Soil Mechanics Fundamentals - A
Review
John Sturman, Rutgers University 2003
Introduction
Soils are three-phase materials consisting
of solid matter (usually rock with some
organic matter), water, and air. The
engineering properties of soils are either
determined by direct testing representative
samples under conditions (stress, saturation,
etc.) of concern or through index properties.
Index Properties and Soil
Classification
Many engineering properties can be correlated
with the index properties and classification
of soils. Key index properties include:
• Grain size distribution, shape, and
uniformity
• Void ratio/porosity
• Water Content and Degree of Saturation
Key Index Properties (continued)
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Specific Gravity
Unit weight or Density
Relative Density (sands)
Liquid and Plastic Limits
Unified Soil Classification Type
Unified Soil Classification
• A system to group soils by similar
constituents to predict behavior.
• Primarily sorts by granular (sands and
gravels) and fine-grained (silts and clays).
• Uses grain size distribution, plastic limit,
and liquid limit mainly.
• In-situ water content and density is
independent of soil classification.
Plasticity
• An index property used to estimate the
behavior of a soil under varying moisture
conditions, especially shrink-swell
• Tested in the laboratory using the Atterberg
Limit Tests
• The term Plasticity Index is defined as the
difference between the liquid and plastic
limits PI = LL - PL
Shear Strength
• A function of normal stress for most soils
under typical conditions
• We use the Mohr-Coulomb Failure envelope
to model the upper bound of stress that can
be imposed on a soil
• The formula S = C + σ (tan Ø) expresses
shear strength of soil
Cohesion
• The interparticle force binding a soil mass
together
• The inherent shear strength of a soil under
no confining stress
• Cohesion is assumed to be zero for clean
sands and normally consolidated clays
(tested under CU conditions)
Internal Angle of Friction
• Relates shear strength to normal stress
• Found from direct shear tests or triaxial
compression tests in the laboratory
• Correlated to density and relative density by
various field tests including the SPT and
CPT
• Greater for granular than cohesive soils
• Increases with density & angularity
Relationship of Ø v. DR
Types of Triaxial Testing
• Consolidated-Drained (CD)
• Consolidated-Undrained (CU)
• Unconsolidated-Undrained (UU)
and Unconfined Compression (UC), a special
type of UU
Seepage and Permeability
Laminar flow through saturated porous media
explained by Darcy Equation
v= ki
v = velocity
k = coefficient of permeability
i = flow gradient = - Δ h/ΔL
Seepage and Permeability
(continued)
• For most soils, k is directly proportional to
grain size & inversely proportional to
density
• k is best correlated with D10
Example: Hazen Eqn k= C D10 where k is
expressed in mm/sec, D10 is in mm and C
ranges between about 10 and 15
Seepage and Permeability
(continued)
• In cohesive soils, permeability is often
anisotropic due to the structure of clays
• Remolded (recompacted) soils can have
“engineered” permeability based on the type
of compactive stress and the moisture
content
Effective Stress
• We are concerned with the forces
experienced by a soil over a cross-sectional
area
• The soil bears the loads through solid-tosolid contacts
• In a saturated soil mass, the pore water
carries the load in the void spaces
Effective Stress (continued)
• In a static water situation, pore water
pressure u is defined as γwater . Hwater
(or ρgH)
• In a seepage condition, the pore water
pressure may be up, down, or horizontal
• Assuming a static condition, the total stress
σ is equal to the pore water pressure u plus
the reduced solid-to-solid stress σ’
Effective Stress (continued)
So, σ = u + σ’ and σ’ = σ - u
We define σ’ as the effective stress (a derived
term)
And we can express γ in saturated conditions
as γ’ where γ’ = γSAT - γWAT = the effective
or submerged unit weight
Effective Stress (continued)
We note that u can be negative due to capillary
forces in unsaturated soils
Capillary force is inversely proportional to
grain size. D10 again is a good correlation
One-Dimensional Consolidation
• Saturated Clays, Silts, and Silt-Clay
mixtures exhibit a time-dependent stressstrain behavior in response to changes in
effective stress.
• We must consider the change in effective
stress at the depth of concern, and know the
one-dimensional stress-strain behavior of
the soil at that depth
Consolidation
• The compressibility of a cohesive soil is
characterized by laboratory consolidation
testing. After adding loads, the additional
pore water is slowly expulsed.
• Measurements are taken to characterize the
void ratio, e, and density state of the soil at
equilibrium with that load. The time
required to reach equilibrium is also an
important measurement.
Consolidation
• The pressure-soil volume relationship is not
a simple linear relationship. We
approximate the relationship as two lines
joining at a single point on a graph of the
void ratio v. the log (base 10) of the
pressure. The point of inflection is termed
the preconsolidation pressure.
Consolidation
• The slope of the e-log P graph for pressures
less than the preconsolidation pressure is
termed the recompression index.
• The slope of the e-log P graph for pressures
greater than the preconsolidation pressure is
termed the compression index.
• The compression index is greater as the
recompression index represents the previous
stress states of the soil.
Consolidation
• When a soil has a preconsolidation pressure,
Pp, greater than it’s current effective
overburden stress, Po, the soil is termed
oversonsolidated. Overconsolidation results
from a variety of factors including previous
glacial loading, removal of previous layers,
rise in the ground water table.
• Overconsolidation ratio OCR = Pp/Po