Abstract: A dam for sealing an underground passage extending along an axis is formed with a pair of end ridges themselves formed by--oppositely inclined inner and outer surfaces and at least one middle ridge also formed by a pair of oppositely inclined surfaces. The surface projected parallel to the axis of the outer end-ridge surfaces is larger than the surface projected parallel to the axis of the dam of the inner end-ridge surfaces and the middle ridge is offset outward, that is into the earth bounding the space, of the end ridges by a distance equal to at least the difference between the sizes of the projections.