TableĀ 7.

Engineering geology of saddle crossings in the Central Cordillera

Typical ground conditionsTypical engineering issuesEngineering solutions and considerations
EarthworksRetaining, revetment and containment structuresDrainage, surface treatment and erosion control
Narrow widthSaddle width may be too narrow to support required road formation widthIf the gradients either side allow, cut down vertical alignment (VA) to increase available width. If the VA is reduced the height and required founding depth for supplementary retaining walls may be reducedIf sufficient width cannot be created by earthworks use retaining walls (incl. earth reinforced structures) founded in in situ ground with sufficient bearing capacity
Topographical low-pointRoad runoff converges from higher elevation sections of road either side with the potential to cause significant erosion below outletsDivert the majority of runoff before it reaches the saddle using relief culverts and side drain turnouts, though adequate (often significant) scour protection should be provided at all drainage outlets
Residual soils often occur on saddlesThese materials may be prone to erosion & slope failure, thus compromising the stability of the saddle. They may be unsuitable for wall foundations as wellRemove unsuitable/unstable materials & replace with rock fill, for exampleRemove unsuitable/unstable materials. Use retaining walls, founded in in situ material below existing and potential failure surfaces & with sufficient bearing capacity. Earth- reinforced structures could prove effectiveEnsure adequate scour protection below any drainage outlets on the saddle. Bio-engineering may be able to contribute on gully sides
Often faulted and weak rock massesUse shotcrete to protect surfaces in weathered rock from erosion and ravelling
Saddle side slopes may be formed in fill and spoil materialsBio-engineering (principally planting) may provide some protection against erosion & shallow slope instability