Soil Mechanics


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Consolidation - Part 2. Consolidation - Part 3. Consolidation - Part 4. Consolidation - Part 5. Consolidation - Part 6. Consolidation - Part 7. Consolidation - Part 8. Consolidation - Part 9. Consolidation - Part Shear Strength - Part 1. Shear Strength - Part 2. Shear Strength - Part 3. Shear Strength - Part 4. Shear Strength - Part 5. Shear Strength - Part 6. Shear Strength - Part 7. Shear Strength - Part 8. Lateral Earth Pressure - Part 1.


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Lateral Earth Pressure - Part 2. Lateral Earth Pressure - Part 3. Lateral Earth Pressure - Part 4. Stay tuned! The surface tension of water explains why the water does not drain out of a wet sand castle or a moist ball of clay. Negative water pressures make the water stick to the particles and pull the particles to each other, friction at the particle contacts make a sand castle stable. But as soon as a wet sand castle is submerged below a free water surface, the negative pressures are lost and the castle collapses.

The negative pore pressure pulls the particles together and causes compressive particle to particle contact forces. Negative pore pressures in clayey soil can be much more powerful than those in sand. Negative pore pressures explain why clay soils shrink when they dry and swell as they are wetted.

The swelling and shrinkage can cause major distress, especially to light structures and roads. Later sections of this article address the pore water pressures for seepage and consolidation problems. Consolidation is a process by which soils decrease in volume. It occurs when stress is applied to a soil that causes the soil particles to pack together more tightly, therefore reducing volume. When this occurs in a soil that is saturated with water, water will be squeezed out of the soil.

The time required to squeeze the water out of a thick deposit of clayey soil layer might be years. For a layer of sand, the water may be squeezed out in a matter of seconds. A building foundation or construction of a new embankment will cause the soil below to consolidate and this will cause settlement which in turn may cause distress to the building or embankment. Karl Terzaghi developed the theory of consolidation which enables prediction of the amount of settlement and the time required for the settlement to occur.

When stress is removed from a consolidated soil, the soil will rebound, drawing water back into the pores and regaining some of the volume it had lost in the consolidation process. If the stress is reapplied, the soil will re-consolidate again along a recompression curve, defined by the recompression index. Soil that has been consolidated to a large pressure and has been subsequently unloaded is considered to be overconsolidated. The maximum past vertical effective stress is termed the preconsolidation stress.

A soil which is currently experiencing the maximum past vertical effective stress is said to be normally consolidated. The overconsolidation ratio , OCR is the ratio of the maximum past vertical effective stress to the current vertical effective stress. The OCR is significant for two reasons: firstly, because the compressibility of normally consolidated soil is significantly larger than that for overconsolidated soil, and secondly, the shear behavior and dilatancy of clayey soil are related to the OCR through critical state soil mechanics ; highly overconsolidated clayey soils are dilatant, while normally consolidated soils tend to be contractive.

The shear strength and stiffness of soil determines whether or not soil will be stable or how much it will deform. Knowledge of the strength is necessary to determine if a slope will be stable, if a building or bridge might settle too far into the ground, and the limiting pressures on a retaining wall. It is important to distinguish between failure of a soil element and the failure of a geotechnical structure e. Different criteria can be used to define the "shear strength" and the " yield point" for a soil element from a stress—strain curve.

One may define the peak shear strength as the peak of a stress—strain curve, or the shear strength at critical state as the value after large strains when the shear resistance levels off. The shear stiffness is important, for example, for evaluation of the magnitude of deformations of foundations and slopes prior to failure and because it is related to the shear wave velocity.

The slope of the initial, nearly linear, portion of a plot of shear stress as a function of shear strain is called the shear modulus. Soil is an assemblage of particles that have little to no cementation while rock such as sandstone may consist of an assembly of particles that are strongly cemented together by chemical bonds. The shear strength of soil is primarily due to interparticle friction and therefore, the shear resistance on a plane is approximately proportional to the effective normal stress on that plane. But in addition to friction, soil derives significant shear resistance from interlocking of grains.

If the grains are densely packed, the grains tend to spread apart from each other as they are subject to shear strain. The expansion of the particle matrix due to shearing was called dilatancy by Osborne Reynolds. Furthermore, as dilative soil grains dilate, they become looser their void ratio increases , and their rate of dilation decreases until they reach a critical void ratio.

Contractive soils become denser as they shear, and their rate of contraction decreases until they reach a critical void ratio. The tendency for a soil to dilate or contract depends primarily on the confining pressure and the void ratio of the soil. The rate of dilation is high if the confining pressure is small and the void ratio is small.

The rate of contraction is high if the confining pressure is large and the void ratio is large. As a first approximation, the regions of contraction and dilation are separated by the critical state line.

The peak strength of the soil may be greater, however, due to the interlocking dilatancy contribution. This may be stated:. However, use of a friction angle greater than the critical state value for design requires care. The peak strength will not be mobilized everywhere at the same time in a practical problem such as a foundation, slope or retaining wall.

The critical state friction angle is not nearly as variable as the peak friction angle and hence it can be relied upon with confidence. Not recognizing the significance of dilatancy, Coulomb proposed that the shear strength of soil may be expressed as a combination of adhesion and friction components: [10]. In addition to the friction and interlocking dilatancy components of strength, the structure and fabric also play a significant role in the soil behavior.

