achieving the purpose of reinforcing the weak surrounding rock around the steel pipe. (3) Annular groove effect: The propagation of explosion shock wave and the expansion of detonation gas generated by tunnel face blasting are reflected, absorbed or diffracted when they meet the dense annular holes and grooves in the pipe roof, which greatly reduces the damage degree and disturbance range of surrounding rock caused by reverse stretching wave. (4) Ensure the safety of construction: the support rigidity of the pipe roof is relatively large. In case of landslide during construction, the collapse debris will also fall on the rock debris on the upper part of the pipe roof, which will play a buffer role. Even if the pipe roof is unstable, its damage will be relatively slow. List of advance support method for tunnel According to the influence of reinforcement measures on the characteristics of surrounding strata and stress distribution, the advance support can be divided into stratum improvement method and pre-support method. Stratum improvement method is a method to improve the characteristics of the stratum soil around the excavation face, which includes grouting, soil reinforcement, drainage and stratum freezing, etc. Pre-support method is a method to reinforce the surrounding rock in advance before tunnel excavation, so as to increase the self-stability of the surrounding rock and minimize the stress interference around the excavation face. Advanced support methods mainly include: pipe shed method, mechanical pre-cutting method, pre-lining method, horizontal jet grouting method, advanced small conduit method, advanced anchor method, freezing method and so on. Among them, pipe roof method, horizontal jet grouting method, small conduit method and other support methods also improve and reinforce the stratum. Pipe shed grouting support A group of steel pipes are driven into the stratum along the drilled holes outside the excavation contour, and are combined with the steel arch to form a strong scaffolding pre-supporting and reinforcing system to support the load from the upper part of the pipe shed, and grouting is pressurized into the stratum through the quincunx grouting holes of the steel pipes to reinforce the weak and broken stratum and improve the self-stability of the stratum. Pipe roof grouting is a long-distance advanced support method with long advanced distance and large rigidity, which is suitable for the stratum where the tunnel face can not be self-stabilized and contains water, and has good effect of controlling surface settlement and anti-seepage and water sealing, and has high requirements for construction technology. If the pipe roof grouting is combined with the small pipe supplementary grouting method, in addition to the characteristics of the large pipe roof, it can prevent the collapse of the triangular soil below the pipe roof, and this combination of long and short pre-support effect is more ideal. Applicable scope of pipe shed support According to the construction practice at home and abroad and the actual cases of the application of pipe shed support in underground engineering in China at present, the pipe shed support can be applied to soft sandy soil strata, sandy gravel strata, expansive soft flow plastic and hard plastic silty clay strata. The support for the construction of underground structures with abundant geological and groundwater conditions, such as fissure developed rock mass, mud and water burst section, fault fracture zone, collapse section, broken soil and rock pile section, shallow buried and large bias pressure, and the support for the excavation of tunnel entrance and exit sections are also mostly used in the excavation of underground projects such as subway crossing urban areas. It can be used as an auxiliary method for building tunnels under existing buildings, highways, railways and underground structures; As an auxiliary construction method for the construction of tunnel portal section and large-section tunnel, and as an auxiliary construction method for other construction, it is also commonly used for projects with special requirements for construction settlement, such as shallow-buried but not suitable for open-cut sections or shallow-buried tunnels, when there are buildings on the surface, or when the tunnel is close to underground structures. Design of pipe shed support The design parameters of pipe shed support mainly include: steel pipe diameter, length, spacing, elevation, horizontal lap length, steel frame spacing, grouting parameters, etc. When the strength and rigidity of the steel pipe need to be increased, the reinforcement cage can be set in the pipe and then filled with cement mortar. The Code for Construction of Railway Tunnels in China stipulates that the diameter of the steel pipe used for the pipe shed should be φ70-φ127mm; the distance between the centers of the steel pipes should be 2-3 times of the pipe diameter; the length of the pipe shed should be selected according to the stratum conditions and should not be less than 10m; and the overlapping length of two groups of longitudinal pipe sheds should be more than 3m. The parameters of pipe shed support can be determined according to the engineering analogy method and adjusted according to the actual situation during construction. Selection of pipe diameter The diameter of steel pipes