Engineering Characterisation of Soil and Rock

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Engineering Characterisation of Soil and Rock

• Rock Core Logging

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Rock Core Logging

• Introduction
• Obtaining rock core is expensive
• Must therefore maximise amount of data recorded
• Should be presented in a readily understood form
• This involves the careful systematic logging of
  rock cores by a qualified engineering geologist
• Geological Society Engineering Group Working
  Party (1970). The logging of rock cores for
  engineering purposes. Quarterly Journal of
  Engineering Geology, 3, 1-24.

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Rock Core Logging

• Introduction
• It is recommended that the final log should
  include
• General Information
• Borehole number
• Location and orientation of borehole
• Drilling technique
• Contract details
• Drilling progress
• Drillers daily record
• Flush returns
• Standing water levels

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Rock Core Logging

• Introduction
• State of recovery of core
• Systematic rock sample description
• Rock grade classification
• We have covered the systematic rock sample
  description and grade classification

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Rock Core Logging

• Handling labelling core preservation
• Expensive information may be lost if cores are
  not properly labelled
• Cores are contained in core boxes
• Must be sound and robust
• Core is heavy so must be an appropriate size to
  be handled by two people
• Normally 1.5m long
• Should be made of hard board for durability

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Typical Core Box during drilling
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Rock Core Logging

• Core box labelling
• Core box reads as a book i.e. shallow core start
  at top left
• Box must have
• Site name
• Borehole number
• Contractors name
• Contract
• Labels should appear on the box lid plus on the
  side

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Typical core box fully labelled
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Rock Core Logging

• State of Recovery of the Core
• State of core recovery is dependent on the
  drilling methods used and the care taken
• This needs to be taken into account when
  analysing core recovery
• The state of the rock recovered is a valuable
  indicator of the in situ conditions and mass
  behaviour

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Rock Core Logging

• State of Recovery of the Core fracture state
• The core recovered can be divided into five
  categories
• Solid core gt 0.1m in length
• Solid core lt 0.1m in length
• Fragmented rock not recovered as core
• Additional rock lost from the core above
• Reduced length/diameter due to core loss (weak or
  friable rock)

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Rock Core Logging

• Solid Core
• This is the key term to be defined in the
  assessment of fracture state
• CORE WITH AT LEAST ONE FULL DIAMETER MEASURED
  ALONG THE CORE AXIS BETWEEN TWO NATURAL FRACTURES

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Rock Core Logging

• Quality of recovered core
• Total core recovery
• Solid core recovery
• Rock Quality Designation (RQD)
• Fracture Index

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Rock Core Logging

• Total Core Recovery
• Percentage of the rock recovered during a single
  core run
• Gives indication of material that has been washed
  into suspension of the presence of natural voids
• Solid Core Recovery
• Percentage of full diameter core recovered during
  a single core run
• Gives indication of fracture state

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Rock core logging

• Rock Quality Designation
• Percentage of constant diameter solid core
  greater than 0.1m in length recovered during a
  single core run
• Can give indication of fracture state
• Does not take into account changes in diameter of
  core
• Diameter of core should preferably not be less
  than 55mm

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Rock core logging

• Fracture Index (or fracture log)
• A count of the number of natural fractures and
  discontinuities measured over an appropriate
  length
• Gives indication of fracture state
• Logger decides on how to split the core into
  zones associated with different fracture spacing

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Rock Core Logging
GEO (1988). Geoguide 3 Guide to Rock and Soil
Descriptions. Geotechnical Engineering Office of
the Hong Kong SAR.
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Rock Core Logging

• Identification of man-made fractures
• Natural discontinuities will normally be
• Planar
• Discoloured - weathered
• Form in sets
• Sometimes in filled
• Where-as man made fractures will normally be
• Irregular
• Fresh
• random

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Drilling Induced Fracture