1, timeliness
NATM adopts shotcrete and bolt support as the main means, which can keep up with the excavation face to the maximum extent. Therefore, the space-time effect of excavation face can be used to limit the deformation development before support and prevent surrounding rock from entering loose state. Advance support can be carried out when necessary, and the early strength and comprehensive cohesiveness of shotcrete ensure the timeliness and effectiveness of support.
Shotcrete support construction immediately after roadway blasting can effectively prevent the development of rock deformation, control the extension of stress reduction zone, reduce the bearing capacity of support and enhance the stability of rock stratum.
Step 2 close
Because the anchor-shotcrete support can be constructed in time, it is a comprehensive and dense support, which can effectively prevent the destruction and peeling of surrounding rock by water and weathering, prevent the deliquescence and expansion of expansive rock mass, and protect the strength of original rock mass.
After roadway excavation, the surrounding rock may be deformed or collapsed at any time due to the new cracks caused by blasting and the cracks in the original geological structure. When the shotcrete support is fired at the rock surface at high speed, the cracks, joints and cavities of surrounding rock are well filled, and the strength of surrounding rock is greatly improved.
At the same time, the bolt-shotcrete support plays the role of sealing the surrounding rock, isolating the contact between water and air and the rock stratum, so that the crack filler will not soften and disintegrate, which will open the crack and lead to the instability of the surrounding rock.
3. Flexibility
Shotcrete-anchor support belongs to flexible thin support and can work closely with surrounding rock. Because of the flexibility of bolting and shotcreting support, it can deform with the surrounding rock, forming a certain range of inelastic deformation zone in the surrounding rock, effectively controlling and allowing the moderate development of the plastic zone of the surrounding rock, and giving full play to the self-supporting capacity of the surrounding rock.
On the other hand, the bolt-shotcrete support is compressed in the same deformation as the surrounding rock, which produces more and more supporting reaction to the surrounding rock, which can restrain the excessive deformation of the surrounding rock and prevent it from loosening and destruction.
Disadvantages of NATM:
1, the implementation not only requires good construction organization and management, but also requires technicians and surveyors to be very skilled, without which mistakes are easy to occur; The quality of homework is related to everyone's careful operation.
2. The geology exposed by excavation will immediately change its state, so the construction geologist is required to inspect the site in order to find the problem;
3. Construction measurement controlled by energy consumption often brings inconvenience to construction;
4. The damage caused by dust and chemicals produced by dry spraying must be protected, especially for eyes. Although wet spraying can avoid this shortcoming, the supporting effect of rock mass is not as good as dry spraying under the same conditions.
Extended data:
Basic points:
1, after the excavation of the cavern, the surrounding rock itself should bear the main supporting role, while the lining only strengthens the surrounding rock, making it a whole and working together with * * *.
Therefore, it is necessary to maintain the inherent strength of surrounding rock to the maximum extent in order to give play to the self-supporting ability of surrounding rock. If shotcrete is used to seal the rock wall in time, the surrounding rock can be effectively prevented from loosening without greatly reducing its strength, and there is no deformation and relaxation of surrounding rock caused by displacement support. In a word, it is ideal to keep the surrounding rock in triaxial stress constraint state.
2. When the surrounding rock is expected to have large deformation and relaxation, a protective layer should be applied on the excavation surface and laid at an appropriate time, not too early or too late. Its stiffness should not be too large or too small, and it must be closely attached to the surrounding rock, but it should be made into a flexible thin layer, allowing some deformation, so that the surrounding rock can play an unloading role when releasing stress and try not to be damaged by bending moment.
This kind of bearing is different from the traditional bearing, which is not destroyed by bending moment but by compression and shear. Because the compressive and shear strength of concrete is much greater than the tensile and bending strength, it has a high bearing capacity. After the initial support displacement converges, a high-quality waterproof layer can be laid on its smooth surface as a secondary support to improve safety. There is only radial force between front and rear support and surrounding rock.
3. In the section where the lining needs to be strengthened, instead of increasing the thickness of concrete, steel mesh, steel support and anchor rod are added, so that the whole length of the tunnel adopts roughly the same excavation section. In addition, because NATM does not erect a strut in the tunnel, the space is spacious and the safety and work efficiency are improved.
4, in order to correctly grasp and evaluate the time characteristics of surrounding rock and support, indoor tests can be carried out at the same time, and measurements can be made at the scene. The measurement contents include the stress in the lining, the contact stress between surrounding rock and lining, the displacement of surrounding rock, etc., so as to determine the most important parameters such as the stability time, deformation speed and classification of surrounding rock, so as to adapt to the changes of geological conditions and change the design and construction in time.
Measurement monitoring is the basic feature of NATM, and the focus of measurement is the dynamic change of mechanical characteristics of surrounding rock and support with time. The lining method and time are determined by the displacement measurement of surrounding rock.