TECHNICAL
Over the past two decades, various non-destructive testing solutions have
been developed for different applications in geotechnical engineering.
collapse of the superstructure. The consistency and integrity
of piles and deep foundations need to be regularly evaluated
from the beginning.
Over the past two decades, various NDT solutions have
been developed for different applications in geotechni-cal
engineering. The article will briefly discuss three NDT
methods that are widely used for evaluating the integrity of
concrete piles and deep foundations.
NDT of piles and foundations
As discussed earlier, visual examination of piles is nearly
impossible, due to the fact that a large section of pile systems
is often buried under ground. Whether it is a driven concrete
pile or bored cast-in-place pile, contractors and geotechnical
engineers are looking for methods to quantitatively assess the
load bearing capacity of piles, as well as quality and integrity.
NDT techniques provide a practical and convenient solu-tion
for testing piles and deep foundations (2010). In contrast
to traditional intrusive methods such as drilling and coring,
NDT methods are relatively fast, repeatable and accurate
(when conducted properly), which makes them very popular
(ACI 228.2R). Different NDT solutions have been developed
and commercialized over the years.
Integrity evaluation is a constant challenge in concrete
pile construction. Cast-in-place piles can suffer from defi-ciencies
in materials and pour quality, while precast concrete
piles may experience damages during transportation and
installation. NDT methods can be used to evaluate the integ-rity
and consistency of piles. This article presents and briefly
discusses three different methods of integrity evaluation:
• Low strain impact integrity testing (ASTM D5882)
• Ultrasonic cross hole testing (ASTM D6760) for piles with
an accessible tip
• Parallel seismic (ACI 228.2R) for piles covered up by a
pile-cap
Low strain impact integrity testing
Low strain impact integrity testing provides acceleration or
velocity data that can be used to assess the integrity and con-sistency
of piles. The test is also applicable to long structural
elements that function in a manner similar to any deep founda-tion
unit – such as driven piles, augured piles or drilled shafts.
A hand-held hammer – or instrumented hammer – is used
to impact the pile head. The impact should be applied axi-ally
and perpendicularly to the pile head surface. A motion
transducer, capable of detecting and recording the reflected
echoes, is placed over the pile head. The distance between the
impact location and the sensor should be no larger than 300
mm. It is recommended to collect a minimum of 10 measure-ments
for every single pile to have a better understanding of
the pile’s integrity problems.
The test results can be used to evaluate the pile cross-sectional
area and length, determining the integrity and
continuity of piles and assessing the consistency of pile materi-als.
However, this method has certain inherent limitations: pile
integrity testing does not provide any information on the load
bearing capacity of piles. This test cannot be conducted over
pile caps and is not reliable for evaluation of unfilled steel pipe
piles, H-piles or steel sheet piles. In some cases, it is difficult to
distinguish the soil response and the pile toe response.
Figure 1
OR
Figure 2
Ultrasonic cross hole testing
Ultrasonic cross hole testing provides information about the
homogeneity and integrity of concrete. This method is used
to identify the regions of poor quality concrete and over-comes
the limitation of low strain impact integrity testing.
This method requires a number of vertical holes (tubes)
made during concrete placement using parallel metal or plas-tic
tubes. Depending on the pile diameter, the number and
configuration of these holes can be determined (see ASTM
D6760). The holes should be filled with water. An acoustic
wave emitter transducer is lowered to the bottom of one tube
while another acoustic wave receiving transducer is placed
at the bottom of second tube. Both transducers are pulled
upward at the same rate. The signals from the transmitter
and receiver probes and the depth-measuring device are
transmitted to a field computerized apparatus for recording,
processing and displaying the data in the form of an ultra-sonic
profile. This profile is used for the integrity evaluation
of the pile along the pile length (Fig. 2).
The signals are analyzed to determine the location of
defects, as well as identifying the extent of the defects. The
test can be expanded using several holes, enabling a full cross
section analysis of the pile. Ultrasonic cross hole testing is an
ideal test when evaluating large diameter piles.
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