Table 6.

Engineering geology of steep, sidelong ground in the Central Cordillera

Typical ground conditionsTypical engineering issuesEngineering solutions and considerations
EarthworksRetaining, revetment and containment structuresDrainage, surface treatment and erosion control
Often faulted and weak rock massesRock falls and rock slides onto the roadRemoval of overhangs and cutting back of slopes may be of some effect depending upon access and daylighting considerationsRetaining and containment walls may be of some effect but space restrictions may limit the capacity of free-standing structures. Revetments may prove successful in preventing ravelling and surface instability. Wire netting may perform an effective dual role in helping to retain jointed rock material in situ and prevent rock debris from affecting the roadAnchored structural shotcrete can be very effective in jointed rock masses. Access may prove a constraint. Wire netting may perform an effective dual role in helping to retain jointed rock material in situ and prevent rock debris from affecting the road
Variable thicknesses of spoil (from original construction, road widening and debris clearance) covering slopes belowCreates conditions highly vulnerable to erosion and slope failure. Not suitable for foundationsRemoval of spoil from the road subgrade and from foundation excavations for retaining wallsBio-engineering (principally planting) may provide some protection against erosion and shallow slope instability
Deep gullies, both naturally occurring and the result of erosion below culvert outlets and side drain turnouts and locations of uncontrolled road runoffUndermining of culvert outlet structures and ultimately the road edge by headward erosionFoundation of road edge retaining walls in erosion-resistant slope materials at depth, if they existDeeply founded outlet protection works (e.g. cascades and checkdams). Continuous to a ‘hard point’ in the gully bed. Shotcrete may be able to prevent erosion and ravelling of jointed rock masses. Bio-engineering may be able to contribute
Shallow (usually rapid) and deep-seated (usually slow-moving) slope failureUndermining of road edge, cracking, subsidence and total failure of road pavementRealignment into the cut slope may trigger or aggravate instability problems above the road and will not address the problems below. However, in some cases, this may be the only feasible optionConstruction of road edge retaining walls with foundations in competent materials beneath zone of movement where possible. These foundations may need to be several metres deep
Debris flows originating on hillsides aboveBlockage and damage to the road and its structuresContainment walls at road level and checkdams in affected stream channels above may provide some protection to the road from the smaller flowsIt may be appropriate to introduce soil conservation (including bio-engineering) measures in debris flow source areas, if practicable