Table of Contents
1. Introduction to Geotechnical Reports
A geotechnical investigation report is the final deliverable of a subsurface exploration program. It synthesizes field observations, laboratory test results, and engineering analyses into a single document that advises on foundation design, earthwork, slope stability, and groundwater control. Every structural engineer, contractor, and project owner must be able to read and interpret these reports correctly.
The report translates complex soil behavior into actionable design parameters: bearing capacity, settlement estimates, lateral earth pressures, liquefaction potential, and soil stiffness. It is governed by standards such as ASTM D3740 (Standard Practice for Minimum Requirements for Agencies Engaged in Testing and/or Inspection of Soil and Rock) and Eurocode 7 (EN 1997 — Geotechnical Design), which define quality assurance and reporting formats.
A poorly interpreted geotechnical report is one of the most common root causes of foundation failures. Understanding the structure, terminology, and limitations of these reports is therefore essential for every civil engineer. The Civil Engineering Handbook and Geotechnical Engineering learning path provide complementary resources.
2. Phases of Site Investigation
A complete site investigation progresses through five distinct phases. Each phase informs the next, and skipping or compressing any phase introduces uncertainty that propagates into the foundation design.
Phase 1 — Desk Study
Review of existing topographic maps, geological maps, aerial photographs, historical records, previous site investigations, and local building codes. The desk study identifies anticipated soil types, groundwater conditions, natural hazards (faults, landslides, floodplains), and potential contaminants. It informs the scope of the field investigation and reduces the risk of unexpected subsurface conditions.
Phase 2 — Site Reconnaissance
A site walkover to observe surface conditions: existing structures, utilities, access roads, drainage patterns, visible rock outcrops, soil exposures, vegetation, and signs of instability such as cracks, slumping, or seepage. Photographs and notes are collected to refine the investigation plan and identify access constraints for drilling equipment.
Phase 3 — Field Investigation
Boring, sampling, and in-situ testing (SPT, CPT, vane shear, pressuremeter, geophysical surveys). The number, depth, and spacing of boreholes are determined by the project type and site variability. For a typical building, boreholes extend to a depth where the stress increase from the foundation is less than 10% of the overburden stress, or until competent bearing stratum is encountered.
Phase 4 — Laboratory Testing
Classification tests (water content, Atterberg limits, grain size distribution, specific gravity), strength tests (UU triaxial, CU triaxial, CD triaxial, unconfined compression, direct shear), consolidation tests (oedometer), and permeability tests (constant head, falling head). The test program is tailored to the soil types encountered and the design parameters required.
Phase 5 — Reporting
Synthesis of all findings into a geotechnical investigation report with boring logs, soil profiles, laboratory test results, engineering analyses, and foundation recommendations. The report concludes with design parameters, construction considerations, and references to applicable standards. See also our guide on soil investigation methods.
3. Boring and Sampling Methods
The quality of a geotechnical report depends directly on the boring and sampling methods used. Disturbed samples are adequate for classification tests, while undisturbed samples are required for strength and consolidation testing. The choice of method influences the reliability of every parameter in the report.
Standard Penetration Test (SPT) — ASTM D1586
The most widely used in-situ test worldwide. A 63.5 kg hammer is dropped 760 mm to drive a standard split-spoon sampler 450 mm into the soil. The blow count for the final 300 mm is recorded as the N-value. SPT provides both a soil sample (disturbed) and an index of soil density/consistency. Corrections for overburden pressure (N60, N1,60) are applied for advanced analyses. The SPT N-value is the single most widely correlated geotechnical parameter.
Cone Penetration Test (CPT)
A cone with a 60° apex and 10 cm² base area is pushed into the ground at 20 mm/s while measuring tip resistance (qc), sleeve friction (fs), and pore pressure (u). CPT provides continuous soil profiling with high resolution and is faster than SPT. Soil behavior type (SBT) charts (Robertson 1990) classify soils from the cone measurements. CPT does not retrieve a physical soil sample, so it is often paired with SPT boreholes for correlation and sample recovery.
