chapter 3 soil mechanics part i

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)


Chapter 3: Soil Mechanics

Lecture No.2 3.5 Compressibility and

settlementLecture No.3 3.6 Strength


Soil: A 3-Phase Material

Solid

WaterAir


The Mineral Skeleton

Volume

Solid Particles

Voids (air or water)


Three Phase Diagram

Solid

Air

Water

Mineral Skeleton Idealization:Three Phase Diagram


Fully Saturated Soils

Fully Saturated

Water

Solid

Mineral Skeleton


Dry Soils

Mineral Skeleton Dry Soil

Air

Solid


Partly Saturated Soils

Solid

Air

Water

Mineral Skeleton Partly Saturated Soils


Three Phase System

Volume Weight

Solid

Air

WaterWT

Ws

Ww

Wa~0

Vs

Va

Vw

Vv

VT


Weight Relationships Weight Components:

Weight of Solids = Ws

Weight of Water = Ww

Weight of Air ~ 0

%100(%), s

w

W

WwContentWater


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


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


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


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


Specific Gravity, Gs


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


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


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


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


Typical Unit weights


TWO KINDS of Soil...

Two kinds of soil in this world… COARSE FINE

Basis for division is...


Fine-Grained vs. Coarse-Grained Soils

U.S. Standard Sieve - No. 200 0.0029 inches 0.074 mm

“No. 200” means...


Sieve Analysis (Mechanical Analysis)

This procedure is suitable for coarse grained soils

e.g. No.10 sieve …. has 10 apertures per linear inch


Hydrometer Analysis

Also called Sedimentation Analysis

Stoke’s Law

18

)(2Lsw GGD

v


Grain Size Distribution Curves


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


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)


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


Engineering Characterization of Soils

Soil Properties that Control its Engineering Behavior

Particle Size

Particle/Grain Size DistributionParticle Shape

Soil Plasticity

fine-grained

coarse-grained


Clay Morphology Scanning

Electron Microscope (SEM)

Shows that clay particles consist of stacks of plate-like layers


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


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)


Casagrande Apparatus


Liquid Limit Determination


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)


Plastic Limit (PL, wP)


Plasticity Index ( PI, IP )

PI = LL – PL

or IP=wL-wP

Note: These are water contents, but the percentage sign is not typically shown.


Plasticity Chart


USCS Classification Chart


Plasticity Chart


Groundwater

U = porewater pressure = wZw

Zw

+


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.


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


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.


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.


Geostatic Stresses


See p.79

chapter 3 soil mechanics part i

Documents

soil composition soil

water ce

dry weight of soil

weight of soil sample

soil classification

soil mechanicslecture

kinds of soil

soil pat