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.
Understanding Engineering: Geotechnical Site Investigation
Investigation starts before a drill rig arrives. Engineers study old maps, aerial photographs, previous building records, flood history, geology maps, and information from nearby projects. A site walkover can reveal clues such as cracks in pavements, tilted walls, ponds, drainage ditches, steep slopes, or made ground from earlier construction.
The planned building matters too. A light one-storey warehouse creates a different ground problem from a tall apartment block, a bridge, or a water tank.
Engineers use this early evidence to decide where to investigate and how deep to explore. They focus on places where the ground may change, including the edges of old quarries, filled basements, river deposits, and slopes.
A borehole log is more than a list of soil names. It records the depth of each layer, its colour, texture, smell, moisture, and condition. Soft clay may be squeezed easily, while dense sand resists penetration.
Some samples are disturbed during drilling and are useful for identifying material. Other samples are carefully taken in tubes so a laboratory can test their natural structure. This difference is important because remoulded clay can behave very differently from clay that has stayed undisturbed in the ground.
Engineers may use cone penetration tests to measure resistance continuously with depth. On soft clay, a vane test can estimate how much twisting force the soil resists. Each test has limits, so reliable conclusions come from several pieces of evidence rather than one number.
Water changes soil behaviour in ways that are easy to underestimate. Soil grains carry part of the load, while water in the pores pushes outward between them. If pore water pressure rises, the grains carry less load and the soil can lose strength.
This is why a slope can fail after long rainfall and why excavation beside a river needs careful planning. Pumping water from an excavation can create another problem. It may draw fine soil particles toward the excavation, cause settlement nearby, or lower water levels beneath neighbouring buildings.
Water readings need time because a freshly drilled hole may not show the true groundwater level immediately. Seasonal rain, tides, leaking pipes, and perched water above a clay layer can all produce misleading conditions.
The final engineering decision includes uncertainty. Ground conditions exist between boreholes, where no one has directly observed them. Engineers therefore allow for variation and specify checks during construction.
A contractor may expose a different layer at foundation level, find old rubble, or encounter water sooner than expected. The design may then need adjustment. Students should pay attention to the link between observations, measured properties, and decisions.
A soil description alone does not tell an engineer what to build. The key is how that soil responds to loading, water movement, excavation, and time. This reasoning appears in everyday places such as house extensions, retaining walls, road embankments, railway cuttings, and wind turbine foundations.
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.