Penetration through existing brick wall

[bugbus]

Client wants to cut a large penetration, approx. 800 mm wide x 300 mm high (32" x 12"), through the footing of a brick wall, in order to feed through several conduits into a building. The footing is around 400 mm wide (16"), appears to be 4 wythes thick, and supports a brick cavity wall on top. The brick wall is one storey and supports a roof only. The footing is directly on sandstone.

After the conduits are fed through the penetration, the idea is then to fill this back up with plain concrete.

Clearly 800 mm (32") is too wide not to install a lintel or temporary props without risk of cracking the wall. So, one possibility is to do this in two stages:

First, cut half the penetration, i.e. 400 mm wide (16"), install half the conduits, then fill with concrete.
After concrete has reached say 15 MPa (2000 psi), come back and cut the other half of the penetration, install the remaining conduits, and again fill with concrete.

Do you see any issues with the proposed plan?

A few questions:
What is the widest penetration you would be comfortable cutting through an existing wall without strengthening? Australia's National Construction Code says 500 mm is OK.
Any issue with filling the penetration back up with plain concrete? The intention is for this to restore the vertical capacity of the wall to avoid any cracks forming. Will any small gaps between the bricks and the concrete caused by shrinkage have an effect on the wall?

 

[GeorgeTheCivilEngineer]

Your last comment is what my concern would be - although 400mm height of infill is not very large, I feel there is enough volume there to cause shrinkage and loss of support.

The approach used in traditional mass underpinning is to cast mass concrete up to a level 75mm below the line of the masonry, wait 48 hours, then pack with dry mix and slate.

 

[JAE]

If there's adequate "roughness" between the brick foundation and the brick walls above, I'd not be very worried about a 32" wide opening presuming you have at least 32" of untouched masonry on either side and 32" of solid brick (no openings) above.

The interlock between brick and brick should allow for quite a bit of arching action over your opening. Some small movement might occur in the bricks directly over the opening itself but this also shouldn't be a problem in my view.

Packing back with grout or concrete would be fine and certainly help restore interlock between the bricks above and below your temporary opening.
Shrinkage over that small of distance would be very small in absolute terms so again - not a worry to me.