highwall
The unexcavated face of exposed overburden and coal in a surface mine.
highwall: the vertical cliff left behind by surface mining
A highwall is the near-vertical or steeply sloped wall of rock and overburden left standing after a surface mine or quarry removes material from a seam or deposit. In coal mining, the highwall marks the active mining boundary, separating excavated ground from undisturbed strata. Heights range from 30 feet in shallow operations to over 200 feet in deep strip mines, depending on geological conditions and the thickness of the coal seam or ore body being worked.
The highwall comprises both overburden (the layers of rock, clay, and soil above the economically valuable deposit) and the exposed edge of the deposit itself. This layering creates a visible geological cross-section and makes the highwall useful for geotechnical mapping. Mining engineers inspect highwalls for signs of weakness, such as water seepage, fractured strata, or slippage planes, which indicate instability and potential collapse.
Stability and Failure Modes
Highwalls are engineered to specific slope angles, typically between 45 and 70 degrees, calculated from the strength of the rock type, joint spacing, and groundwater conditions. Failure occurs when internal water pressure exceeds the cohesion of the material, when bedding planes dip toward the void, or when vibration from blasting disturbs weakened zones. Failures range from small sloughs of loose material to catastrophic slides involving millions of tons. Adequate benching (creating stepped terraces) and proper blast design are essential controls.
The term "highwall" emphasizes the height and wall-like character of this feature. It contrasts with the "lowwall," the corresponding slope on the opposite side of the pit where overburden is being placed. Modern surface mining operations maintain detailed highwall monitoring using surveying, drone imagery, and inclinometers to detect movement before failure occurs.
Highwall stability becomes a long-term liability in reclamation; many mined sites must grade or stabilize exposed highwalls before the land can be released. In some jurisdictions, regulations require that highwalls be trimmed back to stable slopes or benched during closure, adding significant cost to the final stages of a mine's life.