Stabilising Continuous Flight Auger Pile Excavations with Compressed Air
A commonly used piling method in Australia is continuous flight augering, with an experienced operator it can be relatively simple to install, cost effective, with limited noise and vibration.
The interesting aspect of CFA piling by AVO is their use of compressed air to stabilise the excavation. Avo use a Bauer BG 24 rig, which has the ability to inject compressed air from the tip of the auger. Compressed air is injected near the discharge pump at 150kPa increasing by 100kPa for every 10m of depth. The pressure helps maintain the stability of the excavation prior to concrete being introduced through the stem of the auger.
As the soil is excavated from the pile shaft, the horizontal effective stress (σ’3) on a soil particle on the shaft wall reduces to zero. The air pressure applies an equalising horizontal force to the walls of the excavation of about 250kPa, this is larger than the 146kPa which is being exerted by the soil around the shaft near the toe (see table 1). The air pressure helps stabilise the excavation from collapsing by applying a passive force against the active force from the soil around the shaft walls. Excess pressure is forced up the shaft helping lift soil up the flights of the auger.
This method helps increases the pile skin resistance by limiting voids along the pile shaft.
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How much is the technique affected by the soil properties?
Specifically, I was wondering about permeability?
James,
Why does inserting concrete not work as normal to stabilise the bore? There is normally no gap beneath the auger if the concrete is flowing in correctly.
Also if you are putting in significant air pressure how does this affect ability to install piles nearby?
James, I hope you and Sophie are well.
I’m slightly confused by this, what soil layers are your drilling in? Are you in a cohesive material hence the Cu on the sketch but then why not use a support fluid?
Does the air pressure help lift the pile arisings up and could therefore speed up drilling with less RPM?
Sorry just read the bottom comment so ignore the lifting comment.
Thank you all very much for your comments, these have helped me further develop my understanding of the topic.
Firstly Brad, you are correct, I have used the wrong parameters, the soil is a coarse grained sand below the water table. I should have used c’ and Φ’ as the shear strength parameters. I was going down the garden path of cohesion being from the cementation between the soil particles as the rock flour particles of sandstone have formed what is called coffee rock near the toe of the pile.
Damien, interestingly I would have thought that the concrete would have been sufficient to provide the passive pressure within the pile excavation. On talking to Avo Piling about their method they have found that on extraction of the auger, if the concrete is not supplied sufficiently quick enough a void can occur between the auger head and concrete delivery, so to treat the risk they use the air to help compensate for the lack of passive pressure on the shaft walls. The air pressure also helps ground water and soil pass up the flights more smoothly.
From B63 (The Australian Specification for CFA Piling) they cannot drill piles which haven’t cured with spacing’s less than 4 diameters. Therefore every even pile was drilled on a Saturday and every odd pile on a Sunday.
Dickie, I haven’t been able to find any quantifiable information as of yet about how it is affected by permeability. I would have to surmise that this technique seems to be more useful in a coarse grained material with low shearing resistance as it aims to stop the collapse of shaft and helps push the water and soil up the flights.
If you have any other thoughts on this process please share them with me as I believe it is quite innovative.
PS Brad, Sophie and I are very well thanks, she has just started an exciting new job for Cisco Systems as Partner Marketing Manager. I hope you and Charlotte have settled into London life well.