An apparatus for cutting soil and constructing sub-surface containment barriers, such as for constructing subsurface containment walls or basins around and under contaminated soils, comprises an elongated beam for abutting an extended length of soil. The beam comprises a cutting assembly that creates a cutting action against the extended length of abutting soil. The cutting assembly preferably comprises a conduit containing a plurality of jet ports through which high pressure fluid is ejected to impact the soil to be cut. The cutting assembly is maintained adjacent to the face of the soil to be cut. A method of cutting soil comprises generating cutting action along an extended locus of soil, and advancing the cutting action along a descending locus of the soil in response to gravity, or along a path determined by pre-placed pulling assemblies. Subsurface containment barriers are formed by when a jetted slurry or other suitable material cuts and mixes with the soil.