Every building, road, bridge, dam and pipeline ends up resting on the ground. Yet the ground is the one construction material that nobody manufactures, nobody can specify in advance and that changes from one metre to the next. A geotechnical investigation before construction replaces assumptions with evidence. It tells you what lies beneath the site, how strong and how compressible it is, where the groundwater sits and what could go wrong.
Skipping or under-scoping this work is one of the most expensive mistakes in construction. Cracked walls, differential settlement, flooded basements, failed retaining walls and redesigned foundations usually trace back to ground conditions that were never properly understood. This guide explains what a geotechnical investigation involves, what it delivers and how to get real value from it.
What Is a Geotechnical Investigation?
A geotechnical investigation, also called a site investigation or soil investigation, is the systematic study of subsurface conditions at a site. It combines desk research, site reconnaissance, drilling or excavation, in-situ testing, sampling, laboratory testing and engineering analysis. The output is a report with the data and recommendations that structural and civil designers need to design safe, economical foundations and earthworks.
In practical terms, the investigation answers questions such as:
- What soil and rock layers are present, and how thick are they?
- How strong is the ground, and how much will it settle under load?
- Where is the groundwater table, and does it fluctuate seasonally?
- Is the soil expansive, collapsible, corrosive, liquefiable or otherwise problematic?
- What foundation type is safest and most economical?
- Is the site stable, or are there slope, flood or fill hazards?
- What construction difficulties, such as excavation support or dewatering, should be expected?
Why It Matters So Much
Safety. Foundation failure can endanger lives. A proper investigation reduces the risk of collapse, excessive settlement and slope failure.
Cost control. Investigation typically costs a small fraction of total construction cost, commonly cited as roughly 0.5 to 1 percent for ordinary projects and more for complex sites. The cost of fixing foundation problems after construction can be many times higher. Good data also prevents over-design. Without it, engineers must be conservative, and conservatism costs money.
Better design. Foundation type, depth, bearing capacity, retaining wall design and pavement thickness all depend on ground data. The same building may need shallow footings on one site and piles on the next.
Fewer surprises. Unforeseen ground conditions are among the leading causes of construction delays and claims. Good investigation moves risk from the contractor’s and owner’s pockets into a plan.
Regulatory and financing requirements. Building authorities, lenders and insurers often require geotechnical reports, particularly for multi-storey buildings, industrial structures and infrastructure.
Kenyan and Regional Ground Conditions Worth Knowing
Ground conditions vary enormously across East Africa, and local experience matters.
- Black cotton soils: expansive clays found across large parts of Kenya, including areas around Nairobi, the Athi Plains, the lake basins and parts of the Rift Valley. They swell dramatically when wet and shrink when dry, cracking floors and walls if foundations are not designed for them.
- Residual lateritic soils and weathered volcanic rocks: common in the highlands, often variable in thickness and strength.
- Volcanic ash and tuff deposits: found around the Rift Valley, sometimes collapsible when saturated or loosely cemented.
- Alluvial and lacustrine deposits: soft, compressible clays and loose sands along rivers and lake shores, with shallow groundwater.
- Coastal coral limestone, sands and clays: variable cementation, voids and saline groundwater.
- Seismic activity: the Rift Valley is tectonically active, so earthquake loads and ground response may need to be considered.
- Steep and landslide-prone slopes: in highland and escarpment areas, particularly during heavy rains.
Local geology should shape both the investigation plan and its interpretation.
The Main Stages of an Investigation
A well-run investigation follows a logical progression. Each stage informs the next.
1. Define the project requirements. Establish what will be built, the expected loads, the building footprint, levels and sensitivity to settlement. A lightly loaded warehouse and a ten-storey tower need very different investigations.
2. Desk study. Review geological maps, previous investigation reports, aerial and satellite imagery, topographic data, flood and landslide records, utility information and the history of land use. This stage is cheap and often reveals hazards that guide the field work.
3. Site reconnaissance. An engineer or engineering geologist walks the site, noting topography, drainage, vegetation, existing structures and cracks, evidence of fill, slopes, springs and access constraints.
4. Plan the ground investigation. Decide on the number, depth and type of boreholes, trial pits and in-situ tests, along with sampling and laboratory testing programmes.
5. Fieldwork. Carry out drilling, test pitting, in-situ testing such as the standard penetration test or cone penetration test, groundwater measurement and sampling.
6. Laboratory testing. Test soil and rock samples to measure classification, strength, compressibility, swelling, permeability and chemical properties.
7. Analysis and reporting. Interpret the data, define the ground model, calculate bearing capacity and settlement, and prepare recommendations for design and construction.
The sequence from desk study to report is explained further in the next article in this series.
Common Investigation Methods
Trial pits. Excavated pits, typically 2 to 4 metres deep, allow direct observation of soil layers and collection of disturbed and block samples. They are quick, cheap and ideal for shallow foundations and for examining expansive soils.
Boreholes. Drilled holes provide information at depth. Common methods include:
- Cable percussion (shell and auger) drilling, widely used for soils and weak rock.
- Rotary drilling and coring, used for rock and hard materials, retrieving continuous cores.
- Auger drilling, quick and economical in soft to firm soils.
- Window sampling or dynamic sampling, used where access is restricted.
In-situ tests. These measure ground properties directly in the field.
- Standard penetration test (SPT): widely used to estimate soil density and strength.
- Cone penetration test (CPT): provides continuous profiles of resistance, helpful in soft soils.
- Dynamic probing: a quick, low-cost way to profile consistency with depth.
- Vane shear tests: measure the undrained strength of soft clays.
- Pressuremeter and plate load tests: measure stiffness and bearing behaviour.
- Permeability and packer tests: measure how easily water flows through soil or rock.
Geophysical methods. Electrical resistivity, seismic refraction and multichannel analysis of surface waves (MASW) help map bedrock depth, voids, groundwater and soil stiffness between boreholes. They complement but do not replace direct sampling.
Groundwater monitoring. Standpipes and piezometers record water levels over time, which is essential because levels fluctuate seasonally.
Laboratory tests. Typical tests include:
- Classification: moisture content, grading and Atterberg limits
- Strength: unconfined compression, triaxial and direct shear tests
- Compressibility: consolidation (oedometer) tests
- Swell and shrinkage: free swell and swell pressure for expansive soils
- Compaction: Proctor tests and California bearing ratio (CBR) for earthworks and pavements
- Chemical tests: pH, sulphates, chlorides and organic content to assess concrete and steel durability
How Many Boreholes, and How Deep?
There is no universal answer, but several principles guide the choice.
Number and spacing. Typical practice places boreholes at spacings of roughly 15 to 30 metres for buildings, closer for variable ground or heavy loads. Linear projects such as roads and pipelines use spacing along the route, often 50 to 200 metres depending on variability. Relevant codes and guidelines, such as Eurocode 7 and BS 5930, provide frameworks, and project specifications often add requirements.
Depth. The investigation should extend below the depth that will influence foundation behaviour, often called the zone of influence. A common rule is to drill to a depth of 1.5 to 3 times the foundation width below the founding level, or to competent rock or very dense or hard strata. Pile foundations need investigation well below the proposed pile toe.
Variability. If early boreholes show highly variable ground, add more. Saving money on boreholes at the start often leads to costlier surprises during construction.
Purpose. A high-rise tower, a bridge abutment and a single-storey house justify different intensities of investigation.
What the Report Should Contain
A professional geotechnical report generally includes:
- Project description and scope
- Site description, geology and desk study findings
- Fieldwork summary, with borehole locations and logs
- Laboratory test results
- Ground model, describing soil and rock layers and groundwater conditions
- Geotechnical parameters for design, such as unit weight, strength and stiffness
- Foundation recommendations, including type, founding depth, allowable bearing pressure and expected settlement
- Earthworks and retaining structure advice, where relevant
- Groundwater and drainage considerations
- Seismic considerations and liquefaction assessment where relevant
- Construction considerations, such as excavation support, dewatering and soil improvement
- Risks, limitations and recommendations for further investigation or monitoring
- Appendices with logs, test data and photographs
Insist on a report that gives clear recommendations, not just raw data. The most useful reports speak directly to the design problem.
Matching the Investigation to the Project
Residential and light commercial buildings. Trial pits, shallow boreholes with SPT, and basic laboratory testing for classification, strength and swell are often adequate, with special attention to black cotton soils.
Multi-storey buildings. Deeper boreholes with SPT or CPT, undisturbed sampling, consolidation testing, groundwater monitoring and often seismic assessment.
Industrial facilities. Investigation must address heavy and concentrated loads, machine foundations, floor slab behaviour and, sometimes, chemical contamination.
Roads and pavements. Test pits and shallow boreholes along the alignment, CBR testing, subgrade classification and material source investigation.
Bridges, dams and major infrastructure. Deep borehole programmes, rock coring, geophysics, permeability testing and detailed engineering geological mapping.
Slopes and landslide-prone areas. Investigation of failure surfaces, groundwater conditions, geological structure and monitoring instruments such as inclinometers and piezometers.
Choosing a Geotechnical Consultant
Quality varies, so choose carefully.
- Look for relevant experience in your region and ground type, not just general credentials.
- Check qualifications: a registered engineer or geologist should lead and sign off the work.
- Ask about equipment and laboratory accreditation. Reliable testing depends on calibrated equipment and quality-controlled laboratories.
- Request a clear scope listing the number and depth of boreholes, tests and report content.
- Beware of the cheapest quote. An under-scoped investigation saves little and risks a lot.
- Involve the consultant early, ideally before the building design is fixed, so findings can influence layout and structural system.
- Ensure site safety and environmental compliance, including utilities clearance, reinstatement and waste handling.
Common Mistakes
- Skipping the investigation or relying on neighbours’ experience.
- Too few boreholes or insufficient depth.
- Testing only in the dry season and missing seasonal groundwater and swelling behaviour.
- Ignoring expansive soils until cracks appear.
- Failing to share the structural loads with the geotechnical engineer.
- Treating the report as a formality instead of a design input.
- Not planning for variability between boreholes.
- Neglecting construction-stage verification, such as inspecting foundation excavations.
Conclusion
A geotechnical investigation turns unknown ground into a known quantity. It protects safety, improves design, controls cost and reduces the chance of delays and disputes. Match the scope to the project, benefit from local geological knowledge, choose a competent consultant and treat the report as a living design document. The modest cost of knowing what lies beneath your site is among the best investments you can make before breaking ground.
