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Consents and Engineering
Retaining walls over 1.5m high will require a building consent from the Local Body Council. Walls that carry extra load on top from traffic, nearby buildings or steeply sloping banks also require a building consent. This guide is prepared on the basis of no traffic, non-boundary and with a maximum slope of 1 in 10 above the wall.
Soil types can have a significant effect on how a wall performs. For the purpose of this guide the ground has been taken to be a firm clay soil type. Other soils may require specific engineering design. If in doubt, check with your local council.
This information is provided as a guide only and does not constitute a complete design aid. As such Crofts accept no responsibility with the design and construction of pole retaining walls or their suitability for any purpose. The advice of a suitably qualified engineer should be obtained.
Retaining Wall Concepts
A retaining wall has to be strong enough to support the load of soil pressing against it. This load determines the size of poles and the thickness of retaining timber to be used so that the poles or timber do not break. However, the most important part of the wall is the part below the ground that anchors it. If the footings, foundations or the soil is weak then there is nothing stopping it from falling over. One of the key things is to drain water from the bank behind the wall. If a wall doesn’t drain well then the bank behind and footings can become waterlogged and the wall fall over. Good drainage is critical to the success of a retaining wall.
Building your wall
- Draw your plans and calculate the quantity of materials required.
- Assess your design and check whether it will require specific engineering and or building consent. Check that the proposed wall is clear of sub soil drains, electrical, water or gas services.
- Excavate the bank to give at least 250mm gap between the back of the wall and the bank.
- Make sure the ground behind the wall is level or slopes to one end to provide drainage.
- Dig or auger the foundation holes. Depth of the poles will normally be at least equal to the height of the wall above the ground. Allow 100mm extra for the base concrete.
- Put 100mm of concrete into the two end holes. Set posts on top of this concrete making sure that only uncut or preservative treated ends go into the holes. Check that the top of the posts sit high enough.
- Set the posts at the recommended 1:10 inclination from vertical towards the bank. Fix the poles in place with temporary braces and ground pegs. (Small, low garden walls may be installed vertically as long as sufficient depth into the ground is provided)
- Use two string lines (top and bottom) run from the end poles to align the back side of the intermediate poles. Set these poles in place in a similar fashion to the end poles.
- Check the poles are all vertical and plumb when viewed from the front.
- When all poles are set in their final position pour in the concrete foundation. Tamp the concrete down with a piece of timber and leave to set for at least two days.
- Nail the horizontal rails to the back of the poles, starting at the bottom and working up to the top of the wall. Stagger and make joins only at the back of the poles.
- Once the wall is complete lay 50mm of clean free draining metal along the bottom at the back of the wall and then lay over the drain coil (with sock). If possible protrude both end of the drain-coil so it can be flushed. Cover with another 250mm of free draining metal.
- The back of the wall can have a polystyrene sheet attached or backfilled with for 250 to 300mm with a scoria drainage material to within 300mm of the top of the wall. Finish to level with topsoil. (geotextile cloth can be used between the topsoil and the scoria if drainage is a concern)
Poles or Piles
Poles or piles should be H5 treated high density pine to meet the “ground contact load bearing” requirements.
The timber for the wall should be H4 treated and can be either a rough sawn, machined or tongue and groove profile. If the wall has been engineered the specifications may require specific timber treatment or stress graded material.
Pole and Timber Sizes
Typical Sizes for a Non-boundary, no traffic wall with a 1:10 slope above the wall and 2 to 2.4m of level ground in front of the wall in stiff clay type soil.