Geotechnical Investigation
What Is a Geotechnical Investigation and Why It Matters
A Geotechnical Investigation is the structured process of understanding subsurface conditions—soil, rock, and groundwater—to safely and economically design foundations, earthworks, pavements, retaining structures, and underground works. It reduces uncertainty, reveals hazards, and provides parameters for analysis and construction. For owners and designers, a well-scoped investigation often produces the largest lifecycle savings on a project by preventing overdesign, delays, and failures.
This page is your engineer-focused guide: we outline investigation objectives, how to plan scope by risk, what to look for in desk studies, how borings and CPTs complement each other, which in-situ and lab tests to specify, how to treat groundwater, and how to synthesize everything into a defendable ground model that drives design. It’s written to answer the questions people actually search for—What tests do I need? How many borings are enough? How do I turn logs into design parameters? What are typical deliverables and standards?
The value of a geotechnical investigation is measured by decision clarity: fewer unknowns, realistic parameters, and constructible designs.
Investigation Objectives, Scope & Risk-Based Planning
The right scope depends on project risk, variability, and consequences of failure. A low-rise building on uniform sands needs a different approach than a deep excavation near critical infrastructure or a bridge over soft clay. Start with clear objectives, then match methods and intensity.
- Define decisions to be made: Foundation type and depth, allowable bearing/settlement, lateral pressures, slope stability, liquefaction potential, excavation support, dewatering strategy.
- Anticipate hazards: Collapsible or expansive soils, organics/peat, soft clays, karst, variable fill, boulders/obstructions, contamination, compressible layers, seismic and cyclic issues.
- Scale scope to risk: Higher consequence → more borings/CPTs, deeper exploration, redundant test types, and more lab testing for variability and anisotropy.
- Plan phasing: Preliminary (feasibility), design-level (parameter definition), and construction-phase verification (test pits, additional CPTs, proof testing).
Quick Scoping Heuristics
Uniform sands with shallow spread footings → CPT-led program with select borings for sampling. Soft clay sites for piles → borings with high-quality Shelby tubes, vane shear, and consolidation tests. Urban deep basements → combined borings, CPT/DMT, groundwater response, and instrumentation for excavation staging.
Desk Study: Build the First Ground Model
A robust investigation starts at your desk. Collate geologic maps, historical aerials, previous reports, utility records, flood maps, and LiDAR; interview site personnel; review adjacent project data. The goal is to create a conceptual ground model highlighting likely strata, groundwater regimes, geomorphology, and anthropogenic influences (fills, preloading, contamination).
- Geomorphology & geology: Identify depositional environment (fluvial, marine, glacial), structural geology, and potential rock weathering profiles.
- Historic use: Landfills, borrow pits, basements, fuel stations, and industrial operations suggest variability and environmental constraints.
- Hydrology: Rivers, tidal influence, artesian aquifers, perched zones; seasonal groundwater fluctuation expectations.
- Seismicity: Regional hazard, liquefaction susceptibility mapping, and fault proximity.
Did you know?
Early access to adjacent project data often reduces required new exploration by focusing boreholes where variability is greatest—not where it’s convenient.
Field Exploration: Borings, CPT, Test Pits & Geophysics
Field work validates and refines the desk study. Combine methods to capture both continuity and sample quality. Place exploration points to intercept critical strata transitions, structure footprints, and load paths; extend depth to where stresses dissipate (often 1–2 times foundation width for shallow footings, well below pile tips, or to competent rock for end-bearing designs).
- Test pits: Cost-effective for shallow variability, fill characterization, and bulk sampling; excellent for pavement and utility corridors.
- Borings (SPT-based): Provide disturbed samples, N-values (with energy corrections), groundwater observations, and Shelby tubes in fine-grained soils.
- CPT/CPTu: Continuous tip and sleeve data with pore pressure; ideal for profiling sands and soft clays, pile design, and liquefaction evaluation.
- DMT/PMT: Dilatometer and pressuremeter tests for deformability, horizontal stress, and direct stiffness inputs for settlement and lateral response.
- Geophysics: MASW, seismic CPT, GPR, and ERT to map bedrock depth, rippability, voids/karst, and stratigraphic continuity between points.
Exploration Spacing (Rule of Thumb)
In-Situ Testing: Getting Parameters Right
Choose tests that best represent the soil behavior governing your design: strength for stability and capacity, stiffness for settlement and deformation, and permeability for groundwater control.
- SPT (N): Ubiquitous; correct to \( N_{60} \) for hammer energy, borehole conditions, and overburden; use with caution in gravels and very soft clays.
- CPT/CPTu: Direct correlations to undrained strength and relative density; pore pressure aids soil type interpretation and pile design.
- Vane shear (VST): High-quality undrained strength in soft clays; apply appropriate sensitivity and anisotropy corrections.
- DMT: Provides constrained modulus and horizontal stress index—excellent for settlement and lateral response of foundations and retaining systems.
- Pressuremeter (PMT): Yields limit pressure and shear modulus; powerful in sands, chalk, and weak rock for deformability and capacity checks.
- Permeability tests: Rising/falling head, packer tests for rock, and pumping tests to support dewatering design.
Typical SPT Energy Correction
Sampling Quality & Laboratory Testing
Laboratory data are only as good as the samples. Use undisturbed samples (Shelby tubes, block samples) for clays when strength and compressibility govern; obtain representative disturbed samples for index and gradation tests. Protect moisture state during handling and storage.
- Index tests: Moisture content, Atterberg limits, unit weight, grain size, hydrometer, specific gravity for classification and initial parameters.
- Strength: UU, CU, and CD triaxial; direct shear for granular soils and interfaces; unconfined compression for quick clay checks.
- Compressibility: 1-D consolidation (oedometer) to derive \( C_c, C_r, C_\alpha \), preconsolidation stress, and settlement predictions.
