Geotechnical Soil Testing

What Is Geotechnical Soil Testing?

Geotechnical soil testing is the collection of field and laboratory measurements used to characterize subsurface conditions for foundations, earthworks, pavements, and retaining structures. The right test plan converts unknown ground into a quantified ground model: stratigraphy, classification, groundwater regime, strength, stiffness, and permeability. Results inform design topics such as Bearing Capacity, Settlement Analysis, Slope Stability, and Retaining Wall Design.

This guide explains how to scope investigations, obtain high-quality samples, choose field and laboratory tests, and translate results into design parameters. For fundamentals, see Site Characterization and Soil Mechanics. For stable external references that rarely change, consult FHWA, USACE, and testing standards from ASTM International.

Great designs start with great measurements—specimen quality, groundwater control, and appropriate test selection matter more than test quantity.

Scoping an Effective Test Program

Align testing with project risk, ground variability, and the decisions to be made. Begin with desktop studies (geology, prior borings, utilities), then craft a phased plan that can adapt as the ground “talks back.”

  • Define decisions: Foundation type and depth? Excavation support? Earthwork moisture/compaction strategy? Permeability for dewatering?
  • Match test to soil behavior: Cohesive soils → strength/consolidation; granular soils → relative density/drainage; mixed profiles → index + targeted advanced tests.
  • Iterative approach: Start with reconnaissance borings/CPT, refine with focused sampling and lab suites.

Related Pages

Tie your plan to Geotechnical Risk Assessment and downstream Geotechnical Modeling.

Sampling & Specimen Quality

Sampling governs data reliability. Poor tube driving, trimming, or transport introduces disturbance—reducing measured strength and stiffness, particularly in soft clays and sensitive silts.

  • Disturbed samples: Split-spoon (SPT), bulk grab—adequate for index tests (classification, moisture, Atterberg Limits).
  • Undisturbed samples: Thin-walled Shelby tubes, piston samplers—required for consolidation and high-quality triaxial testing.
  • Care & handling: Seal tubes (wax/poly caps), keep cool, minimize vibration. Log recovery, tube length, and visible disturbance.
  • Groundwater: Install piezometers where needed; see Groundwater in Geotechnical Engineering.

Important

Don’t use disturbed clay samples for consolidation or undrained shear strength—specimen disturbance can lower results dramatically.

In-Situ Field Tests

Field tests map variability quickly and capture stress-history effects. Combine methods to cross-check parameters and reduce uncertainty.

  • SPT (Standard Penetration Test): N-values for sand density/corrosponding strength; obtain samples for index tests. Correct to energy and overburden.
  • CPT (Cone Penetration Test): Continuous profiles of tip resistance, sleeve friction, and pore pressure; excellent for stratigraphy, liquefaction screening, and modulus trends.
  • Vane shear: Undrained shear strength in soft clays; evaluate sensitivity with remolded readings.
  • Dilatometer (DMT): Intermediate soils; estimates of constrained modulus and K0 trends.
  • Pressuremeter/PMT: In-situ stress–strain curve; high-value for deformation-based design.
  • Plate load test (PLT): Bearing/deformation for shallow foundations and slabs-on-grade.
  • DCP & LWD: Quick compaction control and near-surface stiffness screening during earthworks.

SPT Energy/Overburden Correction (indicative)

\( N_{60} = N_{\text{meas}} \cdot C_E \cdot C_B \cdot C_S \cdot C_R \)
\(C_E\)Hammer energy ratio
\(C_B, C_S, C_R\)Borehole, sampler, rod length

Laboratory Tests: Index & Engineering Properties

Laboratory testing quantifies classification, compaction behavior, compressibility, shear strength, and durability. Choose tests to match the governing design limit states.

  • Index & classification: Sieve Analysis, hydrometer, natural moisture/Unit weight, Atterberg Limits, specific gravity.
  • Compaction & moisture density: Standard Proctor Test (MDD/OMC), Modified Proctor for heavy-duty fills.
  • Shear strength: Direct shear (drained), and triaxial suites—UU, CU w/ pore-pressure, and CD (see Triaxial Test).
  • Compressibility: Oedometer consolidation for settlement/time rate; recompression vs. virgin compression behavior.
  • Durability/chemical: Sulfates, pH, chlorides for material compatibility; slake durability for shales.

Did you know?

Even in granular soils, a few percent fines can dominate drainage and consolidation behavior—always test the fine fraction, not just the coarse sieve splits.

Hydraulic Testing & Permeability

Permeability governs seepage, dewatering, drainage, and contaminant transport. Match test method to expected k-range and specimen integrity.

  • Constant-head: For sands/gravels with higher k; maintain laminar flow and stable gradients.
  • Falling-head: For silts/clays with lower k; correct for temperature/viscosity.
  • Flexible-wall permeameter: Minimizes sidewall leakage in fine-grained soils and allows effective stress control.
  • Field slug/pumping tests: Aquifer parameters for dewatering design—coordinate with Groundwater.

