DEFINITON• In construction, underpinning is the process of strengthening and stabilizing the foundation of an existing building or other structure.
Reasons for Underpinning• The original foundation is simply not 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.• It is more economical, due to land price or otherwise, to work on the present structures foundation than to build a new one.
Process Of Underpinning• Underpinning is accomplished by extending the foundation in depth or in 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. All of these processes are generally expensive and elaborate.
Types of Underpinning• Mass Concrete Underpinning• Beam and base underpinning• Mini-piled underpinning
Mass Concrete Underpinning• Also known as traditional underpinning, This underpinning method strengthens an existing structures foundation by digging boxes by hand underneath and sequentially pouring concrete in a strategic order. The final result is basically a foundation built underneath the existing foundation. This underpinning method is generally applied when the existing foundation is at a shallow depth, however, the method still works very well even at fifty feet 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 called for in this method due to the tight nature of the boxes being dug.
Beam and base underpinning• The beam and base method of underpinning is a more technically advanced adaptation. A reinforced concrete beam is constructed below, above or in replacement of the existing footing. The beam then transfers the load of the building to mass concrete bases, which are constructed at designed strategic locations. Base sizes and depths are dependent upon the prevailing ground conditions. Beam design is dependent upon the configuration of the building and the applied loads
Mini-piled underpinning• Mini-piles have the greatest value where ground conditions are very variable, where access is restrictive.Mini-piled underpinning is generally used when the loads from the foundations need to be transferred to stable soils at considerable depths - usually in excess of 5.0 metres. Mini-piles may either be augured or driven steel cased, and are normally between 150mm and 300mm in diameter.
Underpinning the foundations of a railway bridge using a timber box crib to support the bridge. A completed concrete pad underpinning can be seen at the bottom right.]