Table of Contents
1. Introduction to Soil Testing
Soil testing is the foundation of geotechnical engineering. Every structure — from a small residential footing to a high-rise tower, bridge, dam, or highway embankment — relies on the soil beneath it for support. Soil tests provide the engineering parameters needed for foundation design, slope stability analysis, earth-retaining structure design, pavement design, and construction quality control.
Soil tests are broadly classified into field tests (performed at the site in the natural soil conditions) and laboratory tests (performed on soil samples recovered from the site and transported to a laboratory). Field tests provide in-situ soil properties without sample disturbance, while laboratory tests allow more controlled measurements under varying conditions. A complete site investigation program typically includes both field and laboratory testing.
The selection of appropriate tests depends on the project type, the governing design code, the soil conditions anticipated, and the level of geotechnical risk acceptable for the project. The ASTM standards provide the authoritative test protocols for both field and laboratory soil testing. The Geotechnical Engineering Learning Path offers a structured introduction to soil mechanics fundamentals.
2. Field Tests
Standard Penetration Test (SPT) — ASTM D1586: The most widely used in-situ test worldwide. A 63.5 kg hammer drops 760 mm to drive a standard split-spoon sampler 300 mm into the soil. The blow count N (blows per 300 mm) indicates soil density and strength. Corrections: N60 = N × CE × CB × CS × CR (hammer energy, borehole diameter, sampler type, rod length). For bearing capacity correlations, use N60. The SPT is suitable for all soil types but provides only intermittent data at 1.5 m depth intervals.
Cone Penetration Test (CPT) — ASTM D5778: A cone (10 cm² base area, 60° apex) is pushed into the ground at a constant rate of 20 mm/s. Continuous measurements of tip resistance qc, sleeve friction fs, and pore pressure u2 provide a detailed soil profile. CPT is faster than SPT, provides continuous data, and is excellent for identifying thin soil layers. Soil behavior type (SBT) is classified from qc and friction ratio Rf = fs/qc. Limitations: difficult in gravelly soils and very dense sands.
Plate Load Test (PLT) — ASTM D1194: A steel plate (300-750 mm diameter) is loaded in increments, and the load-settlement curve is measured. PLT directly determines bearing capacity and modulus of subgrade reaction. For sands, the bearing capacity for a full footing is estimated from plate test results using size correction: qf = qp(Bf/Bp), where Bp is the plate width. For clays, no size correction is needed. PLT is expensive but provides the most reliable bearing capacity data.
Vane Shear Test (VST) — ASTM D2573: A four-bladed vane is pushed into soft clay and rotated at a standard rate (0.1 deg/s). The peak torque is converted to undrained shear strength su = T / K, where K is the vane constant. VST is ideal for soft to firm clays where undisturbed sampling is difficult. Corrections for anisotropy and strain rate may be applied per Bjerrum's correction factors.
Dynamic Cone Penetrometer (DCP) — ASTM D6951: A lightweight cone (20 mm diameter, 60° apex) driven by an 8 kg hammer dropping 575 mm. The penetration rate (mm per blow) correlates with CBR for pavement design. DCP is quick, portable, and ideal for compaction control of earthworks and subgrade evaluation. The Soil Bearing Capacity Calculator accepts SPT N-values and CPT qc values for bearing capacity estimation.
3. Laboratory Classification Tests
Atterberg Limits — ASTM D4318: The liquid limit (LL), plastic limit (PL), and shrinkage limit define the consistency states of fine-grained soils. The plasticity index (PI = LL - PL) is a key indicator of clay behavior: low PI (less than 15) indicates low compressibility clay (CL), high PI (greater than 35) indicates high compressibility clay (CH). The liquidity index (LI = w - PL / PI) indicates the natural consistency relative to the plastic range. Soil classification per USCS uses Atterberg limits and grain-size distribution.
Grain Size Distribution — ASTM D6913 (sieve) and ASTM D7928 (hydrometer): Sieve analysis determines the percentage of soil passing various sieve sizes (75 mm to 0.075 mm). Hydrometer analysis extends the distribution into the silt and clay range (below 0.075 mm). The coefficient of uniformity Cu = D60/D10 and coefficient of curvature Cc = D30²/(D60×D10) classify well-graded (Cu ≥ 6 for sands, Cu ≥ 4 for gravels) versus poorly graded soils. The Sieve Analysis Calculator automates gradation analysis.
