PILED FOUNDATION WHAT IS A PILED FOUNDATION? A pile can be loosely defined as a column inserted in the ground to transmit the structural loads to a lower level of subsoil. WHERE ARE PILED FOUNDATIONS USED? Piled foundations are used when there is unsuitability of the upper regions of the subsoil caused by: Ÿ Low bearing capacity of the subsoil. Ÿ Heavy point loads of the structure exceeding the soil bearing capacity . Ÿ High water table giving rise to high permanent dewatering costs. Ÿ Presence of layers of highly compressible subsoils such as peat and recently placed filling materials which have not sufficiently consolidated. Ÿ Subsoils which may be subject to moisture movement or plastic failure. CLASSIFICATION These foundations can be broadly classified as: Ÿ End Bearing Ÿ Friction END BEARING PILES These piles transfer their load on to a firm stratum located at a considerable depth below the base of the structure and they derive most of their carrying capacity from the penetration resistance of the soil at the toe of the pile. The pile behaves as an ordinary column. Even in weak soil a pile will not fail by buckling and this effect need only be considered if part of the pile is unsupported, i.e. if it is in either air or water. Load is transmitted to the soil through friction or cohesion. But sometimes, the soil surrounding the pile may adhere to the surface of the pile and causes "Negative Skin Friction" on the pile. This, sometimes have considerable effect on the capacity of the pile. FRICTION PILES These piles also transfer their load to the ground through skin friction. The process of driving such piles does not compact the soil appreciably. These types of pile foundations are commonly known as floating pile foundations. DISPLACEMENT PILES These are often called driven piles since they are usually driven into the ground displacing the earth around the pile shaft. These piles can be either preformed or partially preformed if they are not cast in-situ and are available in a wide variety of ty pes and materials. The pile or forming tube is driven into the required position to a predetermined depth or to the required `set' which is a measur e of the sub soils r esistance to the penetration of the pile and hence its bearing capacity by noting the amount of penetration obtained by a fixed number of hammer blows. REPLACEMENT PILES These are often called bored piles since the removal of the spoil to form the hole for the pile is always carried out by a boring technique. They are used primarily in cohesive subsoils for the formation of friction piles and when forming pile foundations close to existing buildings where the allowable amount of noise and/or vibration is limited. DISPLACEMENT PILINGMETHODS Ÿ Pre cast concrete driven piles Ÿ Thick wall driven steel tubes Ÿ Thin wall bottom driven piles Ÿ Timber piles Ÿ Screw piles Ÿ Helical displacement piles Ÿ Vibro concrete columns (d rive cast in-situ) ADVANTAGES OF DISPLACEMENT PILING Ÿ Self testing as driven to refusal or "set” Ÿ No pile arising to dispose of Ÿ Little disturbance Ÿ Limited access Ÿ High production DISADVANTAGES OF DISPLACEMENT PILING Ÿ Cannot penetrate obstructions Ÿ Cannot always penetrate desiccated clay Ÿ Vibration and noise may be an issue Ÿ Replacement Piles Ÿ Displacement Piles REPLACEMENTPILINGMETHODS Ÿ Open hole auger piles Ÿ Continuous flight auger (CFA) Ÿ Large diameter rotary piles Ÿ Odex piles Ÿ Tripod piles ADVANTAGES OF REPLACEMENT PILING Ÿ Effectively vibration free Ÿ Installed into non cohesive and water bearing soils Ÿ High production Ÿ Restricted access DISADVANTAGES OF REPLACEMENT PILING Ÿ It produces excavated material which requires remov al off site.