Mini pilling is also called drilled piers or Cast-in-drilled-hole piles (CIDH piles) or Cast-in-Situ piles. Rotary boring techniques offer larger diameter piles than any other piling method and permit pile construction through particularly dense or hard strata. Construction methods depend on the geology of the site. In particular, whether boring is to be undertaken in 'dry' ground conditions or through water-logged but stable strata - i.e. 'wet boring'. 

 

Boring is done until the hard rock or soft rock layer is reached in the case of end bearing piles. If the boring machine is not equipped with a rock auger, then socketing of the hard rock layer is done with the help of a heavy chisel which is dropped from a height of about 1.5 metres (depends on the weight of the chisel and design requirements) by suspending it from a tripod stand attached to a winch crane. The socketing is carried out until the desired depth within the rock layer has been attained. Usually, the required depth within the rock layer is considered to be equal to the diameter of the pile in hard rock layers and is taken to be equal to 2.5 times the diameter of the pile in soft rock layers. 

 

'Dry' boring methods employ the use of a temporary casing to seal the pile bore through water-bearing or unstable strata overlying suitable stable material. Upon reaching the design depth, a reinforcing cage is introduced, concrete is poured in the bore and brought up to the required level. The casing can be withdrawn or left in situ. 

 

'Wet' boring also employs a temporary casing through unstable ground and is used when the pile bore cannot be sealed against water ingress. Boring is then undertaken using a digging bucket to drill through the underlying soils to design depth. The reinforcing cage is lowered into the bore and concrete is placed by tremmie pipe, following which, extraction of the temporary casing takes place. 

 

The reinforcement cage may need to be lapped with another cage if the depth of the pile exceeds 12 metres as that is the standard length of reinforcement bars of diameter 16mm and above. 

 

In some cases there may be a need to employ drilling fluids (such as bentonite suspension) in order to maintain a stable shaft. Rotary auger piles are available in diameters from 350 mm to 2400 mm or even larger and using these techniques, pile lengths of beyond 50 metres can be achieved. 

 

Such piles commonly fail due to the collapse of the walls of the shaft resulting in the formation of a reduced section which may not be able to bear the loads for which it had been designed. Hence at least a third of piles in projects with a large number of piles are tested for uniformity using a "Pile Integrity Tester". This test relies on the manner in which low intensity shock waves are affected as they pass through the pile and are reflected to judge the uniformity and integrity of the pile. A pile failing the integrity test is then subjected to a pile load test.