Hollow or dense blocks for a 1m high retaining wall?

7 Oct 2014
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West Lothian
United Kingdom
Hi all,
Can I ask for guidance on which would be most appropriate for a garden retaining wall:
- single skin of hollow blocks with vertical reinforcing bar & concrete fill
- double skin of dense blocks with concrete fill (and rebar if necessary?)
- dense block rear skin and facing brick front skin (with concrete fill?)
I would likely finish the first two options with brick cladding or roughcast. Which option would be easiest for a first-time bricklayer, yet strong enough for the job? Footings of 600mm wide x 300mm deep plus one block course below ground, with 5 courses above ground.
Thanks in advance!
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You either need careful design to prevent overturning or heavy mass. I'd keep it simple. Build a heavy backing from blocks laid flat (215mm walling) for about half the height and then revert to 100mm blocks on edge. Face the whole thing with bricks tied with wall ties.
Wow - sounds rather heavy duty given the footings extend 600mm below ground. Do you believe the reinforced/cement filled options might fail?
I'll be running a field drain along the base behind the wall, and the retained ground will be level?
According to your original description the foundation would be 500mm below ground (300mm plus 200mm) which I doubt is enough. Plus it would only resist overturning if the wall and foundation was fully mechanically connected. That's what I meant by careful design. Just placing a reinforced wall on a foundation won't work - it will just overturn at the base. Plus a reinforced cavity filled wall is generally more tricky - it involves more operations and more materials. Laying blocks on flat for a few courses is easy. Facing up with brick is easy and the foundation is less critical.
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I am a firm believer in a construction incorporating a so called toe where the foundation is constructed into the lower area of land, and the reinforcing bars that will go into the cavity or into the hollow block are laid into the toe the toe should extend about a meter beyond the face of the wall.

Suggest you Google retaining wall toe.

it is worth considering a mix of hollow block later in filled with concrete, a cavity where the reinforcing can rise up from the foundation within the cavity, I believe it is easier to get the reinforcing rising up in this cavity rather than inside the hollow block, a little persuasion is often needed, to get some of the reinforcing bars sited where you want them. it is sometimes problematic, getting the reinforcing in the correct position?

The use of solid block to complete the wall and the subsequent facing on the wall.

As has been mentioned it is important to carefully consider the design and installation of a drain at or near the base of the wall, a drain will reduce the hydraulic load on the wall. the drain should be surrounded in pea gravel and the gravel contained within an envelope of geotextile again a Google search for draining a retaining wall will give some ideas
Build a heavy backing from blocks laid flat (215mm walling) for about half the height and then revert to 100mm blocks on edge. Face the whole thing with bricks tied with wall ties.
You've convinced me John. Should the cavity between these be filled with concrete (and even rebar into the footings), or just use wall ties? Perhaps dampness would be an issue and I should use a DPC on the facing skin 1 course above ground? I dont suppose the back of the dense block skin requires any such waterproofing - just the pea gravel and field drain?

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