World Stress Map Conference - International Lithosphere Program ...

Wednesday, October 15 th
Oral Session II: Stress modelling and interpretation at reservoir scale
Lithospheric stresses and their effects on mine stability in
the coalfields of the United States
Christopher Mark, Murali Gadde and Sandin Phillipson
National Institute for Occupational Safety and Health (NIOSH, USA)
cnm7@cdc.gov
During the past 25 years, mining engineers have realized that many of the severe stability
problems they observe in underground coal mines are related to lithospheric stresses. Signs of
horizontal stress in underground mines include: (1) Persistent roof “cutting” that follows a
directional pattern; (2) Roof falls that can be 300 m long, preferentially running perpendicular
to the orientation of the major lithospheric stress; (3) Severe stress concentrations that occur
around mined-out and caved areas. Although US coal mines tend to be relatively shallow (less
than 450m deep), analysis of a data base of stress measurements collected by the authors from
approximately 60 US coal mines shows that distinct regional trends correspond roughly to the
regional stress fields identified by the World Stress Map project. For example, in eastern
North America, the greatest horizontal stresses measured in coal mines are usually ENE, and
their magnitudes are usually 2-3 times as great as the vertical, gravity stress. The authors have
also conducted regression analysis of the stress measurement data base to develop equations
that predict the stress magnitude as a function of overburden depth and elastic modulus of the
rock in which the measurement was made. The paper compares these results from the US to
stress measurements compiled by others from the coalfields of Australia, western Europe, and
South Africa. Horizontal stresses can be measured directly with instrumentation, or indirectly
through the technique known as stress mapping. This paper reports the results obtained from
stress mapping studies conducted at 25 mines throughout the eastern US, and compares those
measurements with the trends observed in the direct measurements. The paper concludes with
a discussion of how mines can minimize the effect of lithospheric stresses through proper
mine planning, extraction sequence, and ground support.
32 3 rd World Stress Map Conference, 15.-17. October 2008 in Potsdam