Effects of Bentonite and polymer support fluids on clay-concrete interface friction.

Support fluids are extensively used to support excavations and pile bores in unstable soils, such as soft clays. The most used support fluid is bentonite, however, alternative support fluids, such as polymer, have increasingly been used in practice due to several advantages, including less costly operation, smaller site footprint and a reduced environmental impact. Despite the extensive use of these support fluids, the effect of bentonite and polymer on the interface friction between soil and pile are not well known, particularly with regard to the adhesion parameter (α) to be used for design. In fact, the adhesion parameter (α) shows large variability with type of support fluid and soil conditions, with polymer-supported bores showing either higher or lower α than bentonite-supported bores. This study has been undertaken to further understand the complex interaction between support fluid, clay and pile shaft. The research work comprised of experimental tests to investigate the clay-concrete interface friction of clay samples exposed to different support fluids, case study analysis to back-calculate α based on pile load tests available in the literature and numerical modelling to simulate the percolation of the support fluid into the bore walls and its effect on α. The experimental results showed that clay samples exposed to polymer exhibited larger interface friction than those exposed to bentonite, especially for normally-consolidated clays. The back-calculated α values, obtained by analysing 309 case studies data, show that boreholes supported by polymer perform better than those supported by bentonite. The numerical simulations carried out using the multi-physics software COMSOL, reveal that an increase in the duration of exposure of clay to the bentonite and polymer support fluids increased the percolation depth of these fluids leading to a significant reduction in α. The numerical simulation results confirm the findings of the experimental investigation and case study analysis as it was found that α for pile bores supported by polymer was larger as compared to the pile bores supported by bentonite. Finally, by combining the results of the above three analyses, simple, easy-to-use, and new equations and design charts have been produced, which provide relationships between adhesion parameter and normalised undrained shear strength for polymer- and bentonite-supported pile bores. It is envisaged that these equations and design charts will help practicing engineers in choosing appropriate adhesion parameter (α) suitable for design of piles in soft clays.