Majority of cast-in-situ pile fails because of defective pile shaft necking, intrusion of foreign matters and improper toe formation due to contamination of concrete at base with soil particles, discontinuity of concrete, leaching of concrete due to high water current, improperconstruction method, poor quality control of concreting etc. Cracks are developed during handling operation of precast concrete piles, this may also lead to failure of precast concrete piles. Thus, to ensure the proper placement of piles, its strength and durability, pile integration test is conducted before completion of pile caps. This integration test of piles ensures proper functioning of pile foundations without failure.
Sonic Echo Testing (IS 14893:2001) has been used successfully for checking integrity of piles after installation.
Piles Integrity Testing is quick and simple method and it enables number of piles to be tested in a single working day. This method provides information about continuity, defects such as cracks, necking, soil incursions, changes in cross section and approximate pile lengths (unless the pile is very long or the skin friction is too high). Integrity tests provide an indication of soundness of concrete but they should be undertaken by persons experienced in the method and capable of interpreting the results with specific regard to piling.
In piles integrity test, a small metal / hard rubber hammer is used to produce a light tap on the top of the pile. The shock travels down the length of the pile and is reflected back from the toe of the pile and recorded through a suitable transducer / accelerometer (also held on the top of the pile close to the point of impact)in a computer disk for subsequent analysis.
The primary shock wave which travels down the length of the shaft is reflected from the toe by change in density between the concrete and the sub strata. However, if the pile has any defects or discontinuities within its length these will set up secondary reflections which will be added to the return signal.
By a careful analysis of the captured signal and a knowledge of the conditions of the ground, age of the concrete etc. a picture of the location of such problems can be built up.
Normally more than one recording of signals is done until repeatability of signals is achieved. In case of large diameter piles, the tests are conducted at 5-6 places to cover the entire section of the pile.
General Requirements of the piles integrity tests:
i) Pile shall be trimmed to cut off level or sound concrete level before the test with all laitance removed.
ii) The area surrounding the pile should be free from standing water and should be kept dewatered during the tests.
iii) The cast in situ piles should not normally be tested before 14 days of casting.
iv) The test piles, if available at site can be used for determination of pulse velocity and characteristic or reference signal generated. This can also be done from those piles whose depth is accurately recorded.
The following are the complimentary tests to the low strain integrity test method. These can be used to check the soundness of concrete at test level and to assess the stress wave velocity of pile concrete depending on concrete density and its in-situ condition.
a) Ultrasonic Pulse Velocity Test – This test is carried out at the head of the pile in order to arrive at the speed of sound propagation through the cast concrete of the pipe.
b) Penetration Test – The test is used to assess the strength of the concrete at the pile head to ensure that the pile is sufficiently cured and ready for NDT.
c) Density Testing Of Cast Cubes – In order to determine the density of concrete used each cube should be measured for density by weighing in air and water.
Limitations of Piles Integrity Tests:
i) The present experience of NDT of piles is upto diameter of 1500 mm.
ii) This is applicable to cast in situ concrete bored and driven piles. The test cannot be conducted after provision / casting of pile cap.
iii) This method is not suitable for piles surrounded by water as it may not give correct results.
iv) It does not provide information regarding verticality or displacement (in position) of the piles.
v) Local loss of cover, small intrusions or type of conditions at the base of piles are undetectable.
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