SOIL- Mechanics - Lecture notes lec 1 PDF

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#WeLearnAsOneCourse Title : GEOTECHNICAL ENGINEERING 1 Course Code : CIEN 30183 Course Credit : 3 Units, (2 Hrs Lecture & 3 Hrs Laboratory) Pre-Requisite : Mechanics of Deformable BodiesThe Overview:Soil Formation ad Identification, Engineering properties of soils, Fundamental aspects of soil ch...

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PUP Civil Engineering Department #WeLearnAsOne

Course Title Course Code Course Credit Pre-Requisite

: : : :

GEOTECHNICAL ENGINEERING 1 CIEN 30183 3 Units, (2 Hrs Lecture & 3 Hrs Laboratory) Mechanics of Deformable Bodies

The Overview: Soil Formation ad Identification, Engineering properties of soils, Fundamental aspects of soil characterization and response, including soil mineralogy, soil-water movement, effective stress, consolidation, soil strength, and soil compaction. Use of soils and geosynsynthetics in geotechnical and geo-environmental applications. Introduction to site investigation techniques, Laboratory testing and evaluation of soil composition and properties. This instructional material in Soil Mechanics (Introduction to Geotechnical Engineering) is consists of six (6) modules/lessons. Each module tackles the different topics of Soil Mechanics that deals with Soil Composition, Soil Classification, Physical Properties of Soil, Atterberg Limit, Permeability, Movement of water through soil, In situ stresses of soil, stresses in soil mass, and the shear strength of soil. Introduction to site investigation techniques, Laboratory testing and evaluation of soil composition and properties.

Course Objectives At the end of this course, the students will be able to: • Apply basic mathematics, science, and engineering principles to solve engineering problems. • Calculate the stresses transferred to underlying soils applied by the super structural loads. • Deal with the estimation of compressibility and settlement properties of soils for shallow foundation footings design. • Design and conduct experiments, as well as to analyze and interpret data • Be familiar with soil mechanics tests and determine which test is needed in designing civil engineering projects and/or solving engineering problems. • Use word processors in writing and finishing lab report. • Use soil laboratory equipment properly. • Demonstrate the ability to work in groups. Course Grading System Class Standing • • • •

70%

Quizzes Attendance Recitation/Group Dynamics Projects/Assignments/Seatwork/Special Report

Midterm / Final Examinations

30% 100%

Midterm Grade + Final Term Grade 2

=

1

FINAL GRADE

PUP Civil Engineering Department #WeLearnAsOne TABLE OF CONTENTS MODULE

TITLE

Page

1

Introduction to Soil Mechanics

4

2

Soil Properties and Composition

6

3

Soil Classification

14

4

Soil Tests and Indices

26

5

Permeability of Soil

37

6

Stresses in Soil

45

2

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Module 1 | Introduction to Soil Mechanics Learning Objectives At the end of this module, the students will be able to: • Define Soil Mechanics. • Explain the Origin of Soil Mechanics. • Differentiate the four (4) basic soil types. • Explain how the soil is being transported.

Course Material Soil mechanics is a branch of soil physics and applied mechanics that describes the behavior of soils. It differs from fluid mechanics and solid mechanics in the sense that soils consist of a heterogeneous mixture of fluids (usually air and water) and particles (usually clay, silt, sand, and gravel) but soil may also contain organic solids and other matter. Along with rock mechanics, soil mechanics provides the theoretical basis for analysis in geotechnical engineering, a subdiscipline of civil engineering, and engineering geology, a subdiscipline of geology. Soil mechanics is used to analyze the deformations of and flow of fluids within natural and man-made structures that are supported on or made of soil, or structures that are buried in soils. Example applications are building and bridge foundations, retaining walls, dams, and buried pipeline systems. Principles of soil mechanics are also used in related disciplines such as engineering geology, geophysical engineering, coastal engineering, agricultural engineering, hydrology, and soil physics.

