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