SOIL MECHANICS × LECTURE NOTES

 

SOIL MECHANICS

LECTURE NOTES

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LECTURE # 1

SOIL AND SOIL ENGINEERING

* The term Soil has various meanings, depending upon the general field in which it is being considered.

*To a Pedologist ... Soil is the substance existing on the earth's surface, which grows and develops plant life.

*To a Geologist ..... Soil is the material in the relative thin surface zone within which roots occur, and all the rest of the crust is grouped under the term ROCK irrespective of its hardness.

*To an Engineer .... Soil is the un-aggregated or un-cemented deposits of mineral and/or organic particles or fragments covering large portion of the earth's.............................................. crust.

SOIL MECHANICS

LECTURE NOTES

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LECTURE # 1

SOIL AND SOIL ENGINEERING

* The term Soil has various meanings, depending upon the general field in which it is being considered.

*To a Pedologist ... Soil is the substance existing on the earth's surface, which grows and develops plant life.

*To a Geologist ..... Soil is the material in the relative thin surface zone within which roots occur, and all the rest of the crust is grouped under the term ROCK irrespective of its hardness.

*To an Engineer .... Soil is the un-aggregated or un-cemented deposits of mineral and/or organic particles or fragments covering large portion of the earth's crust.

** Soil Mechanics is one of the youngest disciplines of Civil Engineering involving the study of soil, its behavior and application as an engineering material.

*According to Terzaghi (1948): "Soil Mechanics is the application of laws of mechanics and hydraulics to engineering problems dealing with sediments and other unconsolidated accumulations of solid particles produced by the mechanical and chemical disintegration of rocks regardless of whether or not they contain an admixture of organic constituent."

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* Geotechnical Engineering ..... Is a broader term for Soil Mechanics.

* Geotechnical Engineering contains:

- Soil Mechanics (Soil Properties and Behavior)

- Soil Dynamics (Dynamic Properties of Soils, Earthquake Engineering, Machine Foundation)

- Foundation Engineering (Deep & Shallow Foundation)

- Pavement Engineering (Flexible & Rigid Pavement)

- Rock Mechanics (Rock Stability and Tunneling)

- Geosynthetics (Soil Improvement)

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Soil Formation

 

* Soil material is the product of rock

* The geological process that produce soil is

WEATHERING (Chemical and Physical).

* Variation in Particle size and shape depends on:

- Weathering Process

- Transportation Process

* Variation in Soil Structure Depends on:

- Soil Minerals

- Deposition Process

* Transportation and Deposition

Four forces are usually cause the transportation and deposition of soils

1- Water ----- Alluvial Soil 1- Fluvial

2- Estuarine

3- Lacustrine

4- Coastal

5- Marine

2- Ice ---------- Glacial Soils 1- Hard Pan

2- Terminal Moraine

3- Esker

4- Kettles

3- Wind -------- Aeolin Soils 1- Sand Dunes

2- Loess

4- Gravity ----- Colluvial Soil 1- Talus

What type of soils are usually produced by the different weathering & transportation process????????????????????????????????????????????????????????????????????????????

- Boulders

- Gravel Cohesionless

- Sand (Physical)

- Silt Cohesive

- Clay (Chemical)

* These soils can be

- Dry

- Saturated - Fully

- Partially

* Also they have different shapes and textures

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LECTURE # 2

SOIL PROPERTIES

PHYSICAL AND INDEX PROPERTIES

 

1- Soil Composition

- Solids

- Water

- Air

2- Soil Phases

- Dry

- Saturated * Fully Saturated

* Partially Saturated

- Submerged

3- Analytical Representation of Soil:

For the purpose of defining the physical and index properties of soil it is more convenient to represent the soil skeleton by a block diagram or phase diagram.

4- Weight - Volume Relationships:

Weight

W_t = W_w + W_s

Volume

V_t = V_v + V_s = V_a + V_w + V_s

1- Unit Weight - Density

* Also known as

- Bulk Density

- Soil Density

- Unit Weight

- Wet Density

Relationships Between Basic Properties:

 

Examples:

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Index Properties

Refers to those properties of a soil that indicate the type and conditions of the soil, and provide a relationship to structural properties such as strength, compressibility, per

meability, swelling potential, etc.

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1- PARTICLE SIZE DISTRIBUTION

* It is a screening process in which coarse fractions of soil are separated by means of series of sieves.

* Particle sizes larger than 0.074 mm (U.S. No. 200 sieve) are usually analyzed by means of sieving. Soil materials finer than 0.074 mm (-200 material) are analyzed by means of sedimentation of soil particles by gravity (hydrometer analysis).

