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Vibro low-down


Henley Wheadon's Talking Point (GE December 2003) discusses engineers' tendency to rely upon the ICE specification for ground treatment and chapter 4.6 of the NHBC standards in the procurement process for vibro stone column treatment.

He rightly concludes that vibro replacement involves much more than filling holes with stone and requires a depth of understanding and range of design and application skills commensurate with any other foundation solution.

The ICE document provides what is effectively a concise pro-forma contract document. It is a specification of materials and workmanship and does not attempt to specify design criteria.

Consequently, it offers little guidance on the appropriateness of various techniques with respect to ground conditions of the suitability of the specific technology employed.

Chapter 4.6 of the NHBC standards, published in 1988, provided inspectors with the 'dos and don'ts' of vibro treatment to ensure basic design and installation requirements were being fulfilled at a time when the system was becoming very widely used for low-rise housing. The additional sample (annotated) guidance proved very useful and this document became widely quoted for many applications in addition to low-rise housing.

However, both documents have significant limitations. Understanding the fundamental principles of treatment and why techniques will work under certain ground conditions and not others, is vital to the design process. A suitable depth of knowledge about the ground conditions on a site is therefore also prerequisite to safe, cost-effective treatment. Methodology and workmanship are also crucial.

It is with these and other factors in mind that BRE published BR391 Specifying vibro stone columns in 2000. This document provides for a much tighter specification in which the objective(s) of the treatment must be stated and the level of site investigation required to meet the objective(s) set out. It further states: 'The design-shall quantify target performance and shall describe the manner in which treatment will improve the particular ground conditions'.

Thus the designer (who is often the specialist contractor) is committed to explaining the rationale for treatment as well as the choice of method proposed.

Important design issues such as layout and depth of treatment must be justified by calculation.

Where partial depth treatment is proposed, the design must demonstrate that the depth of treatment is adequate to fulfil the bearing capacity and differential settlement criteria of the structure. Critically, the risk of movement occurring below the depth of treatment must be assessed.

Further considerations covered are suitability of materials, execution of treatment, records and performance testing. BR391 also includes comprehensive notes for guidance which explain the rationale behind all the clauses of the specification along with supporting technical information.

More recently, the appropriate specification of dynamic compaction has been addressed in a similar way, in BR458 Specifying dynamic compaction. This document has also been structured to encourage the clear definition of a rationale for treatment (the geotechnical principle of improvement) and the technical means (method of compaction) by which this improvement will be achieved.

The aim of both specifications is to promote best-practice standards within a framework of fair competition for specialist contractors and to provide value for money for clients by reducing risk and whole-life costs. The guidance notes should provide a much clearer technical understanding of the processes involved.

Ken Watts, principal consultant, Centre for Structural and Geotechnical Engineering, BRE

BRE is running a one-day seminar, Ground treatment: getting the best from difficult sites in February (see Geodiary p9).

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