Cross UK Report 1210 & 1228

Collaborative reporting for safer structures. Report 1210: Unqualified engineer’s unsafe computer-aided design of a retaining wall.

A check found retaining walls designed by a person not qualified as a civil or structural engineer and who relied solely on a computer program.

The reporter, a chartered engineer, was asked by a surveyor to check the design of 1.2-metre-high 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 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 were connected with either institution. The reporter believes the underlying cause of the problem is that unqualified or partially qualified designers are practising as if they are chartered structural engineers. The reporter hopes that the GB Health and Safety Executive (HSE) will have the power to prohibit such people from practising.

Comments

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. Computer programs should be used by those who have the knowledge and experience to check whether the results are sensible. 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 higher.

Surface vegetation, root growth and increased height of retained materials can all contribute to the failure. 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 suggest an over-reliance on quantity of output over quality. CROSS has previously issued several 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.

Collaborative reporting for safer structures. Report 1228: Non-approved sealants are used with CPVC pipes and fittings

Sealants causing leaks to chlorinated polyvinyl chloride (CPVC) sprinkler system pipework and fittings

A reporter is concerned that, if non-approved sealants are used with CPVC sprinkler pipework and fittings, failures and/or leaks in those pipes can be caused due to environmental stress cracking or plasticisation. This occurs when semi-volatile organic substances migrate from the sealant into the pipe or fitting. This can happen within a short time after installation or after several years. When leaks occur, this results in sprinkler circuits being turned off/drained down while failures are investigated and repairs made. This can take several days or months, during which time there will be no protection from the sprinkler system.

In the opinion of the reporter, only sealants recommended by the specific sprinkler pipe manufacturers should be used. They report having seen failures in many premises because of this issue, where non-approved sealants have been used. They suggest that the frequency of failures is increasing due to the increased use of residential sprinklers.

The reporter continues to describe other variables that affect the time to failure. These include environmental temperature (higher temperature favours failure) and lack of movement accommodation in the system. Hanger type and spacing also need to be in accordance with the pipe supplier’s recommendations. The problem is further exacerbated when faced with overcrowded services at wall penetrations, eg through a letter box-type slot in the wall very close to the ceiling, with very little space to carry out removal and replacement of failed sections. This will inevitably lead to systems being shut off for longer to carry out remedial works.

Comments

It is important to make sure that installers only use a type of sealant that is specified for that particular type of pipe. Different types of pipe have different lists of approved sealants. While this is a relatively simple issue, failure can have disastrous consequences such as failure of sprinklers during operation, flooding and inoperative sprinklers leading to expensive and invasive remediation.

It is important to use contractors who can demonstrate their competence. Oversight of works should also pick up these relatively simple non-compliances.

Further reading

• Report 129 Responsibility for boundary retaining wall: b.link/CROSS_129

• Report 134 Deadly retaining wall: b.link/CROSS_134

• Report 189 Retaining wall concerns and the stance of a local authority: b.link/CROSS_189

• Report 989 Dangerous design of a retaining wall: b.link/CROSS_989

• Report 1119 Boundary retaining wall collapse: b.link/CROSS_1119

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Image credit | Alamy