in most projects is between φ50 and φ180mm. Some scholars classify the pipe shed support into small pipe shed and large pipe shed according to the pipe diameter. The diameter of small pipe shed is generally between φ30 and φ50mm, and the diameter of large pipe shed is between φ89 and φ159mm. Steel pipes of φ108 mm are often used in projects, and the circumferential spacing should not be more than 3-5 times the diameter. The selection of steel pipe for pipe shed is based on the calculation results and the analysis of technical and economic factors. For the soft stratum with high requirements for support conditions, φ127mm steel pipe should be selected. For the cohesive soil with high soil cohesion, φ89mm steel pipe can be selected. In most cases, φ108mm steel pipe is selected for the general soil layer. Determination of Circumferential Spacing of Steel Pipe in Pipe Roof The conventional method of equidistant setting of pipe shed along the tunnel excavation contour line is not scientific, and it should be designed reasonably according to different situations. The spacing of steel arch can be adjusted and selected within the range of 40-80 cm according to the looseness of the collapse body, the difficulty of excavation and the construction effect, and it needs to be densified under special circumstances. According to the summary and comparative analysis of the geological conditions, engineering section size, buried depth and other influencing factors of the engineering example, the following empirical conclusions are drawn, which can be used as the construction basis of underground engineering support: Pipe shed steel pipe diameter range is generally φ70-180mm, we can classify the pipe shed support into small pipe shed, medium pipe shed and large pipe shed according to the pipe diameter. The pipe diameter of the small pipe shed is generally within the range of φ32 ~ 50mm, and the steel pipe with the pipe diameter of φ42mm is mostly used. The pipe length is preferably 3.5-5m, the circumferential spacing is generally 0.3-0.4m, and the horizontal overlapping length is 1-1.5m. The pipe diameter of the middle pipe shed is generally in the range of φ50-φ89mm, the pipe length is generally not more than 20m, the circumferential spacing is generally 0.3-0.4m, and the horizontal overlapping length is 1-2m. φ89-φ159 mm steel pipe can be used for large pipe shed. The common pipe diameter is φ108 mm, and the pipe length should not exceed 40 m. The steel pipe is generally 4m or 6m long in sections and connected by screw threads. The length of screw threads is not less than 150 mm, and the circumferential spacing is generally not more than 3-5 times of the pipe diameter. Reasonable Determination of the Extrapolation Angle If the angle is too small, the far end of the pipe shed may droop into the tunnel excavation area and affect the later construction; on the contrary, if the angle is too large, the distance between the pipe shed and the excavation area will be too large, and the collapse of the triangular soil below the pipe shed will bring great difficulties to the excavation and support of the tunnel body. It should also be determined according to the space size of the drilling room of the pipe roof and the length of the drill rod. The external insertion angle of the small pipe shed is usually 5º-15º, that of the medium pipe shed is usually 2º-8º, and that of the long pipe shed is 1º-3º. Steel arch support is generally made of I-steel, or I-steel and grid steel frame are used at intervals, the spacing is generally not more than 1 meter, and encryption is required under special circumstances. Pipe shed layout form Door-type arrangement Full circumference arrangement and upper one side arrangement Pipe shed layout form Pipe shed construction process flow Drilling process Grouting process Hole forming method Commonly used construction methods include pipe ramming, pipe jacking, drilling, etc. However, the drilling method is the most commonly used hole-forming method at present. With the specialization of drilling equipment becoming more and more detailed, the special drilling rig for pipe shed emerges as the times require. However, the pipe roof drilling is nearly horizontal drilling, which is different from the exploration drilling in nature, and requires high accuracy of the drilling direction (spatial position). Once the hole deviation is found or the design allowable deviation is exceeded in the final hole measurement, it will cause serious consequences. Drilling method Down-the-hole hammer drilling with casing Saturn simultaneous casing drilling method "Neptune" simultaneous casing method Dth hammer drilling tools with casing can be divided into two categories according to whether the diameter of the drill bit can be changed: variable diameter drilling tools with casing and non-variable diameter drilling tools. At present, there are mainly single-eccentric variable-diameter simultaneous casing drilling tools, double (three) eccentric variable-diameter simultaneous casing drilling tools and radial variable-diameter simultaneous tubing drilling tools. Except for the single-eccentric variable-diameter simultaneous casing drilling tool, the bit variable-diameter simultaneous casing drilling tools of other structures belong to the concentric simultaneous casing drilling tool because the wing petals of the bit