Pile foundation - Classification

HOW TO CLASSIFY PILES BASED ON CONSTRUCTION MATERIALS?

A pile is a slender, structural member installed in the ground to transfer the structural loads to soils at some significant depth below the base of the structure. Structural loads include axial loads, lateral loads, and moments. Another term commonly used in practice for pile foundations is deep foundations. Structures that cannot be supported economically on shallow foundations are normally supported by pile foundations.

On the basis of materials used for the construction of pile foundation, it can be classified in to following 5 types.

1. Concrete piles
2. Steel piles
3. Timber piles
4. Plastic piles
5. Composite piles

1. CONCRETE PILES

There are several types of concrete piles that are commonly used. These include cast-in-place concrete piles, precast concrete piles, drilled shafts, and barrette piles. Cast-in-place concrete piles are formed by driving a cylindrical steel shell into the ground to the desired depth and then filling the cavity of the shell with fluid concrete. They are called displacement piles. The steel shell is for construction convenience and does not contribute to the load transfer capacity of the pile. Its purpose is to open a hole in the ground and keep it open to facilitate the construction of the concrete pile. Plain concrete is used when the structural load is only compressive. If moments and lateral loads are to be transferred, then a steel reinforcement cage is used in the upper part of the pile.

Precast concrete piles usually have square or circular or octagonal cross sections and are fabricated in a construction yard or a factory from reinforced or prestressed concrete. They are preferred when the pile length is known in advance. The disadvantages of precast piles are problems in transporting long piles, cutting, and lengthening. A very popular type of precast concrete pile is the Raymond cylindrical prestressed pile. This pile comes in sections, and lengths up to 70 m can be obtained by stacking the sections.

Typical design loads are greater than 2 MN.

Micropiles (also called minipiles, pin piles, needle piles, or root piles) are small-diameter (50 mm to 340 mm) pipe piles (pushed or driven) or grouted (jet or post or pressure) piles.

They are particularly useful for

• Sites with low headroom,
• Congested areas,
• Sites with restricted access, and
• Foundation repair or strengthening.

2. STEEL PILES

Steel piles come in various shapes and sizes and include cylindrical, tapered, and H-piles. Steel H-piles are rolled steel sections. They are non-displacement piles. Steel pipe piles are seamless pipes that can be welded to yield lengths up to 70 m. They are usually driven with open ends into the soil. A conical tip is used where the piles have to penetrate boulders and rocks. To increase the load capacity of steel pipe piles, the soil plug is excavated and replaced by concrete. These piles are called concrete-filled steel piles. The soil plug may adhere to the pile surface and moves down with it during driving. This is called plugging.

3. TIMBER PILES

Timber piles have been used since ancient times. The lengths of timber piles depend on the types of trees used to harvest the piles, but common lengths are about 12 m. Longer lengths can be obtained by splicing several piles. Timber piles are susceptible to termites, marine organisms, and rot within zones exposed to seasonal changes. Timber piles are displacement piles.

4. PLASTIC PILES

Plastic piles comprise a variety of composite materials that include polymer composites, PVC, and recycled materials. These piles are used in special applications such as in marine environments and within soil zones exposed to seasonal changes.

5. COMPOSITES

Concrete, steel, and timber can be combined to form a composite pile. For example, the portion of a timber pile above groundwater level that is likely to suffer from decay due to termites or rot may be replaced by concrete. Similarly, the portion of a steel pile within a corrosive environment can be covered with concrete or other protective materials.