Compaction grouting involves the injection of stiff grout that does not penetrate the ground. Bulbs of grout are formed. It is applied to free-draining granular soils. The aim is to increase the density of the soil being treated by applying high local pressure. The design of compaction grouting is based largely on experience and empiricism. If the soil is not sufficiently permeable for consolidation to occur as it is treated, excess pore pressures may be generated, which will dissipate after treatment. The potential effect of such excess pore pressures on the compaction achieved is considered here. It is found that the efficiency of treatment may be reduced substantially.