Use of Shale in Embankments : Technical Paper

Guidelines for the design and construction of soil embankments are sufficiently developed so that unsatisfactory performance by these fills is relatively rare. The same is true for rock fills, where the sound and durable rock is placed in large chunks, and with large voids between the chunks. However, there are transition materials or "soft rocks" for which placement in large chunks may lead to highly unsatisfactory embankment performance. Shales afford the prominent example, since the large pieces may degrade (slake) into soil in service. This soil may in turn sift down into the large voids, with the net result that large settlements, and even slope instability, may occur. Embankment failure due to the above mentioned circumstances resulted in closing lanes on Interstate 74 in Indiana and required costly repairs. Similar failures have occurred in other states in the midwest. The harder and more durable shales can probably be placed as rock fills with certain safeguards. The shales of very low durability must be throughly degraded at the end of compaction, i.e., must be treated as Soil fills. A full spectrum of durabilities exists between these limits. The engineer obviously needs a classification system which will establish where, in the possible range of relative durabilities, a potential embankment shale lies. To develop such a classification for Indiana shales, materials were sampled and subjected to a battery of durability, stability and miscellaneous type tests. The durability tests were those used as standard for mineral aggregates, but modified in severity to account for the "soft rock" being evaluated. It was concluded that the desired classification into four groupings, viz., soil-like, intermediate-1, intermediate-2, and rock-like shales, could be accomplished with no more than four rather simple tests: one cycle slaking in water, slake durability on an initially dry sample, slake-durability on a soaked sample, and a modified sodium sulfate soundness test. The paper describes the Indiana shales tested, the tests proper, and the response of the Indiana shales in the tests. It concludes with a flow chart showing how the tests are used to accomplish the shale classification.