The Importance of Geology

Mine site geotechnical engineering is fraught with uncertainties, with risk management key to driving a balance between safety and production. One of the largest risks to personnel onsite is fall of ground. Whether this includes rock falls, subsidence, minor to major structural failures, all of these falls are rooted in geology.  

Most geotechnical engineers will be familiar with the notion that the geotechnical model is split into the following constituents: 

• Geology Model 

• Structural Model 

• Rock Mass Model 

• Hydrogeological Model 

The first component is straightforward in its relevance. The structural model is rooted in geological concepts. Rock mass is primarily an engineering section. And hydrogeology deals with the nature and behaviour of the groundwater in the system. Three of these components are heavily embedded in geology. So why is there not a greater focus on geology? 

There is an extensive list of geotechnical hazards and problems that require geological input. Within the design phase, input from all four parts of the model is crucial, depending on the commodity and mining method. A pit shell is built using geological horizons, in a position and orientation that favours the intrinsic structural framework, using geometries that are within tolerances for the strength parameters of the rock units, and that also suits the hydrogeological aquifers and pressures of the system.  

The outcome of these assessments is of greater importance then they are often given. One unfavourable fault in an open cut coal deposit may signify the movement of a highwall design back a few hundred metres, resulting in an increase in stripping costs. The position in which this fault is modelled, relies on sufficient information and time to make the geological interpretation that determines in which direction it propagates and which geological horizons the fault is limited to.  

It must be fervently stressed that this is not always clear. With limited drillhole or seismic information, geologists must apply a +/- accuracy on their interpretations. The inaccuracy depends on two things; the expensive data that needs to be acquired, and the skill necessary to make an educated interpretation. In a world where the stripping ratio makes or breaks the development of a mine, it is an understatement to say that changes to these values will cause a few headaches. 

The learning curve to be an effective geotechnical practitioner is extremely high, given the sheer amount of content to learn between the engineering and geology doctrines. In fact, this is partially why there is the split between geotechnical engineering, and engineering geology. Just like the nomenclature of sandy clays and clayey sands in soil classification, a geotechnical engineer is more of an engineer, and an engineering geologist is more of a geologist. But this still doesn’t answer the question about why geology remains as an afterthought in the industry? 

Within the industry itself, a mine site will commonly employ geologists and geotechnical engineers as separate entities. The geologist is primarily responsible for maintaining a geological model, supervising the adherence of production mining to said model, mapping any structures in the pit, and in some cases also dealing with product blending and logistics (trains, shipping, etc). The site geotechnical engineers rely on the geologists to maintain a lot of the information. Geological data feeds into what can be multi-million-dollar design and hazard remediation plans created by the geotechnical engineers. 

In a production setting, production will always take precedent in a geologist’s time. From experience on both the geotechnical and geological sides of operations, the primary focus will be on mining recovery, plant throughput, and grade recovery. Quality Control is not without its own merit of importance, as the flow on effects of missing a train, for reasons like coal ash content being over specification, can amount to millions of dollars in itself. But there is only so much time in a day. When it comes to deciding between spending the time meticulously mapping out a fault system on an exposed highwall, or crunching blend numbers to optimise product, the latter will nearly always win. 

This leads onto the next problem. In times of hardship in the economy, there is a consistent pattern of belt-tightening that goes on throughout the technical services department. Among the first to be downsized are usually the environmental team, project teams, and of course, geology. An unappreciated portion of the geology services comes from the exploration budget – funds spent on developing the resource for improving tonnages and granularity in the ore zones. When this is cut, the mine geologists, and by extension the geotechnical engineering team, suffer greatly.  

Any case study involving unknown structures and geological domaining can be used as an important learning tool here. Take, for example, an operating open-cut coal mine progressing from one strip to the next. The planning phase for this strip turnover highlights a potential for a pit-ward dipping fault, unfavourably oblique to the wall. The lithology is comprised of predominantly weaker siltstone, with intermittent beds of hard sandstone with variable thickness. The overburden lithology is not mapped, and feature mapping of joints along the face is minimal. 

The geotechnical engineer needs to use all the available information to make an informed estimate on the risk of that next strip.  If the geologist cannot be counted on to provide additional data due to their own workload, then it must be up to the geotechnical engineer to do what they can with what they have. Here is where the geological knowledge base is so important. 

Can the geotechnical engineer make inferences around the lithology ahead from drillholes in the area – and build stability analysis models from this? Can they build a correctly filtered kinematic model from the available joint sets mapped from the face, and have the knowledge to discount low quality data? Can they ascertain strength properties from the fault and the surrounding rockmass from any downhole acoustic logs, and quantify the confidence of that structure in strips ahead? These skills are a selection of many that do not often fit into role descriptions or training programs for geotechnical engineering. It is a lot to ask from the geotechnical engineer to have the tools to do all these things. But the value is clear.  

Piecing this all together, we have a highly intricate geotechnical design process. So, what can we change to make improvements in the industry? 

• The first, but not always easiest, step is to ensure there is sufficient funding and resources given to the geology team; 

• Second, to provide geotechnical awareness training to the site geologists to provide context on the value and importance of the information they are gathering for the geotechnical engineers;  

• Additionally, provide geological training to the site geotechnical engineers to bridge the gap between the two departments of information; and, 

• A cultural shift in the management team, to recognise that uncertainty of geology plays a major factor in mining costs, both proactively and reactively 

There will be some that read these options and immediately see them as a cost to the business. Resources, whether it be additional personnel, training costs, and tech tools for the job, cost money. The challenge for the geoscience teams is to show the value of this. In an effective team, with the right resources, can the cost of the team offset the cost of geotechnical failures or last-minute design changes? 

The fact is, that no one has x-ray eyes. Our best hope in understanding the risks ahead in the mining process is to trust the geology. The geology community will never have the exact answer, but we will give the best answer with the time and the data that is provided to us.