GEOTECHNICAL DATA REQUIREMENTS AND CONFIDENCE FOR MINING STUDIES

Geotechnical considerations in JORC and NI 43-101

JORC (2012) defines an ore reserve as “the economically mineable part of a Measured and/or Indicated Mineral Resource. It includes diluting materials and allowances for losses, which may occur when the material is mined or extracted and is defined by studies at Pre-Feasibility or Feasibility level as appropriate that include application of Modifying Factors. Such studies demonstrate that, at the time of reporting, extraction could reasonably be justified”. In Section 29 it further elaborates that The studies (PFS or FS) will have determined a mine plan and production schedule that is technically achievable and economically viable and from which the Ore Reserves can be derived.

In terms of geotechnical considerations JORC 2012 hints that geotechnical stability must be considered without being specified. The word geotechnical is mentioned three times in JORC Table 1. The first two times are with respect to what geotechnical data has been collected and used. The third is within Section 4 and it states that you must provide “The assumptions made regarding geotechnical parameters (e.g. pit slopes, stope sizes, etc)…”.

NI 43-101 (2011) has one mention of geotechnical requirements. Item 16 specifies that geotechnical parameters need to be considered and included where relevant in the mineral reserves technical report.

These vague mentions and one-liners do not adequately prepare companies for the requirements of a geotechnical study.

In this post I provide some guidance to the type of data you require and to what level of confidence the data must be to fulfill the obligations of a typical study. Please not that these are very general and generic. They are not intended for the experienced Geotechnical Engineer. These comments are provided to assist Mining and Exploration houses that may be considering economic studies now or into the future.

The aim of data collection

The aim of geotechnical data collection is to provide a generalised characterisation of the rockmass that identifies the likely failure mode that will be encountered. This will then enable risk mitigation strategies to be put in place to ensure the safety of the personnel and the economic viability of the proposed mine.

Whilst there are quite a few geotechnical failure modes published I like to group them into 4 categories:

• Intact rock failure – this is generally weak and weathered rock and will be determined by UCS and other laboratory testing.
• Rockmass failure – this occurs in heavily fractured rock masses and is determined using rock mass classification systems.
• Structural failure – this is sliding discontinuities including joints, faults, shears, or other such structure.
• Stress driven failure – This can be caused by high stresses or strains with respect to the capacity of the rockmass. This includes dynamic failure and yielding type failure. This type of failure is difficult to anticipate without numerical modelling.

Each of these failure mechanisms have required data and specific method of analysis.

Data confidence

Usually data confidence would follow the data requirements but I have decided to include it first as data collection and the number of drill holes logged feed into the data types.

As a guide for the level of confidence required for these studies I use Table 8.1 from Read and Stacey (Guidelines for Open Pit Slope Design, CSIRO 2009) reproduced below. I am using the 2009 version because this is the version I have and because they are a guideline. It is by no means a hard and fast regulation. These levels of confidence were developed for open Pit but they are applicable for underground in lieu of anything else (if there are other guidelines out there I would be keen to get the reference).

Without a sufficient number of drill holes at varying angles around the deposit, you cannot collect the data to the level of confidence required. Geotechnical characterisation should consider the hanging wall and footwall of the deposit, the location of the infrastructure and any different geological environments. For example, if you have 3 dominant lithologies, then you must have sufficient information in these lithologies to inform characterisation. Minor lithologies should be considered if they are likely to impact on the stability of the mine (for example data regarding a thin weak intrusive may be necessary).

One conversation I had with a colleague was regarding the use of 5 drill holes to characterise a km long deposit deep Greenfields deposit. This would not meet the above requirements if it was the only information available. However, if you have a nearby operation or are expanding an existing operation with exposed excavations, 5 drill holes may be sufficient. The purpose of the few drill holes is to confirm that the deposit is similar to the existing operational area. There must be sufficient existing data and the geological environment (lithology) must be sufficiently represented in the data and be proven to be similar. If there is any variation from the known environment, additional data may be required.

Specific data requirements

The question remains - what type of geotechnical data should be collected. The table from Read and Stacey above provides an indication to the type of models that are required. Over the years I have determined my own more specific list to assist with direct requests to site.