Specific Gravity — ASTM D854: The specific gravity of soil solids Gs is needed for consolidation calculations, compaction control, and phase relationship computations. Typical values: 2.65-2.68 for sands, 2.70-2.80 for clays. The test uses a pycnometer or volumetric flask with de-aired water. Specific gravity affects the calculation of void ratio, degree of saturation, and dry unit weight in compaction control.
4. Shear Strength Tests
Triaxial Compression Test — ASTM D7181: The most versatile shear strength test. A cylindrical soil specimen (typically 38-100 mm diameter, 2:1 height:diameter) is confined under cell pressure and loaded axially until failure. Three standard test types: (a) Unconsolidated-Undrained (UU) — no drainage during confining or shearing; gives undrained shear strength su = (σ1 - σ3)f/2; (b) Consolidated-Undrained (CU) — drainage during consolidation, No drainage during shear; measures effective stress parameters c' and φ'; (c) Consolidated-Drained (CD) — full drainage throughout; gives drained strength parameters. The Mohr-Coulomb failure criterion is τf = c' + σ' tan φ'.
Direct Shear Test — ASTM D3080: A soil sample (typically 60 mm square) is sheared along a horizontal plane under a constant normal load. The test is simple, quick, and cost-effective, but the failure plane is forced to a horizontal plane (may not be the weakest plane). It is suitable for sands and non-cohesive soils. The results give c and φ directly from the Mohr-Coulomb envelope. Limitations: drainage conditions are not fully controlled; stress concentrations at shear box edges.
Unconfined Compression Test (UCS) — ASTM D2166: A cylindrical clay specimen is loaded axially without confining pressure. The unconfined compressive strength qu = Pmax/A is used to estimate undrained shear strength su = qu/2. The test is applicable only for cohesive soils that can stand unsupported (typically for qu > 25 kPa). Sensitivity St = qu(undisturbed) / qu(remolded) measures the strength loss on remolding; sensitive clays have St > 4. The Soil Bearing Capacity Calculator uses shear strength parameters for bearing capacity computation.
5. Consolidation and Permeability Tests
One-Dimensional Consolidation Test (Oedometer) — ASTM D2435: A soil specimen (50-75 mm diameter, 20 mm thick) is loaded incrementally in an oedometer ring with double drainage. Each load increment is maintained for 24 hours (or until primary consolidation is complete). Results define: (a) preconsolidation pressure σ'p from the Casagrande construction — the maximum past pressure that distinguishes overconsolidated from normally consolidated clay; (b) compression index Cc = Δe / Δlog σ'v (0.2-0.5 typical for clays); (c) swell index Cs = 0.1 to 0.2 Cc; (d) coefficient of consolidation cv = T50 × H²dr / t50 used for settlement rate predictions.
Consolidation parameters for settlement calculation: Primary consolidation settlement Sc = Cc × H₀ / (1 + e₀) × log[(σ'₀ + Δσ) / σ'₀] for normally consolidated clay. For overconsolidated clays, use Cs in the recompression range. The coefficient of secondary compression Cαe governs creep settlement after primary consolidation and is important for soft clays and peats.
Permeability Tests: The coefficient of permeability k is measured using: (a) Constant Head Test (ASTM D2434) — suitable for coarse-grained soils (k > 10⁻⁴ cm/s); (b) Falling Head Test — suitable for fine-grained soils (k < 10⁻⁴ cm/s); (c) In-situ permeability tests — packer tests in rock boreholes, slug tests in wells, and pumping tests for aquifer characterization. Typical k values: clean gravel 1-10 cm/s, clean sand 10⁻²-10⁻³ cm/s, silt 10⁻⁴-10⁻⁶ cm/s, clay less than 10⁻⁶ cm/s. The Soil Permeability Calculator automates constant and falling head computations.
