Short note: Weathering profile

A weathering profile is the vertical section from fresh bedrock up to the ground surface, showing zones of increasing degree of weathering.

 Zone         Description
 ---------------------------------------
 VI  Residual soil   (fully altered, no rock fabric)
 V   Completely weathered (soil-like, fabric seen)
 IV  Highly weathered (more than half is soil)
 III Moderately weathered (less than half soil)
 II  Slightly weathered (discoloured joints)
 I   Fresh rock

• The classification is that of ISRM/BS 5930 (grades I to VI).
• The thickness of the profile depends on climate, rock type, structure and topography. It is deepest in warm, humid regions, where it can reach tens of metres (e.g. the thick red residual soils of the Nepal Midlands).
• Strength, bearing capacity and the safe cut-slope angle fall as the grade increases.
• It is important for foundation depth, tunnel portals and slope design, because the weathered zone is weak, permeable and easily eroded.