Pre-boring is a commonly referenced method for easing the passage of some driven piles into the ground. However, its use can also be misunderstood or misguided. It is not a satisfactory way of overcoming significant obstructions to enable piles to be driven because that which impedes the driven pile will also in general impede progress of the pre-boring tool.

Pre-boring in sand and gravel presents a problem because of the inherent instability of the soil through which the pre-bore passes. When such soil is dense, pre-bores may stand open temporarily because of arching and the influence of temporary pore water suction.

However, as soon as a piling tube or pile enters the bore and the hammer begins striking, the upper granular soil collapses into the lower part of the bore. The lower section of the bore will possibly not collapse in this circumstance at the initial driving strokes because the soil is relatively more dense and the hammer influence more remote.

The result is frequently that because of re-compacted debris in the lower bore, piles will not drive back to the same depth as originally bored. Only if the bore is temporarily cased to prevent collapse, and if the casing is of large enough diameter to allow access for the final pile, can a satisfactory load bearing unit be inserted, albeit with loss of potential friction resulting from loss of displacement effects and the need for in-filling around the pile.

As an alternative to trying to form an open hole in sand soils, the pre-boring tool is sometimes used simply to stir up the ground, leaving disturbed soil in position. This may be sufficient to deal with dense soil near ground level.

However, if deep bores are attempted after this manner, again when a piling tube or pile is entered and driving begins, the loosened material is compacted down into the lower part of the bore and becomes virtually indistinguishable from the original natural soil. Piles will frequently not drive back to the depth of the pre-bore or may behave inconsistently under applied load.

It is therefore not generally satisfactory to use deep pre-bore methods in sands, for example, for the purpose of ensuring that piles reach a deeper stratum such as rock unless special temporary casing methods are adopted.

Pre-boring sockets into rock or very hard soils for the supposed purpose of enhancing end bearing or reaching strong soil, where there are overlying fill, sand or clay layers, is also generally futile. For the same reasons as stated above, it will be found that without guaranteed bore stability and measures to prevent soil from collapsing into the socket, a satisfactory load bearing and consistent unit cannot be formed because of debris falling before the pile arrives.

Pre-bores are satisfactory only under specific circumstances:
1 To loosen dense upper crust soils and enable long piles to be driven without breakage. Long piles struck at the head are really slender columns and so the possibilities of buckling failure can be very real.

2 To make an open hole in stiff clays or similar cohesive soils into which a pile is pre-entered. The purpose in this instance is to avoid or diminish soil heave. If using the method for the purpose of eliminating ground heave, it is generally legitimate to choose the area of the bore so that the pile cross-sectional area is just slightly larger.

Jobs with pre-boring are frequently associated with claims and cost overruns, partly because it is difficult to synchronize the activities of boring and driving machines with consequent delay, and partly because, where the motivation is to achieve stringent ‘sets’ this may be a major source of damage to equipment.  

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Carter Madson said...

Do you have a pre drill pilings model or sketch? I'm trying to figure out the best way to begin drilling.

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