Site Characterization

What Is Site Characterization?

Site characterization is the end-to-end process of defining the subsurface conditions, groundwater regime, and geohazards that govern the performance of civil works. It transforms raw evidence—maps, borings, CPTs, lab tests, and monitoring—into a defensible ground model that supports design, construction, and long-term asset management. Done well, it reduces cost and schedule risk more than almost any other geotechnical activity.

This guide answers the questions practitioners ask most: What are the minimum datasets for foundations, slopes, retaining structures, and earthworks? How many borings and where? Which in-situ tests add the most value? How do you manage groundwater uncertainty? And how do you communicate findings so designers, contractors, and owners can make decisions quickly? We link to essentials like Soil Mechanics, Groundwater in Geotechnical Engineering, Site Characterization (this page), and downstream topics such as Foundation Design, Slope Stability, and Retaining Wall Design.

For stable, evergreen reference frameworks, see FHWA, USACE, and AASHTO.

A clear ground model—layers, properties, groundwater and hazards—is the #1 deliverable of site characterization.

Objectives, Questions & Scope Definition

The scope of characterization should be driven by decisions that must be made: foundation type and depth, earthwork balance, retaining solutions, drainage strategies, and construction risks. Turn questions into measurable objectives and acceptance criteria.

• Performance targets: Allowable settlements, factors of safety, deformation limits for adjacent assets.
• Key uncertainties: Layer continuity, soft/loose lenses, rock quality, seasonal groundwater, contamination.
• Required outputs: Stratigraphy sections, property tables (c′, φ′, γ, k, E), design water levels, hazard screening (e.g., Liquefaction, expansive soils, landslides).

Desktop Study & Data Mining

Start by harvesting publicly available data to bound expectations and target fieldwork. Typical sources include published geology and soils maps, historical aerials, LiDAR, well logs, floodplain maps, seismic hazards, and nearby project reports.

• Geology & geomorphology: Identify depositional environments, landslide deposits, karst, expansive clays, and rock types that control weathering.
• Hydrology: Watersheds, streams, FEMA flood zones, and recharge features that influence groundwater levels and flow directions.
• Historic development: Former fills, cuts, and demolished structures that affect variability.

Field Reconnaissance (Walk-Over)

A structured site walk validates desktop hypotheses, identifies hazards, and optimizes exploration locations. Log surface geomorphology, drainage paths, seepage zones, scarps/tension cracks, and man-made fills. Capture access constraints and environmental sensitivities that affect drill rig selection and safety.

• Features to document: Outcrops, springs, erosion, existing walls, utility corridors, and heavy loads near slopes.
• Rapid tests: Hand augers, DCP, pocket penetrometers, and moisture checks to inform first-pass stratigraphy.

Subsurface Exploration Strategy

The exploration plan balances data needs with access, safety, and budget. Combine borings with continuous sampling where variability is high and add CPT soundings to improve stratigraphic resolution in sands and soft clays. Adjust iteratively as results come in.

• Borings: Split-spoon sampling (SPT), Shelby tubes in cohesive soils, rock coring (RQD, recovery) for depth to competent material.
• CPT/CPTu: Continuous stratigraphy, tip resistance, sleeve friction, pore pressure; ideal for liquefaction screening and soft soil profiling.
• Test pits & trenches: Useful for near-surface variability and collecting block samples for strength/compaction trials.
• Geophysics: MASW/CSW for shear wave velocity (Vs), ERT for moisture contrasts, GPR for utilities and shallow features.

In-Situ Testing: Strength, Stiffness & Permeability

In-situ tests reduce sample disturbance biases and deliver continuous profiles. Select methods based on soil type, depth, and design decisions.

• SPT (N-value): Correlates to density/strength; apply energy corrections and overburden normalization.
• CPT/CPTu: Derive qc, fs, Bq; estimate Dr, OCR, undrained shear strength s_u, and hydraulic conductivity bands.
• Vane shear (VST): Direct s_u for sensitive clays; correct for rate and plasticity.
• Pressuremeter/DMT: Stiffness and strength for settlement-critical designs and lateral response.
• Permeability tests: Falling/rising head, packer tests in rock, slug tests in wells for dewatering and drainage design.

Laboratory Testing: Index & Performance

Lab programs quantify classification, strength, compressibility, and hydraulic behavior. Prioritize high-quality undisturbed samples where stiffness and settlement control performance.

