Introduction to 0
Tunnel is a construction project under complex geological conditions, which is greatly influenced by natural geological conditions such as geostress, geophysical parameters, groundwater and geological faults, and manual excavation operations such as excavation method, support method and support time. Because the tunnel is located in different geological environment, its surrounding rock stability characteristics are also different, so the supporting methods and excavation methods should also be different. At present, tunnel excavation methods mainly include full-face excavation method, bench method, bench excavation method, pilot tunnel method, unilateral pilot tunnel method, bilateral pilot tunnel method and so on. Full-face excavation method is suitable for first-class surrounding rock, and bench method is suitable for working-class surrounding rock. The distance between the upper and lower steps can meet the normal operation of the machine and reduce the workload of slag turning. The bench excavation method is suitable for the first-class surrounding rock, and the general round well drilling footage is suitable for the first-class surrounding rock. The process arrangement is compact to ensure the construction safety. The single and double side-wall heading method is suitable for tunnels with poor surrounding rock and controlled settlement. However, the support methods usually include anchor shotcrete, anchor mesh shotcrete, anchor mesh truss, anchor mesh truss grouting, steel frame support, reinforced concrete support, grouting reinforcement and prestressed anchor cable support. In practical application, multiple supports and combined supports are often used. This paper mainly analyzes the excavation and support technology of highway tunnel.
The basic idea of 1 NATM
NATM construction method put forward in 1960s has become one of the main design and construction methods of underground engineering. Since 1970s, a new type of modern support technology-bolting and shotcreting, which originated from the concept of NATM tunnel construction, has been widely promoted in various fields of underground engineering in China. So far, great progress has been made in both theoretical research and engineering practice. The main features of NATM tunnel construction are as follows: ① The concept of tunnel construction is different from the traditional supporting theory, and the main principle of NATM can be interpreted as a concept, that is, changing the rocks around the tunnel section from a load generator to a load carrier. ② It can not only effectively support the surrounding rock, but also allow the surrounding rock to deform. NATM believes that the thin auxiliary concrete lining should be built first, and the deformation process should be monitored, and the permanent support can only be built after the deformation reaches a stable state. (3) The broken rock mass is supported by bolting and shotcreting. The traditional concept of bolt support is that the bolt can only be used to anchor the rocks that may fall off in the upper part of the tunnel, but not to the fractured rock mass with developed joints. (4) Deformation dynamics and monitoring construction. NATM realizes secondary balance by releasing surrounding rock pressure, so controlling tunnel stability by measuring deformation dynamics of surrounding rock and support is the core content of NATM.
2 Tunnel excavation
The basic principle of tunnel excavation is to choose appropriate excavation methods and methods to improve the excavation speed as much as possible on the premise of ensuring the stability of surrounding rock or reducing the disturbance to surrounding rock. That is to say, when selecting excavation methods and tunneling methods, on the one hand, we should consider the geological conditions and changes of tunnel surrounding rock, and choose ways and methods that can well adapt to the geological conditions and changes and maintain the stability of surrounding rock; On the other hand, we should consider the hardness of the rock mass in the tunnel and choose the methods and means that can quickly excavate and reduce the disturbance to the surrounding rock.
Tunnel excavation method actually refers to excavation molding method. According to the division of tunnel section, the excavation methods can be divided into full-face excavation method, bench excavation method, step excavation method with core soil, division excavation method and so on. In the bench excavation method, the design section is generally divided into the upper half and the lower half, and the arc pilot hole at the upper part of the bench is also used for advance excavation. The bench method is suitable for class Ⅲ and Ⅳ surrounding rocks with weak interlayer or joint. According to the broken degree of surrounding rock, bench method can be divided into long bench method, short bench method and ultra-short bench method. It is changed from the bench method, and the upper bench is several times ahead of the tunnel span, which is mainly used in areas where the short bench method is used for digging, water gushing and tunnel face collapse. The characteristic is that the construction adjustment is not big, and it can be used first when the surrounding rock changes in a short distance, which can quickly improve the safety of construction, but the process is increased and the footage is short (generally not more than 1m). Division excavation method is to gradually shape the tunnel section through division excavation, and generally excavate a certain part in advance, so it can be called pilot tunnel advance excavation method. The commonly used methods are advanced excavation method of upper and lower pilot tunnels, advanced excavation method of upper pilot tunnels and advanced excavation method of single (double) side wall pilot tunnels.
