Retaining walls : The Australian Norms

A retaining wall structure is constructed to withstand the lateral pressure of soil. The term is generally used for cantilever as it is a freestanding structure without lateral support at the top. The International Building Code demands the design of retaining walls to be stable against sliding, overturning, water uplift and foundation pressure having a safety factor of 1.5.

Retaining Wall Types

Retaining Wall is of various types. Gravity retaining walls is mass oriented. It uses heavy materials like stone and concrete to withstand pressure from behind.  Cantilevered Wall has the shape of inverted T. It is built with steel reinforced internal stem covered with mortared or concrete masonry. Sheet-pile walls are built from vinyl, wood planks or steel. It is used in tight spaces with soft soil. Bored pile retaining walls are assembly of a sequenced bored piles followed by digging out excessive soil. Anchored walls are basically walls of the aforementioned types but are given additional strength through cable or some other stays anchored in the soil behind or the rock.  Such slender wall is used in areas of high load by integrating additional strength through anchoring.

Alternative Retaining Techniques

There are alternative retaining techniques also. In soil nailing, the retaining walls are reinforced by inserting slender steel bars.  Gambian wall is a free draining retaining structure and consists of wire mesh filled with stone or some other hard material.  There are several other systems by which soil is strengthened to reduce the earth pressure on the wall.

Besides the above, soil has to be constructed with artificial reinforcing using horizontal steel mats or steel straps in layer duly fixed at the end. This is known as mechanically stabilized earth (MSE). This is done to provide additional internal shear strength to the simple gravity wall structure. Cellular confinement system called Geocells are employed for stabilization of steep earth in reinforced as well as gravity retaining walls. These walls become more stable under its own weight together with external loads offering great seismic strength.

Some Important Standards

There are some important Australian standards to be religiously followed while constructing  retaining walls failing which there may be catastrophe if the wall is unable to hold against a flood resulting in subsidence of nearby structures and buildings.

AS4100 prescribes minimum requirements for design, erection, fabrication and modification of steel structures. The standard stipulates various qualities that the steel must have regarding its tensile strength, percentage elongation and compression, hardness and brittleness, toughness and impact value.

AS4680:2006 is the latest Australian standard for galvanizing steel. This specifies the minimum adequate coating thickness based on the type of steel being galvanized. For example a steel of 65-85 micron thickness must have a coating of 500-600 gram per unit area.

AS 3600:2009 standard is about the minimum requirements for the design and construction of plain and reinforced concrete structures and members.

Weak or damaged retaining walls may lead to wall failure due to saturated backfill, subsidence or  wall separation at the corners. An improper design or substandard material used in the construction may also lead to such failure.