Sections of the new Alexandria-Ashland (M) highway in northeastern Kentucky must be located on and through natural slopes of the Kope and Crab Orchard Formations that consist mainly of shales. Numerous highway embankment and cut slope failures have occurred In these two geologic formations In past years. The shales of these formations have very poor and undesirable engineering properties. Many stability problems encountered with the Kope and Crab Orchard shales are caused primarily by the tendency of those shales to breakdown when exposed to water and produce clays and clayey silts of relatively low shear strengths. Construction of highway embankments with, through, and on the shales of the Kope and Crab Orchard Formations has been necessary because of the vast aerial presence of these shales and the Jack of more suitable and economical alternate construction materials. In the design of the AA highway, geotechnical engineers were faced with three difficult problems. First, In forming embankments, there was a question of how the shales should be compacted and what constituted proper compaction. Second, the most difficult problem, perhaps, was the selection of appropriate shear strength parameters of the overconsolidated clays and clayey shales of the natural slope foundations and the embankments constructed of the shales. Numerous landslide studies Involving overconsolidated clays and clay shales show that use of peak strengths from triaxial tests may lead to underconservative designs while use of residual shear strengths may lead to uneconomical designs. A third factor complicating the design of shale embankments on weathered shale slopes Is the seepage of water Into the embankment. Exposure of the lower portions of the embankments composed of Kope and Crab Orchard shales to seepage and rapid-drawdown conditions created by the Ohio River make the shales susceptible to breakdown and swelling and may produce a progressive "softening• and decrease In shear strengths. The design of certain sections of the AA highway passing through the Kope and Crab Orchard shales was further complicated by the fact that some natural slopes and existing highway fills having slopes of 3 horizontal to 1 vertical are failing. The design problem became one of placing new highway embankments on existing failing natural slopes. This paper presents case histories to document existing failures and discusses the treatment of the three factors -- compaction, shear strength, and seepage. A discussion of the different aspects of the geotechnical design for new fill placement is presented. Emphasis is placed on the selection and justification of design parameters for foundation and embankment materials as well as the construction procedures and compaction specification that were finally adopted.
Digital Object Identifier
Bishop, Charles S.; Armour, Donald W. Jr.; and Hopkins, Tommy C., "Design of Highway Embankment on Unstable Natural Slopes" (1986). Kentucky Transportation Center Research Report. 553.