Sleeper retaining wall - Sufficient strength?

[welshsonic]

We are hoping to terrace our back garden using railway sleepers to create a series of small retaining walls. After reading the main site and some other posts, I'd like to solicit input / opinion on our proposed project as I don't want to spend a lot of money only to have it fall down in a few years time or cause other serious problems....

Garden is currently on 2 levels (roughly), with house on higher of the 2. There is about a 6ft height change between the levels and it is currently just grass covered sloped ground. The slope is quite steep and would like to create a series of levels for added interest, better planting, etc. Soil is clay.

Each retaining wall would be 600mm high, made of 4 courses of oak sleepers 2600mm x 250mm x 150mm. Thus the 250mm is the width, 150mm is the height of each sleeper. Sleepers would be laid in brick pattern connected with Timberlok screws. The length of each wall would be 15m with steps somewhere in between. The wall ends within approx 4-5m of the side boundary, where the ground would remain sloped as is current.

There would be 3 retaining walls as per above and starting from the highest point (where the house is) and working down:

House; <2.5m distance> 600mm high retaining wall; <2.0m distance> 600mm high retaining wall; <2.0m distance> 600mm high retaining wall; Lowest level of garden

Would I need to use any 'tie-backs' since the wall is so long? If so, how would you recommend construction of that & what materials would you use? What size foundations would you recommend? Would you recommend any sort of DPC or other membrane to protect the sleepers from the soil/gravel that is backfilled behind them?

I would be leaving small gaps between the sleepers on each row to act as weep holes.

Thanks for any opinion/advice.

[Tony McC]

Sounds OK to me - the only suggestions I'd make are...

1 - batter the sleepers at an angle of 5 degeres for added stability

2 - incorporate a geo-mesh from the joint between course 2 and course 3 and run it back beneath the build-up area for enhanced stability

3 - backfill the first 200-300mm behind the sleepers with clean granular material (a pea-gravel or similar)

4 - bed the lowest sleeper on 100mm of ST1 concrete for stability.

...none of these are critical, and it would be no trouble to omit (1) if you prefer true verticals.

I know I may have baffled you with science with this answer - if so, let me know and I'll try to do you a drawing.

[ABILITY]

Timberlok are good for this, but you may struggle if using hard wood sleepers - often shear the bolt when tightening up fully.
We have used lengths of M16 studding bar ( off cuts in fact ) and pre drill sleepers with auger at 15mm Dia and smack the bar in - rock soild.

Also have used built in sleepers at right angles to the face as tie backs - if possible back to under / into next step of terrace, thus all becomes a giant crip type structure.

Are you sure you are ok to terrace that close to house? Sleeper walls won't do much to support foundations!

[welshsonic]

Thanks to both Tony McC and Ability for the replies. Had a few more questions based on your responses, so if you wouldn't mind....


1) Where would I be able to purchase these M16 studding bars and/or offcuts? How difficult are they to cut to size?

2) I thought having the 1st retaining wall 2.5 m from the house was sufficient for not interfering with the house foundation stress. What sort of distance would you think is safe if 2.5 isn't?

3) Not sure I understand the 'crip type structure'. Any way to elaborate? I think you mean connecting 1 wall with the next one down with a sleeper but under the soil??

4) For foundations, 100mm ST1 concrete -- is that width depth or both?

5) For backfill, 200-300mm gravel behind the sleepers -- is that width, depth or both?

6) If I understand the geo-mesh component correctly -- Did you mean lay it inbetween courses 2 & 3 so it is in effect a horizontal sheet? How far back into the soil would it go? Did you mean a geo-net/geo-grid type material (like on the main site) or something else?


Thanks again for your help.

[ABILITY]

1) Does not need to be M16 any diameter M10 or M12 will do, its threaded rod sold in most merchants, we just happen to use alot of M16 with alot of off cuts. Can be cut with a hacksaw or small angle grinder.

2) Depends on the house foundations / ground conditions.

3) Think you've got it, the weight of wall behind then helps to stop front wall 'rolling' forward ( alternative to geo-mesh )

4) Depth under sleeper - depends on ground, if carving into good solid ground then may not be needed.

5) Width behind wall to aid drainage.