Types of Piles Based on Load Transfer, Installation Methods and Materials
Types of Piles
Piles can by classified on the basis of following characteristics, following are the types of piles:
- Mechanism of Load Transfer (Also See: Load Capacity of Piles)
- Method of Installation
- Type of Materials
Classification of Piles on the basis of load transfer
Types of piles based on the mechanism of Load Transfer:
End/Point Bearing Piles:
If a bedrock or rocklike material is present at a site within a reasonable depth, piles can be extended to the rock surface. In this case, the ultimate bearing capacity of the pile depends entirely on the underlying material; thus the piles are called end or point bearing piles. In most of these cases the necessary length of the pile can be fairly well established.
Instead of bedrock, if a fairly compact and hard stratum of soil is encountered at a reasonable depth, piles can be extended a few meters into the hard stratum.
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 ate driven through the softer material to specified depth.
Friction cum end bearing piles
In the majority of cases, however, the load-carrying capacity is dependent on both end-bearing and shaft friction.
Classification of Piles according to the Method of Installation of piles
Driven or displacement piles
They are usually pre-formed before being driven, jacked, screwed or hammered into ground. This category consists of driven piles of steel or precast concrete and piles formed by driving tubes or shells which are fitted with a driving shoe. The tubes or shells which are filled with concrete after driving. Also included in this category are piles formed by placing concrete as the driven piles are withdrawn.
(Also See: Types of Soil Settlement )
Bored or Replacement piles
These types of piles require a hole to be first bored into which the pile is then formed usually of reinforced concrete. The shaft (bore) may be eased or uncased depending upon type of soil.
Types of Piles based on Materials
- Timber piles are made of-tree trunks driven with small end as a point
- Maximum length: 35 m; optimum length: 9 - 20m
- Max load for usual conditions: 450 kN; optimum load range = 80 - 240 kN
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., dm
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.
Disadvantages of steel piles:
- 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 - 15 m for precast, 20 - 30 m for prestressed.
- Optimum length 10 - 12 m for precast. 18 - 25m prestressed.
- Loads for usual conditions 900 for precast. 8500 kN for prestressed.
- Optimum load range: 350 - 3500 kN.
Advantages of concrete piles:
- 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.
Disadvantages of concrete piles:
- 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.
In general, a composite pile is made up of two or more sections of different materials or different pile types. The upper portion could be eased cast-in-place concrete combined with a lower portion of timber, steel H or concrete filled steel pipe pile. These piles have limited application and arc employed under special conditions.