Hard landscaping experts - heavy troughs on gravel advice please

johnnypenstemon

Hi all, I’m going to build two 45cm high wooden troughs to act as garden ‘room’ dividers. The first pic shows similar on a pavement. They will be surrounded by gravel, and the second pic shows the lowest of my three timber layers mocked-up in situ.

What would you place under these to ensure stability? Is stability an issue given the weight? I can’t work out if they are more or less likely to subside given how heavy they will be.

Should I put 7.5/10cm of compacted hardcore on compacted earth where timbers will sit?

The timber is treated C24 kiln dried timber, for exterior use as long as it doesn’t touch soil.

Any pointers much appreciated. I’m keen not to take short cuts, nor to over engineer...

Thanks, Johnny

johnnypenstemon

I should add that the gravel can be lifted. I’m concerned the gravel will shift long term, causing the troughs to sink unevenly. There’s a membrane under the gravel.

Ferdinand2000

I think the only issue I can see is if your troughs are tall and narrow, which would make them unstable.

Rather than constructing an elaborate base to keep them stable, I think I would either attach a couple of strong flat metal cross pieces underneath that would poke out at the sides by a few inches and cover them with the gravel, or even screw a couple of robust L-brackets (eg for shelves) on the outside at the bottom to do the same thing. Like stabilisers on a bike.

The top pic ones look less stable than the bottom pic ones. I think yours are the wider set which look admirably stable to my eye, though it has to be judged on the spot.

My only other comment would be that if you may need to move them slightly (eg to let something big through the gap), make sure you know how you will do it. (eg scrape out a bit of gravel just in from one end and use a trolley jack or pallet truck).

F

Fairygirl

They'll be fine. 
The top ones aren't the OP's @Ferdinand2000 - it's just an example.

If you're worried about them moving, you could always stick a couple of metal rods down through them, into the ground. One at each end - opposing corners. In the same way it's done with sleepers to keep them stable, and in line vertically.
Or - a batten in each corner, through a hole in the base, and concreted into the ground.

As @philippasmith2 says, unless there's a problem with the ground underneath, there's really no reason for them to move. 

johnnypenstemon

Thanks phillippa, ferdinand and fairygirl, lots of ideas to ponder there. @philippasmith2 the potential surrounding gravel disturbance I hadn’t anticipated, good point. @Ferdinand2000 , as you say my troughs are wider than the other pictured so there won’t be an issue with them toppling.

Rather than moving laterally, I was concerned that the troughs could drop and look uneven over time as gravel is not a consistent material for a base - in the same way that brick edging set just on sand rarely stays level after a year or two. Now I have some options to consider short of deploying mini trenches, hardcore, tamper etc.

Thanks again, Johnny

Fairygirl

@johnnypenstemon - a simple alternative is to put in a very basic concrete plath under them. It wouldn't need to be very thick, and you can buy the ready to use stuff, which saves mixing etc. It would just be enough to stabilise the ground if you're worried. 
The only drawback is that you need to have a couple of, or probably three, battens on the bottom of the troughs, just to ensure they were clear of the concrete, and could drain, but that's easy enough to do. The gravel would hide it all. 

johnnypenstemon

@Fairygirl Thanks again FG, I’ll add that to the roster for consideration. Right, I’d better crack on with it..!

Fairygirl

Aye - no slacking....

Ferdinand2000

johnnypenstemon said:

Thanks phillippa, ferdinand and fairygirl, lots of ideas to ponder there. @philippasmith2 the potential surrounding gravel disturbance I hadn’t anticipated, good point. @Ferdinand2000 , as you say my troughs are wider than the other pictured so there won’t be an issue with them toppling.

Rather than moving laterally, I was concerned that the troughs could drop and look uneven over time as gravel is not a consistent material for a base - in the same way that brick edging set just on sand rarely stays level after a year or two. Now I have some options to consider short of deploying mini trenches, hardcore, tamper etc.

Thanks again, Johnny

That thing that I love about gravel is that it covers a multitude of sins.

If the earth moves, you can just rake it flat or add some more.

I have done patios of pressed council slabs laid onto gravel in an edging. When it is a little less flat you just lift up the corner of each paver and add a handful. Not a Chelsea or Margot Ledbetter solution but useful if you are on top of ground that will be compacting differentially.

If they are strong, you could just put a 2’ x 2’ x 2” concrete slab on your gravel or with one in the middle. They are only about £5-7 each at builders’ merchant.

But they weigh 40-45kg each, about 20% more than a big 2.4m concrete 100mm x 100m fence post, so you need the right sort of assistance to hand to get it in your (soon to be ex) friend’s car, out again and move it around at home. You do not want to drop it on your fingers or toes.

But they don’t move much by mistake.  

johnnypenstemon

Ferdinand2000 said:

If they are strong, you could just put a 2’ x 2’ x 2” concrete slab on your gravel or with one in the middle. They are only about £5-7 each at builders’ merchant.

Funny you should mention that, I found a few leftover robust slabs in the garden and settled on your ploy. I’ll place them on compacted soil with the troughs sat on atop. Then fill around and above the level of the slabs with gravel to conceal.

I clocked that the issue boiled down to the gravel around the troughs not being contained. When it’s disturbed over time it will likely displace some of the gravel under the troughs and hey presto, one or both may drop and be too heavy to raise.

A friend will be relieved not to be driving me on a little trip for “nothing much”  .

Fairygirl

I should really have thought of that @johnnypenstemon
@Ferdinand2000 was way ahead of me. Much simpler idea 

If they move at all, which is unlikely, you can always mortar them down too. Keep us updated once you have them all done and planted. It's always nice to see finished results, and I think it's always a great idea for simple screening