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100+ Free Certificate in Strata Control — Coal Exam Practice Questions
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Sample Certificate in Strata Control — Coal Exam Practice Questions
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1In the Salamon-Munro coal pillar strength formula S = 7.18 * (w^0.46 / h^0.66), what units must be used for pillar width (w), pillar height (h), and the resulting strength (S)?
A.w and h in meters (m), S in Megapascals (MPa)
B.w and h in feet (ft), S in pounds per square inch (psi)
C.w and h in meters (m), S in Kilopascals (kPa)
D.w and h in millimeters (mm), S in Megapascals (MPa)
Explanation: The Salamon-Munro formula derived in 1967 for South African coal mines requires pillar width w and mining height h in meters, yielding pillar strength S in Megapascals (MPa). Using imperial units or incorrect metric prefixes invalidates the empirical constant 7.18.
2Calculate the Salamon-Munro pillar strength for a square coal pillar with width w = 16 m and mining height h = 4 m. (Use S = 7.18 * (w^0.46 / h^0.66)).
A.10.30 MPa
B.14.50 MPa
C.7.18 MPa
D.18.25 MPa
Explanation: w^0.46 = 16^0.46 = 3.5799 and h^0.66 = 4^0.66 = 2.4967. Pillar strength S = 7.18 * (3.5799 / 2.4967) = 7.18 * 1.4339 = 10.30 MPa.
3Using Tributary Area Theory with an overburden gradient of 0.025 MPa/m, calculate the average vertical stress (load) on a square coal pillar at depth H = 80 m, with pillar width w = 12 m and bord width B = 6 m.
A.4.50 MPa
B.2.00 MPa
C.6.75 MPa
D.3.20 MPa
Explanation: Virgin vertical stress sigma_v = 0.025 * 80 = 2.0 MPa. Center-to-center distance C = w + B = 18 m. Pillar load s = sigma_v * (C / w)^2 = 2.0 * (18 / 12)^2 = 2.0 * (1.5)^2 = 2.0 * 2.25 = 4.50 MPa.
4A coal seam mined at depth H = 100 m uses bord width B = 6 m, pillar width w = 14 m, and mining height h = 3.5 m. Calculate the Safety Factor (SF = S / s) of the pillars.
A.2.07
B.1.52
C.2.85
D.1.18
Explanation: S = 7.18 * (14^0.46 / 3.5^0.66) = 7.18 * (3.3562 / 2.2829) = 10.56 MPa. Load s = (0.025 * 100) * (20 / 14)^2 = 2.5 * 2.0408 = 5.10 MPa. Safety Factor SF = 10.56 / 5.10 = 2.07.
5What is the areal extraction ratio e for a square bord and pillar panel where pillar width w = 18 m and bord width B = 6 m?
A.43.75%
B.25.00%
C.56.25%
D.33.33%
Explanation: Extraction ratio e = 1 - (w^2 / (w + B)^2) = 1 - (18^2 / 24^2) = 1 - (324 / 576) = 1 - 0.5625 = 0.4375 or 43.75%.
6In South African coal strata control, what standard vertical stress gradient is used to estimate overburden pressure prior to mining?
A.0.025 MPa per meter depth
B.0.010 MPa per meter depth
C.0.050 MPa per meter depth
D.0.100 MPa per meter depth
Explanation: An overburden density of approximately 2500 kg/m^3 yields a vertical stress gradient sigma_v / H = rho * g = 2500 * 9.81 = 24.5 kN/m^3/m, standardized in SA practice as 0.025 MPa/m depth.
7According to the Salamon-Munro formula, if mining height h is increased while pillar width w remains constant, how does pillar strength S change?
A.Pillar strength decreases proportionally to h^(-0.66)
B.Pillar strength increases proportionally to h^(0.66)
C.Pillar strength remains constant because width is unchanged
D.Pillar strength increases linearly with height
Explanation: Because mining height h appears in the denominator with exponent 0.66 (h^0.66), increasing height increases slenderness, reducing triaxial confinement and lowering pillar strength.
8What width-to-height (w/h) ratio threshold defines the transition to a 'squat pillar' in South African coal mining practice?
A.w/h > 5.0
B.w/h > 2.0
C.w/h > 10.0
D.w/h > 3.5
Explanation: In South Africa, Madden (1991) and Salamon established that pillars with w/h > 5.0 exhibit rapid non-linear strength gain due to high confinement in the pillar core, defining the squat pillar regime.
9Why does the standard Salamon-Munro power formula under-predict pillar strength at width-to-height ratios w/h > 5.0?
A.It does not account for the high triaxial confinement developed in the central core of wide pillars
B.It assumes coal behaves as a perfectly fluid material at high pressure
C.It overestimates the tensile strength of ribside cleats
D.It ignores overburden load variations across the panel
Explanation: As w/h exceeds 5.0, friction at roof and floor contacts constrains lateral expansion, creating high triaxial compressive confinement in the core. The power formula derived from narrower pillars underestimates this exponential strength increase.
10A coal pillar has width w = 20 m and mining height h = 2.5 m. What is its aspect ratio (w/h) and which strength formula approach should be evaluated?
A.w/h = 8.0; Madden squat pillar formula extension should be evaluated
B.w/h = 4.0; standard Salamon-Munro power law applies
C.w/h = 12.5; soft-floor punching formula applies
D.w/h = 6.0; linear rock mass formula applies
Explanation: Aspect ratio w/h = 20 / 2.5 = 8.0. Since 8.0 > 5.0, the pillar falls into the squat pillar range, requiring Madden's squat pillar formula extension to avoid underestimating load capacity.
About the Certificate in Strata Control — Coal Exam Practice Questions
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