Old vs New Crib Wall Design

The changes to enhance design life and durability.
New Crib Wall Design

Concrete Crib Retaining Walls

Concrete crib walls have been used in retaining wall applications in Australia since the 1950’s.

These walls were manufactured with a lower grade concrete strength and constructed with varying face slopes, generally filled with onsite clay materials.

Notwithstanding the relative inferior materials and construction practices utilised, these walls still managed to achieve an extended design life. For these older walls, the use of lower grade concrete strength has resulted in the crib wall components spalling over time where moisture has been allowed to penetrate the concrete through to the steel.

The use of clay material as infill has resulted in the material within the wall silos being washed out over time as the fines are carried away with the ingress of water. The practice of constructing these walls at a steep grade also added to the lack of overall stability once the wall lost its infill material and consequently its weight and ability to resist the loads it was originally designed to retain.

This has resulted in a number of these older walls failing and not functioning as they were originally designed to do.

These practices, the design and construction of concrete crib walls have changed dramatically over the past 20 years which are now required to be designed and built in accordance with Australian Standard AS 4678 – 2002. This standard prescribes the durability requirements for the design where the crib wall components are required to be manufactured with a 40Mpa concrete strength to adhere to a B1 Classification for walls located from 1km to 50 km from the coastline which provides for a 40 to 60 year design life.

For walls located in coastal zones within the 1km distance and not within the splash zone, the standard calls for a 50Mpa concrete strength which meets the B2 Classification requirement for durability.

All concrete crib walls have over the past 20 years been constructed with a 1H:4V face slope using a durable shot stone aggregate of between 20mm to 40mm diameter and is required to meet AS2758.4 -2017 which is a durability standard for aggregates.

This allows for the wall to be free draining which does not result in additional loads on the wall due to the build-up of pore pressure. The easy dissipation of ground water through the wall does not result in the wall losing any of its weight due to the loss of the infill material within the wall silos which has been the case with the older wall construction in the past.

Concrete crib walls designed and built today have a solid design life and do not suffer from the same issues the older walls have encountered.


Click on the photos below for a visual insight: