The stability of any structure begins with the ground beneath it. Soil bearing capacity is a fundamental measurement in civil engineering, representing the maximum pressure the soil can support before it shears or experiences excessive settlement. This value is paramount for designing safe and cost-effective foundations, ensuring the imposed load does not overwhelm the supporting material. Determining this capacity requires field investigation, laboratory analysis, and theoretical calculation. This process translates physical soil characteristics into quantifiable engineering parameters used in established formulas to predict the soil’s ultimate strength.
Defining Soil Bearing Capacity
Soil bearing capacity is discussed using two terms: ultimate and net. The ultimate bearing capacity is the maximum pressure a soil can withstand before it fails abruptly via a shear failure mechanism, where a mass of soil pushes out from beneath the foundation. This is the theoretical maximum load the ground can hold without catastrophic collapse.
The ultimate value includes the weight of the soil originally removed to place the foundation, known as the overburden pressure. The net bearing capacity is the ultimate capacity minus that overburden pressure, representing only the additional pressure the foundation imposes. The net value is the more practical figure for design, as it focuses solely on the new stress introduced by the structure.
Preventing shear failure is only half the goal; the other half is controlling settlement. Even if the soil does not fail catastrophically, excessive or uneven settling can damage the structure above. Therefore, the calculation must ensure the foundation remains stable and within acceptable deformation limits.
Gathering Input Data Through Field Tests
Before calculation, engineers must gather specific data about subsurface soil layers through in-situ (field) testing. Necessary soil parameters, such as the angle of internal friction, cohesion, and unit weight, cannot be directly measured from the surface. These parameters are determined through empirical correlations with data collected from standardized tests performed on site.
The Standard Penetration Test (SPT) is a common method where a sampler is driven into the ground using a standard hammer. The critical output is the \(N\)-value, the number of hammer blows required to drive the sampler a specific distance. This \(N\)-value indexes the soil’s density and resistance to penetration, which is then correlated to estimate the soil’s angle of internal friction (\(\phi\)) for granular soils or its undrained shear strength (cohesion, \(c\)) for cohesive soils.
The Cone Penetration Test (CPT) involves pushing a cone-tipped rod into the ground at a constant rate. The CPT continuously measures two resistances: the cone resistance (\(q_c\) or \(q_t\)) at the tip and the friction along the sleeve. This test provides a continuous profile of the soil layers and their consistency. The measured cone resistance is used in specialized correlations to determine the soil’s unit weight and shear strength properties, which are inputs for theoretical bearing capacity equations.
Theoretical Calculation Approaches
The ultimate bearing capacity is calculated using theoretical models developed by geotechnical pioneers, notably Karl Terzaghi and Hans F. Meyerhof. These models combine soil parameters obtained from field tests with the proposed foundation geometry. The formulas fundamentally represent the sum of three main components of soil resistance that prevent failure:
Components of Soil Resistance
The first component accounts for the soil’s cohesion (internal bonding), which is most relevant in clay.
The second term incorporates the surcharge pressure, which is the confining pressure provided by the weight of the soil located above the foundation base.
The third component represents the resistance generated by the weight and frictional strength of the soil directly beneath the foundation.
These three resistance terms are multiplied by specific bearing capacity factors (\(N_c\), \(N_q\), \(N_{\gamma}\)), which are dimensionless values dependent on the soil’s angle of internal friction (\(\phi\)). Meyerhof’s approach expanded on Terzaghi’s model by introducing modifying factors for conditions like deeper foundations or inclined loads. Inputting the field-derived soil properties and foundation dimensions into these frameworks calculates the ultimate pressure the soil can resist before shear failure.
Key Factors Modifying Capacity
The ultimate bearing capacity calculated by theoretical equations must be adjusted for several external factors influencing the soil’s actual performance. The depth of the foundation (embedment depth) significantly increases capacity because the soil provides greater confining pressure. This deeper embedment increases the surcharge term, requiring the application of a depth factor modifier.
The presence and location of the groundwater table also requires modification. If the water table is at or near the foundation level, it reduces the effective stress within the soil mass due to buoyancy effects. This reduction lowers the soil’s frictional resistance, requiring the use of a modified, or buoyant, unit weight (\(\gamma’\)) in the calculation.
Finally, the foundation’s geometry (shape and size) modifies the bearing capacity. Since theoretical equations are often derived for a strip footing, shape factors must be applied to accurately represent square, circular, or rectangular foundations.
Calculating Allowable Bearing Pressure
The final step is translating the calculated ultimate capacity into a safe design value, known as the allowable bearing pressure. The ultimate capacity represents a theoretical failure point and is unsuitable for structural design. To account for uncertainties in soil testing, variations in properties, and potential long-term settlement, a Factor of Safety (FS) is applied.
The Factor of Safety is a numerical divisor, typically ranging from 2.5 to 3.0, applied to the net ultimate bearing capacity. Dividing the net ultimate capacity by this factor yields the allowable bearing pressure. This is the maximum pressure the foundation can safely exert on the soil without risk of shear failure or excessive settlement. This allowable pressure is the value used by structural engineers to determine the final size and area of the foundation.