6. Compaction Tests
Standard Proctor Test — ASTM D698: Soil is compacted in a 101.6 mm diameter mold (944 cm³) in three layers, each layer receiving 25 blows from a 2.5 kg hammer falling 305 mm. The test establishes the maximum dry density (MDD) and optimum moisture content (OMC) relationship. Typical MDD for silty clay: 1.6-1.8 g/cm³ at OMC of 14-20%. For sandy soils: MDD 1.8-2.0 g/cm³ at OMC 10-14%.
Modified Proctor Test — ASTM D1557: A higher compactive effort: 4.5 kg hammer, 457 mm drop, five layers, 25 blows per layer. This simulates heavier compaction equipment (sheepsfoot rollers on thick lifts). The MDD is 5-15% higher and OMC 3-8% lower than Standard Proctor for the same soil. The modified test is used for highway embankments and where higher density is required.
Compaction specifications require achieving at least 95% of MDD (Standard Proctor) for general fill and 98-100% for structural backfill and pavement subgrade. Field compaction is verified by sand cone test (ASTM D1556), nuclear density gauge (ASTM D6938), or rubber balloon method (ASTM D2167). The Proctor Compaction Calculator generates complete compaction curves and checks field density requirements.
7. Test Methods Reference Table
| Test | ASTM Standard | Parameter | Typical Application |
|---|---|---|---|
| SPT | D1586 | N-value, φ, density | Bearing capacity, liquefaction |
| CPT | D5778 | qc, fs, u2 | Stratigraphy, pile design |
| Plate Load | D1194 | qa, modulus of subgrade | Direct bearing capacity |
| Vane Shear | D2573 | su (undrained) | Soft clay strength |
| DCP | D6951 | Penetration rate, CBR | Pavement design, compaction QC |
| Atterberg Limits | D4318 | LL, PL, PI | Soil classification |
| Sieve Analysis | D6913 | Gradation, Cu, Cc | Soil classification |
| Triaxial (UU) | D7181 | su, φu | Short-term stability |
| Triaxial (CU/CD) | D7181 | c', φ' | Long-term stability |
| Direct Shear | D3080 | c, φ | Quick strength estimate |
| UCS | D2166 | qu | Clay strength, sensitivity |
| Consolidation | D2435 | Cc, Cs, cv, σ'p | Settlement analysis |
| Permeability (Constant) | D2434 | k | Drainage, seepage |
| Standard Proctor | D698 | MDD, OMC | General fill compaction |
| Modified Proctor | D1557 | MDD, OMC (high effort) | Highway embankments |
Typical Soil Properties by Type
| Soil Type | SPT N | φ' (deg) | su (kPa) | k (cm/s) |
|---|---|---|---|---|
| Clean Gravel | 40-60+ | 38-42 | N/A | 1 to 10 |
| Medium Sand | 15-30 | 32-36 | N/A | 10⁻² to 10⁻³ |
| Fine Sand / Silt | 5-15 | 28-32 | N/A | 10⁻⁴ to 10⁻⁶ |
| Soft Clay | 2-4 | N/A | 12-25 | less than 10⁻⁶ |
| Stiff Clay | 10-20 | N/A | 50-100 | less than 10⁻⁷ |
8. Worked Example: SPT N-value Interpretation for Bearing Capacity
Interpret SPT Results for a Shallow Foundation in Sandy Soil
Given: SPT was performed at 1.0 m intervals in a sand deposit. The borehole log shows: Depth 1.5 m — N = 8 (loose), Depth 3.0 m — N = 14 (medium), Depth 4.5 m — N = 22 (medium-dense), Depth 6.0 m — N = 35 (dense). Foundation depth Df = 1.5 m. Groundwater at 6.0 m depth. Footing width B = 2.0 m (proposed).
Step 1 — Correct N-values to N60. Assume: hammer efficiency CE = 0.60 (safety hammer), borehole CB = 1.0, sampler CS = 1.0 (standard sampler with liner), rod length CR = 0.75 (for 3-4 m rods). N60 = N × 0.60 × 1.0 × 1.0 × 0.75. For N = 14 at 3.0 m: N60 = 14 × 0.45 = 6.3. For N = 22 at 4.5 m: N60 = 22 × 0.45 = 9.9. The corrected values are lower, indicating looser conditions than raw N suggests.
