Bearing Capacities, What are they and are they important ?

In geotechnical engineering, foundation bearing capacity is the ability of subgrade soil to support the designed structural loads. The bearing capacity of soil is the maximum contact pressure between the concrete foundation and the soil which should not produce shear failure. Bearing capacity of a foundation is also described as the maximum load that can be applied on a foundation before failure or uncontrolled deformations occur. Professional geotechnical inspection of bearing pressure on foundation footings is a requirement of the Alberta National Building Code.

The calculation of foundation bearing capacity requirements are typically defined by these parameters:

  • foundation dimensions;
  • foundation depth;
  • thickness and shear strength of any fill;
  • natural foundation soil present; and
  • groundwater tables

Checking the foundation soil bearing capacity can easily be done with geotechnical engineering analytical tools, and will identify any bearing capacity problems which could lead to foundation failures.

Geotechnical engineers typically utilize a hand-pushed penetrometer to verify the strength of the subgrade. This portable tool quickly and accurately assesses foundation bearing capacity soil conditions for various construction purposes. The penetrometer can be pushed into the foundation soils using the weight of one’s own body.

The penetrometer provides very accurate information on soil strength, or resistance to penetration, primarily used to determine the bearing capacity of the upper 0.3 meter of the soil directly below the foundation. The acquired resistance data makes it possible to determine the bearing capacity of the subgrade. However, strength measurements are affected by moisture, porosity, and rock content. These factors are considered by the geotechnical engineer when determining the foundation bearing capacity. When combined with other information, such as soil moisture, structure, and texture, data from penetrometers can contribute to a very accurate picture of foundation bearing soil properties.

Wet cohesive soils are particularly prone to rapid softening and therefore may not be able to support foundation bearing capacity loads. The bearing capacity may also become an issue when the subgrade soil consists of cohesive fine-grained material, which typically shear under less force than cohesionless soils at optimum moisture content.

When selecting a safety factor, the allowed deformations are indirectly defined. A lower safety factor may result in larger deformation probabilities. A higher designated safety factor against loss of bearing capacity will generally result in a design where deformations remain limited.

Eventually, all loads in a structure are transferred to the subgrade. As the designed load under a footing spreads on subgrade soils, pressure on the soil is distributed. Soil directly under the footing absorbs the most load pressure and should be thoroughly compacted or left undisturbed, if suitable. To ensure that an adequate area of ground is being used to support the load, footings must have a certain minimum area to distribute the load.

Below the footing at a depth equal to the footing width, the unit soil pressure will have been reduced by approximately 50%. Continuing deeper into the subgrade, approximately twice the footing width, and the force of pressure will be reduced by approximately 66%. The bearing soil directly beneath each footing is the most critical.

When excavating for footings, equipment can potentially disturb existing subgrade soil, decreasing the natural density. Soil from surrounding embankment walls may also fall into the proposed footing area and compromise strength. Loose soil has much less bearing capacity than original (compact) soil. It can prove critical to compact the trench bottom (use a vibrating plate compactor for sand or gravel soils, and a jumping jack compactor for silt or clay). Failure to compact soil could result in settlement. In projects where the soil was compacted, the hand-held penetrometer has proved to be an excellent tool in evaluating how well the compaction of the soil was completed.

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