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Improving Temporary Working Platform Verification Using Plate Load Testing

Temporary working platforms are critical for the safe operation of cranes, piling rigs, and other heavy plant. Yet, they often receive the least engineering attention. The lack of adequate engineering design and verification of temporary working platforms has lead to undesirable outcomes and danger to those in the vicinity of the construction site, this is a global issue. An example of this undesirable behaviour can be seen on the cover the Guide to Working Platforms (Figure 1).


Figure 1: The EFFC DFI Guide to Working Platforms: 2nd Edition (May 2025). 
Figure 1: The EFFC DFI Guide to Working Platforms: 2nd Edition (May 2025). 

Key issues include:


  • If heavy construction equipment topples over, there is an obvious danger to crane operator and those around, as well as property damage.

  • Failures frequently occur due to a platform with inadequate strength and stiffness.

  • Current verification methods such as the Dynamic Cone Penetrometer (DCP) or Nuclear Density Meter (NDM) are not suitable for many coarse granular fills.

  • Industry practice commonly relies on assumed stiffness (E) and strength (φ) parameters rather than measured values.

  • These verification methods and assumptions can lead to designs that are either overly conservative, inconsistent, or unsafe.


To help address this gap, Cook Costello assessed Plate Load Test (PLT) results from sites across New Zealand to develop an improved methodology and representative acceptance criteria for locally used platform materials.


Limitations of the Current Approach


Limits of current practice relating to Temporary Working Platforms (TWP) :


  • Oversimplified or incorrect load modelling and platform design.

  • Variable or weak subgrade that is not adequately characterised or treated.

  • Poorly specified fill materials used in platform construction.

  • Incorrect verification methods selected to check strength or compaction.

  • Insufficient testing coverage across the platform.

  • Inconsistent interpretation of results and application of acceptance criteria.


Common test limits:


DCP → suitable for Dmax ≈ 6 mm

NDM → suitable for Dmax ≈ 40 mm

PLT → suitable for Dmax ≈ 75–200 mm with Dplate = 300–762 mm


ree

Figure 2: The Scala penetrometer should

not be used for testing gravel platforms.


The Proposed Approach – Plate Load Testing


The Plate Load Test provides a direct, practical means of measuring in-situ platform performance, effectively bridging the gap between laboratory testing and real-world behaviour


Figure 3: Example PLT Test results showing load, unload, reload cycle. 
Figure 3: Example PLT Test results showing load, unload, reload cycle. 

Consistent Interpretation of PLT Results:


  • DIN 18134 Standard.

  • Follow method to evaluate Ev1 (Load) and Ev2 (Reload).

  • Modular Ratio, Ev2/Ev1.

  • Important parameters for characterising the material stiffness and state of compaction of granular fill.


Plate Load Testing Adapted for New Zealand


To better understand local performance, Cook Costello analysed Plate Load Test (PLT) data collected from gravel platforms across New Zealand.


In total, 41 tests were completed at seven sites, with each platform constructed under controlled conditions and compacted to local standards, corresponding to a density ratio (Dpr) of 95 %. Test pressures of up to 1,400 kPa were applied using a modified DIN 18134 procedure.


The results were used to derive Ev₁ and Ev₂ values to evaluate stiffness and compaction quality, and to estimate the friction angle (φ) from load–settlement curves following the Corke et al. method.


Analysis of the data found no clear correlation between Ev₁ and φ, confirming that stiffness and friction must both be measured independently.


Figure 4: Results from 41 Plate Load Test results, showing the lack of correlation between strength and stiffness of the gravel platforms. 
Figure 4: Results from 41 Plate Load Test results, showing the lack of correlation between strength and stiffness of the gravel platforms. 

Acceptance Criteria and Practical Application


  • From PLT, a reload modulus Ev₂ ≥ 80 MPa and modular ratio of Ev₂ / Ev₁ ≤ 2.8 is recommended as a practical New Zealand acceptance threshold for well-compacted platforms corresponding to Dpr ≥ 95 % (traditional target in NZ).


  • The research confirms these as broadly suitable for NZ gravel platforms but highlights that both modulus (E) and friction angle (φ) should be verified.


Figure 5: Two PLT results showing the estimated friction angle for a very loose and a very dense material. The strength does not capture the variable stiffness and likely failure mechanism of the platform. 
Figure 5: Two PLT results showing the estimated friction angle for a very loose and a very dense material. The strength does not capture the variable stiffness and likely failure mechanism of the platform. 

Moving from Assumptions to Measurements


The research demonstrates that Plate Load Testing:


  • Provides a cost-effective, scalable, and defensible verification tool for gravel platform design and verification testing.

  • Supports safer crane and piling operations through known ground performance.

  • Reduces the risk of failure, project delays, and insurance exposure.


By measuring both stiffness and strength, the industry can confidently move away from assumptions, achieving safer and more predictable outcomes across projects of all scales.


The Road Ahead


Temporary works platforms may be temporary, but their performance is critical. Incorporating Plate Load Testing into design verification, and aligning with the EFFC/DFI 2025 Guide, offers a clear path toward safer, more efficient, and evidence-based practice in New Zealand’s construction industry.


Those looking for the more detailed technical background for the proposed method can download our full NZGS Symposium paper here:



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