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CROSS Safety Report

Unqualified engineer's unsafe computer aided design of a retaining wall

Report ID: 1210 Published: 20 February 2024 Region: CROSS-UK


Overview

A chartered engineer's check found retaining walls, designed by a person not qualified as a civil or structural engineer and who relied solely on a computer program, to be inadequate. It is likely the walls will need to be demolished and rebuilt.

Key Learning Outcomes

For clients:

  • Only suitably qualified and experienced engineers should design boundary retaining walls
  • Note that there have been numerous failures associated with freestanding and retaining masonry walls with serious consequences. The Safety Alert, Preventing the collapse of freestanding masonry walls, was issued by CROSS (then SCOSS) in 2014

For civil and structural engineering designers:

  • Design retaining walls in accordance with good practice and consult the references in this Safety Report if in doubt
  • Computer programs should be used by those who have the knowledge and experience to check whether the results are sensible
  • Where there are possibilities for change such as the height of retained materials wall designs should be conservative

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The reporter, a chartered engineer, was asked by a surveyor to check the design of retaining walls for their client's land. They checked the drawings and calculations provided to Eurocode 7 and concluded that the walls could overturn. The walls in question were only 1.2 metres high and, while they accept it could be argued they did not need to comply with Eurocode 7, the reporter believes the designer had a duty to their client and to the neighbours to ensure the walls were safe.

The reporter found the designer had failed to calculate the bending moment on the walls correctly.  Although more than 40 pages of computer output had been produced, it was clear the designer did not understand how to design a retaining wall to Eurocode 7 - having failed to demonstrate that the wall was in equilibrium.  The reporter considers the walls in question would need to be demolished and a much heavier structure provided.

On checking with the Institution of Structural Engineers and the Institution of Civil Engineers, the reporter found that neither the designer nor the checking designer was connected with either institution.  The reporter believes the underlying cause of the problem is that unqualified or partially qualified designers are practicing as if they are chartered structural engineers, and there is no means of stopping them doing so.

The reporter is concerned the public may be deceived by unqualified persons posing as designers, and that if designs such as this one go to Building Control errors may not be picked up as many Building Control bodies do not employ checking engineers.  

The reporter hopes that when the Building Safety Act is implemented, the Health & Safety Executive (HSE) will have the power to prohibit such people from practicing.

Expert Panel Comments

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The Building Safety Act provides a statutory framework for competence from building control inspectors, but does not extend to other professions involved in the construction process. It is therefore difficult to envisage that the HSE would be able to prevent people trading. However, in the aftermath of an incident, a prosecution might be pursued against a designer.

There is a requirement for a designer of any structure to provide a safe design. The need for a design to be carried out by appropriately qualified persons, to determine the actual and not assumed ground conditions and site constraints, is vital if safety is to be maintained.  Where retaining walls pose a significant risk to life due to their location or height, a suitably qualified and experienced engineer can produce a design to mitigate these circumstances.

There are empirical rules for the construction of garden retaining walls, such as those given in Building Research Establishment's Good Building Guide 27, however such standard designs are only appropriate if they are used within  their stated  assumptions.  They might not be appropriate for boundary walls, particularly those adjacent to a public footpath or between two gardens at different levels, where the consequences of failure are potentially higher.

Surface vegetation, root growth and increased height of retained materials can all contribute to the failure of a retaining wall and any designs should be such that they are not sensitive to these.  In particular, the effect of increased height of retained material can drastically increase the forces a retaining wall must resist. For instance, an increase from 1.1 metres to 1.2 metres increases the stem bending moment by 30%.  The 40 pages of calculations for a retaining wall in this report example suggests an over reliance on quantity of output over quality.

CROSS has previously issued a number of Safety Reports concerning retaining walls, their construction and design.  Reports 129, 134, 189, 989 and 1119 all deal with similar issues to those raised in this Report.  Report 989 also highlights the risk of using design programs with incorrect data entry, and Report 1119 gives a comprehensive list of further reading for the design of both freestanding and retaining walls.

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