Underpinning is a construction process which strengthens the foundation of a structure. There are several different reasons a structure needs to be underpinned, such as a change in the soil properties, economic reasons, or a change in what the structure is used for. Underpinning stretches the foundation so it rests on a firmer area of soil or so its load is distributed over a larger area. There are several different standard underpinning techniques, including traditional, needle beams, cantilever beams and an underpinning raft.
Dig underneath the existing foundation and fill in the excavated soil with concrete. Construction crews dig in a sequence down to where firm soil exists so intermediate piers can be used. The supervising officer of building inspector must assess an appropriate bearing stratum when the first sequence of bays is constructed.
Install piles to stabilise wall foundations if digging beneath the foundation isn't feasible --- this is known as the needle beam method. Piles are put in on either side of an existing wall. Then a pocket is dug out below ground level and a reinforced concrete needle beam is inserted to connect the piles and the supporting wall. Need beams are used when the traditional method is not appropriate because of existing foundations that are too deep or when it costs too much to dig because the good bearing strata is too deep.
Install two mini-piles to stabilise wall foundations in place of the needle beam method when appropriate. This is known as the cantilever beam method. One mini-pile is compression and the other is tension. A pocket is dug out into an existing wall and a reinforced concrete beam is inserted to link the two piles. The Cantilever beam method is used when the foundations are too deep for the traditional method or access constraints do not allow the use of needle beams.
Use the underpinning raft method for highly loaded structures. Throughout the property, mini-piles are installed and then topped with a reinforced concrete raft. Need beams are then projected through the slabs and walls below ground level. This method works well for highly loaded structures and for when existing foundations are deep.