Combigrid® Geocomposite Geogrid
In 2011, it was planned to build granular piling rig access platforms to facilitate the installation of piles for support of the new Harvey Norman bulky goods project in Maroochydore in Australia to increase the low subgrade strength and to provide sufficient bearing capacity for the imposed loads of the cranes. Subsequent continual wet weather was not allowing the perched groundwater to drain away, leaving the platform saturated as a result of the 2010 local flooding.
The surface soil alone was 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 350 mm above the existing subgrade elevation to avoid interference with the intended levels of the proposed basement car parking concrete slab on ground.
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 (minimum CBR value 1%) and the thickness of the platform was limited, the geosynthetic reinforced platform on existing subgrade was chosen as the preferred solution.
Upon final approval, some 50000m2 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. A 350mm thick granular working platform was then constructed on top of the Combigrid®.
Combigrid® consists of a Secugrid® geogrid with a high stiffness and low installation damage 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. Combigrid® combines reinforcement, separation and filtration into one single product thus reducing supply and installation costs considerably.
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 contractor, client and pile rig operator were extremely happy with the results and reported that the final solution exceeded their expectations.