Piling Mastermind

Mercia Mudstone often is very clayey at the top, but then with depth becomes harder more broken and often (key) water bearing. If the pile depth progresses below the water table and one of those rigs is less powerful than the other, then the rate of penetration will also be much less and the fighting action during drilling may leave the pile shaft being smeared with soft clayey mud, lessening the shaft friction.

The ability to penetrate hard strata with an auger is dependant on the downward force generated to cause penetration, combined with the rotational torque available to rip out the material. 

The ability of a CFA auger to generate downward force is dependant upon the friction developed between the ground and auger by the “screw thread” effect of the rotating flight. If the auger is “cork-screwed in”, then high friction and downward forces would be developed, but if it is drilled with a lot of flighting then the friction and downward force would be quite low. An obvious advantage of a Rotary rig however is the ease with which digging tools may be changed, making the rotary technique generally superior for hard strata.

Creating a laboratory design mix to achieve the required strength and durability is not a problem.

Anticipating the variability of the site conditions which may exist (and how these affect the concrete) should be the focus of your attention, for example the possibility of delivery delays and the likely temperature (e.g. will this cause a flash set?) or whether ground conditions will absorb water from the mix, or whether bleed might occur?

Probably the most important aspect is the ability of the concrete to maintain its structure and workability throughout placing until set.

We have constructed  1050mm piles by 38m deep in the past, but in theory this is not the limit in either diameter or depth.  The Martello rotary system only ever takes small bites of 0.75m or similar, and 1500mm diameter and 50.0m are well within development consideration.

Ideally there should be no difference! Firstly, this depends upon the strata, and secondly upon the drilling technique. For example, if you are in a fine water-bearing sand which is prone to flighting from the Archimedean screw effect of the auger, then potentially there could easily be a difference, and the reason here is, that a larger diameter auger has a shallower flight angle at the auger/soil inter-face, than a smaller diameter auger. The shallower the angle of the flight at the auger/soil inter-face, the greater the propensity for flighting, so potentially (with a larger diameter auger) the flighting could be greater, leading to the loosening of the surrounding ground being greater, and hence the shaft friction lower (with potentially the concrete usage being proportionally higher too).

This would depend upon the engineering requirements of that pile; Ideally nothing, but we have addressed this as an engineering thought process on this site. See “GOOD PILING PRACTICE” and “Good Tremie Technique”