Increasing surface levels within an RPA

[benedmonds]

I have a client developing a property. They have a mature old oak tree growing at the top of a sunken garden, they wish to fill the area so that the front garden is level. The sunken garden is approx 1m deep and has a patio base with decorative rock around the perimeter.

 

Is there a way we can fill it up to ground level and ensure we don't damage to rooting environment below? The ground level around the trunk will remain the same..

[kevinjohnsonmbe]

I have a similar scenario where ground level has to be raised within RPA of a TPO Beech Ben. Still talking with architect / LA to finalise an acceptable solution. If what has been proposed is not acceptable, I'll be getting in touch with Laura at

 

Geosynthetics - Cellweb® Tree Root Protection

 

Let us know if you get a suitable solution?

[Paul Barton]

What do they want the top surface to be?

 

If it could be a gravel surface, could you fill the hole with large stones, cover with a membrane and then dress with gravel? That would still allow water to percolate down to the existing soil, and gaseous exchange.

 

If they want to turf the top it could be more tricky trying to create a sub-base that allows the above but is stable enough. Perhaps large stone base, some Silva cell crates, membrane then turf?

 

Let us know how you get on.

[Paul Barton]

I don't know if you can fill structural cells with stone instead of soil. Silva Cell Tree and Stormwater Management System From Deeproot

[benedmonds]

They are planning to have it as lawn.

 

My concern even with some crate system to prevent compaction it is going to end up reducing gas exchange. I will see what the cell web folk try and sell me..

[Chris at eden]

Sounds like an engineering operation so you may need planning consent although LPA's usually try to dodge it.

 

What about a concrete raft on mini piles with a clear void beneath. Fence posts and gravel boards around the outside and infill with what you like. Cell web may be best. You would need to redistribute surface water underneath somehow. Irrigation pipes connected to weep holes maybe or drainage holes in the bottom with something to stop them clogging.

 

A bit out there bit I don't see why it wouldn't work. Expensive also and needs an engineers input maybe.

[benedmonds]

Plan of site..

[benedmonds]

So cellwebs idea, is to put down membrane, then cellweb filled with clean angular stone then membrane and top soil on top...

 

How much topsoil do you think would be OK? The issue is that too much soil is going to impede gas exchange, but to little and the grass is never going to grow on what will be a very free draining spot.. 30cm?

[Paul Barton]

You're going to need alot of cellweb to fill a 1m deep hole!

 

How about a fake grass - some of them are pretty convincing now and you'd get round needing much soil as I think you lay it on sand.

[treeseer]

How badly does client need a lawn? Any way to restore sunken garden to acceptable appearance?

If not, remove hardscape before filling with soil (good advice above) so the oak roots can colonise the fill. Could be anet benefit over time for oak if done well.