Foundations provide support for structures, transferring their load to layers of soil or rock that have sufficient bearing capacity and suitable settlement characteristics. They can be used to help to prevent settlement and other movements of structures and can permit construction on ground that might otherwise have insufficient bearing capacity.
- The nature of the load requiring support.
- Ground conditions.
- The presence of water.
- Sensitivity to noise and vibration.
- Proximity to other structures and loading
- Project time-frames.
Very broadly, foundations can be categorised as shallow foundations or deep foundations. Shallow foundations are typically used where the loads imposed by a structure are low relative to the bearing capacity of the surface soils. Deep foundations (more than 3 m from the surface) may be necessary where the bearing capacity of the surface soils is insufficient to support loads imposed.
Pad foundations are generally shallow foundations, but can be deep depending on the ground conditions. They are a form of spread foundation formed by rectangular, square, or sometimes circular concrete ‘pads’ that support localised single-point loads such as structural columns, groups of columns or framed structures. This load is then spread by the pad to the bearing layer of soil or rock below. Pad foundations can also be used to support ground beams.
They are generally of a uniform thickness, but sometimes the upper face may be sloped or stepped. Their plan shape will depend on the nature of the applied load and the allowable bearing capacity of the layers below. Their thickness must be sufficient to distribute the load across the plan shape. They are generally reinforced on all but the smallest structures, with the reinforcement allowing higher loads to be imposed and the construction of shallower pads which require less excavation and use less concrete.
The arrangement of pad foundations will vary depending on the nature of the structure they are supporting, the loads imposed, the allowable bearing capacity of the layers below and the space available on site. They may be:
- A series of discrete, well-separated pads.
- Balanced base pads that support more than one point load.
- Continuous pads, where there are a number of point loads close together.
- Pad and beam, where a series of pads support a continuous beam.
Pad foundations can be selected as they do not require much excavation, and are generally suitable where the bearing capacity of ground is sufficient at relatively low depths. However, they can be large in plan shape and may not be effective against differential settlement, uplift forces or wind forces.
See also: How to design pad foundations.
See also: Types of pad foundation.
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