Cyclic stress-strain characteristics of calcareous sand improved by polyurethane foam adhesive

Calcareous sand is a common foundation soil which is usually used for offshore or island infrastructures such as highway embankments and airport runways. These structures are often subjected to cyclic loads (such as waves, traffic, pipeline driving, and earthquakes). However, particle breakage commonly occurs in calcareous sands, which has posed a great challenge to offshore structures in terms of increased differential settlements. To address this issue, an eco-friendly agent, i.e., Polyurethane Foam Adhesive (PFA), instead of the commonly used alkaline stabilization agents (e.g. lime, cement), was proposed to improve the engineering properties of calcareous sands. The PFA is a rigid foam consisting of Isocyanate and polymer Polyol. To examine the effectiveness of using PFA in improving the resilient properties of calcareous sands, a series of triaxial shear tests were performed using a medium-scale true triaxial apparatus equipped with a cyclic loading facility. Besides, scanning electron microscope (SEM) tests were conducted to reveal a microstructural mechanism of using PFA for improving calcareous sand. The experimental results provided crucial insights into the cyclic strength, resilient modulus Mr, equivalent damping ratio λ of modified calcareous sand for a range of PFA contents Rw and confining pressures σ3, as well as the mechanism of PFA solidified calcareous sand from a microscopic point of view. Useful empirical relationships among the governing parameters were established. A high-resolution field emission scanning electron microscope was used to study the internal pore and particles arrangement of PFA modified specimen. The experimental findings are crucial for improving the understanding of the fragility of calcareous sand and their improvement using an eco-friendly solution in the form of PFA