Engineers Australia

E.H. Davis Lecture 2007: Foundations (piles) in Rock – Serviceability Rules

Dr Chris Haberfield

Regardless of recent advances in testing, analysis, construction methods and materials, the geotechnical design of foundations in rock for many applications is still dominated by empirical correlations, rules of thumb and traditional values. These methods were developed at a time when our knowledge was less, analysis methods were relatively simplistic, our construction equipment was of significantly lower capacity, methods for quality control during construction were less advanced, specialist testing methods were not available, prohibitively expensive or not considered and structures and loads were considerably smaller.

The application of these “tried and true” methods to current day projects can result in foundations in rock being grossly over-designed with respect to geotechnical performance. While an overly conservative design approach may have no significant impact on construction program or cost for some projects, there are many projects where such approaches can add significant time and cost and in some instances may make the foundation system impractical to install. Of equal importance is that the geotechnical design of the foundation may be based on assumptions that are inappropriate, leading to misplaced confidence and potentially greater risk. One example of this is that the installation process for a foundation in rock is often not considered at the design stage and yet it is one of the main factors affecting the performance of the footing.

There appears to be a widespread reluctance by geotechnical practitioners to move away from the traditional approaches even when provided with prudent alternative design solutions which are logical and defensible and in many cases offer a reduced risk at a reduced cost.

This lecture will challenge the conventional approaches to foundation design in rock. It will provide some insights into the geotechnical issues and performance of foundations in rock and will seek to provide a design approach which considers the whole process from ground investigation through design and construction to in-service performance.

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