1-1 MECHANICAL METHOD

U.S. Standard Sieve:

Sieve No. 4 10 20 40 60 100 140 200 -200

Opening in mm 4.76 2.00 0.84 0.42 0.25 0.149 0.105 0.074 -

Cumulative Curve:

* A linear scale is not convenient to use to size all the soil particles (opening from 200 mm to 0.002 mm).

* Logarithmic Scale is usually used to draw the relationship between the % Passing and the Particle size.

Example:

Parameters Obtained From Grain Size Distribution Curve:

1- Uniformity Coefficient C_u (measure of the particle size range)

Cu is also called Hazen Coefficient

Cu = D₆₀/D₁₀

C_u < 5 ----- Very Uniform

C_u = 5 ----- Medium Uniform

C_u > 5 ----- Nonuniform

2- Coefficient of Gradation or Coefficient of Curvature C_g

(measure of the shape of the particle size curve)

C_g = (D₃₀)²/ D₆₀ x D₁₀

C_g from 1 to 3 ------- well graded

3- Coefficient of Permeability

k = C_k (D₁₀)² m/sec

Consistency Limits or Atterberg Limits:

- State of Consistency of cohesive soil

1- Determination of Liquid Limit:

2- Determination of Plastic Limit:

3- Determination of Plasticity Index

P.I. = L.L. - P.L.

4- Determination of Shrinkage Limit

5- Liquidity Index:

6- Activity:

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SOIL CLASSIFICATION SYSTEMS

 

* Why do we need to classify soils ???????????

To describe various soil types encountered in the nature in a systematic way and gathering soils that have distinct physical properties in groups and units.

* General Requirements of a soil Classification System:

1- Based on a scientific method

2- Simple

3- Permit classification by visual and manual tests.

4- Describe certain engineering properties

5- Should be accepted to all engineers

* Various Soil Classification Systems:

1- Geologic Soil Classification System

2- Agronomic Soil Classification System

3- Textural Soil Classification System (USDA)

4-American Association of State Highway Transportation Officials System (AASHTO)

5- Unified Soil Classification System (USCS)

6- American Society for Testing and Materials System (ASTM)

7- Federal Aviation Agency System (FAA)

8- Others

1- Unified Soil Classification (USC) System:

 

The main Groups:

G = Gravel

S = Sand

.........................

M = Silt

C = Clay

........................

O = Organic

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* For Cohesionless Soil (Gravel and Sand), the soil can be Poorly Graded or Well Graded

Poorly Graded = P

Well Graded = W

* For Cohesive Soil (Silt & Clay), the soil can be Low Plastic or High Plastic

Low Plastic = L

High Plastic = H

Therefore, we can have several combinations of soils such as:

GW = Well Graded Gravel

GP = Poorly Graded Gravel

GM = Silty Gravel

GC = Clayey Gravel

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Passing Sieve # 4

SW = Well Graded Sand

SP = Poorly Graded Sand

SM = Silty Sand

SC = Clayey Sand

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Passing Sieve # 200

ML = Low Plastic Silt

CL = Low Plastic Clay

MH = High Plastic Silt

CH = High Plastic Clay

To conclud if the soil is low plastic or high plastic use Gassagrande's Chart

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2- American Association of State Highway Transportation Officials System (AASHTO):

 

- Soils are classified into 7 major groups A-1 to A-7

Granular A-1 {A-1-a - A-1-b}

(Gravel & Sand) A-2 {A-2-4 - A-2-5 - A-2-6 - A-2-6}

A-3

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More than 35% pass # 200

A-4

Fine A-5

(Silt & Clay) A-6

A-7

Group Index:

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3- Textural Soil Classification System (USDA)

* USDA considers only:

Sand

Silt

Clay

No. Gravel in the System

* If you encounter gravel in the soil ------- Subtract the % of gravel from the 100%.

* 12 Subgroups in the system

Example: ********

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MOISTURE DENSITY RELATIONSHIPS

(SOIL COMPACTION)

 

INTRODUCTION:

* In the construction of highway embankments, earth dams, and many other engineering projects, loose soils must be compacted to increase their unit weight.

* Compaction improves characteristics of soils:

1- Increases Strength

2- Decreases permeability

3- Reduces settlement of foundation

4- Increases slope stability of embankments

* Soil Compaction can be achieved either by static or dynamic loading:

1- Smooth-wheel rollers

2- Sheepfoot rollers

3- Rubber-tired rollers

4- Vibratory Rollers

5- Vibroflotation

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General Principles:

* The degree of compaction of soil is measured by its unit weight, , and optimum moisture content, w_c.

* The process of soil compaction is simply expelling the air from the voids.

or reducing air voids

* Reducing the water from the voids means consolidation.

