Skip to content 
Every building, bridge, tank, and wall ends up resting on the ground. The question every foundation designer must answer is: how much load can the soil safely carry? That capacity is called bearing capacity, and it is one of the most important concepts in geotechnical engineering.
This article explains what bearing capacity means, the different types, how it is determined using theory and tests, and what factors can change it in Kenyan ground conditions.
What Is Bearing Capacity?
Bearing capacity is the ability of soil to support the loads applied to it by a foundation without failing in shear and without excessive settlement. It is usually expressed as a pressure, in kilonewtons per square metre (kN/m², or kPa).
Think of standing on firm ground versus standing on soft mud. On firm ground you barely sink, while in soft mud you sink quickly. Bearing capacity quantifies this difference in engineering terms.
Types of Bearing Capacity
Several related terms appear in soil reports, and they are often confused.
Ultimate bearing capacity (q_ult) is the maximum pressure the soil can sustain at the point of shear failure. Beyond this value, the foundation sinks or tilts as the soil fails.
Net ultimate bearing capacity is the ultimate value minus the pressure already present from the weight of soil removed at foundation level.
Safe bearing capacity is the net ultimate value divided by a factor of safety, typically between 2.5 and 3 for permanent structures, to provide a margin against uncertainty.
Allowable bearing pressure is the maximum pressure that may be applied to the soil considering both shear failure and settlement limits. In practice, settlement often controls design in sands and in compressible clays, so the allowable value is the lower of the safe bearing capacity and the pressure that produces tolerable settlement.
Failure Modes Under Foundations
Soil beneath a loaded foundation can fail in different ways. General shear failure occurs in dense or stiff soils: a clear failure surface forms and the soil bulges at the surface, leading to sudden collapse. Local shear failure occurs in medium-density soils, with partial development of the failure surface. Punching shear failure occurs in loose or soft soils, where the foundation sinks with little surface disturbance. Understanding the likely mode helps the engineer select appropriate equations and safety margins.
Terzaghi’s Bearing Capacity Theory
Karl Terzaghi’s classic equation, developed in the 1940s, remains a foundation of design. For a strip footing, it expresses the ultimate bearing capacity as the sum of three contributions:
q_ult = cNc + qNq + 0.5γBNγ
Here, c is soil cohesion, q is the overburden pressure at foundation level (unit weight multiplied by depth), γ is the soil unit weight, and B is foundation width. Nc, Nq, and Nγ are bearing capacity factors that depend on the soil’s angle of internal friction.
The first term represents the contribution of cohesion, the second the surcharge from soil above the footing base, and the third the weight and friction of the soil under the footing. Modified versions include shape factors for square and circular footings and depth factors for deeper foundations. Other researchers such as Meyerhof, Hansen, and Vesic refined the approach with additional factors for inclination, shape, and depth.
Determining Soil Strength Parameters
The equations require cohesion (c) and friction angle (φ). These are measured through laboratory tests on samples, such as direct shear tests, unconfined compression tests, and triaxial tests, or estimated from in-situ tests. For clays under short-term loading, undrained shear strength (cu) is used, with φ taken as zero. For sands, c is typically zero and φ is the key parameter. Long-term conditions in clays use effective stress parameters.
The quality of sampling and testing strongly affects the result, which is why disturbed or poorly preserved samples can give misleading strengths.
Using the Standard Penetration Test
In practice, many bearing capacity values for building foundations are derived from SPT N-values. Empirical charts and equations, such as those by Terzaghi and Peck or Meyerhof, link N-values to allowable bearing pressure for given foundation width and settlement limits (commonly 25 mm for isolated footings). The N-values are corrected for factors like overburden, groundwater, and hammer efficiency.
This approach is popular because SPT is widely available and gives data at multiple depths. It is less reliable in gravels, very soft clays, and expansive soils, so engineers cross-check with other methods.
Plate Load Test
The plate load test is a direct field method. A steel plate, commonly 300 mm to 600 mm square or circular, is placed at foundation level in a pit and loaded in increments using a hydraulic jack against a reaction frame or kentledge. Settlement is measured at each load stage, and a load-settlement curve is plotted.