Origin The primary mechanism of soil creation is the weathering of rock. All rock types (igneous rock, metamorphic rock, and sedimentary rock) may be broken down into small particles to create soil. Weathering mechanisms are physical weathering, chemical weathering, and biological weathering, as well as Human activities such as excavation, blasting, and waste disposal, may also create soil. Over geologic time, deeply buried soils may be altered by pressure and temperature to become metamorphic or sedimentary rock, and if melted and solidified again, they would complete the geologic cycle by becoming igneous rock. Physical weathering includes temperature effects, freeze, and thaw of water in cracks, rain, wind, impact, and other mechanisms. Chemical weathering includes dissolution of matter composing a rock and precipitation in the form of another mineral. Clay minerals, for example can be formed by weathering of feldspar, which is the most common mineral present in igneous rock. The most common mineral constituent of silt and sand is quartz, also called silica, which has the chemical name silicon dioxide. The reason that feldspar is most common in rocks, but silica is more prevalent in soils is that feldspar is much more soluble than silica. Silt, Sand, and Gravel are basically little pieces of broken rocks. According to the Unified Soil Classification System, silt particle sizes are in the range of 0.002 mm to 0.075 mm and sand particles have sizes in the range of 0.075 mm to 4.75 mm.

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PUP Civil Engineering Department #WeLearnAsOne Gravel particles are broken pieces of rock in the size range 4.75 mm to 100 mm. Particles larger than gravel are called cobbles and boulders. Soil is relatively thin surface layer of the Earth’s crust consisting of mineral and organic matter that is affected by agents such as weather, wind, water, and organisms. In general, soils are formed by weathering of rocks. The physical properties of a soil are dedicated primarily by the minerals that constitute the soil particles and hence the rock from which is derived.

Soil is the most misunderstood term in the field. The problem arises in the reasons for which different groups or professions study soil. Soil scientists are interested in soils as a medium for plant growth. So, soil scientists focus on the organic rich part of the soil horizon and refer to the sediments below the weathered zone as parent material. Classification is based on physical, chemical, and biological properties that can be observed and measured. Soils engineers think of a soil as any material that can be excavated with a shovel (no heavy equipment). Classification is based on the particle size, distribution, and the plasticity of the material. These classification criteria more relate to the behavior of soils under the application of load – the area where we will concentrate. Soil mechanics deal with the determination and analysis of forces that act on a soil mass. It is a relatively new engineering discipline having been developed only in the 1940's. It seeks to understand how a soil responds to being exposed to an engineered works or to being used in the works as a construction material. 'Geotechnical Engineering' is a new term used to describe soils engineering within the realm and knowledge of geologic processes. Geotechnical engineering is concerned mainly with foundations and basic soil engineering properties pertaining to slope stability, retaining walls, open pit mines, etc. Soil Transport Soil deposits are affected by the mechanism of transport and deposition to their location. Soils that are not transported are called residual soils — they exist at the same location as the rock from which they were generated. Decomposed granite is a common example of a residual soil. The common mechanisms of transport are the actions of gravity, ice, water, and wind. Wind-blown 4

PUP Civil Engineering Department #WeLearnAsOne soils include dune sands and loess. Water carries particles of different size depending on the speed of the water, thus soils transported by water are graded according to their size. Silt and clay may settle out in a lake, and gravel and sand collect at the bottom of a riverbed. Wind-blown soil deposits (aeolian soils) also tend to be sorted according to their grain size. Erosion at the base of glaciers is powerful enough to pick up large rocks and boulders as well as soil; soils dropped by melting ice can be a well graded mixture of widely varying particle sizes. Gravity on its own may also carry particles down from the top of a mountain to make a pile of soil and boulders at the base; soil deposits transported by gravity are called colluvium. The mechanism of transport also has a major effect on the particle shape. For example, low velocity grinding in a riverbed will produce rounded particles. Freshly fractured colluvium particles often have a very angular shape. Activity/Assessment 1. Illustrate the geologic rock cycle and explain each of its phases. 2. In your own words, briefly explain the importance of studying soil mechanics and its impact to a building structure. 3. Sketch completely and accurately the process of how a soil is being transported.

