Geotechnical modelling and analysis involve the mathematical representation of soil and rock behaviour to predict how the ground will respond to applied loads, excavations, and environmental changes. It bridges the gap between site investigation data and engineering design — transforming borehole logs and lab test results into actionable design parameters.
What Is Geotechnical Modelling?
A geotechnical model (or ground model) is a simplified representation of the subsurface conditions at a site. It synthesises geological mapping, borehole logs, geophysical surveys, and laboratory test data into a coherent understanding of:
- Stratigraphy — layer geometry and continuity
- Material properties — strength, stiffness, permeability
- Groundwater conditions — water levels, flow regime, pore pressures
- Geological structure — faults, joints, bedding planes
- Geohazards — reactive soils, acid sulfate soils, fill, mine voids
Levels of Modelling
| Level | Description | When Used |
|---|---|---|
| Conceptual model | Block diagram or cross-section showing interpreted geology | Early project stages, feasibility |
| 1D model | Borehole-based soil profile with depth-dependent properties | Simple foundation design, site classification |
| 2D model | Cross-sectional model along a specific alignment | Slopes, embankments, retaining walls, tunnels |
| 3D model | Volumetric representation of the full site | Complex geology, major infrastructure, BIM integration |
The Ground Model Development Process
Step 1: Data Compilation
All available data is assembled:
- Borehole logs and CPT traces
- Geophysical survey interpretations
- Laboratory test results
- Topographic and LiDAR data
- Geological maps and memoirs
- Aerial photography and satellite imagery
- Previous investigation reports
Step 2: Geological Interpretation
The data is interpreted to define:
- Layer boundaries and thickness variations
- Geological structures and discontinuities
- Weathering profiles and alteration zones
- Groundwater regimes
Step 3: Parameter Assignment
Characteristic design parameters are assigned to each material unit:
| Parameter | Source | Application |
|---|---|---|
| Unit weight (γ) | Lab measurement | All stability and deformation analyses |
| Friction angle (φ') | Triaxial / direct shear | Strength-based analyses |
| Cohesion (c') | Triaxial testing | Strength-based analyses |
| Undrained shear strength (su) | UU triaxial / shear vane | Short-term stability in clays |
| Young's modulus (E) | Triaxial / oedometer | Settlement and deformation analysis |
| Poisson's ratio (ν) | Triaxial / empirical | Settlement and deformation analysis |
| Permeability (k) | Lab / in-situ testing | Seepage and dewatering analysis |
| Coefficient of consolidation (cv) | Oedometer | Time-rate of settlement |
Step 4: Model Validation
The model is checked against:
- Adjacent site data and published geological maps
- Historical performance of nearby structures
- Sensitivity analysis to key parameters
Common Analysis Methods
Limit Equilibrium Analysis
Used for slope stability, bearing capacity, and retaining wall design.
| Method | Description | Best For |
|---|---|---|
| Bishop Simplified | Circular slip surface, method of slices | Homogeneous or layered slopes |
| Janbu Simplified | Non-circular slip surfaces | Irregular slopes, weak layers |
| Spencer | Satisfies moment and force equilibrium | All slope geometries |
| Morgenstern-Price | Non-circular, arbitrary interslice force function | Complex slope geometries |
| Sarma | Non-vertical slice boundaries | Blocky rock slopes |
Output: Factor of Safety (FoS)
Finite Element Analysis (FEM)
Provides stress-strain analysis of the full soil-structure system.
| Software | Developer | Key Features |
|---|---|---|
| Plaxis 2D / 3D | Bentley | Industry standard, soft soil model, hardening soil |
| RS2 / RS3 | Rocscience | 2D/3D, slope stability, tunnel support |
| ABAQUS | Dassault | Advanced constitutive models, coupled analysis |
| Z_SOIL | Zace Services | Unsaturated soil, dynamics |
| DIANA | DIANA FEA | Tunnel lining, soil-structure interaction |
Typical outputs:
- Displacement contours and vectors
- Stress distributions (effective stress, pore pressure)
- Plastic strain zones (failure progression)
- Structural forces in retaining walls, piles, tunnel linings
- Ground settlement troughs
Finite Difference Analysis
| Software | Developer | Key Features |
|---|---|---|
| FLAC 2D / 3D | Itasca | Explicit solution, large deformation, dynamic analysis |
Discrete Element Method (DEM)
For granular materials and jointed rock masses.
| Software | Developer | Key Features |
|---|---|---|
| PFC 2D / 3D | Itasca | Particle-based, fracture propagation |
| UDEC / 3DEC | Itasca | Distinct element, jointed rock, blocky systems |
Seepage Analysis
Modelling groundwater flow and pore pressure distribution.
