Drilled shafts, also referred to as drilled piers, caissons or bored piles, are deep foundation solutions used to support structures with large axial and lateral loads by excavating cylindrical shafts into the ground and filling them with concrete
How Drilled Shafts Works :-
Drilled shafts are typically designed and constructed to support axial forces through a combination of side friction and end bearing resistance. They range in diameter from 24” to 120” and can be installed to depths in excess of 300 feet.
Why You Need Drilled Shafts
Drilled shafts are an economic solution that can be installed in a variety of different ground conditions to accommodate large axial, lateral and overturning forces.
CONNECTIONS BETWEEN DRILLED SHAFTS AND COLUMNS
The connection between the drilled shaft reinforcement and the column raises another constructability concern. There are several possible approaches to the connection design.
A major consideration that all contractors must take into account is the tolerance in design of a splice near the top of the driller shaft or the base of the column. This may present a concern for ductility in a high moment area for seismic loading.
If the design allows for a lap splice at the base of the column, a relatively simple approach is to leave the shaft reinforcement sticking above the top of the shaft by a length sufficient to form the splice. This design works best for round columns with shaft and columns of similar size.
Alternatively, the connection can be made at the top of the column for the same offset relevance of the drilled shaft.
This can be done to accommodate the location tolerance of the drilled shaft and to maintain the required concrete cover for the drilled shaft rebar cage. It allows the drilled shaft re bar cage to remain centered in the drilled shaft, while the column steel can be spliced directly to the drilled shaft re bar cage.
If a continuous longitudinal cage extending from the shaft into the column with no splices near the ground line is required, then the contractor may need to work over and around a cage that extends many feet above the shaft.
This will increase costs, due to the need for bigger cranes and more complicated concrete placement.
In certain cases, a drilled shaft that is significantly larger than the column is part of the design so that any damage from seismic over-stress conditions is confined to the base of the column above grade.
This type of connection is used in seismic areas, with a column reinforcement extended into the top of the shaft to form a “non-contact” lap splice to develop the strength of both the column and the shaft reinforcement.
If a drilled shaft reinforcement includes a connection to a cap, grade beam or abutment wall, the cage for the shaft should not include out-hook bars or other obstacles when a temporary casing is used.
If possible, these can be turned inward during installation and then rotated into position after the concrete has been placed.
Longitudinal bars can also be field bent hydraulically after the casing has been removed and both L-shaped bars or out-hooks could be included in a secondary splice cage.