Most gabion retaining walls don’t need concrete foundations
Depending on the wall height, a 2″ to 4″ layer of compacted 1″ road base, crushed from either blue metal or basalt rock is all that is required for most gabion walls.
Geotech filter fabric stops fine silts and clay migrating into the wall
The filter fabric ensures the gabion remains free draining, and eliminates hydro-static pressure behind the retaining wall.
Small gabion walls under 3ft tall
For small walls under 3′ tall, with flat ground behind, you can place the soil against the gabion provided you install a geotech filter fabric between the soil and the gabion.
4″ diameter jettable drainage pipe at the rear
The drainage pipe is often encased in a filter sock, to prevent silts and clay clogging up the pipe.
Gabions with flat land behind, built using 2:1 stability ratio
Smaller gabion walls under 5′ tall, built to the 2:1 stability ratio, are able to be built vertical and still pass engineers overturning calculations.
Retaining wall profiles and design factors
The retaining wall toe prevents the gabion wall sliding forwards, the depth of the toe needs to increase as the wall gets bigger. When building gabions on softer soils, both the depth of the toe and the size of the base needs to be increased to spread the load over a wider area. An engineer’s design will consider design limit states and specify the dimensions of the toe and base for your wall.
Any excavations should be checked for any flows or seepage that require drainage measures. Any flows, seepage or standing water should be directed to a suitable outfall as soon as they are encountered. The wall drainage must be inspected and maintained (rodded/jetted) annually and after particularly heavy rainfall events. This is essential to prevent the pore water pressures increasing behind any retaining wall.
For larger retaining walls and difficult sites, it is recommended that a soil investigation is undertaken and that the wall is designed by a qualified geotechnical engineer.
If a safety fence or barrier is placed along the top of a gabion wall, to prevent falls. It is assumed that this fence will be post and mesh, or open boarded, so as to not add any wind or impact loads to the gabion structure. Possible wind and impact loads need to be considered and the design revised to accommodate these loads on the gabion retaining wall.
Temporary excavations have the potential to collapse rapidly and without warning, and may become unstable during wet weather. Contractors should plan their work to reduce this risk of collapse and consider the use of temporary propping during construction.