UNDERPINNING EXPLAINED

Today we’re going to swing by the crawl space conversion job and see what new information we can glean about this process. In this vlog we’re going to discuss:

• What is Underpinning?
• How are the new footers placed?
• How are dowels placed within the new footers?
• How does the old foundation tie to the new foundation?

What is Underpinning?

We’ll answer these questions in real time as we go down into the basement. Please come along. We arrive as Tim and JC are preparing the pump hose to shotcrete new footers. They drop the hose down into a small 4’X4′ opening that has been saw cut out of the side of the existing foundation of a beautiful house in the South Denver area, just below ground level. Tim and JC both climb down into the hole and pull the hose down with them.

We jump down in there as with the camera and get in on some of the action. The crew has excavated five feet below the foundation or “footer” of the former crawl space area. They’ve dowelled into the existing foundation, tied in the rebar and formed up sections of new footer to be poured. It has to be done in sections so the structure continues to have support. This is called Underpinning.

How are the new footers placed?

Inside, we designate four foot wide sections around the parameter of the entire area where the floor will be lowered. The first four foot stretch will designated as an A section, the following stretch will be designated as a B section, the following an A, then a B, A, B, A, B, and so on. First all A sections are excavated to the required depth for new footers. The engineer plans will call out the exact depth, thickness and other detail.

The B sections will remain in order to provide support to the existing foundation. Once A sections have been poured, B sections can be excavated and prepared for placement of new footers. Vapor barrier and rebar are placed, and B sections are then poured or shotcreted into place. The new, deeper foundation is now in place.

How are dowels placed within the new footers?

The plans for this design calls out for a dowel every 12 inches into the side of the existing footer, and for it to then curve down in an L shape and proceed the depth of the new footer. Also at every 12 inches a dowel is placed underneath the existing footer and continues down the depth of the new footer. So there are two sets of rebar running vertical.

Along these vertical rebar, another rebar is placed horizontally and dowelled into the adjacent footers at every 12 inches. This creates two grid locked rebar systems for a very strong new foundation. The plans may call out different requirements depending on several factors, but this one is good to go.

How does the old foundation tie to the new foundation?

Dowels are used to tie the old foundation together with the new foundation. In addition to the weight of the entire structure sitting on a new foundation, rebar is running throughout the concrete in a particular format and tying it all together. This added reinforcement will hold the house together for many many years to come.

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