A practical step-by-step guide to designing a working platform for tracked plant using the BRE470 Appendix A calculation methodology. From ground investigation to final platform thickness.
Step 1: Assess the Subgrade Conditions
The first step in any BRE470 working platform design is to characterise the natural ground (subgrade) on which the platform will be placed. The subgrade type determines which branch of the calculation methodology to follow.
Cohesive subgrades (clays and silts): These are characterised by their undrained shear strength (cu), measured in kilopascals (kPa). Typical values range from 20 kPa for soft clay to 80 kPa for stiff clay. BRE470 Table B1 provides guidance on typical cu values for different soil descriptions. The cu value should ideally come from ground investigation data — either from in-situ testing (vane shear, cone penetration tests) or laboratory testing (triaxial tests, unconfined compression tests).
Granular subgrades (sands and gravels): These are characterised by their angle of shearing resistance (φ'), measured in degrees. Typical values range from 25° for loose sand to 40° for dense gravel. The φ' value is usually estimated from SPT N-values or CPT cone resistance using published correlations.
If ground investigation data is not available, conservative estimates should be used. For cohesive soils, a cu of 20-40 kPa is often assumed for soft to firm clay. The designer must clearly state the assumed parameters and their basis in the design certificate.
Step 2: Select the Platform Material
The platform material is the granular fill placed on the subgrade to form the working platform. The two key parameters are:
Angle of shearing resistance (φ'p): This determines how effectively the platform spreads the load from the tracked plant to the subgrade. Higher values mean better load spreading and thinner required platforms. Typical values are: - Well-graded crushed rock: 40-45° - Crushed concrete: 38-42° - Gravel: 32-38° - As-dug sand and gravel: 30-35°
BRE470 recommends using well-graded crushed rock with a minimum φ'p of 40° for best performance. The material should be angular, well-graded, and free from clay lumps, organic matter, and other deleterious material.
Unit weight (γp): The self-weight of the platform material, typically 18-22 kN/m³ for crushed rock. This is used in the calculation to account for the weight of the platform itself bearing on the subgrade.
Geosynthetic reinforcement: BRE470 allows for the use of geosynthetic reinforcement (geogrid or geotextile) at the base of the platform to improve load spreading. When reinforcement is used, the effective angle of load spread through the platform is increased, resulting in a thinner required platform. This can be economically beneficial on sites with poor subgrade conditions.
Step 3: Determine the Plant Loading
The design load comes from the tracked plant that will operate on the platform. For piling rigs, the critical parameters are:
Track width (W): The width of one track shoe, typically 0.6-1.0 m for piling rigs. This defines the width of the loaded area.
Track length — overall (L1): The full length of the track in contact with the ground. This is used for the outrigger loading case.
Track length — loaded (L2): The loaded length of the track under slewing conditions. This is typically shorter than L1 as the load concentrates under the mast during slewing.
Maximum track pressure (q1k): The maximum ground pressure under the track during outrigger operations, in kPa. This is the most onerous loading case for many rigs.
Slewing track pressure (q2k): The maximum ground pressure during slewing operations, in kPa. This can be more onerous than q1k for some rig configurations.
EN 996 provides standardised track pressure data for common piling rigs. Our design tool includes 23 pre-loaded rigs from Liebherr, Bauer, and Soilmec with all parameters auto-filled from EN 996 data.
Step 4: Run the BRE470 Appendix A Calculation
With the subgrade, platform material, and plant loading parameters defined, the BRE470 Appendix A calculation determines the required platform thickness. The calculation follows these steps:
1. Apply partial factors: The characteristic track pressures are multiplied by the load factor γF (typically 1.26) to obtain design track pressures. The tangent of the platform material's angle of shearing resistance is divided by the material factor γM to obtain the design angle of load spread.
2. Calculate the spread area: Using the design angle of load spread, the calculation determines how the track pressure spreads through the platform thickness to the subgrade surface. The spread area increases with platform thickness.
3. Check bearing capacity: The pressure at the subgrade surface (after spreading through the platform) must not exceed the bearing capacity of the subgrade. For cohesive subgrades, the bearing capacity is based on cu and the Nc bearing capacity factor. For granular subgrades, it is based on φ' and the Nγ bearing capacity factor.
4. Iterate to find minimum thickness: The calculation iterates to find the minimum platform thickness T that satisfies the bearing capacity check. The final thickness is rounded up to the nearest 25 mm for practical construction.
Our BRE470 Piling Mat Designer performs this entire calculation automatically, presenting each step clearly in the design certificate so the Temporary Works Coordinator can follow and verify the design logic.
Step 5: Document and Certify the Design
A BRE470 working platform design is not complete until it is properly documented and certified. Under BS 5975, the design must be:
Produced by a competent designer: The Temporary Works Designer (TWD) must be competent in the BRE470 methodology and experienced in working platform design.
Independently checked: The design must be checked by an independent checker who is also competent in BRE470. The checker verifies the calculation methodology, input parameters, and results.
Approved by the TWC: The Temporary Works Coordinator reviews the design and check certificate, confirms the design is appropriate for the site conditions, and issues approval for construction.
Communicated to site: The approved design must be communicated to the site team, including the required platform thickness, material specification, and any special requirements such as geosynthetic reinforcement.
Our design certificates include all of this documentation in a single professional output: the full calculation with all parameters and steps, a cross-section diagram, and a check certificate signed by David Miller, Temporary Works Designer. This gives the TWC everything needed to review and approve the design.
