Post on 18-Nov-2014
Priyantha Jayawickrama,
Ph.D.Associate Professor
Chapter 3: Soil Mechanics/Review
Texas Tech UniversityDepartment of Civil and Environmental Engineering
CE 4321: Geotechnical Engineering Design
Chapter 3: Soil Mechanics
Lecture No.1 3.1 Soil Composition
Soil-a 3-phase material Soil Characterization (particle size,
soil plasticity) 3.2 Soil Classification 3.3 Groundwater 3.4 Stress (Total vs. Effective)
CE 4321: Geotechnical Engineering Design
Chapter 3: Soil Mechanics
Lecture No.2 3.5 Compressibility and
settlementLecture No.3 3.6 Strength
CE 4321: Geotechnical Engineering Design
Soil: A 3-Phase Material
Solid
WaterAir
CE 4321: Geotechnical Engineering Design
The Mineral Skeleton
Volume
Solid Particles
Voids (air or water)
CE 4321: Geotechnical Engineering Design
Three Phase Diagram
Solid
Air
Water
Mineral Skeleton Idealization:Three Phase Diagram
CE 4321: Geotechnical Engineering Design
Fully Saturated Soils
Fully Saturated
Water
Solid
Mineral Skeleton
CE 4321: Geotechnical Engineering Design
Dry Soils
Mineral Skeleton Dry Soil
Air
Solid
CE 4321: Geotechnical Engineering Design
Partly Saturated Soils
Solid
Air
Water
Mineral Skeleton Partly Saturated Soils
CE 4321: Geotechnical Engineering Design
Three Phase System
Volume Weight
Solid
Air
WaterWT
Ws
Ww
Wa~0
Vs
Va
Vw
Vv
VT
CE 4321: Geotechnical Engineering Design
Weight Relationships Weight Components:
Weight of Solids = Ws
Weight of Water = Ww
Weight of Air ~ 0
%100(%), s
w
W
WwContentWater
CE 4321: Geotechnical Engineering Design
Volumetric Relationships Volume Components:
Volume of Solids = Vs
Volume of Water = Vw
Volume of Air = Va
Volume of Voids = Va + Vw = Vv
s
v
V
VeRatioVoid ,
%100(%), T
v
V
VnPorosity
CE 4321: Geotechnical Engineering Design
Volumetric Relationships Volume Components:
Volume of Solids = Vs
Volume of Water = Vw
Volume of Air = Va
Volume of Voids = Va + Vw = Vv
%100(%), V
w
V
VSSaturationofDegree
CE 4321: Geotechnical Engineering Design
Specific Gravity
Unit weight of Water, w w = 1.0 g/cm3 (strictly accurate at 4° C) w = 62.4 pcf w = 9.81 kN/m3
WaterofVolumeEqualanofWeight
ceSubsaofWeightGravitySpecific
tan
WaterofWeightUnit
ceSubsaofWeightUnitGravitySpecific
tan
CE 4321: Geotechnical Engineering Design
Specific Gravity Iron 7.86 Aluminum2.55-2.80 Lead 11.34 Mercury 13.55
Granite 2.69 Marble 2.69 Quartz 2.60 Feldspar 2.54-2.62
CE 4321: Geotechnical Engineering Design
Specific Gravity, Gs
CE 4321: Geotechnical Engineering Design
Example: Volumetric Ratios
Determine void ratio, porosity and degree of saturation of a soil core sample
Data: Weight of soil sample = 1013g Vol. of soil sample = 585.0cm3
Specific Gravity, Gs = 2.65 Dry weight of soil = 904.0g
CE 4321: Geotechnical Engineering Design
Solid
Air
Water
Wa~0
Volumes Weights
1013.0g585.0cm3
904.0g
s =2.65
109.0g
341.1cm3
109.0cm3
243.9cm3
134.9cm3
W =1.00
Example
CE 4321: Geotechnical Engineering Design
585.0cm3
Solid
Air
Water
Volumes
s =2.65341.1cm3
109.0cm3
243.9cm3
134.9cm3
W =1.00
%7.441009.243
0.109%100(%)
%7.411000.585
9.243%100(%)
72.01.341
9.243
v
w
T
v
s
v
V
VS
V
Vn
V
Ve
Example
CE 4321: Geotechnical Engineering Design
Soil Unit weight (lb/ft3 or kN/m3)
Bulk (or Total) Unit weight = WT / VT
Dry unit weightd = Ws / VT
Buoyant (submerged) unit weightb = - w
CE 4321: Geotechnical Engineering Design
Typical Unit weights
CE 4321: Geotechnical Engineering Design
TWO KINDS of Soil...
Two kinds of soil in this world… COARSE FINE
Basis for division is...
CE 4321: Geotechnical Engineering Design
Fine-Grained vs. Coarse-Grained Soils
U.S. Standard Sieve - No. 200 0.0029 inches 0.074 mm
“No. 200” means...
