Friday, November 23, 2012

Nondestructive Tests - Drilled Shafts.

b.   Nondestructive tests.  Routine inspection with nondestructive  tests (NDT) using wave propagation shall be performed  to check the quality of the installed drilled shafts.

Additional special tests as indicated in the following paragraphs are performed if defects are suspected in some drilled shafts.

Routine tests performed as part of the inspection procedure are typically inexpensive and require little time.  Special tests to determine  defects, however, are often time consuming, expensive, and performed only for unusual situations.

(1)  Routine inspection tests.  The most common routine NDT  is  to externally vibrate the drilled shaft by applying a sudden load as from a hammer or heavy weight dropped from a specified height.  Signals from the wave are recorded by transducers and accelerometers installed near the top of the shaft or embedded in the concrete at some location in the length of the shaft.  Access tubes may  also be installed in the shaft for down-hole instrumentation to investigate the concrete between access tubes.  Refer to FHWA-HI-88-042

(a)   The PDA procedure as discussed for driven piles may also be used for drilled shafts, even though it cannot be considered  a routine test for NDT.  The force-time and velocity-time  traces ofo the vibration recorded on the oscilloscope caused by a dynamic load can be interpreted by an experienced technician to determine discontinuities and their location in the concrete.

(b) The wave pattern of large displacements caused by dropping  sufficiently large weights from some specified height  can be analyzed by the PDA procedure and CAPWAPC  method to determine the ultimate bearing capacity and load-displacement behavior.

(c)  Vibration from a hammer blow measured with embedded  velocity transducers (geophones) can confirm
any  possible irregularities in the signal and shaft defects.

The  transducers are inexpensive and any number can be readily installed and sealed in epoxy-coated aluminum cases on the reinforcing cage with no delay in construction.  The embedded  receivers provide a much reduced noise level that  can eliminate much of the requirement for signal processing.

(d)   Forced vibrations induced by an electrodynamic vibrator  over a load cell can be monitored by four accelerometers installed near the shaft head (Preiss, Weber, and Caiserman 1978).  The curve of v0  / F0  , where v0  is the o o o maximum velocity at the head of the drilled shaft and F0  is o the applied force, is plotted.  An experienced operator can determine the quality of the concrete such as discontinuities and  major faults if the length of the shaft is known. Information below an enlarged section cannot be obtained.

(2)  Access tubes and down-hole instruments.  Metal or plastic tubes can be cast longitudinally into a drilled shaft that  has been preselected for special tests.  These tubes usually extend full length, are plugged at the lower end to keep  out concrete, and are fastened to the rebar cage.

Various instruments can be lowered down the access tubes to generate and receive signals to investigate the quality of the concrete.

(a)  A probe that delivers a sonic signal can be inserted down a tube and signal receivers inserted in other tubes.

One tube can check the quality of concrete around the tube or multiple tubes can check the concrete between the tubes.

(b)   An acoustic transmitter can be inserted in a fluid-filled tube installed in a drilled shaft and a receiver inserted to the same depth in an adjacent tube.  This test can also be performed on a drilled shaft with only a single tube using a probe that contains the receiver separated by an acoustic isolator.  A single tube can be used to check the quality of concrete around the tube.

(c)  A gamma-ray source can be lowered down one tube and a detector lowered down to the same depth in another tube  to check the density of concrete between the source and detector.  A change in the signal as the instruments are lowered indicates a void or imperfection in the concrete.

(3)  Drilling and coring.  Drilled shafts that are suspected  of having a defect may be drilled or cored to check the quality of the concrete.  Drilling is to make a hole into the shaft without obtaining a sample.  Coring is boring and removal of concrete sample.  Drilling and coring can  indicate  the nature of the concrete, but the volume of concrete that is checked is relatively small and drilling or coring  is time consuming, costly, and sometimes misleading.  The direction of drilling is difficult to control, and the hole may run out the side of the shaft or might run into  the reinforcement steel.  Experienced personnel and proper equipment are also required to ensure that drilling is done correctly and on time.

(a)   Drilling is much faster than coring, but less information is gained.  The drilling rate can infer the quality
of  concrete and determine if any soil is in the shaft.  A caliper can measure the diameter of the hole and determine any anomalies.

(b)  Coring can determine the amount of concrete recovery and the concrete samples examined for inclusions
of  soil or slurry.  Compression tests can be performed to determine the strength of the concrete samples. 

The cores can  also be checked to determine the concrete to soil  contact at the bottom of the shaft.

(c)  Holes bored in concrete can be checked with  a television  camera if such an instrument is available.  A portion of a borehole can also be packed to perform a fluid pressure  test to check for leaks that could be caused by defects.

(d)  Defects of large size such as caused by the collapse of the excavation prior to concrete placement or if concrete is absent in some portion of the shaft can be detected by drilling or coring.  Defects can be missed such as when the 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 indicate weak concrete or poor material, or poor contact with the end bearing soil or rock in the region of the core, but  the remaining shaft could be sound and adequately supported by the soil.

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