Engineering Characterisation of Soil and Rock

1
Engineering Characterisation of Soil and Rock

• Engineering Description of Soil

2
Engineering Description of Soil

• Introduction
• New International Standard ESN ISO 14688 - 2002
• Details of part 1 of new standard

3
Engineering Description of Soil

• Formal system of soil description
  classification
• Comprehensive, meaningful concise
• Description of soil
• A statement describing the physical nature
  state of the soil.
• Sample or a soil in situ
• Visual examination
• Simple tests
• Observation of site conditions, geological
  history, etc

4
Engineering Description of Soil

• Soil description for engineering purposes must
  follow a consistent system of order and terms so
  that when a soil is described any other qualified
  person will understand what is meant
• In the UK we have been using the system defined
  in BS59301999
• This BS has now been replaced by a new
  International Standard
• International Standard EN ISO 14688 12002.
  Geotechnical investigations and testing
  identification and classification of soil

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Engineering description of soil

• EN ISO 14688 12002 is split into two parts
• Part 1 Identification and Description
• Part 2 Principles for Classification
• Part 1 is covered in this course and deals with
  the identification and description of soils in
  the field
• EN ISO 14688 12002 is a highly flexible
  framework as may be expected it allows adoption
  of existing systems of description with some
  subtle changes
• Unlike BS5930 is does not force the user to
  describe the soil in any particular order but
  defines terms and simple tests to aid in the
  description

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Engineering description of soil

• EN ISO 14688 12002
• Split into three main parts
• Definitions
• Identification of Soil
• Methods for identification and description
• Definition of Soil
• Assemblage of mineral particles and/or organic
  matter in the form of a deposit but sometimes of
  organic origin, which can be separated by gentle
  mechanical means and which does include variable
  amounts of water and air

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Methods of identification description of soil

• Soil description examples
• Firm to stiff high strength fissured dark
  reddish-brown slightly sandy high plasticity CLAY
  (Reading Beds). Fissures closely spaced (0.1 to
  0.4m) and smooth with occasional slickensides.
• Medium dense light orange brown slightly silty
  very sandy medium to fine GRAVEL (Taplow Gravel).
  Gravel is sub-angular, smooth of quartz and sand
  is coarse angular of quartz.

8
Methods of identification description of soil

• This system allows us to utilise the BS5930
  description order
• Consistency/Field Strength (fine soils only)
• Density/compactness (coarse soils only)
• Discontinuities
• Bedding
• Colour
• Composite soil types
• Principal soil type or types in CAPITALS
• Stratum Name
• Geological formation, age and type of deposit
  (optional) ONLY USE IF CERTAIN - in brackets

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Figure 1 EN ISO 14688 12002. Flow chart for
the identification and description of soils

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Identification of Soil

• Soils can be split into two distinct groups
• Very coarse coarse soils boulders, cobbles,
  gravels and sands
• Fine soils silt and clay
• The boundary between the two is at 0.063mm
  (BS5930 0.06mm)

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Table 1 EN ISO 14688 12002.
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Identification of Soil

• Composite soils - primary fraction
• Soils generally are composite. They are
  designated by a main term describing the
  principle fraction and by qualifying terms to
  describe the secondary fraction sandy GRAVEL or
  clayey SILT
• Controls the engineering behaviour
• Coarse and very coarse soils in terms of mass
• Fine soil in terms of what predominates the
  engineering properties of the soil

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Identification of Soil

• Composite soils secondary fraction
• Secondary fraction does not determine but will
  affect the engineering properties of the soil
• Secondary fractions placed with the term
  describing the principle fraction in order of
  importance
• Fine sandy CLAY
• Coarse sandy fine GRAVEL
• Slightly sandy clayey fine SILT
• Fine fraction silt or clay can be determined by
  plasticity properties
• For coarse soils of equal proportions then
  describe as GRAVEL/SAND or fine/medium SAND

