Masonry cavity fill retaining walls are increasingly becoming the preferred method of construction for walls up to 3m in height. Due to their vital function, and the impact of failure, it is imperative that they are designed to ensure they fulfil the required performance criteria.
An important consideration is the construction method for the concrete fill. This will use either ‘low lift’ or ‘high lift’ methodology: with both options requiring a differing specification. A lack of adhesion to these requirements could potentially result in failure of retaining walls during the construction period.
Below are some key differences between ‘low’ and ‘high’ cavity fill construction techniques:
• Concrete infill should be placed at maximum vertical intervals of 450 mm
• Infill concrete should be placed in layers to within 50 mm of the level of the last course laid
• Concrete infill should be compacted immediately after pouring • Avoid raising the wall too rapidly as this causes disruption due to excessive lateral pressure
• Spacing of wall ties should follow BS5628-1 recommendations (450mm vertical, 900mm horizontal)
• Walls should be built up to a maximum of 3m high
• Clean out holes to the base of the walls (minimum dimension 150mm x 200m, spaced at 500mm centres) should be allowed for in one leaf of masonry.
• Prior to infilling with concrete, debris should be removed from the cavity and the clean-out holes blocked off.
• The concrete infill should be introduced no later than 3 days after the masonry is completed. The infill should be placed and compacted, usually in two lifts, with a one day curing time between successive lifts. Recompaction of the concrete in each lift may be necessary after initial settlement, due to water absorption by the masonry, but before setting.
• Wall ties should be spaced at no greater than 900 mm centres horizontally and 300 mm vertical centres, with each layer staggered by 450mm. The ties should also be designed by an engineer to ensure enough capacity to hold the masonry leaves together.
If any movement takes place during construction, the walls must be taken down and rebuilt. An understanding of the above could avoid costly remedial measure, saving both time and money.