When surface observations and shallow pits are not enough, geotechnical drilling provides the direct evidence of what lies deep below a site. Boreholes allow engineers to see, sample and test soil and rock at depth, and rock coring recovers cylindrical samples of bedrock that reveal its strength, fractures and weathering. Geotechnical drilling in Kenya supports projects from high-rise buildings and bridges to dams, tunnels, towers and mines. This guide explains the main drilling methods, what rock coring involves and how to get reliable results.

Why Drill?

Drilling is used when engineers need information that cannot be obtained at the surface or by shallow methods. Typical objectives include:

• Determining the sequence and thickness of soil and rock layers.
• Finding the depth to bedrock or competent bearing layers.
• Obtaining samples for laboratory testing.
• Carrying out in-situ tests such as SPT, permeability and pressuremeter tests.
• Measuring and monitoring groundwater.
• Characterising rock quality for foundations, excavations, tunnels and slopes.
• Detecting voids, weak zones or faults.

Common Geotechnical Drilling Methods

Hand auger drilling. Suitable for shallow depths in soft soils. It is low-cost but limited by depth and ground conditions.

Shell and auger (light percussion) drilling. A traditional method for soils, using a cable-operated rig with a shell or clay cutter. Common for shallower investigations and where access is limited.

Rotary drilling with wash boring or mud flush. A rotating bit advances the hole while drilling fluid carries cuttings to the surface and stabilises the borehole. Widely used for deeper soil boreholes combined with SPT.

Continuous flight auger. A helical auger advances the hole and brings soil to the surface, useful in cohesive soils above the water table.

Rotary core drilling. A hollow diamond or tungsten carbide bit cuts a cylindrical core of rock, which is retrieved in a core barrel. This is the standard method for sampling rock and for investigating hard layers.

Reverse circulation and percussion drilling. Produce rock chips rather than intact core and are faster for depth exploration, but give less detailed structural information.

Sonic and specialist methods. Used in some projects where sample quality or difficult ground demands alternatives.

Rock Coring Explained

In rock coring, a core barrel fitted with a diamond-impregnated bit rotates and advances, cutting an annulus of rock and leaving a solid cylinder, the core, inside the barrel. The core is brought to the surface, laid out in labelled boxes, photographed and logged. Core sizes vary, and larger diameters generally give better recovery and are preferred for weak or fractured rock.

Triple-tube core barrels use an inner liner to protect fragile core in weathered or fractured rock, improving recovery. Drilling fluid, usually water, cools the bit and flushes cuttings, and additives may be used to stabilise the hole.

What Is Logged and Measured in Rock Core

• Rock type and description: colour, texture, mineralogy and grain size.
• Weathering grade: from fresh rock to completely weathered, often critical for foundation design.
• Core recovery: the percentage of core retrieved from each drill run.
• Rock Quality Designation (RQD): the percentage of core pieces longer than a set length, a simple indicator of fracturing.
• Discontinuities: joints, fractures, faults and bedding, with orientation, spacing, roughness, infill and aperture.
• Strength estimates: field assessments of hardness and later laboratory testing.
• Water loss and groundwater observations: indicating permeability and fracture connectivity.

Good logging, ideally by an engineering geologist, is as important as the drilling itself.

Laboratory Tests on Rock and Soil Samples

• Uniaxial compressive strength (UCS) of rock cores.
• Point load index test as a quick strength indicator.
• Density and porosity.
• Slake durability for rocks that weather quickly.
• Soil classification tests, such as grain size and Atterberg limits.
• Shear strength and consolidation tests on soil samples.
• Chemical tests for aggressive soil or water.

In-Situ Tests Carried Out in Boreholes

• Standard Penetration Test (SPT) in soils and weathered rock.
• Permeability (packer or falling head) tests to assess groundwater flow and grouting needs.
• Pressuremeter and dilatometer tests for stiffness and strength in soil and weak rock.
• Borehole geophysical logging such as natural gamma, resistivity or caliper, where specified.
• Piezometer installation for long-term groundwater monitoring.

Applications in Kenya

High-rise and heavy buildings. Deep boreholes establish soil profiles and depth to rock for piled or raft foundations, particularly in Nairobi’s volcanic and sedimentary sequences and in coastal areas.

Bridges and viaducts. Boreholes at piers and abutments define foundation depth and rock quality in river crossings.

Dams, pans and water infrastructure. Drilling and permeability testing assess foundation conditions, seepage risk and construction materials.

Towers and tall structures. Steel towers, masts and water towers impose concentrated loads and uplift, requiring reliable data on soil and rock at foundation level.

Roads and railways. Drilling investigates cuttings, embankments, bridge sites and soft ground areas.

Tunnels and underground works. Rock core drilling informs excavation methods, support design and groundwater control.

Mines and quarries. Geotechnical drilling supports pit slope design, waste dump stability and tailings facilities.

Geothermal and energy projects. Site investigation for plant foundations and pipelines in rift areas with variable volcanic ground.

Challenges in Kenyan Ground Conditions

• Black cotton soils can cause borehole instability and sample disturbance.
• Boulders and cemented layers in volcanic and alluvial deposits can stop progress or mimic bedrock.
• Highly variable weathering profiles make it hard to define the rock surface.
• Fractured and vesicular volcanic rock may cause loss of drilling fluid and poor core recovery.
• High groundwater and artesian conditions require careful borehole control.
• Remote or restricted access affects rig mobilisation and logistics.

Experienced drillers and engineers adapt methods, casing and fluids to manage these conditions.

Quality Drilling Practice

A reliable service provider will:

• Use well-maintained rigs and appropriate bits, barrels and casing.
• Follow recognised standards and project specifications for drilling, sampling and logging.
• Employ trained drillers and a qualified supervising engineer or geologist.
• Record drilling parameters, water levels and depths accurately.
• Preserve and label samples properly, with core boxes marked for depth and orientation.
• Carry out in-situ tests correctly, with calibrated equipment.
• Ensure health and safety, including traffic control, utility checks and equipment guarding.
• Backfill or complete boreholes appropriately, protecting groundwater and rehabilitating the site.

Choosing a Drilling Contractor

1. Verify company registration, experience and references.
2. Ask about rigs, tooling and the range of methods available.
3. Check qualifications of supervisors and logging staff.
4. Request a sample borehole log and core photographs.
5. Confirm laboratory arrangements for testing.
6. Compare quotations by scope, rates per metre by ground type, and treatment of provisional items such as rock coring.
7. Discuss contingencies for refusal, loss of circulation and unexpected depth.

Common Mistakes to Avoid

• Terminating boreholes in boulders or cemented layers mistaken for bedrock.
• Drilling to insufficient depth.
• Poor core handling, resulting in lost or mixed-up samples.
• Skipping in-situ tests or using uncalibrated equipment.
• Inadequate supervision and logging.
• Failing to record groundwater levels over time.
• Leaving boreholes open or poorly backfilled.

Conclusion

Geotechnical drilling and rock coring provide the direct, depth-specific evidence that designers need for foundations, slopes, tunnels and excavations. In Kenya’s variable ground, from soft clays to fractured volcanics and hard basement rock, the right method, a competent crew and careful logging make the difference between reliable data and costly guesswork. Choose a contractor with proven equipment, qualified supervision and a commitment to quality, and make sure the results feed straight into design.