I. Project Overview This tunnel is an extra-long tunnel, with a total length of 3795 meters on the left side, a slope of1.1%1.885 meters and -0.6%/ 1.945 meters. The openings are all end-wall tunnels, and the open-cut tunnel is 27 meters, with grade V surrounding rock. The openings are all end-wall tunnels, open-cut tunnels 13m, with 932m grade ⅴ surrounding rock and grade ⅴ surrounding rock. Among them, there are 9 crosswalks, 4 crosswalks and 4 emergency parking areas. Building clearance of vehicle main tunnel (single tunnel): 1 1.0m (width) ×5.0m (height). Among them, the net width of the roadway is 3.75m× 2 = 7.5m, and the transverse width: l left = =0.75m, l right = =1.0m; ; Sideways (including width c) are set on both sides, J left = =0.75m, J and J right = =1.0 m. Second, analysis of the construction principle of advanced small duct Before the tunnel construction, the soil structure is undisturbed and the surrounding rock is also in a stressed state. With the gradual development of tunnel excavation, the surrounding rock around the tunnel will be disturbed, the original surrounding rock stress will be released, and the stress will be redistributed. Through the advanced small conduit construction technology, the broken surrounding rock around the tunnel project is promoted to form a whole, and the elastic modulus and internal friction are improved, thus ensuring the stability of the surrounding rock structure. According to the different principles, there are three advanced small conduit construction technologies: the first is the role of small conduit anchor, which fixes the broken surrounding rock around the tunnel on the surrounding rock and connects it into a whole. The second is to strengthen the strength of tunnel arch through arch group effect and improve the bearing pressure. The third is to improve the stability of surrounding rock structure through the inherent characteristics of slurry. Three. Application of advanced small conduit construction in Zhuyuan tunnel project (I) Analysis of application scope The advanced single-layer small conduit is mainly used for shallow-buried eccentric pressure of Grade V surrounding rock with poor self-stabilization ability, as well as fault fracture zones and their influence areas of Grade IV and V surrounding rock. (2) Advance single-layer small conduit advance support is technically necessary, and it is suitable for ⅳ and ⅴ surrounding rock sections with loose and broken arch strata. In the construction of this project, the construction parameters of the leading small conduit are: hot-rolled seamless steel flower tube with a wall thickness of 3.5mm and a diameter of 42mm is selected, the length is controlled at about 4.5m, and the circumferential spacing is controlled at about 20cm. Extrapolation angle: steel pipe extrapolation angles are10 ~15 respectively; Grouting material: water cement ratio of cement slurry: 1.0: 1.0 (weight ratio); Setting range: Arch 120. The steel pipe is grouted by circumferential drilling, with the hole diameter 10mm and the hole spacing 150mm, and the grout stopping section without drilling at the tail is not less than 30cm. (3) Construction preparation According to the engineering characteristics and the objective conditions of the construction site, choose the appropriate construction method, reduce the mutual interference of each process in the construction, determine the materials, specifications and models used for grouting, and select reasonable auxiliary materials according to the actual construction needs. If silt and quicksand are encountered in tunnel construction, double liquid grouting with sodium silicate and cement can be used. Although some chemical slurries can also meet the construction requirements, they are relatively expensive and have certain toxicity, which is not conducive to environmental protection and should be used with caution. When selecting grouting equipment, reasonable drilling equipment, grouting materials, testing equipment and instruments should be selected according to the engineering characteristics. The grouting parameters of the advanced small conduit construction in this project are: cement slurry water-cement ratio1:1; The initial grouting pressure is 0.5 MPa ~1.0 MPa; After grouting, M20 cement mortar is used to fill the steel pipe of the pipe shed to enhance the strength of the pipe shed. The strength grade of grouting cement is P.O42.5 ordinary portland cement; The beauty of sodium silicate liquid is 30 and the modulus is 2.4; Adding 5% sodium silicate, if the amount of water is large, it should be determined by field test. (IV) Production of steel pipes In order to improve the construction efficiency and convenience, the front end of the small pipe can be made into a pointed cone, and the tail part can be welded with reinforcing ribs with a diameter of not less than 8mm for fastening. A plum blossom borehole