01 Jul 97
for further information.
performed on a drilled shaft with only a single tube using
a probe that contains the receiver separated by an acoustic
(a) The PDA procedure as discussed for driven piles
isolator. A single tube can be used to check the quality of
may also be used for drilled shafts, even though it cannot be
concrete around the tube.
considered a routine test for NDT. The force-time and
velocity-time traces ofo the vibration recorded on the
(c) A gamma-ray source can be lowered down one tube
and a detector lowered down to the same depth in another
by an experienced technician to determine discontinuities
tube to check the density of concrete between the source
and their location in the concrete.
and detector. A change in the signal as the instruments are
lowered indicates a void or imperfection in the concrete.
(b) The wave pattern of large displacements caused by
dropping sufficiently large weights from some specified
(3) Drilling and coring. Drilled shafts that are
height can be analyzed by the PDA procedure and
suspected of having a defect may be drilled or cored to
CAPWAPC method to determine the ultimate bearing
check the quality of the concrete. Drilling is to make a hole
capacity and load-displacement behavior.
into the shaft without obtaining a sample. Coring is boring
and removal of concrete sample. Drilling and coring can
(c) Vibration from a hammer blow measured with
indicate the nature of the concrete, but the volume of
embedded velocity transducers (geophones) can confirm
concrete that is checked is relatively small and drilling or
any possible irregularities in the signal and shaft defects.
coring is time consuming, costly, and sometimes
The transducers are inexpensive and any number can be
misleading. The direction of drilling is difficult to control,
readily installed and sealed in epoxy-coated aluminum cases
and the hole may run out the side of the shaft or might run
on the reinforcing cage with no delay in construction. The
into the reinforcement steel. Experienced personnel and
embedded receivers provide a much reduced noise level
proper equipment are also required to ensure that drilling
that can eliminate much of the requirement for signal
is done correctly and on time.
(a) Drilling is much faster than coring, but less
(d) Forced vibrations induced by an electrodynamic
information is gained. The drilling rate can infer the quality
vibrator over a load cell can be monitored by four
of concrete and determine if any soil is in the shaft. A
caliper can measure the diameter of the hole and determine
and Caiserman 1978). The curve of vo /Fo , where v is the
maximum velocity at the head of the drilled shaft and Fo is
the applied force, is plotted. An experienced operator can
(b) Coring can determine the amount of concrete
determine the quality of the concrete such as discontinuities
recovery and the concrete samples examined for inclusions
and major faults if the length of the shaft is known.
of soil or slurry. Compression tests can be performed to
Information below an enlarged section cannot be obtained.
determine the strength of the concrete samples. The cores
can also be checked to determine the concrete to soil
(2) Access tubes and down-hole instruments. Metal or
contact at the bottom of the shaft.
plastic tubes can be cast longitudinally into a drilled shaft
that has been preselected for special tests. These tubes
(c) Holes bored in concrete can be checked with a
usually extend full length, are plugged at the lower end to
television camera if such an instrument is available. A
keep out concrete, and are fastened to the rebar cage.
portion of a borehole can also be packed to perform a fluid
Various instruments can be lowered down the access tubes
pressure test to check for leaks that could be caused by
to generate and receive signals to investigate the quality of
(d) Defects of large size such as caused by the collapse
(a) A probe that delivers a sonic signal can be inserted
of the excavation prior to concrete placement or if concrete
down a tube and signal receivers inserted in other tubes.
is absent in some portion of the shaft can be detected by
One tube can check the quality of concrete around the tube
drilling or coring. Defects can be missed such as when the
or multiple tubes can check the concrete between the tubes.
sides of a rock socket are smeared with remolded and weak
material. Coring can also detect defects that appear to be
severe but are actually minor. For example, coring can
filled tube installed in a drilled shaft and a receiver inserted
indicate weak concrete or poor material, or poor contact
to the same depth in an adjacent tube. This test can also be
with the end bearing soil or rock in the region of the core,