"There is no way out of a bad design"

Pit designing is one of the things that I enjoy the most about consulting work. There are a lot of factors to consider to ensure that your pit design is not only geotechnically compliant, but it can also support economic and environmentally friendly exploitation and it operates within other set constraints such as the mining rights area. It is fundamental to ensure that you get the pit designs right before you can publish them and implement them on the ground. Once you get in there, it is too late to go back!!!

The main goal of a mine is to operate in the most safe and efficient way, which can be achieved by increasing the production while reducing the cost. A good pit design can contribute positively to the safety and economics of an operation. On the other hand, a poor design may add unnecessary safety risks and result in an inefficient and costly operation.

Drawing back to the article titled “Strategic Mine Planning for Open Pit Mines – The Integrated Way!!! (Part 1)”, I emphasised that after running the pit optimisation and you have selected the pit shell that speaks to your corporate objective(s), you will have to design a detailed, operative pit design. The aim is to generate an operative design that will allow for exploitation of the defined reserve in a safe, economic, and environmentally responsible manner while deviating as little as possible from the selected mathematical pit shell outline.

Let me re-iterate that the operative design has to ensure that the mine will be operating efficiently, operating fully within the mining rights and away from exclusion zones, minimises the haulage distances, maximises ore recovery, support production targets and it is compliant with the geotechnical and safety restrictions. Sometimes it may not be possible for all the factors mentioned above to be fully executed on one design, and as a design engineer you may have to trade off one factor for another. That is where the fun begins…

A good design is one that conforms to all geometrical slope considerations and geometric ramp design to comply with the geotechnical and safety restrictions while still ensuring the mine can operate economically and in an environmentally responsible manner.

Geometrical slope considerations

The overall slope of the pit wall is one of the major elements which affects the size and shape of the pit. The walls of the pit needs to be designed such that they can remain stable as long as mining activity is planned for the safety of people, equipment, and machinery. The configuration of an open pit slope is based on three spatial aspects: the bench configuration, the inter-ramp angle, and the overall slope angle. The geometrical components of a slope are in direct relationship with safety conditions of the mine since they are associated with acceptable design criteria, which allow the slope stability to be achieved

Slope geometry design is an important aspect as it related directly to operational risk management. Some of the key aspects to model in the design are summarised below:

Bench Configuration – bench configuration is defined on Figure 1 below:

Figure 1: Typical bench configuration

• Bench height (BH) – bench height is the vertical distance between the toe and crest of a single bench. The determination of the optimum bench height is a fundamental concept in open pit mining. The optimum bench height depends on a number of factors, including the cutting height and bucket capacities of loading machines, the capacity of drilling machines, the rock properties, necessity of selective extraction, slope stability, required production etc. (Sami et al., 2010). The height of the bench has an impact on selectivity and dilution and consequently the economics of the mine. In some instances, mine operations may double bench or triple bench in order to have steeper inter-ramp angle (improving economics) without compromising on the safety of the operation.
• Bench Face Angle (BFA) – the bench face angle is the angle formed between the horizontal plane and the bench wall. In weaker rocks, bench face angles are often controlled by the equipment and digging technique used while mining. In hard rock, which cannot be mechanically ripped but must be blasted, the stable bench face angles are controlled primarily by the stability of the local geologic structure (Ryan & Pryor, 2000)
• Berm Width (BW) – there are two types of berms to take into consideration on your designs as follows: Standard Berm Width (SBW) is the horizontal distance between the toe and the crest of a bench. Catchment (safety) Berm Width (CBW) is the wider, safety berms that designed at defined intervals in order to catch falling rocks from benches above and prevent them from rolling down the pit wall and creating a safety hazard.

Various combinations of the bench height, bench face angle and berm width can be used in order to define an overall slope angle that satisfies the stability criteria as shown on Table 1:\

Table 1: Various combinations of the bench height, bench face angle and berm width used to calculate the overall slope angle of 45°.

• Inter-ramp angle (IRA) – The inter-ramp angle is defined by the bench configuration discussed above. The inter-ramp angle portion of a pit slope is the toe-to-toe slope between two ramp sections. The inter-ramp angle can be calculated in numerous ways, depending on the bench height (BH), bench face angle (BFA), and the berm width (SBW/CBW) as shown on Table 2.?Different combinations of these can be used to develop the same inter-ramp angle:

Table 2: Calculating the inter-ramp angle using different combinations of the bench face angles and berm withs at a bench height of 10m.

As discussed, to steepen the inter-ramp slope angle, the bench height can be doubled (or tripled), and the berm width can be adjusted accordingly. Because berm widths do not have to be doubled for a higher final bench, the inter-ramp slope angle can be steeper while maintaining similar levels of rockfall containment and stability (Ryan & Pryor, 2000).

Double benching (or triple benching) is used where the inter-ramp slope angles are steep enough that single benching would result in an overly flat overall slope. (Kuchling, 2015). For example, if the BH is 5m, the BFA is 80° and the inter-ramp slope angle is 50°, then the corresponding calculated catch berm width would be 3.3 metres to achieve the 50° IRA.?However, such a small catch bench would be ineffective in catching falling rock. Triple benching to 15m benches would result in a catch berm width of 10 meters, which has a higher chance of catching falling rock.

