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Why are we still designing using standard penetration tests: Chris Milne

Last year excellent guidance explaining the need for high quality in situ testing data was provided in the form of the revised BS5930:2015 Code of Practice for Site Investigations. Despite all this, honestly, how many designs are still based around counting the number of times it takes to bash a metal rod into the ground?

Talking point chris milne

Chris Milne is a director of Ground Stiffness Surveys

Yes, of course simple site investigation techniques have their uses where there is already good understanding of ground conditions and where the alternatives are limited; but in the UK and for major projects?

We all understand the standard penetration test (SPT) cannot be a good measure of shear strength, let alone stiffness. BS5930 is clear that inferred soil parameters from SPT results “are at best approximate” and any correlations “should be made with great care”. Furthermore, the end of the SPT has been a regular theme in GE throughout my over 20 years as a geotechnical engineer. Nevertheless, here we are in 2016 and the steady beat of the hammer still accompanies most borehole investigations.

Inherently we are a conservative profession, in many ways for good reason – who would not want us to err on the side of caution when we are designing a building’s foundations, a railway earthwork, a retaining wall to a highway? But we need to accept that this tendency leads us to feel that what we have always done and what others continue to do is safe and reasonable, sometimes in the face of the evidence.

In my own field of stiffness measurement understanding has moved substantially over the last 30 years. Chris Clayton’s 2011 Rankine Lecture provides an excellent explanation of small strain stiffness, and the variety of effective means of measurement and strain softening approaches for design – none of which are embodied in the multiplication of SPT N values. There is good published data to support the use of advanced, and very cost effective techniques such as continuous surface wave (CSW) testing. As a result, the potential benefits of optimising designs using accurate stain softened stiffnesses are significant even for small schemes.

However, some blame has to attach to those of us promoting these “new” approaches. Academics may not be fully aware of the everyday issues and constraints facing busy practicing engineers. Similarly, comparing reliable and proven geophysical-based tests like CSW with sometimes difficult to interpret geophysical surveys puts off many engineers. Sometimes guidance on use of new technologies is lacking, poor or not well known, tempting engineers to stick with what they can easily reference, however poor its basis.

While the principle of good in situ testing is set out in the revised BS5930, we need better case studies, practical guidance and straightforward explanations by practicing engineers to convince our colleagues of the benefits for them on their projects. Work therefore needs to be done by all of us involved in promoting better investigation techniques to explain from a geotechnical engineering perspective the reliability, economy and practicality of developments in in situ testing if we are to consign SPT kit to the museum piece it should surely be in the 21st century.

Chris Milne is a director of Ground Stiffness Surveys

 

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