This is a combination process where the placement of the fill material (usually limestone gravel) is done by pneumatic stowing followed by injection of the stowed material with Polyurethane grouts providing tight permanent seals. Where surface soils require cessation of movement, topical application of Polyurethane can form a surface crust to stabilize them. Deeper injection provides coverage to sub-levels consolidating the mass of material. Very high compressive strengths are achieved when used with certain soils.