Roof Support 1. Stand-Up Time (RMR):

Transcription

1 Roof Suppot 1 Enty Design is a complex polem. 1. One can use a Roof Classification System. o one can use Beam Fomulas Stand-Up Time (RMR): Maximum Unsuppoted Span (Q): Accoding to the Q system, the maximum unsuppoted span (in m) can e otained fom the following equation: Maximum span 0.4 ESRQ Beam Theoy: 1. It is ecommended to use the Simple Beam equation fo Shallow oveuden (< 300 ft). And the Clamped Beam equation fo Deep cove (> 300 ft). 3. In oth cases, the Shea equation should also e evaluated to see if it poduces a smalle width, which would then e used.

2 Simple Beam - Fo a simply suppoted oof eam of unit width, the Allowale Span, ased on tensile failue, can e calculated as follows: L 4t a 3w L Allowale span t Beam thickness σ Allowale woking stess (tensile stength / safety facto) a w unifom load pe unit length of eam It follows that given the span, thickness and unifom load, the Exteme Fie Stess can e calculated as: 3L w σe 4t Exteme fie stess L Allowale span σ e t Beam thickness w unifom load pe unit length of eam Clamped Beam - Fo a clamped oof eam of unit width, the Allowale Span, ased on tensile failue, can e calculated as follows: L t σ w a σ L Allowalespan t Beam thickness a Allowale woking stess (tensile stength / safety facto) w unifom load pe unit length of eam It follows that given the span, thickness and unifom load, the Exteme Fie Stess can e calculated as: σ e L w t σ e Exteme fie stess L Allowalespan t Beam thickness w unifom load pe unit length of eam

3 Beam Shea - Fo eithe a simply suppoted o a clamped oof eam of unit width, the Allowale Span, ased on shea failue, can e calculated as follows: L Allowalespan t Beam thickness a 4 t τ L 3w Allowaleshea stess (shea stength / safety facto) w unifom load pe unit length of eam a Beam Theoy The poceeding eam equations equie the use of the in situ tensile stength of the oof mateial. In many situations, the oof mateial is cacked o jointed and the in situ tensile stength may e much less than the laoatoy value. It is geneally consideed an uneliale value.

4 (Polem Beam Span) The immediate oof in a new mining section of a limestone mine unde shallow cove has een found to e fee of any majo defects. Its thickness is 4 ft, its modulus of uptue is 100 psi, it shea stength is 000 psi, and its specific gavity is.6. Utilizing a safety facto of 8, what is the maximum safe oof span in feet? Fist, we calculate the allowale span using the simple eam fomula: L 4t a 3w l in 4*(4 ft) * 100 /8 * 144 in ft l 3* 4 ft *.6 * ft 6.6 ft To doule check, we calculate the exteme fie stess: σ e 3L w 4t 3* 6.6 ft l ft 1,500 * ft 144 in l * 4 ft *.6*6.4 3 ft 4* 4 ft 150 psi Finally, we calculate the allowale span ased on shea stength (not a limiting facto): 4 t τa L 3w 000 l 144in 4 *4 ft * * in ft 6.4 l 3* 4 ft *.6* 3 ft,366 ft

5 Coal Mine Empiical Design: In addition, it is impotant to conside that although it is called Enty design, it may e moe accuately called Intesection design. Typically, the lagest spans in an undegound coal mine ae the diagonals at the intesections. Nominally, the intesection diagonal would e 41% (1/sin(45 )) wide than the enty; howeve, the pilla cones ae known to slough, and fequently the pilla cones ae cut to incease haulage moility. Both of these activities incease the intesection span. In an analysis of oof falls at 37 coal mines whee numeous intesections wee measued, Molinda et al. (000) found that the intesection spans aveaged 4 to 6 ft wide than the nominal width in shallow (depth <400 ft) mines and 6 to 8 ft wide than the nominal span in deep (depth >400 ft) mines. In the same study, Molinda et al. (000) found that 70% of all the oof falls occu in the intesections, although the intesections only total aout 0% to 5% of the development enty lengths. They concluded that intesections ae 8 to 10 times moe likely to have a fall than an equivalent length of enty. Theefoe, if the mine enginee can design a stale intesection span, then the enties in etween the intesections should natually e stale. Fom thei analysis of the oof falls ates at the study mines, Molinda et al. (000) detemined that a mine with a given CMRR would avoid high oof fall ates (>3 falls pe 10,000 ft of enty development) if the sum of the diagonals of the intesection span (Is), in feet, was kept elow I s *CMRR (EQ 3.51) If the CMRR > 65, then it should e set equal to 65 in Equation Theefoe, if the CMRR is 45 at a 600-ft-deep mine, then the maximum intesection span should e I s *CMRR * ft (EQ 3.5) And given that the actual span aveages 7 ft wide than nominal (Molinda et al. 000), the enty width (we) should e less than *sin45 w e 19.0 ft (EQ 3.53)

6 Roof Bolting Geneal types of oof olts: 1. Point-Ancho. Resin Gouted / Cement Gouted 3. Fiction Bolts a. Split-Sets. Swellex 4. Cale Bolts

7 Figue. Mechanical-Ancho Bolt. Figue. Resin-Assisted, Mechanical-Ancho Bolt.

8 Figue. Fully Gouted Bolt. Figue. Tensioned Rea Bolt

9 Figue. Comination Bolt. Figue. Cale Bolts.

10 Roof Bolt Paametes: 1. Length. Diamete a. (No. 5 a = 5/8 in dia.) 3. Stength / Gade a. (gade 40 = 40,000 psi yield) 4. Anchoage Type a. Expansion shell. Resin c. Comination Suppot Pinciples - Roof olting suppots the oof using thee geneal pincipals 1. Suspension. Beam Building 3. Reinfocement

11 Roof Suspension - Fo designing olts fo suspending the oof, the ock weight suppoted y each olt is calculated and compaed to the allowale esistance of the olt.

