GeoStrength — GSI & Slope Stability

Geological Strength Index

Evaluate rock mass quality from structure and discontinuity surface condition.

1 Rock mass structure

Select one structure type · five-point scale

2 Discontinuity surface condition

Rate each factor from 1 (poorest) to 6 (best)

Roughness of joints

Degree of weathering

Infilling materials

Limit Equilibrium Method

Planar failure — factor of safety against sliding on a daylighting joint plane.

1 Slope geometry

Slope height H (m)

Failure plane dip β (°)

Unit weight γ (kN/m³)

Block thickness t (m)

2 Strength along failure plane

Cohesion c (kPa)

Friction angle φ (°)

3 Optional loads dry slope default

Pore water force U (kN)

External force T (kN)

III

Hoek–Brown Parameters

Derive rock mass strength envelope and deformation modulus from GSI, intact UCS, and rock type.

1 Rock mass classification

GSI is carried from the GSI tab automatically — or override it here

GSI (0–100)

Disturbance D (0–1)

2 Intact rock properties

Intact UCS σci (MPa)

m_i constant

Typical m_i: granite 32, granodiorite 29, gneiss 28, sandstone 17, limestone 12

3 Confining stress (for σcm)

Depth / overburden z (m)

Unit weight γ (kN/m³)

4 Schmidt hammer → UCS field converter

Rebound value R

Rock density g/cm³

Enter R to estimate UCS

Hoek–Brown Parameters

m_b: —
s: —
a: —

Rock mass UCS σcm: — MPa
Tensile strength σt: — MPa
Deformation modulus Erm: — GPa
Confinement σ₃ at depth: — MPa

Equiv. Mohr-Coulomb c': — MPa
Equiv. Mohr-Coulomb φ': — °

m_b = m_i·exp((GSI−100)/(28−14D)) | a = 0.5 + (e^−GSI/15−e^−20/3)/6

Wedge Failure Analysis

Factor of safety for a rock wedge defined by two intersecting joint planes daylighting on a slope face.

1 Slope face

Slope dip αf (°)

Slope dip direction (°)

Slope height H (m)

Unit weight γ (kN/m³)

2 Joint plane A

Dip (°)

Dip direction (°)

Cohesion cA (kPa)

Friction φA (°)

3 Joint plane B

Dip (°)

Dip direction (°)

Cohesion cB (kPa)

Friction φB (°)

About this app

GeoStrength combines rock mass classification, planar and wedge stability analysis, Hoek–Brown derivation, and Najd structural zonation.

Overview

GeoStrength provides linked calculators for GSI classification, planar LEM stability, Hoek–Brown rock mass characterisation, wedge failure analysis, and Najd fault structural zonation. Results update instantly. Advanced mode reveals additional panels and deeper outputs; Standard mode keeps only the core GSI and planar LEM tools.

Module I — GSI

Chart-based method: structure type (score 1–5) plus three surface condition sliders (roughness, weathering, infilling, each 1–6). GSI = interpolated from anchor points. Advanced mode adds tunnel class, RMR cross-check, and Q rough estimate.

J_s = R + W + F | GSI = f(structure, J_s)

Module II — Planar LEM

Planar block on a daylighting joint. Advanced adds tension crack, crack water, and pseudo-static seismic force. For bolt spacing and target FS design charts, use the linked Rock Slope Reinforcement Helper (GeoStrength extension).

FS = (c·A + (W cos β − U) tan φ) / (W sin β + T + k_h·W)

Module III — Hoek–Brown (Advanced)

Derives m_b, s, a from GSI, D, m_i. Computes rock mass UCS (σcm), tensile strength (σt), deformation modulus (Erm), and equivalent Mohr-Coulomb c′, φ′ at the given depth. Schmidt hammer → UCS converter uses the Deere-Miller / ISRM correlation.

m_b = m_i·exp((GSI−100)/(28−14D)) | E_rm = (1−D/2)·√(σci/100)·10^{(GSI−10)/40}

Module IV — Wedge Failure (Advanced)

Two-plane wedge using vector algebra. The line of intersection (LOI) is found from the two plane normals. FS is computed as total resisting force on both planes divided by the component of weight acting along the LOI.

Module V — Najd Δθ Zonation (Advanced)

For each slope, Δθ = min(|slope_dir − Najd_dir|, 360° − |slope_dir − Najd_dir|). Red: Δθ ≤ 20° (Najd planes can daylight — high planar/wedge risk). Amber: 20° < Δθ ≤ 60° (wedge-forming). Green: Δθ > 60° (Najd planes dip into slope — favourable). The compass diagram visualises all slopes relative to the Najd reference direction.

Note

These tools support preliminary assessment and research. All design should follow site investigation, applicable codes, and qualified engineering judgement. Najd reference direction and Hoek-Brown anchor values should be calibrated to each project site.