Module 2 | Soil Properties and Composition Learning Objectives At the end of this module, the students will be able to: • Identify the particles making up the soil. • Differentiate and Explain the different phases of Soil. • Discuss the different properties of soil and phase relationship. • Solve problems related to physical properties of soil. Course Material One of the common mistakes committed by a non-soil engineer is thinking that the term soil referred to the solid particle of the soil only. In fact, The term soil refers to the combination of the solids, liquids, and gaseous particles making up the soil.

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PUP Civil Engineering Department #WeLearnAsOne Soil phase describes the percentage by volume and weight of the constituent members of the soil, namely solids, liquids, and air. Each of these constituents are represented on a two-part phase diagram as shown below:

Physical Properties of Soil 1. Void Ratio (e) – the ratio of the volume of the voids to the volume of solid. 2. Porosity (n) – the ratio of the volume of the voids to the total volume of soil. 3. Degree of Saturation (S) – the ratio of the volume of water to the volume of the voids. Note that degree of saturation must be in percent. 4. Water Content / Moisture Content (ω) – the ratio of the mass of water to the mass of solid. It is present in percent form. 5. Unit Weight – the weight of soil per unit volume. a. Moist / Wet / Bulk Unit Weight (γm) – the weight of moist soil per unit volume. b. Dry Unit Weight (γd) – the weight of solid in the soil per unit volume. c. Saturated Unit Weight (γsat) – the weight of saturated soil per unit volume. Saturated define as the voids is fill up with water. d. Submerged / Effective / Buoyant Unit Weight (γ’) – due to the buoyant principle it is define as the effective weight of soil under submerged condition per unit volume. 6

PUP Civil Engineering Department #WeLearnAsOne e. Zero Air Void Unit Wieght @ Dry State. (γzav) – dry unit weight of soil under zero air void condition. Zero air void means saturated state. 6. Air content (AC) – Volume of air in the voids. 7. Air Void (AV) – volume of air in the soil.

The total volume of a given soil sample can be expressed as: 𝑉 = 𝑉𝑠 + 𝑉𝑣 = 𝑉𝑠 + 𝑉𝑤 + 𝑉𝑎 Where:

Vs = volume of soil solids Vv = volume of voids Vw = volume of water in the voids Va = volume of air in the voids

Assume that the weight of air is negligible, the total weight of the sample as:

where:

𝑊 = 𝑊𝑠 + 𝑊𝑤 Ws = weight of soil soilds Ww = weight of water

Phase Relationships: The water content or moisture content ( w) is the ratio of the weight of water to the weight of soil solids, i.e. w=

Ww Ws

The degree of saturation or saturation ratio (S) is the ratio of the volume of water to the total volume of void spaces, i.e. 𝑆= 7

𝑉𝑤 𝑉𝑣

PUP Civil Engineering Department #WeLearnAsOne The degree of saturation can range between the limits of zero for a completely dry soil and one (or 100%) for a fully saturated soil. The void ratio (e) is the ratio of the volume of voids to the volume of soil solids, i.e. 𝑒=

𝑉𝑣 𝑉𝑠

The porosity (n) is the ratio of the volume of voids to the total volume of soil, i.e. 𝑛=

𝑉𝑉 𝑉

As V = VV + Vs, void ratio and porosity are interrelated as follows: and

𝑒=

𝑛=

𝑛 1−𝑛 𝑒 1+𝑒

The specific volume (v) is the total volume which contains a unit volume of solids, i.e. 𝑉

𝑣=

𝑉𝑠

= 1+𝑒

The air content or air void ratio (A) is the ratio of volume of air to the total volume of soil, i.e. 𝐴=

𝑉𝑎 𝑉

The bulk unit weight (γ) of a soil is the ratio of the total mass to the total volume, i.e. 𝛾=

𝑀 𝑉

The specific gravity of the soil particles (Gs) is given by 𝐺𝑠 =

𝑀𝑠 𝜌𝑠 = 𝑉𝑠 𝜌𝑤 𝜌𝑤

where ρs is the particle density.