Continuous Sampling and Rock Coring
Thin-walled tube samplers (Shelby tubes) are pushed hydraulically to retrieve undisturbed samples for laboratory testing. Piston samplers minimize disturbance in soft clays. Rock coring uses a diamond-impregnated core barrel (NX, HQ, PQ sizes) to recover intact rock cores. The Rock Quality Designation (RQD) — the percentage of sound core pieces longer than 100 mm — is reported to assess rock mass quality. RQD below 25% indicates very poor rock quality; above 90% is excellent.
Key Point: SPT and CPT results are complementary. SPT provides physical samples and direct N-values; CPT offers continuous profiling and better resolution in soft soils. Most comprehensive investigations use both methods. The Engineering Glossary contains definitions of all sampling terminology.
4. Laboratory Test Programs
Laboratory testing converts field samples into engineering design parameters. The required tests depend on the soil types encountered and the foundation system being designed. The table below summarizes typical laboratory test requirements by project type.
| Project Type | Required Tests | Key Parameters | Standard |
|---|---|---|---|
| High-Rise Building | CU triaxial, consolidation, SPT/CPT, rock coring | c', φ', E50, Cc, Cr | ASTM D4767, D2435 |
| Low-Rise Residential | Classification, UU triaxial, direct shear | Su, φ, LL, PL, PI | ASTM D2850, D3080 |
| Embankment / Dam | CD triaxial, compaction, permeability | c', φ'cv, OMC, MDD, k | ASTM D7181, D698 |
| Pavement / Highway | CBR, Proctor compaction, sieve analysis | CBR, MDD, OMC, D10, D60 | AASHTO T100, T180 |
| Bridge Foundation | Rock coring, point load, triaxial on rock | UCS, RQD, Em, φjoint | ASTM D5731, D7012 |
Classification testing per ASTM D2487 (USCS) and ASTM D4318 (Atterberg limits) is the starting point for all projects. The Atterberg Limits Calculator automates plasticity index and liquidity index computations. For compaction-sensitive projects, the Proctor Compaction Calculator determines optimum moisture content and maximum dry density.
Important: Laboratory test results are only as good as the samples they come from. Poor sampling, improper handling, delayed testing, or sample disturbance during transport can render test results meaningless. Always check the sample quality designation (SQD) and the sampling method when reviewing lab data. See our standards reference for detailed quality criteria.
5. Understanding SPT N-Values and Correlations
The SPT N-value is the blow count recorded during the Standard Penetration Test. It serves as the primary index for soil strength, density, and stiffness in most geotechnical reports. Raw N-values must be corrected for overburden pressure, hammer efficiency, borehole diameter, rod length, and sampler configuration to obtain N60 (corrected to 60% energy efficiency) and N1,60 (further corrected to a reference effective overburden of 100 kPa).
| N60 (blows/ft) | Relative Density (Sand) | Consistency (Clay) | Approx. qu (kPa) | Foundation Type |
|---|---|---|---|---|
| 0 – 4 | Very Loose | Very Soft | < 25 | Deep foundation required |
| 4 – 10 | Loose | Soft | 25 – 50 | Mat or deep foundation |
| 10 – 30 | Medium Dense | Firm / Stiff | 50 – 150 | Shallow footings possible |
| 30 – 50 | Dense | Very Stiff | 150 – 400 | Shallow footings |
| > 50 | Very Dense | Hard | > 400 | Shallow footings (preferred) |
Engineering Judgment Required: SPT N-value correlations should be used as preliminary estimates only. The actual bearing capacity and settlement behavior depend on soil fabric, stress history, groundwater conditions, and the specific foundation geometry. Always verify correlations with laboratory test data when available. See Soil Bearing Capacity Explained for detailed correlation methods.
Common correlations include undrained shear strength Su = k × N60 (with k ranging from 4 to 8 for clays), friction angle φ' from N1,60 using the Peck-Hanson-Thornburn or Kulhawy-Mayne relationships, and constrained modulus Es = α × N60 (where α is 0.3–0.5 MPa for sands and 0.2–0.3 MPa for clays). The Soil Bearing Capacity Calculator implements these correlations automatically.
6. Foundation Recommendations
The geotechnical report recommends a foundation system based on soil conditions, structural loads, and project constraints. The fundamental decision is between shallow foundations (isolated footings, combined footings, strip footings, mat/raft foundations) and deep foundations (driven piles, drilled shafts, micropiles, screw piles).
