Engineering Challenge

As most of my friends know, my brother bought a way cool cottage up north. And since he didn’t mind getting a fixer-upper, well, that’s what he got. It’s not in bad shape at all, really. But does need some work here and there. Just small things, like preventing it from sliding into the lake 😀

I exaggerate. But when we put up the first pictures of the cottage, and our good friend Walter saw them, he sent us email. He was very concerned about the supports. We were able to reassure him – my brother is a civil engineer, like him, and he knew about the problem and in fact it was one of the first things he fixed. The picture below is the before picture. If you look closely, you can see one of the wood beams getting very close to the edge of the support.

tony-cottage-09-pano

He also decided to level the cottage last year. Since he wanted to add some windows and replace patio doors, it needed to be leveled. So, a little at a time, he shored up each post and made the cottage level.

Then this year, after he replaced the patio doors, he was under the cottage and realized that the whole cottage was leaning out further. He figured that the load on the supports had changed, and as a result, the “piers” it was resting on started to lean out. He hadn’t thought of this happening, and I think he was a little upset because it diverted him from his other plans. He now had to put everything on hold until he could figure out what to do.

He spoke briefly about building a basement under the cottage (something he’d love to have), but it’s far too expensive. Then he started making plans to dig out the piers and straighten them out. I could tell it was plaguing him, he was thinking of it constantly. (It’s a thing we Perrella’s do — we worry over a problem, almost consumes us, until we’ve got a solution.)

At some point I realized he had stopped talking about it, and so I asked him how that was going. And he said, “Oh, I’ve got that worked out”. And he sent me these pictures. Jeez.

Cottage Support

He’s been calling this one the “Leaning Pier of Tony”. This one looked pretty frightening to me.

Leaning pier of Tony

He says this will hold the cottage steady through the winter and heavy snows, and he’ll have to dig out the piers and re-set them vertically next year.

Any of you civil engineers out there have some advice??

9 thoughts on “Engineering Challenge”

  1. Tony did a great job on the temporary support! I’m assuming that the movement of the pier is the base sliding\kicking out and not that the top is being pushed over. I’d guess that the toe of the pier is probably sliding on an inclined rock surface. Any guess as to how far down to the toe of the pier? If this is the problem, the best/easiest solution probably is to positively connect the toe(s) of the pier(s) to the bedrock by installing some dowels into the bedrock. One possible solution would be to install three or four dowels tight around the circumference of a pier toe. That by itself may be enough to stop lateral movement by I would probably try ‘attaching’ them to the pier toe with a couple of steel bands or something. Ideally when the piers were originally poured, rock dowels would have been an integral part of the pier toes, but cottage construction is often a little haphazardly done so guess you just have to make the best of it. 🙂

  2. Walter – thanks for the comment -appreciated. Wish the piers were on rock. Given the surrounding terrain, my guess is that there is about 10 feet of pit-run fill at the front (pictured) pier row and way less at other 2 rows. I asked original owners if they placed the pier footings below the original groundline. Thirty years ago, they went down about 4 feet and placed the piers on precast pads (maybe 16 inches square – there’s one in the picture). My take is the pads are on uncompacted fill that have sunk at the front piers, way more (up to 4 inches) than at the other 2 rows of pier. Think that as they “sunk” they pulled the other rows of piers over (distance increased and support beams can’t stretch so something had to happen), until they were all leaning in some kind of load equilibrium. When I re-leveled the cottage I likely re-started or sped up the process. The cottage has a truss roof so all the snow load goes to the front and back set of piers.

    My solution is to expose the pier pads front and back, and jack the pier/pad bottom forward so the piers are vertical, and hopefully jimmy another pad under 1/4 to 1/2 under the existing pad as I push it forward. Benefit – cottage is closer to the lake. I’m hoping that doing the front row only will be sufficient – also wondering if leaning the piers in the opposite direction will counteract the horizontal forces. I also plan to place a reinforced concrete “collar” just below the groundline of each pier to add to the lateral stability and as well as some bearing to the front piers. Finally hoping the tires will allow higher fill around the piers and therefore also help to support the cottage for the next 30 years, when I may need to worry about it again. I’ll be around 90 so, will need something to keep me going. Think you will have to visit to do a proper inspection.

  3. Hi Tony – partially unconsolidated fill – lucky you! 🙂 I think your analysis is probably bang on. I’ve seen lots of settlement problems that “suddenly” develop or re-occur in unconsolidated fills after long periods of stability. The very tenuous equilibrium that existed has been disturbed and the consolidation/settlement process is activated/re-activated. If the movement had been purely vertical I guess you could have just reshimmed the pier/beam connection every year or two ’till things stabilized again. The horizontal movement is something else. 🙁

    Increasing the bearing surface of the pier footing with the new pads should go far to reduce or eliminate the settlement movements. I’m assuming the pit-run is a clean free-draining material so that frost heave of the ‘collars’ is not an issue.

    With regards to adding more fill around the piers – I can see the lateral support benefit of this but would be concerned that this may set off a more global consolidation of any underlying unconsolidated fill. I had a slope failure investigation a few years back that involved a house in a residential subdivision that backed on a deep ravine. No issues for over 40 years and then “suddenly” tension cracks in the backyard, movement of the rear wall of house, and major angst for the elderly couple who had been there since the house was built. The slope investigation revealed that the slope had always had a factor of safety of just over 1.0 (which is the norm for most naturally occurring slopes). The equilibrium buster – landscaping – the couple had the backyard landscaped and the addition of approximately 0.4 to 0.6 m of soil to level the backyard and create some plant beds had been enough to set off slope movements. So I’d be wary of adding any surcharge to the fill as that may set off or accelerate consolidation.

    Site visit – that’s the ticket – Luisa and I will come up to help with the remedial works! Right Luisa?! 😀

  4. No problem. I have no idea what you’re talking about, but what the heck!! I’d hate to have his cottage get a much, much closer view of the lake, if you know what I mean.

  5. Hello Tony, this is Nic Perrella from Perth in Western Australia “down under”. I love the pictures – I have seen a few beauties in my time and I have reckon you have one on your hands, but I can just imagine the view over the lake.

    I can see you are a true Perrella that likes a construction challenge (must be in our blood), let me know if you need a hand – my tip from “down under” – there is nothing that a good piece of solid concrete in the right spot won’t fix. “30 years to fix it” – you are definitely a Pereella. If you shout me a trip to Canada I reckon we would have it sorted out in no time.

    I think Luisa is right we do worry over a problem until we have it sorted, I am the same and often will over do the solution.

    All the best

    Nic

    Regards

    Nic

  6. Nic – nice to hear from you, and condolences (kidding) on the Perrella afflictions. I definitely will be pouring concrete to fix it. I also like the spotting a trip/helper idea, but then I started thinking of those ocean fish pictures you sent. Hard choice – concrete mixing or fishing? How about – you spot me, I spot you! You have the Perrella frugalness (cheap) affliction too?

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