1

Strength ofrock and rock masses

Ridho K. Wattimena

Dept. Mining EngineeringInstitut Teknologi Bandung

Intact to Heavily Jointed Rock Mass

2

IntroductionThe stability of an underground excavation depends upon the structural conditions in the rock mass and relationship between the stress in the rock and the strength of the rock.In order to utilise the knowledge of stresses induced around underground excavations, it is necessary to have available a criterion or a set of rules which will predict the response of a rock mass to a given set of induced stresses.

Hoek-Brown CriterionHoek and Brown (1980) proposed a method for obtaining estimates of the strength of jointed rock mass.The method was modified over the years (Hoek, 1983; Hoek & Brown, 1988). The application of the method to very poor quality rock masses required further changes (Hoek, Wood and Shah, 1992)The development of new classification called the Geological Strength Index (Hoek, Kaiser and Bawden, 1995; Hoek, 1995; Hoek and Brown, 1997).Historical development of the criterion can be found in Hoek (2002).

3

Hoek-Brown Criterion

Intact Rock Properties

The relationship between the principal stresses at failure for a given rock is defined by σci and mi.The range of σ3’ values is critical.Hoek & Brown (1980) used 0 < σ3’ < 0.5 σci

At least five data points should be included in the analysis.

4

Triaxial CellDeveloped by Hoek and Franklin (1968)Does not require draining between testsMore sophisticated cells are available for research purposesDept. Mining Eng. ITB:

Wattimena & Kramadibrata(1997)Kramadibrata, Wattimenaand Simangunsong (1998)

Determination of σci and mi

y = mσcix + sσci

x = σ3’

y = (σ1’ – σ3’)2

5

Determination of σci and mi

Field estimates of UCS

6

Field estimates of UCS (continued)

Values of mi for intact rock

7

Values of mi for intact rock (continued)

NotesShort-term laboratory tests on very hard brittle rocks tend to overestimate the in situ rock mass strength.Laboratory tests and field studies on excellent quality Lac du Bonnet granite (Martin and Chandler, 1994) show that the in situ strength of this rock is only 70% of that measured in the laboratoryAnisotropic and foliated rocks present particular difficulties in the determination of the UCS.

8

Influence of loading direction on the strength of graphitic phyllite (Salcedo, 1983)

Influence of sample size(Hoek & Brown, 1980)

9

Peak Strength of Moura DU Coal (Medhurst & Brown, 1996)

σci = 32.7 MPa

Peak Strength of Moura DU Coal (Medhurst & Brown, 1996)

10

Geological Strength Index

Estimates of m and s using GSI(Hoek, 1994; Hoek et al., 1995)

mb = mi exp [(GSI – 100)/28]For GSI > 25

s = exp [(GSI-100)/9]a = 0.5

For GSI < 25s = 0a = 0.65 – (GSI/200)

11

Estimates of m and s using GSI(Hoek, 1994; Hoek et al., 1995)

For better quality rock mass (GSI>25), the value of GSI can be estimated directly from 1976 version of Bieniawski’s RMR with groundwater rating set to 10 (dry) and adjustment for joint orientation set to 0 (very favourable).Bieniawski’s RMR should not be used for estimating the GSI values for poor quality rock masses.If the 1989 version of Bieniawski’s RMR is used:

GSI = RMR89’ – 5RMR89’ has the groundwater rating set to 15 and the adjustment for joint orientation set to zero

What is the GSI ???

Controlled blasting Bulk blasting

12

Estimates of m and s using GSI(Hoek et al, 2002)

mb = mi exp [(GSI – 100)/(28-14D)]

s = exp [(GSI-100)/(9-3D)]

D = Disturbance Factor

a = 1/2 + 1/6 [exp(-GSI/15)-exp(-20/3)]

Disturbance Factor, D

D = 0

Excellent quality controlled blasting or excavation by Tunnel Boring Machine results in minimal disturbance to the confined rock mass surrounding a tunnel.

Suggested value of D

Description of rock massAppearance or rock mass

13

Disturbance Factor, D

D = 0

D = 0.5(no invert)

Mechanical or hand excavation in poor quality rock masses (no blasting) results in minimal disturbance to he surrounding rock mass. Where squeezing problems result in significant floor heave, disturbance can be severe unless a temporary invert, as shown in the photograph, is placed.