Shallow foundations are preferred when competent bearing strata exist within 3–5 m of the surface, with allowable bearing capacity typically in excess of 150 kPa. They are economical and straightforward to construct. The report specifies minimum footing width, embedment depth, and bearing stratum identification. The Footing Size Calculator sizes individual footings from allowable bearing capacity and column loads.
Deep foundations are recommended when surface soils are weak (N60 < 5), when high loads must be transferred to deeper strata, or when settlement of shallow foundations would exceed tolerable limits. The report specifies pile type, diameter, tip elevation, estimated capacity (skin friction + end bearing), and installation criteria (set criteria or PDA testing). Driven pile set criteria are often specified as blows per inch for a given hammer energy.
For intermediate cases, ground improvement (stone columns, deep soil mixing, preloading with vertical drains) may be recommended to enable shallow foundations. See Foundation Types and Selection Guide and Common Foundation Failures for further reading.
7. Soil Bearing Capacity and Settlement Estimates
Every geotechnical report provides the allowable bearing capacity (qa) and anticipated settlement for the recommended foundation system. Allowable bearing capacity is the ultimate bearing capacity (qult) divided by a factor of safety (typically 2.5–3.0 per Eurocode 7 and 3.0 per many national codes).
Ultimate bearing capacity is computed using Terzaghi's, Meyerhof's, Hansen's, or Vesic's bearing capacity equations, depending on the code and soil type. These equations account for soil cohesion, friction angle, footing width, embedment depth, and shape, depth, and inclination factors. For cohesive soils (undrained conditions), qult = 5.14 × Su (Prandtl's solution) with modifications for footing shape and depth.
Settlement estimates include immediate (elastic) settlement, primary consolidation settlement, and secondary compression (creep). Immediate settlement is computed using elastic theory with Young's modulus from SPT correlations. Consolidation settlement is computed from the oedometer test results (compression index Cc, recompression index Cr, preconsolidation pressure σp'). The Settlement Calculator computes both immediate and consolidation settlement from soil parameters.
Differential settlement between adjacent footings is typically limited to 1:300 to 1:500 of the span for framed structures and 1:200 for load-bearing walls. The report should compare predicted settlement to these acceptability criteria. The Consolidation Degree Calculator estimates the time required for consolidation settlement under a given drainage condition.
8. Liquefaction Potential Assessment
For projects in seismically active regions, the geotechnical report must assess liquefaction potential. Liquefaction occurs in saturated, loose to medium-dense sands and silts during earthquake shaking. The pore water pressure rises to equal the confining stress, and the soil loses its shear strength, behaving like a liquid.
The assessment follows the simplified procedure of Seed and Idriss (1971) updated by Youd et al. (2001) and the NCEER/NSF workshops. The cyclic stress ratio (CSR) induced by the earthquake is compared to the cyclic resistance ratio (CRR) of the soil. The factor of safety against liquefaction is: FSliq = CRR / CSR.
CRR is determined from corrected SPT N1,60 values or CPT tip resistance using established curves (e.g., Youd et al. 2001 for SPT; Robertson & Wride 1998 for CPT). Soils with N1,60 > 30 are generally considered non-liquefiable. The report presents liquefaction potential for the design earthquake (typically 2% probability of exceedance in 50 years) and may recommend mitigation measures such as ground densification, stone columns, deep soil mixing, or deep foundations extending below liquefiable layers.
The Standards Reference provides relevant seismic code provisions, and the Geotechnical Engineering learning path includes a module on liquefaction assessment.
9. Geotechnical Report Structure per ASTM D3740 and Eurocode 7
ASTM D3740 establishes minimum quality assurance requirements for geotechnical testing agencies, while Eurocode 7 (EN 1997-1 and EN 1997-2) defines the framework for geotechnical design and reporting in Europe. A compliant geotechnical investigation report includes the following sections:
- Project Information: Project name, location, owner, design team, date, report number, and revision status.
- Scope of Work: Objective, applicable standards (ASTM, Eurocode 7, IS 1892, BS 5930), and limitations of the investigation.
- Site Description and Geology: Regional geology, site topography, existing structures, and subsurface conditions based on desk study.
- Field Investigation: Boring locations (plan), drilling methods, sampling intervals, groundwater observations, in-situ test procedures and results.
