Abstract
Engineering behavior of shales is extremely variable and is markedly influenced by their structure and composition. To investigate in detail the influence of structure and composition, fresh shale samples with varying degrees of fissility were collected from ten exposed sites in Northeast Ohio. In addition, siltstone samples were collected from three of these sites. The shales and siltstones ranged in age from Upper Devonian to Pennsylvanian. Based on their breaking characteristics, representative samples from each site were ranked as least fissile, moderately fissile, and highly fissile.
A series of laboratory tests were performed to determine the engineering properties of each shale including the determination of natural water content, absorption, specific gravity, plasticity characteristics, slake durability, point load index, and x-ray diffraction analysis. Standard and modified compaction tests were performed on each shale individually. In addition, compaction and strength characteristics of mixed samples, containing varying proportions of shale and siltstone, were investigated to study the influence of the proportion of siltstone on the engineering behavior of shale embankments.
Results showed the shales ranged form silty to clayey in nature with illite and kaolinite as the dominant clay minerals. Generally speaking, highly fissile shales tended to have lower absorption values, higher point load strengths, and higher void ratios after compaction than the less fissile shales. Fissility showed no apparent relationship with the natural water content, silt and clay content, durability characteristics, and unconfined compressive strengths of compacted samples. In terms of composition, silty shales were generally found to be less durable than clayey shales. When compacted, clayey shales showed the best compaction with the lowest void ratios. For mixed samples, the void ratio tended to increase with increasing proportions of siltstone in most cases. The unconfined compressive strength decreased with increasing proportions of siltstone up to 60 percent siltstone content, after which the strength increased with increasing proportions of siltstone. Both fissility and durability seemed to be influenced more by origin, nature of cementation, and degree of lithification than by proportion of silt or clay.