• Overall Slope Angle (OSA) – For mine planning and mining engineering design, the overall slope angle is the angle made with the horizontal of the line connecting the lowest most toe to the upper most toe. This angle considers all ramps and inter-ramps. Figure 2 illustrates the relationship between the inter-ramp angle (IRA) and the overall slope angle (OSA).

Figure 2: Relationship between the inter-ramp angle (IRA) and the overall slope angle (OSA).

Haul road geometric design considerations

Mining haul roads (or ramps) are a critical component of surface mining infrastructure, and the design of these haul roads has a direct impact on operational efficiency, costs, and safety. Material haulage represents a significant proportion of a mine’s cost and as such, well-designed and managed roads have a direct contribution to reductions in cycle times, fuel burn, tyre costs and overall cost per tonne hauled and critically, promote a safe transport system (Roger et al.,2019).

Haul road design is an important consideration in the design of open pits. There is usually inherent loss of value between the optimised surface and the operative design due to the inclusion of ramps and berms. Generally speaking, addition of haul roads during the pit design exercise results in the addition of waste material and/or reduction of ore material extracted from the defined ultimate pit. This has a significant impact on the economic value of the project. It is important to think carefully around the design and location of haul roads.

Geometric design of haul roads

Haul road design is critical to the longevity of the mine. This article is focused on the geometric design of ramps. It does not discuss any aspects related to the structural, functional and/or maintenance design of haul roads. The geometric design is the main starting point for any haul road design and refers to the layout and alignment of the haul roads in both the horizontal (road width, curve radius, etc.) and vertical (ramp gradients, cross-fall/camber, super-elevation etc.) plane (Thompson & Visser, 2000). All geometric elements of haulage roads should be designed to allow for safety, reduction in fuel consumption and agility in operation.

• Haul road width – The width of the haul roads is designed based on the width of the largest and least agile truck that will travel in that particular haul road and the number of lanes that are required to provide proper access for haul trucks, ancillary equipment, and small vehicles while minimising congestions. Haul roads that are too narrow create uncomfortable driving conditions which, in effect, slows traffic and increases production cycle times. On the other hand, haul roads that are too wide increases the strip ratio i.e. amount of waste mined and consequently the mining cost. In most cases, the design exercise is concerned with trying to find a balance between operational flexibility and economics.
• Haul road gradient – The haul road gradient is the grade line profile along the road centre line, measured in the vertical plane. The gradient of the haul roads has an impact on the cycle times and the operating costs (wear on machines and tyres). Gradients that are excessively steep reduce the length of the haul road but require the use of low gears for all types of vehicles. Not only does this put more stress on machinery, but it also increases the equivalent flat haul distances and production cycle times. In addition to that, steeper haul roads can result in loss of material during transportation. Gradients that are too gentle requires more excavation and construction for the longer road, and the added travel distance means cycle times are inefficient. As a design engineer you should ensure that the gradient of the haul roads is smooth, has even grade and does have grade “breaks” (combination of grades) (Thompson & Visser, 2000).
• Switchbacks – Haul roads can be designed as spiral ramps (spiralling around the pit) or as ramps with switchbacks or a combination of both. Since haul roads are used as catch berms to catch falling rocks, a design that includes switchbacks is generally more stable in geotechnical terms. However, a significant number of switchbacks (especially over the same wall) increases the amount of stripping required, compromising the profitability of the mine. Also, switchbacks have an impact on the operating cost and safety of haulage truck’s operation. For example, switchbacks reduce the speed of trucks and consequently, the productivity of the trucks. This causes greater tire wear and require periodic maintenance, increasing the operational cost. Switchbacks that are not designed properly cannot be navigated with ease and they pose a safety threat. If switchbacks are to be included in the design, flat switchbacks are preferred since they provide the least load on the truck’s drive train (Poniewierski, 2018). If switchbacks are designed on grade, the gradient on the inside curve radius should be set to a flatter gradient than the ramp to compensate for increased rolling resistance (Thompson & Visser, 2000). It is important to also ensure that designed switchbacks have sufficient turning radius. A larger radius curve can be far more safe and operationally efficient than a sharp switchback – particularly for long life ramps. Sharp switchbacks result in slower cycle times and higher overall costs. Lastly, where possible, it is important consider designing switchbacks which coincide with the catch berm to reduce additional waste stripping (Poniewierski, 2018) as shown on Figure 3:

Figure 3: Swithcback design coincing with a catchment berm

Other important aspects to consider for a good design…

• Should the design have multiple haul roads for improved productivity and safety? If yes, at what extra stripping cost? Remember, more access points mean more flexibility, but the added stripping costs could be high. This is one of the biggest trade-off exercises that a design engineer goes through when producing a design;
• Where should the haul roads exit at the top of the pit? Should the haul roads exit closer to the waste dumps to minimise waste haulage (considering that the waste tonnes are likely to be much higher than ore tonnes), should the haul roads exit closer to the processing plant? Should you have two separate exits, one for ore and one for waste?;
• Where should you place haul roads on the actual pit design? As a design engineer, is important to be aware and to understand the geotechnical risks associated with the deposit in question, especially areas with high risk of failure. These are areas that are best to avoid when adding haul roads to the designs. Where possible (and it’s not always possible) design ramps in a pit to be anti-clockwise upwards (when hauling loaded material upwards) (Poniewierski, 2018). This allows for right-handed trucks to drive loaded upwards against the pit wall. Having the loaded truck against the wall means less weight is placed on the likely less stable ramp edges;
• Are the haul roads connecting all the way to the bottom of the pit? As a design engineer you need to follow the progress of the pit development to check if all the haul roads are connected all the way to the bottom as shown on Figure 4. Some haul roads may be redundant after some time, it is important to have another haul road available to maintain access to the bottom benches;

Figure 4: Pit development analysis to check if all ramps are connecting all the way to the bottom of the pit

• Where haul roads intersect, the intersection area should be flat with lead sections that are also flat. Y-intersection should be avoided as they concentrate stresses and could result in geotechnical instabilities. Where cross-roads are used, the intersections should be at 90° creating a square orientation (Federal Highway Administration [FHWA], 2000);
• Should your design use one-way ramps for the bottom 2-4 benches as shown on Figure 5? If possible, as the design engineer you should consider designing parking and passing areas for trucks that are awaiting access to the shovel (on switchbacks for example) (Poniewierski, 2018). In some cases, one can push the gradient of the last 2-4 benches depending on the truck types that are used to haul the material that lies in those 2-4 benches;

Figure 5: Pit design with bottom benches that operate with single lane, narrower haul roads.

• How many haul roads do you have servicing each area? The rule-of-thumb is to have at least two independent ramps servicing all working areas for at least two-thirds for the pit from surface to the final pit depth (Thompson, 1996). This is an important risk mitigation measure as a failure or blockages of a ramp cuts off access to the working areas, unless a second ramp is available. I am sure when reading this you can easily think of a mine that is operating with one access for the whole pit. I wish I could gamble like that ??;
• The in-pit haul road design must allow for machinery to access geotech berms and flat areas as shown on Figure 6. This is to ensure that there is access for cleaning and maintenance of the berms. In pit infrastructure (transformer, holding dams, pumps, equipment storage) is normally placed on flat areas and access would permit servicing these installations;

Figure 6: Different design approaches to access the catch berms

• When producing design for long-term planning, we generally do not worry about operational details such as road camber and cross-fall. However, when producing designs at an operational level you need to make sure these issues can be dealt with (Poniewierski, 2018). Crossfall is the difference in elevation between the crest (crown) and the road edge. It is a widely used technique that effectively drains water from road surfaces and is to be incorporated in all road designs;
• Does the design have sufficient safety features such as flat areas on switchbacks, safety ramp run-offs etc.;
• Does the design have any sharp bends or bullnoses? These should be avoided as they concentrate stresses and often lead to a weak geotech structure with a high failure risk. Sharp bends and bullnoses poses a safety risk and should be avoided;
• Mining through underground workings presents a number of potential hazards that must be accounted for in the pit design. As a design engineer, you need to ensure that design does not violate the minimum pit floor pillar thickness that has been defined to ensure that mining equipment and personnel can safely traverse during normal mining operations;
• Does that start of all of the designed ramps have a sufficient entry distance to the bench as shown on Figure 7? Ramps with narrow entry distances results in operational issues and higher operating costs; and lastly

Figure 7: Narrow vs broad ramp entry distances

• Does the produced design respect the minimum pit bottom width that is required to support efficient operation of the available machinery? To improve economics, you want to consider designing a “good-bye” cut at bottom (an excavation “trench” of ore by a backhoe excavator without an access ramp, dug retreating from the excavation with trucks backed up to excavator) (Poniewierski, 2018). The minimum pit bottom width can be reduced drastically when using ADTs and this could allow you to extract more ore (extract deeper benches) at no additional waste.

There are other pit design critical factors that I might have missed, and I will be happy to hear from you.

As stated before, on the first run it will be difficult to follow the selected optimisation pit shell. Depending upon difference between the pit optimisation shell and the final design, it may be necessary to re-iterate through the pit optimisation stage using the new OSA parameters from the just completed design.

References:

1. Thompson, R.J. 1996. The design and maintenance of surface mine haul roads.
2. Ryan, T.M. & Pryor, P.R. 2000. Designing catch benches and interramp slopes.
3. Kuchling, K. 2015. Pit wall angles and bench width – how do they relate.
4. Thompson, R.J. & Visser, A.T. 2000. The functional design of surface mine haul roads.
5. Thompson,R. & Visser,T. 2015. Mining haul roads: Theory and practice.
6. Poniewierski,T. 2018. Guidelines and considerations for open pit designers.
7. Soltanmohammadi,H., Osanloo,M. & Sami,A. 2010. Selection of practical bench height in open pit mining using a multi-criteria decision making solution.
8. Federal Highway Administration [FHWA], 2000. Highway design handbook for older drivers and pedestrians