Underpinning

Underpinning


Overloading, poor drainage, erosion, tree roots or expansive soils that expand and shrink with changes in ground-moisture levels produce stresses that can damage a buildings foundation and its overall structure.

What are foundations and footings?

A foundation is a lower portion of building structure that transfers the weight of the structure to the ground supporting it. Foundations are generally broken into two categories; shallow foundations and deep foundations, each referring to the depth of soil in which the foundation is made.

Shallow foundations are made in excavations just below the surface of the ground. They are generally used for smaller buildings such as residential housing and have a wider bottom portion than the walls they support. This wider ‘footprint’ spreads the weight of the structure over a greater area so that the weight of the building does not exceed the safe bearing capacity of the soil. Most common types of shallow footings are strip footings and raft foundations.

Deep foundations are typically formed from a series of piles driven deep into the ground. They are generally used for buildings with heavy or concentrated loads. Piles transfer building loads to the earth at a greater depth than a shallow foundation does. Pile foundations are used in situations where there the soil at the surface that cannot support the weight of the building. They bypass this layer and transfer loads to a deeper layer of stronger soil or rock. There are two types of driven foundations; end-bearing piles and friction piles. In practice, however, each pile resists load by a combination of end bearing and friction.

Why do foundations fail?

Foundation failure can be attributed to several types of ground movement according to soil type:

    • Cohesive soilssuch as clay
      Ground-movement is commonly caused by moisture variations in in the clay soils beneath the foundation footings. In dry conditions clay soils consistently lose moisture and shrink, causing settlement. When moisture levels are high clay soils swell, causing heave. The effect of nearby trees also plays an important role in reducing water levels in cohesive soils.
    • Non-cohesive soilssuch as sand and gravel
      Water flowing around foundation footings, such as may be caused by faulty drainage, can wash away fine particles in sandy soils and cause the soils to become over-saturated to the extent that they and lose bearing strength. When the ground becomes unable to support the weight of this part of the building, the structure may settle or sink into the ground.

Repairing Foundations and Footings

The Association has members who specialise in foundation repair systems. Their expertise covers an array of repair strategies that range from underpinning and soil consolidation to remedial piling techniques.