Damage characteristics of tunnel surrounding rock

Since tunneling is a kind of underground project and is greatly affected by the underground environment, the factors affecting the stability of the tunnel face are summarized and mainly divided. The one is the geological conditions under natural conditions, including the state of rock and soil structure, groundwater status, etc.; the other is design and construction factors, or human factors, including tunnel shape and size, construction methods, support methods, etc.

State of the Geotechnical Structure
During the long-term geological movement, the rock-soil structure is constantly developing and changing, which plays a major role in the influencing factors of stability analysis.
The structural state of the surrounding rock is usually divided into broken and intact. Due to the action of tectonic movement, the rock and soil body will be broken and lose their original integrity, often accompanied by various fractures and deformations. The structural state of the surrounding rock reflects the movement of the geological structure and plays an important role in the stability of the surrounding rock of the tunnel. The practice has proved that under the same lithology conditions, the more broken the rock mass, the more unstable the tunnel will be.
Rock and soil masses can be classified according to how the rock blocks are divided by structural planes. The surrounding rock is mainly divided into rock mass and soil mass. Rock masses include monolithic (crack spacing greater than 1.0m), massive (crack spacing between 0.4~1.0m.), and crushed stone ( The distance between the cracks is less than 0.2m).

Groundwater
In actual construction, groundwater is an important factor affecting the nature of the project. In severe cases, the stability of the surrounding rock of the tunnel will be extremely poor, resulting in a tunnel collapse. If there is groundwater in the stratum, due to the effect of time, the quality of the surrounding rock will be softened and its strength will become lower. In some weak and broken surrounding rocks, the fluidity of water will take away some fillings in the cracks, and at the same time, it will also accelerate the liquefaction of rock and soil, and the strength of the surrounding rocks will decrease. When encountering expansive rock mass, the entire rock mass will swell due to the effect of water absorption, thereby losing stability. Groundwater is a factor that must be considered when grading surrounding rock.

The Influence of Human Factors
Human factors are an important factor affecting the stability of the tunnel. The shape and size of the designed tunnel determine the size of the cavern excavation. Especially the span, which has a significant impact on the stability of the entire underground cavern. The practice has proved that in the same formation, the larger the tunnel span, the worse the stability of the tunnel surrounding rock. During the construction process, the degree of fragmentation of the surrounding rock will increase correspondingly with the increase of the span. The support type and erection time for the tunnel is also important to factor affecting the stability of the surrounding rock.
After the excavation of the tunnel, the surrounding rock has a certain self-stabilization time without support. However, as time goes on, the surrounding rock changes to different degrees of movement and deformation. Supporting the tunnel in time can well control the deformation of the surrounding rock. When the nature of the surrounding rock is extremely poor, advanced pre-support methods should be adopted to ensure the safety of tunnel construction before excavation.
In addition, different construction methods also have an impact on the stability of the tunnel surrounding rock. In the same rock stratum, the impact on the stability of the entire stratum is different. For example, the ordinary blasting method or the controlled blasting method, large section excavation or small section excavation, and the Mine Tunneling Method or the Shield Method has different impact on the stratum stability.

It is important to choose reliable support methods during tunnel excavation. Self-drilling pipe roofs consisting of hollow anchor bolt and drill bits are now widely applied in many tunnel projects.