How does the ultimate bearing capacity of a shallow footing on granular soil change if the groundwater table rises to the level of the footing base, compared to a completely dry condition?

The ultimate bearing capacity of a shallow footing on granular soil is the maximum pressure the soil can support without undergoing shear failure. A shallow footing is a foundation whose depth below the ground surface is less than or equal to its width. Granular soil, such as sand or gravel, is cohesionless, meaning its shear strength is primarily derived from inter-particle friction rather than cohesion. In a completely dry condition, the ultimate bearing capacity of granular soil is determined by its dry unit weight, its internal friction angle, and the footing's geometry. The soil particles transmit stress directly to each other, resulting in high effective stresses within the soil mass below the footing. When the groundwater table rises to the level of the footing base, the soil below the footing becomes fully saturated with water. This saturation introduces pore water pressure, which is the pressure exerted by the water within the soil's voids. According to the principle of effective stress, the stress carried by the soil skeleton (the effective stress) is equal to the total stress minus the pore water pressure. The presence of pore water pressure effectively reduces the inter-granular forces between soil particles. Consequently, two main changes occur. First, the unit weight of the soil used in bearing capacity calculations changes. Instead of the dry unit weight, the *effective unit weight(also known as buoyant unit weight) is used for the saturated soil below the groundwater table. The effective unit weight is calculated as the saturated unit weight (the total weight of soil solids plus water per unit volume) minus the unit weight of water, effectively accounting for the buoyant effect of the water. This effective unit weight is significantly lower than the dry unit weight. Second, the reduction in effective stress directly lowers the soil's shear strength. While the intrinsic internal friction angle of the granular soil itself (a property reflecting particle shape and surface roughness) remains constant for a given density, the actual shear strength mobilized at any given depth, which is dependent on the effective stress (τ = σ' tanφ'), is reduced because the effective stress (σ') is diminished by the pore water pressure. Therefore, when the groundwater table rises to the footing base, the ultimate bearing capacity of the shallow footing on granular soil significantly decreases. This reduction is primarily due to the decrease in the effective unit weight of the soil, which directly reduces the overall confining pressure and, consequently, the soil's ability to resist shear failure. This leads to a substantial decrease, typically around 50%, in the ultimate bearing capacity compared to a completely dry condition.