CROSS Safety Report
Boundary retaining wall collapse
Overview
A boundary blockwork retaining wall, with a fence on top, partly collapses due to inadequate design.
Key Learning Outcomes
For the construction team:
- Be aware of any level changes on-site and the need for retaining walls
- For the design of retaining walls, seek the advice of a competent structural or civil engineer
- The addition of a fence on top of a retaining wall will need to be considered within the design
For civil and structural engineering consultants:
- Design retaining walls in accordance with good practice and refer to the references in this report if in doubt
For the client:
- Ensure that a competent engineer is appointed to undertake the design of any retaining walls
- Note that there have been numerous failures associated with freestanding and retaining masonry walls with serious consequences. A previous Alert was issued by the Standing Committee on Structural Safety (SCOSS) –Preventing the collapse of freestanding masonry walls.
Full Report
Find out more about the Full Report
The Full Report below has been submitted to CROSS and describes the reporter’s experience. The text has been edited for clarity and to ensure anonymity and confidentiality by removing any identifiable details. If you would like to know more about our secure reporting process or submit a report yourself, please visit the reporting to CROSS-UK page.
A small housing development with a public footpath along the boundary was temporarily closed during construction works. The site levels were reduced resulting in the need for a retaining wall around part of the site.
The wall was estimated to retain between 1.0 and 1.5 m of ground and had a close-boarded timber fence on top. It was constructed using blocks laid flat.
The reporter says that the wall rotated and partly collapsed from what appeared to be flexural failure of the mortar bed-joint. They also say that the wall had not been designed but was based upon an ill-formed belief, by the builder, that the form of construction and thickness of the wall would be adequate and the fence would have no adverse effect.
The reporter suggests that all retaining walls should be designed by suitably qualified personnel to suit site conditions.
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Walk any urban area and you will see many examples of free-standing or retaining masonry walls failing, often the culprit is trees pushing them over, but many of these walls are not designed to resist the forces to which they are subjected. There is a misguided belief, outside the engineering community, that masonry must be ‘strong’ and subsequently many of these walls are built, contrary to the CDM regulations 2015, without any competent design. However, even the collapse of a low wall can lead to tragic consequences.
the collapse of a low wall can lead to tragic consequences
A further common misconception, outside the engineering profession, is that because an excavation face may stand vertically for a short period, the ground will not impose any force on a wall built in front of it.
Boundary walls (in England) which do not form part of a building are not within the scope of the Building Regulations 2010. The government provided a divisional circular letter to building control bodies in 2013 about the safe design, construction and maintenance of free-standing boundary and retaining walls. If walls do become ‘dangerous structures’, local authorities have enforcement powers under the Building Act 1984. However, there is often a lack of clarity about which party is responsible for boundary walls. If retaining walls are built adjacent to highways, then the Highway Authority may be able to exert some control during the design process through the Approval in Principle process.
National Highways and Local Highway Authorities will be providing further clarity about responsibilities for boundary walls adjoining highways in their upcoming publication Definition of Asset Management Responsibilities: Bridges and Structures.
The design of a retaining wall is a function of its height, ground conditions, groundwater and drainage conditions, and surcharge loading. The addition of wind load acting on a fence on top of a retaining wall can also contribute to the overturning that needs to be considered. Therefore, ‘standard designs’ for retaining walls should be considered with care and reviewed for each situation.
Further reading
References giving guidance on good practice include:
The Design of Brickwork Retaining Walls - Brick Development Association
Design of Free-Standing Walls - Brick Development Association
A Reinforced Brickwork Freestanding Boundary Wall - Brick Development Association
Building simple brick or blockwork freestanding walls - Building Research Establishment, Good Building Guide - GBG 14
Surveying brick or blockwork freestanding walls - Building Research Establishment GBG 13
Building brickwork or blockwork retaining walls – Building Research Establishment GBG 27
It is worth noting that these documents predate the Eurocodes and the industry should look to provide updated versions.