A tunnel is an underground structure which is constructed through the surrounding mass of soil or rock. Typically, a tunnel’s length is much greater than its width.
Tunnels are designed to withstand earth forces applied from all sides. They are mostly used in mining (ore extraction), public utilities (water, sewage and fuel transportation) and transportation (roadways and railways).
Tunnels are complex structures as they must be designed to sustain the loads applied by the weight of the soil mass as well as to bear loads associated with their serviceability (vehicles, fluid pressures, explosions, temperature loads etc.). Moreover, a detailed investigation must be conducted to evaluate the excavated material properties, recognize faults and shear zones and determine the groundwater conditions.
This technique is used in shallow tunnels. First, the soil/rock mass is excavated. The tunnel is constructed inside the excavation which is later covered with the backfill material.
The Jacked tunnel is a special category of shallow tunneling that is usually constructed where the ground cannot be excavated (e.g. roadways that cross major highways). Their length is relatively small. In this tunnel type, a prefabricated rectangular structure is inserted through the soil mass using hydraulic jacks. The soil mass inside the tunnel is excavated afterward and the ground outside the tunnel remains undisturbed. The technique was inspired by pipe jacking.
The tunnel is constructed inside the earth using excavating techniques that either involve: i) blasting, ii) mechanical excavators (conventional method), or iii) mechanized excavation using Tunnel Boring Machines (TBM's).
In a bored tunnel, the ground is usually supported to avoid large displacements or failure. The type of support depends on the ground conditions and the project’s requirements (e.g. maximum settlements at the ground surface).
In conventional methods, temporary support is applied at a small distance from the face of the excavation and consists of shotcrete, steel ribs, and anchors/bolts if necessary. A final lining is implemented at a subsequent phase.
In mechanized tunnel excavation, as the cutter head rotates, the TBM moves forward and the excavated ground is supported by a precast liner that consists of concrete segments. Once the liner is finished, a grout mixture is injected between the excavated ground and the liner to cover the over-excavation spaces.
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