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.     

Cut Slope Design Recommendations for Sub-Horizontal Hard Sedimentary Rock Units in Ohio, USA

Although most cut slopes in Ohio consist of inter-layered, sub-horizontal units of hard and soft sedimentary rocks (sandstone, limestone, dolostone, shale, claystone, mudstone), slopes consisting of relatively thick hard rock units are not uncommon. Design of stable cut slopes in hard rock units needs to consider rock mass strength and orientation of discontinuities with respect to slope face. Results of kinematic stability analyses show that hard-rock cut slopes are less likely to have conventional plane and wedge failures, caused by unfavorable orientation of discontinuities. The main cause of failure is identified to be the undercutting-induced toppling, which is not amenable to traditional kinematic or rock mass strength-based analyses. Therefore, to recommend a suitable slope angle, numerical models, using UDEC software, were employed to study how various slope angles affect the process of undercutting-induced toppling failures. The UDEC models showed a slope angle of 45° (1H:1 V) to be the most stable angle. However, a 63° (0.5H:1 V) slope angle can significantly reduce the potential for such failures and is therefore more appropriate than the widely used angle of 76° (0.25H:1 V).