Testing pile foundations

[edit] Introduction

Pile foundations are deep foundations. They are formed by long, slender, columnar elements typically made from steel or reinforced concrete, or sometimes timber. A foundation is described as 'piled' when its depth is more than three times its breadth.

The bearing capacity of a pile is determined by several factors, including the size, shape and type of pile, as well as the particular soil properties. The calculation method that is used as a means of testing pile foundations depends upon the magnitude of the work involved, the soil type, and the engineer’s specification. Methods of calculation include:

• Dynamic pile formulae.
• Static formula.
• Test loading.

The integrity of, and presence of defects in, new and existing piles can be assessed by carrying out a pile integrity test.

[edit] Dynamic formulae

In mainly non-cohesive soils, dynamic formulae can be used to calculate the approximate bearing capacity of piles. These are based on assumptions, including:

• The resistance to being driven into the soil is determined from the energy delivered by the hammer together with the pile movement when struck by the hammer.
• Resistance to being driven into the soil is equal to the ultimate bearing capacity for static loads.

The basis of the formulae is that the ability to overcome the ground resistance to penetration is equated with the energy delivered by the hammer on impact. A factor of safety must be applied when the ultimate bearing capacity has been achieved, before calculating the safe working load. This can vary according to the pile’s permitted settlement rate at working load, which is determined by the pile size and the soil compressibility.

[edit] Static formulae

For non-cohesive soils, the static formulae that can be used to test piles include the standard penetration test and the cone penetration test.

The standard penetration test involves measuring the soil’s resistance to penetration under static or dynamic loading. For more information, see Standard penetration test.

The cone penetration test involves a cone enclosed in a tube. This is pushed into the soil, and measurements are taken of the forces that are required to independently advance the cone and tube. The resistance of the cone to being driven into the soil is taken as being equal to the ultimate bearing capacity.

Laboratory tests are more suitable for estimating the shear strength values of cohesive soils.

[edit] Test loading

It is advisable to test load at least one pile per scheme by forming a trial pile that is in close proximity but does not form part of the actual foundations. The pile should be overloaded by at least 50% of its working load and held for 24 hours. This provides a check on the ultimate bearing capacity of the pile as well as the workmanship involved in forming the pile.

[edit] Related articles on Designing Buildings Wiki

• Bearing capacity.
• Bored piles.
• Driven piles.
• End-bearing piles.
• Geophysical survey.
• Ground conditions.
• How deep should foundations be?
• Insitu testing of soils.
• Pile foundations.
• Pile integrity test.
• Piling equipment.
• Soil survey.
• Structural engineer.
• Tension piles.
• Testing construction materials.