Page 19 - Landscape SA 102

ALLIED TRADE FEATURE

Terraforce step block seating for over 1000 students at Reddam School in Cape Town

RETAINING WALL Terraforce L16 blocks at a children’s
hospital in Dubai. The blocks were
installed with an alternating pattern to
accommodate plants in a near vertical
SAFETY
retaining wall.

Retaining walls are typically used for supporting soil
laterally so that it can be retained at different levels on the
two sides. The structures are designed to restrain soil to a
slope that it would not naturally keep to. Landscape SA
posed the questions below on the safety of retaining walls
to Remacon, Mondo Civils and Terraforce, three companies
involved in the design and construction of retaining walls.

For Remacon – Silvio Ferraris (SF) from water flowing through the retained
For Mondo Civils – Sean Dowds (SD) soil. Surface water drainage in the form of a
For Terraforce – Holger Rust (HR) drain channel will ensure that water flows
away from the top of the wall, preventing
What safety considerations damage. Where walls are constructed with
are taken into account when bulk fill, the quality of that fill must be
designing a retaining wall? considered; what is usually required is a
G7 fill compacted to no less than 90% mod
SF: The walls have to be structurally AASHTO.
designed to consider the type of soil
being retained, what wall angles are SD: Engineers have a complex series of
required (60-87 degrees), and keeping in design checks to carry out to ensure that
mind that the steeper the wall, the more the wall is safe. They would first decide
expensive it becomes. What also needs on what type of design category the wall
to be considered is whether there are any falls into, namely gravity retaining walls be calculated and the wall then designed
buildings, roads or parking bays above, or reinforced earth walls. The former to resist those forces. The weight of the
and how far these are behind the walls. are generally low and lean backwards wall is critical for resisting pressure from
Subsurface water drainage also needs to against the soil they support, relying on the retained soil, and the foundation must
be taken into account to ensure that the their weight to resist possible failures. The be designed to resist sliding forward under
fill behind the block walls does not get wet forces imposed by the retained soil must pressure from the soil behind it. This can

Landscape SA • Issue 102 2021 17