Rubble concrete

Rubble Concrete

Rubble concrete is one of the most satisfactory materials for spillway construction. Its cost is more than that of timber and rock, but it is more permanent and impervious. It has the additional advantage of resisting the abrasive effect of waterborne sand and gravel if the rubble is made of boulders placed as closely together as possible with their outer surface projecting into the water to protect the concrete mortar which there used to fill the interstices.

The investigations herein reported indicate that the best rubble structure is one that will stand without mortar and in which concrete is used merely to fill the voids and to decrease leakage. When available, pear-shaped boulders, 2 feet or more in length, should be used. For the floor and crest, they should be laid with the smaller ends tilted downward and pointing upstream. After one course is laid, concrete should be spaded between the rocks to within 6 or 8 inches of the surface. The second course is laid so that each rock rests upon two boulders in the lower course.

The low cost of rubble masonry has appealed strongly to those in charge of flood-control undertakings. Rubble masonry has been placed by common labor furnished by persons who would receive the benefits and who were glad of the opportunity to work out their assessments.