CE 4321: Geotechnical Engineering Design
Sieve Analysis (Mechanical Analysis)
This procedure is suitable for coarse grained soils
e.g. No.10 sieve …. has 10 apertures per linear inch
CE 4321: Geotechnical Engineering Design
Hydrometer Analysis
Also called Sedimentation Analysis
Stoke’s Law
18
)(2Lsw GGD
v
CE 4321: Geotechnical Engineering Design
Grain Size Distribution Curves
CE 4321: Geotechnical Engineering Design
Soil Plasticity
Further classification within fine-grained soils (i.e. soil that passes #200 sieve) is done based on soil plasticity.
Albert Atterberg, Swedish Soil Scientist (1846-1916)…..series of tests for evaluating soil plasticity
Arthur Casagrande adopted these tests for geotechnical engineering purposes
CE 4321: Geotechnical Engineering Design
Consistency of fine-grained soil varies in proportion to the water content
Atterberg Limits
Shrinkage limit
Plastic limit
Liquid limit
solid
semi-solid
plastic
liquid
PlasticityIndex
(cheese)
(pea soup)
(pea nut butter)
(hard candy)
CE 4321: Geotechnical Engineering Design
Liquid Limit (LL or wL)
Empirical Definition
The moisture content at which a 2 mm-wide groove in a soil pat will close for a distance of 0.5 in when dropped 25 times in a standard brass cup falling 1 cm each time at a rate of 2 drops/sec in a standard liquid limit device
CE 4321: Geotechnical Engineering Design
Engineering Characterization of Soils
Soil Properties that Control its Engineering Behavior
Particle Size
Particle/Grain Size DistributionParticle Shape
Soil Plasticity
fine-grained
coarse-grained
CE 4321: Geotechnical Engineering Design
Clay Morphology Scanning
Electron Microscope (SEM)
Shows that clay particles consist of stacks of plate-like layers
CE 4321: Geotechnical Engineering Design
Soil Consistency Limits Albert Atterberg
(1846-1916) Swedish Soil Scientist ….. Developed series of tests for evaluating consistency limits of soil (1911)
Arthur Casagrande (1902-1981)
……Adopted these tests for geotechnical engineering purposes
CE 4321: Geotechnical Engineering Design
Arthur Casagrande (1902-1981)
Joined Karl Terzaghi at MIT in 1926 as his graduate student
Research project funded by Bureau of Public Roads
After completion of Ph.D at MIT Casagrande initiated Geotechnical Engineering Program at Harvard
Soil Plasticity and Soil Classification (1932)
CE 4321: Geotechnical Engineering Design
Casagrande Apparatus
CE 4321: Geotechnical Engineering Design
Casagrande Apparatus
CE 4321: Geotechnical Engineering Design
Casagrande Apparatus
CE 4321: Geotechnical Engineering Design
Liquid Limit Determination
CE 4321: Geotechnical Engineering Design
The moisture content at which a thread of soil just begins to crack and crumble when rolled to a diameter of 1/8 inches
Plastic Limit (PL, wP)
CE 4321: Geotechnical Engineering Design
Plastic Limit (PL, wP)
CE 4321: Geotechnical Engineering Design
Plasticity Index ( PI, IP )
PI = LL – PL
or IP=wL-wP
Note: These are water contents, but the percentage sign is not typically shown.
CE 4321: Geotechnical Engineering Design
Plasticity Chart
CE 4321: Geotechnical Engineering Design
USCS Classification Chart
CE 4321: Geotechnical Engineering Design
USCS Classification Chart
CE 4321: Geotechnical Engineering Design
Plasticity Chart
CE 4321: Geotechnical Engineering Design
Groundwater
U = porewater pressure = wZw
Zw
+
CE 4321: Geotechnical Engineering Design
Stresses in Soil Masses
Area = A
= P/A
X X
Soil Unit
P
Assume the soil is fully saturated, all voids are filled with water.
CE 4321: Geotechnical Engineering Design
Effective Stress
From the standpoint of the soil skeleton, the water carries some of the load. This has the effect of lowering the stress level for the soil.
Therefore, we may define effective stress = total stress minus pore pressure
′ = - u where, ′ = effective stress = total stressu = pore pressure
CE 4321: Geotechnical Engineering Design
Effective Stress
′ = - u The effective stress is the force carried by
the soil skeleton divided by the total area of the surface.
The effective stress controls certain aspects of soil behavior, notably, compression & strength.
CE 4321: Geotechnical Engineering Design
Effective Stress Calculations
′z = iHi - u where,
H = layer thicknesssat = saturated unit weight
U = pore pressure = w Zw
When you encounter a groundwater table, you must use effective stress principles; i.e., subtract the pore pressure from the total stress.
CE 4321: Geotechnical Engineering Design
Geostatic Stresses
CE 4321: Geotechnical Engineering Design
See p.79
CE 4321: Geotechnical Engineering Design
See p.79