What is the bearing capacity of a foundation in geotechnical design and analysis? (2024)

The ultimate bearing capacity (UBC) is the theoretical limit of the load that the soil can sustain before it collapses. It is calculated by applying a general equation that considers the soil strength, the foundation geometry, and the soil-foundation interaction. The equation is: UBC = cN_c + qN_q + 0.5BN_{\gamma} where c is the soil cohesion, q is the effective overburden pressure, B is the foundation width, and N_c, N_q, and N_{\gamma} are dimensionless bearing capacity factors that depend on the soil friction angle and the foundation shape factor.

The UBC is usually expressed in units of force per unit area, such as kN/m^2 or psi.

The net ultimate bearing capacity (NUBC) is the difference between the UBC and the effective overburden pressure at the foundation level. It represents the additional load that the soil can carry beyond its own weight. The equation is: NUBC = UBC - q The NUBC is also expressed in units of force per unit area.

The safe bearing capacity (SBC) is the maximum load that the soil can support without causing excessive settlement or damage to the foundation or the structure. It is obtained by dividing the NUBC by a factor of safety (FS) that accounts for the uncertainties and variations in the soil properties, the load conditions, and the design assumptions. The equation is: SBC = NUBC / FS The SBC is usually lower than the UBC and the NUBC, and it is also expressed in units of force per unit area.

The allowable bearing pressure (ABP) is the actual load that is applied to the foundation by the structure. It is calculated by dividing the total weight of the structure by the foundation area. The equation is: ABP = W / A where W is the total weight of the structure, and A is the foundation area.

The ABP is also expressed in units of force per unit area.

Many factors influence the bearing capacity of a foundation, such as the soil type and its properties, the foundation shape and size, the depth and location of the foundation, the groundwater level and its fluctuations, and the load characteristics. These must be carefully evaluated in the geotechnical design and analysis of a foundation, utilizing appropriate methods, tests, and standards. The soil's cohesion, friction angle, density, moisture content, compressibility, and permeability all need to be considered. Additionally, the foundation's width, length, depth, aspect ratio, and orientation should be taken into account. Furthermore, it is important to consider the depth and location of the foundation in relation to the slope or edge of the site. Finally, one must assess the magnitude, direction, distribution, duration, and frequency of the load.

Estimating the bearing capacity of a foundation involves different methods, depending on the available data, soil conditions, and design criteria. Empirical methods, such as Terzaghi's equation, Meyerhof's equation, and Hansen's equation are based on correlations and formulas derived from field observations and experience. Analytical methods use mathematical models and equations based on soil mechanics principles and assumptions. Numerical methods rely on computer software and algorithms to simulate the soil-foundation behavior and solve complex problems. Experimental methods measure the soil parameters and foundation performance through laboratory tests or field tests. Each method has its advantages and limitations, so they should be used with caution and verification.

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