Ground conditions might not be the first concern for scaffold contractors, designers and temporary works managers – but they should be up there on the list. Paul Thompson reports.
Scaffolding from the ground up Ground conditions can play a fundamental role in determining the stability and structural integrity of temporary works and scaffold design.
Without understanding and mitigating the conditions and capacity of the ground beneath the sole plate of a scaffold system, designers and contractors are leaving themselves open for liability against any mishap or accident that might occur during the period it is in situ.
Scaffolding Association chief executive Robert Candy is concerned that during the cut-and-thrust of day-to-day business, some of the information vital for the safe founding of scaffolding systems might be wrongly assumed. Information such as bearing pressure test results, water table depth, service location, and the type, depth and level of backfill material that has been placed are all vital to know.
“All the information on the ground conditions should be made available to designers and scaffolders by the principal contractor,” Mr Candy explained. “Often, though, some information is unavailable or more might be required. By that time, the pressure is on to get the contract underway. It can be difficult for designers and scaffolders.”
Sharing of information
Ryan Berry, managing director at Rotherham based scaffold design and temporary works consultant Creator, is adamant that the principal contractor should be responsible for delivering all the required information to the designer prior to contract start.
He said: “The principal contractor must deliver all that information to us. If they provide us with an allowable bearing pressure, we will design to that, but generally temporary works designers will not have the knowledge, experience or insurance to cover the geotechnical side of things.”
Whether that information is there and freely available to temporary works designers can depend on the client and the nature of the work involved. On major civil engineering and highways projects, or when working with clients such as Network Rail, the full regime of geotechnical and bearing capacity tests are carried out and engineers have a raft of information on which to base their designs – but things can differ, according to Mr Berry.
“On those sorts of projects, there will have been comprehensive site investigation work so there will be lots of information available. Of course, there may be some projects where intrusive tests cannot be carried out. In those cases, we will provide leg loads for our design and it is down to the principal contractor to make sure the ground conditions are capable of handling those loads,” he said.
In the residential sector, major housebuilders are increasingly developing on brownfield sites thanks to planning restrictions and a shortage of available greenfield land. Any uncertainty over the bearing capacity of the ground should be mitigated in these instances by the ground remediation design but, again, designers will supply a permissible leg load for contractors.
“On housebuilding projects, the leg loads are relatively small and the risk of failure is lower. We will provide the load on these schemes, but the risk will still be carried by the principal contractor and it is up to them to make sure that the strata beneath the sole plates is suitable for bearing any loads. We still accept no liability for us, though – our insurance wouldn’t cover it,” explained Mr Berry.
Dr Mike Webster is a specialist in construction risk management and health and safety at MPW R&R. He is often brought in as an expert witness in legal proceedings.
“When a project is initially designed, some level of site investigation work and trial pits will have been carried out. In a traditional form of contract, it will be the client who has that information and it is up to them to pass it on through to the principal contractor,” said Dr Webster.
“On design and build projects, it may well be the principal contractor themselves that oversees the site investigation and interprets the results. Either way, that information should be available to designers and subcontractors as part of the pre-construction information.”
And Dr Webster is keen to point to the Health and Safety Executive’s Scaffold Checklist. It highlights information that should be supplied to the scaffold contractor to ensure an ‘accurate and proper design process’ and includes the ‘nature of the ground conditions or supporting structure’ as well as calling for designers or scaffold contractors to supply safe working loads and leg loads.
“Scaffold failures are more usually ‘peeling’ failures where ties have pulled away from supporting structures but we don’t know if ground settlement has been a contributory factor,” added Dr Webster.
“Where scaffolders might be criticised is if they don’t ask the principal contractor for that bearing capacity information. The key for scaffold and temporary works companies is to make sure they have done what is reasonably practicable to gather that information.”
Both Dr Webster and Mr Candy agree that if there are drawings and calculations to be submitted to a principal contractor’s temporary works co-ordinator, at the very least designers should make it clear that the designed leg load and sole plate size is only applicable provided a given bearing capacity is met.
“It is important that scaffold companies make sure they have shown that they have taken all steps to provide as much information as possible in order to mitigate any risk,” concluded Mr Candy.