|
Tips for Retaining Walls
The example illustrated below is for a simple timber pole retaining wall at a maximum height of 1000mm in firm, stable soil. Retaining walls over 1000mm high within your property yard or on boundaries, may need appropriate consents. We recommend that you contact your local council authority for their regulations and advice before you begin.
A registered civil engineer should specifically design timber retaining walls higher than 1200mm. This also applies for walls supporting roads, driveways, parking areas or specific loads.
Check that the proposed wall is clear of sub soil drainage, electrical or gas services.
Low, non-load bearing garden retaining walls may not have to be set at the suggested inclined angle. It is advisable though, as the wall may be subjected to ground pressure behind it (see illustration 2).
Not all poles are a consistent diameter and most are tapered. To avoid misalignment, poles should be turned end for end alternately.
Set end (or corner) poles first. This will ensure that the wall is aligned properly.
Retaining Wall Principles
The proposed wall will have to be strong enough to withstand the ground load pressing against it. The poles below ground provide the anchor and restraint.
A simple rule of thumb always applies to timber retaining walls: The depth of the embedded poles should equal the retaining wall’s height above the ground (see illustration 2).
If the foundations are not sufficient, nothing will stop the wall overturning.
Always ensure that there is good drainage to release water from the cut bank or ground behind the wall, otherwise contained water pressure can build up and could overturn the wall (see illustration 3).
Mitre 10 Handy Hints
Save effort by buying timber cut to length at the timber yard, but remember to buy longer posts if you are setting them in concrete instead of using post brackets.
Always remember to measure twice – cut once.
Materials and Tools
Determine the dimensions of the wall (length and height) to calculate your materials list.
Materials
|
 |
H5 treated timber 150 diameter poles @ 900mm centres of wall length |
|
H4 treated timber 150 diameter, half rounds, 1.8m long rails or alternatively H4 treated timber, 150 x 50mm or 200 x 50mm thick rough sawn or TG&V planks |
|
Concrete for the poles – bagged Quikcrete or 1 part cement to six parts builder’s mix |
|
Nails – 100mm, 125mm and 150mm glavanised |
|
Plastic sub soil drainage coil the length of the wall plus extra length to discharge points |
|
Free flowing backfill metal |
|
Bracing timber, pegs and nails |
|
Timber preservative |
| |
|
Tools
|
|
Spade or post hole borer |
|
Shovel |
|
Hammer |
|
Circular saw or handsaw |
|
Tape measure, square and pencil |
|
Spirit level |
|
Stringline |
|
Sledge hammer |
|
Old paint brush |
| |
|
Building your wall
The wall in this example is for a number of poles set in the ground with horizontal timber rails fixed to them for a closed face, cantilevered wall.
Allow 250mm at the base of the bank or cut to be retained behind the vertical poles (see illustration 2).
Shape and cut the bank face to suit. Grade ground at base to allow water in drainage coil to discharge where you want it or into a storm water system.
Set up stringline and pegs, mark end pole positions, measure out along string line and on the ground mark the pole centres (see illustration 1).
ILLUSTRATION 1: Timber Pole Retaining Wall
Dig 450mm diameter holes or 450mm square holes at the 900mm pole centres. Holes should be as deep as the retained height above ground, plus 100mm for the base metal or concrete below the pole end (see illustration 2). Place only uncut ends of poles into holes. Paint cut ends of poles liberally with timber preservative. Adjust base metal for correct pole wall height.
Set posts at the recommended 1 to 10 (5 degree) inclination from vertical to bank to be retained (see illustration 2). Place spirit level against pole to plumb vertically. Using the 1 to 10 formula, measure 1000mm above ground level. At this point, lean the pole 100mm (from the vertical) towards the bank or cut face. Fix poles with temporary braces, ground pegs and temporary rails in correct position. Check constantly and adjust before final set position (see illustration 2).
Use string lines, run from the end poles, to align the intermediate poles. Set these poles in place as for the end poles (see illustration 1).
ILLUSTRATION 2: Retaining Wall Section
When all poles are set in their final position, pour in concrete foundation. Compact concrete down with piece of timber to remove air pockets and leave to set (for at least 2 days).
Place sub soil drainage coil. Lay to fall at base of wall on top of metal blinding. Encase with free draining metal (see illustration 3).
ILLUSTRATION 3: Retaining Wall Section with Sub Soil Drainage
Place horizontal H4 half round rails (or H4 150 x 50mm or 200 x 50mm thick rough sawn or TG&V planks) in position starting at base of wall. If using rough sawn rails or half round rails fix in place with galvanised nails. Stagger rail joints on poles and also lay rails alternatively end for end to allow for taper. This will ensure a reasonably level finished line. Span rails over two poles or more (see illustration 4).
Back fill with free draining metal to 300mm within the top of the wall, finish to level with topsoil.
ILLUSTRATION 4: Half Rounds
|
|
|