Geological information

• Basic geological description of the deposit
• Geological database (see below)
• Surface topography models
• Stratigraphy (lithology) models
• Weathering profiles

Geological database

The following data tables from the geological database

• Collar
• Survey
• Lithology
• Geotechnical (RQD, joint orientation measurements, Q etc)

Note: generally assay data is not required unless other parameters are included in the assay table of the database

Mine scale major structures

• Models of mine scale major structures

Joint Set data

Measured discontinuity orientation data from oriented drill core. This should included as a minimum:

• Characteristics of the measured discontinuity including:
• Type (fault, joint, bedding)
• Infill
• Lithological setting
• Confidence in orientation line

Mapping data where available

Rock mass measurements

Q and / or RMR parameters including:

• RQD
• Joints per metre
• Joint spacing
• Number of sets
• Joint roughness
• Joint infill

Intact rock strength data

• Unconfined compressive strength? (UCS) testing including elastic parameters (Poissons ratio and Youngs modulus)
• Triaxial tests
• Joint shear strength tests
• Uniaxial tensile strength testing

Stress measurements

Where available

Hydrogeological data

If applicable (see below)

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Prior to undertaking any analysis of geotechnical data, the geological setting of the whole mining area needs to be known. This includes and understanding of the various lithologies and weathering profiles that require mining infrastructure. This will dictate what type of data and how many samples will need to be collected. The geological environment will also impact the type of mining method that us viable. For example, an open stope inside the weathered zone is unlikely to be viable and may ultimately result in surface subsidence.

Specific geotechnical data generally includes all the parameters relating to the calculation of Q or RMR. In very general terms Q is generally used for underground whilst RMR is used in open pit design (purists will disagree here but this article is not for the purist). Whilst being empirical, evaluations of the individual components allows rock mass characterisation, whilst calculation of the final number provides a recognised classification system. Please ensure that every parameter is collected separately. A Q or RMR value alone does not allow us to understand the rockmass and the critical factors that will cause potential failure.

Joint orientations are required to determine how the blocks will form within an excavation (development drive or slope face). Kinematic failures are failures along joints These include wedge failures, sliding failures and toppling failures. These can create significant stability issues and when sufficiently large enough can be difficult to control. It is also critical to understand the characteristics of the joints as this influences the shear strength of the joint.

Geotechnical sampling is critical to determining the type of failure that may or may not occur. Having sufficient samples including UCS (including elasticity parameters), UTS, Triaxial and shear box tests for each lithology is critical to ensuring statistical viability of a study. This type of testing is expensive and the correct sample collection is critical to ensure that the test results are viable and can be used in a study. Foliated or bedded deposits are particularly challenging as the aim is to test the rock not the discontinuity interface.

Stress measurements are critical in deep deposits and in some locations where high stresses have been encountered at very shallow depths (such as the in some areas around Kalgoorlie, Western Australia). I am a proponent of Acoustic emission testing simply because you can gain information regarding the likely failure modes prior to having develop into the area. Once development is in place, it can extremely difficult to ensure sufficient pillars are left or to completely rearrange the mining sequence to effectively manage stress. On shallower sites where stresses are know to not really cause issues, stress measurements are less critical at the early stages of evaluation.

Arguably and very generally, hydrogeology is less important in underground than in open pits. I have been known to quip “you just need bigger pumps”. This comes with a huge caveat. It assumes that the mine is effectively dry. A little running water here or there has very little influence on the geotechnical stability of a mine although corrosion of ground support becomes the concern. However, there have been many cases where a single water bearing structure has been intersected and caused significant delays and redesigns, not to mention stability issues. Wet mines can be particularly challenging as product installation issues can cause significant issues and ground support integrity can be compromised very quickly if the water is acidic in any way.

Data collection

For comparative geotechnical studies diamond drill core is required to be of specific dimensions and must be oriented. Thicker core diameters break differently from thinner core diameters and care must be taken that the results are consistent. The minimum diameter for geotechnical analysis is 50mm (NQ2) and even this is debatable. Anything less than this tends to break more easily and is not considered representative). RQD was originally developed with the assumption of a core diameter of 57.4mm (Wikipedia). Larger core diameters may be required where the rock is very broken or very weak. In oxides, it is typical to use triple tube PQ to ensure samples are obtained.

In the past geotechnical data collection has been undertaken by specialist trained people (often geotechnical engineers) within the core yard. Trained core loggers are rare and generally booked a long time in advance. The Western Australian ground control group recently held a workshop to train geotechnical engineers in core logging as these skills are limited within operations geotechnical engineers.

It is often problematic that data is not collected during the earliest phases of drilling. Typically by feasibility study level, the majority of geological drilling has been completed and the drill core cut and sampled. This means that geotechnical logging cannot take place. If you spend the money drilling it should be imperative that as much information is ascertained from the core as possible.

Technology is slowly catching up with several companies already recording RQD for core photos or scanning. However, gaining joint orientations and joint characteristics is going to be more problematic. One company presentation I have seen suggests they can estimate the waviness of the joint from the core profile. They can also determine joint orientations providing the orientation line is visible. The potential for error margins in this data is high, however, there is not currently enough geotechnical engineers to fulfil the operations market. Tying up critical resources in data collection is difficult to justify. It is likely that the geotechnical community will have to modify its accepted practices to account for modern data collection methods.

Conclusion

The information above comes from my experience in conducting these studies and reviewing a number of studies throughout my career. It hopefully has provided the reader with a guide to data requirements for geotechnical studies. The earlier the data collection starts, the more likely it will be that sufficient data will exist when the reporting stages come around.

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