Connecting Adjacent Underpinning Pours with Dowel Bars

When local underpinning is carried out in stages — with separate pours at different positions along the foundation — the joints between those pours need to be considered. While they are not typically structurally critical in the same way as a beam connection, connecting adjacent underpinning pours with dowel bars is good practice and helps maintain monolithic behaviour.

Why Dowel Bars Are Used

Underpinning pours are staged: you excavate one bay, pour concrete, allow it to cure, then excavate the adjacent bay. This staged process creates cold joints between pours.

Under normal loading, those joints are in compression and do not separate. But good practice — and the avoidance of any differential settlement between adjacent pours — suggests that a small number of mild steel dowels should be cast into each pour before the next one is poured.

The dowels span across the joint, tying the two pours together loosely and ensuring that if one pour settles very slightly more than the other, there is no sudden crack or step at the joint surface.

Specification

PorthouseDean’s recommended specification for underpinning pour joints:

– 2 No. H20 (20 mm diameter high-yield) bars per joint
– Total bar length: 600 mm
– Embedment into each pour: 200 mm on each side of the joint (with 200 mm in Pour 1 and 200 mm in Pour 2)
– Bars should be installed approximately centrally in the pour depth and positioned at the time of the first pour, projecting out into where the adjacent pour will be

Practical Installation

When pouring the first underpinning bay, position the two dowel bars so that they project horizontally out of the face of the pour at the intended joint location. When the adjacent bay is subsequently poured, the concrete encases the projecting ends of the bars, completing the connection.

No grouting or mechanical fixing is required — the bond between the bars and the fresh concrete in the second pour is sufficient.

Not Structurally Critical

This detail is described as “good practice” rather than a structural requirement in most domestic underpinning scenarios. However, omitting it is not recommended — it costs effectively nothing to install and eliminates a potential weak point in the foundation.

If your project involves underpinning and you have questions about the details, contact PorthouseDean.