DODGING BOULDERS IS LIKE DODGING BULLETS?
To dodge a bullet is to barely avoid something bad, harmful, or unpleasant.... This certainly applies to boulders and cobbles in the geotechnical report for your new build!
WHY?
Piling contractors of any methodology will understand the inherent risks when installing piles through boulders and cobbles.
- DRIVEN PILES - they may struggle to push past the boulder and it refuses. Equally, the pile may get damaged (top or bottom), or even damage the plant.
- BORED PILES - the auger/barrel will have to core through the "very hard" boulder. This requires core barrels and takes longer - so costs more. The boulder may also collapse/fall into the open bore, and so grabs are required to pick the boulder up (add time, add money). Voids in the bore may become larger and require casings. But be careful withdrawing them (temp casings) as water may fill the void, but cause necking afterwards. Permanent casings = expensive!
The same applies with screw piles - which may sit and spin above the boulder, or have the helix hit the boulder (and refuse) or might even push the boulder inwards and get caught within the helix flight. That sudden change may cause the powerhead to have a pressure spike and, if an unaware operator doesn't drop the pressure smartly, cause the helix to be ripped off or goose neck the pipe.
So, in the past we have tended to recommend to our clients that screw piles is not the best path forward when seeing boulders/cobbles.
However in recent times - we have begun to help clients manage risk - but requiring our clients and their consultants to be collaborative in their approach. In doing so, solutions to the risks can be navigated:
1) Hit a cobble - unscrew, and then re-screw to the side. This necessitates the structural engineer to have greater horizontal tolerances to the pile positioning.
2) Design for installation - the pile is likely to take some "bash and crash" so oversize the pipe, the welds and the helix. More spent in materials, but less spent in re-work!
3) Be ready - you don't mobilize dinky toys, you bring a sledgehammer and the kitchen sink ready (and able) to give it more if and when required.
There are additional tricks, but critically it requires the geotechnical engineer and structural engineer to collaborate with the pile designer and the excavator operator - as when all parties understand the needs of the other, a best-for-client solution can be found!
Finally - a good operator is essential! Good operators can almost feel (and therefore see!) what is happening deep below the ground. Their intuition is incredibly helpful to engineers who often sit in the office with no idea of what the hell is happening in the field.
So, if you know bullets (boulders!) are coming your way, engage a specialist that can help you dodge them.
Dodging boulders.