Other important relationships are listed below: 𝐺𝑠 𝑤 = 𝑒𝑆

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For a completely dry soil (S = 0):

𝑒 − 𝐺𝑠 𝑤 𝐴 = 1 + 𝑒 ; 𝐴 = 𝑛 (1 − 𝑆 ) 𝐺𝑠 + 𝑆𝑒 𝛾 𝛾= 1+𝑒 𝑤 𝛾𝑑 =

For a completely saturated soil (S = 1): 𝛾𝑠𝑎𝑡 =

𝐺𝑠 𝛾𝑤 1+𝑒

𝐺𝑠 + 𝑒 1+𝑒

𝛾𝑤

The relative density (ID) is used to express the relationship between the in-situ void ratio (e), or the void ratio of a sample, and the limiting values emax and emin representing the loosest and densest possible soil packing states respectively. The relative density is defined as: 𝑒𝑚𝑎𝑥 − 𝑒 𝐼𝐷 = = 𝑒𝑚𝑎𝑥 − 𝑒𝑚𝑖𝑛

1 1 𝛾𝑑(𝑚𝑖𝑛) − 𝛾𝑑 1 1 𝛾𝑑(𝑚𝑖𝑛) − 𝛾𝑑(𝑚𝑎𝑥)

Thus, the relative density of a soil in its densest possible state (e = emin) is 1 (or 100%) and in its loosest possible state (e = emax) is 0.

Sample Problem 1: A soil sample weighs 14.46 N and has a volume of 0.000991 m3. The specific gravity is 2.65. The volume of air is 0.000167 m3. Find the dry unit weight of the original soil sample (kN/m3). Solution:

VOLUME (m3)

WEIGHT (N)

0.000167

AIR

0

Vw

WATER

Ww

Vs

SOIL

Ws

0.000991

14.46

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PUP Civil Engineering Department #WeLearnAsOne From the relationship on the volume of soil solids, water and air: Vs + Vw + Va = 0.000991 m3 Vs + Vw + 0.0000167 = 0.000991 Vs + Vw = 0.000824 → eq. 1 From the relationship on the weight of soil solids and water (weight of air is negligible): Ws + Ww = 14.46 N → eq. 2 Remember that the unit weight of a certain object is the ratio of its weight to the volume; hence, the weight in terms of its volume is: W γ = → W = γV V Ws = γs Vs ; Ww = γw Vw Also, remember that the specific gravity of a certain object is the ratio of the unit weight of the object to the unit weight of water, since the object is soil, the unit weight of soil in terms of specific gravity is: γs → γs = Gs γw Gs = γw Substituting the weight of soil solids and the weight of water in terms of their volume to equation 2, yields to: Gs γw Vs + γw Vw = 3.25 2.65 (9810)(Vs ) + (9810)Vw = 14.46 25996.5Vs + 9810Vw = 14.46 → equation 2 Solving the two equations simultaneously, we have: Vs = 0.000394 m3 ; Vw = 0.000430 m3 To solve for the dry unit weight of the soil sample: Ws 2.65(9.81)(0.000394) γd = = 𝟏𝟎. 𝟐𝟒 𝐤𝐍/𝐦𝟑 = 0.000991 VT Sample Problem 2. A mold having a volume of 0.10 ft3 was filled with moist soil. The weight of the soil in the mold was found to be 12.00 lb. The soil was oven-dried and the weight after drying was 10.50 lb. The specific gravity of solids was known to be 2.70. Determine the water content, void ratio, porosity, degree of saturation, total unit weight, and dry unit weight. Solution: 1. The water content is: 12.0 − 10.5 Ww Wwet − Wdry = = 0.1429 = 𝟏𝟒. 𝟐𝟗% w= = Wdry 10.5 Ws 10

PUP Civil Engineering Department #WeLearnAsOne 2. The void ratio can be computed by:

e=

γd =

5.

