Introduction to Piling

Introduction

The principle behind piling is to transfer the loads from a structure through strata of low bearing capacity to deeper soil strata having higher bearing capacity, or directly on a rock. When piling is employed for foundation, the loads from the structure are either transferred to the underlying soil either by end-bearing or friction or a mixture of both depending on the soil conditions and the magnitude of the loads been envisaged from the super-structure.

Principles of Piling

As described in the preceding section, the principle behind piling is to be able to transmit the loads from a structure through strata of weak soil to either a solid and/or to rely on friction between long and slender piles as illustrated in figure 1.

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 while friction piles transmit most of their load to the soil through skin friction between the pile itself and the soil stata.

There are several types of pile that all follow the same principle. They can be made from a variety of materials including timber, steel and reinforced concrete; the last of which being the most common.

Types of Piling

Piling methods can be placed into two categories: displacement and replacement.

Displacement piles shift soil away from the pile itself and their installation typically generates significant noise and vibration. The exceptions to this are displacement augers and bottom driven steel tubes.

Replacement piles extract the soil to form a shaft, which is then filled with a material that creates the pile. This method of piling does not generate as much noise or vibration. More specifically, piling methods can appear in the following forms:

Driven Piles: Driven piles are considered to be displacement piles. A pre-assembled pile is driven into the soil. The pile is made up of sections and is linked together via couplings.

Bored Piles: Bored piles (Replacement piles) are generally considered to be non-displacement piles a void is formed by boring or excavation before piles is produced by pouring in concrete into the void.

Jack Piles: Piles that are pressed into the soil via a jack.

Selecting a Pile Type

The selection of a pile type will depend on certain factors, these are;

Load envisaged from the super-structure

Soil conditions (both geotechnical and environmental)
Site access
Construction Programme

Site Constraints (e.g. availability of space)

For example, a 5 storey building in a residential area that is to be built on a clay soil with layers of silt in it will likely have concrete piles placed via a bored process. This reduces the risk of piles failing as the soil collapses into the dug shaft, as a result of the concrete being installed while the pile shaft is excavated. This method does not generate much in the way of noise or vibration either, which is important for a site located within a residential area

Design Responsibility

The pile design responsibility can fall into either the designer of the structure’s remit or to a specialist contractor. In the case of the former, all of the design information is fed directly to the specialist contractor who does not take on any of the design responsibility of the piles.

The other scenario places the responsibility for designing the piles on the specialist contractor, and the amount of information provided by the designer will vary in terms of comprehensiveness.

In some instances, a pile layout along with loads to each pile is defined and a limit on settlement provided. At other times it is only the loads from the superstructure that are explained and it is up to the specialist contractor to develop the layout of the piles and their loading. In both cases, the loads specified must be both un-factored and permanent in nature.

This latter approach places a great deal of design responsibility onto the specialist contractor, but it does afford them the opportunity to exercise skills and expertise that the designer of the super-structure is unlikely to have. This can in theory lead to more economical designs over those of a designer, with less of a focus on substructure design, which would typically be the responsibility of the engineer overseeing the super-structure.

Pile Testing

Clause 7.5 in BS EN 1997-1 defines which pile load tests need to be carried out on all forms of piling. Specifically, pile tests need to be carried out when the following conditions arise:

Where the piles have not been tested in similar soil and loading conditions
When piles are subjected to loading, that theory and experience do not give confidence in the pile design. Load testing will simulate the design loading condition

During installation the behavior of the piles is different from what was expected, i.e. from the assumptions made from the soil property data gleaned from the earlier site investigations
A method of pile installation and/or type of pile is being used where there is no experience of it being employed

In addition to mandatory testing, specific tests may form part of the pile design and installation process. Testing can inform the suitability of the chosen pile type, how the soil is interacting with the pile and the overall design of the sub-structure.

The extent of testing does have an impact on the design factor of safety of the piles. If a significant amount of testing is to be carried out during construction, then the factor of safety can be reduced, while the opposite is also true.

There are two forms of testing: Load and Integrity.

Load Tests

With respect to load testing; piles that are primarily subjected to axial compressive loads can be tested via two methods. One is the constant rate penetration test (CRP). This sees a force placed upon the pile that is slowly increased until the pile fails as it penetrates the soil. The other method is the maintained load (ML) test. This has a pile loaded to up to twice the design load and a time vs. settlement chart is plotted every time the load is applied to the test pile and removed.

CRP testing is destructive and is therefore reserved for piles that are installed for that purpose only. ML testing on the other hand can be carried out on piles that will ultimately form part of the foundations, although it is possible to test piles to destruction in this manner.

Integrity Tests

Integrity tests are typically not destructive and set out to determine the overall quality of the pile along its length. Six of the most commonly used testing methods are summarised here:

Acoustic test – A tube (or sometimes a pair of tubes) is cast into the pile and the integrity checked using a sonde, which has an acoustic transmitter and receiver at both ends. This is dropped into the hole(s), which is ﬁ lled with a liquid (water or possibly something more viscous) in order to attain the best signal from the sonde. The sonde is dropped to the base of the pile and the time it takes for the sounds being emitted from it and received back are recorded. Short time periods mean the pile is sound, long periods indicate cracks and voids are present within the pile.

Stress wave test – This testing method is based on a dynamic pulse that is emitted through the pile via a repeatedly dropped mass applied to the top of pile. The mass must be large enough to induce a deflection in the pile in order for the test to be valid. The application of the load creates a stress wave within the pile that is measured. This then determines the integrity of the pile. It has been mooted by some that this test can be used instead of a load test, but its results can be inaccurate.

Electrical test – These tests measure the electric potential within the pile and the soil it is inserted into. The results from these measurements are then used to determine the integrity of the pile.

Dynamic response test – This testing method subjects the pile to a continuous stream of vibrations across a broad set of bandwidths. The method by which the integrity of the pile is determined is somewhat similar to seismic test methods, although the magnitude of the force being applied to the pile is much less.

Seismic test – This is based on measuring the eff ect of a hammer blow at the top of the pile. An accelerometer is installed at the top of the pile to measure the Rayleigh waves that travel through the pile. The time it takes for the waves to travel through the pile is measured, from which the soundness of the pile can be determined.

Radiometric test – This test detects flaws in the pile by measuring the radiation within it. Like acoustic tests, a tube is cast into the pile and a sensor is placed within the tube. Rather than sound, radiation is measured which is used to determine the integrity of the pile.