Step 2 — Estimate φ from N60 (Meyerhof correlation). φ = 27.1 + 0.3N60 - 0.00054N60². At 3.0 m: φ = 27.1 + 0.3(6.3) - 0.00054(6.3)² = 27.1 + 1.89 - 0.02 = 28.97° ≈ 29°. At 4.5 m: φ = 27.1 + 0.3(9.9) - 0.00054(9.9)² = 27.1 + 2.97 - 0.05 = 30.02° ≈ 30°.
Step 3 — Estimate allowable bearing pressure. For shallow footing on sand, using Terzaghi with FOS = 3. For φ = 30°, average within 2B = 4 m below base: Nc = 37.2, Nq = 22.5, Nγ = 19.7. Use conservative estimates: γ = 18 kN/m³ (moist above water table). qu = 1.3cNc + γDfNq + 0.4γBNγ = 0 + 18(1.5)(22.5) + 0.4(18)(2.0)(19.7) = 607.5 + 283.7 = 891.2 kPa. qa = 891.2 / 3.0 = 297 kPa.
Step 4 — Use direct SPT-bearing capacity correlation. For sand: qa = 12N60 (kPa) for B up to 1.2 m. For B = 2.0 m, reduce: qa = 8N60. Use weighted average N60 over depth 2B = 4 m below base: (6.3 + 9.9) / 2 = 8.1. qa = 8 × 8.1 = 64.8 kPa. This is much lower than the 297 kPa from Terzaghi because the SPT correlation includes settlement control (25 mm settlement limit typically governs for footings on sand).
Step 5 — Determine governing bearing capacity. Terzaghi bearing capacity: 297 kPa (shear failure). SPT settlement correlation: 65 kPa (settlement control). The governing value is 65 kPa — settlement will govern the footing design. Required footing area for a column load of 800 kN: A = 800 / 65 = 12.3 m² → B = 3.5 m. Verify using the Soil Bearing Capacity Calculator and Footing Size Calculator.
When Each Test Is Required
Subdivision / Residential: Hand augers + limited SPT for bearing capacity. Atterberg limits for shrink-swell assessment. Standard Proctor for fill compaction control.
Commercial / Industrial Buildings: Full SPT or CPT program to 3× footing width depth. Plate load test for critical footings. Triaxial CU for long-term stability of retaining walls. Consolidation test for slab-on-grade settlement.
Highways / Pavements: DCP for subgrade CBR. Modified Proctor for embankment compaction. Permeability for drainage layer design. CBR test (ASTM D1883) for pavement thickness design.
Dams / Levees: Triaxial CD for long-term stability. Consolidation for settlement. Permeability for seepage analysis. SPT for liquefaction assessment in seismic zones.
Deep Foundations / Piles: CPT preferred for pile design (continuous qc profile). SPT with N60 for shaft and base resistance correlations. Triaxial CU for side friction estimates in clay.
Best Practices for Soil Testing Programs
- Plan the test program based on the project type, anticipated soil variability, and the design parameters needed — avoid over-testing or under-testing.
- Use multiple test methods for cross-verification: SPT + CPT + lab tests provide a more reliable soil model than any single method.
- Engage a certified geotechnical testing laboratory (ASTM E329 or equivalent) for all laboratory testing.
- Document all test procedures and corrections — raw and corrected N-values must be reported separately.
- Reference applicable codes: Eurocode 7 (EN 1997-2 for ground investigation), ASTM standards for test methods, and local building codes for specific requirements.
- Use the Proctor Compaction Calculator, Soil Permeability Calculator, and Consolidation Degree Calculator for efficient data reduction.
9. Frequently Asked Questions
What is the difference between SPT and CPT?
SPT uses a driven split-spoon sampler providing discrete blow counts at intervals; CPT uses a pushed electronic cone providing continuous tip and sleeve resistance profiles. CPT gives higher resolution data but does not provide a soil sample. SPT gives actual soil samples for lab testing.
What is the N-value correction for SPT?
The raw N-value is corrected to N60 = N × CE × CB × CS × CR, where CE (hammer energy ratio 0.45-1.0), CB (borehole diameter 1.0-1.15), CS (sampler type 1.0-1.2), CR (rod length 0.75-1.0). The corrected N60 should be used for all correlations.
