| Chapter 2 - Rock Mechanics |
| Number |
Topic |
Rule of Thumb |
| 2.01 |
Ground Stress |
The vertical stress may be calculated on the basis of depth of overburden with an accuracy of ± 20%. This is sufficient for engineering purposes. Source: Z.T. Bieniawski |
| 2.02 |
Ground Stress |
Discs occur in the core of diamond drill holes when the radial ground stresses are in excess of half the compressive rock strength. Source: Obert and Stephenson |
| 2.03 |
Ground Stress |
The width of the zone of relaxed stress around a circular shaft that is sunk by a drill and blast method is approximately equal to one-third the radius of the shaft excavation. Source: J. F. Abel |
| 2.04 |
Ground Control |
The length of a rock bolt should be one-half to one-third the heading width. Mont Blanc Tunnel Rule (c.1965) |
| 2.05 |
Ground Control |
In hard rock mining, the ratio of bolt length to pattern spacing is normally 1½:1. In fractured rock, it should be at least 2:1. (In civil tunnels and coalmines, it is typically 2:1.) Source: Lang and Bischoff (1982) |
| 2.06 |
Ground Control |
In mining, the bolt length/bolt spacing ratio is acceptable between 1.2:1 and 1.5:1. Source: Z.T. Bieniawski (1992) |
| 2.07 |
Ground Control |
In good ground, the length of a roof bolt can be one-third of the span. The length of a wall bolt can be one-fifth of the wall height. The pattern spacing may be obtained by dividing the rock bolt length by one and one-half. Source: Mike Gray (1999) |
| 2.08 |
Ground Control |
The tension developed in a mechanical rock bolt is increased by approximately 40 Lbs. for each one foot-Lb. increment of torque applied to it. Source: Lewis and Clarke |
| 2.09 |
Ground Control |
A mechanical rock bolt installed at 30 degrees off the perpendicular may provide only 25% of the tension produced by a bolt equally torqued that is perpendicular to the rock face, unless a spherical washer is employed. Source: MAPAO |
| 2.10 |
Ground Control |
For each foot of friction bolt (split-set) installed, there is 1 ton of anchorage. Source: MAPAO |
| 2.11 |
Ground Control |
The shear strength (dowel strength) of a rock bolt may be assumed equal to one-half its tensile strength. Source: P. M. Dight |
| 2.12 |
Ground Control |
The thickness of the beam (zone of uniform compression) in the back of a bolted heading is approximately equal to the rock bolt length minus the spacing between them. Source: T.A. Lang |
| 2.13 |
Ground Control |
Holes drilled for resin bolts should be ¼ inch larger in diameter than the bolt. If it is increased to 3/8 inch, the pull out load is not affected but the stiffness of the bolt/resin assembly is lowered by more than 80%, besides wasting money on unnecessary resin. Source: Dr. Pierre Choquette |
| 2.14 |
Ground Control |
Holes drilled for cement-grouted bolts should be ½ to 1 inch larger in diameter than the bolt. The larger gap is especially desired in weak ground to increase the bonding area. Source: Dr. Pierre Choquette |
| 2.15 |
Ground Control |
Every 100° F rise in temperature decreases the set time of shotcrete by 1/3. Source: Baz-Dresch and Sherril |
| 2.16 |
Mine Development |
Permanent underground excavations should be designed to be in a state of compression. A minimum safety factor (SF) of 2 is generally recommended for them. Source: Obert and Duval |
| 2.17 |
Mine Development |
The required height of a rock pentice to be used for shaft deepening is equal to the shaft width or diameter plus an allowance of five feet. Source: Jim Redpath |
| 2.18 |
Stope Pillar and Design |
A minimum SF of between 1.2 and 1.5 is typically employed for the design of rigid stope pillars in hard rock mines. Various Sources |
| 2.19 |
Stope Pillar and Design |
For purposes of pillar design in hard rock, the uniaxial compressive strength obtained from core samples should be reduced by 20-25% to obtain a true value underground. The reduced value should be used when calculating pillar strength from formulas relating it to compressive strength, pillar height, and width (i.e. Obert Duval and Hedley formulas). Source: C. L. de Jongh |
| 2.20 |
Stope Pillar and Design |
The compressive strength of a stope pillar is increased when later firmly confined by backfill because a triaxial condition is created in which σ3 is increased 4 to 5 times (by Mohr’s strength theory). Source: Donald Coates |
| 2.21 |
Subsidence |
In Block Caving mines, it is typical that the cave is vertical until sloughing is initiated after which the angle of draw may approach 70 degrees from the horizontal, particularly at the end of a block. Source: Fleshman and Dale |
| 2.22 |
Subsidence |
Preliminary design of a block cave mine should assume a potential subsidence zone of 45-degrees from bottom of the lowest mining level. Although it is unlikely that actual subsidence will extend to this limit, there is a high probability that tension cracking will result in damage to underground structures (such as a shaft) developed within this zone. Source: Scott McIntosh |
| 2.23 |
Subsidence |
In hard rock mines employing backfill, any subsidence that may occur is always vertical and nothing will promote side sloughing of the cave (even drill and blast). Source: Jack de la Vergne |
| 2.24 |
Rockbursts |
75% of rockbursts occur within 45 minutes after blasting (but see below). Source: Swanson and Sines |
| 2.25 |
Rockbursts |
The larger the rockburst, the more random the pattern in time of occurrence. Microseismic data from many areas shows that the smaller microseismic events tend to be concentrated at or just after blast time, on average (see above). However, the larger the event, the more random its time of occurrence. Source: Richard Brummer |
| 2.26 |
Rockbursts |
In burst prone ground, top sills are advanced simultaneously in a chevron (‘V’) pattern. Outboard sills are advanced in the stress shadow of the leading sill with a lag distance of 24 feet. Source: Luc Beauchamp |
| 2.27 |
Rockbursts |
Seismic events may be the result of the reactivation of old faults by a new stress regime. By Mohr-Coulomb analysis, faults dipping at 30 degrees are the most susceptible; near vertical faults are the safest. Source: Asmis and Lee |
| 2.28 |
Rockbursts |
There can be little doubt that it is possible to control violent rock behavior by means of preconditioning or de-stressing under appropriate circumstances. This technology, therefore, has the potential to be profitably harnessed for use in the mining of deeper orebodies, particularly hazardous situations such as highly stressed high grade remnants, or development into areas known to be prone to bursting. Source: Board, Blake & Brummer |