THE PROBLEM
An apartment block and town house development required piled foundations through made ground which overlies the river terrace deposits. The conforming design called for a 900mm working platform and this was regarded as worthy of a design review.
THE SOLUTION
The contractor, ISG contacted Tensar International and requested that the tender design be reviewed. The design guidelines, ‘BR 470’, provide scope for alternative methodologies in certain circumstances, and this avenue was exploited. The result was that ISG adopted a Tensar MSL design of 300 mm thickness, incorporating granular capping material for most of the site.
PROJECT DESCRIPTION
The cleared site which has had several former uses was covered with a layer of made ground underlain by sand and gravel glacial deposits on geological strata. The made ground could be regarded as cohesive, (undrained shear strength of 30 kPa) or frictional (friction angle of 28 degrees). Ground water was observed at a depth of around 2.5 m but due to uncertainty of the water table caused by tidal influence, the ground water depth was assumed at 1.0m depth below the base of a working platform.
The apartment blocks required an 18,000m2 piled foundation and the design loads for the piling rigs were provided by the client for the load cases that were to be considered for working platform.
Over part of the site, above archaeological excavations, the Contractor made up the levels to the underside of the platform using a site won Victorian granular backfill, which further minimised the need for removal and importation of fill.
The contractor invoked Tensar’s Design & Supply service using a short form sub-consultancy agreement. The design was carried out according to BR 470 guidelines, where alternative design methodologies can be used for the incorporation of ‘structural geosynthetic reinforcement’. The variety of alternative approaches that can be used include load spread, load factor and bearing capacity improvement. To use an alternative method, the requirements are:
- The objective of safety is preserved
- Alternative approaches are based on credible and representative research work
- A person competent in both geotechnical engineering and geosynthetics is made responsible for the design
Kevin French, Project Manager at the contractor ISG remarked, “We approached Tensar International for a design service and with the expectation that an engineered solution could be provided which would significantly reduce the platform thickness. We received local support from Tensar’s Engineers, obtained a ‘result’ and we got this project off to a good start”.
