Content Improvement—The content of each chapter has been improved by adding updated material and more explanations. A quiz at the end of each chapter is at www.wiley.com/college/budhu to test your general knowledge of the subject.
CONTENTS
INTRODUCTION TO SOIL
MECHANICS AND FOUNDATIONS
Any application of soil mechanics involves uncertainty due to soil variability—its stratification, composition, and engineering properties. Experience and approximate calculations are essential for the successful application of soil mechanics to practical problems.
GEOLOGICAL CHARACTERISTICS AND PARTICLE SIZES OF SOILS
- Earth’s Profi le
- Plate Tectonics
- Composition of the Earth’s Crust
- Discontinuities
- Geologic Cycle and Geological Time
Chemical sedimentary rocks are minerals such as halite (rock salt), calcite, and gypsum that have formed from elements dissolved in water (eg, the material found in Death Valley, California). Principle of original horizontality, which states that sediments are deposited in layers parallel to the earth's surface.
Knowledge of geology is important for the successful practice of geotechnical engineering
The earth’s surface (lithosphere) is fractured into about 20 mobile plates. Interaction of these plates causes volcanic activity and earthquakes
Sedimentary rocks are of particular importance to geotechnical engineers because they cover about 75% of the earth’s surface area
Rock masses are inhomogeneous and discontinuous
- Soil Formation
- Soil Types
- Clay Minerals
- Surface Forces and Adsorbed Water
- Soil Fabric
A central silica cation (positively charged ion) is surrounded by four oxygen anions (negatively charged ions), one at each corner of the tetrahedron (Figure 2.6a). Illite consists of repeated layers of an aluminum oxide plate interlaced by two silicate plates (Figure 2.7b).
Soils are derived from the weathering of rocks and are commonly described by textural terms such as gravels, sands, silts, and clays
Physical weathering causes reduction in size of the parent rock without change in its composition
Chemical weathering causes reduction in size and chemical composition that differs from the parent rock
Clays are composed of three main types of mineral—kaolinite, illite, and montmorillonite
- D E T E R M I N AT I O N O F PA R T I C L E S I Z E O F S O I L S — AST M D 4 2 2
- Particle Size of Coarse-Grained Soils
- Particle Size of Fine-Grained Soils
- Characterization of Soils Based on Particle Size
The second category is fine-grained soil, which is marked if more than 50% of the soil is finer than 0.075 mm. The second coefficient is the curvature coefficient, Cc (other terms used are gradation coefficient and concavity coefficient), defined as 2.5), where D30 is the diameter of the soil particles in which 30% of the particles are smaller.
A sieve analysis is used to determine the grain size distribution of coarse-grained soils
A soil with a uniformity coefficient of .4 contains particles of uniform size (approximately one size). A soil with a uniformity coefficient of 0.4 is described as a well-graded soil and is indicated by a flat curve (Figure 2.12).
For fi ne-grained soils, a hydrometer analysis is used to fi nd the particle size distribution
The absence of certain grain sizes, called graded-graded, is diagnosed by a curvature coefficient outside the range of 1 to 3 and an abrupt change in slope in the particle size distribution curve, as shown in the figure. The coefficient of uniformity and the coefficient of concavity apply strictly to coarse-grained soils.
Particle size distribution is represented on a semilogarithmic plot of % fi ner (ordinate, arithmetic scale) versus particle size (abscissa, logarithmic scale)
The minimum Cu value is 1 and corresponds to a group of particles of the same size. The diameter D10 is called the effective floor size and was described by Allen Hazen (1892) in connection with his work on floor filters.
The particle size distribution plot is used to delineate the different soil textures (percentages of gravel, sand, silt, and clay) in a soil
The effective size is the diameter of an artificial sphere that will give approximately the same effect as an irregularly shaped particle. In Chapter 4, you will learn about how particle size distribution is used along with other physical properties of soils in a classification system designed to help you select soils for specific applications.
- CO M PA R I S O N O F COA R S E - G R A I N E D
In the next section, a broad comparison between coarse-grained and fi ne-grained soils is presented. The engineering properties of coarse-grained soils are mainly controlled by the particle grain size and their structural arrangement.