• Index tests: Moisture, unit weight, Atterberg limits, particle size (Sieve Analysis), specific gravity; classify soils for engineering behavior.
• Strength: Consolidated undrained triaxial (CU w/ pore pressure), direct shear, unconfined compression for clays; derive c′, φ′, s_u.
• Compressibility: Oedometer/consolidation for c_v, M_v, preconsolidation stress; critical for embankments and mats.
• Hydraulic: Permeability (falling/constant head) and suctions for fine-grained soils.

Groundwater Characterization & Drainage Implications

Groundwater drives stability and serviceability. Characterize seasonal high/low water levels, artesian conditions, perched horizons, and anisotropy in hydraulic conductivity. Install standpipe piezometers or vibrating wire piezos early so trends inform design and construction staging.

• Dewatering & drawdown: Predict inflows and settlements; coordinate with Ground Improvement Techniques or cutoff walls if needed.
• Retaining systems: Design back drains, weep holes, and outlets—see Retaining Wall Design.
• Earthworks: Moisture conditioning and compaction windows—see Geotechnical Earthworks.

Refresh fundamentals in Groundwater in Geotechnical Engineering.

Building the Ground Model & Integrating Data

The ground model is a living, testable hypothesis of subsurface conditions. It integrates borings, CPTs, geophysics, lab results, and monitoring into 2D/3D layers with property ranges and groundwater surfaces. Maintain clear data provenance and version control.

• Data fusion: Cross-plot CPT qc vs. lab strengths; reconcile layer picks; map uncertainties spatially.
• Parameters for design: Provide characteristic values (mean or fractile) for c′, φ′, γ, k, stiffness moduli, and s_u with rationale.
• Compatibility: Prepare models for Geotechnical Modeling and Geotechnical Design Software (LE, FE, seepage).

Managing Uncertainty & Geotechnical Risk

All subsurface models are uncertain. Make uncertainty explicit, quantify ranges, and reduce it where the project is most sensitive. Use monitoring to validate assumptions and adapt.

• Sensitivity studies: Vary groundwater, strength, and stiffness; track impacts on settlements and factors of safety.
• Contingency planning: Define triggers from piezometers and inclinometers for staged works.
• Risk register: Document likelihood, consequence, and mitigation—see Geotechnical Risk Assessment.

Reporting, Figures & Communication

Great reports are concise, decision-oriented, and traceable. Summarize what matters, show how you know it, and state residual risks and recommended monitoring. Include boring/CPT logs, sections, lab summaries, property tables, design water levels, and model assumptions.

• Clarity: Provide a one-page executive summary and a “How to Use This Report” section for designers and contractors.
• Traceability: Link parameters to tests; show ranges and chosen design values with rationale.
• Next steps: Specify additional exploration/testing required during design-build or construction.

Structure deliverables per your agency’s frameworks—see FHWA, USACE, and incorporate our guidance on Geotechnical Reporting.

FAQs: Quick Answers on Site Characterization

How many borings do I need?

Use an initial grid or along-alignment spacing based on geology and structure spacing, then adapt as variability emerges. Add CPTs for continuity, and cluster explorations at load concentrations (e.g., columns, abutments).

What’s the most common cause of design surprises?

Unanticipated groundwater regimes. Install piezometers early and observe seasonality; revisit Groundwater assumptions during detailed design.

Which tests give the most “bang for buck”?

CPT/CPTu (continuous profiling), high-quality undisturbed tubes in compressible clays (for settlements), and targeted permeability tests where drainage/dewatering matters. For earthworks, index testing and Proctor/compaction trials (see Compaction Test).

How should I format parameters for designers?

Provide characteristic values with ranges, applicable stress states, drainage conditions, and load cases. Note any correlations or model-dependent values used in Geotechnical Design Software.

Where should I go next on this site?

Explore Geotechnical Investigation, Soil Consolidation, Geotechnical Earthworks, and Earth Retaining Structures.

Conclusion

Site characterization is more than drilling holes—it’s a disciplined process of forming, testing, and communicating a ground model that decision-makers can trust. Begin with a targeted desktop study and a purposeful field reconnaissance. Design an exploration and testing program that answers specific performance questions, prioritize high-value in-situ and lab tests, and monitor groundwater early. Integrate all data into a coherent model with property ranges, quantify uncertainty, and manage risk through sensitivity studies and monitoring triggers. Present findings clearly so structural, civil, and construction teams can act with confidence. For durable reference frameworks that rarely change, consult FHWA, USACE, and AASHTO. Continue your learning with our pages on Foundation Design, Slope Stability, Retaining Walls, and Geotechnical Modeling.