Holdfast Training Services Trial Blog

Site Two Fifty One – Drainage testing and Assessment of plunging reinforcement in piles

Drainage testing

As I mentioned before the underslab drainage needed testing before the concrete slab was cast. The method for this was actually very simple. There are 2 common options: air and water tests. We have done air tests. The method is to bung the openings in a run (such as gullies, rodding eye, down pipes, etc) pressurise one end with a pump to 100mm on a U tube Manometer. Over a 5 minute period the drop should be no more than 25mm. If this is the case move on to the next run, if not, test shorter sections until the leakage point is found.

Manometer used to confirm pipes do not leak.

Tension

How to get tension anchorage. We have installed our first tower crane base this week. The 80m3 pour is impressive, not least because of the 0mm tolerance on the crane base but more because of the W bars used to provide tension reinforcement within the pile cap.

If you open this image up and look at the 4 tower crane legs you can see the “W” reinforcement.

The other interesting point is that dywidag bars the length of the piles (about 26m) have been installed to avoid the whole pile cap rotating. However the anchorage length required is greater than the height of the pile cap. To solve this, large (676cm2) plates and nuts have been used to give greater anchorage. Has anyone else seen this method used?

Square plates used to provide additional anchorage.

Piling methods

The piling method at Two Fifty One was to install piles at ground level, plunge the pile reinforcement and then excavate ground to formation level (approximately 5m lower), breaking the piles are we go.

2-Year old’s impression of plunging pile reinforcement cages

This has a significant disadvantage. Let’s assume 300 piles, 5m waste, 0.75m diameter, £140 per m3 of concrete equals a little under £100k in waste concrete, plus time to break piles, plus muck away time.

However the advantage is that the piles can actually be installed. If the site was deeper the props would be in the way of the piling rig and getting the rig out of the site would have been pretty tricky. Bearing in mind the piling package was about £4M, this represents a 2.5% fraction of the cost.

Further issues. Submerging pile reinforcement cages to an exact cut-off level means it was not done very well. Mostly are within tolerance for anchorage requirements but some are too low.

Spot the difference – something is not right about this pile…

Moreover, in order to dig deeper pits we have had to cut some reinforcement down (to facilitate excavator arm length reaching to dig level). Therefore to extend the reinforcement back to the correct length couplers have been used. Bar diameter dictates coupler size. The common MPT couplers seem to be what everyone has heard of, they are British Board of Agreement approved, but unfortunately not Certification Authority for Reinforcing Steels (CARES) approved. Laing O’Rourke only allow CARES suppliers – due to adhering to certain quality standards and also as a wider corporate image piece. Therefore once this hurdle was overcome, the couplers were installed. As a final point the locking bolts should shear off to demonstrate correct installation.

Couplers used to extend pile reinforcement.