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DEEP FOUNDATION TESTING – QA/QC Measures Post Installation

admin — Wed, 27 Jul 2022 02:46:58 +0000

PILE INTEGRITY TEST

Pile Integrity test (PIT) is a low strain, non-destructive integrity test method for foundation piles. This test can be performed on various types of piles including concrete piles, steel piles, and timber piles. The purpose of the test is to detect any damage or defects in the pile that may affect its load-bearing capacity.

In this method, a small hammer is used to strike the top of the pile, which creates a stress wave that travels down the pile. At the same time, an accelerometer is placed on the pile to record the wave that is reflected back up the pile. The reflected wave data is then analysed to identify any issues such as cracks, voids, or changes in cross-section in the deep foundation. The amplitude and shape of the wave can also provide information on the length of the pile and the type of material it is made of. For cast in place concrete piles or concrete filled pipe piles integrity test is carried no sooner than 7 days of cast of piles or after achieving a minimum of at least 75% of the design strength or whichever occurs earlier.

Pile Integrity Tester (PIT) Consists following equipment:

A computer-based PIT data acquisition system for display of analog signals during data acquisition, with atleast 12-bit resolution (16 bit or higher resolution is preferred) with a sampling frequency of at least 25 000 Hz each for the motion sensor and the optional instrumented hammer, if used.

Accelerometer – An accelerometer is a sensing device that measures the vibration, or acceleration of motion of a pile. Obtain velocity data from integration of signals from (one or more) accelerometers, provided the acceleration signal(s) can be integrated to velocity in the analog display for reducing data. The accelerometer(s) should be placed at (or near) the pile head and shall have their sensitive axis parallel with the pile axis. Accelerometers shall be linear to at least 50 g.

Impact Force Application – The impact may be delivered by a hand-held hammer that will produce an input force pulse of generally less than 1ms duration and should not cause any local pile damage due to the impact. A hammer with a very hard nylon tip can induce a short input force pulse without causing local pile damage. The impact should be applied axially to the pile (normally on the pile head).

The test method is as outlined in ASTM D 5882-16 (Low strain impact integrity testing for deep foundations).

The pile top is prepared to obtain clean and hard surface by chipping/grinding. The accelerometer is firmly fixed on pile top using wax and small impacts were given using a hand held hammer. Pile top motions as a function of time was measured using the collector model Pile Integrity Tester (PIT-X or PIT-Q collector), which is a precision instrument developed by PDI Inc. USA. The record of pile top motions (together with all the information provided viz., soil profile, pile geometry, etc.) were further analysed using PIT-W Software in time domain and the integrity evaluated although the method is approximate and not exact. This method will not give information on pile bearing capacity.

It is important to ensure that the pile top is above the water level and the pile is free from the loose material and other foreign materials. The pile head is to be grinded to smooth surface for fixing of the motion sensor and for striking. The accelerometer is to be attached to the prepared pile head and ensure for its firmness and at selected locations away from the edges of the pile head. In case of larger pile diameters (> 500mm) the accelerometer shall be fixed firmly at various positions that an integrity evaluation near the pile head may be made for each section of pile. The position of the handheld hammer is to be in such a way that the impact is applied axially to the pile and the spacing between the hammer impact & accelerometer shall not be greater than 300mm. It is set up the PIT instrument for displaying and recording of data. Multiple impacts, as desired by the testing engineer, are given to capture the consistent velocity signals. The data quality is verified by the operator for all the impacts & its consistency.

The interpretation for pile length depends on the wave speed used. Generally, wave speed for a good quality concrete ranges from 3500m/sec to 4300m/sec. For piles where multiple reflections occur near toe, interpreted length is decided based on the pile length input provided and the allowable wave speed limits. Interpretation for pile length cannot be performed if clear toe reflection is not identified. Early positive reflections are evaluated for a possible defect. Change in impedance due to reduction in cross-sectional area (necking), honeycombs, inclusions, voids etc. can be termed as defects. The following figure shows the difference between good and bad pile.

The limitations of the test are,

The accuracy of length determination depends on the assumed wave speed which can vary from 3500m/sec to 4300m/sec for a good quality concrete, hence a variation in length up to ± 10% is expected.

Significant change in pile cross-sectional area (neck or bulge) and soils stiffness might cause repetitive multiple positive reflections making the interpretation inconclusive.

The length up to which reflections could be clearly seen depends on L/D ratio and the soil resistance. Generally, clear toe reflections can be obtained up to an L/D ratio of 40.

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admin — Mon, 18 Jul 2022 02:56:37 +0000

“
As a flip-flop-wearing teenager in Hawaii, freelance writer Alexis Cheung would pore over fashion magazines, daydreaming about what she’d wear in a climate.
Arnold Joe

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admin — Wed, 13 Jul 2022 15:07:47 +0000

“
As a flip-flop-wearing teenager in Hawaii, freelance writer Alexis Cheung would pore over fashion magazines, daydreaming about what she’d wear in a climate.
Arnold Joe