Lessons Learned from Rates of Mudrock Undercutting Measured over Two Time Periods

Differential weathering is a common source of rock slope instability on the Appalachian Plateaus, where topography is often steep and widespread cyclothemic deposits contain repeated interfaces between weak mudrocks and more resistant beds of sandstone and limestone. Relatively rapid weathering of various mudrocks can undermine more competent overlying strata and lead to instability and failure in cut slopes. Rates of undercutting of the resistant units by weak units were measured at seven highway cuts ranging in age from 33 to 40 years. The average annual rate, 0.79 in. ( 2.0 cm), is approximately half that determined for the same sites by Shakoor and Rogers ( 1992) when the cuts were 18-25 years in age. At six of the seven sites, the length of measurable road cut had been reduced, in part because of new rock fall, and at four sites there had been no additional undercutting. Two failure modes are proposed for collapse of the overlying beds: 1) interaction of undercut surfaces with vertical jointing and 2) delayed tensile failure. The first failure mode is consistent with pre-1992 rock fall at all of the sites, as well as post-1992 rock fall at the three sites where undercutting has remained active. The second mode, less common overall, is consistent with rock fall at sites where undercutting has been inactive since 1992. The current study also confirmed two significant aspects of the previous study: 1) a correlation between slake durability and rates of weathering, and 2) extreme variability in weathering characteristics among sub-types of mudrocks.