Smooth blasting has been widely used in tunnel engineering, which can not only improve the driving speed, but also control blasting scientifically and effectively, avoid overbreak and ensure the later support effect. The essence of smooth blasting is to arrange dense peripheral blast holes on the contour line of tunnel excavation design section, reduce the charge and charge diameter, control the blasting energy and its function by using explosives with low density and low detonation velocity, reduce the peak effect of explosion shock wave, reduce the stress wave intensity caused by it in rock, avoid the broken area around blast holes, concentrate the blasting action on the rock mass on the side to be blasted and weaken the damage to the original rock mass.
3. Support method 3. 1 shotcrete. Spraying concrete on the inner surface of the cave can make the rock blocks divided by cracks stick together, keep the occlusion and mosaic function of the rock blocks, effectively prevent the surrounding rock from loosening by improving the adhesion and friction of the rock blocks, avoid or reduce the occurrence of stress concentration, and give resistance and shear to the surrounding rock surface, so that the surrounding rock is in a stable triaxial stress state, and prevent the collapse of unstable bodies by spraying the structural rigidity of the concrete layer itself. Shotcrete itself has a certain stiffness, which can resist the collapse of rock and soil, and often bears the compression deformation of supporting structure together with other supporting methods. Therefore, shotcrete support method is one of the most commonly used methods in modern tunnel construction. (refer to the Chinese website of architecture)
3.2 Bolt support. Driving the anchor into the rock mass can restrain the deformation of the rock mass. By exerting pressure on the surrounding rock, the surrounding rock on the inner surface of the cave room, which was originally in a biaxial stress state, can maintain a triaxial stress state, thus preventing the deterioration of the surrounding rock stiffness, especially in the loose area. Through the function of system anchor in rock and soil, a reinforced circle of rock and soil is formed in rock and soil, and the bearing arch of rock and soil can bear the external load together with rock and soil, thus enhancing the stability of rock and soil.
3.3 hanging steel mesh. Steel mesh is usually connected with anchor rod. Because the anchors are arranged at a certain distance, the rock and soil between the anchors is very weak and easy to collapse. Therefore, connecting anchor with steel mesh can make loose rock and soil in triaxial stress state and maintain its stability. At the same time, the anchors are connected with each other, which reduces the possibility of local collapse caused by the failure of a single anchor.
3.4 steel support. Steel support is to stabilize rock and soil and control the deformation of rock and soil by using the stiffness of supporting structure. Generally, it is installed immediately after the excavation of the working face, which can give full play to the role of steel support and stabilize the rock and soil. Steel support is mainly used in strata with extremely poor geotechnical stability, and there are usually two forms, one is a grid steel frame structure made of steel bars, and the other is an I-beam support made of section steel. Moreover, steel supports are often used in conjunction with shotcrete, anchor bolts and steel mesh to ensure the stability of rock and soil.
3.5 Advance support of grouting conduit. The advanced grouting conduit is similar to the advanced anchor rod in the advanced support method. The loose rock mass is supported longitudinally, and the construction is carried out on the excavation contour line of tunnel excavation. The rock mass in the middle part is supported by the steel in the back and the rock mass in the front, which plays the role of longitudinal beam. At the same time, due to the effect of grouting in the conduit and mortar anchor, the slurry will enter the cracks in the rock and soil, forming a rigid soil layer reinforcement circle, which improves the stability of the rock and soil. This method has a good effect on massive rock mass with developed cracks, and the grouting reinforcement effect of advanced grouting conduit on rock and soil mass is more prominent than that of advanced anchor rod. Grout is injected into the rock mass to fill the cracks in the rock mass, which not only improves the mechanical properties of the rock mass, but also plays a waterproof role. Groundwater often reduces the grade of surrounding rock, and it is not easy to make the rock unstable and collapse. Therefore, this method is particularly effective for supporting and strengthening aquifers.
3.6 pipe roof advanced support. Pipe shed construction method is mainly used in rock and soil strata with poor arch forming effect. Due to the gravity of the stratum, the stratum will generate large lateral pressure of rock and soil, and longitudinal collapse will occur in the tunnel, resulting in large lateral pressure of rock and soil due to the gravity of the stratum, and longitudinal collapse will occur in the tunnel, resulting in the collapse of the stratum ahead. In order to ensure the safety of entering the tunnel, firstly, a pipe shed should be built at the entrance of the tunnel, and steel pipes with larger diameter should be arranged on the excavation contour in advance, so as to form the role of longitudinal steel beams, which can effectively reduce the lateral pressure caused by the self-weight of rock and soil and stabilize the stratum ahead.
4 conclusion
At present, NATM construction has become one of the main design and construction methods of underground engineering, and the excavation and support methods guided by NATM are quite different in different surrounding rock grades. This paper mainly analyzes the excavation and support of highway tunnel to promote the development of dynamic information construction of highway tunnel.
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