Alternative Rock Mass Classification for Tunnel

Design in Sydney

Robert Bertuzzi and Andrew de Ambrosis

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16th Australasian Tunnelling Conference

30 October - 1 November 2017, Sydney

Pells Sullivan Meyninkfrom Pells et al (1998) and Bertuzzi & Pells (2002)

Sydney Classification System

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Mapped as

Class I

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Mapped as

Class II

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Mapped as

Class III

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Scale

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Class I

Class II

Class III

Class IV

Class V

0.001 0.01 0.1 1 10 100 1000

Q

Ash

fiel

d S

hal

e

Class I

Class II

Class III

Class IV

Class V

0.001 0.01 0.1 1 10 100 1000

Q

Haw

kesb

ury

San

dst

on

e

Q values from Bertuzzi (2014)

𝑄 =𝑅𝑄𝐷

𝐽𝑛×𝐽𝑎𝐽𝑟×𝑆𝑅𝐹

𝐽𝑤

Insensitive to defect spacing category

Relatively insensitive to Class

Overlap

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Failure mechanisms

Shallow stress induced failure in combination with discontinuity and gravity controlled failure

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Discontinuity controlled, gravity induced falling and sliding of blocks, occasional local shear failure on discontinuities

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Stress-induced spalling

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Target

Factors important to tunnel design in Sydney

• The magnitude of insitu stress, which to the most part is

dictated by tunnel depth

• Intact strength

• Fracturing of the rock mass

• The position of bedding plane partings relative to the

excavated boundary of the tunnel

• The size of the tunnel.

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Intact strength

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Fracturing

In underground excavation, 25 MPa Hawkesbury Sandstone with joints at an

average spacing of 0.5 m behaves differently to massive 10 MPa Hawkesbury

Sandstone.

Yet both are Class III.

The presence of bedding plane partings or clay seams or fragmented zones close

to the excavated crown has a marked impact on tunnel support.

Dependent on the tunnel span. A bedding plane parting 1 m above the crown has

less of an impact on a 5 m wide tunnel than it does on a 20 m wide cavern.

It is suggested that critical height above the tunnel crown is 10% of the tunnel span

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Proposed Matrix

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Comparison

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