1GS235 GEOTECHNICS

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• 1GS235 GEOTECHNICS

SCHOOL OF EARTH ENVIRONMENTAL SCIENCES Soil
Description and Classification Part 2
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Soil Description Classification

• Specific Gravity
• Volume Weight Properties
• Volumes of solid, water air the soil model
• Degree of saturation
• Air - voids content
• Masses of solid and water water content
• Densities and unit weights
• Current state of soil
• Soil history deposition and erosion
• Soil history ageing
• Density index (relative density)
• Liquidity Index
• Predicting stiffness and strength from index
  properties

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Specific Gravity

• Specific gravity (Gs) is a property of the
  mineral or rock material forming soil grains

http//www.mineralab.com/AboutSG.htm
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Volume - Weight Properties

• Volumes of solid, water air The soil model
• Masses of solid Water Water content
• Densities unit weights

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Volumes of Solid, Water Air The Soil Model

• Degree of saturation
• Air-voids content

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Volume - Weight Properties
Masses
Volumes
VA VW Sr e
Vv e VS 1
Mw Sr e ?w w Gs ?w Ms Gs ?w
V 1 e
The volume-weight properties of a soil define its
state. Measures of the amount of void space,
amount of water and the weight of a unit volume
of soil are required in engineering analysis and
design.
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Volume - Weight Properties
Soil comprises three constituent phases Solid
rock fragments, mineral grains or flakes, organic
matter. Liquid water, with some dissolved
compounds (e.g. salts). Gas air or water
vapour. In natural soils the three phases are
intermixed. To aid analysis it is convenient
to consider a soil model in which the three
phases are seen as separate, but still in their
correct proportions.
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Volumes of Solid, Water Air The Soil Model
Masses
Volumes
VA VW Sr e
Vv e VS 1
Mw Sr e ?w w Gs ?w Ms Gs ?w
V 1 e
The soil model is given dimensional values for
the solid, water and air components Total
volume, V Vs Vw Va
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Volumes of Solid, Water Air The Soil Model
Masses
Volumes
VA VW Sr e
Vv e VS 1
Mw Sr e ?w w Gs ?w Ms Gs ?w
V 1 e
Since the amounts of both water and air are
variable, the volume of solids present is taken
as the reference quantity. Thus, the following
relational volumetric quantities may be defined

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Volumes of Solid, Water Air The Soil Model
Masses
Volumes
VA VW Sr e
Vv e VS 1
Mw Sr e ?w w Gs ?w Ms Gs ?w
V 1 e
Note also that n e / (1 e) e n / (1 -
n) v 1 / (1 - n) Typical void ratios might
be 0.3 (e.g. for a dense, well graded granular
soil) or 1.5 (e.g. for a soft clay).
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Degree of Saturation
Masses
Volumes
VA VW Sr e
Vv e VS 1
Mw Sr e ?w w Gs ?w Ms Gs ?w
V 1 e
The volume of water in a soil can only vary
between zero (i.e. a dry soil) and the volume of
voids this can be expressed as a ratio For
a perfectly dry soil Sr 0 For a saturated
soil Sr 1
Note In clay soils as the amount water increases
the volume and therefore the volume of voids will
also increase, and so the degree of saturation
may remain at Sr 1 while the actual volume of
water is increasing.
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Air - Voids Content
Masses
Volumes
VA VW Sr e
Vv e VS 1
Mw Sr e ?w w Gs ?w Ms Gs ?w
V 1 e
The air-voids volume, Va , is that part of the
void space not occupied by water. Va Vv - Vw
e - e.Sr e.(1 - Sr)
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Air - Voids Content
Masses
Volumes
VA VW Sr e
Vv e VS 1
Mw Sr e ?w w Gs ?w Ms Gs ?w
V 1 e
Air-voids content, Av Av (air-voids volume) /
(total volume) Va / V e.(1 - Sr) / (1e)
n.(1 - Sr) For a perfectly dry soil For a
saturated soil Av n Av 0
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Masses of Solid Water Water Content
Masses
Volumes
VA VW Sr e
Vv e VS 1
Mw Sr e ?w w Gs ?w Ms Gs ?w
V 1 e
The mass of air may be ignored. The mass of solid
particles is usually expressed in terms of their
particle density or grain specific gravity.
Grain specific gravity
Hence the mass of solid particles in a soil Ms
Vs .Gs .?w (?w density of water 1.00Mg/m³)
Range of Gs for common soils 2.64-2.72
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Masses of Solid Water Water Content
Masses
Volumes
VA VW Sr e
Vv e VS 1
Mw Sr e ?w w Gs ?w Ms Gs ?w
V 1 e
The mass of air may be ignored. The mass of solid
particles is usually expressed in terms of their
particle density or grain specific gravity.
Particle density ?s mass per unit volume of
particles Gs . ?w The ratio of the mass of
water present to the mass of solid particles is
called the water content, or sometimes the
moisture content.
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Masses of Solid Water Water Content
Masses
Volumes
VA VW Sr e
Vv e VS 1
Mw Sr e ?w w Gs ?w Ms Gs ?w
V 1 e
The mass of air may be ignored. The mass of solid
particles is usually expressed in terms of their
particle density or grain specific gravity.
Water Content
From the soil model it can be seen that w
(Sr .e .rw) / (Gs .rw) Giving the useful
relationship w .Gs Sr .e
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Densities Unit Weights
Masses
Volumes
VA VW Sr e
Vv e VS 1
Mw Sr e ?w w Gs ?w Ms Gs ?w
V 1 e
Density is a measure of the quantity of mass in a
unit volume of material. Unit weight is a
measure of the weight of a unit volume of
material. There are two basic measures of
density or unit weight applied to soils Dry
density is a measure of the amount of solid
particles per unit volume. Bulk density is a
measure of the amount of solid water per unit
volume.
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Densities Unit Weights
Masses
Volumes
VA VW Sr e
Vv e VS 1
Mw Sr e ?w w Gs ?w Ms Gs ?w
V 1 e
The preferred units of density are Mg/m³, kg/m³
or g/ml.
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Densities Unit Weights
Masses
Volumes
VA VW Sr e
Vv e VS 1
Mw Sr e ?w w Gs ?w Ms Gs ?w
V 1 e
The corresponding unit weights are
Also, it can be shown that ? ?d (1 w)
and ? gd (1 w)
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Laboratory Measurements

