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How can we control the weight of the immediate roof?
1. If the weak roof is thin, we can suspend it from an overlying strong bed with bolts.
2. Reinforce the rock and create a self-supporting beam with bolts.
3. Suspend the broken roof from stable ground (with cable bolts or trusses).
4. Carry the weight of the broken roof with standing support.
Cable Bolts and Trusses
Grout Failure
Bolt Failure
Grout Failure
Loadon Plate
(A) (B) (C)
Head Failure
Rod Failure
Anchor Failure
A
Grout Failure
Loadon Plate
Plate/Head Failure
BoltFailure
BGrout Failure
C
Rod Failure
C
“Anchor” Failure
Design of Cable Bolt Systems
Design Considerations• System Capacity• Cable Length• Anchorage Length• Surface Control
Cable bolt support capacity for entries
Estimation of Dead Weight Load for Cable Bolt Design
Roof fall angle
Fall height
Entry width
Dead Weight Load Calculator for Straights (Version 7.0)Input Data Output Data
Entry Width (ft)
Thickness of roof to be supported (ft)
Angle of Roof Fall (degrees)
Cable Bolt Capacity* (ton)
Bolts/Row Stability Factor
Row Spacing (ft)
Total Rock Load (tons/ft)
20 5 30 27 2 1.0 7.9 7*For a nominal 30 or 40 ton cable bolt the minimum strength required by ASTM F432 is 27 or 36, ton respectively. This can be adjusted at users discretion
Angle of Roof Fall
Thickness of roof to be supported
Entry Width
Cable bolt support capacity for intersections
Roof fall angleFall height
Intersection Span
Input Data Output DataMeasured Sum of
Diagonals (ft.)Thickness of roof to be supported (ft.)
Angle looking down entry
Angle looking down cross-cut
Cable Bolt Capacity* (ton)
Entry Height(ft.)
Stability Factor
# of Supports
Total Rock Load (tons)
66 5 30 30 27 4 1.0 6.2 168
*For a nominal 30 or 40 ton cable bolt the minimum strength required by ASTM F432 is 27 or 36 ton, respectively. This can be adjusted at users discretion.
Entry Width (ft.) Cross-cut Width (ft.) Calculated Sum of Diagonals (ft.)
20 20 56.6
Dead Weight Load Calculator for Intersections (Version 7.0)
Cable Length = Rock Fall Height + Anchor Length
Ensure integrity of plate-roof contact.
Truss Bolts
Truss bolts suspend the broken rock from stable ground above the pillars.
A “30 ton truss” can actually carry more than 40 tons because there are two anchors.
P = Rock Load Carried by Bolts
30 Ton CableBolt
30 Ton CableBolt
Roof MatPillar Pillar
Rock Load Carried by Truss
Force in Truss
Truss Angle
P = Rock Load Carried by Truss
P = 2 * T (cos(truss angle))
= 2 * 30 tons * 0.7
~ 42 tons
Truss advantages: Anchorage in confined rockCan avoid groundwater
Disadvantages: Connection hardwareIntersections
Standing Supports
• Support capacity• Support stiffness• Residual capacity• Support system
design
Elements of Standing Support Design
• Stiffness is a measure of how quickly a support develops its load carrying capacity.
• Stiffer supports develop capacity quicker (with less convergence).
Stiffness
A REAL EXAMPLE
CONFINED CORE CRIB (3C) SUPPORTNIOSH SAFETY STRUCTURES TESTING LABORATORY
600
500
1,000
1,500
2,000
2,500
0 10 20 30 40 50
DISPLACEMENT, inches
SUPP
OR
T LO
AD
, ki
ps
Would you call this a 1,000 ton support?
Wood cribs also have a low stiffness, a high capacity, and can withstand large deformations.
Wood posts are stiff, but have little residual strength.
Many modern supports combine a high stiffness with large residual capacity.
PERFORMANCE CHARACTERISTICSARE WELL KNOWN
HOW TO APPLY THEM
? ? ?
Support Technology Optimization Program (STOP)
• One hardwood timber post can carry about 50 tons• Two posts on 5 ft centers can carry 20 tons per ft• Support per intersection = 400 tons• (Compare to 6 cable bolts = 180 tons)
Installation quality is always important with standing supports (especially wood).
Beams
The capacity of a beam depends heavily on the span.
Beams
Arches