A thorough site investigation is the foundation of every successful geotechnical design. Without reliable subsurface data, even the most sophisticated analysis can lead to unsafe or overly conservative solutions.

Stages of Site Investigation

A typical investigation progresses through several stages:

1. Desk study — review of geological maps, previous reports, aerial photography, and historical land use
2. Walkover survey — visual inspection of the site and surrounding area
3. Preliminary investigation — limited borings or probing to identify major strata
4. Detailed investigation — comprehensive borings, sampling, and in-situ testing
5. Laboratory testing — determination of index and engineering properties
6. Interpretation and reporting

Borings and Sampling

The most common method of subsurface exploration is the borehole. Techniques include:

• Auger boring
• Wash boring
• Rotary drilling
• Percussion drilling

Samples are classified as:

• Disturbed samples — useful for classification tests
• Undisturbed samples — required for strength and compressibility testing (e.g., thin-walled Shelby tubes, piston samplers)

Standard Penetration Test (SPT)

The SPT is one of the most widely used in-situ tests worldwide. A standard sampler is driven into the ground by a 63.5 kg hammer falling 760 mm. The number of blows required to advance the sampler 300 mm (after a seating drive) is the N-value.

Correlations exist between SPT N-values and relative density, undrained strength, and friction angle, although they should be used with caution and local experience.

Cone Penetration Test (CPT)

The CPT provides continuous profiles of tip resistance ($q_c$), sleeve friction ($f_s$), and often pore pressure ($u_2$). Advantages include:

• High resolution and continuous data
• Excellent for stratigraphic profiling
• Good correlations with strength and stiffness parameters
• Ability to detect thin layers

Piezocone (CPTu) testing is particularly valuable for evaluating soil type, undrained strength, and consolidation characteristics.

Other In-Situ Tests

• Pressuremeter test — measures in-situ stress-strain behavior
• Dilatometer test (DMT) — useful for settlement and lateral stress evaluation
• Vane shear test — measures undrained strength in soft clays
• Plate load test — direct measurement of bearing capacity and modulus

Geophysical Methods

Non-invasive geophysical techniques complement intrusive methods:

• Seismic refraction and reflection
• Electrical resistivity tomography (ERT)
• Ground-penetrating radar (GPR)
• Multichannel analysis of surface waves (MASW)

These methods are especially useful for detecting anomalies, mapping stratigraphy over large areas, and identifying groundwater conditions.

Best Practices

• Plan the investigation based on the proposed structure and expected ground conditions
• Combine multiple methods for a more complete picture
• Engage experienced geotechnical engineers in the interpretation of results
• Document groundwater levels carefully (both during and after drilling)
• Consider spatial variability and the need for additional investigation during construction

Closing Remarks

Investment in a high-quality site investigation almost always pays for itself through more economical and safer designs. In the next post, we will explore techniques used to improve poor ground conditions when the natural soil is inadequate.