Compliance Testing and Instrumentation of Piles

This paper is intended to provide an overview of the issue of foundation pile instrumentation, and to help the practitioner understand some of the options that are available, and some of the issues which are pertinent to the choice, specification and interpretation of piling instrumentation.

The paper takes a wide view of what constitutes piling instrumentation. In some, this may be generic devices which are utilized in piling instrumentation – for instance to measure strain or displacement. In other cases, the paper looks at piling-specific technology or measurement systems which have been developed to collect particular information.

In general, piling instrumentation is used to ensure construction works compliance testing. It is used as part of a testing regime, or a quality assurance system in order to ensure that the completed pile, as a representation of the whole foundation, will perform satisfactorily for the life of the structure which is to be supported.

Although this is by no means a unique problem in geotechnical engineering, the fundamental problem that we face with ensuring compliance with foundations is that the entire system is buried. The quality and fitness of the foundation is dependent on a large number of issues:

• the type of foundation element;
• the quality of the construction process;
• the experience of the supervisory and construction personnel;
• the quality of the site investigation;
• the variability and sensitivity of the ground conditions discovered in that investigation;
• the designer’s awareness of and response to those conditions;

Our confidence in the foundation also depends on :

• the type of verification testing which is undertaken
• the frequency of that verification testing

The Australian Standard, AS2159-1995: Piling – design and installation, accounts for these issues in Section 4 – Geotechnical Design. In particular, Table 4.1 and Table 4.2 provide guidance to the designer on the selection of the geotechnical strength reduction factor. These tables are reproduced in Figure 2and Figure .3

The primary function of these tables is to draw the designer’s attention to the risk factors and issues which affect foundation construction. The use of a range of ɸ_g values instead of a single value is intended to highlight and encourage the benefits that accrue from good practice – in site investigation, in design, in construction and in verification. Ideally, the additional costs of god practice should be more than compensated by the lower foundation cost which results from the higher ɸ_g values which can be justified.

Pile loads – particularly in Australia – are becoming ever higher for a given section. Reinforced concrete piles which 20 years ago may have been good for a 40 tonne working load are now expected to carry working loads of 200 tonne or more in some instances. Part of this may be the result of improvements in material technology, but ti is more due to economic imperatives which have forced us to exploit piles to their limit in a need for design efficiency, and hence cost efficiency. Increased loads and stresses are typically associated with reduced redundancy and higher risk. tI is important that the increased risk is compensated by increased monitoring and verification.

As intimated above, at a basic level, there are two approaches to ensuring compliance of foundations. These will hopefully be applied in combination rather than isolation. The first is to ensure compliance of the completed foundation with the performance specification. The second is to ensure the compliance of the construction process with good practice. We will deal with these matters separately.