T-Value: A new direction for temporary works design

Working platforms and piling mats

Temporary working platforms are important aspect of many construction projects, providing stable and safe working areas. Piling mats are working platforms designed specifically to carry the heavy static and dynamic loads from piling rigs, their support cranes and associated equipment.

Often placed over weaker subgrades, temporary working platforms are typically built using well-graded, compacted granular fill, such as natural gravels and crushed rock, as well as recycled demolition material. Geogrids can be incorporated in the granular material to stabilise it and increase bearing capacity, and platforms are designed to be free-draining, to prevent a build-up of water on the surface.

The importance of safety

The UK’s Federation of Piling Specialists (FPS), estimates that a third of all dangerous occurrences reported by its members are related to piling mats – a soft spot just 1m² can cause a rig to topple, with the potential for devastating consequences. Safety in design, construction and maintenance is obviously of paramount importance.

Source: http://www.heavyliftnews.com/accidents/tragic-crane-accident-vungtau-vietnam

The FPS recommends that piling mats should be inspected daily to ensure they are in proper working condition. If any excavations, trenches or holes have formed in the surface, they must be properly backfilled to ensure they are as stable as the rest of the mat.

Every site with an operational piling rig has to have a Working Platform Certificate, which confirms the piling mat has been properly designed, and constructed in accordance with the design, and that it will be regularly inspected and maintained. This is signed by the principal contractor and provided to the piling contractor before work starts.

Design guidance for working platforms and piling mats

The key document for UK designers is BR470 Working platforms for tracked plant, which includes guidance on the design, construction, operation and maintenance of working platforms. It was written by the Building Research Establishment under the direction of the FPS.

In 2011, BRE published a supplementary document, Use of structural geosynthetic reinforcement – A BRE review seven years on. This recognises that alternative design methods can be used, including platforms built using granular material mechanically stabilised with geogrid, as long as designs are based on ‘credible and representative’ research and project case studies.

Other guidance includes:

• Federation of Piling Specialists Guide to Working platforms
• The European Federation of Foundation Contractors and Deep Foundations Institute’s A guide to working platforms
• The Institution of Civil Engineers Temporary Works Forum’s Working platforms – Design of granular working platforms for construction plant – A guide to good practice

The benefits of stabilising geogrids in working platforms

Incorporating stabilising geogrids in the granular material used to form a working platform can improve bearing capacity, while being up to half the thickness of a non-stabilised granular layers.

This is because of the interlocking mechanism and particle confinement that develops between the aggregate and the geogrid. This prevents lateral movement of the granular material, creating a mechanically stabilised layer that increases bearing capacity and controls differential settlement.

Additionally, lower quality and recycled granular fill can be used and together, this can save time and money through reduced excavation and imported materials, as well as reducing the platform’s carbon footprint.

However, while the 2011 supplementary document to BR470 Use of structural geosynthetic reinforcement – A BRE review seven years on, considers geosynthetics, because their effects are based upon individual manufacturers’ guidance, this can lead to inconsistency and uncertainty for designers and engineers carrying out design checks.

This can mean that geosynthetics are not specified, so potential benefits and savings are missed. This missed opportunity led to the development of the Tensar T-Value design method.

How T-Value differs from other Piling mat design methods

While BR470 considers geosynthetics, their effects are based upon manufacturers’ guidance, which can lead to inconsistency and uncertainty during design checks. Existing methods are either overly-simplified (eg a common load spread angle for all cases) or use obscure, hard-to-understand empirical input parameters.

Furthermore, a granular material’s bearing capacity and a geosynthetic’s tensile strength are considered separately. This is inappropriate to stabilising geogrids because, as discussed, rather than acting in tension to reinforce granular material, the aggregate and geogrid perform as a composite, due to interlocking and particle confinement in the geogrid’s apertures.

What makes the T-Value design method different from other methods is that, for the first time, the performance of a composite of granular material and stabilising geogrid can be analysed. This gives a truer prediction of performance and also allows designers to compare this performance with that of non-stabilised materials.

While in theory, the T-Value Method can be applied to other granular materials and geogrids, the relationship between the T-Value and subgrade strength that has been derived is for Tensar geogrids alone. If other aggregates and geogrids are to be used, the relationship must be derived through FEA and laboratory testing and validated using full-scale testing appropriate to the platform or foundation being built.

Verification of the T-Value design method through field trials

The T-Value design method has also been tested in the real world, with Tensar teaming up with the University of Saskatchewan in Canada to carry out full-scale plate load tests of a trial section of granular working platform on a clay subgrade.

Testing, using two 20t trucks as a reaction force, was carried out on 0.25m and 0.5m thick granular platforms, on sections with, and without, stabilising geogrid.

A 1m square plate was used to create a pressure bulb similar in size, and depth, to the one created by the track of a piling rig or crane. This, gave a more accurate assessment of ultimate (and safe) bearing capacity of the platform, compared with conventional tests using 0.3m or 0.6m diameter plates.

Applying the T-Value design method in practice at Green Park Primary Academy

The T-Value design method enabled design and construction of two working platforms for the construction of Green Park Primary Academy in King’s Lynn, when very weak ground meant standard approaches could not be used.

Norfolk County Council awarded the £8m design and build contract for the new academy to Cocksedge Building Contractors.

Consultant Richard Jackson was tasked with designing two working platforms, covering a total area of 2,150m2, to support a maximum load of 272kPa from the rig carrying out ground improvement of the underlying highly variable and very weak peat and clay soils. The platforms also provided access for the piling rig installing foundations for the school buildings.

However, the underlying soils’ minimum undrained shear strength (Su) of 9kPa fell outside BR470 guidance, so Tensar was asked to provide a solution to allow piling operations to take place safely.

The T-Value design method produced the designs for the working platforms, one for the area with a minimum Su of 9kPa, and another, where the minimum Su was 25kPa.

Designs were based on rig loadings, soil conditions and granular fill grading. On-site validation testing confirmed the T-Value design method accurately predicted safe bearing capacity at the top of the working platforms, built using 6F1 aggregate, mechanically stabilised with Tensar geogrid.

The platforms were successfully installed and, based on a comparison with similarly derived non-stabilised platform thicknesses, saved around 35% in construction costs, 20% in construction time and around 50% in carbon emissions.

Download the case study of Green Park Academy project here.