Darcy’s Law (1-D)

\( q = k \, i \, A \)
\(k\)Coefficient of permeability
\(i\)Hydraulic gradient
\(A\)Flow area

For procedures and acceptance criteria, consult agency guidance at FHWA and test methods cataloged by ASTM International. Also see our primer on the Permeability Test.

Strength & Compressibility for Design

Strength parameters (c′, φ′, and undrained su) and compressibility (Cc, Cr, mv) drive foundation capacity and settlement. Choose drained vs. undrained conditions based on loading rate and drainage path.

  • Direct shear: Drained interface/soil friction; common for retaining wall backfill and geosynthetic interfaces (Geosynthetics).
  • Triaxial UU/CU/CD: UU for quick undrained strengths; CU with pore-pressure for total/effective interpretations; CD for drained sands or long-term clay behavior.
  • Oedometer consolidation: Determine preconsolidation stress and time rate for Soil Consolidation assessments.

Primary Consolidation (1-D, simplified)

\( S = \frac{C_c}{1+e_0} \, H \, \log \left( \frac{\sigma’_{0} + \Delta \sigma’}{\sigma’_{0}} \right) \)
\(C_c\)Compression index
\(e_0\)Initial void ratio
\(H\)Clay layer thickness

Use strength/settlement outputs directly in Deep Foundations, Shallow Foundations, and Earth Retaining Structures.

QA/QC & Data Management

Quality assurance ensures traceable, defensible results. Require calibration records, field/lab data sheets, and chain-of-custody. Digitize all logs and results for analysis and future reuse.

  • Field QA: Hammer energy calibration for SPT; CPT zero checks; accurate elevations and groundwater readings.
  • Lab QA: Repeat tests on duplicates, moisture equilibrium for clays, temperature-viscosity corrections.
  • Data systems: Centralize results and perform outlier checks—see Geotechnical Data Analysis.
  • Reporting: Capture methods, locations, results, and interpretation; refer to Geotechnical Reporting.

Did you know?

Most disagreements over soil parameters trace to specimen disturbance and undocumented corrections—write the assumptions directly into tables and figures.

From Test Results to Design Parameters

Convert raw measurements into the parameters needed by structural and civil teams. Use multiple lines of evidence, reconcile field vs. lab results, and document variability with ranges and characteristic values.

  • Sands/gravels: Use corrected SPT/CPT trends for φ′ and modulus; validate with plate load or DMT where settlement control is critical.
  • Clays/silts: Combine high-quality tube tests (UU/CU, consolidation) with vane/CPT profiles; identify overconsolidation and sensitivity.
  • Groundwater: Map seasonal highs; evaluate seepage/boiling risks for excavations and retaining structures.
  • Documentation: Summarize parameters in a design soil profile used consistently across Geotechnical Design Software.

Characteristic Value (concept)

\( X_k = \mu_X – k \, \sigma_X \quad (\text{choose } k \text{ per code/risk}) \)
\( \mu_X \)Mean parameter
\( \sigma_X \)Standard deviation

Where to Use the Numbers

Plug parameters into Bearing Capacity, Settlement Analysis, Slope Stability, and Liquefaction checks.

FAQs: Quick Answers on Geotechnical Soil Testing

How many borings or CPTs do I need?

Enough to capture variability and de-risk key decisions. Start with a grid or transects (e.g., one per structure line) and add tests where logs diverge or risk is high. Increase frequency in complex geology, fills, or soft ground.

Which lab tests are essential?

Minimum: classification (sieve/hydrometer), moisture/unit weight, and Atterberg Limits. Add Proctor for earthwork, consolidation for compressible clays, and triaxial/direct shear based on design limit states.

When do I choose drained vs. undrained strength?

Use undrained (UU/CU) for short-term loading of low-permeability clays and drained (CD/direct shear) for long-term or granular behavior where pore pressures dissipate.

How do I handle variable fill?

Expect heterogeneity. Use dense spacing of CPT/SPT, frequent sampling for classification, and treat design with conservative characteristic values. Consider ground improvement if variability threatens performance (see Ground Improvement Techniques).

What should I read next?

Explore adjacent topics: Site Characterization, Geotechnical Reporting, Geotechnical Data Analysis, and test-specific guides like Standard Proctor Test, Atterberg Limits, and Permeability Test.

Conclusion

Geotechnical soil testing turns uncertain ground into actionable parameters for design and construction. Start with a risk-based scope, obtain quality samples, and pair efficient field profiling (CPT/SPT) with targeted laboratory suites for classification, compaction, strength, compressibility, and permeability. Convert results into consistent design soil profiles and document assumptions and corrections transparently. Anchor methods to enduring references such as FHWA, USACE, and ASTM International. Continue your learning with our pages on Geotechnical Modeling, Geotechnical Design Software, and downstream applications including Deep Foundations and Shallow Foundations. With disciplined testing and interpretation, you’ll deliver safe, economical, and buildable designs.

Scroll to Top