ROCK MECHANICS: L6

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ROCK MECHANICS L6

• Rock and Soils 2006
• Don Cameron

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ROCK MASS RATING SYSTEM or RMR

• Bieniawski (1972)
• Numerous amendments since
• For assessing the stability of rock slopes

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RMR Basic System RMRbasic

• strength of the intact rock
• RQD
• Groundwater
• Discontinuities
• Spacing, length, roughness
• Aperture width, infill, weathering
• 9 ratings to add ? RMRbasic 100 max

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RMR for slopes or tunnels

• Additional factors applied to RMRbasic
• Accounts for excavation method
• BUT moreover,
• Accounts for joint orientation wrt the excavation

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RMR and tunnels

• Stand up time for various tunnel spans based on
  RMR
• Unreinforced tunnels
• No advice re shotcrete or rockbolts/anchors

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Evaluation of Tunnels based on RMR
Example 10 m span RMR 80 Stand up time gt 4
years RMR 50 Stand up time ? 2 days
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Q or NGI index

• RQD Rock quality designation
• Jn joint set number (0.5 20 massive rock,
  few joints to a crushed rock
• The ratio RQDJn ? block size

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Q System

• Jr joint roughness number (0 -20)
• Ja the joint alteration number (0.75
  4 hard to soft filling)
• The ratio JrJa ? joint roughness friction

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Q System

• slightly rough, planar v undulating
• rough/regular and undulating

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Q System

• Jw joint water reduction factor
  (1 0.05 dry to water under pressure)
• SRF the stress reduction factor
  (1 400 low stress favourable orientation to
  high stress)

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1. RQD
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2 . JOINT SET NUMBER, Jn

• Notes refer to tunnelling possibly greater Jn

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3. JOINT ROUGHNESS NUMBER, Jr (not for open
joints)
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4. JOINT ALTERATION NUMBER, Ja
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5. JOINT WATER REDUCTION, Jw
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6. STRESS REDUCTION FACTOR
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Tunnels and the Q rating

• Require ESR and De
• De equivalent dimension
• De ratio of excavation span or heightESR
• ESR excavation support ratio
• ESR is a function of the tunnel use and
  acceptable level of risk

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ESR Values (Barton et al 1974)
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Tunnels and the Q rating
Example 10 m span ESR 2 Q 40 10 m span ESR
1 Q 40
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Evaluation of Tunnels based on Q rating

• Example
• 10 m span ESR 2
• Q 40
• Requires rockbolts at 3 m spacing, 2.4 m long
• 10 m span ESR 1
• Q 40
• Requires rockbolts at 3 m spacing, 3 m long

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Tunnels based on Q rating

• Final Example
• 10 m span ESR 1
• Q 10
• Requires rockbolts at 2.3 m spacing, 3 m long in
  40 mm shotcrete!!

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Tunnels and the Q rating
Example 10 m span ESR 1 Q 10
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Notes

• Rock bolt lengths are for an ESR of 1.0
  (conservative)
• Lengths (L) can be modified based on the
  excavation span (B)
• L 2 0.15 (B/ESR)

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KEY POINTS?

• Rock mass rating systems are a useful way of
  forming an evaluation of rock masses
• The Q or NGI system was based on tunnelling
• The RMR (CSIR) system is more commonly used for
  slope stability
• The strength of rock masses can be judged from
  these sytems