Open Faced Tunnelling

This is a simple but economic form of mechanised tunnelling especially for short tunnels provided the ground is self supporting with SPTs of below 40. It can generally be divided into two methods backactor or digger shields and drum diggers.

The backactor machine consists of a telescoping digger arm mounted in a rotating turret built into the roof of a steel can the outside diameter of the pipe. For alignment purposes this can is articulated or has pockets at the rear to allow the use of steering rams. This can also contains the power pack for powering all excavation operations, plus the operators station.

The telescoping arm allows the bucket to dig the face, dragging the spoil down onto muck plates which then direct the excavated material onto a belt conveyor, on which it passes to the rear of the can and into skips.

The skips are hauled up and down the tunnel using winches in the rear of the machine and in the pit bottom.

The jacking rams in the pit bottom are positioned such that the skip will fit between them, from which point it is lifted out by crane and an empty skip positioned in its place by use of a davit arm, from where it is dragged back into the tunnel by the winch in the rear of the machine.

A laser set to tunnel line and level is positioned in the pit bottom acting on a target adjacent to the machine operator who uses the information obtained to set the steering rams to give the required orientation.

As the spoil is removed from the face the whole pipeline is jacked forward from the pit bottom on the command of the machine operator, thus ensuring the ground is supported at all times. When the full length of a pipe has been pushed, the pit bottom rams are retracted, a pipe lowered in front of the rams and connected to the last pipe pushed and then the whole process of excavation and pipe pushing is repeated.

The drum digger which can also be used in weak mudstones uses rotating radial arms fitted with point attack bits or spades to excavate the face instead of the backactor bucket. From there on, the systems are similar in operation and method.

A E Yates Trenchless Solution Ltd have a number of Open Faced Tunnelling Case Studies

Open Faced Tunnelling rigOpen Faced Tunnelling

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