In an attempt to reduce the damage to ventilation stoppings caused by strata convergence, many mines are now using new stopping construction techniques that include compressible materials. This Bureau of Mines study concentrated on compressible stoppings in use in underground trona mines, all of which had overburden in excess of 1,000 ft. This overburden depth, in conjunction with the relatively soft floor in trona mines, has commonly created excessive floor heave throughout mined-out entries. Five different compressible stopping assemblies were examined: metal, wood, a wood- polyisocyanurate hybrid, and "small" and "large" expanded polystyrene (eps) blocks. Testing was done in both elliptical and rectangular openings, and both rigid urethane foam (ruf) and a mortar-based sealant were used in different tests--for sealing either the perimeter (and joints) or the entire stopping face. Results showed that the eps stoppings sealed with ruf were superior in prevention of air leakage in the mine openings evaluated. Eps has the advantage of being capable of being cut to any size and shape required. The mortar-based sealant was susceptible to degradation, especially around perimeter interfaces with the mineral surface, because of the hygroscopic nature of trona.