Retaining wall combining concrete blocks and PT lumber
New to the forum and looking for advice on retaining wall. I've removed an above ground pool from my property. The pool was located on a flat pad (sand over clayey soil) set into a sloping lot (grade of ~5"/ft). The pad sits above a more steeply sloped section of the lot (slope of 8"/foot). I need to build a retaining wall to support the soil, limit erosion, and level the soil sloping toward the pool pad. This wall will be ~40 ft long and 2.5 ft high. My options are interlocking cement blocks or timber retaining wall.
For aesthetic reasons and because the lot is hard to access, I prefer timber retaining wall. There is no way to get machinery into the property so blocks will have to be transported by hand over a long distance. Part of the reason for removing the pool was to decrease the load on the ground above the steeper section of the lot. I'd like to avoid reloading the ground with 200+ concrete blocks. Timber seems easier to move around also. I am considering a design such as this:
Instead of the usual stacked and pinned 6x6 designs. I would add deadmen 3/4 of the way up the wall, anchored to the post and the angled brace and extending back ~5-6 ft.
Because the lot slopes toward the pool pad, and the soil is clayey, I worry about excessive hydrostatic pressure build-up. As well, I live in a cold climate so I need to get drainage right to avoid freeze-thaw damage.
My question is the following. I would rather not lay down PT timber in direct contact with the soil. I was wondering if anyone had an opinion on running a single course of interlocking cement blocks (on top of a gravel-filled trench with drain pipe - the whole thing wrapped in geotextile) and then setting the PT lumber on the blocks, resting against the raised lip of the block (Allan Block in this case). I would obviously use deadmen along the wall for proper reinforcement.
I am also wondering if it would be useful to use a waterproof membrane against the lumber on the inside of the wall. The idea would be to tie this membrane into the gravel trench and drain pipe to limit moisture contact with the lumber (or is gravel sufficient and water percolates to the trench before pooling against the wall).
Any thoughts on this block/timber combo? Am i asking for trouble with the timber wall?
Thanks for whatever advice and help that can be provided.
A gravel bed 3" or so deep should be more than adequate.
Drill as appropriate and then drive 1/2" rebar through the wood and gravel.
any one stone will weigh a LOT less than any one timber. ;)
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