CVEN3202 Notes – Soil Mechanics
Phase Relationships
PHASE DEFINITIONS
• Soil is defined as the mixture of three phases; namely solid, water and
air
• The volume of water and air within a soil is known as void
• The phase diagram displays graphically the volume proportions of each
of the three phases
o Volume is depicted on the left hand side of the phase diagram,
mass is shown on the right side
• Two systems of measurements are commonly used in soil mechanics, SI
units and the CGS system
o The SI system revolves around metres, kilograms and Newtons
o The CGS system is comprised of centimetres, grams, seconds and
Dynes (0.001 Newtons)
PHASE RELATIONSHIPS
• The volumetric relationships commonly used in soil mechanics are as
follows:
o Void ratio (e) = VV/VS
o Porosity (n) = VV/V
o Saturation ratio (Sr) = VW/VV
• It follows that a soil with a saturation ratio of zero will be completely
dry, whilst a saturation ratio of one means a fully saturated soil
• The void ratio can theoretically be greater than one, whereas porosity
always will be a value between zero and one
• Water content is a mass proportion given as: WW/WS (expressed as a
percentage)
UNITS
• Basic unit definitions in soil mechanics are as follows:
o Density = Mass/Volume
o Unit weight = Weight/Volume = Mass*g/Volume
o Soil grain unit weight = Solid Weight/Solid Volume
o Specific gravity (GS) = Solid Density/Water Density
o Saturated unit weight = Total Weight (saturated)/Total Volume
o Dry unit weight = Total Weight (dry)/Total Volume
• The following assumptions can be made regarding phase relationships:
o If no information is provided, the specific gravity of a soil can
generally be assumed to equal 2.65 (the density of the soil is 2.65x
that of water)
o If no other relevant information is provided, VS can be assumed
to equal one, which will simplify the above relationships
o The weight of air is usually considered to equal zero
• Accuracy standards specific for CVEN3202 are below:
o To 0.1cm3 accuracy for volumes
o To 0.1g accuracy for weights
, o To 0.01g accuracy for unit weights
Classification
CLASSIFICATION STANDARDS
• Various classification systems have been developed to accurately define
soil samples, such as the MIT standard and the Australian standard
• The most commonly used standard for classifying soils is the Unified Soil
Classification System (USCS)
• According to USCS, a course grained soil is easily visible to the naked eye
and includes sands, gravel, cobbles and boulders
• Fine grained soils include silt and clay (are less than 0.075mm in size)
MAJOR SOIL TYPES
• The remainder of this section will be relevant only to the USCS standards
of soil classification
• The USCS system categorizes soil granules based on their size AND
behaviour in the presence of water
• Plasticity is the ability of soil to be moulded in shape
• Cohesiveness is the ability of a soil to hold together
• The definition of clay:
o Fine soil grains
o Highly plastic and cohesive
o Poor water drainage capability
o Highly compressible
o Hardens when dried
• The definition of silt:
o Also fine soil grains
o Non-plastic and not at all cohesive
o Reasonable water drainage capabilities
o Not compressible
• The size categorization scale of the USCS system is as follows:
o Cobble if >75mm grain size
, o Otherwise gravel if >4.75mm grain size
o Otherwise sand if >0.075mm grain size
o Otherwise clay or silt
GRAIN SIZE DISTRIBUTION
• Most soils found in nature have a wide variety of grain sizes, known as a
grain size distribution
• A well-graded soil will have a wide range of grain sizes of relatively even
distribution, whereas a poorly-graded soil will have an excess or
deficiency of certain grain sizes
• The grain size distribution and more specifically, the proportion of soil
grains of each size can be obtained by sieve analysis
o The proportion of grains that fall through each sieve can be
tabulated, then the grain size distribution graph is plotted
o There are 200 sieves used with gradually decreasing sizes
• Until now, the difference between well-graded and poorly-graded soil has
been largely subjective (not quantitative)
• To create a formal definition of well and poorly graded soil, two new
values are introduced:
o Coefficient of uniformity: Cu = D60/D10
o Coefficient of curvature: Cc = D230/(D10*D60)
• For gravels, the soil is well-graded only if Cu>4 and 1<Cc<3
• For sands, the soil is well-graded only if Cu>6 and 1<Cc<3
• Relative density compares the density of the sample (through void
ratios) to the range of densities exhibited by the given soil and can be
calculated as: Dr = (emax – e)/(emax – emin)
o The relative density essentially describes the level of compactness
of a soil compared to its loosest and densest states
• The grain size distribution is far less important for fine grained soils,
where plasticity is instead the critical characteristic
• The physical behaviours of a soil can be related to its water content
through the Atterberg limits
o Depending on the water content, a soil may be present in one of
four states: solid, semi-solid, plastic or liquid
o The Atterberg limits define the water content percentage at which
the soil sample changes between these four states
o The Atterberg limits can be ordered in terms of increasing water
content: Shrinkage Limit (SL), Plastic Limit (PL) and Liquid Limit
(LL)
• The plasticity index (PI) is defined as the difference between the liquid
limit and plastic limit
o A high plasticity index typically depicts a clay, whereas a low
plasticity usually indicates a silt-dominated soil
• The shrinkage limit (SL) is the water content at which any further loss of
water will no longer affect the volume of the soil
• The plastic index (PL) is the moisture content level at which a 3mm
thread of soil begins to crumble