Mechanism of Soil Compaction:

* By reducing the air voids, more soil can be added to the block. When moisture is added to the block (water content, w_c, is increasing) the soil particles will slip more on each other causing more reduction in the total volume, which will result in adding more soil and, hence, the dry density will increase, accordingly.

* Increasing W_c will increase

Up to a certain limit (Optimum moister Content, OMC)After this limit

Increasing W_c will decrease

Density-Moisture Relationship

Knowing the wet unit weight and the moisture content, the dry unit weight can be determined from:

The theoretical maximum dry unit weight assuming zero air voids is:

I- Laboratory Compaction:

* Two Tests are usually performed in the laboratory to determine the maximum dry unit weight and the OMC.

1- Standard Proctor Test

2- Modified Proctor Test

In both tests the compaction energy is:

1- Standard Proctor Test

Factors Affecting Compaction:

 

1- Effect of Soil Type

2- Effect of Energy on Compaction

3- Effect of Compaction on Soil Structure

4- Effect of Compaction on Cohesive Soil Properties

II- Field Compaction

 

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Flow of Water in Soils

Permeability and Seepage

 

* Soil is a three phase medium -------- solids, water, and air

* Water in soils occur in various conditions

* Water can flow through the voids in a soil from a point of high energy to a point of low energy.

* Why studying flow of water in porous media ???????

1- To estimate the quantity of underground seepage

2- To determine the quantity of water that can be discharged form a soil

3- To determine the pore water pressure/effective geostatic stresses, and to analyze earth structures subjected to water flow.

4- To determine the volume change in soil layers (soil consolidation) and settlement of foundation.

* Flow of Water in Soils depends on:

1- Porosity of the soil

2- Type of the soil - particle size

- particle shape

- degree of packing

3- Viscosity of the fluid - Temperature

- Chemical Components

4- Total head (difference in energy) - Pressure head

- Velocity head

- Elevation head

The degree of compressibility of a soil is expressed by the coefficient of permeability of the soil "k."

k cm/sec, ft/sec, m/sec, ........

Hydraulic Gradient

Bernouli's Equation:

For soils

Flow of Water in Soils

1- Hydraulic Head in Soil

Total Head = Pressure head + Elevation Head

h_t = h_p + h_e

- Elevation head at a point = Extent of that point from the datum

- Pressure head at a point = Height of which the water rises in the piezometer above the point.

- Pore Water pressure at a point = P.W.P. = g_water . h_p

*How to measure the Pressure Head or the Piezometric Head???????

Tips

1- Assume that you do not have seepage in the system (Before Seepage)

2- Assume that you have piezometer at the point under consideration

3- Get the measurement of the piezometric head (Water column in the Piezometer before seepage) = h_{p(Before Seepage)}

4- Now consider the problem during seepage

5- Measure the amount of the head loss in the piezometer (Dh) or the drop in the piezometric head.

6- The piezometric head during seepage = h_{p(during seepage)} = h_{p(Before Seepage)} - Dh

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GEOSTATIC STRESSES

&

STRESS DISTRIBUTION

 

Stresses at a point in a soil mass are divided into two main types:

I- Geostatic Stresses ------ Due to the self weight of the soil mass.

II- Excess Stresses ------ From structures

I. Geostatic stresses

I.A. Vertical Stress

Vertical geostatic stresses increase with depth, There are three 3 types of geostatic stresses

1-a Total Stress, s_total

1-b. Effective Stress, s_eff, or s'

1-c Pore Water Pressure, u

Total Stress = Effective stress + Pore Water Pressure

s_{total =} s_eff + u

Geostatic Stress with Seepage

When the Seepage Force = H g_sub -- Effective Stress s_eff = 0 This case is referred as

Boiling or Quick Conditon

I.B. Horizontal Stress or Lateral Stress

s_h = k_o s'_v

k_o = Lateral Earth Pressure Coefficient

s_h is always associated with the vertical effective stress, s'_v.

never use total vertical stress to determine s_h.

II. Stress Distribution in Soil Mass:

When applying a load on a half space medium the excess stresses in the soil will decrease with depth.

Like in the geostatic stresses, there are vertical and lateral excess stresses.

1. For Point Load

The excess vertical stress is according to Boussinesq (1883):

- I_p = Influence factor for the point load

- Knowing r/z ----- I₁ can be obtained from tables

According to Westergaard (1938)

where h = s (1-2m / 2-2m) m = Poisson's Ratio

2. For Line Load

Using q/unit length on the surface of a semi infinite soil mass, the vertical stress is:

3. For a Strip Load (Finite Width and Infinite Length):

The excess vertical stress due to load/unit area, q, is:

Where I_l = Influence factor for a line load

3. For a Circular Loaded Area:

The excess vertical stress due to q is: