Table of Contents
1. Introduction to Site Investigation
Soil investigation—also called geotechnical site investigation—is the systematic process of collecting subsurface information to characterize ground conditions for engineering design. It answers critical questions: what soil and rock types exist at the site, what are their engineering properties, where is the groundwater table, and are there any geological hazards such as sinkholes, landslides, or expansive soils?
An inadequate site investigation is one of the most common causes of foundation failures, cost overruns, and construction delays. The cost of a thorough investigation (typically 0.5-2% of total project cost) is negligible compared to the potential cost of unexpected ground conditions. Every civil engineering project—from residential buildings to dams and tunnels—requires a site investigation proportionate to the scale and complexity of the works.
The investigation follows a phased approach: desk study and site reconnaissance, preliminary investigation, detailed investigation (main fieldwork and laboratory testing), and supplementary investigation during construction as needed. The Proctor Compaction Calculator and Atterberg Limits Calculator support interpretation of lab test results.
2. Investigation Planning and Scope
The desk study collects existing information: geological maps, aerial photographs, previous site investigation reports, records from nearby construction, and topographic data. This informs the preliminary ground model and identifies potential issues. The site reconnaissance walkover confirms surface conditions, visible rock outcrops, drainage patterns, and access constraints.
The number and depth of boreholes depend on the project type and ground variability. Typical guidelines: one borehole per 200-500 m² for buildings, with depths extending at least 1.5 times the foundation width below footing level, or to a depth where stress increase is less than 10% of the applied pressure. For deep foundations, boreholes should extend below the pile tip by at least 3-5 pile diameters or through the full depth of compressible strata.
International standards for site investigation include ASTM D420, BS 5930, Eurocode 7 (EN 1997-1 and 1997-2), and IS 1892. These standards specify sampling frequency, in-situ testing methods, and quality assurance requirements. For seismic sites, investigation depth must extend to a stiff layer as defined by the shear wave velocity profile per ASCE 7 site class criteria.
3. Field Exploration Methods
The standard penetration test (SPT) is the most widely used in-situ test. A 63.5 kg hammer drops 760 mm to drive a split-spoon sampler 300 mm into the soil. The blow count (N-value) is the number of blows for the final 300 mm penetration. SPT N-values correlate with sand density (N < 4 very loose, 4-10 loose, 10-30 medium, 30-50 dense, >50 very dense) and clay consistency (N < 2 very soft, 2-4 soft, 4-8 medium, 8-15 stiff, 15-30 very stiff, >30 hard).
Cone penetration testing (CPT) uses a 60° cone with a 35.7 mm diameter pushed at 20 mm/s, measuring tip resistance qc and sleeve friction fs. CPT provides continuous profiles with high resolution, excellent for identifying thin layers and determining soil type via the friction ratio Rf = (fs/qc) × 100%. Piezocone (CPTu) adds pore pressure measurement for hydrostatic conditions and permeability estimation.
Geophysical methods supplement direct exploration. Seismic refraction measures P-wave and S-wave velocities to determine depth to bedrock and dynamic soil properties. Electrical resistivity surveys detect groundwater, voids, and contamination plumes. Ground-penetrating radar (GPR) maps shallow utilities, voids, and stratigraphy. Geophysical methods are non-invasive and cost-effective for large areas but require calibration with borehole data.
The Soil Permeability Calculator assists in analyzing field permeability test data from constant-head or falling-head tests conducted in boreholes.
4. Soil Sampling Techniques
Disturbed samples are collected from boreholes or test pits for soil classification, moisture content, and compaction tests. The SPT split-spoon provides disturbed samples suitable for index properties. Bulk disturbed samples (25-50 kg) are taken for compaction and fill quality testing. Undisturbed samples preserve the in-situ structure and void ratio for strength and compressibility testing. Thin-walled Shelby tube samplers (50-100 mm diameter) are pushed hydraulically for cohesive soils.
Sampling quality is classified by the degree of disturbance per ASTM D4220. Class I samples (undisturbed) are required for triaxial strength, consolidation, and permeability tests. Class II samples have some disturbance but preserve in-situ water content. Class III and IV are disturbed samples for classification only. Specific recovery techniques—piston samplers for soft clays, block sampling for stiff clays—minimize sample disturbance.
Sample handling, sealing, transportation, and storage significantly affect test quality. Samples must be sealed immediately with wax or plastic caps to prevent moisture loss, transported in rigid containers with cushioning, and stored in a temperature- and humidity-controlled environment. Testing should commence within 14 days for sensitive soils.