are symmetrical along the central axis of the drilling tool in the process of expansion and convergence. Both the inner and outer bit structure is adopted for the drilling tool with constant diameter and casing. Generally, the casing needs to be rotated. This kind of drilling tool belongs to the concentric drilling tool with casing. Relatively speaking, bit variable diameter simultaneous casing drilling tools are used in a large proportion, accounting for about 95% of the market, which is determined by its structural characteristics. Main defects of drill bit with constant diameter casing 1. When the drilling tool is working, the inner and outer pipes rotate at the same time, which is easy to cause the annular gap between the inner and outer pipes to be blocked by rock powder or stuck by large rock debris, resulting in the failure of the inner pipe to rotate reversely and the failure to separate the inner pipe from the outer pipe. This method requires a rotary drilling rig with large torque, which consumes a lot of energy and is not conducive to environmental protection; 2. Because the drill bit does not change the diameter of the casing drilling tool, it is very difficult to drill in the same diameter, especially when encountering boulders. Even if the drilling is successful, it is very troublesome to pull out the casing due to the shrinkage of the drilling hole, and even often causes accidents in the hole such as casing fracture. Turn head valve open Closure of head turning valve Single eccentric drill bit with casing-type I Open state Single eccentric drill bit with casing — Ⅱ type The advantage of this type of bit 1. The cross pin that connects the eccentric bit to the bit body only serves to suspend the bit It does not bear the torque, thus avoiding the accidents in the hole caused by the deformation and breakage of the pin; 2. At present, the eccentric casing bit used in China needs to be rotated. The expansion and contraction of the drill bit can be realized by about 175 degrees, and the diameter of the drill bit can be increased to the set reaming diameter by rotating the bit body of this type of drill bit forward by a very small angle (about 8 degrees), and when the drill bit is withdrawn, the diameter of the drilling bit can be reduced to the set diameter by rotating reversely by a very small angle, thereby reducing the probability of being stuck when the drilling bit is withdrawn; 3. Single eccentric casing bit The torque of type II is transmitted through the cooperation of a kidney-shaped oblong hole on the bit body and a kidney-shaped oblong shaft on the eccentric bit; 4. It is easy to realize larger reducing requirement. 。 Long screw drilling with casing Pipe shed borehole trajectory control method Once the hole deviation occurs or exceeds the design allowable deviation, it will hinder the drilling of adjacent steel pipes, resulting in uneven shape of the hole body and poor support effect. If the steel pipe sinks to a certain extent, it needs to be cut off during excavation, resulting in increased spacing and easy collapse. Therefore, medium pressure feeding, medium rotating speed and medium circulating fluid volume can be adopted during drilling; the radial error of the borehole plane shall be controlled within 20cm, and the angle error shall be less than 1 °, so as to avoid deflection and downward bending of the steel pipe of the pipe shed due to too large aperture. In the actual construction, it is generally difficult to avoid horizontal borehole bending, mining dth bit , so in addition to improving the positioning accuracy of the pipe roof, appropriate uplift (determined according to the geological conditions of the site) can be given to compensate for part of the borehole sag. Pipe shed borehole trajectory control method Once the hole deviation occurs or exceeds the design allowable deviation, it will hinder the drilling of adjacent steel pipes, resulting in uneven shape of the hole body and poor support effect. If the steel pipe sinks to a certain extent, it needs to be cut off during excavation, resulting in increased spacing and easy collapse. Therefore, medium pressure feeding, medium rotating speed and medium circulating fluid volume can be adopted during drilling; the radial error of the borehole plane shall be controlled within 20cm, and the angle error shall be less than 1 °, so as to avoid deflection and downward bending of the steel pipe of the pipe shed due to too large aperture. In the actual construction, it is generally difficult to avoid horizontal borehole bending, so in addition to improving the positioning accuracy of the pipe roof, appropriate uplift (determined according to the geological conditions of the site) can be given to compensate for part of the borehole sag. Grouting Pipe roof grouting is one of the key links in pipe roof construction, and the grouting effect also affects the construction quality of the whole pipe roof system. Therefore, the appropriate grout, pressure and reasonable flow should be selected according to the geological conditions. In the process of grouting, if the pressure is low, the grouting will not be able to enter, and if the pressure is too high, it will cause the ground to bulge and crack, which will affect the ground buildings in shallow urban areas. During grouting, low pressure and medium flow are generally adopted. During