We regularly undertake working platform designs for various developments. Working platforms provide temporary surfaces on which construction plant can travel and operate without a risk of instability. These are generally required where development takes place above soft and compressible ground such as in former alluvial settings, or where thick Made Ground is present.
Tracked plant on development sites, particularly those of significant height such as cranes, piling and vertical drain installation rigs can topple due to a poor quality formation on soft and compressible soils. Working platforms are used to provide a stable formation on which plant can safely travel and operate.
Working platforms are temporary works where a granular layer is constructed across a working area on which plant can operate. Thickness is governed by the nature and shear strength of the underlying soils, and the characteristics of the tracked plant. A working platform suitable for one form of plant (e.g. a mobile crane) may not be suitable for all other types of plant (e.g. a piling rig). Therefore, a good understanding of the plant involved and ground and groundwater conditions (the Conceptual Ground Model) is needed; during working platform construction conditions may be different to those identified earlier, so care is needed.
A short clip of vertical band drain installation on site last week. The process involves the construction of artificial drainage paths into thick soft compressible soils, e.g. estuarine alluvium #geotechnics #earthworks #engineeringgeology #development #civilengineering pic.twitter.com/bAKYhxYYMq
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If site levels are to be raised (e.g. for flood relief purposes or for the mitigation of risk from contaminated soils), the working platform can be designed to be incorporated into permanent works. Working platform design is undertaken using methods published by the Building Research Establishment (BRE, 2004) and the Temporary Works Forum (TWf, 2019).
We recently provided a working platform design to allow the safe installation of vertical drains at a residential development site underlain by soft and compressible alluvial soils in Weston Super Mare.
The vertical drains were to be installed across the development area to accelerate settlements from the rising of site levels for flood relief purposes. Underlain by a steep sided ’basin’ of alluvial soils (Tidal Flat Deposits) comprising a higher strength Desiccated Crust over soft and compressible fine-grained soils, including a peat band of up to 2.7m thickness. The full depth of the alluvial succession is up to 8m.
Limestone aggregate excavated from a nearby development site was used for the working platform; we undertook an assessment of the aggregate suitability. The working platform design for the vertical drain rig used the methodology published by the BRE, and information on track loadings were provided by the installation contractor.
A working platform thickness of 500mm was recommended; we considered the feasibility of the working platform materials being incorporated into the permanent works as a drainage layer for pore pressure relief above the vertical drains, as a founding stratum for the proposed dwellings, and suitability for use as shallow soils within private gardens. We also supervised the construction of the working platform, across which the installation rig was able to operate safely.