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Ground Stability, Foundations and Substructures 119
Kelly bar rotates and the core
barrel
Tracked excavator
The loose soil is trapped within
the barrel
The barrel is removed from the
excavated shaft and the granular
non-cohesive soil is released
(b)
Figure 3.50 (Continued)
Rotary drilling equipment is commonly used for piles to be cast in cohesive soils. A
tractor-based rig supports a diesel engine and crane jib. A cable run from the motor up
the jib supports a large, square drilling rod or kelly bar that passes through a turntable,
which rotates the bar to which is attached a drilling auger. The weight of the rotating kelly
bar causes the augur to drill into the soil. The augur is withdrawn from time to time to
clear it of excavated soil. Where the subsoil is reasonably compact, the reinforced concrete
pile is cast in the pile hole and consolidated around the reinforcing cage. In granular
subsoil, the excavation may be lined with steel lining tubes that are withdrawn as the pile
is cast in place. This type of pile is often used on urban sites where a number of piles are
to be cast, because it will cause the least vibration to disturb adjacent buildings and create
the least noise disturbance. See the series of Photograph 3.7a–f for illustration of the plant
and equipment associated with bored cast-in-place piles.