Jacking this Old Leaning House

[Loki2]

Though originally a geological engineer, I now practice in the environmental field, and as such, my foundation and soils engineering knowledge is now somewhat faded... so in the hopes of getting some sage advice, here is my foundation rehabilititation issue:

We purchased (at a great price!) a post-war 24'X30' bungalow with a lean to it -- 6" lower along one side. We have dug a few test pits and found that the problem appears to be differential settling due to: 1) no footings at base of perimeter wall (crawlspace, no basement), and 2) about 2/3 of the perimeter wall base sitting on a clayey/peaty lens, versus a fine dry sandy strata under the 1/3 that has settled minimally.

We are considering, given that there is no footing, of jacking both house and the concrete perimeter wall up as a unit. We are looking at jack points every 6-8', strategically under supporting beams and existing cracks in the wall. The plan is to dig down from the outside of the house at the jack points, pre-pour jack pads, raise the wall to the desired level, then pour footings under the remainder of the wall.

I have two questions for you experts:
1) If anyone has experience with this particular strategy of jacking house and wall as a unit, please comment on your success or failure, give any advice for strategies.

2) Given the clay soil conditions, a one-story woodframe bungalow with asphalt shingle roof, and the settlement history of 6", what are the opinions out there of a desirable footing design? We do not intend to put a second story on the structure, and just want to find a cost-effective solution to this problem. I can do more detailed sizing calculations myself, but really want more your ideas of options we could consider.

Thanks very much!

 

[civilmsu]

You mentioned the prescence of peat at the site. Peat typically undergoes decomposition, resulting in large secondary compression (depending on the amount of peat at the site). One method (and it may be costly) would be to found the residence on piles that extend into suitable soil.

 

[MTHelical]

A more cost effective alternative would be the use of screw piles to give you a guaranteed deep foundation, with a bracket to raise the bungalow to the desired elevation.

 

[Ron]

Using helical anchors/screw piles at reasonably spaced centers will get you below the poor soil conditions and you can use the supports to jack the house from there. For these purposes, you might want to consider fairly close spacing (48 inches) and using an "L" bracket at the top for attachment and jacking. Once at the level you desire, then you can encapsulate the system in a concrete stemwall.

Another option is to hire a house mover to pick up and support the house, move it a few feet so that you can get good access to the foundation area, and then put in whatever foundation system you like. One easy system that will work for this based on the soil conditions you have described is an augered concrete pile.

Good luck.

 

[Loki2]

Thanks very much for the responses to date -- you all agree that the best course of action is to extend piles of some type down to better soil. We're therefore considering doing a deep test hole to determine if there is a more favorable horizon within 10' of surface. Based on my understanding of the area geomorphology before development, I think that this clayey lense is not extensive in area, therefore, hopefully it does not extend to great depth.

Thanks again! I will keep you all posted on progress.

 

[jdmm]

Loki2

If the "peaty layer" is the problem (you need to be sure) and if it is not very deep then underpinning with jack points as you have described would likely be an economical solution. If the problem is deeper or below the water table then screw piles become more economical. You should be sure that the peaty layer is the problem by comparing the amount of settlement vs thickness of the layer and by exploring below the peaty layer to confirm good ground to depth.

Depending the details of your particular case, a very cheap alternative might be to disconnect the plate of the house from the foundation wall and shim the settled portion level then reconnect the plate. This does not require that you lift the foundation wall. Depending on the soil conditions in your 'peaty layer' it might not prevent further settlement. We have used this approach where we have assessed further settlement as being small compared to the initial settlement. If decomposition of the peat is a question then the alternative leveling below the peat layer would be the best approach.

regards

 

[geonerd]

A cablelock or permalock shoring solution would be great in this case. It would probably cost between $5000 to $10000, depending on your location and size of your home. These methods are used extensively in the New Orleans are, where we have very soft to soft clay foundations. See the website.

http://www.cablelock.com/

Permalock is similar except instead of a cable, each cylinder has a treaded 3/4 inch rod cast in it. See the website below:

http://www.jmgraphics.net/permalock/Perma_Lock/perma_lock.html

 

[boo1]

Using helical anchors/screw piles and using an "L" bracket at the top cost ~$1000 US in NW FL for typical installation.

 

[phillyfrank]

Before going to al of this expensive and potentially damaging remedial foundation work, I would get a quote from a housemover to move the house to a nearby site where you have drilled to see if peat and silt are present.

If your lot is too small, have the house moved off the lot, excavate out the foundation area, replace it with compacted fill, and put the house back.

 

[clayR]

You could also relevel the structure and improve the soil with compaction grout. The problem would be finding someone in your area to do the work. cr