Check my spec: 5' post and plank retaining wall

23 Oct 2020
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United Kingdom
Hi folks,

I've done a bit of homework and have designed a retaining wall for our home, and thought I'd post the design here for constructive critique. Any feedback most welcome.

The house is set back up on a slope, with a mostly flat 15m long front garden which then slopes down to a space full of brambles and holly trees in the front and then a private driveway used by neighbours terminated by a cul de sac. My goal is to clear the plants in front of the garden which will free up a space that is 12.5m wide, 2.5m deep and about 1.5m tall, remove a bit of soil, secure the space with a (pressure-treated) post and plank retaining wall, and then establish parking in front. Here's how I have designed things so far:

Retaining wall:
- I'm going to use 10 (spaced at 1.25m intervals) 6x6" square posts at 3.1m long, buried in augured out post holes at 1.6m deep, with 4" allowance at the bottom for 3/4" gravel and then filled in with concrete in 12" diameter post holes. I'll aim to fix posts in concrete to lean at about 10 degrees backwards towards slope. I'm hoping to use 2x4s and timber stakes temporarily nailed to the posts to hold them in place until concrete sets.
- Planks will be 2x6 pressure treated timbers, secured to the posts using double-dipped galvanised 3" screws
- I'll fix 6x6" dead-man beams of 3m long to the top four flights of the wall at 8' intervals (every second post) staggered across the top four courses secured with rebar or horizontal reinforcements
- I'll lay filter-mesh at the front of the newly excavated embankment and then backfill...
- Behind the planks, going back 12-18" - I'll lay 12" of 20mm gravel, topped with a french drain using 100mm perforated drainage pipe, and then remaining space filled with 20mm gravel up to garden level
- To be safe, I'll drill 1-1/2-inch drainage holes in bottom plank at 4ft intervals
- The wall to be topped with a 2x10" timber, chamfered at the edges for a nice visual

In front of the wall in the space where I've removed all the plants, I'll excavate out soil (down 250mm or so) to compacted subsoil. In that space, I'll add:
- Hardcore / Aggregate (MOT Type 1) to 150mm depth (about 4.5m3) - to be compacted with a whacker
- Sand to 50mm depth (1.5m3)
- Filter fabric
- 3cm deep gravel grids filled with 20mm gravel at 50mm deep

This all comes in at about £2k and that's my budget for the project.

A few open questions for me:

- Are there additional things I should do to strengthen / manage the zone of influence in front of the wall where cars will be parked? We won't have any structures above the wall, just grass and the occasional picnic.
- Has anyone had experience working with horizontal deadman beams on a wall like this? Is rebar or horizontal timber reinforcement more effective? I'd rather not install these all the way down the 5' of the wall, even at 45 degrees as this will be a lot of excavation and the slope is currently quite stable.
- I don't necessarily want to reopen the debate about plastic sheeting, but in general am not planning on putting plastic directly behind the timbers, but instead aim to let the gravel and drain handle water. Anyone have experience that recommends a different approach?

Thanks in advance for any comments that folks can share from experience on similar projects.
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