| Software | Developer | Key Features |
|---|---|---|
| SEEP/W | GeoStudio | Steady-state and transient, unsaturated flow |
| Plaxis (seepage) | Bentley | Coupled deformation-seepage |
| MODFLOW | USGS | Regional groundwater modelling |
| SVFlux | SoilVision | Unsaturated flow, climate boundary conditions |
Consolidation and Settlement Analysis
| Software | Key Features |
|---|---|
| Settle3 | 1D and 3D consolidation, staged construction, PVDs |
| Plaxis 2D/3D | Coupled consolidation, soft soil creep |
| GEO5 Settlement | Immediate and consolidation settlement |
| CDS (Consolidation Design System) | PVD design, staged preloading |
Dynamic / Seismic Analysis
| Software | Key Features |
|---|---|
| DEEPSOIL | 1D site response analysis, liquefaction triggering |
| Plaxis / FLAC | Dynamic time-history, earthquake loading |
| OpenSees | Open-source, research-grade seismic analysis |
| QUAKE/W | Tailings dam seismic assessment |
Constitutive Soil Models
| Model | Parameters | Best For |
|---|---|---|
| Mohr-Coulomb | E, ν, c', φ', ψ | Simple elastic-perfectly plastic, general use |
| Drucker-Prager | E, ν, α, k | Simplified 3D Mohr-Coulomb |
| Modified Cam Clay | λ, κ, M, e₀ | Soft clay consolidation |
| Hardening Soil (HS) | E₅₀, E_ur, E_oed, m, c', φ' | All soils — best general-purpose model |
| HS Small (HSS) | HS + G₀, γ₀.₇ | Small-strain stiffness (excavations, tunnels) |
| Soft Soil Creep (SSC) | λ*, κ*, μ*, M | Long-term creep in soft soils |
| Hypoplastic | Granular parameters | Sands, cyclic loading |
| Barcelona Basic | Unsaturated parameters | Expansive and collapsible soils |
Model Selection Guide
| Problem Type | Recommended Model |
|---|---|
| Simple bearing capacity | Mohr-Coulomb |
| Retaining wall deflection | HS Small |
| Deep excavation | HS Small |
| Tunnel-induced settlement | HS Small |
| Soft clay embankment | Soft Soil Creep or Modified Cam Clay |
| Slope stability (FoS only) | Mohr-Coulomb (with phi-c reduction) |
| Liquefaction assessment | UBC3D-PLM or PM4Sand |
| Unsaturated soil | Barcelona Basic or extended Mohr-Coulomb |
| Dynamic / cyclic | Hypoplastic or PM4 models |
Applications
| Application | Analysis Type | Key Outputs |
|---|---|---|
| Shallow foundation | Settlement + bearing capacity | Allowable bearing pressure, settlement |
| Deep foundation (pile) | Axial and lateral capacity | Pile capacity, load-settlement curve |
| Retaining wall | Earth pressure + structural | Wall deflection, bending moment, anchor loads |
| Slope stability | Limit equilibrium or FEM | Factor of safety, failure surface |
| Excavation support | FEM, soil-structure interaction | Wall deflection, ground settlement |
| Tunnel design | FEM or DEM | Ground loss, lining forces, settlement |
| Embankment / dam | Consolidation + stability | Settlement, pore pressure, stability |
| Liquefaction | Cyclic stress + site response | CRR, CSR, liquefaction potential, settlement |
| Ground improvement | FEM | Composite ground behaviour, effectiveness |
| Seepage / dewatering | Seepage analysis | Flow rates, drawdown, pore pressure |
Australian Standards
| Standard / Guideline | Relevance |
|---|---|
| AS 1726-2017 | Geotechnical site investigations (data for modelling) |
| AS 4678-2002 | Earth retaining structures — analysis requirements |
| AS 2870-2011 | Residential slabs (design soil parameters) |
| AS 2159 | Piling — analysis methods |
| AS 1170.4 | Earthquake loads — site response analysis |
| AS 3798 | Earthworks — compaction and fill parameters |
| TfNSW QA Specification | Modelling requirements for road projects |
Frequently Asked Questions
What is the difference between a ground model and a geotechnical model?
A ground model describes the geological conditions (layers, structure, groundwater). A geotechnical model assigns engineering properties to the ground model units and is used directly in analysis.
How many boreholes are needed for a good 3D model?
It depends on geological complexity. Simple sites: 1 borehole per 1,000–2,000 m². Complex sites (e.g., variable alluvium, faulted rock): 1 per 500–1,000 m². Geophysics can supplement sparse borehole coverage.
Can numerical modelling replace site investigation?
No. Site investigation provides the input parameters for modelling. A model built on poor data will produce unreliable results — "garbage in, garbage out."
What is the most common numerical modelling mistake?
Using over-simplified constitutive models with generic parameters. Always calibrate models against site-specific laboratory test data and use the simplest model that captures the essential behaviour.