Soil Mechanics

SOIL

An uncemented aggregate of mineral grains and decayed organic matters (solid

particles) with liquid and gas in the empty spaces between the solid particles.

SOIL MECHANICS

Branch of science that deals with the study of the physical properties of soil and the

behavior of soil masses subjected to various types of forces

SOIL ENGINEERING

Application of the principles of soil mechanics to practical problems.

GEOTECHNICAL ENGINEERING

Subdiscipline of civil engineering that involves natural materials found close to the

surface of the earth.

PRECLASSICAL SOIL MECHANICS (1700 – 1776)

Concentrated on the studies relating to natural slope and unit weights of

various types of soils as well as the semiempirical earth pressure theories.

Henri Gautier

studied the natural slopes of soils when tipped in a heap

for formulating the design procedures of retaining walls.

Bernard Forest de Belidor

published a book proposing a theory for lateral

earth pressure on retaining walls following up Gautier’s original study.

Charles Augustin Coulomb

used the principles of calculus for maxima

and minima to determine the true position of the sliding surface in soil

behind a retaining wall.

Jean Victor Poncelet

extended Coulomb’s theory by providing a

graphical method for determining the magnitude of the lateral earth

pressure on vertical and inclined retaining walls with arbitrarily broken

polygonal ground surfaces.

William John Macquorn Rankine

provided a study on a notable theory

on earth pressure and equilibrium of earth masses.

Henri Philibert Gaspard Darcy

defined the term coefficient of

permeability or hydraulic conductivity of soil.

Joseph Valentin Boussinesq

developed the theory of stress distribution

under load-bearing areas in a homogeneous, semi-infinite, elastic, and

isotropic medium.

Albert Mauritz Atterberg

defined clay-sized fractions as the percentage

by weight of particles smaller than 2 microns in size. He explained the

consistency of cohesive soils by defining liquid, plastic, and shrinkage

limits.

Arthur Langley Bell

developed relationships for lateral pressure and

resistance in clay as well as bearing capacity of shallow foundations in

clay.

Wolmar Fellenius

developed the stability analysis of undrained saturated

clay slopes with the assumption that the critical surface of sliding is the arc

of a circle.

Karl Terzaghi

developed the theory of consolidation for clays.

Karl Terzaghi

Father of Modern Soil

Mechanics

(ISSMFE)

International Society of Soil Mechanics and

Foundation Engineering

American Society for Testing Materials (ASTM)

carries out the laboratory

and field tests on soils, as for any other materials per standard method.

04.08 and 04.09

The test methods for soils, rocks, and aggregates are bundled into the

two volumes –

IGNEOUS ROCK

Formed by the solidification of

molten magma ejected from

deep within the earth’s mantle.

Plutons

intrusive igneous rocks

firmed when magma ceases its

mobility below the earth’s surface

and cools.

WEATHERING

Process of breaking down rocks by mechanical and chemical processes

into smaller pieces.

glacier ice, wind, running

water (streams and rivers), ocean waves.

Physical Agents that help disintegrate rocks

eldspars, ferromagnesians,

and micas.

Clay minerals are product of weathering of f

Glacial Soils

formed by transportation and deposition of glaciers

Alluvial Soils

transported by running water and deposited along streams

Lacustrine Soils

formed by deposition in quiet lakes

Marine Soils

formed by deposition in the seas

Aeolian Soils

transported and deposited by wind

Colluvial Soils

–formed by movement of soil from its original place by

gravity, such as during landslides.

Detrital Sedimentary Rocks

formed when deposits of gravel,

sand, silt, and clay formed by

weathering may become

compacted by overburden

pressure and cemented by agents

like iron oxide, calcite, dolomite,

and quartz.

Chemical Sedimentary Rocks

sedimentary rocks formed by

chemical processes and can have

either clastic or nonclastic texture.

Metamorphism

process of

changing the composition and

texture of rocks (without melting)

by heat and pressure.

Gneiss

igneous rocks, such as

granite, gabbro, and diorite

Slate

– shales and mudstones

Phyllite

slate (250 to 300°C)

Marble

calcite and dolomite

Quartzite

quartz-rich sandstone

Gravel

pieces of rocks with occasional particles of quartz, feldspar, and

other minerals.

Sand

particles made of mostly quartz and feldspar and other minerals at

times.

Silt

microscopic soil fractions that consist of very fine quartz grains and

some flake-shaped particles that are fragments of micaceous minerals

Clay

–mostly flake-shaped microscopic and submicroscopic particles of

mica, clay minerals, and other minerals

Kaolinite

consists of repeating layers of elemental silica-gibbsite sheets in

a 1:1 lattice and held together by hydrogen bonding.

Illite

consists of a gibbsite sheet bonded to two silica sheets – one at the

top and another at the bottom bonded by potassium ions.

Montmorillonite

consists on one gibbsite sheet sandwiched between

two silica sheets where there is isomorphous substitution of magnesium

and iron for aluminum in the octahedral sheets.

SPECIFIC GRAVITY, Gs

Ratio of the unit weight of a given material to the unit weight of water.

Light-colored sand

2.65

Clayey and silty soils

2.6 to 2.9

MECHANICAL ANALYSIS

Determination of the size range of particles present in a soil, expressed as

a percentage of the total dry weight.

