Page 28 - Build 152

DESIGNING AND BUILDING DIFFERENT TYPES OF LOW RETAINING WALLS
BUILD
Low retaining walls
There can be large forces at work on retaining walls so they need to be well designed and built to avoid failure.
mass concrete may have stone rubble or brick facing
weepholes
RIGHT
BY ALIDE ELKINK,
FREELANCE TECHNICAL WRITER, WELLINGTON
RETAINING WALLS support the ground where there is a slope or change of grade. They must withstand the combined loads of the soil, water content and any surcharge or extra load such as sloping back ll, building or vehicle loads.
Consents when over 1.5 m
A building consent is not required under clause 1(c) Exempt building work of Schedule 1 to the Building Act 2004 when:
● a wall retains no more than 1.5 m of ground
and
● doesnotsupportanysurcharge.
A building consent must be obtained in all other situations.
Where consent is not required, the wall must still comply with the Building Code including clauses B1 Structure, B2 Durability and F4 Safety from falling.
BRANZ recommends that advice is sought from a chartered professional (structural) engineer for all walls over 1 m high.
Drainage needed behind wall
Retaining walls must have drainage behind the wall to prevent water build-up that increases the load or lateral pressure and reduces its bearing capacity.
Place a minimum thickness of 300 mm of clean metal or gravel drainage material immediately behind the wall. Extend from the base up to within 150–200 mm of the top of the wall.
All retaining walls must have a subsoil drain falling to the stormwater system and covered
Figure 1
Mass gravity retaining wall.
300 mm thickness of granular ll
slotted drain to stormwater
good ground as de ned in NZS 3604:2011
concrete or concrete masonry cantilever retaining walls
gabion walls.
with a lter cloth or sock installed behind the foot of the wall to remove water. Weepholes must be provided through the base of a mass gravity wall and may be installed as a hydrostatic relief in other solid walls. BRANZ also recommends that: ● subsoil drainage is installed behind open
retaining wall systems such as crib walls
● a layer of lter cloth is installed to separate the drainage material and the soil behind the wall.
Types of retaining walls
There are several di erent retaining wall construction systems, including:
● gravity walls, including mass gravity and crib walls ● segmental retaining walls, including mechan-
ically stabilised embankment (MSE) walls
● ●
Select a wall system that suits the site and soil conditions. Other factors to consider include the ability to get materials to the location of the wall, ease of construction, cost, appearance, drainage and water table level, and the distance from any adjacent slope.
Proprietary systems are available. These should be constructed in accordance with the manufacturer’s information.
Mass gravity wall
A mass gravity wall gets its stability from its mass or weight. It has a low height-to-width ratio and is sloped back towards the bank (see
26 — February/March 2016 — Build 152