3. Geotechnical

1.03 Geotechnical.jpg

Standard Practices Advanced Properties
3.1 Rock Properties

Rock Properties-01.png

(University of Saskatchewan)

  • UCS and elastic properties
  • Majority of testing in feasibility
  • Rock mass classification
  • Triaxial testing and intact strength parameters
  • Extensive database to estimate parameter reliability
  • 3.2 Stress Properties

    Stress Properties.png

  • Overcoring and/or core testing during feasibility and mine construction
  • Limited test samples
  • Stress conditions monitored through irregular inspection
  • Extensive test database, including during production
  • Real-time stress monitoring of key areas
  • Borehole camera or acoustic televiewer to interpret borehole breakouts
  • Regular field observations
  • 3.3 Deformation/Damage Monitoring

    Deformation Damage Monitoring.png

    (Kaiser 1996)

  • Regular inspections but only sparsely documented
  • Individual damage incidents investigated and reported
  • Borehole extensometers and crack monitors
  • Deformation and damage database
  • Large array of instruments
  • Laser scanning
  • Automatic reading of instruments
  • 3.4 Ground Support Capacity

    Ground Support Capacity.jpg

    (Cai and Kaiser 2018)

  • Records kept for planned ground support
  • Support condition visually checked over time
  • Support element capacity from supplier data/lab tests
  • Planned and installed support recorded
  • Support condition tracked over time, residual support capacity estimated
  • Support units tested in site conditions
  • Every operation collects a wide range of different types of geotechnical data. Sites with better practices are continuously reviewing the data quality to ensure the analysis and design assumptions are valid. Better geotechnical data systems are also very well organised and can be easily analysed in a variety of software packages. Four geotechnical data categories are considered; rock properties, stress properties, deformation/damage monitoring and ground support capacity.

    3.1 Rock properties

    3.1.1 Standard

    Standard geomechanical properties of intact rock (Uniaxial Compressive Strength (UCS), density (ρ), Young’s modulus (E), Poisson’s ratio (ν)) are generally defined for each geotechnical rock type. Field measurements such as point load tests are often used to supplement laboratory testing. Rock testing is often done during feasibility studies, but additional testing is not conducted regularly during production.

    Core logging is normally done using various rock mass classification systems. Borehole mapping using televiewer and underground mapping is often used to supplement core logging data, as shown in the below figure. Rock mass classification systems are generally used to quantify the reduction in strength between the intact rock and the rock mass. The rock mass properties, as an equivalent continuum, are generally used in mine scale numerical modelling assessments. Rock mass quality block models are normally done with the geological modelling software used on site. This sometimes means the format is difficult to export into seismic analysis software which inhibits the assessment of the variation in seismic hazard as a function of rock mass quality.

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    Figure: Geotechnical logging of drill core (Institution of Civil Engineers)

    3.1.2 Advanced

    Operations with advanced rock property databases continue to use field and lab testing throughout production to confirm the assumptions in the feasibility study, and to characterise the rock properties in proximity of the current mining areas. There are enough test results to get a good distribution of parameters to understand the variation between samples. It is critical that the orientation of testing is recorded; either the absolute orientation of the sample, or the direction of testing relative to the bedding or foliation direction.

    Indirect tensile tests, joint shear tests, toughness and seismic velocity tests are conducted along with the UCS test. Triaxial testing is done to define the stress dependent strength component.

    Advanced rock mass classification models have a dense array of data points. They might be produced in alternate software, but are available in seismic analysis software to assess the seismic hazard with respect to rock mass quality.

    3.2 Stress properties

    3.2.1 Standard

    Pre-mining stresses are assessed during the feasibility or early operating stages of most projects. In mining, the most common stress measurement techniques include overcoring using the Hollow Inclusion (HI) cell, or by core testing under uniaxial load in the laboratory using acoustic emission (AE) activity and deformation rate analysis (DRA). These approaches can be used to estimate the full in situ stress field.

    Feasibility estimates for the relation between principal stresses and depth are often dependent on a very limited number of samples and additional testing is not done regularly during the operation, even when the mining has progressed well outside the initial testing area. In particular, with regard to the orientation of principal stresses, minor errors can have a major influence on the expected stresses around excavations especially as stress fields can change over short distances by faults and contrasting geotechnical properties of the rock.

    The stress conditions during mining are often only inferred from damage monitored by visual inspection and field notes.

    3.2.2 Advanced

    Operations with advanced practices in collecting stress data have a much larger database of test results to better understand the spatial variability and uncertainty of the pre-mining stress field. Stress measurements are taken regularly throughout production, ensuring different depths and areas are assessed.

    Some operations also employ real-time stress monitoring in selected areas such as regional pillars, critical infrastructure and cave footprints. Monitoring boreholes are also used to assess stress conditions over time. Borehole cameras are used to monitor the signs of high stress conditions along the borehole as mining progresses, as shown in the figure below. Large rock mass deformation, plate bending and rock mass deterioration are all signs of high stress conditions in a drive. Regular field observations are recorded to infer the stress level at the surface of new developments. Acoustic televiewers are used to interpret the local stress from breakouts.