grouting, the pressure is gradually increased and the grouting flow is gradually reduced. When the pressure is increased to the final pressure, the grouting is continued for 5min, and then the grouting is finished. In order to ensure the construction quality, the in-situ grouting test should be carried out before the actual grouting construction to determine the optimal grouting parameters to guide the grouting and ensure the grouting effect under the conditions of the grout type and the expected grouting effect given in the design. In order to prevent hole collapse or grouting hole channeling, holes can be drilled at intervals during drilling, the distance between holes can be increased, and the connected holes can be drilled after the grouting is completed and solidified, so that the quality of drilling and grouting can be effectively ensured, the pipe shed and the grouting concrete can form a thick arch, and the bearing effect of longitudinal beam forming and transverse arch forming of the pipe shed can be realized. In order to reduce the mutual influence between adjacent pipe hole grouting, the grouting sequence is usually carried out from bottom to top and symmetrically from left to right; However, due to the mutual support of many block stones in the collapse body, there are many large cavities in the collapse body, some of which are directly connected with the collapse area. During grouting, the grout flows to the collapse area through these cavities under pressure, and can not diffuse evenly near the pipe shed, so that the grouting reinforcement zone can not be formed. Therefore, the intermittent grouting method is adopted according to experience. That is to say, when the grouting pressure does not come up for a long time, it means that the grout flows to the collapse area along the gap. At this time, the setting time of the grout is adjusted to 30-50 seconds. The grouting lasts for 1-5 minutes and stops for 40 seconds. After the original grout is initially set and thickened, the grouting is repeated. The original grout discharge channel is gradually reduced and finally blocked. The grout reaches the purpose of uniform diffusion around the pipe shed. Development Trend of Pipe Roof Advanced Support Technology In the current advanced support methods, there are mainly advanced bolt, advanced small pipe grouting and advanced pipe shed. Grouting with advance anchor and advance small pipe has the advantages of convenient construction, easy mastery of technology and low requirement of mechanized matching degree, but the support length is small (only 3 ~ 5 m), the distance of anchor or small pipe into the front sliding line of the working face is short, and the excavation cycle footage is limited. Generally, the cycle footage is controlled at 15 ~ 17 m in the shallow-buried loose strata, the number of cycles is increased, and the process exchange is frequent, especially in the surrounding rock with poor self-stability, the front end of the bolt and conduit is still in the sliding surface, which can not play the role of advanced support protection, and it is very easy to cause instability of the working face, and there is a greater security risk. Therefore, the application of long pipe shed is becoming more and more common. The support mechanism of the long pipe shed advance support construction technology is mainly the result of the joint action of the steel pipe and the grout consolidation body. On the one hand, drilling and setting down the steel pipe are carried out. When the steel pipe passes through the loose and weak surrounding rock, rock (soil) and other damage areas and extends into the undisturbed soil, it effectively guarantees the stability of the rock and soil mass on the excavation face. And on that other hand, grout is extrude from the holes of the steel floral tube through grouting. Under the action of a certain pressure, it is injected into the loose and weak strata around the steel pipe to form a composite and stable consolidation body, which changes the mechanical properties of the surrounding strata, strengthens the stability, and prevents soil collapse and surface subsidence. Pipe shed drilling rig produced in Italy DDL-300 drilling rig Diamond drilling and cemented carbide drilling in solid deposits. But also can be use for engineering geological exploration drill and foundation pile hole drilling. It is widely used in geological exploration, engineering exploration, anchoring protection, rotary jet grouting, pipe shed support and other engineering construction operations. Grouting construction case of advanced pipe shed The shallow-buried Class II surrounding rock section of the tunnel in this contract section is supported by advanced long pipe shed grouting, and the pipe shed is made of Φ108 mm steel pipe with a wall thickness of 4mm. The pipe shed is used in conjunction with the steel arch and passes through the belly of the arch. Pipe shed holes shall be drilled longitudinally on the arch along the tunnel excavation contour line, with the external insertion angle of about 3 °, so as not to intrude into the tunnel excavation line as small as possible. The hole diameter shall be 20 ~ 30 mm larger than the diameter of the pipe shed steel pipe, and the drilling sequence shall be from high hole position to low hole position. The front end of the steel pipe is processed