Foundations provide support for structures by transferring loads to soil or rock that have sufficient bearing capacity. Speaking broadly, foundations can be categorised as either shallow or deep. Deep foundations are necessary where the bearing capacity of the surface soils is insufficient to support loads so the loads are transferred to deeper layers. Pile foundations are deep foundations, and are formed by long, slender, columnar elements, typically made from steel or reinforced concrete. A foundation is described as ‘piled’ when its depth is more than three times its breadth.
Piles can be used individually to support loads or grouped with a reinforced cap. The pile cap should overhang the outer piles, typically by a distance of 100-150mm on all sides. Pile caps can be linked together using a reinforced concrete ground beam. Capping beams are suitable for distributing the weight of the load-bearing wall. The capping beam should be kept clear of the ground where the purpose of the piles is to overcome the problem of the subsoil swell and shrinkage. This can be done by casting the capping beam on polystyrene or other compressive material, thereby allowing upward ground movement without damage to the beam.
Pile integrity testing
The majority of cast-in-situ piles fail because of defective pile shaft necking, intrusion of foreign matters, improper toe formation due to contamination of concrete base with soil particles, leaching of concrete, discontinuity of concrete, improper construction method or poor quality control. The pile integrity test is conducted before completion of pile caps, and ensures the proper functioning of pile foundations without failure. You can use sonic echo testing to test pile integrity after installation.
The integrity test enables a number of piles to be tested in a day. The information gathered is about the continuity, crack defects, necking, soil incursions, changes in cross section and approximate pile lengths. To carry out the test, a small but hard 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. The primary shock wave is reflected from the toe by change in density between the concrete and the sub strata. However if the pile has any defects within its length these will set up secondary reflections which will be added to the return signal. Normally more than one recording of signals is done until repeatability of signals is achieved.