Underpinning - how to tell a good job?
I’m buying an old brick/brownstone (circa 1899) mid-block 4-story townhouse. The place was completely gutted in the 1980s, including excavating out about 2/3 of the basement – down an extra foot or two and pouring an 18″ concrete “retaining” wall along the perimeter along with the new concrete floor. My inspector called this little retaining…
I’m buying an old brick/brownstone (circa 1899) mid-block 4-story townhouse. The place was completely gutted in the 1980s, including excavating out about 2/3 of the basement – down an extra foot or two and pouring an 18″ concrete “retaining” wall along the perimeter along with the new concrete floor.
My inspector called this little retaining wall by a nickname — a ‘knee’-something. He said that it wasn’t the best way to underpin, but that it was very common, and that it can be perfectly fine. However, there’s no way to tell if it was done properly, and he can’t guarantee that the foundation wasn’t compromised. A couple days later, over the phone, he told me that since the underpinning was done twenty years ago, any problems would have surfaced by now and that there’s nothing to worry about. (He was also impressed by the quality of the plumbing and electrical work, so he said it was a solid building.)
I went to the DOB and pulled all the records anyway. Everything looks like it was done right – permits and plans filed and approved for plumbing, mechanical ventilation, etc., over a few years – except there was no specific “underpinning” work described anywhere.
What do you think? Is this kind of underpinning common? Your experience would be greatly appreciated. Thanks!
thanks, johnife! you explained it better than the inspector. very much appreciated.
The weight of load bearing elements of a building hits the ground and spreads out at about a 45 degree angle each side of the load bearing point. Thus, if your load bearing party walls were built sitting on soil at the original basement floor level (and this is unlikely, they’re probably bearing on soil a foot or two below that) you could excavate your basement by two feet to within 2 feet of the walls and the load would still find its way to undisturbed soil with no problem. So, if your knee wall is set away by a couple of feet from your load bearing walls you’re “safe as houses”. If, however, the extra depth of your basement goes all the way to the load bearing walls and those walls were built directly on soil at the same level as the original basement floor (without a concrete foundation under them – and I believe omitting such a foundation was quite common back in the day) then there should be a reinforced concrete wall directly under the original load bearing walls to take the load down to undisturbed soil. Such a wall needs to be poured in multiple alternate sections, each about 4 feet long (so there’s never more than 4 feet of unsupported wall during construction) and the jointing between those sections should be visually evident.
I strongly suspect from your description that the former situation is the case and that you need have zero worries.