Soil conditioning techniques are used to alter the properties of soils to make them more suitable for excavation by tunnel boring machines. Conditioning agents include foams, polymers and bentonite slurry, either alone or in combination. Conditioning agents may have additional benefits in providing lubrication for the advancing tunnel shield, or in the case of pipejacking for the entire tunnel shaft. The paper concentrates principally on the use of foams as conditioning agents, but also includes some results involving use of polymers and bentonite.