Transcript CVE 308

CVE 308
SOIL MECHANICS
ENGR S.O ODUNFA
DEPT. OF CIVIL ENGINEERING
UNIVERSITY OF AGRICULTURE,
ABEOKUTA
• INTRODUCTION
• HISTORY OF SOIL MECHANICS
• Soil mechanics is the science which studies the
physical, chemical and mechanical properties of
soils to solve problems related civil engineering.
• Prof. Terzaghi was the founder of soil mechanics
• To assure the integrity of structures under impose
loads is one of the reason for its establishment.
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Importance and Applications
Deals with problems related to soils
Highway embankments and cuts
Flow of water
Foundation – Shear failure
Compressibility of soils.etc.
Soil: Engineering Materials
• Soil as constructional materials
• Soil as foundational materials
Origin of Soil
• Soil is the disintegration of rocks through
weathering
• Weathering:
• Biological weathering
• Chemical weathering
• Mechanical weathering
• Transportation and Deposition of materials.
• Soil composition and phase relationship
Physical and Chemical Nature of Soil
• Both physical and chemical characteristics of
soil are the reflection of the parential
materials constituents where the soil is
formed.
Engineering Properties of Soil
• Physical properties e.g Shapes, density, etc
• Mechanical properties e.g Strength,
Deformation, etc
• Mechanical properties are direct functions of
its physical properties.
• Soil as multiphase and particulate.
Soil Structures
• Soil structure is geometric arrangement of the
soil particles and the interparticle forces
among them.
• Geometric arrangement
• Interparticle forces
Types of Soil Structures
• Primary soil structure
• Secondary soil structure
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Major group of soils are
Cohesionless soil e.g sand, mainly exhibit
Single-grained structure and
Honey-combed structure
Cohesive soil e.g clay soil, exhibit
Flocculated and Dispersed structures
Seepage, Capillary flow and
Permeability
• Seepage is the ease at which water flows
through the soil and this is possible due to the
presence of voids within the soil particles.
• Permeability is the property that allows the
flow of water/fluid through the soil.
• The flow of water through the soils is assumed
to follow Darcy’s law:
• Q/t = KAH/l
• Where Q = Quantity of water flowing
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t = time for quantity Q to flow;
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K = coefficient of permeability for the
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soil
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A = area of cross section through
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which the water flows;
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H = hydraulic head across soil
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l = length of flow path through soil.
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Seepage through the soil
• Seepage can be effectively study by the use of
flow nets
• Flow net is a pictorial representation of the
path of flow through a soil.
• Flow net comprises of
• flow lines and equipotential lines
• Construction of flow net
Control of Seepage
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Through embankment:Rock Toe/filter and
Horizontal blanket methods
Through foundation:Impervious cut offs and
Chimmy drain methods
Uses of Flow Nets
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Seepage discharge
Hydraulic pressure
Seepage pressure and
Exit gradient
Determination of the coefficient of
Permeability
• Coarse grained soils- Constant – head
permeameter method
• Fine –grained soils – Variable head
permeameter method
Compaction of Soils
• Compaction is the method of densifying soil
mass through mechanical means.
• Moisture content and Dry density relationship
Objectives of Compacting the soils
compaction
• To reduce the void ratio and thus the
permeability of the soil
• To increase the shear strength and therefore
the bearing capacity of the soil
• To make the soil less susceptible to
subsequent volume changes and therefore the
tendency to settlement under load or under
the influence of vibration.
Methods of compaction
• Standard Proctor compaction Test
• Modified AASHTO compaction Test
Consolidation and settlement
• Consolidation is the removal of water from soil
mass to enable the soil particles become wedged
closer together which results to change in
volume due to reduction in void ratio of the soil
mass.
• Settlement is the vertical downward
displacement brought about by such a volume
change.
• It can takes years for the complition of the
process.
Types of Consolidation
• Normal consolidation – Takes place under
normal condition
• Over consolidation –When the overburden
materials which caused the soil mass to be
consolidated is removed due to erosion and in
these circumstances the soil may have been
subjected to a pressure greatly in excess of its
present overburden.
Determination of settlement in terms
of Compression index
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e of compressibility, Mv first
Mv = av/1=eo
av = coeficient of compressibility
eo = original void ratio of the soil mass
av = ∆e/∆p
∆ e = Reduction in the void ratio after full
consolidation has taken place due to load ∆p
∆p = Additional stress
V = vv+vs
Determine coefficient of volum
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Vs = v/1+ eo
∆v = ∆H.A/HO.A
∆e/1+ eo = ∆H/HO = ∆n = MV∆p
∆H = MV∆Pho
∆H = [cc log(PO + ∆P)/PO
HO = original height
∆H = change in height
A = cross sectional area
∆n change in porosity
Cc = 0.009(LL – 10%)
Typical e – p curves
• Sands
• Clays
Stress Distribution in soil mass
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To determine stresses, Boussinesqs formula
δz = 3P/2πZ²[1/(1+{r/z}²)5/2
Δz vertical normal compressive stress
P = vertical concentrated load
Z vertical distance below P
r = horizontal distance from P
Shear Strength
• Shear strength of most soils is made up of a
combination of
• Friction and
• Cohesion
• Soil mass fails by shear and occurs along a
definite plane
Laboratory Determination of shear
strength
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Methods are
Unconfined compression test
Triaxial
Direct shear test
Field Tests Methods
• Vane shear test
• Standard Penetration test
• Dutch Cone Penetration test
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