14
Identification of Soil

• Plasticity
• Soils which contain enough fine grained
  constituents to allow the PL/LL to be determined
  can be defined as plastic
• Can use the terms
• Low plasticity
• High Plasticity
• Slightly sandy high plasticity CLAY

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Consistency Limits Plasticity

• Consistency varies with the water content of the
  soil
• The consistency of a soil can range from (dry)
  solid to semi-solid to plastic to liquid (wet)

16
Identification of Soil

• Peat and other organic soils
• Peat is low density and has a distinctive odour
  described later
• Described according to degree of decomposition
• If contains mineral constituents then describe as
  
• slightly silty PEAT

EN ISO 14688 12002.
17
Identification of Soil

• Volcanic Soils
• Not so important in the UK but wide variety in
  Europe (France and Italy in particular)
• Characterised by being light weight and normally
  dark in colour
• Described according to particle size, structure
  and colour

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EN ISO 14688 12002.
19
Identification of Soil

• Discontinuities and bedding
• Discontinuities in the field or undisturbed
  fine soil samples
• Joints fissures and shear surfaces
• Spacing
• Surface texture
• Orientation if in situ persistence

Coastal exposure of fissured London Clay Isle
of Sheppey Kent
20
Identification of Soil

• Discontinuities and bedding
• Bedding in the field
• Bedding thickness
• Interbedded
• Laminated (undisturbed samples also)
• Orientation

Thinly bedded and laminated volcanic lacustarine
high plasticity very stiff clay Claremont -
France
21
Identification of Soil

• Origin of deposit
• Geological information, age and type of deposit
• Found on geological maps or memoirs
• Prefer to put this in brackets
• May or may not indicate some engineering
  characteristics

22
Methods of identification description of soil

• Particle size distribution
• Particle shape
• Mineral composition
• Fines content
• Colour
• Dry strength
• Dilatancy
• Plasticity

• Sand silt clay content
• Carbonate content
• Identification of organic soil
• Degree of decomposition
• Volcanic soils
• Consistency or strength

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Methods of identification description of soil

• Very Coarse
• Large Boulders gt630mm
• Boulders gt200 lt 630mm
• only seen complete in pits or exposures
• Cannot identify in boreholes but may be able to
  interpret from boreholes where coring is carried
  out

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Methods of identification description of soil

• Visual Identification of Soils Very Coarse
• Cobbles
• gt 63mm lt 200mm
• difficult to recover from boreholes
• Best observed and described in exposures or in
  trial pits
• Very difficult to recover from conventional
  boreholes

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Engineering Description of Soil

• Visual Identification of Soils Coarse
• Gravel
• gt2mm lt63mm
• visible to the naked eye
• see shape and grading

26
Engineering Description of Soil

• Visual Identification of Soils Coarse
• Sand
• lt2mmgt0.063mm
• visible to the naked eye
• does not stick together when dry or fully
  saturated

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Engineering Description of Soil

• Visual Identification of Soils Fine
• Silt lt0.063 gt0.002mm
• Coarse silt visible with hand lens
• low plasticity and bleeds water when vibrated
  (dilatant)
• disintegrates in water
• dry quickly
• sticks together when wet but powdered easily
  between figures

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Engineering Description of Soil

• Visual Identification of Soils Fine
• Clay lt0.002mm
• dries into hard lumps
• Smooth
• sticks together when wet but is not dilatant
• dries slowly
• shrinks when dries
• Can be rolled into 3mm strands easily

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Methods of identification description of soil

• Particle shape
• coarse grained soils
• Angularity
• Form
• Surface texture
• Mineral Composition

GEO (1988). Geoguide 3 Guide to Rock and Soil
Descriptions. Geotechnical Engineering Office of
the Hong Kong SAR.
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Methods of identification description of soil

• Soil Colour
• Overall impression
• More than one colour is mottled i.e. light grey
  mottled brown
• Colour charts are useful as everyone sees colours
  differently

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Methods of identification description of soil

