Working Platform Design - Verification by Plate Load Test

Safe Working Platforms

The Federation of Piling Specialists, in their ‘Working Platform Guidance’, state that one third of all dangerous occurrences reported by their members are related to working platforms for tracked plant. Each occurrence is of concern in its own right and is in fact a potential fatality. They emphasise the importance of working platforms being properly designed and correctly constructed.

Working Platform Design

In the UK, the key documents for working platform designers are ‘BR470 - Working Platforms for Tracked Plant’ plus the important ‘Building Research Establishment (BRE)’ supplement to BR470 published in 2011. The Institution of Civil Engineers also publish a comprehensive guide for working platform design prepared by the Temporary Works Forum: ‘Working Platforms -Design of Granular Working Platforms for Construction Plant - A Guide to Good Practice’, (ICE/TWF 2019).

Guide to Working Platforms 

The need for properly designed working platforms was subsequently the focus of a document published in 2020 by The European Federation of Foundation Contractors and the Deep Foundations Institute (EFFC/DFI). Their ‘Guide to Working Platforms’ covers the importance of proper design, but also testing. They recognise that Plate Load Testing is most commonly used, but caution that it needs to be representative of the actual load condition.

What is a Plate Load Test

Plate load testing can be used to determine the stiffness and in some cases, the bearing capacity, of the ground. The test involves loading a circular plate and measuring settlement. The diameter of the plate can vary, and this will influence the result. Larger diameter plates mobilise the soil strength to a greater depth and if the test is carried out to failure, then bearing capacity can be determined.

Plate Load Testing to Validate the Design of Working Platforms

The EFFC/DFI concerns with plate load testing relate to the need to use a suitably sized plate. Typical plate load testing uses a 300mm size plate, which is not representative of the load that will be applied by tracked plant. A larger size plate is needed to replicate the load condition that will apply in practice. In our very first Ground Coffee Blog  (How to carry out a plate load test correctly), I explained the importance of plate size for testing of working platforms in the video ‘How to carry out a plate load test correctly’. I also explained why it is important to use a plate diameter that is as close as possible to the track width.  

The ICE/TWF Guide to Good Practice proposes the use of plate load testing to validate the capacity of working platforms, but they state that the plate load testing can only be acceptable if “the plate is of appropriate diameter relative to the actual track/pad”.  

If a plate load test is intended to validate an ultimate limit state platform design, then the test must be carried out to failure, in order to obtain a measurement of the bearing capacity of the platform. This will require the application of a very large load, something that presents practical difficulties on site.

Large Size Plate Load Testing – Krakow, Poland

The Northern Bypass project in Krakow, Poland offered an ideal opportunity for Tensar to conduct a large diameter plate bearing test, to validate a working platform design generated using the T-Value Design Method and incorporating new design parameters for Tensar InterAx geogrid. The main contractor was willing to allow the site to be used, and the ground improvement sub-contractor facilitated the testing and provided the plant needed.

Working Platform Construction

The platform was constructed over a silty clay subgrade, so Tensar InterAx geogrid was used to create a mechanically stabilised aggregate layer.  The aggregate particles interlock with the geogrid and are confined within the apertures, increasing the strength and stiffness of the granular layer, thereby increasing the bearing capacity of the platform. The advantage of this is that considerable savings in aggregate layer thickness can be achieved compared to a non-stabilised design, delivering benefits of lower cost, faster construction, and reduced carbon emissions.

The Plate Load Test

A large, 1.0m diameter plate was used to replicate the track width of the operational plant. Loading needed to be carried out to failure to determine bearing capacity. This required the application of a huge load, and to facilitate this, a reaction frame was constructed with four CMC columns in tension, plus kentledge to oppose the downward force applied to the plate.

The plate load was applied until bearing failure was reached, validating the ultimate limit state design of the platform.

T-Value Design Software for Working Platforms

For details of this software, please click here to visit the website: Tensar+ | Geotechnical Design Software | Free to Use | Tensar]

Tensar+ Geotechnical Design Software includes the T-Value Design Method for working platforms. The free to use software can be obtained from Tensar by registering on tensarplus.com