Classification of piles with respect to load transmission and functional behavior.
- End bearing piles (point bearing piles)
- Friction piles (cohesion piles)
- Combination of friction and cohesion piles
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 and should be designed as such. 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. Negative skin friction is caused by the drainage of the ground water and consolidation of the soil.
In these types of piles, the load on pile is resisted mainly by skin/friction resistance along the side of the pile (pile shaft). Pure friction piles tend to be quite long, since the load-carrying. Capacity is a function of the shaft area in contact with the soil. In cohesion less soils, such as sands of medium to low density, friction piles are often used to increase the density and thus the shear strength. When no layer of rock or rocklike material is present at a reasonable depth at a site, point/end bearing piles become very long and uneconomical. For this type of subsoil condition, piles at driven through the softer material to specified depth.
Timber piles are made of-tree trunks driven with small end as a point
Maximums length: 35m; optimum length: 9 – 20m
Max load for usual conditions: 450kN; optimum load range = 80 – 240kN
Disadvantages of using timber piles:
Difficult to splice, vulnerable to damage in hard driving, vulnerable to decay unless treated with preservatives (If timber is below permanent Water table it will apparently last forever), if subjected to alternate wetting & drying, the useful life will be short, partly embedded piles or piles above Water table are susceptible to damage from wood borers and other insects unless treated.
Comparatively low initial cost, permanently submerged piles are resistant to decay, easy to handle, best suited for friction piles in granular material.
Maximum length practically unlimited, optimum length: 12-50m.
Load for usual conditions = maximum allowable stress x cross-sectional area.
The members are usually rolled HP shapes/pipe piles. Wide flange beams & I beams proportioned to withstand the hard driving stress to which the pile may be subjected. In HP pile the flange thickness = web thickness, piles are either welded or seamless steel pipes, which may be driven either open ended or closed end. Closed end piles are usually filled with concrete after driving.
Open end piles may be filled but this is not often necessary.
Advantages of steel piles:
Easy to splice, high capacity, small displacement, able to penetrate through light obstructions, best suited for end bearing on rock, reduce allowable capacity for corrosive locations or provide corrosion protection.
Vulnerable to corrosion.
HP section may be damaged/deflected by major obstruction.
Concrete piles may be precast, prestressed, cast in place, or of composite construction.
Precast concrete piles may be made using ordinary reinforcement or they may be prestressed.
Precast piles using ordinary reinforcement are designed to resist bending stresses during picking up & transport to the site & bending moments from lateral loads and to provide sufficient resistance to vertical loads and any tension forces developed during driving.
Prestressed piles are formed by tensioning high strength steel prestress cables, and casting the concrete about the cable. When the concrete hardens, the prestress cables are cut, with the tension force in the cables now producing compressive stress in the concrete pile. It is common to higher-strength concrete (35 to 55 MPa) in prestressed piles because of the large initial compressive stresses from prestressing. Prestressing the piles, tend to counteract any tension stresses during either handling or driving.
Max length: 10 – 15m for precast, 20 – 30m for prestressed.
Optimum length 10 – 12m for precast. 18 – 25m prestressed.
Loads for usual conditions 900 for precast. 8500kN for prestressed.
Optimum load range: 350 – 3500kN.
High load capacities, corrosion resistance can be attained, hard driving possible.
Cylinder piles in particular are suited for bending resistance.
Cast in place concrete piles are formed by drilling a hole in the ground & filling it with concrete. The hole may be drilled or formed by driving a shell or casing into the ground.
Concrete piles are considered permanent, however certain soils (usually organic) contain materials that may form acids that can damage the concrete.
Salt water may also adversely react with the concrete unless special precautions are taken when the mix proportions are designed. Additionally, concrete piles used for marine structures may undergo abrasion from wave action and floating debris in the water.
Difficult to handle unless prestressed, high initial cost, considerable displacement, prestressed piles are difficult to splice.
Alternate freezing thawing can cause concrete damage in any exposed situation.