They are two distinct methods used for different ground conditions. Polyurethane grouting injects a dual-component hydrophobic polyurethane grout through small-diameter port pipes — it expands to fill voids and densify loose near-surface material, and is ideal for slab lifting, void fill, and near-surface foundation stabilization. Minimal cure time, small footprint, no excavation required. Compaction grouting injects a stiff, low-mobility grout mass under pressure to mechanically densify weak or loose soils at depth — used when the primary problem is deeper bearing capacity failure that polyurethane cannot reach. GSI evaluates both options and recommends based on subsurface conditions and depth of treatment required.