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Getting to grips with vibro stone

The BRE has published a specification document for ground treatment using vibro stone columns. Ken Watts reports.

THE MOST commonly adopted form of ground treatment in the UK involves the introduction of vibro stone columns to reinforce and, in some circumstances, to improve the properties of loose fill materials and weak natural soil deposits. The aim is to create a composite soil/stone column system with enhanced bearing capacity and settlement characteristics.

Despite its widespread use, there is no British Standard for vibro ground treatment and the Specification for Ground Treatment, published by the ICE in 1987, does not address many of the fundamental issues concerning vibro techniques. However, it is important that those specifying vibro stone column ground treatment fully understand the nature of the technique, its likely benefits and limitations for the specific ground conditions being considered.

A new guide, Specifying vibro stone columns , has been published by BRE to provide this information. It was prepared under DETR's Partners in Innovation (PII) initiative, with guidance and assistance from a steering group, which included senior representatives from the main UK specialist ground improvement contractors.

The guide provides a technically prescriptive specification for vibro stone columns, including the essential elements of design, and will:

save considerable time at tender stage;

avoid misunderstandings between those specifying the works and specialist contractors, particularly in the use of standard terminology; and provide common best practice benchmarks for all parties to a vibro stone column contract.

To enhance the specification, the Notes for Guidance and Information section presents a rationale for the particular clauses and provide up-to-date supporting technical information and the latest thinking on all matters concerned with the provision of vibro works.

The aim of the document is to raise technical standards, provide a framework for fair competition between specialist contractors and to give clients value for money.

The notes take into account innovations in vibro technology and should lead to greater understanding by industry professionals, resulting in more informed and better design.

In addition, UK industry will be well placed to influence the development of international codes and standards based on a strengthened and coherent UK specification.

The document does not attempt to impose particular contractual terms and conditions for vibro ground treatment works. However, it does provide comprehensive referencing to relevant standards, codes of practice, reports and other literature, including those dealing with contractual issues.

The report covers the full range of activities normally associated with vibro works, from objectives to the specific treatment method and testing procedures to verify the degree of ground improvement achieved.

The role of site investigation and the provision of adequate soils information at tender stage is emphasised. This vital step is often inadequately addressed and the minimum acceptable requirements for investigations for the designer to provide a safe and economical foundation solution are explained.

From this assessment, the suitability of the ground conditions can be judged. The specification stipulates that the designer shall state the principle by which the ground will be improved, while the notes highlight a range of possibly unsuitable conditions where the designer should critically appraise the use of vibro methods.

The specification of an appropriate vibro technique for the particular ground conditions is crucial and again the notes provide comprehensive explanations of the different methods, the principles of column construction and their normal range of uses. The basic elements of design, including treatment layout and depth, are covered.

It is important to understand that successful application of vibro techniques relies on a clear understanding of the fundamental principles of the process, how the ground will be modified by column construction and the complex interaction between soil, column and foundations. The notes give the main elements for numerical analysis and discuss key issues such as the use of full or partial depth treatment.

Stone for the columns often accounts for a sizeable proportion of the overall cost of treatment and is vital to the long-term performance. Issues of stability, grading and storage are addressed. A section on execution of treatment covers a wide range of practical issues, including minimum acceptable levels of record keeping and the notes discuss the advantages and increasing use of automatic incab monitoring for process control and contract purposes.

The report is essential reading for specialist contractors, consulting engineers and any building professionals concerned with the specification, procurement and supervision of vibro treatment.

Reader offer: 20% discount for Ground Engineering readers. Complete the flyer in this issue or mention Ground Engineering when ordering to receive special price of just £32 including postage - saving £6.50 on the normal price.

Specifying vibro stone columns (ref. BR391) is available from CRC, tel. (020) 7505 6622; email: crc@construct. emap. co. uk Ken Watts is project manager at the BRE Centre for Ground Engineering & Remediation

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