Underpinning

December 4, 2019

What is Underpinning?

Underpinning is the process of lifting or supporting the footing of a building or structure, so as to provide a more stable foundation.

Underpinning is commonly used to stabilise houses when they experience severe cracking.

There are several different types and methods of underpinning. Two of the most common are:

  • Traditional concrete underpinning, which involves piering or excavating under a footing to install a concrete support for the footing.
  • Chemical underpinning, using an expanding urethane foam or similar to create piers in the soil under a footing, to provide lift to it.

These methods are quite different and are best used in different situations, the choice dependent on the cause of the cracking.

Traditional concrete underpins

Traditional concrete underpins used to be dug by hand and were typically 1m cubes of concrete placed under a footing beam. The footing beam could then be jacked up, using the concrete underpin as a stable base.

However, the cracking in some houses is related to seasonal moisture variation in the soil which can extend down to over 3m in depth. This seasonal moisture variation is the drying/shrinkage of the clay soils over summer and the wetting/expanding of the clay soils over winter. Hence, due to the depth of soil moisture variation, the 1m cube concrete block also experiences some lift and settlement over the seasons. Hence, a 1m cube underpin may not be a stable foundation.

It is better practice to install a concrete underpin as a concrete-filled bored pier to a depth of at least 4m, founding the pier in soils which are stable over the seasons. Please note that this depth may vary due to other considerations, such as the permanent water table depth, soil conditions or shallow rock, proximity of trees, etc.

The footing under the building may then be jacked off the underpin if jacking is viable.

The location of underpins will affect the decision to use traditional concrete underpins or another method, as it is preferable to use a machine such as a small excavator to auger or dig the underpin. It may not always be possible or practical to position an excavator where required to dig the underpin – you don’t want an excavator in your hallway!

Jacking over underpins

Underpins are often installed as a stable base so that the footings of the building can be jacked up and re-levelled. This can only be done using traditional concrete underpins.

However, there are many limitations to this jacking, which include:

  • The concrete strength of the footing. If this is too soft, the footing concrete may crush rather than lift.
  • If the footing is bluestone rather than concrete, it may not be recommended to jack as it is not structurally continuous, or strong
  • The footing may crack if lifted too far, which in turn may cause additional cracking to the building over
  • Soil suction on the footing may prevent lift.

Chemical Underpinning

Chemical underpinning is undertaken by injecting an expanding urethane foam (or similar) into the soil at selected locations under the footing. This creates a pier of foam in the soil to lift and support the footing over. This type of underpinning incorporates jacking.

Chemical underpinning has some advantages and some disadvantages when compared to traditional concrete underpins. These are discussed below.

The advantages of Chemical Under-pinning are:

  • It can be done relatively quickly – only one day is required for a normal house, whereas a week may be required using traditional concrete underpins.
  • The method provides a large degree of control over the lift undertaken to the footing.
  • The injection can be undertaken at numerous points close together.
  • The injection can be undertaken under internal walls more easily than the installation of traditional concrete underpins.

The disadvantages of chemical underpinning are:

  • The chemical is injected using a wand or tube inserted into the soil under the footing. This limits the founding depth of the pier and the resultant pier is likely to be founded in soil which may show seasonal lift and settlement. This is undesirable (see above).
  • The system requires the use of a very high-pressure pump.

Which underpinning method to choose? Chemical or Traditional?

The decision whether to use traditional concrete underpins or chemical underpins should depend on the reason for the movement and cracking occurring, and location of the cracking.

If the movement is due to seasonal wetting and drying of soils around the exterior of a house, traditional concrete underpins are better as they are founded at a deeper depth.

If the movement is in the centre of a house and is not related to seasonal wetting and drying of the soils, chemical underpins may be a better option.

Underpinning Costs

The relative cost of chemical underpinning vs traditional concrete underpins is variable and depends on many factors, but the order of magnitude of both options is similar. The exact relative costs are best explored with individual contractors.

Magryn recommends both types of underpinning, with the choice depending on the details and requirements of the individual project. We recommend that you engage a structural engineer to review any project which may require underpinning, to ensure that:

  • You receive an unbiased and professional opinion on the need for underpinning.
  • You receive a report documenting the scope of underpinning you require. You can then take this scope to several underpinners to obtain easily comparable quotes. The report should detail the number and location of underpins required, as well as the details of these underpins (depth, size, reinforcement, etc).

Magryn would be pleased to assist you by providing an underpin report and design for your house. We can even recommend quality reliable contractors to undertake the work.

Give us a call on 8295 8677 and discuss your problem and requirements with us.