Suggested value of D

Description of rock massAppearance or rock mass

Disturbance Factor, D

D = 0.8

Very poor quality blasting in a hard rock tunnel results in severe local damage, extending 2 or 3 m, in the surrounding rock mass.

Suggested value of D

Description of rock massAppearance or rock mass

14

Disturbance Factor, D

D = 0.7Poor

blasting

D = 1.0Good

blasting

Small scale blasting in civil engineering slopes results in modest rock mass damage, particularly if controlled blasting is used as shown on the left hand side of the photograph. However, stress relief results in some disturbance.

Suggested value of D

Description of rock massAppearance or rock mass

Disturbance Factor, D

D = 1.0Production

blasting

D = 0.7Mechanical excavation

Very large open pit mine slopes suffer significant disturbance due to heavy production blasting and also due to stress relief from overburden removal. In some softer rocks excavation can be carried out by ripping and dozing and the degree of damage to the slopes is less.

Suggested value of D

Description of rock massAppearance or rock mass

15

Mohr-Coulomb parameters

Mohr-Coulomb parameters

16

A Brief History of the Development of Hoek-Brown Failure Criterion

1980Hoek E. and Brown E.T. 1980. Underground Excavations in Rock . London: Institution of Mining and Metallurgy 527 pages.Hoek, E. and Brown, E.T. 1980. Empirical strength criterion for rock masses. J. Geotech. Engng Div., ASCE 106(GT9), 1013-1035.

1983Hoek, E. 1983. Strength of jointed rock masses, 23rd. RankineLecture. Géotechnique 33(3), 187-223.

A Brief History of the Development of Hoek-Brown Failure Criterion

1988Hoek E and Brown E.T. 1988. The Hoek-Brown failure criterion - a 1988 update. Proc. 15th Canadian Rock Mech. Symp. (ed. J.H. Curran), pp. 31-38. Toronto: Civil Engineering Dept., University of Toronto.

1990Hoek, E. 1990. Estimating Mohr-Coulomb friction and cohesion values from the Hoek-Brown failure criterion. Intnl. J. Rock Mech. & Mining Sci. & Geomechanics Abstracts. 12(3), 227-229.

17

A Brief History of the Development of Hoek-Brown Failure Criterion

1992Hoek, E., Wood, D. and Shah, S. 1992. A modified Hoek-Brown criterion for jointed rock masses. Proc. rock characterization,symp. Int. Soc. Rock Mech.: Eurock ‘92, (J.Hudson ed.). 209-213.

1994Hoek, E. 1994. Strength of rock and rock masses, ISRM News Journal, 2(2), 4-16.

A Brief History of the Development of Hoek-Brown Failure Criterion

1995Hoek, E., Kaiser, P.K. and Bawden. W.F. 1995. Support of underground excavations in hard rock. Rotterdam: Balkema

1997Hoek, E. and Brown, E.T. 1997. Practical estimates of rock mass strength. Intnl. J. Rock Mech. & Mining Sci. & GeomechanicsAbstracts. 34(8), 1165-1186.

1998Hoek, E., Marinos, P. and Benissi, M. (1998) Applicability of the Geological Strength Index (GSI) classification for very weak andsheared rock masses. The case of the Athens Schist Formation. Bull. Engg. Geol. Env. 57(2), 151-160.

18

A Brief History of the Development of Hoek-Brown Failure Criterion

2000Hoek, E. and Marinos, P. (2000) Predicting Tunnel Squeezing. Tunnels and Tunnelling International. Part 1 - November Issue 2000,. 45-51, Part 2 - December, 2000, 34-36. Marinos, P.G. and Hoek, E. (2000): "GSI: A geological friendly tool for rock mass strength estimation", Proceedings of the International Conference on Geotechnical & Geological Engineering (GeoEng 2000), Technomic Publishing Co. Inc., p.p. 1422-1440, Melbourne, Australia.

2001Marinos. P, and Hoek, E. (2001) - Estimating the geotechnicalproperties of heterogeneous rock masses such as flysch, Bull.Engg. Geol. Env. 60, 85-92.

A Brief History of the Development of Hoek-Brown Failure Criterion

2002Hoek, E., Carranza-Torres, C.T., and Corkum, B. (2002), Hoek-Brown failure criterion – 2002 edition. Proc. North American Rock Mechanics Society meeting in Toronto in July 2002.