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Distinct element modeling and analysis of mining-induced subsidence

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Summary

The influence of rock discontinuities on mining-induced subsidence is addressed in this paper. A two-dimensional rigid block computer model was used to simulate discontinuities within strata overlying a longwall coal mine. Input for the model was available from a previous field study and numerical experiments were performed by varying the simulated joint stiffness, joint roughness, and vertical joint density. A comparison of simulated and measured displacements both within the overburden and on the surface provides insight into the influence of rock discontinuities. For the case in which all contacts had a relatively low stiffness, the maximum simulated subsidence was 293 mm whereas the case involving variable, but higher contact stiffness produced a maximum subsidence of only 73 mm reflecting the influence of increased overall stiffness. By comparison, the maximum measured subsidence was 580 mm. Consequently, the model behaved more stiffly than the actual rock mass but still provided a reliable simulation of block caving and strata separation. A comparison of simulated and observed displacements within the overburden suggests that horizontal discontinuities not included in the rigid block mesh above the zone of caving controlled rock mass compliance.

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Abbreviations

c :

joint stiffness ratio [dimensionless]

d :

strata thickness ratio [dimensionless]

e :

strata modulus ratio [dimensionless]

E a :

modulus of stratum “a” [MPa]

E b :

modulus of stratum “b” [MPa]

E equiv :

equivalent rock mass modulus [MPa]

E u :

unconfined compression modulus of intact rock [MPa]

F n :

contact normal force [N]

h :

overburden thickness [m]

k a :

normal spring stiffness of stratum “a” [N/m]

k b :

normal spring stiffness of stratum “b” [N/m]

k equiv :

equivalent rock mass spring stiffness [N/m]

K n :

normal material stiffness of joint [MPa/m]

K s :

shear material stiffness of joint [MPa/m]

m :

mined thickness of coal seam [m]

q u :

unconfined compressive strength of intact rock [MPa]

Sh:

shale

Ss:

sandstone

Sh/Ss:

shale/sandstone interbeds

S max :

maximum subsidence [m]

SLEX:

Slope Indicator inclinometer/Sondex extensometer

T a :

thickness of stratum “a” [m]

T b :

thickness of stratum “b” [m]

T j :

joint thickness [m]

u a :

compression of stratum “a” [m]

u b :

compression of stratum “b” [m]

u j :

compression of joint [m]

u total :

total compression of strata and included joint [m]

w :

width of longwall panel [m]

σ n :

normal stress [MPa]

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Author notes

  1. K. M. O'Connor (Civil Engineer)

    Present address: Bureau of Mines, Twin Cities Research Center, 55417, Minneapolis, MN, USA

  2. Ch. H. Dowding

    Present address: Department of Civil Engineering, Northwestern University, 60208, Evanston, IL, USA

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    1. K. M. O'Connor
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    2. Ch. H. Dowding
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    O'Connor, K.M., Dowding, C.H. Distinct element modeling and analysis of mining-induced subsidence. Rock Mech Rock Engng 25, 1–24 (1992). https://doi.org/10.1007/BF01041873

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