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“How To”
Retaining Wall Guide
Before you start:
Consents and Engineering
Building Consent
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.
Engineering Design
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 Design Concepts
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.
Materials
Poles or Piles
Poles or piles should be H5 treated high density pine to meet the “ground contact load bearing”
requirements.
Timber Boards
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.
Square
Poles
Wall
Height
700
800
950
1000
1250
(Small Walls
and gardens)
Sawn Timber
Square Pile Size
1.5 / 100x100
1.8 / 125x125
2.1 / 125x125
2.4 / 125x125
2.7 / 150x150
Max
Spacing
1200
900
1200
900
900
Hole
depth
900
950
1100
1250
1350
Hole
Auger
300
300
350
350
350
Retaining
timber
150 x 50
150 x 50
150 x 50
150 x 50
150 x 50
Round
Poles
Wall
Height
Large Walls
Construction
Pole
(Length x small
end diameter)
Max
Spacing
Hole
depth
Hole
Auger
Retaining
timber
Retaining timber
1st Layer
2nd Layer
750
1.8m x 150 SED
1200
900
300
150 x 50
850
1.8m x 150 SED
1200
950
300
150 x 50
1100
2.4m x 150 SED
1200
1300
300
150 x 50
1350
3.0m x 175 SED
1200
1500
350
150 x 50
1700
3.6m x 200 SED
1200
1900
400
150 x 50
2000
4.2m x 225 SED
1200
2200
450
150 x 50
2250
4.8m x 250 SED
1200
2500
450
150 x 50
2500
5.4m x 275 SED
1200
2750
500
150 x 50
2700
6.0m x 300 SED
1200
3000
550
150 x 50
2900
6.0m x 325 SED
1200
3100
550
150 x 50
150 x 50 - below
750mm
150 x 50 - below
750mm
150 x 50 - below
750mm
150 x 50 - below
750mm
150 x 75 - below
1900mm
150 x 75 - below
1900mm
150 x 75 - below
1900mm
150 x 75 - below
1900mm
Concrete
Poles should be set in 17Mpa concrete. Pre-bagged mix or 6:1 ratio of builder mix to cement.
Timber and Pegs
Timber and pegs will be required to temporarily brace the poles while the concrete sets
Nails
100mm, 125mm or 150mm flat head Hot dipped galvanised nails to fix the boards to the back of the
piles. Nails should be at least twice the thickness of the timber being fixed.
Timber Preservative
Any cut surfaces should be liberally painted with timber preservative (Copper Napthenate) before
installation.
Drainage Pipe
100mm diameter perforated polyethylene pipe, preferably fitted with a filter sock, to drain behind
the entire length of the wall. You may need to add extra to reach appropriate discharge points for
the drains.
Backfill
A free draining metal or scoria back fill will be required at least 250mm thick behind the wall.
Geotextile cloth or polystyrene sheeting can also be added to assist with drainage.
Beer
A supply of ice cold lager at the end of each day will encourage assistance from “Mates”. Crofts
recommends you drink in moderation as this can adversely affect the quality of the wall.
Instructions- Building your wall.
1
Draw your plans and calculate the quantity of materials required.
2
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.
3
Excavate the bank to give at least 250mm gap between the back of the wall and the
bank.
4
Make sure the ground behind the wall is level or slopes to one end to provide drainage.
5
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.
6
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.
7
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)
8
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.
9
Check the poles are all vertical and plumb when viewed from the front.
10
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.
11
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.
12
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.
13
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)