Fine-grained soils have much larger surface areas than coarse-grained soils and are responsible for the major physical and mechanical differences between coarse-grained and fi ne-grained soils
The technical properties of fine-grained soils are determined by mineralogical factors rather than by grain size. Fine-grained soils have a much larger surface area than coarse-grained soils and are responsible for the large physical and mechanical differences between coarse-grained and fine-grained soils.
The engineering properties of fi ne-grained soils depend mainly on mineralogical factors
Vibrations accentuate volume changes in loose, coarse-grained soils by rearranging the soil material into a dense configuration. Fine-grained soils are practically impermeable, change volume and strength with variations in moisture conditions, and are susceptible to frost.
Coarse-grained soils have good load-bearing capacities and good drainage qualities, and their strength and volume-change characteristics are not signifi cantly affected by changes in moisture
Coarse-grained soils have good bearing capacity and good drainage properties, and their strength and volume change properties are not significantly affected by changes in moisture conditions under static loading. Thin layers of fine-grained soil, even within thick deposits of coarse-grained soil, have been responsible for many geotechnical failures, and therefore you need to pay special attention to fine-grained soil.
Fine-grained soils have poor load-bearing capacities and poor drainage qualities, and their strength and volume-change characteristics are signifi cantly affected by changes in moisture
2.1 (a) What three layers make up the internal structure of the earth. b) What is the composition of each layer. 2.9 (a) What are the six categories of soil types defined in the ASTM classification system.
SOILS INVESTIGATION
A soils investigation is necessary to determine the suitability of a site for its intended purpose
A soils investigation is conducted in phases. Each phase affects the extent of the next phase
A clear, concise report describing the conditions of the ground, soil stratigraphy, soil parameters, and any potential construction problems must be prepared for the client
- Soils Exploration Methods
- Soil Identifi cation in the Field
- Number and Depths of Boreholes
- Soil Sampling
- Groundwater Conditions
- Soils Laboratory Tests
- Types of In Situ or Field Tests
- Types of Laboratory Tests
The seismic refraction method is used to determine the depth and thickness of the soil profile and the existence of buried structures. The ratio of the maximum torque to the residual torque is the ground sensitivity, St, where.
A number of tools are available for soil exploration. You need to use judgment as to the type that is appropriate for a given project
Laboratory test samples are always disturbed, and the degree of disturbance can significantly affect test results. You will learn about them and the meaning and significance of the soil parameters they measure in the following chapters of this textbook.
Signifi cant care and attention to details are necessary to make the results of a soils investigation meaningful
Details of the types of studies conducted, soil and groundwater information, including laboratory and field test results, assumptions and limitations of the study, and potential construction issues. A legend of the filling pattern must be included in the soil report. f) Depths at which samples or in situ tests were carried out, with sample or test numbers. G).
Self- A ssessment
Such an investigation is carried out in phases and may include geophysical investigations, boreholes and field and laboratory tests.
PHYSICAL SOIL STATES
AND SOIL CLASSIFICATION
Specifi c volume (V9) is the volume of soil per unit volume of solids
The total volume is decomposed into the volume of solids and the volume of voids, and then both the numerator and denominator are divided by the volume of solids; it is,. If the particles of coarse-grained soils were spheres, the maximum and minimum porosities would be 48% and 26%, respectively. Specific gravity (Gs) is the ratio of the weight of the soil solids to the weight of water of equal volume:.
After removing all air bubbles, fill the container with deaerated water. Add deaerated water to the container and determine the mass of the container and water. The degree of saturation (S) is the ratio, often expressed as a percentage, between the volume of water and the volume of voids:.
- P H YS I CA L STAT E S A N D I N D E X P R O P E R T I E S O F F I N E - G R A I N E D S O I L S
- D E T E R M I N AT I O N O F T H E L I Q U I D, P L AST I C , A N D S H R I N K AG E L I M I T S
- Casagrande Cup Method—ASTM D 4318
- Plastic Limit Test—ASTM D 4318
- Fall Cone Method to Determine Liquid and Plastic Limits
- Shrinkage Limit—ASTM D 427 and D 4943
The bulk weight per unit of soil in the pit is 17 kN/m3 and its natural water content is 5%. Soil in this state is said to exhibit plastic behavior—the ability to deform continuously without rupture. A sample of soil within the enclosure is removed to determine the water content.
