Polyurethane grouting works by injecting dual component polyurethane (PU) foam below or around structures, or into the ground.  Depending on the specific formulation, the grout then expands up to 30 times to fill voids, and/or cause displacement.  Displacement is managed to densify soils, or correct settlement. 

Just like different concrete mixes have different uses, polyurethane grout formulations ("systems") are designed for specific applications.  Each system varies by how much it expands, whether it continues expanding after it gets hard enough to stop flowing, and whether it's designed for use in wet or dry environments. 

For example, CJGrout 20SDB is designed to expand a lot, and continue flowing even after it's stopped expanding, so is really well suited for filling large voids around sensitive structures.  Alternatively, CJGrout 48NHL doesn't expand as much, and stops flowing before it stops expanding, so is designed to cause displacement of heavy structures, so is used for lifting things like railroad crossings, or densifying loose soils.

Have more questions about how polyurethane grouting works?