Instrumentation in Tunnelling
The observation of physical and structural properties and behaviour of the soil and the tunnel itself are integral parts of design and construction of underground structures. During the investigation stage, investigations would cover determination of the characteristics of the soil along the alignment pore pressure, and permeability, especially in case of subway tunnels and fault zones. A bore hole is drilled into the ground upto well below the potential influence zone of the tunnel and a Flush coupled rigid PVC Access pipe of smaller diameter is inserted into the same upto base. It is necessary to note the quantum of movement of exposed tunnel walls and roof periodically, for which target anchors have to be fixed as quickly as possible before the tunnel starts to ‘work’. In case of shallow tunnels, done by boring through soil such settlement is countered by stabiiising the soil just ahead of the tunnelling face and on sides by freezing or grouting.