FINDING THE LIQUID LIMIT
Phase Relationships
PHASE DEFINITIONS
• Soil is defined as the mixture of three phases; namely solid, water and
air
• The volume of water and air within a soil is known as void
• The phase diagram displays graphically the volume proportions of each
of the three phases
o Volume is depicted on the left hand side of the phase diagram,
mass is shown on the right side
• Two systems of measurements are commonly used in soil mechanics, SI
units and the CGS system
o The SI system revolves around metres, kilograms and Newtons
o The CGS system is comprised of centimetres, grams, seconds and
Dynes (0.001 Newtons)
PHASE RELATIONSHIPS
• The volumetric relationships commonly used in soil mechanics are as
follows:
o Void ratio (e) = VV/VS
o Porosity (n) = VV/V
o Saturation ratio (Sr) = VW/VV
• It follows that a soil with a saturation ratio of zero will be completely
dry, whilst a saturation ratio of one means a fully saturated soil
• The void ratio can theoretically be greater than one, whereas porosity
always will be a value between zero and one
• Water content is a mass proportion given as: WW/WS (expressed as a
percentage)
UNITS
• Basic unit definitions in soil mechanics are as follows:
o Density = Mass/Volume
o Unit weight = Weight/Volume = Mass*g/Volume
o Soil grain unit weight = Solid Weight/Solid Volume
o Specific gravity (GS) = Solid Density/Water Density
o Saturated unit weight = Total Weight (saturated)/Total Volume
o Dry unit weight = Total Weight (dry)/Total Volume
• The following assumptions can be made regarding phase relationships:
o If no information is provided, the specific gravity of a soil can
generally be assumed to equal 2.65 (the density of the soil is 2.65x
that of water)
o If no other relevant information is provided, VS can be assumed
to equal one, which will simplify the above relationships
o The weight of air is usually considered to equal zero
• Accuracy standards specific for CVEN3202 are below:
o To 0.1cm3 accuracy for volumes
o To 0.1g accuracy for weights
, o To 0.01g accuracy for unit weights
Classification
CLASSIFICATION STANDARDS
• Various classification systems have been developed to accurately define
soil samples, such as the MIT standard and the Australian standard
• The most commonly used standard for classifying soils is the Unified Soil
Classification System (USCS)
• According to USCS, a course grained soil is easily visible to the naked eye
and includes sands, gravel, cobbles and boulders
• Fine grained soils include silt and clay (are less than 0.075mm in size)
MAJOR SOIL TYPES
• The remainder of this section will be relevant only to the USCS standards
of soil classification
• The USCS system categorizes soil granules based on their size AND
behaviour in the presence of water
• Plasticity is the ability of soil to be moulded in shape
• Cohesiveness is the ability of a soil to hold together
• The definition of clay:
o Fine soil grains
o Highly plastic and cohesive
o Poor water drainage capability
o Highly compressible
o Hardens when dried
• The definition of silt:
o Also fine soil grains
o Non-plastic and not at all cohesive
o Reasonable water drainage capabilities
o Not compressible
• The size categorization scale of the USCS system is as follows:
o Cobble if >75mm grain size
, o Otherwise gravel if >4.75mm grain size
o Otherwise sand if >0.075mm grain size
o Otherwise clay or silt
GRAIN SIZE DISTRIBUTION
• Most soils found in nature have a wide variety of grain sizes, known as a
grain size distribution
• A well-graded soil will have a wide range of grain sizes of relatively even
distribution, whereas a poorly-graded soil will have an excess or
deficiency of certain grain sizes
• The grain size distribution and more specifically, the proportion of soil
grains of each size can be obtained by sieve analysis
o The proportion of grains that fall through each sieve can be
tabulated, then the grain size distribution graph is plotted
o There are 200 sieves used with gradually decreasing sizes
• Until now, the difference between well-graded and poorly-graded soil has
been largely subjective (not quantitative)
• To create a formal definition of well and poorly graded soil, two new
values are introduced:
o Coefficient of uniformity: Cu = D60/D10
o Coefficient of curvature: Cc = D230/(D10*D60)
• For gravels, the soil is well-graded only if Cu>4 and 1<Cc<3
• For sands, the soil is well-graded only if Cu>6 and 1<Cc<3
• Relative density compares the density of the sample (through void
ratios) to the range of densities exhibited by the given soil and can be
calculated as: Dr = (emax – e)/(emax – emin)
o The relative density essentially describes the level of compactness
of a soil compared to its loosest and densest states
• The grain size distribution is far less important for fine grained soils,
where plasticity is instead the critical characteristic
• The physical behaviours of a soil can be related to its water content
through the Atterberg limits
o Depending on the water content, a soil may be present in one of
four states: solid, semi-solid, plastic or liquid
o The Atterberg limits define the water content percentage at which
the soil sample changes between these four states
o The Atterberg limits can be ordered in terms of increasing water
content: Shrinkage Limit (SL), Plastic Limit (PL) and Liquid Limit
(LL)
• The plasticity index (PI) is defined as the difference between the liquid
limit and plastic limit
o A high plasticity index typically depicts a clay, whereas a low
plasticity usually indicates a silt-dominated soil
• The shrinkage limit (SL) is the water content at which any further loss of
water will no longer affect the volume of the soil
• The plastic index (PL) is the moisture content level at which a 3mm
thread of soil begins to crumble
FINDING THE LIQUID LIMIT
