Ken Watts of the BRE looks at the latest guidance and research on ground improvement.
Despite wide use of ground improvement techniques in the UK- there is no ritish Standard code of practice for ground treatment. The ICSpecification for ground treatment and accompanying Notes for guidance published in 1--7 do not address many of the fundamental issues concerning the various techniques. There have also been significant advances in the technology since then.
In the UK these advances have been driven by the geotechnical challenges posed by the widening range of poor ground conditions encountered by developers as well as the demands of a fiercely competitive ground improvement market.
Most commonly used methods originated in continental -urope and were adapted for UK conditions. Treatment of miscellaneous fills is commonplace and the traditional use of shallow foundations for low-rise structures- particularly for housinghas helped to shape the local ground treatment industry.
In the UK- vibro stone columnsvibro concrete columns- and dynamic compaction are common techniques. Other proprietary techniques such as soil mixing and grouting are not used as often but this is sure to change. Another method is temporary pre-loading to induce consolidation- which is often overlooked commercially.
To ensure appropriate solutions are chosen for particular problems- decisions must be well informed and often rely heavily on past experience as well as on theory. This means much of the expertise is kept within specialist contractors and larger or specialised geotechnical consultants.
In recent years a number of documents have been written to provide wider access to information about many of the techniques.
Specifying vibro stone columns The National House uilding Council (NHC) published its Standards Chapter 4. 6: Vibratorground improement techniques in 1--5. This lays down the technical requirements and performance standards for the use of vibro techniques acceptable under the NHC warranty scheme. It concentrates on the use of stone columns and applications for lowrise housing.
It was- until recently- the only relatively detailed document to cover a broad range of design issues- including site investigation- suitability of ground conditions and materials and workmanship. Consequently- it has been quoted widely when specifying vibro works outside the particular application for which it was intended.
It is important that those specifying vibro stone column ground treatment understand the nature of the technique- its likely benefits and limitations for the ground conditions being considered.
To provide this informationR- published guidance on specifying vibro stone columns in 2000 (GE July 2000). This was prepared under the D-TR's Partners in Innovation scheme- with guidance from a steering group which included senior representatives from the main UK specialist ground improvement contractorsplus wider industry consultation.
The guide provides a technically prescriptive specification for vibro stone columns- including the essential elements of design.
Additional notes for guidance and information present a rationale for the particular clauses. The aim of the document is to raise technical standards- provide a framework for fair competition between specialist contractors and to deliver value for money for clients. The notes take into account new vibro technology and should lead to greater understanding by industry professionals- resulting in more informed and better design.
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 practicereports and other literatureincluding those dealing with contractual issues.
Specifying dynamic compaction As with vibro- it is important that those specifying dynamic compaction understand the nature of the technique and its likely benefits for the ground conditions being considered.
An authoritative technical specification for ground treatment using dynamic compaction is being prepared by R- for use by specialist contractors- consulting engineers and other building professionals concerned with the design- procurement and supervision of dynamic compaction. It will provide a technically prescriptive specification for the process- including design issueswhich is based on accepted best practice.
The specification has been structured to encourage the clear defining of a rationale for treatment (the geotechnical principle of improvement) and the technical means (method of compaction) by which this improvement will be achieved. It is also being prepared under the DTI's (formerly the D-TR's) Partners in Innovation scheme- with a steering group representing the main UK specialist ground improvement contractors.
There will be wider industry consultation during the drafting process and R- would like to hear from persons or organisations interested in participating.
The new specification is due to be published by June 2003.
Specifying engineered fills for building purposes It has long been recognised that filled ground may present hazards for buildings constructed on itbecause of settlement or heave.
-ven when fill has been engineered to form a foundationmovements have sometimes been unacceptably large and have damaged structures. This may have been due to an inappropriate specification or to inadequate control of fill placement.
Specifications used for engineered fill are usually those developed for highway works and are not necessarily appropriate for buildings. In 1--6- Trenter and Charles published a paper on the specification and control of fills for safely supporting low-rise structures without damaging movements. This includes a model specification.
