Q. Examine the factors that determine slope stability.
Slope stability is the condition in which shear strength along a potential failure surface exceeds the shear stress gravity imposes on it. The ratio of the two is the factor of safety, and every factor below acts by raising one term or lowering the other. The analytical gain lies in separating the preparatory factors that push a slope towards its threshold from the triggers that carry it across.
The Factor of Safety: What Every Factor Acts Upon
- The Coulomb-Terzaghi criterion: shear strength equals cohesion plus effective normal stress times the tangent of the angle of internal friction, effective stress being total normal stress minus pore-water pressure.
- Factor of safety is resisting shear strength divided by driving shear stress: above unity stable, at unity critical, below unity failing. Design requires a margin well above unity.
- The infinite-slope model, used for shallow translational failure in regolith over bedrock, makes stability a function of cohesion, slope angle, regolith thickness and saturation — hence thin, saturated mantles on steep slopes fail first.
- Decisively, pore-water pressure is the only term that changes in hours. Cohesion, friction angle and geometry change over years to millennia, so water is the proximate cause and rarely the underlying one.
Preparatory Factors Against Triggering Factors
- Preparatory or conditioning factors act over years to millennia, lowering the factor of safety towards unity without causing failure: deep weathering, tectonic fracturing, undercutting, deforestation, an unsupported road cut.
- Triggering factors act over minutes to days and supply the final increment: a rainfall burst, an earthquake, rapid drawdown, a blast.
- The distinction divides the response — zonation maps preparatory conditions, early warning can only track triggers. The Char Dham corridor and Joshimath are preparatory failures; the Kedarnath cloudburst of June 2013 was a trigger on ground already prepared.
Inherent and Geological Factors
- Lithology — sheared Lesser Himalayan phyllites and slates and unconsolidated Siwalik molasse fail readily; massive granite, gneiss and Deccan basalt resist unless closely jointed.
- Depth and grade of weathering — thick saprolite retains water and loses cohesion on wetting; Western Ghats lateritic mantles are strong dry and collapse saturated.
- Discontinuities — bedding planes, joints, foliation and the Main Boundary and Main Central Thrust zones supply ready-made failure surfaces far weaker than intact rock.
- Dip relative to the slope is the decisive structural criterion: discontinuities dipping out of the face at an angle lower than the slope but steeper than the friction angle make planar sliding possible; dipping inward permits toppling only.
- Regolith thickness fixes the slip-surface depth and mass mobilised; colluvium-filled hollows are the standard debris-flow initiation sites.
Geometric Factors
- Slope angle — driving stress rises with the sine of the angle while normal stress, and so friction, falls with its cosine, so failure probability climbs sharply past the threshold angle.
- Slope height raises toe stress and deepens rotational slip surfaces, which is why tall cut faces are benched.
- Curvature and aspect — concave hollows converge subsurface flow and colluvium and fail preferentially while convex noses shed both; aspect governs insolation, moisture and vegetation, giving different failure regimes on opposing valley sides.
Hydrological Factors
- Pore-water pressure is the mechanism throughout: it cuts effective normal stress, and with it friction, with no change in geometry or material.
- Rainfall intensity against antecedent moisture — both are needed. Intense bursts trigger shallow debris flows; prolonged antecedent rain raises the water table and triggers deeper failures, so Himalayan slides cluster late in the monsoon.
- Perched water tables at the regolith-bedrock contact concentrate pressure on the plane most likely to fail, while seepage exerts drag on the soil skeleton and artesian pressure at a footslope can lift the mass.
- Rapid drawdown — when a reservoir falls faster than bank pore pressures dissipate, the stabilising water load goes while internal pressure persists; rim slopes such as Tehri’s are managed against this.
Vegetation
- Root cohesion adds apparent cohesion dominating strength in the upper one to two metres, and anchors regolith into bedrock where roots reach it; interception and evapotranspiration also lower antecedent moisture.
- Surcharge and windthrow work the other way: tall trees add weight and transmit wind loading, so forest is not automatically stabilising on steep ground.
- The deforestation effect operates with a lag — root strength decays for roughly three to ten years after felling before replacement roots establish, opening a window of maximum vulnerability. Shallow-rooted tea, rubber and eucalyptus on Wayanad, Idukki and Nilgiri slopes never restore it.
Seismic and Volcanic Factors
- Ground acceleration adds a cyclic horizontal force and can liquefy saturated fine sediment; the 2011 Sikkim and 2015 Gorkha earthquakes each triggered widespread landsliding in the Teesta and Kosi headwaters.
- Cumulative fracturing is the quieter seismic effect — repeated shaking damages rock mass without failing it, a preparatory legacy persisting for years.
- Volcanic controls — edifice loading, hydrothermal alteration to weak clays, eruption-triggered lahars — are the classic case of material weakening.
Anthropogenic Factors
- Road cutting and toe excavation is the largest artificial control, removing lateral support and steepening the face above: on the Char Dham corridor 81 per cent of landslides lie within 100 metres of the road.
- Sidecast muck compounds it — a kilometre of hill road yields 40,000 to 80,000 cubic metres of debris, mostly sidecast.
- Loading at the crest by buildings, embankments and spoil heaps adds mass where it most harms the balance.
- Terracing and irrigation cut runoff but raise infiltration, unlined channels on Himalayan terraces being a recognised cause of saturation failure; reservoir filling saturates the rim and on fractured basement can induce seismicity, as at Koyna in 1967.
- Quarrying and blasting open joints by vibration; quarries adjoin many Western Ghats slopes that failed at Wayanad and Idukki.
- Joshimath instructs because it is mass movement rather than landslide: a town on old landslide debris, flagged by the 1976 Mishra Committee, moved 5.4 centimetres in 12 days to 8 January 2023, cracking over 860 buildings.
Mitigation, Briefly
- Because pore-water pressure moves fastest, drainage — surface, catch-water and sub-surface — returns most per rupee; retaining structures, soil nailing and bio-engineering follow, not the reverse.
- Structural works cannot compensate for alignment: the Landslide Atlas of India (2023) ranks 147 districts by exposure, yet binds no construction decision.
Conclusion
Slope stability is not a property a slope possesses but a running balance it maintains, and the factor of safety is its only honest summary. Geology, weathering depth and structure set the ceiling; geometry, vegetation and human modification adjust it; water decides the day. The discipline this question rewards is the separation of preparatory from triggering factors, because it explains why identical rainfall destroys one slope and spares the next, and why India’s landslide losses track road alignment more closely than rainfall totals.