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Polyurethane foam adhesive (PFA), a product resulting from exothermic chemical polymerization reactions between the diisocyanate functional group and polyol monomers, can form a closed cellular structure with high strength and stiffness. In the current study, the effectiveness of PFA in improving the strength and ductility of a well-graded gravelly soil was investigated. Using drained triaxial compression tests on the unimproved and PFA-improved soil as the basis for comparison, the peak strength and residual-state strength of the PFA-improved soil increased significantly (from 1.5 to 7 times and from 1.4 to 5.4 times of that of unimproved soil, respectively) with increasing PFA content (from 2 to 8%) at a confining pressure of 100 kPa. For the scenarios of low confining pressure, the PFA additive resulted in a change in volumetric response from contractive to highly dilative. The residual (or pseudo-critical) state lines (RSLs) for PFA-improved soils were observed to be parallel in the q∼p′ and e∼p′ planes, with an upward translation with increasing PFA content. Strain-dependent reductions in stiffness were observed to become more pronounced with increases in PFA content and confining pressure. In particular, the brittleness index for the PFA-improved soil (ranging from 0.01 to 0.29) was found to be much lower than that for the lime-cemented soil (ranging from 0.03 to 2.36), indicating that the PFA-improved gravel exhibited much better ductility than similar gravelly soils improved with lime due to the ductile nature of the polymer bonds. From the perspective of ductility, strength, and economy, the optimal PFA content ranges from 4 to 6% (with an optimum value of approximately 5%). Given its ductile response and stress-strain-strength characteristics, PFA-improved gravelly soils could offer a promising alternative for application to high rockfill dams or railway embankments.
Xiao et al. (Sat,) studied this question.