Holdfast Training Services Trial Blog

Finally we have got to the bottom of the shaft. The excavator has been lifted out of the shaft, allowing the base slab to be poured.

So what is the next step?

Well the Waterproofing sheet membrane needs to be installed over the slab and the walls of the Sprayed concrete lining (SCL). However the water proof sheet membrane requires a regulating layer to be sprayed over the reinforced SCL as the steel fibres could pierce the sheeting allowing water ingress. THe waterproofing sheet will be lowered onto the slab and a huge sheet will be rolled to the top of the shaft. Once it has been nailed in place the sheets will be welded together using a heat roller. Once the sheeting has been sealed and connected to the shaft it will be tested for any leaks. They use compressed air to find the leaks, much like looking  for a puncture in a tyre. The gap between the sheeting is filled with compressed air, if the pressure decreases…you have leak.

Once the waterproofing is in place the next step is the In Situ lining. The concrete will be the first non sprayed concrete we have used since the installation of the ring beam. The concrete will be placed into position using ‘climbing shuttering’. This  will allow the first lift of concrete to be poured, once that has cured and reached the correct compressive strength, the shuttering is raised into the next position ready for the following pour.

So far the project has run relatively smoothly, however with this new waterproofing material I predict a few issues next week. Additionally, we are still trying to find a concrete design mix that fits the required specification for the In Situ lining. So far the concrete design that was selected by our Sub contractors has failed to reach the specification. AS a result I am trying to find a legacy  design that meets with the spec, however they are mostly falling short. The major culprit for this seems to be the amount of polypropylene monofilament fibre and the drying shrinkage requirements. Hopefully we will find a mix before the 26th, when we start pouring. unfortunately, the one design that did meet the spec had a working life of 30mins and it takes 45mins to get to site from the batching plant. The amount of concrete required is a problem area as well, as we need 60m3 every 3 days, not many of the batching sites can provide with their current work loads.

Any way I shall let you know how it goes next week. The good news is that the Sprayed Concrete lining is complete and the slab is being poured as I type. Nearly there!!!!