bearing capacity

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bearing capacity Bearing capacity is a general term used in the design of foundations for engineered structures to define the ability of a rock or soil immediately below the foundation to bear the stress that will be placed upon it when the structure is complete. Bearing capacity is defined in more specific terms: ultimate bearing capacity (q_f) is the limit of stress which can be imposed upon a soil or rock without exceeding the strength of the material and thus causing it to fail and, consequently, to move and thus cause the foundation and the structure based on that foundation to fail. The value of q_f for soil or rock increases with depth because of the constraint produced by the weight of the increased thickness of soil or rock material adjacent to it.

Foundation engineers take account of the probability that the ultimate bearing capacity of the material might not be known accurately enough because of deficiencies in sampling or strength testing and because the nature and properties of geological materials can change over small lateral and vertical distances. Foundation engineers also take account of the consequences of inadequate design, which could lead to catastrophic failure of the structure. To allow for such possible inaccuracies and their consequences, a factor of safety (F) is employed. Its value is usually between 1.1 (for a road foundation, for example) and 3 (for a dam). The ultimate bearing capacity q_f is then divided by the chosen factor of safety to give the safe bearing capacity (q_s). Thus, q_s  =  q_f/F. The safe bearing capacity q_s is then used in the design calculations for the foundation. Using the safe bearing capacity in the calculations should ensure that the strength of the geological material will not be exceeded and it will therefore not fail, but because of the imposed stress it will deform. The effect of that deformation is that the structure will settle. The amount and rate of settlement has then to be considered to ensure that the structure can accommodate the settlement without serious damage. If there is a possibility that the deformation of the underlying geological material that is to bear the weight of the structure will be too great under pressure equal to q_s, then the deformation must be reduced by changing the size or shape of the foundation so that the stress is reduced sufficiently to keep the deformation and settlement within acceptable limits. The bearing pressure that takes into account of the demands of the built structure as well as the strength of the geological material under the foundation is termed the allowable bearing pressure (q_a), because that is the limit of stress (pressure) that is allowable if deformation, and consequently settlement, are to be kept within the limits that the structure can accommodate. Thus q_a will be equal to or less than q_s.

J. West

bearing capacity Maximum load per unit area a surface can support in safety without shear failure.