- Laboratory Testing: Test methods, results summary tables, and detailed test reports (gradation curves, consolidation curves, stress-strain plots).
- Soil Profile and Stratigraphy: Boring logs, cross-sections, and description of each soil unit with USCS classification, consistency/density, and color.
- Engineering Analysis: Bearing capacity analysis, settlement estimates, lateral earth pressures, slope stability, liquefaction assessment (if applicable).
- Foundation Recommendations: Recommended foundation type, design parameters (qa, δmax, modulus of subgrade reaction ks), construction considerations, and special provisions.
- Construction Considerations: Dewatering requirements, excavation support, earthwork specifications, compaction criteria, seasonal limitations.
- References: Applicable codes, standards (ASTM D3740, D1586, D2487, D4318, Eurocode 7, IS 1892, BS 5930), and cited literature.
Additional appendices typically include boring location plans, detailed boring logs, laboratory test reports, grain size distribution curves, consolidation curves, and photograph logs. Eurocode 7 additionally requires a Geotechnical Design Report (GDR) that documents design assumptions, calculations, and compliance with limit state design principles.
10. Reading and Interpreting Boring Logs
The boring log is the most fundamental data sheet in a geotechnical report. It records every meter of drilling in a standardized graphical and tabular format. Understanding each column is essential for extracting design parameters.
A typical boring log contains: elevation and depth scale (left margin), USCS soil classification symbol and description (center), graphical column showing soil layers with standard hatching patterns (USCS patterns), SPT N-values at each test depth, groundwater observations (depth at time of drilling and after stabilization), sample type (SPT split-spoon, Shelby tube, core barrel), sample recovery and RQD, lab test indices (wn, LL, PL, PI, γ, Su), and field notes on drilling resistance, color changes, odors, and obstructions.
Key interpretation rules: ignore the seating drive (first 150 mm) of SPT, use the second increment blow count as the N-value; cross-check N-values with the soil description (a loose sand should not show N=40); note groundwater depth carefully — the water level in the borehole during drilling may not reflect the long-term equilibrium phreatic surface; look for trends in N-value with depth to identify stiff layers, bearing strata, and potential looser zones.
Boring logs should include a legend explaining all symbols, hatching patterns, and abbreviations used. The Engineering Glossary provides definitions for all common boring log symbols and abbreviations.
11. Soil Profile and Cross-Sections
Soil profile cross-sections are graphical representations of subsurface stratigraphy between boreholes. They show the lateral continuity of soil layers, the elevation of bearing strata, groundwater conditions, and variability across the site. Cross-sections are constructed by correlating soil units between adjacent boreholes using geological judgment.
When reading cross-sections, examine the elevation datum (typically mean sea level or project benchmark), the vertical and horizontal scales (vertical exaggeration is common, often 5× to 10×), the interpreted layer boundaries (dashed lines indicate inferred boundaries between boreholes), and the groundwater surface (phreatic surface, often shown as a blue dashed line with a piezometer symbol).
Pay special attention to pinch-outs (where a soil layer terminates between boreholes) and lenses (thin, discontinuous layers of different soil within a dominant stratum). These features are critical for foundation design — a sand lens within a clay stratum can provide a drainage path and accelerate consolidation, while a clay lens within sand can impede drainage and complicate dewatering.
If the cross-section shows significant variability between boreholes, the geotechnical engineer should recommend additional boreholes to reduce uncertainty. The Civil Engineering Handbook includes a guide to minimum boring spacing requirements by project type.
12. Common Pitfalls in Report Reading
Even experienced engineers can misinterpret geotechnical reports. The following common pitfalls can lead to incorrect foundation designs or costly change orders during construction.
1. Confusing Disturbed and Undisturbed Samples
Strength and consolidation tests require undisturbed samples (Shelby tubes, piston samplers). SPT split-spoon samples are disturbed and cannot provide reliable strength parameters. Check the sample type column on the boring log before relying on test results for design.
2. Ignoring Groundwater Effects
Groundwater reduces effective stress, which lowers bearing capacity and shear strength. Seasonal fluctuations can be significant — a single water level reading from a borehole may not represent the highest anticipated groundwater level. Always check for piezometer monitoring data and seasonal variation notes in the report.
