The UK's Federation of Piling Specialists (FPS) has previously highlighted [see GDI May 2019] the work that the EFFC and the DFI, together with the support of the FPS, undertook to reduce the quality issues encountered in some bored piles and diaphragm walls cast using tremie methods, which led to the publication of the second edition of the EFFC/DFI Guide to Tremie Concrete for Deep Foundations.
Given the clear linkage between the concrete and the support fluid, the Task Group identified the need for greater review of the characteristics, testing and management of support fluids, which subsequently saw the publication of the first edition of The Guide to Support Fluids for Deep Foundations in March 2019.
The support fluids guide was ground-breaking in many ways. It was the first time that the collective knowledge pool of good practice from around the world was brought together into a single authoritative publication. A second edition is already planned, which will be based on an extensive field research study, funded by EFFC, DFI, contractors, consultants and suppliers. It is also hoped that the study will forge links with academics working on support fluids, which already includes numerical modelling, polymer fluids testing in the UK, trench stability linked with fluid penetration rate, influence of fluid type on shaft friction in Germany and the influence of support fluid on steel / concrete bond strength.
The research itself is divided into two parts. The first is obtaining contractor's standard fluid test data from sites in Europe and North America, and secondly, data obtained from the study, which is being led by two independent specialists. These specialists spend two to three days each on 20 different sites, proposed by contractors (10 sites in Europe and 10 in North America). The study, developed by members of the EFFC/DFI Support Fluid Task Group, involves a series of non-standard tests, new procedures and test equipment to improve support fluid management and develop a better comprehension of support fluid behaviour.
The study will run until mid-2021 and has the following primary objectives:
- Record properties of fresh, excavation and recycled/cleaned fluid
- Establish a property/depth profile for the excavation fluid
- Compare base hardness methods and tools
- Compare commonly used fluid samplers
- Investigate the accuracy of concrete depth indicators
- Record concrete levels during casting inside and outside the cage
- Obtain and test fresh and hardened samples from the Interface Layer
- Test fresh concrete properties
The project stakeholders are grateful to the contractors that have so far volunteered to give access to their projects, so that independent specialists can implement the study. This has been designed specifically to limit possible production time losses on site. Prior to site access, a detailed planning discussion is held with the site team and any necessary inductions are carried out. All information obtained during the site visits is treated as strictly confidential by the specialists and is only disclosed to members of the Task Group who have signed a detailed confidentiality/non-disclosure agreement. Contractors who have allowed testing on their site will be allowed to check and approve the draft second edition before publication.
Site testing involves performing standard tests on the fluid at different stages (fresh, during excavation, after cleaning and prior to concreting). The data collected will complement the contractor's data survey to evaluate actual industry practice for support fluid management. Alternative procedures to measure the fluid density will be evaluated, as the Task Group identified a limitation in the accuracy of the mud balance for measuring fluid density (especially polymer-based support fluid) and the target is to increase the measurement accuracy from 0.01g/ml to 0.005g/ml. Other non-standard tests aimed at measuring the fluid rheological parameters will be carried out such as apparent viscosity, gel strength and silt content in polymer support fluids.
Data acquired to-date is proving invaluable and the Task Group is optimistic that as more sites are visited the quality and volume of data will enable good practice and procedures to be fine-tuned in the second edition. Of course, more sites are required and the FPS would be interested in hearing from anyone who may like to contribute to the study and ultimately to the findings that will define the content of the second edition of EFFC/DFI Guide to Support Fluids for Deep Foundations. Contact can be made directly to the FPS.
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