What is the Casagrande construction used for?
The Casagrande graphical method estimates the preconsolidation pressure σ'p from the oedometer consolidation curve. The overconsolidation ratio (OCR = σ'p / σ'₀) determines whether the clay is normally consolidated (OCR = 1), overconsolidated (OCR > 1), or underconsolidated (OCR < 1).
What is the optimum moisture content in compaction?
OMC is the water content at which the soil achieves its maximum dry density (MDD) under a given compactive effort. It represents the moisture level at which soil particles can be rearranged into the densest packing. Field specifications typically require compaction at OMC ± 2%.
When is a triaxial CU test preferred over UU?
CU tests are preferred when effective stress parameters (c', φ') are needed for long-term stability analysis (slopes, retaining walls, foundations under drained loading). UU tests give undrained strength su for short-term (end-of-construction) stability conditions.
What is the coefficient of consolidation cv?
cv is the rate at which excess pore water pressure dissipates during consolidation. It is determined from the oedometer test using the log-time (Casagrande) or square-root-time (Taylor) method. Typical cv: 10⁻³ to 10⁻¹ m²/year for clays.
What is the difference between Standard and Modified Proctor?
Standard Proctor (ASTM D698) uses a 2.5 kg hammer, 305 mm drop, 3 layers, 25 blows/layer. Modified Proctor (ASTM D1557) uses a 4.5 kg hammer, 457 mm drop, 5 layers, 25 blows/layer. Modified effort gives 5-15% higher MDD and 3-8% lower OMC, simulating heavier compaction equipment.
How do I choose between constant head and falling head permeability tests?
Constant head (ASTM D2434) is used for high-permeability soils (k > 10⁻⁴ cm/s), such as gravels and sands. Falling head is used for low-permeability soils (k < 10⁻⁴ cm/s), such as silts and clays, where the flow rate is too low for accurate constant head measurement.
What is the sensitivity of clay?
Sensitivity St = su(undisturbed) / su(remolded). It measures the percentage of strength loss when clay structure is destroyed by remolding. Insensitive (St < 2), medium sensitive (St 2-4), sensitive (St 4-8), quick clays (St > 16). Quick clays can lose almost all strength when disturbed.
What is the plasticity index and why is it important?
PI = LL - PL. It indicates the range of water content over which the soil behaves plastically. High PI clays (CH, MH) are more compressible, have higher shrink-swell potential, and lower hydraulic conductivity than low PI clays (CL, ML). USCS classification uses PI and LL for fine-grained soil classification.
Related Calculators
Soil Bearing Capacity Calculator
Bearing capacity via Terzaghi, Meyerhof, Hansen, Vesic.
Proctor Compaction Calculator
MDD, OMC, and field density check.
Soil Permeability Calculator
Constant and falling head computations.
Consolidation Degree Calculator
Time-rate of consolidation analysis.
Atterberg Limits Calculator
Liquid and plastic limit determination.
Sieve Analysis Calculator
Gradation, Cu, Cc, and FM computation.
References & Standards
- ASTM D1586. Standard Test Method for Standard Penetration Test (SPT).
- ASTM D5778. Standard Test Method for Electronic Friction Cone Penetration Test (CPT).
- ASTM D4318. Standard Test Methods for Liquid Limit, Plastic Limit, and Plasticity Index of Soils.
- ASTM D698 / D1557. Standard Test Methods for Laboratory Compaction Characteristics of Soil.
- ASTM D2435. Standard Test Methods for One-Dimensional Consolidation Properties of Soils.
- ASTM D7181. Standard Test Method for Consolidated Drained Triaxial Compression Test.
- Bowles, J.E. Foundation Analysis and Design. 5th ed., McGraw-Hill, 1996.
- Holtz, R.D., Kovacs, W.D., and Sheahan, T.C. An Introduction to Geotechnical Engineering. 2nd ed., Pearson, 2011.
- EN 1997-2:2007. Eurocode 7: Geotechnical Design — Part 2: Ground Investigation and Testing.
- Civil Engineering Handbook — Geotechnical Testing chapter.
- Engineering Formula Library — Geotechnical formulas.
- Engineering Glossary — Geotechnical terms.