5. Laboratory Testing
Classification tests establish the basic identity of the soil: water content (ASTM D2216), Atterberg limits (liquid limit, plastic limit, plasticity index per ASTM D4318), specific gravity (ASTM D854), and particle size distribution by sieve and hydrometer (ASTM D422). The plasticity chart classifies fine-grained soils as low (CL, ML), intermediate (CI, MI), or high (CH, MH) plasticity. The unified soil classification system (USCS) per ASTM D2487 is the international standard.
Strength testing determines the shear strength parameters c and φ for bearing capacity and slope stability analysis. Unconfined compression tests (ASTM D2166) provide qu for cohesive soils. Unconsolidated-undrained (UU) triaxial tests (ASTM D2850) measure undrained shear strength. Consolidated-drained (CD) and consolidated-undrained (CU) triaxial tests with pore pressure measurement (ASTM D4767) determine effective stress strength parameters for long-term stability. Direct shear tests (ASTM D3080) are simpler but less versatile.
Compaction testing includes the Standard Proctor (ASTM D698, 600 kN·m/m³) and Modified Proctor (ASTM D1557, 2700 kN·m/m³) tests. These determine the maximum dry density (MDD) and optimum moisture content (OMC) for earthwork quality control. The California Bearing Ratio (CBR) test (ASTM D1883) measures subgrade strength for pavement design. Consolidation (oedometer) tests (ASTM D2435) determine compression index Cc, recompression index Cr, and preconsolidation pressure σ'p for settlement analysis.
The Atterberg Limits Calculator computes plasticity index and soil classification from lab test data, and the Proctor Compaction Calculator analyzes compaction test results.
6. Geotechnical Reporting
The geotechnical interpretive report (GIR) or ground investigation report communicates all findings to the design team. A well-structured report includes: project description and scope of investigation, site description and geology (with geological map and cross-sections), field exploration details (borehole logs with SPT N-values, sample depths, water levels), laboratory test results in tabulated and graphical form, engineering interpretation (soil profiles and geotechnical parameters), and foundation recommendations with allowable bearing pressures, settlement estimates, and construction considerations.
Borehole logs present a graphical record of each borehole with depth scale showing soil/rock types by USCS or ASTM symbols, SPT N-values, sample types and depths, water level observations, and laboratory test results. The factual report (raw data) and interpretive report (analysis and recommendations) may be separate documents. The report must be signed by a qualified professional engineer registered in the jurisdiction.
The Slope Stability Calculator and Soil Bearing Capacity Calculator are valuable tools for developing foundation recommendations from investigation data.
Typical Borehole Log
[SVG Diagram: Simplified borehole log showing a depth scale from 0 to 15 m. Soil profile columns: 0-2 m topsoil/fill (brown silty sand), 2-8 m medium dense sand (SPT N=12-25), 8-12 m stiff clay (SPT N=18-28), 12-15 m weathered rock (SPT N>50 refusal). Groundwater level at 3.5 m marked with a blue triangle symbol.]
7. Frequently Asked Questions
How many boreholes are needed for a building site?
Minimum one borehole for residential buildings on uniform sites, three to six for commercial buildings, and more for variable ground conditions or sensitive structures. Larger building footprints require additional boreholes—typically one per 200-500 m² with a minimum of three for any structure with deep foundations.
What is the difference between SPT and CPT?
SPT (Standard Penetration Test) is a hammer-driven test providing discrete N-values at 1.5 m intervals with a disturbed sample. CPT (Cone Penetration Test) is a continuous push test providing high-resolution profiles of tip resistance and sleeve friction. CPT is faster and more detailed but does not provide a physical soil sample.
What is a geotechnical baseline report?
A geotechnical baseline report (GBR) is a contractual document used in underground construction that establishes the anticipated subsurface conditions that contractors use for bidding. It differs from a GIR by defining specific baseline values for key parameters that form the basis for changed condition claims during construction.
When is geophysical testing preferred over boreholes?
Geophysical methods are preferred for large-area reconnaissance, depth to bedrock determination, detection of cavities or buried objects, and groundwater studies. They are non-invasive, faster, and lower cost per area than boreholes but require borehole calibration for quantitative parameter interpretation.
References & Standards
- BS 5930:2015. Code of Practice for Ground Investigations. British Standards Institution.
- ASTM D420-18. Standard Guide for Site Characterization for Engineering, Design, and Construction Purposes.
- IS 1892:2021. Code of Practice for Subsurface Investigation for Foundations.
- EN 1997-2:2007. Eurocode 7 — Geotechnical Design — Part 2: Ground Investigation and Testing.
- Clayton, C.R.I. et al. Site Investigation. 2nd ed., Blackwell Science, 1995.
- Civil Engineering Handbook — Geotechnical Engineering chapter.
- Engineering Formula Library — Geotechnical formulas.
- Engineering Glossary — Geotechnical and soil mechanics terms.