- Resilient modulus & cyclic tests: For pavements and seismic/ cyclic loading evaluations.
- Chemical/durability: Sulfates, chlorides, pH, organic content—inform concrete and steel protection, and environmental compliance.
Sample Quality Indicator
Groundwater, Permeability & Dewatering Strategy
Groundwater conditions control excavation stability, base heave, piping risk, corrosion potential, and long-term performance. Observe groundwater levels during and after drilling and, where possible, install piezometers to track recovery and seasonal variation. In layered soils, expect perched and artesian conditions that behave differently during construction.
- Permeability characterization: Use lab tests for fine-grained soils and in-situ tests for sands/gravels and rock; estimate anisotropy where stratification is present.
- Dewatering design: Select wellpoints, deep wells, eductor systems, or cutoff walls; consider recharge to limit settlement in urban settings.
- Construction impacts: Evaluate consolidation due to drawdown, base heave in soft clays, and uplift in deep excavations; specify monitoring triggers.
Effective Stress Framework
From Data to Design: Building the Ground Model
Synthesize logs, in-situ and lab results into engineering strata with representative parameter ranges. Each stratum should include strength (drained/undrained), stiffness (small-strain to working strain), unit weight, permeability, and characteristic variability. Document assumptions and correlations, and calibrate parameters with any available performance data or local experience.
- Parameters for shallow foundations: \( \varphi’, c’ \) or undrained strength \( s_u \), modulus \( E \)/\(M\), and settlement coefficients; check bearing, settlement, and punching.
- Pile/shaft design: Use CPT, SPT, and lab data for shaft/base resistance and load-transfer; include setup/relaxation and downdrag where relevant.
- Slopes & retaining walls: Define shear strength envelopes with appropriate drainage conditions; assess seismic and rapid drawdown scenarios where relevant.
- Seismic/liquefaction: Evaluate triggering, lateral spreading, and post-liquefaction residual strength where applicable; plan mitigation if needed.
- Risk & sensitivity: Provide best-estimate and lower-bound cases; show how uncertainty affects design and construction means and methods.
Important
Designs should be traceable: every parameter must map back to a test, correlation, or conservative rationale stated in the report. Explicitly note limitations and data gaps.
QA/QC, Health & Safety in Geotechnical Investigation
Quality and safety underpin reliable results. Field crews work around heavy equipment, open boreholes, utilities, and traffic; labs manage sample chains and precision instruments. A documented QA/QC plan sustains data integrity from site to report.
- QA/QC: Calibrate SPT energy; verify CPT cone factors; maintain slurry and grout logs when applicable; preserve samples with moisture control; duplicate/spot tests for critical parameters.
- Safety: Utility locates, confined space protocols for test pits, traffic control plans, spill prevention, and decontamination where contamination is suspected.
- Documentation: Daily field logs, photos, GPS coordinates, instrument IDs, and weather/groundwater notes; lab CMs and traceability records.
Typical Deliverables & How to Use Them
A clear, construction-ready report accelerates design reviews and contractor pricing. Beyond raw data, deliver decisions: recommended foundation systems, parameters with ranges, monitoring plans, and risk items with mitigation options.
- Boring/test logs & lab summaries: Standardized formats with corrections and metadata.
- Cross-sections & ground model: Interpreted profiles aligning exploration points to structural layouts.
- Design parameters: Tabulated for each stratum (strength, stiffness, unit weight, K, cyclic modifiers).
- Recommendations: Foundation type/levels, allowable pressures, pile types and capacities, lateral earth pressures, pavement sections, excavation/dewatering methods, and instrumentation.
- Monitoring & hold points: Trigger levels for groundwater drawdown, settlement/tilt, and vibration; requirements for additional testing if conditions deviate.
- Standards & references: Cite applicable codes (local building codes, transportation manuals, and geotechnical standards) and correlations used.
Specification Tip
Include re-scoping language: if unexpected conditions arise (e.g., artesian water, uncontrolled fill), the geotechnical engineer should be notified to adjust exploration, testing, and design.
Geotechnical Investigation: Frequently Asked Questions
How many borings or CPTs do I need?
It depends on variability and risk. As a starting point, provide at least one exploration at each column line intersection or grid of 20–50 m for uniform sites, plus additional points near transitions, excavations, and utilities. Increase density for irregular geology or critical structures.
How deep should I explore?
To the depth where stress increases from the proposed loads drop to insignificant levels and where potential failure surfaces or compressible layers are captured. For shallow foundations, this may be 1–2 times the foundation width; for piles, well below tip to confirm bearing stratum and downdrag zones.
SPT or CPT—which is better?
Use both when possible. CPT provides continuous profiling and is excellent for sands and soft clays; SPT supports sampling, visual classification, and is helpful in mixed soils. Choose based on governing design needs and site constraints.
How do I turn test results into design parameters?
Build strata, choose relevant drained/undrained strength and stiffness models, apply validated correlations (with corrections), and calibrate against local experience or load tests. Provide characteristic values and ranges, then perform sensitivity checks.
What drives investigation cost most?
Access constraints, required depth/diameter, groundwater management, traffic control, and lab testing intensity. Early coordination with the design team and contractor reduces re-mobilizations and change orders.
Conclusion
A well-planned Geotechnical Investigation is a decision-making tool, not just a set of logs. Start with a desk study, choose field and lab methods matched to risk and geology, treat groundwater realistically, and translate data into a defendable ground model with clear parameters and recommendations. When uncertainty remains, say so—and design monitoring and contingency plans to manage it.
Projects that invest in the right subsurface information deliver safer designs, fewer surprises, and better value. Use this outline to brief stakeholders, scope proposals, and audit deliverables so the ground you build on is as well understood as the structure you place upon it.