Sieve Analysis

• (for particle sizes larger than 0.075 mm in diameter)

• Consists of shaking the soil

  sample through a set of

  sieves that have

  progressively smaller

  openings.

Hydrometer Analysis

• (for particle sizes smaller than 0.075 mm in diameter

• Based on the principle of sedimentation of soil grains in water.

Effective Size (D10)

diameter in the particle-size distribution curve

corresponding to 10% finer. It is a good measure to estimate the hydraulic

conductivity and drainage through soil.

A well-graded soil has a uniformity coefficient greater than about

4 for

gravels and 6 for sands,

A well-graded soil has a coefficient of gradation between

1 and 3

(for gravel and sands)

Bulky

–formed mostly by mechanical weathering of rock and minerals.

Flaky

have very low sphericity

Weight-volume relationships

include relationships among parameters such as void

ratio, porosity, degree of saturation, moisture content, and weight. The parameters

are fundamental to the study of geotechnical engineering.

void ratio,

porosity, and degree of saturation.

The volume relationships commonly used for the three phases in a soil element are

moisture content and unit weight.

The common terms used for weight relationships are

RELATIVE DENSITY

Commonly used to indicate the in situ denseness or looseness of granular soil.

➢ Grain Size

➢ Grain Shape

➢ Nature of the Grain-Size Distribution Curve

➢ Fine Contents. Fc (fraction smaller than 0.075 mm)

The maximum and minimum void ratios for granular soils depend on several factors:

ANGULARITY OF SAND

Qualitative description of sand particles with the range of angularity (Youd, 1973)

Very Angular

unworn fractured surfaces. (A = 0.12 to 0.17 with mean value of

0.14

Angular

sharp corners having approximately prismoidal or tetrahedral shapes.

(A = 0.17 to 0.25 with mean value of 0.21)

Subangular

blunted or slightly rounded corners and edges (A = 0.25 to 0.35 with

mean value of 0.30)

Subrounded

well-rounded edges and corners (A = 0.35 to 0.49 with mean value

of 0.41)

Rounded

irregularly shaped and rounded with no distinct corners or edges (A =

0.49 to 0.79 with mean value of 0.59)

Well-Rounded

spherical or ellipsoidal shape (A = 0.7 to 1.0 with mean value of

0.84)

ATTERBERG LIMIT

When clay minerals are present in fine-grained soil, the soil can be remolded in the presence of

some moisture without crumbling. This cohesive nature is caused by the adsorbed water surrounding

the clay particles.

solid, semisolid, plastic, and liquid.

depending on the moisture content, the

behavior of soil can be divided into four basic states—

shrinkage limit.

The moisture content, in percent, at which the transition from solid to semisolid state takes place is

defined as the

plastic limit,

The moisture content at the point of transition from semi-solid to

plastic state is the

liquid limit 

from plastic to liquid state

liquid limit devicE.

This device consists of a brass cup and a

hard rubber base. The brass cup can be dropped onto the base by a cam operated by a crank.

plastic limit is defined as

the moisture content in percent, at which the soil crumbles, when rolled

into threads of 3.2 mm (1/8 in.) in diameter.

is the lower limit of the plastic stage of

soil.

plasticity index (PI)

(PI) is the difference between the liquid limit and the plastic limit of a soil,

Shrinkage limit tests

are performed in the laboratory with a porcelain dish about 44 mm (1.75 in.) in

diameter and about 12.7 mm (1/2 in.) high.

Textural Classification

based on the particle-size distribution of the percent of sand, silt, and

clay-size fractions present in a given soil.

U.S. Department of Agriculture (USDA) Classification System

is based on particle-size limits as

described under the USDA System,

Sand Size:

2.0 to 0.05 mm in diameter

Silt Size:

0.05 to 0.002 mm in diameter

Clay Size:

smaller than 0.002 mm in diameter

American Association of State Highway and Transportation Officials (AASHTO)

AASHTO

American Association of State Highway and Transportation Officials (AASHTO)

was developed in 1929 as the Public Road Administration Classification System.

• The present version is proposed by the Committee on Classification of Materials for Subgrades and

Granular Type Roads of the Highway Research Board in 1945

UNIFIED SOIL CLASSIFICATION SYSTEM

• The original form of this system was proposed by Casagrande in 1942 for use in the airfield

construction works undertaken by the Army Corps of Engineers during World War II.

• In cooperation with the U.S. Bureau of Reclamation, this system was revised in 1952.

• At present, it is used widely by engineers (ASTM Test Designation D-2487)

PERMEABILITY.

This properties of soil which permits the water or any liquid to flow through it through its voids is

called

Darcy's Law

describes fluid flow through a porous medium and relates the flow

rate to the hydraulic gradient and the permeability of the medium.

HYDRAULIC CONDUCTIVITY

• depends on several factors: fluid viscosity,

pore size distribution, grain-size distribution, void ratio, roughness of mineral

particles, and degree of soil saturation.

• In clayey soils, structure plays an

  important role in

CONSTANT HEAD TEST

In this type of laboratory setup, the water supply at the inlet is adjusted in such

a way that the difference of head between the inlet and the outlet remains

constant during the test period. After a constant flow rate is established, water

is collected in a graduated flask for a known duration.

FALLING HEAD TEST

Water from a standpipe flows through the soil. The initial head difference, h1, at

time t = 0 is recorded, and water is allowed to flow through the soil specimen

such that the final head difference at time t = t2 is h2.