Building on fill Scarcity and cost of good building land has led to increasing development of sites where there are deep fill deposits. For examplethe regeneration of major coalfields means many sites will involve building on colliery spoil.
Fills have considerable economic significance for land values. R- research has concentrated on monitoring field performance at a large number of filled sites. The emphasis has been on long-term observations of settlement- characterising fills on the basis of observed field performance- to help in the selection of appropriate foundation solutions and assessing the effectiveness of ground treatment techniques.
Bui-ding on fi--: geotechnicaaspects was published in 1--3. It provided a detailed account of R- research findings and their significance for appropriate and successful building developments on fill.
Since then the term 'brownfield' has come into everyday use.
Although brownfield land is a worldwide phenomenon- the issues are particularly acute for Great ritain- a heavily populated island with a long industrial history.
A second edition of Bui-ding on fi--: geotechnica- aspects - updates and expands the first edition.
Three new chapters cover collapse compression on wettingproblems associated with a variable depth of fill- and engineered fill. Records of R- field monitoring have been brought up to date and some recent case studies added.
The report has been reorganised into four parts and five appendices have been added. It provides considerable geotechnical background information to aid the choice and effective design of ground improvement- in particular the non-proprietary technique of preloading with a surcharge of fill.
Recent and ongoing research A number of research programmes on ground improvement techniques have recently been completed or are under way by R- and collaborating partners.
Two major projects on the use and effectiveness of vibro stone columns in miscellaneous fill and soft soil deposits were completed in collaboration with auer Foundations (UK) in recent years.
A similar study carried out with SP- Amec and Pennine Ground -ngineering has provided an assessment of the effectiveness of the rapid impact compactor.
-uroSoilStab- a major -uropean project to study the use of stabilisation methods for organic weak soils- was recently been completed (see box).
R- is also carrying out a longterm assessment of the durability of stabilisation systems with sponsorship from the Highways Agency. It is anticipated that there will be an ongoing requirement for guidance and specification of other existing and new treatment technologies for difficult ground in the years ahead.
Ken Watts is principa- consu-tant at the Centre for Ground Engineering & Remediation, BRE, contact: wattsk@bre-co-uk
References Institution of Civil -ngineers (1--7).
Specification for ground treatment.
Thomas Telford- London.
Institution of Civil -ngineers (1--7).
Specification for ground treatment: Notes for guidance. Thomas Telford- London.
National House uilding Council (1--5).
Standards Chapter 4. 6: Vibratory ground improvement techniques. NHCAmersham.
R- Report R 3-1(2000). Specifying vibro stone columns.
NA Trenter and JA Charles (1--6). A model specification for engineered fill for building purposes. Proc Instn Civ -ngng.
Geotech -ngng 11-- Oct- pp 21--230.
R- Report R 230 (1--3). uilding on fill:
R- Report R 424 (2001). uilding on fill:
geotechnical aspects (second edition).
KS Watts- D Johnson- LA Woods and A Saadi (2002). An instrumented trial of vibro ground treatment supporting strip foundations in a variable fill. Geotechnique Symposium in Print - Ground and soil improvement. Vol 50- No 6- pp 6-7706.
KS Watts and CJ Serridge (2000). A trial of vibro bottom-feed stone column treatment in a soft clay soil. Proc 4th Int Conf on Ground Improvement Geosystems - grouting- soil improvement and geosystems including reinforcement.
Helsinki- June 7--- pp 54--556.
KS Watts and CJ Serridge- RC Chown and MS Crilly (2001). Vibro stone columns in soft clay soil: a trial to study the influence of column installation on foundation performance. Proc 15th Int Conf on Soil Mechanics and Ground -ngineeringIstanbul- August 2001- Vol 3- pp 1-67-1-70.
KS Watts and JA Charles (1--3). Initial assessment of a new rapid impact ground compactor. -ngineered fills. Proc of the Conference on -ngineered Fills 1--3.
Thomas Telford- London- 1--3- pp 3---412.