Last edited 17 Oct 2016

Driven piles

Contents

[edit] Introduction

Driven piles, also known as displacement piles, are a commonly-used form of building foundation that provide support for structures, transfering their load to layers of soil or rock that have sufficient bearing capacity and suitable settlement characteristics.

Driven piles are commonly used to support buildings, tanks, towers, walls and bridges, and can be the most cost-effective deep foundation solution. They can also be used in applications such as embankments, retaining walls, bulkheads, anchorage structures and cofferdams.

A foundation is described as piled when its depth is more than three times its breadth (Atkinson, 2007). A driven pile is a long, slender column made of preformed material and having a predetermined shape and size that can be installed by impact hammering, vibrating or pushing it into the ground to a design depth or resistance. If the soil is particularly dense, pre-drilling may be required to enable the pile to reach the design depth.

Driven piles are very adaptable and can be installed to accommodate compression, tension or lateral loads, with specifications set according to the needs of the structure, budget and soil conditions.

[edit] Types of driven pile

Types of driven pile include:

[edit] Steel

Standard steel sheet pile sections can be used to form box section piles, or H-section piles. These are percussion driven and used mainly in connection with marine structures. These have a load range of 300-1,700 kN and can reach up to 36 m length.

Steel screw piles have a cast iron helix, are rotary driven, and are used for support at shallow depths in soft silts and sands. They have a load range of 400-3,000 kN and can reach up to 24 m length. For more information, see Screw pile foundations.

Steel tube piles are used on marine structures and foundations in soft subsoils over a suitable bearing strata. They are usually bottom-driven with an internal drop hammer.

[edit] Pre-cast concrete

These can be square, octagonal, cylindrical or sheet piles. They are percussion driven piles which are used where bored piles would not be suitable owing to running water or very loose soils. They have a load range of 300-1,200 kN and can reach up to 30 m.

[edit] Timber

These are usually square sawn (but can also be circular, tapered, treated, untreated), and percussion driven. They can be used for small contracts on sites with shallow alluvial deposits overlying a suitable bearing strata (e.g. river banks and estuaries).

The load range of timber piles is 50-350 kN. They can be up to 12 m in length without splicing.

[edit] Composite

These are piles that use a combination, such as a concrete pile with a steel tip extension.

[edit] Quality

Driven piles are built to precise tolerances using high-strength materials and require good quality control. Consistency of quality is achieved by conforming to BS 8004:2015 as well as EC standards, and inspection prior to installation to verify integrity.

It is important that driven piles maintain their shape during installation, and are not damaged by the installation of subsequent piles.

Static or dynamic pile testing can be used to verify pile capacity, that is, the maximum load that a pile can carry without failure or excessive settlement of the ground. Pile capacity depends on three primary factors:

  • The type of soil through which the pile is driven.
  • The method of pile installation.
  • The pile dimensions (cross section and length).

The shaft soil strength usually increases with time post-installation to provide additional load capacity. When incorporated into foundation design, this so-called ‘setup’ can enable the installation of fewer and shorter piles which results in less time, labour and materials being employed.

[edit] Pile installation

A pile hammer is used to drive piles into the ground, which compacts the soil around the side and leads to densification of the mass and increases its bearing capacity. However, with saturated, silty or cohesive, as opposed to granular, soil, poor drainage quality does not allow for the same densification. The water in the soil leads to a decrease in the overall bearing capacity and the pile design must allow for this.

The blow count is the number of times the pile must be struck in order to be driven down to the desired depth. Where there are variations in the subsurface conditions, pile lengths may have to be cut-off or spliced to extend their length.

As there are no special casings required and no delays related to concrete curing, driven piles are well suited to difficult site conditions. They can be used immediately when driven through water, can be installed to create temporary work platforms, and used in a large diameter form in earthquake-prone regions.

[edit] Advantages and disadvantages

The main advantages of using driven piles are:

  • Piles can be pre-fabricated off-site which allows for efficient installation once on site.
  • Driven piles displace and compact the soil which increases the bearing capacity of the pile. Whereas, other deep foundations tend to require the removal of soil which can lead to subsidence and other structural problems.
  • They are cost-effective as a wide variety of materials and shapes can be easily fabricated to specified dimensions, which can result in the need for fewer piles on site.
  • They generally have superior structural strength to other forms of foundation. Their high lateral and bending resistance makes them ideal for challenging conditions such as wind, water, seismic loading and so on.
  • Installation usually produces little spoil for removal and disposal.

The main disadvantages of using driven piles are:

  • Advance planning is required for handling and driving, as well as the heavy equipment on site.
  • To be able to withstand handling stresses during transportation and installation, precast or pre-stressed concrete piles must be adequately reinforced.
  • It may not be possible to determine the exact length required and so splicing or cut-off techniques may be required which has time and cost implications.
  • Driven piles may not suitable be where ground has poor drainage qualities.
  • Driven piles may not suitable for compact sites, where the foundations of structures in close proximity may be affected by the vibrations caused by installation.
  • Driven piles can be noisy to install and vibrations can result in complaints from neighbours, who may become aware of pre-existing problems with their own building that they then blame on piling vibration.

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