A mechanism of rock failure at the walls of large caverns and excavations

1
A mechanism of rock failure at the walls of large
caverns and excavations

• A. V. Dyskin, E Sahouryeh, L. N. Germanovich

The University of Western Australia
2
Plan

• Introduction
• Model of Crack Growth
• Rockburst Initiation
• Rockburst Initiation
• Conclusions

3
Introduction
4
Crack Growth
2-D mechanism
5
3-D crack growth in uniaxial compression
No extensive growth
6
Wing crack near free surface
L
b
2a
a
7
3-D crack growth in biaxial compression
8
Crack Model
(Germanovich et al., 1996)
(1)
KIF(pR)-3/2 (Cherepanov, 1979)
Stable growth
9
Accounting for free surface
Srivastava Singh (1969).
(2)
stable
(3)
unstable
(4)
Rcr h(p/5)1/3 ? 0.85h
(5)
(6)
Critical radius
10
Rcr h(p/5)1/3 ? 0.85h
11
Rockburst Initiation
pz
Averaging (1)
(7)
h

• H is minimum of
• stress concentration depth
• depth of flaws

pz
12
(8)
e.g., Freudenthal, 1968)
For A1/2 , H gtgt 1/N1/3 and H gtgt hmin
(9)
Substituting into (6)
and using (7)
13
(10)
for unstable growth
Using Cherepanovs, 1979 critertion of maximum
circumferential stress
(11)
(12)
(13)
Normalising, following Dyskin et al. (1991),
(14)
(15)
14
Size Effect
(16)
(17)
v00.1 0.46ltA1/2/alt3.5, 2.7ltPcr/stlt11.8 v00.01
0.99ltA1/2/alt11, 2.7ltPcr/stgt37, v00.001 2.1ltA1/2
/alt35 2.7ltPcr/stgt118
15
Rockburst Progression
(e.g., Timoshenko, 1959)
(18)
Substituting (9), (10), (14) into (18)
(19)
From (18) and (19)
(20)
inserting A?Rcr2 into (9), we have
(21)
16
Conclusions

• The proposed model of rockburst initiation
  predicts the scale effect the larger the area of
  stress concentration the smaller the magnitude of
  stress concentration required to initiate
  rockburst.
• The model relates rockburst initiation to the
  quality of the excavated surface the smaller the
  crack concentration at the excavation wall, the
  higher the stress of rockburst initiation.