Shear strength enhancement at the sand-steel interface: A pioneering approach with Polyurethane Foam Adhesive (PFA)

This study explores a novel application of Polyurethane Foam Adhesive (PFA) injection to augment shear resistance at the interface, investigating various parameters such as PFA content, injection pressure, curing time, sand density, and surface roughness. Results demonstrate that injecting 10 % PFA increased the friction angle (phi) from 6.4 to 27.4 degrees and 32.6-42 degrees for sand-smooth and rough surface interfaces, respectively, under relative density (Dr) of 30 %. Correspondingly, cohesion (c) values rose from 0 to 124 and 62 kPa. Samples with lower Dr (30 %) exhibited a more significant increase in shear strength post-PFA injection, attributed to PFA penetration and enhanced shear performance of loose sands, showing a maximum 23 % difference compared to Dr = 90 % for the sand-smooth surface interface. Curing time notably influenced injected samples, particularly at lower normal stress levels, leading to increased mobilized shear strength with longer curing periods, with 28-day shear strength averaging 2.34 and 2.74 times higher than one-hour shear strength for smooth and rough surfaces, respectively. A substantial shear strength increase was observed one hour after injection, with more pronounced behavior after seven days. Additionally, increased injection pressure expanded the stabilized area at the interface, enhancing mobilized shear strength. At the sand-smooth surface interface, increasing injection pressure from 25 to 225 kPa resulted in average mobilized shear strength increases of 84% and 117% for Dr 30 % and 90 %, respectively. Examination under a Scanning Electron Microscope (SEM) revealed that the inclusion of PFA additive serves to interconnect adjacent grains, consequently enhancing the mechanical properties of the mixture.