Summary: The numerical studies suggest that the current CBR design methods can lead to under-design results, especially for mine haul roads that are used by ultra-large trucks and heavy loads. The new proposed design method with stabilized pavement layers can lead to better design in road structure and smaller strain and displacement. The stiffness change from road stabilization also noticeably reduced the rolling resistance and fuel consumption.
Within the mining industry, it is a well-known fact that as much as 50% of a mine’s total operating costs could be attributed to haulage. With mine haul roads playing such a critical role in a mine’s operation, the structural design of the haul roads directly impacts energy consumption, operating costs, and productivity. When these haul trucks stop for any reason, mine operations are at a standstill.
In a research study conducted by the University of Alberta’s Department of Civil and Environmental Engineering, in partnership with Cypher Environmental, their team explored a new design approach for mine haul road pavements using stabilized layers. This new design method would improve on the current design approach to build stronger, higher performance roads. Currently, it is unknown how stabilization impacts structural haul road design, as no current methods address the issue of rolling resistance. Since we know any variation in rolling resistance can substantially impact energy consumption (and thus, productivity), the research set out to determine just how stabilization (with a stabilization product such as ROAD//STABILIZR®) can impact strain and deflection. The study was applied to both mine haul roads and rural municipal roads.
This article will provide a summary of the research study on mine haul roads. The research study in its entirety is available here for your reference. Before we delve into the testing methods and results, here is a glossary of useful terms to aid in your understanding of the significance of the research.
Road Design and Construction Terms
| Stress | Refers to the load applied to a material. For the purpose of this research, stress will refer to the weight of the vehicle (+ load) applied to the road surface. |
| Strain | Refers to the change in the length of a material relative to its initial length. This is measured in micrometers. |
| Critical Strain Limit (CSL) | A strain exceeding what is considered a structural failure. The typical CSL value is 2000 micrometers. |
| Deflection (or Deformation) | The sum of the vertical strains at every point beneath the surface. This is measured in millimeters (mm). |
| Modulus (Elasticity modulus or modulus of elasticity) | Describes the stiffness of a material. Measured in MPa (megapascals), this is a measure of a material’s capacity to bear and spread load. |
| Rolling Resistance | The force (energy) required to accelerate and maintain a constant speed over a given surface. Rolling resistance is measured in Newtons (N). On road surfaces, rolling resistance is created by the combined force of the deformation of the tires and the deformation of the running surface. To understand rolling resistance, imagine the physical resistance you feel as you drive across a hard packed surface versus a muddy or snowy surface. |
| Fuel Burn | fThe amount of fuel (energy) required to move a known mass over a given distance. The more energy required, the higher the fuel burn. |
| Pavement Layers | There are 3 pavement layers that are mentioned in the study –
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Method
First, a numerical model using FEM (Finite Element Model) software was developed and validated against existing literature and data (Coffrey, 2015). The numerical model was used to investigate the effects of stabilization on pavement response (looking at both strain and deflection).
FEM (Finite Element Model) is an approximation method that subdivides a complex problem space, or domain, into numerous small, simpler pieces (the finite elements) whose behavior can be described with comparatively simple equations.
For the research, a new design approach was developed based on the theories of elasticity and Critical Strain Limit (CSL) to improve existing designs.
The new design approach was verified using the validated FEM model and then used, along with a tire model, to estimate rolling resistance and explore the effects of stabilization on rolling resistance.
In total, four design models were developed:
- Reference case with no stabilization
- Stabilized surface layer
- Stabilized base layer
- Stabilized subbase layer
Results
The research team was able to attain the following results:
- Reference case with no stabilization (benchmark)
- Stabilized surface layer: increased CBR % from 100 to 120
- Stabilized base layer: increased CBR% from 60 to 75
- Stabilized sub-base layer: increased CBR% from 15 to 30
These results show us that stabilization of the road layers reduced the max strain in the pavements. What was most interesting was that the strain distribution suggests that the stabilization of lower layers showed more significant influences on the strain distribution in the pavement. This further suggests that the stabilization of the lower layers was more effective than that of the surface layer in reducing the design thickness.
New Design Method for Mine Haul Roads
The numerical studies suggest that the current CBR Design method for mine haul roads can lead to under-designed results, especially for roads used by ultra-large trucks such as those used on mine haul roads. These vehicles could weigh upwards of 600 – 650 tons with cargo – that’s 1.3 million pounds!
CBR Design Method –flexible pavement design to determine the required thickness of pavement based on known CBR values of the various layers (subgrade, subbase and base) and the wheel load (light (3175 kg), medium (4082 kg) and heavy (5443 kg)).
For example, strain calculations using current CBR design model, CAT 797B haul truck gross vehicle weight and numeric modelling demonstrates that the strain exceeds the Critical Strain Limit (CSL) of 2000 microns, max strain was 2137 microns. This demonstrates an under-designed result.
Since the new design method is based on the theories of elasticity and Critical Strain Limit (CSL), rolling resistance is taken into consideration. The results from the research study suggest that this proposed new model will lead to better design in road structure, smaller strain and displacement.
It was also found that that the new design method could also reduce the layer thicknesses of each of the pavement layers and thus reduce associated costs in the road construction. In fact, the stabilization of lower layers can reduce the design thickness even more significantly than that of the surface layer. Sub-base stabilization resulted in an overall reduction in total road thickness from 1.19 m to 1.04 m while stabilization of the surface only reduced the overall thickness to 1.18 m.
Why This is Significant
- The numerical studies suggest that the current CBR design methods can lead to under-design results, especially for mine haul roads that are used by ultra-large trucks and heavy loads.
- The new proposed design method with stabilized pavement layers can lead to better design in road structure and smaller strain and displacement.
- Furthermore, the sub-base stabilization resulted in an overall reduction in total thickness from 1.19 m to 1.04 m while stabilization of the surface only reduced the overall thickness to 1.18 m.
- The stiffness change from road stabilization also noticeably reduced the rolling resistance and fuel consumption.
Ultimately, it was determined that the new proposed design method performed better than the existing CBR Design Method.
Better built roads lead to…
- Reduced rolling resistance
- Reduced strain and displacement
- Reduced road construction and maintenance costs
- More efficient fuel consumption
… resulting in a better bottom line.