News & Views

Support Fluids for Deep Foundations – Guiding Principles

Quality issues in bored piles and diaphragm walls cast using tremie methods, have been well documented, and prompted the EFFC and DFI to undertake a review of the issues back in 2014. The Task Group subsequently established, and which included representation from the Federation of Piling Specialists (FPS), led to the publication of the 1st Edition of the “EFFC/DFI Guide to Tremie Concrete for Deep Foundations”. Whilst the 1st Edition of the ‘Tremie Guide’ did include recommendations on support fluid properties, it was clear at the time [2014] that the characteristics and testing of support fluids required far greater discussion and review.

With this brief, a similar review was undertaken of the effect of support fluids on the performance of deep foundation elements, as they have the potential to impact significantly on both the geotechnical and structural performance. How this impact is considered at the design stage was considered, along with how potential issues are either allowed for and/or mitigated. Of course, it is essential that high quality and rigorous construction methodology, specific to the support fluid being used, is developed and adhered to, as this will optimise the primary support function of the fluid and minimise the effect of the support fluid on the performance of the foundation element in both the temporary and permanent condition, and it was against this backdrop that the intention to publish a definitive guide to support fluids was proposed.

Two-years on and the first edition of guide is ready entitled – “The 1st Edition of the Guide to Support Fluids for Deep Foundations”. A major step forward in establishing a definitive protocol for their use, the guide represents the state-of-the-art of support fluid practice, which if followed and adhered to correctly aims to improve existing design, testing and practices for deep foundation elements – bored piles (drilled shafts), barrettes (LBEs) and diaphragm wall panels. It also represents the first time that knowledge of good practice from around the world has been brought together into a single authoritative publication.

The guide follows the same model as the Tremie publication, with this First Edition detailing accepted industry good practice. It is the result of the dedication and exhaustive work of a dedicated Task Group spanning contractors, suppliers, consultants and leading academics from around the world including FPS Members, in particular Andrew Bell, Stephan Jefferies, Duncan Nicholson, Andrew Tear and Chris Harnan, and numerous worldwide Sponsors. The FPS’s willingness to diligently prepare comments to drafts was welcomed by the Task Group. Together, this group of industry experts have worked for the past 24-months to deliver the guide to the geotechnical industry.

Specifically, it takes a general look at the reasons and purpose for the guide, before examining in detail the many design considerations, such as site investigation and support fluid functions. The rheological properties of support fluids, sampling and testing are also discussed as well as raw materials (bentonite, polymer, blended fluid), environmental considerations, choice of fluid type and cost evaluation, make-up water and water without addition of bentonite or polymer. Equally as important is the section on ‘Execution’, which discusses mixing and storage, pumping, excavation and cleaning, reactions with the ground, treatment and recycling, concreting, spillage and disposal together with safety considerations. There is also a section on quality control, existing acceptance values and detailed appendices and references.

Whilst totally stand-alone in content the project is ongoing and it will be followed by a second edition, based on a comprehensive Research & Development programme. The next phase of the task is to carry out a series of Field Research Studies, which will collect data from active project sites and develop and conduct non-standard tests in order to establish what testing methods have substantial value and to better understand the validity of the compliance values in use. Specifically, the studies will be split into three parts; obtain contractor’s standard test data sets from about 40 sites; develop non-standard test methods and obtain data sets from about 20 sites by independent specialist visits; and work with academic partners carrying out related studies.

It is anticipated that standard data sets will be compiled by the contractors from sites in North America and Europe, including all types of support fluid currently used in practice (bentonite, polymer, and blended fluids). This information will be interpreted by the Support Fluid Task Group to develop a better understanding of the current status of testing on site and the range of compliance values being used on sites. Independent Specialists appointed by the Support Fluid Task Group will work with the Specialist Contractor to review the Contractor’s standard test data sets and to carry out additional non-standard tests and sampling. This non-standard testing will be linked to university research work. It is anticipated that the university researchers will accompany the specialists on some of their site visits and help perform the additional testing. It is hoped that all testing be completed by early 2021.

With the Field Research Studies completed, the Task Group will then produce the Second Edition of the Guide that will include all the additional information and research and development and should be the best authority on the use of support fluids in the industry. The second edition of the guide is expected to be published in 2021/2022.

The 1st Edition of the guide is available for free download at www.dfi.org or www.effc.org