GEOTECHNICAL

Slope Stability

Analysis of factor of safety against landslide. Methods: Bishop's, Fellenius, Janbu. FoS ≥ 1.5 for permanent slopes.

Also calledslope analysiscircular failurewedge failure
Related on InfraLens
CODES
IS 7894IS 14458
Definition

Slope stability analysis evaluates the factor of safety (FoS) against landslide failure of a slope — the ratio of available shear strength on a potential failure surface to the shear stress required for equilibrium. Per IS 7894:1975 (revised 2024) and IS 14458:1998, the principal methods of analysis are: (a) Fellenius (or Swedish circle) method — assumes circular failure surface, simple formula but conservative; (b) Bishop's simplified method — accounts for inter-slice forces, more accurate for routine design; (c) Janbu's method — extends to non-circular surfaces; (d) Spencer's method — most rigorous, satisfies both force and moment equilibrium; (e) Finite-element analysis — for complex slopes with anisotropic soil or 3-D geometry.

For a typical slope in Indian residual soil: failure tends to be circular through the soil with circle centre above the slope. Bishop's simplified method computes FoS by dividing the failure mass into vertical slices, summing the available shear strength on the failure surface (c × ΔL + W × tan φ × m where m is a Bishop factor) and dividing by the required shear stress (W × sin α). FoS minimum criteria per IS 7894: 1.5 for permanent slopes (long-term stability), 1.25 for temporary slopes (during construction), and 1.0-1.1 for emergency (during earthquakes or imminent failure conditions).

Critical slope-stability factors: (a) effective shear strength parameters c and φ — must be from tests on saturated samples representative of the slope; (b) groundwater table — drainage controls slope stability; (c) seismic loading — IS 1893 mandates pseudo-static analysis with seismic coefficient for slopes in zones III-V; (d) loads on top of slope — building, road, cast embankment; (e) toe support — toe excavation reduces FoS dramatically. Indian practice: routine slope analysis via Bishop's simplified method; complex slopes via PLAXIS / SLIDE finite-element analysis. The most common cause of slope failure in Indian construction: post-monsoon soil saturation reducing effective c and φ from drained values.

Typical values
FoS — permanent slope (long-term)≥ 1.5
FoS — temporary slope (construction)≥ 1.25
FoS — under earthquake (with seismic coefficient)≥ 1.0-1.1
Typical natural slope angle (sand)30-35°
Typical natural slope angle (clay)20-30°
Embankment maximum recommended slope (1:V vs H)1.5-2.0 horizontal : 1 vertical
Where used
  • Highway and railway embankments — IRC SP 84
  • Cut-slope design for hillside roads, mines
  • Earth and rock-fill dam design (IS 7894)
  • Levee and flood-protection embankments
  • Mine reclamation and waste-pile slope assessment
Acceptance / threshold
Per IS 7894 + IS 14458: minimum FoS 1.5 (permanent), 1.25 (temporary), 1.0-1.1 (under earthquake). Drainage controls; seismic analysis in zones III-V; saturated effective stress parameters used.
Site example
Site reality: a Himachal Pradesh hillside road had a 12 m cut slope at 1:1 (45°) from horizontal. Bishop's simplified analysis returned FoS = 1.42 — close to but below the 1.5 limit for permanent slopes. The contractor argued '1.42 is fine, just need a stability factor'. The geotechnical engineer correctly insisted on either flatter slope (1:1.25, costing ₹85 lakh extra cut) or rock-bolting and shotcrete (₹42 lakh). The slope was retained at 1:1 with rock-bolting; subsequent two monsoons confirmed adequate stability. Always meet IS 7894 minimum; below-limit FoS is asking for failure during the next event.
Frequently asked
What is slope stability analysis?
Slope stability analysis evaluates the factor of safety (FoS) against landslide failure — the ratio of available shear strength to required shear stress on a potential failure surface. Per IS 7894 + IS 14458: minimum FoS 1.5 for permanent slopes, 1.25 for temporary, 1.0 for under-earthquake. Methods: Fellenius (simple, conservative), Bishop's simplified (standard practice), Spencer's (most rigorous), finite element (complex geometries).
What causes slope failure?
Common causes: (1) Increase in shear stress — added weight on top of slope (building, embankment, fill), toe excavation removing support. (2) Decrease in shear strength — saturation by rain reduces c and φ in cohesive soils, weathering of soil over time, freeze-thaw cycling. (3) External triggers — earthquake (cyclic loading), heavy rainfall (pore-pressure rise), thermal expansion. (4) Construction errors — unstable cut-and-fill geometry, inadequate drainage. Indian context: monsoon-induced saturation is the dominant cause of natural-slope failure.
How is slope stability improved?
(1) Flatter slope — reducing the slope angle is the simplest stabilization. (2) Drainage — surface and subsurface drainage to prevent saturation. (3) Toe support — gabions, retaining walls, sheet piles at the toe. (4) Internal reinforcement — soil nails, geo-textile, geogrid, rock-bolts. (5) Vegetation — root reinforcement, surface erosion control. (6) Slope mass removal — partial cutting reduces driving force. Major Indian projects routinely combine 2-3 of these methods.
Related geotechnical terms
Safe Bearing Capacity (SBC)
Max safe pressure soil can take. 100-500 kN/m² typical.
Standard Penetration Test (SPT)
Soil density test in borehole per IS 2131
California Bearing Ratio (CBR)
Subgrade strength index for pavement design
Soil Classification
Per IS 1498 / Unified Soil Classification System
Geotextile / Geomembrane
Synthetic fabrics for drainage, filtration, soil reinforcement
Dewatering
Removing groundwater from excavations during construction
Pile Foundation
Deep foundation transferring load to firm strata via piles
Retaining Wall Types
Walls retaining earth — gravity, cantilever, counterfort, diaphragm