Longwall Mining

Golder engineers have been working in longwall mines around the world for over 20 years, providing practical design advice, monitoring, instrumentation and day to day geotechnical support. We use a combination of practical experience, numerical modelling and a unique empirical database to help develop and operate safe and productive longwall mines.

Geotechnical Characterisation

Rock mass characteristics impact significantly on underground excavation behaviour and performance.  We focus on defining rock mass parameters and build geotechnical models of underground coal resources utilising geological and geophysical data sets, including targeted programs of geotechnical logging, field and laboratory testing, appropriate to the stage of the project or mine development. 

Our skilled engineers characterise the strength of near-seam, as well as higher overburden strata, on the basis of rock material and structural properties (e.g. bed thickness, jointing). Techniques include:
  • Application of classification systems (indices / ratings) for rock mass competency
  • Profiling
  • Assessment of spatial variation

We couple this to stress-field interpretation, initially utilising depth of cover, plus local and regional stress measurement and borehole breakout data, to establish the magnitude and orientation of the principal stresses and, in particular, that of the major horizontal stress. This is complemented by our pioneering, risk-based approach to geotechnical mapping of underground workings (for stress, structure and deformation).

Underground Stress Measurement and Monitoring 

Measurement exercises aimed at quantifying vertical stress changes in pillars during a variety of extraction operations are conducted to assist in the development of improved design techniques for longwall chain and barrier pillars, pre-driven recovery roads and partial extraction panels. 

This crucial field data may include:  
  • Measurement of abutment loads due to longwall mining on chain pillars to determine a site-specific abutment angle, as input to the rationalisation of chain pillar widths and gate road support design.
  • The measurement of stress changes in the fender and barrier pillars adjacent to longwall pre-driven recovery roads, necessary for the design and optimisation of the support requirements for such roadways.
  • The measurement of stress changes in partial pillar extraction layouts, to confirm long-term stability and assist in the design of such layouts.

Knowledge of these stresses is essential as excavation instability can have major consequences on the safe and efficient performance of mining operations.

Partial Extraction for Subsidence Protection

Mine subsidence can affect natural and man-made surface and sub-surface features. We design and monitor partial extraction systems for coal mining (i.e. miniwall; room and pillar with partial pillar extraction on the retreat) to restrict these effects and impacts in sensitive areas. Our design methodologies typically involve empirical-analytical techniques with respect to pillar and span design together with boundary element modelling (using LaModel) for validation and optimisation in order to: 
  • Design room and pillar workings for the protection of critical surface infrastructure.
  • Analyse the stability of old room and pillar workings for the purpose of surface development in urban areas.
  • Design protection pillars (e.g. for shafts) adjacent to total extraction (e.g. longwall) workings.

Shaft and Drift Design

Underground mining operations require access drifts and ventilation shafts as key parts of development. In many cases this involves excavation through a range of variable materials; from extremely strong basalts and coal measures strata through to sand zones and reactive clays. We investigate development locations using boreholes, supplemented by geophysical, hydrogeological and laboratory testing, to provide parameters to develop a geotechnical model of the sites. Analyses are then carried out using leading industry software and in-house empirical methodologies to prepare designs that take into account the ground conditions from portal to seam, constructability and the required design life. Design drawings and specifications are then prepared for contract tender purposes. We have considerable on site experience in the supervision, mapping and monitoring of construction activities for mine access works including the remediation of poor ground.

Roof Monitoring, Data Processing and Reporting 

Roof monitoring is a legislated requirement for Australian underground coal mines. Ensuring this is done in a timely manner at minimal cost is of benefit to all mines. In addition, the production of good quality data allows for greater security and accuracy in subsequent design work and maintenance of poor roof zones. Golder has a long history of collecting and maintaining such data. In recent years we have teamed with Keith O’Donnell in the development of the “ExtoChart” Access Database and Reporting System to provide a cost efficient method of processing and reporting on large quantities of roof monitoring data. The reporting of data can be modified to suit the requirements of a given site. By combining the database with a team of dedicated data processors, we are able to provide a high quality, fast turnaround service for data processing and interpretation at a low cost. 

Strata Management 

The application of strata management principles by implementing rational, optimised processes for defining and tailoring ground support requirements provides a foundation for hazard management, productivity improvement and regulatory compliance.  Our services include:
  • The estimation of likely geological conditions through the application of characterisation techniques.
  • Assessment of stability  and definition of ground support requirements via the application of composite empirical and analytical approaches based on internationally recognised research, in-house conceptual modelling and global databases of mining experience.
  • On site data collection and investigation, including devising and applying underground monitoring and mapping programs tailored to mine-specific needs.
  • Assistance with the development of detailed ground support plans and associated strata management documentation (Trigger Action Response Plans (TARPs), Authority to Mine (ATM) forms, Codes of Practice and Strata Failure Management Plans).
  • Training of personnel with regard to ground control principles and support processes.
  • Auditing and review of drivage and extraction operations.
  • Incident investigation and reporting.
  • Optimisation support.

Longwall Operations

Longwall geomechanics and ground control activities can assist with pre-feasibility and support design through to troubleshooting in difficult conditions. Our services include: 
  • Geotechnical risk management and optimisation of longwall recoveries (relocations), including ground characterisation and support design for pre-driven roadways and conventional recoveries in Australia and overseas, building on the outcomes of both our ACARP sponsored and in-house research.
  • Panel layout, support design and geotechnical risk management for longwall top coal caving (“LTCC”) operations at depths of 100m to 800m.
  • Panel design for subsidence control.
  • Multi-seam longwall layout design.
Understanding longwall characteristics can assist with increasing the efficiency and safety of operations.