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13 w / l Tiutay Aea w l / l w w Plan View h Coal Pilla w w w / Coal Pilla Coss Section View Figue. Schematic of unifomly spaced, suspension design. Rock Weight - Fo Unifom Spacing of the oof olts, the Rock Weight suppoted y each olt can e calculated as: h w l P ρ *h *w P Rock load pe olt ρ Density of the ock Height of ock load * l Spacing acoss the width of the enty Spacing along the length of the enty

14 Rock Height - The Rock Load Height can e detemined fom Geology, Osevation, o fom the Rock Mass Classification as: RMR h * w e 100 Bolt Stength - The Allowale Load fo each olt is the yield stength times the aea divided y the safety facto. (Polem - Sample Test Question #0) A mine has the following two types of untensioned, esin-gouted ock olts availale. Roof ock at the mine weighs 160 l/ft3. What is the most economical olting patten and olt to suppot a ock column 6 ft thick if the maximum allowale load on the olts is 80% of yield? 1. #6 Rea, gade 40. #6 Rea, gade 60 RMR Width of enty The ock weight fo 6 ft height with a 4 x 4 o 5 x 5 patten is: S σ y h w e Height of ock load S Rock Mass Rating σ y * π d SF Allowale olt stength SF Safety Facto 4 Yield stength of olt d Diamete of olt P ρ *h * w 160*6*4*4 15,360 ls The Bolt Stength of the two olts types ae: S σ y * π d SF *l 40,000*0.80* π * ,137 ls 4 P ρ *h * w *l 160*6*5*5 4,000 ls σ S y * π d SF 60,000*0.80* π * ,05 ls 4 Neithe olting patten is stong enough at the 5 ft spacing and only the gade 60 olt is stong enough at the 4 ft spacing.

15 Tiutay Aea 5.0 Tiutay Aea Plan View Coal Pilla Coal Pilla Coss Section View Figue. Complex suspension design. Example (Complex Suspension Design) - The immediate oof consists of 3.5 ft of weak shale ovelain y a competent sandstone, all with a density of 158 l/ft 3. The enty is 16 ft wide and the ow spacing down the enty is 5 ft. The outside olts ae: 5.5 ft long, #5 ea, gade 60, fully-gouted olts and they ae installed 3 ft fom the i of the enty. The cente olt in this example is assumed to e an 8 ft long, 0.6 in dia. (30 ton) cale olt.

16 Rock Load - the width of the tiutay aea fo the outside olt would e calculated as the sum of half of the distance to the cente olt (.5 ft) and half the distance to the i (1.5 ft), fo a total of 4 ft (see Figue). The width of the tiutay aea fo the cente cale olt would then e 5 ft, and the length of the tiutay aea fo oth types of olts is equal to the ow spacing of 5 ft. With this olting plan, the width of the suppot aea fo the entie olt ow is then 13 ft and the length of the suppot aea fo the entie ow is 5 ft. The Rock Load fo the 3.5 ft height, 13 ft width and 5 ft spacing is then: The Stength of the ea olt is then: The comined olt capacity is then: S And the safety facto is: σ P ρ y *h *w 158 ls/ft 3 35,945 ls * π d 60,000 psi* 18,400 ls 4 *l *3.5 ft *13 ft *5 ft * S *18,400 ls 96,800 ls 0.65 in 60,000 ls S 96,800 ls SF.70 P 35,945 ls /4

17 Wedge Suppot: 1. Rock olting can e used to suppot a sliding lock o a lock susceptile to falling unde gavity.. Assumes that the locks ae disceetly defined. 3. A limit equiliium analysis can e used to detemine olting suppot 4. Commecial softwae facilitate this type of analysis

18 Beam Building: RMR Bolt Design fo tunnels:

19 RMR Bolt Design fo coal:

20 Q Bolt Design fo Had Rock:

21 The Q value is elated to tunnel suppot equiements y defining the equivalent dimensions of the excavation. This equivalent dimension, which is a function of oth the size and the pupose of the excavation, is otained y dividing the span, diamete, o the wall height of the excavation y a quantity called the excavation suppot atio (ESR). Thus, Equivalent dimension Excavation span, diamete,o height (metes) ESR The ESR is elated to the use fo which the excavation is intended and the degee of safety demanded, as shown elow. Excavation categoy ESR No. of cases A. Tempoay mine openings 3-5 B. Vetical shafts: Cicula section.5 - ectangula/squae section.0 - C. Pemanent mine openings, wate tunnels fo hydopowe (excluding high-pessue penstocks), pilot tunnels, difts, and headings fo lage excavations D. Stoage ooms, wate teatment plants, mino highway and aiload tunnels, suge chames, access tunnels. E. Powe stations, majo highway o aiload tunnels, civil defense chames, potals, intesections F. Undegound nuclea powe stations, 0.8 aiload stations, factoies The elationship etween the index Q and the equivalent dimension of an excavation detemines the appopiate suppot measues. Baton et al. (1974) povide 38 suppot categoies which give estimates of pemanent suppot. Fo tempoay suppot detemination, eithe Q is inceased to 5Q o ESR is inceased to 1.5 ESR. Fo selection of the suppot measues using the Q-system, the eade should consult the oiginal pape y Baton et al. (1974) o the ook y Hoek and Bown (1980). The maximum unsuppoted span can e otained as follows: Maximum span (unsuppoted) = (ESR)Q 0.4 (6.7)

22 Q Bolt Length: