A drilled shaft is a cylindrical excavation from which all
of the soil or rock has been removed. The diameters of drilled shafts for
transmission structures are controlled by the size of the anchor bolt circle or
the stub angle geometry, and thus can be from 2 ft (0.6 m) to 10 ft (3 m), or
larger.
A drilled shaft should be excavated to the required depth with equipment that gives a true dimensional hole with straight walls. Where rock is near the ground surface, drilled shafts that penetrate the rock are common.
This excavation can be done by methods that will be
discussed herein. Auger-type drilling rigs are available in a wide variety of
mounting and driving arrangements. The choice normally is the contractor’s
preference, based on experience and the capability of the machine.
The equipment should be suitable to complete the job. In addition
to the auger-type drilling rigs, there are rotary rigs that use circulating
drilling fluid to carry the cuttings out of the excavation. These rigs can be
used in wet-hole construction to drill large diameter, uncased foundations.
Construction by Dry-Hole Method
In noncaving soils where a drilled hole stands open readily,
straight holes are drilled quickly, usually with an open-helix auger having two
or three turns of a single ßight. In these areas, the concrete is placed with
few, if any, problems.
When a hole is being drilled in stable soil and a soft
stratum of clay or organic silt is encountered, this stratum tends to squeeze
in and reduce the diameter of the hole. A casing should be inserted to limit
this action and to protect the hole. The casing can be advanced by driving or
drilling ahead of the excavation through the squeezing zone.
Cohesionless soils, sandy silts, and gravels usually will
cave when drilled by auger, either above or below the water table. The usual
and direct way of coping with a caving soil is to install casing as the hole
proceeds through the caving formation. In some caving soil, it is possible to
drill the hole quickly and insert the casing freely before caving starts.
Placement of Concrete
Immediately prior to the placement of concrete, the bottom
of the drilled shaft should be clean and dry. If the bottom of the hole
contains a few inches of water or mud, enough cement or gravel cement mix must
be added to absorb the water and form a paste or soil-cement mix so rich that
it will protect the concrete as it is placed in the bottom of the foundation.
Past experience has shown that freely-falling concrete does
not segregate, but concrete discharged into the drilled shaft from the chute of
the transit mixer invariably will strike the reinforcing and the side of the
hole as it falls. This type of placement should be avoided. A hopper centered
over the hole with a section of elephant trunk attached to its bottom is
necessary to take the concrete from the chute of the truck or the concrete
bucket and concentrate it in a small stream to the bottom of the excavation.
Concrete slump and maximum aggregate size should be
controlled so that the concrete will flow freely around the reinforcing bars
and completely fill the area outside the cage. The slump should be between 4Ð6
in (100-150 mm). The concrete should be vibrated in the top 10-15 ft (3-4.5 m)
so that it makes complete contact with the reinforcing and anchor bolts. This
vibration also prevents voids in the part of the foundation where the
hydrostatic pressure of the wet concrete is the lowest.
Vibration should not be done until the casing has been
removed, because it could cause packing and wedging of sand and gravel outside
the casing, resulting in considerable difficulty in pulling the casing.
Although not always possible, the placement of concrete in the drilled shaft
should be continuous.
When a partially filled drilled shaft is allowed to stand
until the concrete has taken its initial set, the exposed surface will require
cleaning before more concrete can be placed to ensure a good bond between the
old and new concrete.
The scum or laitance that settles on the top of the
newly-set concrete should be removed, and the surface should be roughened. Just
before the pour is resumed, a slush of cement grout should be spread over the
top of the old concrete. All reinforcing bars should be cleaned of old cement
grout.
Casing
When a hole has been cased to seal out water, the casing
must not be disturbed until enough concrete has been placed to produce a higher
concrete pressure at the level of the casing seal than the water pressure
outside the casing at that level. If this procedure is not followed, water can
enter the concrete column, causing voids or zones where aggregate is washed
clean of cement. Also, the concrete level in the casing must be maintained at a
high level as it is pulled because voids outside the casing will cause concrete
to flow from the casing to fill them.
When telescope casing is used, it is necessary to place
enough concrete in the casing section so that the surface of the concrete does
not fall below the bottom of the next larger casing. This surface will depend
on the relative positions of the top of the bottom casing, the bottom of the
next section, the level of groundwater outside the bottom section, and the
volume of water to be displaced outside the lower casing and below the bottom
of the next sections.
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