Example of Challenges Unforeseen Ground conditions d d Rock Mass - - PowerPoint PPT Presentation

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Example of Challenges Unforeseen Ground conditions d d Rock Mass - - PowerPoint PPT Presentation

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Example of Challenges Unforeseen Ground conditions d d Rock Mass Rating Systems Rock Mass Rating Systems ROCK MASS RATING SYSTEM or RMR RMR Bieniawski (1972) numerous amendments since For assessing the stability of rock slopes For

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Example of Challenges‐ Unforeseen d d Ground conditions

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Rock Mass Rating Systems Rock Mass Rating Systems

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ROCK MASS RATING SYSTEM RMR

  • r RMR

Bieniawski (1972) ‐ numerous amendments since For assessing the stability of rock slopes For assessing the stability of rock slopes

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RMR Basic System = RMRb

i

RMR Basic System RMRbasic

St th f th i t t k

  • Strength of the intact rock
  • RQD
  • Groundwater

Discontinuities: Discontinuities:

  • Spacing, length, roughness
  • Aperture width, infill, weathering

9 ratings to add ⇒ RMRb

i = 100 maximum

9 ratings to add ⇒ RMRbasic = 100 maximum

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Elements of RMR Elements of RMR

1 S h b d (UCS) I

  • 1. Strength based on σc (UCS) or Is50

– 25:1 ratio

15 pts

– σc = 250 MPa or more; 15 points

2 RQD Rock Quality Designation 20 t

  • 2. RQD Rock Quality Designation

– < 25%; 3/20 points only

20 pts

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Elements of RMR

  • 3. Groundwater

Inflow rate

15 pts

ujoint:σ1 If > 0 5; zero points? − If > 0.5; zero points? Dry, damp, wet, dripping, flowing?

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Elements of RMR - discontinuities

4 Spacing

  • 4. Spacing

2 m or more; 20 points

20 pts

< 60 mm; 5 points

  • 5. General condition (refer section E)

30 pts

Roughness Continuity Opening Weathering Weathering

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RMR modified for slopes or tunnels RMR modified for slopes or tunnels

Additional factors applied to RMRbasic

  • Accounts for excavation method

Accounts for excavation method BUT moreover, A t f j i t i t ti t th ti

  • Accounts for joint orientation wrt the excavation

– Unfavourable conditions, deduct points from RMR RMRbasic – refer section F of Table

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Slopes ‐ unfavourable Slopes unfavourable

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Slopes - favourable

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Tunnels - unfavourable

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Tunnels - favourable

  • Widely spaced joints?

Widely spaced joints?

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RMR & Tunnels RMR & Tunnels

  • “Stand up time” for various tunnel spans

based on RMR

  • Unreinforced tunnels

─ no advice re support e.g. shotcrete or rockbolts/anchors Shotcrete = sprayed concrete, lightly reinforced p y , g y

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Evaluation of Tunnels b d RMR

Example: 10 m span RMR = 80 Stand up time > 4 years

based on RMR

RMR = 50 Stand up time ≈ 2 days

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Class of Rock from RMR Class of Rock from RMR

RMR Description Class RMR Description Class 100-81 Very good I 80-61 Good II 60-41 Fair III 40-21 Poor IV 40 21 Poor IV <21 Very poor V

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An Alternative Rating System An Alternative Rating System

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NGI index or Q rating NGI index or Q rating

SRF J J J J RQD Q

w r

= SRF J J

a n

  • RQD as before
  • RQD as before
  • Jn = joint set number

– (0.5 – 20: massive rock to a crushed rock

  • The ratio RQD:Jn ∼ “block size”
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Q System

SRF J J J J RQD Q

w r

= SRF J J

a n

J = joint roughness number (0 6) Jr = joint roughness number (0 - 6) Ja = the joint alteration number

  • 0.75 – 4?: hard to soft filling; Ja =fn(φr)

The ratio Jr:Ja ∼ joint roughness & friction

r a

j g

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Q System

JRC

1-5 5-101 5-101 10-152 15-20

Jr

0.5 1 1.5 3 3 6

  • 1. “slightly rough”, planar v undulating

2 “rough/regular” and undulating 2. rough/regular and undulating

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Q System

SRF J J J J RQD Q

w r

= SRF J J

a n

Jw = joint water reduction factor (1 – 0.05: dry to water under pressure) SRF = the stress reduction factor (0.5 – 20: low stress & favourable orientation t hi h t ) to high stress)

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1 RQD

  • 1. RQD

Very poor 0 25 Very poor 0-25 Poor 25 50 Poor 25-50 Fair 50-75 Fair 50 75 Good 75-90 Excellent 90-100

