Stress-Deformation and Strength Characteristics of a Compacted Shale : Interim Report

Abstract

The report includes the results of laboratory testing of a mechanically hard but non-durable shale from the New Providence Formation in Indiana. The compaction, consolidation and shearing characteristics of the shale are presented, as well as the testing procedures used.The report should be an aid to ISHC engineers in evaluating the stress-deformation characteristics and effective stress strength parameters of compacted shales.The report is submitted as partial fulfillment of the overall objectives of the research project.Copies of the report will also be submitted to the ISHC and the FHWA for their review, comment and similar acceptance. 16. Abstroct vThen shales are encountered in road cuts, economic and environmental considerations usually dictate that they be used in adjoining embankments. However, unless special precautions are taken, the stability of a shale embankment can deteriorate with time on account of the non-durable nature of some shales in the presence of water.Various tests have been developed to classify shales. This report presents the results of a laboratory investigation to determine the influence of compaction and confining pressure on the stress-deformation and strength characteristics of a mechanically hard but non-durable shale from the New Providence Formation.Some construction guidelines are also recommended. Cylindrical specimens formed by kneading compaction were saturated under a low effective confining pressure, consolidated to the desired effective stress and sheared at a constant rate of strain. The compaction characteristics, the volume changes after saturation, the consolidation characteristics, and the undrained shearing response including pore water pressure changes are reported for triaxial tests, The initial gradation of crushed shale aggregate, the molding water content, the compaction pressure, and the pre-shear consolidation pressure were adopted as the test variables.The effective stress strength parameters were found to be essentially independent of the initial conditions when they were evaluated at maximum deviator stress, except for loose uncompacted aggregate. The volume change characteristics, the induced pore water pressures, and the consolidated undrained strength were found to be greatly dependent on the initial conditions. The principal conclusions for the shale studied follow.1.The shale slakes partially when mixed with water and undergoes some mechanical breakdown when compacted.2.The influence of molding water on the compacted dry density is not as well defined as in fine grained cohesive soils. For the experimental range of molding water contents, the mean dry density increased linearly with the logarithm of the compaction pressure.4.The as-compacted dry density ranged from 95 pcf to 132 pcf, the void ratio ranged from 0.829 to 0.296, and the degree of saturation ranged from 5.5% to 95%. 5.For a given initial condition, there is a critical effective confining pressure below which the specimen swells, and above which it compresses, when saturated.6.The ...