Education, Features

How to understand quarry scheduling

A very wise person once told Steve Franklin, founder of Eltirus, “Better to make your mistakes on the computer than in the field.” So, what did he mean by that comment? Steve Franklin explains. 

Quarry extraction planning can be a little like a three-dimensional game of chess and the people who are best at it generally have a good ability to visualise in their mind how the deposit is going to be extracted in such a way as to keep a balance of development and production material and to ensure that critical access is maintained.

These experts can visualise the sequence, run different strategies mentally and come up with an effective way forward. Unfortunately, this tends to be something they keep in their mind and don’t commit to paper. If they leave the operation, that mental plan tends to leave with them and as we progressively lose our older more experienced quarry managers, this is an increasingly important loss to any quarry business.

So, what to do? The answer in short is to build a scheduling model. But what is it and how do you put one together and then make the most of it?

Scheduling blocks are generally the same height as the bench heights in the pit design and approximately the volume of a standard size blast.Image: Eltirus

Scheduling basics

When you create a scheduling model, you are working out what can be extracted and when. You need to know the geology, have a pit design, know how fast you can extract the deposit and what your targets are. From there, the scheduling model can help you understand if what you propose is possible or not. Common questions are, can we extract the material fast enough to meet client demand? How many fleets will we need for development? When is development required?  How much low-quality material do we need to dump (or sell off site etc.). All these questions (and more) can be answered through scheduling.

Geological information

First you need to know your geology. By this I mean that you need to know what type, quality, and quantity of material you have in your deposit. The best schedule in the world will be wrong to the degree that the geology is incorrect, so it is important that this step is not taken lightly. Depending on the work being done, the site may need to do additional drilling to prove up areas of the geology that are poorly understood. Also, take the time to check geological model with site personnel to see that it aligns with their on the ground experience.

Pit design

The next step is to have a well thought out, geotechnically sound pit design. This helps you to understand which parts of the deposit are going to be scheduled for extraction and how they will be accessed.

When we talk about a “sound pit design” we are talking about one that has practical haul roads and access and is designed in accordance with the site geotechnical model and any corporate guidelines. The design should be created in a three-dimensional, mining type design package, not two-dimensional CAD.

Creation of scheduling blocks

With a good understand of the geology and an accurate pit design, you can create a “solid” of the material between the topography and the pit design and cut it into blocks for scheduling.

Scheduling blocks are generally the same height as the bench heights in the pit design and approximately the volume of a standard size blast. The scheduling software then interrogate the geological information and writes to each block its location, volume, tonnes and material type.

When you create a scheduling model, you are working out what can be extracted and when. Image: Eltirus

Dependencies and constraints

It is fairly obvious in open-pit extraction that you generally start from the top down and from an open face. Likewise, you know that you can’t extract a block before taking the one above it. These common sense rules are important in scheduling – we call them dependencies and they help the software understand what is possible to extract at any given time.

Dependencies can be used to help set the sequence of extraction on a bench, enforce the completion of one cut-back before starting another, ensure that the walls are brought down in a geotechnically safe manner and many, many other things. As you can imagine, this can get very complicated with dependency rules, building on rules to give literally thousands of different permutations, all to ensure that access is retained, quarry safety rules are met, and extraction meets targets.

Resource rates

The next step in scheduling is to understand what resources (equipment) you have to extract the deposit and what their extraction rates are.

This step requires you to have a clear idea of how much material a load and haul fleet can move in a given operating scenario. For example, a fleet working in a wide-open area with good haul roads will move more material than the same fleet working on a tight, narrow bench where the trucks have to back long distances and the wheel loader loading them is struggling to maneuverer.

Taking the time to really nail down actual production rates is critical to an effective schedule. I well remember an instance many years ago where I conducted a production study to determine fleet productivity and was appalled by what I saw in the field. Imagine my surprise later when further investigation found that the performance, I saw on the day was actually thirty per cent better than their long term average performance.

Time spent working out real production rates is vital – anything else can result in wishful thinking.

Calendar and time model

The last step is to make sure that you have a good understanding of the site calendar. By this we mean, days worked, actual digging hours in a shift (or a day), downtime for maintenance, allowances for inclement weather etc. This is also the time to really understand any seasonal issues – e.g. can you actually conduct a stripping campaign in the middle of winter or not and if so at what impact on productivity?

Bringing it all together

As you can see, there are a range of factors to consider in building a schedule. You can likely also see that at any one of these steps, a lack of good data will lead to a poor outcome, so time taken to build an effective, robust model is time well spent later on.

Once the model is built, you can then run scenarios to test different ideas about how the deposit could be developed and whether your production targets can be met. This is where the real value of the work comes into play, but it should be noted that it can be highly iterative work (and requires specialist mining engineering skills that are not commonplace).

For example, a schedule might result in an excellent match to sales targets for say three out of five years, but result in poor results in the other two. Likewise, you might have a situation where waste movement is too front loaded (reducing deposit NPV accordingly). An effective scheduler is often able to work through these different types of factors to “even out” a schedule and come up with the best possible result for the site.

Communication with site

Once an acceptable, robust schedule is built, it is vital that it is communicated with the site in such a way that they can understand and put in play. Likewise, you also have to ensure that there is a feedback loop between site and the scheduling team to communicate changes in extraction rate, sales targets, geology and geotechnical conditions. Without this communication, the model can go out of date, lose its relevance or worse, site personnel can lose confidence in it and the value of the work.

If you go to the effort of putting a schedule in place – use it and if you find it wrong or inaccurate, don’t blame the scheduler in the first instance, take a look into the assumptions and then whether it was correctly applied in the field (and adjust accordingly).

You should also take the time to ensure that your sites have the survey equipment (e.g. GPS rover) to mark out the sequence of extraction correctly in the field and an effective, accurate site-wide drone program to be able to keep track of what has been extracted for use in updating the schedule.

A good schedule can really help you understand what your stripping requirements are, how much equipment you need and when and if you can deliver on that big project you have your eye on. •

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