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2 . JOINT SET NUMBER, Jn 2 . JOINT SET NUMBER, Jn

O j i t t 2 One joint set 2 Two joint sets 4 Two joint sets 4 Two joint set + random 6 j Three joint sets 12

Notes refer to tunnelling & possibly greater Jn Notes refer to tunnelling & possibly greater Jn

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  • 3. JOINT ROUGHNESS NUMBER, Jr (not for
  • pen joints)

Description JRC Jr Discontinuous joints 15 20 4 Discontinuous joints 15 - 20 4 Smooth undulating 1 - 5 2 g Smooth planar 1 - 5 1 Slickensided planar 0.5

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  • 4. JOINT ALTERATION NUMBER, Ja

D i ti φ (°) J Description φr (°) Ja

Unaltered joint walls, surface staining

25 - 35 1

Unaltered joint walls, surface staining

  • nly

25 35 1

Slightly altered joint walls, no clay

25 - 30 2

Silty/sandy coatings, some clay

20 - 25 3

Kaolinite, mica, chlorite, talc, gypsum, graphite and/or some swelling clay

8 - 16 4

graphite and/or some swelling clay

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5 JOINT WATER REDUCTION J

  • 5. JOINT WATER REDUCTION, Jw

Description Jw Dry excavation or minor inflow 1 Large inflow, or high pressure in competent rock with unfilled joints 0.5 Exceptionally high inflow or pressure 0.1 - 0.05

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6 STRESS REDUCTION FACTOR

  • 6. STRESS REDUCTION FACTOR

Description σc : σ1 SRF Low stress, near surface rock 200 2.5 Medium stress 200 - 10 1 Mild rockburst (massive rock) 5 2 5 5 10 Mild rockburst (massive rock) 5 – 2.5 5 - 10

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Tunnels and the Q rating Tunnels and the Q rating

R i D d ESR

  • Require De and ESR

– De = equivalent dimension = ratio of excavation span or height to ESR – ESR = excavation support ratio ESR = excavation support ratio ESR = fn(the tunnel use & level of risk chosen) chosen)

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ESR Values (Barton et al 1974) ESR Values (Barton et al 1974)

Tem porary mine openings 3 5 Tem porary mine openings 3 - 5 l Perm anent mine openings, water tunnels for hydro power, etc. 1.6 - 2 Power stations, m ajor road & railw ay tunnels, etc. 1 Underground nuclear power stations, railway stations, etc. 0.8

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Tunnel Support Tunnel Support

Lining, e.g. shotcrete shotcrete Rockbolts

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DE Q

Shotcrete thickness

Q

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Areas within the chart Areas within the chart

  • area 1
  • unsupported
  • area 1
  • area 2
  • area 3
  • unsupported
  • spot bolting
  • systematic bolting (SB)
  • area 4
  • area 5

6

  • SB + 40‐50 mm shotcrete
  • SB + 50‐90 mm FRS
  • SB + 90 120 mm FRS
  • area 6
  • area 7
  • area 8
  • SB + 90‐120 mm FRS
  • SB + 120‐150 mm FRS
  • SB + 150‐120 mm FRS, ribbed
  • area 9

,

  • Cast concrete lining

FRS = fibre reinforced shotcrete FRS = fibre reinforced shotcrete

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Tunnels and the Q rating

Example: 10 m span ESR = 2 Q= 40

Tunnels and the Q rating

10 m span ESR = 1 Q= 40

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Tunnels and the Q rating

Example: 10 m span ESR = 1 Q 1 0

Tunnels and the Q ratingQ = 1.0

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Evaluation of Tunnels b d Q i based on Q rating

E l Example:

  • 10 m span & ESR = 2

Q 40

  • Q = 40

Area 1: UNSUPPORTED

  • 10 m span & ESR = 1
  • Q = 40

Area (2): SPOT BOLTING Requires rockbolts at 3 m spacing, 3 m long (max)

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Draft Layout Guidance for DUSEL Laughton, February 2006

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Rock Bolt length Rock Bolt length

Draft Layout Guidance for DUSEL Laughton, February 2006

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Quantifying shotcrete design Quantifying shotcrete design

Draft Layout Guidance for DUSEL Laughton, February 2006

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Typical Rock Support Typical Rock Support

Draft Layout Guidance for DUSEL Laughton, February 2006

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KEY POINTS? KEY POINTS?

R k ti t f l f

  • 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

( ) y y used for slope stability

  • The strength of rock masses can be judged
  • The strength of rock masses can be judged

from these systems