ABSTRACT:

A large body of geotechnical data has been developed in conjunction with the efforts of the Federal and State Governments to rehabilitate Abandoned Mine Lands (AML) in eastern Kentucky. This data base allowed for the examination of soil profiles and compositions, phreatic levels, and subsurface failures conditions. Soil properties calculated from laboratory procedures were subjected to back-analysis for estimating realistic factors of safety. Several important slope stability factors were noted. This paper examines the difficulties with using laboratory soil properties in AML remediation designs. Shear strength parameters were found to be inadequate due to pre-failure site strain conditions, 'phreatic levels during failure, and, of particular interest to this paper, rock particle influences.

INTRODUCTION

Prior to the Federal Surface Mining Control and Reclamation Act of 1977 (Public Law 95-87), rock, soil, organic material, coal, and even discarded mining equipment from surface and underground mines in Kentucky were simply cast over the sides of strip and adit benches. This coal mine spoil was generally poorly mixed, uncompacted, and allowed to settle at the material's angle of repose. The spoil most often rested on top of the preexisting colluvium soil layers. Many of these slopes were already close to failing or, in some cases? had experienced some slope instability. Weigle (1966) estimated that 12% of eastern Kentucky's strip mine spoil had failed by the mid-1960's. Often, runoff from the abandoned mines drains through the adjacent soil cover, reducing the shear strength of the soil and enhancing the development of subsurface failures. This type of instability has become one of the Nation's most critical Abandoned Mine Land (AML) problems. From 1979 until February 1992, 425 landslides in 9 States were stabilized at a cost of $64,625,044. Kentucky had the most AML landslides, with a total of 268 at a cost of $41.5 million. The average cost of each stabilization project was $152,059.

In eastern Kentucky, the Federal and State Governments share responsibility for remediating AML landslides. Emergency landslides are administered by the Office of Surface Mining, whereas, imminent hazards are administered by Kentucky's Department for Natural Resources, Division of Abandoned Lands. A total of 77 geotechnical reports were collected and analyzed from these two organizations.

Unfortunately, even with this geotechnical information, the soil's shear strength was still difficult to establish. Accurate shear strength characterization of landslide material is absolutely essential for reliable designs. Uncertainty about the range of material strengths experience from different categories of AML landslides affects the engineer?s judgment. Under these conditions, the remediation effort must often be conservatively designed. Underestimating material strength leads to increased remediation cost, while overestimating could lead to inadequate slope stabilization.

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