Holdfast Training Services Trial Blog

Situation

Have a look at the sketch below. As the tunnel is being excavated, above the tunnel there is a ‘umbrella’ of grouting (this is constructed using TAM’s, Tube A Manchette). The grouting is there to mitigate the settlement on the surface. However there are no grouting zone, that ensures the pressure from the grouting does not effect the exposed face (in this case 3 rings, 3m). The horizontal no grouting zone runs parallel to the tunnel at a depth of 4.5m, the vertical zone moves with the construction, however it’s dimensions are dictated by the exposed face (3m) and the angle that the TAM pressure influences (currently at 45 deg)

Issue

The problem is that the amber trigger levels have been hit, ref the surface settlement. The subcontractor want us to reduce the vertical ‘no grouting’ zone. This can only be achieved by reducing the angle from 45 deg to say 30 deg as the exposed face can not be reduced.

Calcs

If the pressure on the face is approximated as conical spread from point of injection. My preliminary calcs suggest that a reduction from 45 deg to 30 deg will impose a 300% increase in the pressure, as the area will reduce from 61m2 to 20m2.

However this is a very rough approximation, as many of the stress counters from the TAM will be contained within the 30 deg. What Im trying to calculate is the % of stress that will be contained within the 30 deg compared to the 45 deg. Does this make sense? How the hell can I do it?