A stretchy garage design to tolerate ground heave.

[epsilonGreedy]

Following a setting out exercise I now know one corner of my house is 13 meters away from a willow tree and after checking with the NHBC foundations app the trench fill depth has been established.

 

The double brick wall double garage 5.6m x 7.2m is closer to the willow tree and I am thinking that rather than fighting ground heave with heavy duty foundations why not redesign the garage to flex. Is there any structural engineering precedence for this type of solution?

 

My current outline solution is an oakframe timber clad structure which I assume could flex a bit without incurring structural damage. Under this would be relatively shallow trenchfill footings, let's say 1.0m laid as 4 independent quarters that could move up to 5cm independent of each other. Looking at oak frame garage examples it seems they all feature a brick dwarf wall and so I think the dwarf wall would need substantial expansion joints at the quartered foundation boundaries.

 

Is my thinking leading me into a structural engineering fantasy land?

Edited March 29, 2018 by epsilonGreedy

[Jeremy Harris]

Have you looked at other options?

 

For example, if thinking of an oak frame, forget about flex and just place the frame supporting pads on something like ground improvement piles, like those that @recoveringacademic used.  These are just holes that go down to a depth well below the heave affected zone and are packed with stone.  The same system could be used to support a reinforced slab foundation, which would allow other construction methods to be considered.

 

Trenches are, in my view, probably one of the least effective ways of dealing with poor ground conditions, but somehow we seemed to have trench foundations engrained in the very DNA of the UK construction industry, for some unknown reason.

 

FWIW, you can also just use a thicker bed of stone under a slab to mitigate poor ground or heave, too.  Needs a deep excavation for really poor ground, but if you're already excavating a lot on site the additional cost may not be that great.

 

Best bet is to talk to an open-minded structural engineer, one who's willing to consider solutions that may not often be seen in a domestic build.  I found a chap with lots of experience of foundation design for motorway structures, and he was a goldmine of ways around difficult ground that could be adapted to domestic construction.

 

Another option would be to look at helical piles, they are quick to install, can bear their full working load as soon as they are installed, and have a long track record of being very reliable.  The Victorians built seaside piers using helical cast iron piles into sand, that are still standing today.

[ProDave]

How close are you building the garage to the Willow tree's?

 

Our house is less than 10M from Willows. The Structural engineer that designed the strip foundations specified we must dig down to the sandy clay sub soil (he had witnessed me dig 3 test puts so he knew the ground conditions to about 2 metres deep) and specified a strong (I think it was C35) concrete mix, and reinforcng meshing in the pour.

 

The reality was when digging the foundations no tree roots were encountered.

 

Our site is unusual that we have a burn running through it and the Willows are all along the edge of the burn, so no prizes for guessing where they get most of their water from.

 

 

[epsilonGreedy]

2 hours ago, ProDave said:

How close are you building the garage to the Willow tree's?

 

 

I failed to measure that one, it is about 8 meters. I was more concerned with the house until a structural engineer at the NEC warned the garage is a building people can be expected to enter so I should not anticipate reduced foundation requirements.

[nod]

Our house is within ten mtrs of large ash trees as is the garages

SE recommended 1.8 strip foundations double skin All to be surrounded by Clay board

[joe90]

Best to do a test hole and see what you have got!

[epsilonGreedy]

19 minutes ago, joe90 said:

Best to do a test hole and see what you have got!

 

 

When I look at the fees for test boring and soil sample analysis I feel like dig & discover might be the cheaper solution.

[Roger440]

What Jeremy suggested. If its an Oak frame jobbie, 6 piles of some sort, then do the floor in block paving. Worst that can happen is the paving moves a bit in the long term. Which can easily be re-levelled.

 

As Jeremy also suggested, we seem to regard trench foundations as the answer to everything, which from an engineering perspective makes no sense to me. We dont even put steel re-inforcement in most of them!

[bassanclan]

You can get claymaster (brand name I think) boards which are effectively soft eps which you line the outside of the trench with to allow the clay to expand a certain amount.

[CC45]

We just used polystyrene sheets - bc was happy with this.

[Dudda]

Just started foundations for a client in a house this week. I want a feature window as close to an old apple tree as possible so it's 2 meters (it's only one of two in the original orchard that remains). We we're going to cantelevering the house foundations over the roots but they're so shallow we're having to move the house 2 meters further away. Hope the poor thing doesn't die now.  

 

Anyway at 8 meter your a lot further.

[Mr Punter]

If the garage is detached and less than 30sq metres you don't need building regs