• Colour description scheme
• Enables a more systematic approach to colour
  descriptions

GEO (1988). Geoguide 3 Guide to Rock and Soil
Descriptions. Geotechnical Engineering Office of
the Hong Kong SAR.
32
Methods of identification description of soil

• Determination of dry strength
• Provides information on the soil plasticity and
  in turn helps identification of silt or clay
• Dry a sample of soil
• Low dry strength disintegrates under light to
  moderate finger pressure - silt
• Medium dry strength disintegrates only under
  substantial finger pressure silt/clay
• High dry strength can only be broken - clay

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Methods of identification description of soil

• Determination of dilatancy
• Indicates silt and clay content
• Moistened sample 10 to 20mm shaken from hand to
  hand
• Sample becomes shiny
• Press sample with finger water disappears
• Water disappears rapidly silt
• Shaking and pressing no effect clay

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Methods of identification description of soil

• Determination of plasticity
• Moistened sample rolled to produce 3mm threads
• Low plasticity having cohesion but cannot be
  rolled into 3mm threads high silt content
• High plasticity sample can be rolled into 3mm
  threads high clay content

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Methods of identification description of soil

• Determination of sand, silt and clay content
• Rub soil between fingers sand feels gritty and
  can be seen with naked eye silt also feels
  gritty but cannot be seen with naked eye
• Cut sample with knife if shiny surface then
  high clay content if dull then silt or clayey
  sandy silt low PI

36
Methods of identification description of soil

• Carbonate content
• Add bench hydrochloric acid
• Carbonate free no effervescence
• Calcareous clear but not sustained
  effervescence
• Highly calcareous strong sustained
  effervescence

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Methods of identification description of soil

• Organic Soils
• Odour is used to distinguish organic from
  non-organic
• Organic soils mouldy odour intensifies with
  heating
• Rotten organic soils hydrogen sulphide

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Methods of identification description of soil

• Degree of decomposition of organic soils
• Squeezing wet sample
• If dry then assess on appearance plant remains
  visible etc

EN ISO 14688 12002.
39
Methods of identification description of soil

• Determination of consistency
• Very soft exudes between fingers
• Soft easily remoulded
• Firm cannot be moulded but can be rolled into
  threads
• Stiff breaks when rolled into 3mm threads but
  can be moulded into lump
• Very stiff crumbles under pressure - can be
  indented by thumb nail

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Methods of identification description of soil

• Soil Strength
• Can only be determined in the field by using a
  simple field test such as a hand vane or pocket
  penetrometer
• Do not get confused between consistency and
  undrained strength

EN ISO 14688 12002.
41
Methods of identification description of soil

• Consistency of coarse grained soils is not
  covered in EN ISO 14688 as field determination
  without in situ tests is subjective
• Density of Coarse soils Only by Standard
  Penetration Test (N-values)
• Very loose - N 0 to 4
• Loose N 4 to 10
• Medium Dense N 10 to 30
• Dense N 30 to 50
• Very Dense N gt 50
• Relation developed by Terzaghi Peck (1948) for
  transported sands only but generally applied to
  all coarse soils in the UK is used for residual
  soils but must be careful - does not take into
  account relict rock structure often observed in
  these soils

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Methods of identification description of soil

• Man-made soils
• Either termed FILL or RECONSTITUTED GROUND
• FILL - placed in a controlled manner - engineered
• RECONSTITUTED GROUND - placed without control
  dumped or fly tipped
• If reworked natural soil then describe as natural
  soil but put (FILL or RECONSTITUTED GROUND in
  brackets). Can be difficult to identify
• Medium dense clayey coarse GRAVEL (FILL)
• If RECONSTITUTED GROUND containing man made
  objects then can put RECONSTITUTED GROUND first
• RECONSTITUTED GROUND Loose light grey gravely
  SAND with many brick fragments, plastic pipes,
  reinforcing bars and timber fragments. Voids up
  to 55mm in diameter common.