The Role of Shale Pores in Settlement

by Luis E. Vallejo, (M.ASCE), Univ of Pittsburgh, Pittsburgh, United States,
Michael K. Robinson, Univ of Pittsburgh, Pittsburgh, United States,
Ann C. Stewart-Murphy, Univ of Pittsburgh, Pittsburgh, United States,

Document Type: Proceeding Paper

Part of: Vertical and Horizontal Deformations of Foundations and Embankments


Waste fills resulting from the mining of coal in Appalachia should ideally consist of large, free-draining sedimentary rock fragments. The successful performance of these embankments is directly related to the strength and durability of the individual rock fragments. When shale is the predominant rock in the fills, some shale fragments will degrade into soil size particles. This degradation is the result of slaking and point load crushing. Slaking takes place when the shales absorb water. Point load crushing is the result of gravity-induced loads that the individual fragments exert on each other at their points of contact. The slaked and crushed material fill the void spaces between the intact shale fragments. This material rearrangement causes settlement. A laboratory testing program, with point load and slake durability tests, as well as thin section examinations on sixty-eight shale samples from the Appalachian region, revealed that pore microgeometry of shales has a major influence on degradation. The smaller the pores, the more the shales slaked in water. Although not generally true for all the samples, the testing program indicated that the larger the pores, the lower the crushing strength under point loads. Thus, the size of the pores in shales appears to have a direct relationship to degradation and settlement.

Subject Headings: Shale | Load tests | Load factors | Strength of materials | Rock fills | Mine wastes | Chemical degradation

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