Semester

Summer

Date of Graduation

2022

Document Type

Dissertation

Degree Type

PhD

College

Statler College of Engineering and Mineral Resources

Department

Mining Engineering

Committee Chair

Brijes Mishra

Committee Co-Chair

Ihsan B. Tulu

Committee Member

Ihsan B. Tulu

Committee Member

Hassan Amini

Committee Member

Bruce S. Kang

Committee Member

G.S. Esterhuizen

Abstract

Roof failure in the Appalachian underground coal fields occurs often in laminated shale. Laminated shale roof in coal mines fails in unique ways, such as cutter failure or delamination failure. Extensive studies have investigated the influential factors that cause laminated roof failure, which include in-situ stress, entry layout, roof span, and roof support. However, cutter failure continues to occur frequently and erratically. This is due to the lack of in-depth understanding of the inherent properties of the laminations, such as bedding plane strength, matrix strength, and bedding plane spacing, which in turn influence the geomechanical behavior of the laminated rock. These inherent properties vary and are therefore the significant factors influencing entry and support design. The objective of this dissertation is to discover the effect of lamination properties on the geomechanical behavior of laminated rocks through experimental and numerical analysis. The experimental approach included the development of synthetic laminated rock (SLR). The SLR included three different cohesive strengths (����). This research conducted biaxial tests and triaxial tests on the cubic laminated rock with a special platen. We analyzed the strength, failure mode, and deformation of laminated specimens with various ���� under varied stress conditions. The experimental results showed that ���� significantly influenced the SLR strength, modulus, and failure modes in biaxial stress conditions. Application of confining stress reduced the damage of SLR specimens and constrained the effect of ���� on SLR behavior. The results from the tests on SLR supported the development of a series of numerical models of underground coal mines with laminated roof. To simulate the laminated roof at different scales, this research used FLAC3D based on the finite difference method (FDM) and PFC3D based on the discrete element method (DEM).

Next, this research developed the coal mine entry model with the laminated roof in the PFC program using laboratory data and investigated the effect of bedding plane spacing, bedding plane strength, and support pressure on roof stability. The results from the numerical analysis showed that the roof stability and stress magnitude inside the roof increased with both bedding plane

spacing and bedding plane strength, and this effect was sensitive to these two properties. PFC then modeled the delamination process of laminated rock under various stress conditions. The results demonstrated that the delamination of an unconfined laminated rock initiates in the inner section of the bedding. Cutter failure initiated with damages that distributed extensively in the roof. We then developed a panel scale longwall model in PFC3D which was then coupled with FLAC3D for analyzing crack propagation in roof as well as understanding large scale failure behavior. Numerical results from the FLAC3D-PFC3D coupled model showed that the bedding plane strength significantly influenced the roof deformation and also modified the fracturing mechanism of the laminated roof. These effects are sensitive to the extraction activity of the entries and panels. These findings will advance knowledge on laminated roof failure and improve entry and support design.

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