If structures are porous or lacking in strength, additional reinforcement is often necessary. Also, if the foundation is not strong enough, the filling injection method is used. This method is often used for mining and tunnel construction, where due to internal cavities, the stability of soil does not meet the requirements for structures. It can also be used for reinforcing floor levelling layers and for reattaching detached layers to the firm substrate. A variety of injection materials can be used for reinforcement or filling of cavities. Depending on the conditions, the following materials are most commonly used: high-performance polyurethane foam (up to 80 N/mm2), epoxy resin, cement glue or cementitious slurry.