with a diameter of 15 mm is arranged on the pipe wall every 15cm, and a slurry stop section is arranged at the tail. The pipe shed adopts φ 108 perforated steel flower pipe and φ 108 non-perforated steel pipe, and the length of steel pipe section is 4m ~ 6m. Pipe shed extension should first slowly jack the previous steel pipe into the hole and then connect with each other. After the current steel pipe enters the drilling hole, leave 30cm~40cm outside the hole, and then start the rock drill for reverse treatment to separate the connecting sleeve from the steel pipe. After that, the rock drill will return to its original position, put down the boom, manually install a section of steel pipe, and then recalibrate the boom. The rock drill should advance slowly, aim at the tail of the previous steel pipe and then connect it manually, so that the two steel pipes can be connected into a whole. Finally, the steel pipe is jacked at low speed with the aid of a rock drill. (V) Installation of small ducts The installation of small ducts is the most important thing leading to the installation of small ducts, and the installation effect and quality directly determine the construction quality and safety of the whole project. Determine the hole location through setting-out measurement, mark it well, and drill with a rock drill, with the hole diameter slightly larger than the pipe diameter by more than 20 mm.. After the completion of drilling, insert the small catheter into the drilling hole according to the design requirements, and control the exposed part at about 20cm, and support it on the dug steel frame to form a pre-support system. (6) Grouting During the grouting operation and construction, the grouting records must be filled in strictly and carefully, and the operation situation should be analyzed and improved at any time, and the supporting work should be strictly observed. The selection of grouting parameters is also the focus of this project. In concrete construction, grouting parameters should be selected according to grouting test results and field investigation, and concrete should be sprayed to seal the subsurface layer during grouting to avoid slurry leakage. For forcibly driven steel pipes, sediment washing must be carried out, and then grouting operation must be carried out. The bottom-up construction method should be selected for grouting operation, and the slurry water-cement ratio can be controlled at three levels: 1.5: 1, 1: 1, 0.8: 1. Gradually increase the mud concentration, considering the excavation immediately after grouting, ordinary cement is used in the construction of this project, and water reducing agent is added during preparation. In the process of drilling and grouting, the construction should be carried out in strict accordance with the method of "outside first and then inside". In order to ensure the strength and rigidity of the drill pipe joint to the greatest extent, the connecting sleeve of the drill pipe and the drill pipe should be made of the same material, with internal threads on both sides, and the wall thickness of the connecting sleeve should be ≥ 10 mm ... (VII) Inspection of grouting effect and quality judgment The inspection of grouting effect can be divided into in-process inspection, inspection at the end and inspection during excavation: After grouting, check the grouting records to see whether the grouting quantity of each hole meets the final hole standard and whether the number of holes is in line with the design; During excavation, check whether the excavation edge is neat, the consolidation of slurry, the penetration radius of slurry, etc. It provides a basis for modifying grouting parameters in the next step. If the quality is not good during the excavation, in addition to improvement in the next cycle, support measures should be strengthened. Four. Conclusion To sum up, this paper analyzes the application of advanced small conduit construction in Zhuyuan tunnel project with engineering examples, and draws the following conclusions: (1) Compared with advanced pipe shed, advanced anchor rod and other construction technologies, the advanced small conduit construction technology has more obvious reinforcement effect and simpler construction technology. (2) Advanced small conduit construction technology can not only effectively reinforce the surrounding rock and improve its own stability, but also improve the anti-leakage ability of surrounding rock structure in tunnel engineering. It can effectively prevent the occurrence probability of safety accidents such as collapse and roof fall. (3) In recent years, there are more and more tunnel projects in China. With its unique advantages and advanced adaptability, advanced small conduit construction has been highly valued by the construction unit and has a very broad development prospect.
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