Soil Sampling and Classification (SPT, Shelby Tube, Auger Sampling)
What is Soil Sampling and Classification?
Soil sampling and classification are fundamental components of geotechnical site investigations, helping engineers understand subsurface conditions to design safe and effective foundations, embankments, retaining structures, and other civil infrastructure. Three common in-situ sampling techniques are:
- SPT (Standard Penetration Test) sampling
- Shelby Tube sampling
- Auger sampling
Each method yields different types and quality of samples, suited to different soil conditions and design needs.
Standard Penetration Test (SPT) Sampling
SPT is both a sampling and in-situ testing method. It provides a disturbed soil sample and a measure of soil resistance (N-value) to dynamic driving, commonly used to estimate relative density and shear strength of soils.
Equipment:
- Split-spoon sampler
- 140-lb (63.5 kg) hammer
- 30-inch drop height
- Drill rod and rotary drilling rig
Procedure:
- Drill to the desired depth using a rotary or hollow-stem auger.
- Drive the split-spoon sampler into the soil using the hammer.
- Record the number of blows for each 6-inch increment of 18 inches.
- The SPT N-value is the sum of blows for the last 12 inches.
Key Outputs:
- Disturbed soil sample for classification
- N-value for estimating:
- Soil density or consistency
- Bearing capacity
- Liquefaction potential (in seismic zones)
Best For:
- Sandy, silty, and some clayey soils
- Preliminary site classification and stratigraphy
Shelby Tube Sampling (Thin-Walled Tube Sampling)
Shelby tube sampling is used to obtain undisturbed samples of cohesive soils (especially clays) for advanced laboratory testing. These samples preserve in-situ structure, void ratio, and moisture content.
Equipment:
- Shelby tube (thin-walled steel tube, usually 3” OD)
- Hydraulic or static push rig
- Push rod assembly
Procedure:
- Push the Shelby tube slowly and continuously into the soil at the target depth.
- Withdraw the tube carefully to avoid sample disturbance.
- Seal the ends for transport to the laboratory.
Key Outputs:
- Undisturbed soil sample
- Laboratory tests:
- Triaxial shear
- Consolidation
- Permeability
- Atterberg limits
- Moisture-density relationship
Best For:
- Soft to medium stiff clays and silts
- Embankment, foundation, and settlement analysis
Auger Sampling
Auger sampling uses hand or mechanical augers to recover disturbed samples of soil. It’s a quick and simple method used mainly for preliminary site assessments and soil profiling.
Types:
- Hand auger – manually rotated; suitable for shallow depths (~6 m)
- Power auger – mounted on rigs; deeper penetration
Procedure:
- Drill using continuous flight or bucket augers.
- Remove the auger and collect soil clinging to the flights or from cuttings.
- Classify samples visually or transport to lab for index testing.
Key Outputs:
- Disturbed samples for:
- Grain size analysis
- Plasticity tests (Atterberg limits)
- Moisture content
- Soil profile logs
Best For:
- Shallow investigations
- Non-engineered fills, silts, clays, and sands
- Preliminary environmental or geotechnical screening
Comparison Table
| Feature | SPT | Shelby Tube | Auger Sampling |
| Sample Type | Disturbed | Undisturbed | Disturbed |
| In-Situ Testing? | Yes (N-value) | No | No |
| Soil Types | Sands, silts, clays | Soft clays, silts | Clays, sands, non-cemented soils |
| Depth Capability | Moderate to deep | Moderate | Shallow (unless power auger used) |
| Lab Testing Suitability | Classification, some strength | Advanced strength, consolidation | Basic index tests |
| Common Use | Foundation and density evaluation | Settlement and shear strength analysis | Preliminary classification |
Soil Classification from Samples
After sampling, soils are classified based on:
- USCS (Unified Soil Classification System) or AASHTO system
- Visual-manual logging
- Lab test results (grain size, Atterberg limits, moisture content, density)
Classifications include:
- Gravel (GW, GP)
- Sand (SW, SP, SM, SC)
- Silt (ML, MH)
- Clay (CL, CH)
- Organic soils (OL, OH)
Summary
- SPT is best for assessing soil resistance and stratigraphy with disturbed samples.
- Shelby Tube is ideal for high-quality, undisturbed samples in soft soils for lab testing.
- Auger Sampling offers a low-cost, quick method for basic site classification in shallow soils.