3. Misapplying SPT Corrections
Using raw N-values without overburden correction for liquefaction or bearing capacity analysis can be unconservative. Ensure the report specifies whether N, N60, or N1,60 is being reported and which correlations were used. Apply corrections consistently throughout the analysis.
4. Overlooking Report Limitations
Geotechnical reports include a limitations section that defines the intended use, identifies data gaps, and restricts liability. Common limitations include: "subsurface conditions between boreholes are inferred," "groundwater observations are instantaneous," and "recommendations are preliminary pending further investigation." Ignoring these can lead to inappropriate use of the data.
5. Neglecting Construction Considerations
The report's construction recommendations (dewatering, shoring, temporary slopes, compaction specifications) are as important as the design parameters. Foundation designs that are technically correct but impossible to construct due to groundwater or access constraints will result in delays and cost overruns.
13. Case Study: Report Interpretation for a 5-Story Building
Project: 5-story office building with one basement level. Column loads up to 3,500 kN. Site in alluvial plain with variable soil conditions.
Boring Program: 6 boreholes to 25 m depth. SPT at 1.5 m intervals. Four Shelby tube samples collected from clay layers. Two piezometers installed.
Soil Profile (Typical BH-3): 0–2.5 m: Fill (silty sand, N = 6, loose). 2.5–8.0 m: Soft clay (CH, N = 3–5, Su = 25–35 kPa, wn = 45%, LL = 60, PI = 35). 8.0–12.0 m: Medium-dense silty sand (SM, N = 18–24). 12.0–18.0 m: Stiff clay (CL, N = 12–16, Su = 80–110 kPa, OCR = 2.5). 18.0–25.0 m: Dense sand (SP, N = 35–50). Groundwater at 3.5 m depth.
Analysis: The soft clay layer (2.5–8 m) with very low Su precludes shallow footings — excessive bearing stress would cause immediate punching failure and long-term consolidation settlement exceeding 100 mm. The medium-dense sand at 8–12 m provides moderate bearing capacity but consolidation of the underlying stiff clay (which is overconsolidated with OCR = 2.5) would contribute additional settlement. The dense sand at 18 m provides an excellent bearing stratum.
Recommendation: Bored cast-in-place piles (900 mm diameter) extending to 20 m depth, socketed at least 2 m into the dense sand. Estimated design capacity: 2,800 kN per pile (skin friction in sand and stiff clay + end bearing in dense sand). Anticipated settlement at working load: 12–18 mm. Mat foundation on improved ground was considered but rejected due to the 5.5 m of soft clay requiring extensive ground improvement (stone columns or deep soil mixing) that would be more expensive than piling.
Lesson: A single soft layer at shallow depth can dictate the entire foundation strategy. Reading the boring log top to bottom reveals the critical design constraint — in this case, the soft clay at 2.5–8 m. Always identify the weakest and most compressible layers first, then evaluate how the foundation will transfer loads through or past them. The Soil Bearing Capacity and Settlement Calculators were used to quantify the shallow foundation limitations.
14. Frequently Asked Questions
What is the difference between a geotechnical report and a soil investigation report?
These terms are often used interchangeably. A geotechnical report is the broader term that includes soil investigation data, laboratory test results, engineering analysis, and foundation recommendations. A soil investigation report may refer specifically to the factual data (boring logs, test results) without the interpretive engineering analysis.
How many boreholes are needed for a building foundation design?
Minimum requirements vary by code. Eurocode 7 recommends at least one borehole per 200–400 m² of building footprint. For a typical 20 m × 30 m building, 3–5 boreholes are common. The number should be sufficient to characterize the variability across the site, with at least one borehole extending below the zone of stress influence.
How deep should boreholes go?
Boreholes should extend to a depth where the net stress increase from the foundation is less than 10% of the effective overburden stress, typically 5–15 m below foundation level. They should also penetrate through any weak or compressible layers into competent bearing stratum. For deep foundations, boreholes should extend at least 5 m below the anticipated pile tip level or a minimum of two pile diameters into the bearing stratum.
What is the difference between N, N60, and N1,60?
