Mining Contract: Building Capacity and Enhancing Long-Term Coal mine Weak Roof Stability through Characterization and Modeling of Time-Dependent and Moisture-Sensitive Failure in Shale

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Contract # 200-2016-90385
Start Date 9/1/2016
End Date 8/31/2020
Research Concept

Research in ground control for underground coal mines spans significantly more than 50 years. Although the problems are multi-faceted, one enduring and persistent issue remains – that of roof failures associated with weak shales due to time-dependent and moisture-sensitive deterioration. Variations in humidity and the duration of roof exposure are known to significantly influence the condition of shale roof strata [1–4]. A study conducted by NIOSH found that roof falls were 15% more frequent in humid months than the annual average for the same time period [5]. This deterioration in strength and stability is time-dependent [6] – allowing failure to occur without obvious precursory signals and therefore amplifying the hazard. Although moisture-induced degradation of shale is a well-known phenomenon, the underlying mechanism of strength reduction is not well understood and a modeling capability for time-dependent shale failure is lacking. These knowledge gaps hinder the selection of engineering solutions for optimized roof support design to prevent failure of weak roofs. In this study we will extend our recent findings on shale roof rock strength deterioration due to air-water-shale interactions to: (i) develop predictive models to define shale failure behaviors over timescale ranging from hours to years under real mine stress and moisture conditions, (ii) apply this improved understanding of shale failure mechanisms and the developed models to mitigate weak roof failures to improve safety conditions in underground coal mines, and complete this by (iii) entraining and educating a cadre of superior graduate students in mining engineering with a specialization in ground control with a goal of capacity building in industry, academia and government.

Contract Status & Impact

This contract is complete. To receive a copy of the final report, send a request to mining@cdc.gov.

The overarching goal of this capacity-building contract is to establish a systematic pathway to improve the long-term stability of coal mine roofs. This will be achieved through characterization and modeling of time dependent shale rock failures. This overarching objective will be achieved through a series of laboratory, modeling and field investigations.

Shale samples from the Energy, Anna, Turner Mine, and Dykersburg shale seams will be collected. The mineral composition and pore structure of the samples will be characterized by XRD (x-ray diffraction) and ASAP 2020 (Accelerated Surface Area Porosimetry system). The failure behavior of shale will be explored through cyclic exposure to moisture and appropriate scaling in time and scale. Time-dependent deterioration of shale will be monitored through wetting/drying cycles using non-destructive techniques. Moisture absorption capacity and its impact on shale swelling will be measured using our sorption-induced -deformation apparatus. Stress free swelling deformations of the shale samples will be visually recorded as a function of controlled humidity. This information will be included in an existing sorption swelling model to quantify shale swelling behavior.

Mechanical properties of p-/s-wave speeds of shales under in situ stress will be measured and calibrated against strength. Cubic samples will be loaded, using a GCTS RTR rapid triaxial rock testing system, under true triaxial stress conditions to measure the failure mode. This information will be used to create a dynamic CMRR roof classification which will be established by linking the dynamic evolution of rock properties with moisture deterioration.

A numerical model (meter scale) will be developed, integrating failure behaviors from laboratory experiments, to define time-dependent skin failure and this model will be integrated to a beam-scale model (decimeter scale) to evaluate long-term roof stability at different time scales (hours to years). Additional key parameters will be identified to include as inputs to the numerical modeling system.

Based upon the previous information, recommendations will be made to control skin failure and its evolution to massive roof failure. Guidelines will be provided to prevent shear-driven cutter roof failure. Ultimately a new time dependent CMRR rock classification system will be developed. The laboratory experiment results that include moisture sensitivity will be used to expand the CMRR capabilities to coal mines with weak shale roof rock.

The knowledge gained through this project will be unique and transformative because it will provide a new and comprehensive understanding of the time-dependent deterioration and strength reduction in shales from first principles and with crucial application to mitigating the pervasive failure hazard in mines. In order to achieve this objective, the following specific aims are defined:

  1. Characterize the mineral compositions and static mechanical properties of Illinois shales;
  2. Quantify the dynamic evolution of rock properties under cyclic wetting and drying;
  3. Develop a time-dependent rock mass classification with dynamic CMRR;
  4. Upscale results through modeling of time-dependent skin failure and long-term roof stability;
  5. Link with field observations for validation and model adjustment.

Page last reviewed: 3/21/2022 Page last updated: 9/18/2020