Fine-grained soils can exist in one of four states: solid, semisolid, plastic, or liquid
The mass of the mercury is determined, and the decrease in volume caused by shrinkage can be calculated from the known density of mercury. The range of water content from the plastic limit to the shrinkage limit is called the shrinkage index (SI).
Water is the agent that is responsible for changing the states of soils
A soil gets weaker if its water content increases
The plasticity index defi nes the range of water content for which the soil behaves like a plastic material
The liquidity index gives a qualitative measure of strength
The soil strength is lowest at the liquid state and highest at the solid state
- S O I L C L AS S I F I CAT I O N S C H E M E S
- Unifi ed Soil Classifi cation System
- American Society for Testing and Materials (ASTM) Classifi cation System
- AASHTO Soil Classifi cation System
- Plasticity Chart
The water content of the sandy soil in the rental site is 15%, and its void ratio is 0.69. For 1 km of embankment length, determine the following: a) Weight of borrowed sandy soil required for embankment construction. The dry mass of the soil and container is 0.29 kg. a) Total, dry and saturated soil weight unit.
SOIL COMPACTION
- BAS I C CO N C E P T
- P R O C TO R CO M PAC T I O N T E ST — AST M D 114 0 A N D AST M D 15 5 7
- I N T E R P R E TAT I O N O F P R O C TO R T E ST R E S U LT S
- B E N E F I T S O F S O I L CO M PAC T I O N
- Compaction is the densifi cation of a soil by the expulsion of air and the rearrangement of soil particles
- The Proctor test is used to determine the maximum dry unit weight and the optimum water content and serves as the reference for fi eld specifi cations of compaction
- Higher compactive effort increases the maximum dry unit weight and reduces the optimum water content
- Compaction increases strength, lowers compressibility, and reduces the rate of fl ow water through soils
- F I E L D CO M PAC T I O N
- CO M PAC T I O N Q UA L I T Y CO N T R O L
- A variety of fi eld equipment is used to obtain the desired compaction
- The sand cone apparatus, the balloon apparatus, and the nuclear density meter are three types of equipment used for compaction quality control in the fi eld
- It is generally best to allow the contractor to select and use the appropriate equipment to achieve the desired compaction
What is the dry unit weight and water content at standard 95% compaction, dry at optimum. The higher compaction effort increases the maximum dry unit weight and reduces the optimum water content. The laboratory test to investigate the maximum dry unit weight and optimum water content is the Proctor test.
ONE-DIMENSIONAL FLOW OF WATER THROUGH SOILS
The fl ow of water through soils is governed by Darcy’s law, which states that the average fl ow velocity is proportional to the hydraulic gradient
The proportionality coeffi cient in Darcy’s law is called the hydraulic conductivity, k
Homogeneous clays are practically impervious, while sands and gravels are pervious
- F LOW PA R A L L E L TO S O I L L AY E R S
- F LOW N O R M A L TO S O I L L AY E R S
- Constant-Head Test
- Falling-Head Test
The flow through the soil as a whole is equal to the sum of the flow through each of the layers. For flow normal to the soil layer, the height loss in the soil mass is the sum of the height losses in each layer:. Calculate the ratio of the equivalent horizontal hydraulic conductivity to the equivalent vertical hydraulic conductivity for flow through the sides of the channel.
The constant-head test is used to determine the hydraulic conductivity of coarse-grained soils
The falling-head test is used to determine the hydraulic conductivity of fi ne-grained soils
- Pumping Test to Determine the Hydraulic Conductivity
- G R O U N D WAT E R LOW E R I N G BY W E L L P O I N T S
The wells are placed in rows and the distance between them depends on the soil type and hydraulic conductivity. Estimate the hydraulic conductivity of a comparable soil with a porosity of 35% based on the results of this test. Assuming the flow is parallel to the slope, estimate the hydraulic conductivity. at the very edge of the excavation should be 0.5 m below the base. a) Calculate the radius of influence.
