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Underpinning

Underpinning

In construction or renovation, underpinning is the process of strengthening the foundation of an existing building or other structure. Underpinning may be necessary for a variety of reasons:

  • The original foundation isn't strong or stable enough.
  • The usage of the structure has changed.
  • The properties of the soil supporting the foundation may have changed (possibly through subsidence) or were mischaracterized during design.
  • The construction of nearby structures necessitates the excavation of soil supporting existing foundations.
  • To increase the depth or load capacity of existing foundations to support the addition of another storey to the building (above or below grade).
  • It is more economical, due to land price or otherwise, to work on the present structure's foundation than to build a new one.
  • Earthquake, flood, drought or other natural causes have caused the structure to move, requiring stabilisation of foundation soils and/or footings.

Underpinning may be accomplished by extending the foundation in depth or breadth so it either rests on a more supportive soil stratum or distributes its load across a greater area. Use of micropiles and jet grouting are common methods in underpinning. An alternative to underpinning is the strengthening of the soil by the introduction of a grout, including expanding urethane-based engineered structural resins.

Mass Concrete Underpinning

'Traditional underpinning,' the mass concrete underpinning method is nearly 100 years in age, and the protocol has not changed. This underpinning method strengthens an existing structure's foundation by digging boxes by hand underneath and sequentially pouring concrete in a strategic order. The result is a foundation built underneath the existing foundation. This underpinning method is generally applied when the existing foundation is at a shallow depth, but works well up to fifty feet (fifteen metres) deep. The method has not changed since its inception with its use of utilitarian tools such as shovels and post hole diggers. Heavy machinery is not employed in this method due to the small size of the boxes being dug. There are several advantages to using this method of underpinning, including the simplicity of the engineering, the low cost of labour, and the continuity of the structure's use during construction.

Underpinning by expanding resin injection

A mix of structural resins and hardener is injected into foundation ground beneath footings. On entering the ground the resin and hardener mix and expansion occurs due to a chemical reaction. The expanding structural resin mix fills any voids and crevices, compacts any weak soil and then, if the injection is continued, the structure above may be raised and re-levelled. This relatively new method of underpinning has been in existence for approximately 30 years, and because it does not involve any construction or excavation set-up, is known to be a clean, fast and non-disruptive underpinning method.

There are other methods available for underpinning also like beam and base underpinning and mini-piled underpinning.

 

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17 September 2019

References

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