Design of Highway Embankment on Unstable Natural Slopes

Abstract

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 stabllijy 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 rapiddrawdown 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 ot 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. INTRODUCTION The stability of natural slopes, cut slopes, and embankment slopes Is a major concem to any highway designer. The AlexandriaAshland (AA) Highway In northern Kentucky will cross two areas that historically have been difficult areas for both highway construction and maintenance activities due to slope stability and settlement • American Engineering Company is providing overall supervision of design and construction of the AA Highway tor the Commonwealth of Kentucky. Geotechnical overview was provided by Mr. Bishop for American Engineering Company. problems (1-16). Weak clayey shales that comprise the bedrock In these two areas are the major contributor to the problem. Major problems which roadways have experienced in the two areas are catastrophic landsliding and significant settlements of several inches to several feet within ent:lankment sections. The purpose of the AA Highway Is to provide a direct link between the Covington-Cincinnati area and the Ashland-Greenup area in northeastern Kentucky as shown in a plan view In Figure 1. The AA Highway begins in Campbell County at the interchange of I