Working platforms are temporary structures made up of granular fill, typically between 300mm to 800mm thick, that provide a suitable foundation for heavy construction plant and machinery to carry out operations such as crane lifts and piling. The aggregate base installed for the platform is designed to reduce the pressure imposed on the underlying formation to an acceptable level.
A poorly constructed working platform with inadequate drainage can quickly deteriorate and become unsafe to support the movements and loading from large equipment, resulting in potential rig overturning accidents or vehicles sinking into the weak sub-soils. Failure to not utilise geosynthetics as part of the working platform design can therefore prove to be a false economy. The cost of implementing a safe working platform is not insignificant however, and so a competitive design is a distinct advantage.
ABG offers a free to use working platform design calculator, to login to use the tool please visit our Design Solutions portal or submit your design request using our project assessment form.
Site access and ramps
Geosynthetic based designs to provide a reliable base for operation and manoeuvre of heavy plant
Use of geosynthetics reduces the thickness of required granular fill by up to 60% and enables construction over soft foundation soils
Reduction in construction waste and off-site removal of material and the associated carbon costs
Improved site accessibility and health and safety speeds up construction programme and reduces vehicle emissions
Reinforcement and separation functions make the re-use of reclaimed aggregates possible
Variety of geosynthetic alternatives available to suit the specific site conditions and the aggregates available on site
ABG has developed a proprietary calculation method in accordance with the BRE 470 health & safety guidance, with the objective of improving safety via the construction of reliable and economic working platform designs. As part of this methodology, geosynthetics are typically included beneath or within a granular platform in order to improve its structural stability and minimise deformations during trafficking and loading.
The ABG range of geosynthetics typically used to construct working platform designs are Terrex geotextiles and a range of geogrids. Geotextiles are primarily used as a protective separation layer to prevent the loss of fines and contamination of the platform material and to provide a secondary degree of reinforcement/stabilisation. Geogrids are specifically required to provide reinforcement and stabilisation of the granular fill, providing a lateral restraint via friction and interlock/confinement. As a rule of thumb, a layer of geogrid is equivalent in cost to 100mm of granular fill and it is generally safe and economically justifiable to incorporate a grid when the platform depth can be reduced by 100mm or more.
For stabilising haul roads, site access routes and working platform areas with soft subgrades, ABG can provide design assistance to strengthen the in-situ soils in accordance with the foundation classes stated in clause 2.7, Note 2 of Design Manual for Roads and Bridges (DMRB) document CD 225.
Geosynthetic cellular confining systems such as Abweb may also be used to provide additional stabilisation of working platforms and to protect tree roots that are to be preserved alongside the installation of temporary work areas (Abweb TRP).
ABG provide a range of punched and drawn polypropylene geogrids with a typical design life in excess of 100 years and with aperture sizes to maximise stone interlock within the grids. They provide reinforcement of loading platforms in areas of weak compressible or variable sub-soils. In addition, by improving interlock/confinement, the geosynthetic acts to increase the load spread angle via increased shear resistance within the platform. Where a reclaimed on site fill is to be used, the geogrid can be selected to suit the specified maximum particle size.
The Terrex geotextile layer primarily acts as a separation layer to prevent contamination and loss of fines content and to allow efficient drainage of the platform. Since it also acts in tension under loading, especially in soft soil conditions and often in combination with a geogrid, it provides additional restraint against the outward movement of the fill material and to confine the lowest layers of fill when they are compacted. Geotextiles used for working platforms must have sufficient puncture resistance to minimise damage from the gravel platform and in order to maintain exclusion of contaminants and retain tensile capacity. Terrex SNW is available where high resistance to puncture is required.
Working platform design is carried out by our experienced geotechnical engineers using ABG’s in-house empirical load spread method and following the HSE guidance as detailed in BRE document ‘BR470 – Working platforms for Tracked Plant’. The layer configuration specified will depend on the subgrade strength, the platform materials and the construction traffic loads. Our designs seek to achieve a balance between platform safety with adequate bearing capacity whilst still being economic and straightforward to construct. In our design proposal we consider the site plan and the possibilities to re-use existing materials as a carbon saving alternative.
Drainage of the platform is also key, and a poorly drained platform will deteriorate more rapidly. ABG has over 30 years’ experience in providing environmental drainage and where compressible subgrade soils require protection from water ingress, ABG can suggest a suitable drainage geocomposite option to drain excess rainwater to nearby channels. A geotextile may also be specified to treat platforms that are close to, or below the ground water table. ABG’s technical service includes initial budget cost estimates and indicative thickness calculations, through to full PI backed designs available on request.
Input parameters required for design
ABG’s design approach promotes health and safety first. A properly designed and installed working platform enhances the efficient working of a site, as well as ensuring a safe environment. The design brief should be developed as required for any other temporary works design, but ideally the following information is required:
What is the subgrade formation like:
Ground investigation report
Cohesive or granular soil
Undrained shear strength
Angle of friction
Ground water table
What is the platform to be made of:
Specific source of gravel from site, or gravel to be specified and delivered
Plan of the working platform area and access roads
Geosynthetics are used in temporary works applications to provide significant cost and time savings throughout the construction process; from stabilising temporary access roads, haul roads, site compounds, embankments and slopes.
ABG’s range of geotextiles and geogrids are quick to install compared to traditional methods, reducing the volume of natural materials required and saving material and transport costs. ABG’s geosynthetic products provide permanent and temporary solutions for ground reinforcement, slope reinforcement, erosion control, pavement reinforcement and structural drainage. Haul Roads / Temporary Access Roads
According to the Design Manual for Roads and Bridges (DMRB) document CD 225 (Revision 1, Apr 2020 formerly IAN 73/06 revision 1, HD 25/94), clause 2.7, Note 2: “Options for improvement of the subgrade include excavation and replacing between 500 to 1,000 mm of the soft subgrade with granular fill, mechanical stabilisation (geogrids and/or geotextiles) and soil stabilisation”. ABG can propose a stabilised foundation solution to compensate for lower CBR strength values (i.e. less than 2.5%) and meet the foundation class requirements. The design for all foundation areas shall be based on achieving a foundation class selected from Table 3.7 in CD 225:
Abweb TRP is a geocellular system for protecting tree roots where a temporary access route is required without damaging nearby trees.
Site compounds require access routes, temporary car parking and storage areas. ABG’s range of Trigrid and Abgrid geogids provide ground stabilisation by mechanically stabilising granular materials effectively. Abtex woven geotextile and Terrex NW geotextile provide separation functions when constructing unpaved access roads and parking areas.
Crane Pads / Piling Mats
ABG geogrids and geotextiles are ideal for temporary applications including stabilising weak ground for mobile cranes, piling rigs and working platforms. Geogrids mechanically interlock with well graded angular stone, reducing subbase thickness and increasing bearing capacity.
Wind farms access roads
ABG Truckcell provides a permeable grass or gravel stabilised surface suitable for the heavy plant involved in the construction and maintenance access of wind farms sites.
Drainage is a major consideration in all Civil Engineering earthworks projects, including temporary works. ABG Fildrain is a Fin Drain system that saves installation time and reduces costs compared to traditional stone based drainage layers.
Where temporary slopes and embankments are constructed, ABG provide a range of erosion control mats and geocells to confine fill material to reinforce the surface structure.
Specific design advice and guidance is available from our experienced in house technical team who will help ensure the right product is specified for your temporary works project.