Remediation of a Peat Settled Foundation

[Ardshael]

I've come across a problem that I've found it difficult to find information on.

My client has a 2 storey residence with a crawl space that was built on at least 25' of mainly peat. Our investigation was performed to a depth of 30' with no sign of the peat ending. The house has differential settlement in the rear of the home. Of course, there was no geotechnical report completed for the home when it was constructed in 1999.

I have been looking for possible solutions for remediation. Can anyone steer me towards some reports or papers that deal with foundation remediation over peat?

Thanks.

 

[msquared48]

Ouch!

You are going to have to identify the bottom, somehow.

It is normal to use 2" to 4" diameter pin pile here.

Overexcavation probably not the answer.

Underpinning definitely not if used alone, but it could work if combined with pin pile and a grade beam system.

Do you have an interior bearing line or interior spread footings in the area of settlement?

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[Ron]

Agree with Mike...you have to find the bottom. With that much peat, consolidation will take "forever" and settlement will continue.

This will not be an easy remediation. Since the structure is "off grade" solution option are more numerous.

 

[MiketheEngineer]

Possibly heli-coil or miniature piles driven to refusal.

Yes - You do have a problem!!

 

[Ardshael]

Well... I'm both happy and sad. Happy that my brain wracking isn't missing something and sad that there isn't something.

I was going to write that the best solution might be to just rebuild the house. Of course, there's still the issue of settlement to deal with but at least it's a fresh slate to work from.

msquared, I don't recall 100% but I believe there may be an interior bearing line. The house is not a simple one floor all at one level building and I can't see enough of the roof in my photos to tell.

One of my thoughts was to use helical piles with regular monitoring and adjustments. It's definitely not an ideal solution which is why I was hoping someone might have a skyhook solution kicking around.

 

[JedClampett]

I'd recommend a gallon of gasoline and a book of matches. And make sure the fire doesn't spread into the peat.
Did your client buy the house from someone? This sounds like a disclosure type issue. I hate to be all litigious, but whoever built the house should of known better. It's not like 1999 was during the dark ages or something.

 

[Ardshael]

Jed, that's rather humorous since the "bonfire" solution was mentioned while discussing what to do with the house.

Yes, the client bought the house a few years or so after it was built. You're also not the first person to mention disclosure of the house. When talking with the neighbour, who built around the same time, an inspector apparently told the contractor that he just needed to make the footings bigger to deal with the poor "soil". Of course we all know that designing for bearing pressure and designing for settlement is not one and the same. The house appears to be failing due to settlement. There were no signs of distress (cracking) in the concrete so it appears that at least the bearing pressure design is holding up.

 

[hokie66]

Ardshael,
I think your scheme of regular monitoring and adjustments is the best way of dealing with an "impossible" situation. That general method has been used in commercial buildings overlying strata subject to long term settlement. It won't be cheap for a house, but retrofitting footings with jacking points may be able to save the house...if it is worth the effort.

 

[FixedEarth]

As a rule of thumb, peat settles 25% of its thickness over many decades. So a 5 ft thick consolidation is what is estimated +/-. Your problem is in the foundation and not the house. Why not take the house off the foundation and then design/build a new foundation (drilled pier with enlarged base) bearing on the stratum beneath the peat? of course you will need a suspended slab. Since you are providing a new foundation, it will be easy to incorporate a basement.

 

[BigH]

A few questions, if you don't mind. When the house was built or subsequently was there any fill placed to raise grade - even a couple of feet? The house, you indicated, was built in 1999 - when did the present owner buy the house? - and was there any noticeable problems of settlement at the time of purchase? How much settlement are you talking about? inches (or less) or feet? Where is the site's water table - at a depth? or at ground surface?

Why didn't you drive a dynamic cone (Canadian style pentest) from the depth of 30 ft to try to ascertain the depth of the peat? It seems that once you went to 30 ft you might have kept going. What does the geology of the area show? (i.e., is this typical of the Burnaby Peat Lake (British Columbia) - I once investigated a muskeg are and ended up shoving a split spoon down - up to 20 ft - through the muskeg and then jammed the spoon into the soil below to obtain a sample (in my case sand).

It would have been nice to have preloaded the site before the house was built and then build the house on a raft-style foundation. Many areas do this as a matter of fact before construction (Richmond BC for example).

As for remedial measures, it will be expensive, I think to try to tackle this problem - whether pin piles, helical piles, or other means.

So thinking a bit out of the box so to speak, if the water table is relatively deep - say 5 or 6 ft - and stable there, one might consider underpinning measures such as underpinning the peat below the foundation (at density of about 60 #/ft3 with a foamed grout (such as Elastizell) at 30 #/ft3). Yes, this would be expensive - but will reduce the loading to a fraction of what is there now - else widen the foundation to . Compare to moving the house (and this can be done) doing remedial measures beneath and then returning the house to new foundation - say a raft of Elastizell - or you can preload and then move the house back . . . As implied, these are expensive operations. Depends on the expense of the house - with the market the way it is now, maybe abandonment might be in order . . .

Anyway - some day off thoughts . . .

 

[Ardshael]

Thx for the thoughts.

Actually, taking the house off the foundation was something I was thinking about as well. The house would need to be completely moved off the site. If we preload, my concern would be the home on the adjacent property.

The house is on the lake and the water table between 4 and 5' deep. There are two creeks about 150m to either side of the property and the neighbours mentioned the area is prone to floods. With that said, I wouldn't be surprised to see the water table come up closer to the underside of footings (approx 3-4' deep to my estimate).

DCPT was performed on the site; however, the drilling contractor only had 30' of rod with them so going deeper wasn't an option unfortunately. Next time I will make sure they bring more when they come out.

BigH, this home is in the Okanagan Valley (British Columbia) on an alluvium fan complex. There are very few places here that are on peat but this area has it. I don't know the date of purchase but it was around 2003 (+/- 2 years). From what I understand, the vast majority of settlement (approx. 2" over 2' to the west and 1" over 2' to the north, perhaps 12-18" total) was post purchase. There was a small cottage built on the property prior to the current home. I believe the foundation was about 6' deep since we hit what appeared to be a buried part of what remained. Fill as well as silt/sand was observed to a depth of 5'.

BigH, I have read some of your past posts regarding Elastizell. It's certainly an interesting product but I'm not familiar with it. I would be concerned about uplift pressures during flooding or higher than normal groundwater if it was used.

Whatever solution I give him, it will definitely be expensive and there are overhead powerlines that also pose a logistics problem.. Whether the solution is worth it will be up to him. It's a lakefront property and I know what he paid for it and what the homes in the area are worth. He wants to completely renovate it so who knows, maybe he'll decide to completely rebuild.

 

[BigH]

agree that Elastizell might be prone to uplift - which is the question on the permanent and high water tables. I used the Elastizel on a structure in the Burnaby Lake Plain years ago - it worked great - we had 25 ft of peat and 25 ft of soft clay below . . . but the structure was on piles - the problem was the first floor was 3 ft above the outside grade necessitating ramps - and it is the ramps that were settling.