A geotechnical site investigation is the process engineers use to learn what is below a proposed structure before design and construction begin. Soil and rock layers can change over short distances, so assuming the ground is uniform can lead to unsafe or expensive foundation choices. Field tests, boreholes, samples, and groundwater measurements help engineers predict settlement, bearing capacity, excavation behavior, and construction risks.
The result is a geotechnical report that connects subsurface evidence to practical design recommendations.
A typical investigation uses drill rigs to make boreholes, collect soil samples, run in-place tests such as the standard penetration test, and measure the water table. Laboratory tests then determine properties such as moisture content, grain size, density, shear strength, and compressibility. Engineers combine field logs, lab results, and site observations to create a subsurface profile and choose foundation types such as shallow footings, mats, piles, or drilled shafts.
Good geotechnical decisions reduce the chance of excessive settlement, slope failure, groundwater problems, and costly construction delays.
Key Facts
- Bearing stress under a footing is q = P/A, where P is load and A is footing area.
- A simplified allowable bearing check is q_applied ≤ q_allowable.
- Standard penetration test resistance is reported as N, the number of blows needed to drive the sampler the last 300 mm.
- Effective stress controls soil strength and is σ' = σ - u, where σ is total stress and u is pore water pressure.
- Below the water table, pore water pressure is often estimated by u = γw h, where γw is the unit weight of water and h is depth below the water table.
- Settlement risk increases when loads are placed on loose sand, soft clay, organic soil, or fill that was not properly compacted.
Vocabulary
- Borehole
- A borehole is a narrow drilled hole used to observe soil and rock layers and collect samples from different depths.
- Standard Penetration Test
- The standard penetration test is a field test that measures soil resistance by counting hammer blows needed to drive a split-spoon sampler.
- Water Table
- The water table is the depth below the ground surface where soil or rock openings are saturated with groundwater.
- Bearing Capacity
- Bearing capacity is the maximum pressure the ground can safely support without shear failure or unacceptable deformation.
- Geotechnical Report
- A geotechnical report summarizes subsurface conditions, test results, design parameters, and recommendations for foundations and earthwork.
Common Mistakes to Avoid
- Using only one borehole for a large or variable site is a mistake because soil layers can change laterally and a single location may miss weak zones or buried fill.
- Treating the SPT N value as a direct soil strength is a mistake because N must be interpreted with soil type, depth, equipment energy, and groundwater conditions.
- Ignoring the water table is a mistake because groundwater changes effective stress, excavation stability, uplift forces, and the difficulty of construction.
- Choosing a foundation before reviewing the geotechnical report is a mistake because footing size, pile depth, settlement limits, and drainage needs depend on measured subsurface conditions.
Practice Questions
- 1 A column load is 900 kN and the allowable soil bearing pressure is 150 kPa. What minimum square footing area is needed, and what is the side length of the footing?
- 2 The water table is 2 m below the ground surface. A soil point is 7 m below the ground surface. Using γw = 9.8 kN/m^3, estimate the pore water pressure at that point.
- 3 A site investigation finds loose sand from 0 to 4 m, soft clay from 4 to 12 m, and dense sand below 12 m, with groundwater at 1.5 m. Explain why deep foundations might be recommended instead of shallow spread footings.