The curve helps determine the load at failure or at the allowable settlement. Because the plate is much smaller than the actual foundation, results must be extrapolated with care, especially for clays, where the stressed zone beneath a large foundation is deeper than for a small plate and may include weaker layers. Plate load tests are valuable for critical foundations or where laboratory testing is difficult, but they test only shallow depth.
Other Methods
Cone Penetration Test (CPT) gives continuous profiles of resistance and is widely used internationally; local availability is limited but growing. Pressuremeter tests measure soil stiffness and strength in boreholes. Presumptive values from codes, such as those in British Standards, give conservative bearing pressures for common soil types, useful for preliminary design but not a substitute for investigation. In rock, bearing capacity is based on rock strength and discontinuities, and typically exceeds that of soils, though fractured or weathered rock needs careful assessment.
Settlement: The Other Half of the Question
A soil may have adequate shear strength and still deform too much. Total settlement and, more importantly, differential settlement between columns can damage buildings. Settlement is estimated using elastic theory for sands and consolidation theory for clays, with parameters from SPT, plate tests, or oedometer tests. Typical limits for isolated footings in buildings are of the order of 25 mm total, with limits on angular distortion to protect finishes and structure. When settlement governs, the engineer reduces the allowable pressure or changes the foundation type.
Factors That Affect Bearing Capacity
Several factors influence real-world capacity:
- Soil type and density or consistency.
- Groundwater level: a high water table reduces effective stress and capacity.
- Foundation size, shape, and depth: deeper and wider footings generally increase capacity.
- Load inclination and eccentricity: off-centre or tilted loads reduce capacity.
- Soil variability and layering: a strong layer over a soft one may fail by punching into the soft layer.
- Moisture changes: expansive soils lose strength when wet.
- Construction quality: disturbance of the founding surface, poor compaction, or water in excavations can reduce actual capacity.
Bearing Capacity in Kenyan Conditions
In Nairobi’s volcanic tuffs and weathered trachytes, bearing capacity can be good at moderate depth but variable, so deep trial pits and boreholes are important. In black cotton soils, apparent strength in the dry state is deceptive, because wetting reduces strength and causes swelling, so design must consider moisture changes. In coastal sands, groundwater and liquefaction risk may govern. In weathered basement rocks, capacity depends on the degree of weathering and structure.
Frequently Asked Questions
What is a “good” bearing capacity? It depends on the structure. Values from under 100 kPa for soft soils to several hundred kPa for dense soils and more for rock are common, but only site-specific data can say.
Can I assume bearing capacity from neighbours? No. Conditions vary, and design assumptions differ.
Why does the engineer recommend deeper foundations? Often because stronger or more stable soil exists at depth, or to avoid moisture-affected zones.
A Simple Illustrative Calculation
To see the idea in numbers, imagine a square footing 2 metres wide placed 1.5 metres deep in a dense sand with unit weight of about 18 kN/m³ and friction angle of about 32 degrees. The overburden pressure at foundation level is roughly 27 kPa (18 multiplied by 1.5). Using Terzaghi-type bearing capacity factors for this friction angle and appropriate shape factors, the ultimate bearing capacity may come out in the order of 1,000 kPa or more. Applying a factor of safety of 3 would give a safe bearing capacity above 300 kPa. However, if settlement checks show that pressure above, say, 250 kPa would cause more than 25 mm of settlement, the allowable bearing pressure would be limited accordingly.
This example is only illustrative. Real designs use site-specific parameters, include groundwater effects, and are carried out by qualified engineers. But it shows how shear strength, depth, width, and settlement all combine to determine the final design value.
Why Bearing Capacity Figures Should Never Be Copied
A bearing capacity value is only valid for the conditions it was calculated for: the soil profile, foundation size, depth, groundwater level, and acceptable settlement. Using a figure from another site, or applying the same figure to a larger foundation, can be unsafe. Always ask for the supporting assumptions and ensure the structural engineer uses the value as intended.
Conclusion
Bearing capacity is the link between soil and structure. It is determined by combining theory, laboratory strength tests, field tests like SPT and plate load tests, and settlement analysis, always with an appropriate factor of safety and engineering judgement. Understanding the concept helps owners and builders appreciate why site-specific investigation is non-negotiable, and why the number in a soil report is only reliable when it rests on sound data.
