Shale constitutes about 50 percent of all the rock types exposed on the earth's surface and comprises 70 percent of all the sedimentary rocks (Ackenheil, 1969). In the southern portion of the State of Indiana (U.S.A.), shales are commonly encountered in construction as they often outcrop or are under a shallow overburden.

The two most important properties of shale in compacted embankments are:

  1. durability (resistance to environmental deterioration) and

  2. strength (resistance to deformation or breakage under load). Many combinations of durability and strength can be expected with shales. Hard and durable shales can be placed as a "rock fill", while soft and nondurable shales are easily broken down and placed in thin layers as a "soil fill". Soft and durable shales have not been encountered in Indiana, but hard and nondurable ones exist, and require special attention during placement in an embankment. These need to be thoroughly broken down by handling and compaction, which produces thin layers without large voids. Because of the strength of the shale pieces this is difficult to accomplish.

The experimentation summarized in this paper was directed toward the selection of additives which could reduce the deterioration caused by slaking during the service life of a compacted shale embankment. The slake durability test was used to evaluate the change in durability of compacted shale samples effected by lime used in the compaction water. The shales were hard and nondurable, and were from the State of Indiana.

Slaking Mechanisms

Slaking is defined (American Geological Institute, 1976) as "…loosely, the crumbling and disintegration of earth materials when exposed to air or moisture. More specifically, the breaking up of dried clay when saturated with water, due either to compression of entrapped air by inwardly migrating capillary water, or to the progressive swelling and sloughing off of the outer layers."

Terzaghi and Peck (1967) attributed the slaking phenomenon to the compression of air entrapped in soil pores as water entered these pores. This entrapped air exerts tension on the solid skeleton, causing it to fail. The behavior has been observed in some slaking shales and mudstones, but not in others (Moriwaki, 1975; Badger et al., 1956; Nakano, 1967).

Clay surface hydration by ion adsorption has been suggested as a second mechanism causing slaking through swelling of illite and montmorillonite minerals (Chenevert, 1970). Other potential mechanisms include simple stress relief, which opens shale fissures, and removal of cementing agents by the dissolving action of moving ground water (Badger, et al., 1956; Moriwaki, 1975). The pH of the percolating ground water, the presence of oxygen and carbon dioxide, and the minerals present in the shales control the degree of dissolution. For Indiana shales, no single mechanism for slaking has been proven to be the dominant one. A combination of mechanisms, with either one triggering another or each acting independently is the most likely (Surendra, 1980).

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