Foundation quality inspection

PURPOSE OF LOW-STRAIN INTEGRITY TEST (PIT)

Low-strain integrity test will be carried out for determination of integrity and defects (if any) of the pile.


METHOD OF LOW-STRAIN INTEGRITY TEST

Low strain dynamic method is based on the premise in one directional propagation of stress wave in elastic piece. The changes of the pile impedance and soil resistance will produce predictable wave reflection at the pile top.

When impacted at the pile top, a stress wave will transmit down the pile shaft at a wave speed, c, which is function of the material elastic modulus, E, and mass density, r, (C2 = E/r). The time needed which the reflection reaches the pile top is directly proportional to the distance from the any position creating reflected wave.

Since pile impedance (Z) is directly related to area and elastic modulus, therefore, it is a measure of pile cross section size and concrete quality.

When the incident stress wave (Wi) encounters impedance with change from Z1 = r1.A1.C to Z2 = r2.A2.C, one part of the wave will reflect up (Wu) and other part will transmit down (Wd) such that both continuity and equilibrium are satisfied:

                        Wd = Wi [2.Z2 / (Z2 + Z1)]

                        Wu = Wi [(Z2 – Z1) / (Z2 + Z1)]

At the free pile toe (Z2 = 0), the impact will be completely reflected, but with an opposite sign and for a uniform pile (Z1 = Z2) the impact wave will transmit at unchanged amplitude.

The stress wave will be reflected wherever the pile impedance changes and received at the pile top after a mount of time is proportional to the distance from the position of impedance change to the top. The reflected wave will cause changes in both force and velocity measured at the pile top. The magnitude of relative change of velocity at the pile top allows for determination of the impedance change.


APPLIED STANDARD

  • TCVN 9397:2012 “Piles – Method of detection of defects by dynamic low- strain testing”.
  • ASTM D5882-07 “Standard Test Method for Low Strain Impact Integrity Testing of Deep Foundations”.
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