6.

γd

−1

2.70 × 62.4 −1 105 𝐞 = 𝟎. 𝟔𝟎𝟓 The porosity in terms of the void ratio is: e n= 1+e 0.605 = 𝟎. 𝟑𝟕𝟕 n= 1 + 0.605 The degree of saturation can be computed by using the formula: Gs w = eS Gs w S= e 2.70 × 0.1429 = 0.6380 = 𝟔𝟑. 𝟖𝟎% S= 0.605 The total unit weight is: W γ= V 12 lb = 𝟏𝟐𝟎 𝐥𝐛/𝐟𝐭 𝟑 γ= 0.10 ft 3 The dry unit weight was already solved in 2: 𝛄𝐝 = 𝟏𝟎𝟓 𝐥𝐛/𝐟𝐭 𝟑 Therefore:

4.

Gs γw 1+e Gs γw

Wdry V 10.5 lb γd = 0.10 ft 3 γd = 105 lb/ft 3

The dry unit weight is:

3.

γd =

e=

Sample Problem 3. One cubic meter of wet soil weighs 19.80 kN. If the specific gravity of soil particles is 2.70 and water content is 11%. Find the void ratio, dry unit weight, and degree of saturation. Solution: 1. The void ratio can be computed by:

γ=

Gs + Gs w

γw 1+e W Gs + Gs w = γw V 1+e 11

PUP Civil Engineering Department #WeLearnAsOne 19.80 kN 1 m3

2.70 + 2.70(0.11) × 9.81 1+e = 𝐞 = 𝟎. 𝟒𝟖𝟓

γ 1+w 19.80 γd = 1 + 0.11 𝛄𝐝 = 𝟏𝟕. 𝟖𝟒 𝐤𝐍/𝐦𝟑 3. The degree of saturation can be computed by: Gs w = eS Gs w S= e 2.70(0.11) S= 0.485 𝐒 = 𝟎. 𝟔𝟏𝟑 2. The dry unit weight is:

γd =

Activity/Assessment Direction: Solve the following problems completely. Show your complete solution. No Solution, no credit points. 1. From the following data of a soil sample: Sample size 3.81 cm dia. × 7.62 cm ht. Wet weight = 1.668 N Oven-dry weight = 1.400 N Specific gravity = 2.70 Determine the water content (%), dry unit weight (kN/m3), bulk unit weight (kN/m3), void ratio, and the degree of saturation (%). 2. The porosity of a soil sample is 35% and the specific gravity of its particles is 2.70. Calculate its void ratio, dry unit weight, saturated unit weight, and the submerged unit weight. 3. A soil sample with a grain specific gravity of 2.67 was filled in a 1000 ml container in the loosest possible state and the dry weight of the sample was found to be 14.75 N. It was then filled at the densest state obtainable and the weight was found to be 17.70 N. The void ratio of the soil in the natural state was 0.63. Determine the density index in the natural state. 4. The dry unit weight of a sand sample in the loosest state is 13.34 kN/m3 and in the densest state, it is 21.19 kN/m3. Determine the density index of this sand when it has a porosity of 33%. Assume the grain specific gravity as 2.68.

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PUP Civil Engineering Department #WeLearnAsOne 5. The mass specific gravity of a fully saturated specimen of clay having a water content of 30.5% is 1.96. On oven drying, the mass specific gravity drops to 1.60. Calculate the specific gravity of clay.

Module 3 | Soil Classification Learning Objectives At the end of this module, the students will be able to: • Differentiate and Explain the different types of soil. • Understand and Perform the particle size distribution of soil using sieve analysis and Hydrometer Analysis. • Explain the different soil classification systems and Classify the soil based on them. • Solve problems related to soil classification. Course Material

Foundations of structures such as buildings, bridges, towers, dams, oil tanks generally requires the knowledge of the behavior and stresses related deformability of the soil that will support the foundation system and the geological conditions of the so...