New Method for Improvement of Rockfill Material with Polyurethane Foam Adhesive

External factors, such as earthquakes or changes in water level, can reduce the safety factor for rockfill dams. Hence, the material in the slope and upper part of a high rockfill dam should be reinforced or strengthened. One way to do this is to use geogrid or rebar mesh to reinforce the rockfill. However, this method may cause difficulties during construction, especially for roller-compacted materials. Another method is to use Portland cement to strengthen the rockfill. However, this method may cause materials in the upper part of the dam to be stiffer than materials in the lower part of the dam, which may lead to differential displacements and cracking. Thus, cement stabilization is undesirable from this standpoint.

A new soil-improvement method that employs polyurethane foam adhesive (PFA) is proposed for rockfill materials. An experimental program of large-scale drained triaxial compression tests was conducted to investigate the strength and deformation behavior of PFA-improved rockfill. The rockfill material had particle sizes ranging from 0.1 to 20 mm and was classified as well-graded gravel (GM) according to ASTM (2006). Specimens with a height of 200 mm and a diameter of 100 mm were prepared by mixing the rockfill material with 8% PFA by weight percentage ($W_p$). The mixed material was divided into five equal parts for compaction inside a split mold. The effective confining stresses for the tests were 0.1, 0.3, 0.5, 0.7, and 0.9 MPa, and the specimens were saturated using the vacuum method ($B\text{-}\mathrm{value}>0.96$). The tests were performed at a constant axial displacement rate of $0.6\mathrm{mm}/\min$ and discontinued at 30% axial strain. Corresponding tests were performed without PFA admixture for comparison.

Shear strength envelopes for the unimproved and PFA-improved rockfill materials are presented in Fig. 1. The envelope for the unimproved rockfill is approximately linear over a large effective stress range and passes through the origin ($c=0$, $\varphi =40.4°$). The envelope for the PFA-improved rockfill is parallel and indicates a cohesion intercept ($c=0.32\mathrm{MPa}$, $\varphi =40.4°$). Thus, the effect of the PFA admixture is to add significant cohesive strength to the material. Fig. 2 shows photographs of unimproved and PFA-improved rockfill specimens after testing for a confining pressure of 0.3 MPa. The unimproved material exhibited contractive behavior during shear, whereas the improved material exhibited expansive behavior and a bulging failure mode. The PFA-improved rockfill material has good ductility for $6\%<W_p<10\%$. The material exhibits brittle deformations for $W_p>10\%$ and lower cohesion for $W_p<6\%$.