Underpinning reinforced stem

Similar in construction methodology to Mass concrete underpinning, reinforced concrete underpins are more typically used for mid rise buildings & also where the underpins are acting as a retaining structure – this is commonly seen in basement construction and extensions.

Construction Sequence

Underpins are excavated in a hit and miss sequence, most commonly each of these underpins will be 1.0m – 1.5m in width, an access pit is excavated & excavations in excess of 1.2m are shored with timber.
Once access has been gained to the correct designed depth the underside of the footing is cleaned, any loose material removed and shutters installed.
Reinforcement cages are installed, to create a watertight, continuous structure continuity strips or couplers are often introduced between underpins alongside bentonite waterbars where required.
Concrete is then poured 75mm shy of the underside of existing foundation, once cured the residual is dry packed with a sand/cement mix to ensure load transfer into the underpin.
1. For particularly complex loading jacks may be introduced between the new formation and existing footing to ensure transfer.

Applications

Domestic/commercial subsidence & heave foundation failures, basement retrofits, more commonly used where underpins may act as a permanent retaining structure.