Evaluating selected factors affecting the depth of undercutting in rocks subject to differential weathering

One of the most important considerations in designing cut slopes in sub-horizontal, inter-layered, sedimentary rocks subject to differential weathering is predicting the total depth of undercutting. Undercutting-induced rockfalls are a major problem for many roadways in Ohio. The total depth of undercutting was measured for 59 profiles from 18 cut slope sites in Ohio with references to cut faces of hard rock units containing pre-split blast-hole traces. The presence of blast hole traces ensured that the rock face represented the slope surface that was cut during construction. Additionally, initial design plans were used for reference. Step-wise regression was used to determine the geological, geotechnical, and geometrical factors that have the highest influence on the total depth of undercutting. Selected independent factors used in the regression analysis included the vertical distance of the undercut unit from the slope crest, the relative position of the undercut unit from the slope crest, the total thickness of the undercut unit, the spacing of orthogonal joints within the undercut unit, the slake durability index value of the undercutting unit, the initial slope angle, and the age of the road cut. Factors that showed the most significant correlation with the total depth of undercutting were found to be the vertical distance of the undercut unit from the slope crest, the relative position of the undercut unit from the slope crest, the total thickness of the undercut unit, joint spacing within the undercut unit, and the slake durability index of the undercutting unit. The regression analysis resulted in an R2 value of 0.61, with the depth of undercutting correlating most strongly with a closer relative position of the undercut unit to the slope crest above it and to a closer spacing of orthogonal joints within the undercut unit. These results can be attributed to the fact that more porous and fractured rock units closer to the slope crest intercept and transport greater amounts of infiltrating groundwater, producing greater depths of undercutting. Also, closely jointed rock units are more permeable, allowing more groundwater seepage, which also leads to greater undercutting.

► Road cuts in heterogeneous hard/soft strata fail due to complex undercutting.
► 61% of variation in depth of undercutting can be explained statistically.
► Close jointing and being high in the cut favor undercutting by capturing seepage.
► Slake durability index & initial slope angle correlate modestly with undercutting.
► Rate and concentration of seepage require further investigation.