N is the raw SPT blow count. N60 corrects for hammer energy efficiency (typically 30–80% for automatic vs. rope-and-cathead systems) to a reference 60% efficiency. N1,60 further corrects N60 for overburden pressure to a reference stress of 100 kPa, using the formula N1,60 = CN × N60 where CN = (100/σvo')0.5 (typically capped at 0.5–2.0). N1,60 is used for liquefaction assessment and relative density correlations.
Can I use SPT N-values directly for bearing capacity calculation?
Only as a preliminary estimate. For final design, bearing capacity should be verified using laboratory strength tests (triaxial or direct shear) on undisturbed samples. SPT-based correlations (e.g., Terzaghi and Peck, Bowles) provide conservative estimates suitable for preliminary sizing. Use the Soil Bearing Capacity Calculator with both SPT correlations and lab test inputs.
How do I check if the geotechnical report is compliant with ASTM D3740?
Verify that the testing agency has current ASTM D3740 accreditation, that all test methods reference the specific ASTM standard (not just "ASTM"), that sample handling and transport procedures are documented, that calibration records are available, and that the report includes a statement of limitations and a quality assurance/quality control section.
What is modulus of subgrade reaction (ks) and why is it important?
The modulus of subgrade reaction (ks) is the ratio of bearing pressure to settlement, expressed in kN/m³ or pci. It is used in mat/raft foundation design and soil-structure interaction analysis using the Winkler spring model. Typical values range from 10–30 MN/m³ for medium sands to 40–80 MN/m³ for dense sands and stiff clays. The value depends on footing size and shape — ks decreases with increasing footing width.
What should I do if the report recommendations do not match the structural design assumptions?
Coordinate directly with the geotechnical engineer of record. Common mismatches include: assumed bearing depth different from the competent stratum depth, column loads exceeding the recommended capacity, incompatible foundation systems (e.g., the structural team designed raft foundation while the report recommends piles). A design coordination meeting between structural and geotechnical teams at the 30% design stage prevents costly late-stage changes. Structural engineers should also consult Soil Bearing Capacity Explained and Foundation Types for alignment with geotechnical recommendations.
Related Calculators
Soil Bearing Capacity Calculator
Compute bearing capacity from SPT and lab data.
Settlement Calculator
Immediate and consolidation settlement analysis.
Footing Size Calculator
Size footings from bearing capacity and loads.
Consolidation Degree Calculator
Time-rate of consolidation analysis.
Atterberg Limits Calculator
Plasticity index and liquidity index.
Proctor Compaction Calculator
OMC and MDD from compaction tests.
Related Articles
References & Standards
- ASTM D3740. Standard Practice for Minimum Requirements for Agencies Engaged in Testing and/or Inspection of Soil and Rock as Used in Engineering Design and Construction. ASTM International.
- ASTM D1586. Standard Test Method for Standard Penetration Test (SPT) and Split-Barrel Sampling of Soils. ASTM International.
- ASTM D2487. Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System). ASTM International.
- ASTM D4318. Standard Test Methods for Liquid Limit, Plastic Limit, and Plasticity Index of Soils. ASTM International.
- AASHTO T100. Standard Method of Test for Specific Gravity of Soils. AASHTO.
- EN 1997-1:2004. Eurocode 7: Geotechnical Design — Part 1: General Rules. CEN.
- EN 1997-2:2007. Eurocode 7: Geotechnical Design — Part 2: Ground Investigation and Testing. CEN.
- IS 1892. Indian Standard Code of Practice for Subsurface Investigation for Foundations. BIS.
- BS 5930. Code of Practice for Ground Investigations. BSI.
- FHWA Geotechnical Engineering Circulars (GEC 1–7). Federal Highway Administration.
- Youd, T.L., et al. (2001). Liquefaction Resistance of Soils: Summary Report from the 1996 NCEER and 1998 NCEER/NSF Workshops. ASCE Journal of Geotechnical and Geoenvironmental Engineering.
- Seed, H.B. and Idriss, I.M. (1971). Simplified Procedure for Evaluating Soil Liquefaction Potential. ASCE Journal of the Soil Mechanics and Foundations Division.
- Civil Engineering Handbook — Geotechnical Engineering section.
- Engineering Standards Reference — Geotechnical codes and standards.
- Engineering Glossary — Geotechnical terms and definitions.