into a sharp cone, and the tail is welded with a Φ10 stiffener for reinforcement. Φ12 mm grouting holes are arranged in quincunx shape on the part of the steel pipe entering the rock, and the spacing of the grouting holes is 15 cm. The direction of the steel pipe is parallel to the center line of the route, and the longitudinal overlapping length is greater than 1.5m. Tunnel construction process flow Surveying, setting out, drilling, charging, on-line blasting, ventilation, pumice removal, inverted arch loader loading of gangue (central ditch excavator loading of gangue), dump truck discharging of gangue construction system, bolt shotcreting 1. Main process flow (1) Drill and install grouting pipe. Cast C25 concrete protection arch outside the tunnel excavation contour line. Erect the drilling rig platform. Drill with the engineering horizontal drilling rig. After reaching the design depth, clean the hole. Install the grouting pipe. Install a rubber sleeve with the same diameter as the borehole at 1.5m ~ 2.5m of the orifice. Seal the orifice with cement mortar to prevent the slurry from extruding along the gap between the grouting pipe and the borehole wall. Drilling-grouting sequence shall be adopted for construction. (2) Prepare grouting liquid with superfine cement for slurry preparation and grouting. Adopt backward sectional grouting. Close the orifice valve and start the grouting pump for pipeline water pressure test. The test pressure is equal to the final grouting pressure. If there is water leakage, repair it in time. During grouting, low pressure and medium flow are adopted. During grouting, the pressure rises gradually and the flow decreases gradually. When the pressure rises to the final pressure, continue to inject for 5 minutes and then finish grouting. The grout in the grouting hole shall be removed in time after grouting and filled with No.30 cement mortar to enhance the rigidity and strength of the pipe shed. (3) Grouting completion standard: If the grouting construction of each section is normal, the grouting pressure reaches the designed final pressure, and the grouting amount reaches 80% or more of the designed grouting amount, or although the designed final pressure is not reached, but the grouting amount has reached the designed grouting amount, then the single-hole grouting can be completed. All the design grouting holes meet the completion standard, and there is no missing injection. The water absorption rate of the water injection test stratum is 0.05L/min. When the clay consolidation strength is greater than 0.4Mpa, the grouting of the whole section can be finished. 2. Construction of advanced small conduit Φ42mm seamless steel pipe with a wall thickness of 3.5mm shall be used as the advanced small conduit. Φ6 mm grouting holes are arranged in quincunx shape on the part of the steel pipe entering the rock, and the spacing of the grouting holes is 12 cm. The front end of the conduit is processed into a sharp cone, and the tail is welded with a Φ6.5 stiffener for reinforcement. Hydraulic grouting machine shall be selected. During construction, firstly use instruments to measure and set out, draw the excavation contour line, and determine the position of the center line of the small conduit. When the conduit is drilled, the depth of the hole shall be greater than the length of the conduit; when hammering or drilling is used for jacking, the jacking length shall not be less than 90% of the length of the conduit. Hydraulic grouting machine is used for grouting. Before grouting, shotcrete shall be sprayed to seal the working surface to prevent mortar leakage, and the thickness of the shotcrete layer shall not be less than 5cm. During the grouting of the conduit, the sealing plug shall be set at the tail. During the grouting of cement slurry, the grouting hole and vent hole shall be set on the sealing plug. During grouting, the grouting shall be stopped immediately after the grout is discharged from the vent hole. 3. Anchor rod construction 1) φ22 mortar advanced anchor bolt The hole position shall be determined and marked according to the design requirements and surrounding rock conditions before drilling the anchor bolt. The drilling hole shall be round and straight, and the hole diameter and depth shall meet the design requirements. The accumulated water and rock powder in the hole shall be blown clean. The rock surface of the anchor bolt orifice shall be leveled, and the rock surface shall be perpendicular to the drilling direction. If it is not perpendicular, it can be adjusted with a special backing plate when installing the anchor bolt, so that the backing plate is close to the rock surface. During the construction of anchor rod, the pre-tensioning force should be 50% ~ 80% of the anchoring force. After installation, it should be checked regularly and tightened in time if looseness is found. 