Engineering Characterisation of Soil and Rock

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Engineering Characterisation of Soil and Rock

• Weathered Rock Masses

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Weathered Rock Masses

• Outline
• BS EN ISO 14689 12003 Guidelines on the scale
  of weathering of rock mass
• Classification for Granite
• Other systems

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Weathered Rock Masses

• Physical or mechanical weathering rock
  disintegrated by temperature changes, frost
  action and organisms
• Chemical weathering chemical changes in which
  rock minerals are decomposed, dissolved and
  loosened by water, oxygen and carbon dioxide of
  the atmosphere and ground water, and by organisms
  the products of decay
• Normally act in tandem i.e mechanical action
  fractures rock allowing chemical agents to
  penetrate deeper into the rock mass/material

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Weathered Rock Masses

• BS EN ISO 14689 12003
• Gives generic guidance on weathering
  classification
• Allows for the use of unambiguous systems
  developed for specific rock types
• These may be project dependent
• Country specific
• Internationally recognised as being applicable to
  all regions for a specific rock type

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Weathered Rock Masses
BS EN ISO 14689 12003
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Weathered Rock Masses

• Specific guidelines for chemically weathered
  granite
• Developed in Hong Kong but now universally used
• Need to remember that there is rock material and
  rock mass weathering

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Weathered Rock Masses

• Granite weathering
• Coarse grained intrusive igneous rock composed
  primarily of quartz, feldspars, mica, mafic
  minerals
• Formed at high temperatures and pressures
• Granite body uplifted and exposed at ground level
• High temperature minerals decompose to minerals
  that are more stable at the ambient temperatures
  and pressures

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Weathered Rock Masses

• Granite weathering
• Feldspars and micas
• revert to low temperature clay minerals
• kaolin (kaolinite halloysite)
• illite and chlorite

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Weathered Rock Masses

• Granite weathering
• Chemical weathering due to percolation of acidic
  ground water in the tropics this process is
  accelerated due to high ambient temperatures
• Drastically affects the strength and permeability
  of the rock material and mass

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Weathered Rock Masses

• Model for chemical weathering of granite
• Weathering starts at discontinuity boundaries
• Works its way in to the centre of the
  discontinuity bounded block
• Often leaves isolated corestones within a
  weathered matrix
• Highly complex geometry

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Weathered Rock Masses
Typical corestone profile in coarse grained
granite Mount Butler Quarry Hong Kong 2007
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Weathered Rock Masses
Typical corestone profile in coarse grained
granite Mount Butler Quarry Hong Kong 2007
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Weathered Rock Masses
Weathered granite profile cut slope at Deep Bay
Link Hong Kong
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Weathered Rock Masses
Weathering fronts picked out by iron staining in
completely decomposed granite Deep Bay Link
Hong Kong
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Weathered Rock Masses
Corestone surrounded by completely decomposed
granite Northern Spain
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Weathered Rock Masses
Typical shallow weathering profile in a well
jointed fine grained volcanic Tuff High Island
Reservoir Hong Kong 2007
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Weathered Rock Masses
Sharp interface between weathered and
un-weathered fine ash Tuff Chai Wan Estate
Hong Kong
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Weathered Rock Masses
GEO (1988). Geoguide 3 Guide to Rock and Soil
Descriptions. Geotechnical Engineering Office of
the Hong Kong SAR.
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Weathered Rock Masses
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Weathered Rock Masses

• Guide to identification of granite weathering
  grade by visual examination
• Note
• Loss of rock fabric
• Change in colour

GEO (1988). Geoguide 3 Guide to Rock and Soil
Descriptions. Geotechnical Engineering Office of
the Hong Kong SAR.
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Weathered Rock Masses
GEO (1988). Geoguide 3 Guide to Rock and Soil
Descriptions. Geotechnical Engineering Office of
the Hong Kong SAR.
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Weathered Rock Masses
40m high cut slope in weathered granite Deep
Bay Link Hong Kong June 2005 note large
partially weathered zone within completely
decomposed rock mass
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Weathered Rock Masses - Chalk
Table 3.1 Classification of Chalk at the Mundford
Site in Lord, J.A., Clayton, C.R.I. and
Mortimore, R.N. (2002). Engineering in Chalk,
C574. CIRIA.
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Weathered Rock Masses - Mudrocks
Classification of weathered mudrocks after Crips
J.C. and Taylor R. K. (1981) The engineering
properties of mudrocks. Quarterly Journal of
Engineering Geology. 14, 325-346
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Weathered Rock Masses - mudrocks
Mechanical weathering of a calcareous fissile
mudrock (Terre Noir) Supersuaze French Alps
2007. Weathering by freeze thaw and dissolution
of calcite from matrix