Thin Combigrid® Platforms Carry Heavy Loads
In the construction of heavily trafficked areas such as working platforms, a stable subgrade with sufficient bearing capacity is required. Economical and environmental advantages of construction methods with geosynthetics, especially on soft soils are already well known. Soil masses that need to be excavated, transported and installed can be dramatically reduced by the inclusion of geosynthetics. The best example is the improvement of soft subgrades with geogrids or geocomposite products. Geo-textiles are normally required to separate a granular platform from a cohesive subgrade and to act as a filter.
In 2011, it was planned to build granular piling rig access plat-forms to facilitate the installation of piles for support of the new Harvey Norman bulky goods project in Maroochydore, Queensland, Australia, to increase the low subgrade strength and to provide sufficient bearing capacity for the imposed loads of the cranes.
After site investigations, the consultant confirmed that the current condition of the site surface would not be satisfactory for the safe operation of the proposed precast piling rig imposed load of 280kPa. It was also advised that the grey clay exposed on the surface of the site had been badly affected through saturation as a result of the 2011 Queensland floods. Subsequent wet weather was not allowing the perched groundwater to drain away, leaving the platform saturated. These surface soils alone were not capable of supporting the required 280kPa bearing capacity required for the pile rig. A further constraint was that the platform thickness was limited to 350mm above the existing subgrade elevation to avoid interference with the intended levels of the proposed basement car park.
Global Synthetics were asked to provide a geosynthetic alternative design to provide a suitable (i.e. safe and stable) working platform for the piling rig. The original design proposal was to excavate 200mm and construct a 500mm thick platform consisting of good quality unbound pavement gravel. As the subgrade was weak, and the thickness of the platform was limited, the geosynthetic reinforced platform on existing subgrade was chosen as the preferred solution. With reference to the in-formation given, a subgrade CBR value of minimum 1% (equal to cu > 30kN/m2) had to be considered as the existing bearing capacity of the in-situ soft sub-grade. The design by BBG Bauberatung Geokunststoffe GmbH & Co. KG focused on the specific machine type to be used with a total mass of 77 tonne. The design of the reinforced working platform was carried out in accordance with the BRE (Building Research Establishment, UK 2004) design manual and the design principles defined in DIN 4017:2006 to achieve an adequate safety factor against bearing failure.
Upon final approval, some 50,000m2 of Combigrid® geocomposite was installed successfully on site beneath the crane platform. The material was placed directly on the subgrade to improve the bearing capacity and to prevent fines from migrating into the platform materials. The appropriate strength of geogrid was selected according to appropriate stress and strain properties required and the subgrade bearing capacity. A 350mm thick granular working platform was then constructed on top of the Combigrid®.
Combigrid® consists of a Secugrid® geogrid as the reinforcement component; combined with a needle-punched nonwoven geotextile firmly welded between the reinforcement bars offering separation and filtration in one single layer. It offers the functions of two different materials with the simplicity of installing a single product.
The use of Combigrid® made the construction of a thin granular platform layer on the very soft subgrade possible, reducing construction costs and construction time for the client. By omitting the 200mm subgrade excavation and removal procedure, and reducing the platform thickness from 500mm to 350mm substantially reduced the number of truck movements to and from site ensuring significant additional environmental benefits as a result. The con-tractor, client and pile rig operator were extremely happy with the results and reported that the final solution exceeded their expectations.