Skip to main content

CROSS Safety Report

Unsafe alternative designs for retaining wall

Report ID: 1245 Published: 12 June 2024 Region: CROSS-AUS


Overview

A contractor proposed a precast concrete design as an alternative to a documented limestone block wall, and engaged a precast supplier to design and construct the alternative design. Several submitted designs, including certified drawings, could have posed a real risk to the community if constructed.

The reporter notes that suppliers regularly state their expertise in the field but this may not always be correct.

Key Learning Outcomes

For clients, project managers and specifiers:

  • Alternative options offered by contractors must be subject to appropriate scrutiny as they may have less oversight than the documented design and contractors are often impatient for answers during the construction period
  • Ensure that the scope of provision of services during the construction phase includes all professional responsibilities, including those of rejection of submissions and processes that are not compliant with statutory and technical requirements

For Civil and Structural Engineers:

  • Recognise that both structural design and stability are integral to the function of structures. Interfaces between soils and structures are complex topics, with high technical levels of understanding of design being required
  • Understand the particular requirements of retaining structures including drainage and soil properties. For example, understanding that the angle-of-friction of soil has a significant influence on stability of a retaining wall (high angles-of-friction should be validated with on site testing or specified compaction requirements)
  • Provide a comprehensive briefing for any proposed alternative design process
  • Ensure that any changes to the structure proposed by others are signed off by the original designer

For contractors:

  • Be cautious of suppliers who provide in-house design, and be sure to insist on external design validation and certification as required
  • If validation and/or oversight of proposed alternatives is not possible, it may be prudent to keep with documented design solutions

Full Report

Find out more about the Full Report

Our secure and confidential safety reporting system gives professionals the opportunity to share their experiences to help others. If you would like to know more, please visit the reporting to CROSS-AUS page. 

The reporter, a construction-phase structural engineer in a company responsible for the detailed design and construction support of a project, identified a safety issue related to retaining structures within the project.

The original design specified a traditional limestone block retaining wall to support a new road with moderate vehicle surcharge. The contractor wanted to adopt a precast concrete retaining wall as it would provide improved constructability and faster construction than the limestone block wall. Accordingly, the reporter's company provided a concept scheme for review.

Typically, concrete retaining walls have bases beneath the retained soil to provide a counterbalance to overturning forces, and increased frictional resistance to sliding due to the weight of the soil imposed on the base. However, the contractor requested the base face away from the retained soil, i.e. on the low side of the wall. This would help with construction sequencing but would also create more onerous design conditions. The scheme provided to the contractor accounted for these design conditions but needed increased cover and an extended retaining wall base.

The reporter's company advised the contractor that calculations were essential in order to provide confidence the design solution was adequate

To avoid longer lead times for custom-designed precast retaining walls, the contractor wanted to use standard panels and chose to engage a precast supplier both to prepare an alternative design and to construct the wall. The design requirements (retaining heights, surcharge, geotechnical conditions) were provided to the supplier. The precast supplier provided two pricing options to the contractor; one provided certified drawings for fabrication, and the other included design calculations as well as the certified drawings. The second option would have an extended lead time. The reporter's company advised the contractor that calculations were essential in order to provide confidence the design solution was adequate.

The first design for fabrication submitted by the precast supplier indicated the base of the retaining wall beneath the retained soil which did not necessarily align with the design requirement. This design had been certified by independent engineers. Feedback was provided on the design, requesting confirmation that the wall stability had been appropriately considered in the design. The supplier stated they had no concerns.

Shop drawings were submitted to the reporter's company for approval without any updates from the design previously provided, and without any calculations. Calculations were later provided and ultimately the proposal was rejected.

Several incomplete design iterations were submitted by the supplier and detailed feedback (outlining areas of concern) at each iteration was provided to the supplier.

The reporter notes contractors regularly propose alternatives which deviate from the original design, and these can involve products and external suppliers’ designs that may not undergo sufficient design scrutiny. While suppliers regularly state their expertise in the field, this may not always be correct. In this case, a detailed understanding of the geotechnical/structural interface appeared to be lacking.

This experience greatly concerned the reporter and their colleagues for several reasons. Several submitted designs could have posed a real risk to the community if constructed.

The reporter has particular concern with regard to the following issues:

  • The design capability of the precast retaining wall supplier and their certifying engineers
  • The general attitude taken by the precast supplier in the face of the feedback received

Expert Panel Comments

Find out more about the Expert Panel

An Expert Panel comment on the reports we receive. They use their experience to help you understand what can be learned from the reports. If you would like to know more, please visit the CROSS-AUS Expert Panel page.

The reporter is to be congratulated on raising this issue as well as seeking appropriate management and ensuring integrity throughout the design and construction process. This report presents legitimate concerns and recommendations for practitioners facing similar scenarios.

While it is commonplace for contractors to seek alternative approaches to particular construction items, generally for cost and/or programming reasons, it is imperative that alternative designs provided by contractors and subcontractors, and their respective engineers, are subject to appropriate design processes and serious scrutiny. If the reporter had not exercised their responsibility in this case, the outcome may have been disastrous.

The reported dismissal of stability concerns and design non-compliances is cause for alarm

There are two aspects of this report that warrant particular consideration. The first is the level of understanding and apparent lack of appreciation by the sub-contractor and their design engineer for the design constraints and requirements of this structure. The reported dismissal of stability concerns and design non-compliances is cause for alarm. Although the majority of the design issues raised by the reporter involve standard aspects of good design (which ought to be standard practice for any competent design engineer), this lack of understanding identifies the importance of a comprehensive briefing to be prepared in any re-design process.

The second aspect relates to safety and risk; the report demonstrates how pressure can be applied to engineers to accept an alternative that is not to the required standard. Any design change involves risk, and that risk should be carefully considered by all parties involved including the client. In this particular case, given the wall supports a road, the consequences of failure could well have been more than just the cost and inconvenience of remediating and strengthening the failed structure.

This report reinforces the imperative for a full evaluation of any contractor-led alternatives, and the need for thorough independent review of alternative designs

Other risks involved in entertaining design alternatives during the delivery phase of a project include:

  • Time pressure leading to potential error in design and design philosophy
  • An incomplete process of developing the concept with appropriate review, and loss of design integrity including inconsistent quality practices, blurred ownership of design, and incomplete capture of all required inputs including those of an interdisciplinary nature (e.g. drainage, architectural, civil, geotechnical)

This report reinforces the imperative for a full evaluation of any contractor-led alternatives, and the need for thorough independent review of alternative designs.

Submit a report

Your report will make a difference. It will help to create positive change and improve safety.

Our secure and confidential safety reporting system gives professionals the opportunity to share their experiences to help others.