ROCK MECHANICS: L7

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ROCK MECHANICS L7

• Rock and Soils 2006
• Don Cameron

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HORIZONTAL VERTICAL STRESS

• Unlike soils, the ratio of lateral to vertical
  stress is commonly gt1 in the top 1 km
• high horizontal stresses are the result of
• movement of crustal plates
• plastic yield subsequent elastic deformation on
  unloading during denudation
• creep under sustained loads (on a geological time
  scale)
• topography
• tectonic forces
• general contraction of the earths crust

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Rock Earth Pressure Coefficient

• Empirically
• where h is the depth in metres below the surface

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Data from Brown Hoek 1978
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Australian Experience
Please note ?v ?mgh where ?m density of the
rock mass Density can vary between 2.12 t/m3 for
chalk to 5.17 t/m3 for magnetite
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ESTIMATING ROCK MASS STRENGTH

• The Hoek Brown rock mass yield criterion 1988-1992

?'1 the major principal stress for yield of the
rock mass ?'3 the minor principal stress ?c
the uniaxial compressive strength of the intact
rock mb, s a empirical constants for the
rock mass
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The Constants GSI

• The parameters mb, s a are defined by the
  empirically derived Geological Strength Index, or
  GSI
• GSI from either Q or RMR
• a) RMR
• Obtain RMRbasic assuming a groundwater rating of
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• For RMRbasic gt 23, GSI (RMRbasic 5)
• For RMRbasic lt 23, GSI from Q mass rating system

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The Constants GSI

• b) Q System
• Jw and SRF are both set to 1
• i.e. a dry rock under medium stress
• GSI 9lnQ 44

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Constants s a

• For GSI gt 25,

• For GSI lt25,

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Constant mb
mi fn(rock type) Typical values are tabulated
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Tested normal to foliation The value of mi
will be significantly different if failure occurs
along foliation
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Example

• SANTOS house in Currie Street in the city sits
  on Hallett Cove sandstone. A sewer pipe located
  10 m below the building has ruptured. You suspect
  that this rupture may have occurred because the
  rock surrounding the pipe has yielded.
• Determine the factor of safety of the rock at
  the depth of the pipe.

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Information

• Depth of pipe (h) 10 m
• Bearing pressure from building 1 MPa
• RMRbasic for the sandstone 33
• intact sandstone, ?c 35 MPa
• Density of the sandstone rock mass 1,800 kg/m3

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Solution

• ?v 1,176 kPa
• ?H K ?v (269/h 0.8) ?v 4,886 kPa
• ?H ?1 and ?v ?3
• GSI RMRbasic 5 28 gt 25

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Solution shear strength parameters

• mb mie((GSI 100)/28)
• mb 19e(27.7)
• mb 1.45
• s e((GSI 100)/9)
• s e-8 0.0003
• a 0.5

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Solution
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Solution

• ?1 yield 8,925 kPa
• FoS 1.8

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KEY POINTS?

• Lateral stresses in rock masses can be very high
  relative to vertical stresses
• The failure envelope for rocks is non-linear
• Empirical approximations are available for rock
  mass strength based on RMR or Q