construction note no.6001
subject: construction of log pole retaining walls
For related notes and articles, see Timber Walls Thread
fig one. Drawing of bank section showing of log pole retaining wall, creating two level tiers by cut and fill.
fig. two. Site under construction using log poles as retaining walls to terrace the steep slope. Poles have yet to be trimmed to height, wired and backfilled with drainage medium and soil.
fig. three. The finished terraces
We have seen in design note 2073: changes of level that the terracing of banks is a good method of dealing with steep slopes, increasing the stability of the slope, the visual display area and minimising maintenance. There are many ways and methods of building such walls: log walls are suitable in an informal situation where the ground undulates or the design is of a curvilinear nature.
An easy and effective method of construction is as follows:
- Dig out a trench wide enough for the base of the poles plus 200mm/8" each side. depth will depend upon height but should be approximately one third of the pole's length. ie a pole 1200mm/4ft out of the ground should be buried 600mm/2ft under, making total length 1800mm/6ft. On very steep banks, make this 50/50.
- Place poles to trench on a bed of wet concrete, ensuring they are vertical in front elevation but leant back slightly into the bank on side elevation. Allow height of tops to be uneven.
- Pack poles tightly together, four or five at a time, place concrete mix around them to depth of trench (minimum 200mm). As you fill the trench, you can add up 20% rubble to the mix to help wedge the poles in place.
- Wedge end pole with a length of batton whilst preparing the next batch.
- Once concrete has set, (minimum 48 hours) wire backs of poles together using 3mm galvanised wire and 30mm fencing staples. Do this just below the soil line.
- On long straight stretches, or walls over 0.9m/3ft height, horizontal tie-backs may be needed into the bank.
- Gaps between poles act as drainage outlets.
- Backfill rear of trench with drainage medium (gravel) .
- Place topsoil to top 300mm/1ft (over drainage medium), tread down firmly.
- Trim poles to (staggered) height with saw, leaving some 100mm/4" above the soil level to allow for a mulch to be applied.
summary:
- Use peeled chestnut where available
- If using softwood poles soak butts in preservative
- Bury 33% of pole in trench
- Place 4 - 6 poles to trench at one time
- Lean poles back slightly into the bank
- Ensure they are vertical to front face
- Once set wire together along backs
- Use tie-backs to bank on long stretches or tall walls
- After 72 hours, backfill with drainage medium and soil
- Cut poles down to height (if excessive)
For more information, view the Timber Walls Thread
