Underpinning 3

[VictorAK]

Can the top 2" of non shrink grout be omitted and instead vibtrate the conrete to the underside of the existing footing when underpinning?

My hesitation in allowing this is that some of the voids under the existing foundation will not be filled with concrete which may result in some settlement in the future.

Does anyone have any experience in this?

 

[PEinc]

If you were worried about the soil falling in before you got the concrete poured, then you were probably doing it wrong. When underpinning, the excavation is done by hand and the pit is shored on all four sides with horizontal lagging boards as the pit is dug, from the top down, in lifts of about 3 to 4 feet. The hole must be safely shored.

Some "underpinners" dig a sloped excavation downward from the edge of the footing and then dig an open cut into the slope and under the footing in order to form and pour the pier. This is wrong even though many try to do it this way. It is wrong because the unshored sides of the excavation can collapse which can damage the building. Also, in order to form the front face of the pier, someone has to enter the excavation. If your unshored excavation is over 5' deep, you are probably in violation of OSHA regulations and someone will get hurt.

 

[jheidt2543]

PEinc:

Good points, but just to clarify, we NEVER let anyone into the underpinning pit under an existing footing. A 2' to 4' wide footing can be hand excavated without getting someone under the footing.

We face-formed the underpinned area and braced it back to the outside. And you are also right about the depth of excavtion and the OSHA requirements. I think the deepest we went in one lift was 8', one form panel.

 

[PEinc]

Sorry, but IMHO your method is a problem waiting to happen. It is common, but it is the way underpinning is done by people who are not specialists.

Just this week, a client of mine updated me about a job where a building facade was being underpinned by your method. One contractor was hired to dig the pits. Another contractor was hired to install the concrete. No one had overall responsibility for the underpinning. The contractor wanted my client to install tiebacks in the underpinning. I told my client to stay away from the job. Luckily, he did as I suggected. The "underpinning contractor" was trying to install 17' deep concrete underpinning piers in stacked lifts of about 5' per lift. Essentially, the contractors were going to underpin the underpinning several times. The tieback anchors were going to be installed in the upper lift of underpinning. I don't know what the contractor thought was going to brace the lower sections of underpinning. Over the weekend, rain caused excavated, unshored, underpinning pits to collapse. This further undermined the foundation wall. The building facade is now significantly damaged.

There are good reasons why underpinning should be done today the same way as it's been done for many, many years. The proper method is low tech but it works. No one has found a better, surer way to do underpinning.