2) D25 hollow grouting anchor bolt D25 hollow grouting system anchor bolt shall be used for various surrounding rock anchor bolts, with the length of 3.0 ~ 3.5m and quincunx layout. Use the pneumatic drill to drill the hole, after drilling to the specified depth, blow the hole with high-pressure air, drive in the anchor rod, and then use the grouting pump to fill the mortar from the middle hole of the anchor rod to the bottom of the hole, and install the base plate bolt. Key points and precautions for anchor rod construction ① The hole position shall be arranged according to the design, with the deviation less than 10 cm and the hole depth error of ± 10 cm. The diameter of the drill hole is 15 mm larger than the diameter of the anchor rod. The insertion length of anchor rod shall not be less than the design length. ② The borehole itself shall be straight and not bent. The direction is radial to the tunnel perimeter, but not parallel to the rock face. ③ During the construction of anchor rod, the positioning anchor rod of grid steel frame shall be embedded at the same time. After the anchor rod is inserted, it is not allowed to knock at will, and it is not allowed to hang heavy objects within three days. ④ Clean the hole before grouting, and use high-pressure air to remove the accumulated water, rock powder, debris and other sundries in the hole along the anchor rod hole. ⑤ The grouting tank and grouting pipe shall be used for grouting, and the orifice pressure shall be less than 0.4MPa. Grouting shall be carried out along the anchor bolt hole until grout flows out of the orifice. Mortar shall be ready for use, and shall be used up before the initial setting of mortar. ⑥ Three anchor rods shall be randomly sampled from every 100 anchor rods for pull-out test, so as to understand the anchoring quality of anchor rods. 4. Reinforcement mesh construction Reinforcement mesh shall be arranged for various surrounding rocks. Reinforcement mesh is made of φ6 round steel, with spacing of 5 cm × 15 cm. Reinforcement mesh is installed on site by single piece. The reinforcement mesh shall be firmly connected with the anchor rod and laid with the fluctuation of the shotcrete surface. The mesh reinforcement shall be firmly lapped with the mesh reinforcement of the sprayed concrete section, and the lapping length shall not be less than 200mm. The reinforcing mesh shall be firmly attached and shall not shake during shotcreting. 5. Construction of shotcrete (wet shotcrete) A. Design principle of shotcrete mix proportion When selecting the mix proportion of shotcrete, the requirements of concrete strength and other physical and mechanical properties as well as the requirements of shotcrete technology shall be considered. Cement, sand, gravel and various admixtures must meet the design and specification requirements. Design steps: determine the maximum particle size and sand ratio of aggregate, determine the amount of cement, determine the water-cement ratio and water consumption, determine the amount of sand and stone, adjust the trial spraying, and determine the construction mix proportion. B. Spray process flow: ① Inspection of raw materials: check the quality of various raw materials used, and use them only after they are qualified. ② Mechanical equipment inspection: technical inspection shall be carried out for the machinery, and test run shall be carried out for the water, air and circuit, and the equipment can be operated only after it is qualified. ③ Construction site inspection: inspect the surface to be sprayed, remove the dangerous rock and floating soil, sweep the rock surface with high-pressure air, and bury the thickness mark. C. Shotcrete sequence Shotcreting sequence: from bottom to top in the vertical direction, from left to right or from right to left in the horizontal direction, moving in a spiral track, pressing half a circle in one circle, and proceeding in order in the longitudinal direction. The rotation radius is generally 15 cm, and the length of each snaking is 3 ~ 4 m. When the rock surface is uneven, the concave part shall be leveled first. When spraying concrete, the speed of spraying concrete should not be too slow or too fast, and it should be adjusted in time. D. Spray quality inspection: check the spray surface for looseness, cracking, falling, slippage and other phenomena according to the specifications. If any, remove the re-spray in time. After the spray reaches a certain strength, the sound can be heard by hammering, and the hollowing and shelling can be treated in time. Drill and measure, and re-spray where the thickness is not enough. Timely measure the rebound rate and the actual mix proportion to guide the next construction. Carry out mechanical test on the spray body test piece. Detailed explanation of pictures of tunnel pipe shed construction process The difference between the advanced small conduit and the pipe shed: the treatment range of the advanced small conduit is generally about 6 meters. The small conduit is a steel pipe with a wall thickness of 5 mm and a diameter of 42 mm. It is a method to consolidate the relatively broken surrounding rock in front of the tunnel face by small pipe grouting, so that the surrounding rock can form a whole before the next step. The pipe shed is used when the geological conditions at the entrance of the tunnel are very poor (such as sandy soil, seriously broken rocks, loess, etc.). Generally, the length is about 20-30 meters. The pipe shed is a steel pipe with a wall thickness of 3.5mm and a diameter of 108mm. Of course,dhd drill bit, the pipe shed can also be used when the geological conditions in the tunnel are very poor (for example, the length along the axis of the tunnel caused by roof fall in the tunnel is large, and the volume is large). Return to Sohu to see more Responsible Editor:. wt-dthtools.com