Table C1.1.

Landform and land use features, slope stability and construction difficulty

Alignment typeLandform and land use featuresTypical problems encounteredLikelihood of existing landslidePotential for road construction to cause landslide
Ridge topRounded reliefDeeply weathered soils likely; some erosion potentialUnlikelyPossibly
Sharp reliefRock at surface; costly and difficult rock excavation possibleUnlikelyUnlikely
Irregular reliefDifficult alignment along ridge top between high points and low pointsPossiblyPossibly
Asymmetric reliefJoint-controlled slopes will influence stability of alignments and cut slopesPossibly – check for failed debris downslopePossibly on dip slope
Ridge lines generallyMay be subject to greater rainfall than valley sidesPossiblyPossibly
Ridge lines generallyMay be more affected by seismicity (topographic amplification)PossiblyPossibly
Valley sideSlopes are steeper than 40°Probably underlain by rock, therefore alignment likely to be more costly to construct but less costly to maintainUnlikelyPossibly, depends on jointing
Slopes are 35–40°Potential to be shallow taluvium on rockPossiblyPossibly
Slopes are 20–35°Potential to be deep taluvium, colluvium or failed slopePossiblyPossibly
Continuous rock slopes with persistent jointing approximately parallel to slopeDepending on strength of rock mass this joint set could be problematic in excavations and foundationsPossibly – check for failed debris downslopeLikely
EmbaymentsEither erosional in origin or formed by landslide(s)ProbablyPossibly, reactivated movements in landslide debris
Areas of irrigated paddy fieldDrainage problems likely; soils possibly taluvial/colluvial in origin and potentially unstable locallyPossibly, but mass as a whole may be stablePossibly
Forest/jungle areas on otherwise cultivated hillsidePossibly areas of wet ground, steep slopes, instability that cannot be cultivatedPossiblyPossibly
Rounded spursProbably formed in residual soils and stableUnlikelyUnlikely
Elongated mid-slope benchesEither ancient river terraces or rock benches; both stable and ‘easy’ for road constructionUnlikelyUnlikely
Local mid-slope benchesCould be as above, or part of deep-seated landslidePossiblyUnlikely
Valley floorSteep slopes forming margins of river channel (i.e. no river terrace)Possibly unstable; difficult for road alignments, especially on meander bends; possible flood risk and high water tableLikelyPossibly
Steep slopes forming valley side margins to river terracePossible ancient landslides and high water tablePossiblePossibly
River terracePossible flood risk, soft soils and terrace edge scour; high water tableUnlikely, except at terrace edgesUnlikely
Tributary streamsPossibly active debris flows and debris fan deposition causing scour and blockage/damage to road structures; possible flood risk and high water tableDebris flows onlyDebris flows only