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Arno Hefer Administrator Posts: 19
11 days ago
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The Rubicon Toolbox Layered Elastic Theory (LET) Tools provide an effective framework for the design of gravel mine haul roads and the use of limiting strain criteria in general. This post outlines a mechanistic-empirical design approach originally proposed by Thompson and Visser (1996) and refined with expanded design criteria by Thompson et al. (2019).
Design Criteria
This mechanistic design method focuses on limiting design criteria to guide performance optimisation. For layers below the wearing course, the key design criterion is vertical compressive strain, which traditionally correlates to the failure mechanism of plastic deformation (or rutting) in the subgrade and selected layers. This approach ensures the strategic use of available materials, including typical waste materials such as dump rock, which is often accessible at mines. Table 1 summarises vertical compressive strain limiting criteria for various haul road operational conditions.
Design Process
- Select Limiting Design Criteria: Define vertical compressive strain limits from Table 1 for the layers below the wearing course to mitigate rutting and ensure structural integrity.
- Determine Material Properties and Layer Configuration: Haul road design relies on the effective use of available construction materials, including mine waste such as dump rock, which provides a cost-effective and structurally sound option for the underlying layers. While these materials are integral to the design, the wearing course – excluded from mechanistic modelling due to its dynamic nature and regular maintenance – plays a critical role in the road’s performance. The wearing course must provide acceptable stability and functional performance, particularly in terms of resistance to corrugation, stoniness, and dustiness. Assumptions from Thompson and Visser (1996) suggest that a California Bearing Ratio (CBR) of at least 60, and preferably 80, can result in stiffness values in the range of 300 to 400 MPa when placed over a stiff underlying layer. Configuring the underlying layers to optimise load distribution and leverage material stiffness is essential for minimising strain and ensuring structural integrity.
- Performance Evaluation: 1). Ensure wearing course material properties meet minimum requirements to ascertain performance between maintenance cycles; 2). Validate the vertical strain limits for critical layers below the wearing course to ensure compliance with the design criteria.
- Enhance Structural Stiffness: Where applicable, incorporate a rockfill or dump rock layer directly beneath the wearing course to enhance stiffness and improve load distribution. Research by Thompson and Visser (1996) suggests modulus values for these layers can range between 1500 MPa and 3000 MPa, depending on material quality and compaction. To optimise stiffness, dump rock size should be limited to approximately two-thirds of the lift thickness (200 - 300 mm). Effective compaction using heavy vibratory rollers or impact compaction until refusal is recommended to achieve the desired stiffness properties.
Design Example
- In this example a haul road category II is applicable, and the traffic is less than 100kt/day. A maximum limiting vertical strain in all layers below the wearing course of 2000 microstrain is selected.
- Make sure the load configuration for your design vehicle has been set up under Settings/Manage Load Setups and is available for use under Settings/Select Load Setups. The mine haul truck used as the design vehicle in this example is a CAT793D.
- The output report containing the setup and assumptions for this example is shown below. Note that you can define your own materials with specific descriptions. For guidance on this aspect, go to the Help Post: Adding Materials.
- Since the design criterion is maximum vertical compressive strain, you only need to select a failure criterion that returns this parameter for each layer. Any traditional subgrade criterion is ideal for this purpose, such as XXX or XXX. Note that the reliability and structural capacity in terms of MESA, E80s or years are not applicable when using limiting strain criteria as outlined in Table 1 above.
- For our example, the pavement configuration comprising a 200 mm wearing course and a 650 mm Dump Rocklayer, supported by a 500 mm pioneering layer on a relatively weak subgrade produces acceptable compressive strain responses below 2000 microstrain in all layers.
References
Thompson, R.J. and Visser, A.T. (1996) Towards a mechanistic structural design method for surface mine haul roads. South African Institute of Civil Engineering (SAICE) Journal, 38(2), pp. 13-21.
Thompson, R.J., Peroni, R. and Visser, A.T. (2019) Mining Haul Roads. Taylor & Francis Group, London, UK.
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