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    Figure: Borehole breakout can be monitored with borehole camera and acoustic televiewer logging (Goodfellow et al. 2017)

    3.3 Deformation/damage monitoring

    3.3.1 Standard

    Rock mass deformation is normally monitored during regular inspections, but often the observations are only sparsely documented and not collated into a searchable format. Incidents of damage are generally reported individually and there is no easily accessible database to facilitate long term analysis of damage trends across many incidents.

    Instrumentation is often used to supplement observational data. Borehole extensometers are used to monitor differential displacement beyond the excavation surface. They are often installed in targeted areas with excessively large spans and key infrastructure. Crack monitors are another common instrument to monitor the ongoing movement across observed cracks.

    3.3.2 Advanced

    Operations that have better deformation and damage data keep detailed records of the fracturing and deformation at the surface of all excavations. These records are stored in a single formatted database including the Date-Time and 3D-coordinates. Mines with fibre-reinforced shotcrete (FRS) have a similar system, except they use an FRS damage scale to quantify the deformation.

    Detailed damage observations are made regularly, sometimes using electronic tablets, in addition to significant incidents. The formatted database of quantitative damage ratings can be searched and assessed for trends and correlations with geotechnical conditions. The software CIGT developed by El Teniente is an example of such a database, as shown in the figure below.

    Figure 11.png

    Figure: Example of CIGT rock mass and support damage software

    Deformation readings from instrumentation are well tracked over time to monitor the response to mining. Readings are stored and organised to facilitate analysis of the link between seismicity and deformation. Laser scanning (LiDAR) has been introduced at a number of operations to quantify surface deformation over time.

    Laser scanning has much better spatial coverage than single borehole instruments. Baseline laser scans are kept of all active development areas and scans are redone regularly, or after large seismic events. An example of laser scanning results is shown in the figure below. Displacement is measured relative to a previous baseline scan.

    Figure 12.png

    Figure: Laser scan cloud comparison, floor heave at intersection (Hancock 2018)

    3.4 Ground support capacity

    3.4.1 Standard

    Planned ground support is generally documented for all development areas although the actual installed support is not confirmed, or the original planned support standard may not be readily available for all areas.

    The changes to support conditions over time are generally monitored by inspection. Observations may be sparsely documented and are usually qualitative in nature and not organised and formatted in a regular manner. Estimating the residual support capacity over time is difficult given frequent mine equipment-related damage, corrosion and other signs of loading.

    The capacity of individual ground support elements is often estimated using generic parameters from supplier testing and lab tests and not confirmed with site installation and ground conditions.

    3.4.2 Advanced

    Planned and installed support is documented in a database at operations with advanced ground support data practices. This includes the location and date of installation, as well as all the support details. Secondary support installation and rehabilitation is also tracked. The capacity of support elements is tested onsite under local installation for QA/QC purpose, and ground conditions to understand in situ behaviour. This may include static pull testing and in situ drop testing (as shown in figure below table) to determine the dynamic capacity in specific ground conditions.

    Support damage classification schemes in the table below are useful and provide a more systematic and comparable scale of support damage, which can contribute in the assessment of the residual capacity and the need for rehabilitation.

    Damage Level General Description Support Damage Shotcrete Damage
      Conditions unchanged
      No new damage or loading
      No new damage or loading
      Support undamaged but first signs of distress detectable
      No damage to any support component
      Shotcrete shows new cracks, very fine or widely distributed
      Slight damage to support
      Loading clearly evident but full functionality maintained
      Plates and wooden washers on some rockbolts are deformed, showing loading
      Individual strands in mesh broken
      Mesh bagged but retains material well
      Shotcrete cracked, minor flakes dislodged
      Shotcrete is clearly taking load from broken rock mass (mostly drummy)
      Moderate damage to support
      Support shows significant loading and local loss of functionality; retaining function primarily lost (except in laced or shotcreted areas)
      Plates, wooden washers, and wood blocking on rockbolts are heavily deformed, showing significant loading; bolt heads may be ‘sucked’ into rock
      Mesh torn near bolt heads with some strands broken and mesh torn or opened at overlapping edges
      Moderate bagging of mesh and isolated failures of rockbolts
      Cable lacing performs well
      Shotcrete fractured, often debonded from rock and/or reinforcement
      Major flakes possibly dislodged
      Holding elements mostly intact
      Substantial damage to support
      More extensive loss of retaining and holding functions (except for lacing systems)
      Mesh is often torn and pulled over rockbolt plates; if it did not fail, it is substantially bagged (at capacity)
      Many rockbolts failed
      Rock ejected between support components
      Cable lacing is heavily loaded with bagged mesh
      Shotcrete heavily fractured and broken, often separated from the rock mass with pieces lying on the ground or hanging from reinforcement
      Connections to holding elements often failed or holding elements failed locally
      Severe damage to support
      Support retaining, holding, and reinforcing functions failed
      Most ground support components broken or damaged
      Most rockbolts fail and rock peels off cable bolts
      Shotcrete non-functional
      Mesh without cable lacing heavily torn and damaged
      Cable lacing systems heavily stressed and often failed
      For damage level S5, shotcrete fails to be functional and the left-hand column applies

    Table: Support damage classification scale (Heal 2010)

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    Figure: In situ testing of rock reinforcementy a) quasi static pull test (photo courtesy of Atlas Copco); b) drop testing rig to assess dynamic ground support capacity in specific ground conditions (Carlton et al. 2013)