STRESSES, STRAINS, AND ELASTIC DEFORMATIONS OF SOILS
- Normal Stresses and Strains
- A normal stress is the load per unit area on a plane normal to the direction of the load
- A shear stress is the load per unit area on a plane parallel to the direction of the shear force
- Normal stresses compress or elongate a material; shear stresses distort a material
- A normal strain is the change in length divided by the original length in the direction of the original length
- Principal stresses are normal stresses on planes of zero shear stress
- Soils can only sustain compressive stresses
- An elastic material recovers its original confi guration on unloading; an elastoplastic material undergoes both elastic (recoverable) and plastic (permanent) deformation during loading
- Soils are elastoplastic materials
- At small strains soils behave like an elastic material, and thereafter like an elastoplastic material
- The locus of the stresses at which a soil yields is called a yield surface. Stresses below the yield stress cause the soil to respond elastically; stresses beyond the yield stress cause the soil to
- Hooke’s law applies to a linearly elastic material
- As a fi rst approximation, you can use Hooke’s law to calculate stresses, strains, and elastic settlement of soils
- For nonlinear materials, Hooke’s law is used with an approximate elastic modulus (tangent modulus or secant modulus) and the calculations are done for incremental increases in stresses or
- A plane strain condition is one in which the strain in one or more directions is zero or small enough to be neglected
- An axisymmetric condition is one in which two stresses are equal
The elastic modulus or initial tangent elastic modulus (E) is the slope of the stress-strain line for linear isotropic material (Figure 7.5). The tangent elastic modulus is the slope of the tangent to the stress-strain point under consideration. The elastic modulus of the secant is the slope of the line connecting the origin (0, 0) to a desired stress-strain point.
The prevalent form of structural anisotropy in soils is transverse anisotropy; the soil properties and the soil response in the lateral directions are the same but are different from those in the vertical
You need to fi nd the elastic parameters in different directions of a soil mass to determine elastic stresses, strains, and displacements
We can draw an analogy of the strength of materials with the strength of a chain. The state of stress at a point in a soil mass due to applied boundary forces can equal the strength of the soil and thereby initiate failure. We will subsequently discuss stress and strain states using your knowledge of Mohr's circle in the strength of materials.
Mohr’s circle is used to fi nd the stress state or strain state from a two-dimensional set of stresses or strains on a soil
The pole on a Mohr's circle identifies a point through which any plane passing through it will intersect the Mohr's circle at a point representing the stresses on that plane.
The pole on a Mohr’s circle identifi es a point through which any plane passing through it will inter- sect the Mohr’s circle at a point that represents the stresses on that plane
Draw a line from P to s1 and measure the angle between the horizontal plane and this line. Draw a line M1N1 through P with an inclination of 308 from the principal plane of the principal stresses, as CPN9. In the next section, we will discuss the effective stress principle, which accounts for soil pore pressures.
- The effective stress represents the average stress carried by the soil solids and is the difference between the total stress and the porewater pressure
- The effective stress principle applies only to normal stresses and not to shear stresses
- Deformations of soils are due to effective, not total, stress
- Soils, especially silts and fi ne sands, can be affected by capillary action
- Capillary action results in negative porewater pressures and increases the effective stresses
- Downward seepage increases the resultant effective stress; upward seepage decreases the resultant effective stress
On the right side of the wall, the numerical stress is upward and the effective stress is reduced. The effective stress represents the average stress carried by the soil solids and is the difference between the total stress and the pore water pressure. Calculate the horizontal effective stress and the horizontal total stress for the soil element at 5 m in Example 7.
- The increases in stresses below a surface load are found by assuming the soil is an elastic, semi- infi nite mass
- Various equations are available for the increases in stresses from surface loading
- The stress increase at any depth depends on the shape and distribution of the surface load
- A stress applied at the surface of a soil mass by a loaded area decreases with depth and lateral distance away from the center of the loaded area
- The vertical stress increases are generally less than 10% of the surface stress when the depth-to- width ratio is greater than 2
Determine the vertical stress increase at a depth of 3 m (a) below the center of the slab, point A (Figure E7. You need to find the vertical stress increase for a rectangle and multiply the result by 2. The constraints on the problem are the maximum vertical stress increase and the slope of the violence.
