Pile Construction in Porous Rock

[steveyeung]

I have a question about secant bored pile construction:

The secant piles (of 10-15m long) are supposed to be constructed in a layer of rock which is porous and low compressive strength. Another constraint is the water table is very high.

Does anybody know there is any method of keeping the borehole dry during concreting if bentonite is not allowed to use?

 

[GeoPaveTraffic]

I would expect the contractor to use augered cast-in-place, ACP, piles. In this type of construction the pile is drilled and filled with concrete at the auger is removed. Therefore, no bentonite or other drill fluid is needed.

 

[BigHarvey]

Real secant piles are executed with cased holes which should answer your question. Doing secant wall with CFA ( ACP in the US )is bad practice: the auger is too flexible and therefore your piles will be more tangent that secant, with the risk of leaving "windows" where materials + water could cave in during excavation.

 

[GeoPaveTraffic]

I agree "windows" can be a problem, however, depending on the soils and the quality of the contractor, ACP piles can be used to make very good walls.

I would say that for most contractors a 30 foot tall wall is approaching the limit for a high quality end product.

 

[steveyeung]

So, what is your solution....?

 

[dmoler]

Use polymer slurry instead of bentonite? Much better anyway to avoid cake build-up.

Why not use a pressure tremie that starts at the bottom.

 

[SmokeyBear]

A dual-head drilling system, where the casing and the auger are counter-rotated would seem the way to go. If you use a CFA method (recommended) the hole will stay relatively dry and the concrete will be tremmied into the base of the hole during extraction. A hard/soft system would seem best - possibly with only the hard piles reinforced.(Maybe some nominal reinforcing in the soft piles.)

The casing "stiffens the drill string and limits bit wander.

I would be very careful using uncased secant piles to this depth. Very good drilling in an uncased hole would be expected to wander at least 1%. Therefore at 15m depth you should expect at least +/- 150mm wander from pile head. This results in the "windows" mentioned in previous posts.

On a recent project, we found a maximum horizontal wander of only 30mm between adjacent piles. This makes placing the walers a breeze, instead of cutting back, chocking and wedging. This was in sands overlying low strength sandstones.

Good luck.