• Water content
• Unit weight
• It is important to quantify the state of a soil
  immediately it is received in the testing
  laboratory just prior to commencing other tests
  (e.g. shear tests, compression tests, etc.)
• The water content unit weight are particularly
  important, since these could change during
  transportation storage
• Some physical state properties are calculated
  following the practical measurement of others
  e.g. void ratio from porosity, dry unit weight
  from unit weight water content

http//hjs.geol.uib.no/hovedlab/analysis_core_wate
r_content_procedure_eng.html
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Current State of Soil

• Soil History
• Deposition erosion
• Ageing
• Density Index (Relative Density)
• Liquidity Index

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Current State of Soil

• The state of soil is essentially the closeness of
  packing of the grains in the range
• Closely packed ? Loosely packed
• Dense ? Loose
• Low water content ? High water content
• Strong stiff ? Weak and soft

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Current State of Soil

• The important indicators of the current state of
  a soil are
• Current stresses
• Vertical and horizontal effective stresses
• Current water content
• Effecting strength and stiffness in fine soils
• Liquidity Index
• Indicates state in fine soils
• Density Index
• Indicates state of compaction in coarse soils
• History of loading Unloading
• Degree of overconsolidation

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Current State of Soil

• Engineering operations (e.g. excavation, loading,
  unloading, compaction, etc.) on soil bring about
  changes in its state
• Its initial state is the result of processes of
  erosion deposition
• It is possible for the engineer to predict
  changes that could result from a proposed
  engineering operation
• Changes from the soil's current state to a new
  future state

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Soil History Deposition Erosion

• Original deposition
• Most soils are formed in layers or lenses by
  deposition from moving water, ice or wind
• One - dimensional compression occurs as overlying
  layers are added
• Vertical and horizontal stresses increase with
  deposition

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Soil History Deposition Erosion

• Erosion
• Erosion causes unloading stresses decrease some
  vertical expansion occurs
• Plastic strain has occurred the soil remains
  compressed, i.e. overconsolidated

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Soil History Deposition Erosion

• Subsequent changes
• Subsequent changes may occur in the depositional
  environment
• Further loading/unloading due to glaciation, land
  movement, engineering ageing processes

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Soil History Ageing

• The term ageing includes processes that occur
  with time, except loading unloading
• Ageing processes are independent of changes in
  loading

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Soil History Ageing

• Vibration compaction
• Coarse soils can be made more dense by vibration
  or compaction at essentially constant effective
  stress

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Soil History Ageing

• Creep
• Fine soils creep continue to compress distort
  at constant effective stress after primary
  consolidation is complete

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Soil History Ageing

• Cementing bonding
• Intergranular cementing bonding occurs due to
  deposition of minerals from groundwater,
• e.g. calcium carbonate disturbance due to
  excavation fractures the bonding reduces
  strength

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Soil History Ageing

• Weathering
• Physical chemical changes take place in soils
  near the ground surface due to the influence of
  changes in rainfall and temperature

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Soil History Ageing

• Changes in salinity
• Changes in the salinity of groundwater are due to
  changes in relative sea land levels, thus soil
  originally deposited in sea water may later have
  fresh water in its pores, such soils may be prone
  to sudden collapse

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Density Index (Relative Density)

• The void ratio of coarse soils (sands gravels)
  varies with the state of packing between the
  loosest practical state in which it can exist
  the densest
• Some engineering properties are affected by this
• e.g.shear strength, compressibility, permeability
• It is therefore useful to measure the in situ
  state this can be done by comparing the in situ
  void ratio (e) with the minimum maximum
  practical values (emin emax) to give a density
  index ID

emin is determined with soil compacted densely in
a metal mould emax is determined with soil
poured loosely into a metal mould
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Density Index (Relative Density)

• Density index is also known as relative density
• Relative states of compaction are defined
• Density Index State of compaction
• 0 - 15 Very loose
• 15 - 35 Loose
• 35 - 65 Medium
• 65 - 85 Dense
• 85 - 100 Very dense

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Liquidity Index
Volume
Liquid
In-situ Water Content, w
Plastic
Semi - Solid
Solid
Va Vs
Water Content
WS WP W WL
In fine soils, especially clays, the current
state is dependent on the water content with
respect to the consistency limits (or Atterberg
limits). The Liquidity Index (IL or LI) provides
a quantitative measure of the current state
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Liquidity Index
Volume
Liquid
In-situ Water Content, w
Plastic
Semi - Solid
Solid
Va Vs
Water Content
WS WP W WL
where wP plastic limit wL liquid limit
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Liquidity Index
Significant values of IL indicating the
consistency of the soil are IL lt 0 ?
Semi-plastic solid or solid 0 lt IL lt 1 ?
Plastic 1 lt IL ? Liquid
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Further Reading
GeotechniCAL Educational Technology for Ground
Engineering http//fbe.uwe.ac.uk/public/geocal/geo
web.htm