When driving large SCL tunnels in London Clay where the face is stepped it is important to ensure a good connection in the sprayed concrete primary lining between the Top Heading and the bench/invert steps.
Traditionally in SCL tunnelling this is normally achieved through the use of “L” shaped steel bars or reinforcement continuity systems (pull-out boxes) like Kwikastrip and Ancon Eazistrip.
However the preparation and installation of the bars and pull out boxes during construction of the top heading exposes personnel to the risk of falls of freshly sprayed concrete and clay at the excavated tunnel face (note Crossrail nozzleman fatality in Feb 2014).
At Bank Station Capacity Upgrade project we have developed, designed and implemented what is known on the project as the “Type A” joint. This method utilises a mechanised approach to preparing the construction joint by spraying the bottom of the top heading in a stepped profile and later during excavation of the bench/ invert trimming the bottom edge of the concrete using a breaker mounted on a small excavator.
The Type A joint has the following advantages over the conventional pull-out boxes:
- Much safer working environment close to the tunnel face
- No need to enter the high-risk area immediately in front of the tunnel face area during construction of the top heading
- Fully mechanised, no manual handling needed
- Better quality joint (i.e. less voids, no trapped concrete rebound)
- Quicker preparation, more productive and cost efficient