The structure and fabric include factors such as the spacing and arrangement of the solid particles or the amount and spatial distribution of pore water; in some cases cementitious material accumulates at particle-particle contacts. Mechanical behavior of soil is affected by the density of the particles and their structure or arrangement of the particles as well as the amount and spatial distribution of fluids present e. Other factors include the electrical charge of the particles, chemistry of pore water, chemical bonds i.

The presence of nearly incompressible fluids such as water in the pore spaces affects the ability for the pores to dilate or contract. If the pores are saturated with water, water must be sucked into the dilating pore spaces to fill the expanding pores this phenomenon is visible at the beach when apparently dry spots form around feet that press into the wet sand. Similarly, for contractive soil, water must be squeezed out of the pore spaces to allow contraction to take place. Dilation of the voids causes negative water pressures that draw fluid into the pores, and contraction of the voids causes positive pore pressures to push the water out of the pores.

Soil Mechanics and Foundation Engineering

If the rate of shearing is very large compared to the rate that water can be sucked into or squeezed out of the dilating or contracting pore spaces, then the shearing is called undrained shear , if the shearing is slow enough that the water pressures are negligible, the shearing is called drained shear. During undrained shear, the water pressure u changes depending on volume change tendencies.

From the effective stress equation, the change in u directly effects the effective stress by the equation:. It follows then that the undrained shear strength of a soil may be smaller or larger than the drained shear strength depending upon whether the soil is contractive or dilative.

LESSON 1. Introduction of Soil Mechanics

Strength parameters can be measured in the laboratory using direct shear test , triaxial shear test , simple shear test , fall cone test and hand shear vane test ; there are numerous other devices and variations on these devices used in practice today. Tests conducted to characterize the strength and stiffness of the soils in the ground include the Cone penetration test and the Standard penetration test. The stress—strain relationship of soils, and therefore the shearing strength, is affected by: [22]. Lateral earth stress theory is used to estimate the amount of stress soil can exert perpendicular to gravity.

This is the stress exerted on retaining walls. There are three coefficients: at-rest, active, and passive. At-rest stress is the lateral stress in the ground before any disturbance takes place. The active stress state is reached when a wall moves away from the soil under the influence of lateral stress, and results from shear failure due to reduction of lateral stress. The passive stress state is reached when a wall is pushed into the soil far enough to cause shear failure within the mass due to increase of lateral stress. There are many theories for estimating lateral earth stress; some are empirically based, and some are analytically derived.

The bearing capacity of soil is the average contact stress between a foundation and the soil which will cause shear failure in the soil. Allowable bearing stress is the bearing capacity divided by a factor of safety. Sometimes, on soft soil sites, large settlements may occur under loaded foundations without actual shear failure occurring; in such cases, the allowable bearing stress is determined with regard to the maximum allowable settlement.

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It is important during construction and design stage of a project to evaluate the subgrade strength. The California Bearing Ratio CBR test is commonly used to determine the suitability of a soil as a subgrade for design and construction. The Modulus of subgrade reaction ks is used in foundation design, soil-structure interaction studies and design of highway pavements. The field of slope stability encompasses the analysis of static and dynamic stability of slopes of earth and rock-fill dams, slopes of other types of embankments, excavated slopes, and natural slopes in soil and soft rock.

As seen to the right, earthen slopes can develop a cut-spherical weakness zone. The probability of this happening can be calculated in advance using a simple 2-D circular analysis package A recent finding in soil mechanics is that soil deformation can be described as the behavior of a dynamical system. DSSM holds simply that soil deformation is a Poisson process in which particles move to their final position at random shear strains. The basis of DSSM is that soils including sands can be sheared till they reach a steady-state condition at which, under conditions of constant strain-rate, there is no change in shear stress, effective confining stress, and void ratio.

The steady-state was formally defined [26] by Steve J. Poulos an Associate Professor at the Soil Mechanics Department of Harvard University, who built off a hypothesis that Arthur Casagrande was formulating towards the end of his career. The steady state condition is not the same as the "critical state" condition. It differs from the critical state in that it specifies a statistically constant structure at the steady state. The steady-state values are also very slightly dependent on the strain-rate. Many systems in nature reach steady-states and dynamical systems theory is used to describe such systems.

Soil shear can also be described as a dynamical system. Because of its origins in the steady state concept DSSM is sometimes informally called "Harvard soil mechanics. DSSM provides for very close fits to stress—strain curves, including for sands.

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Because it tracks conditions on the failure plane, it also provides close fits for the post failure region of sensitive clays and silts something that other theories are not able to do. From Wikipedia, the free encyclopedia. Main article: Soil gradation. For other uses, see Seep disambiguation. Main article: Flownet. Main article: Effective stress. Main article: Pore water pressure. Main article: Consolidation soil. Main article: shear strength soil. Main article: Lateral earth pressure. Download files for later.

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Modify, remix, and reuse just remember to cite OCW as the source. This class presents the application of principles of soil mechanics. It considers the following topics: the origin and nature of soils; soil classification; the effective stress principle; hydraulic conductivity and seepage; stress-strain-strength behavior of cohesionless and cohesive soils and application to lateral earth stresses; bearing capacity and slope stability; consolidation theory and settlement analysis; and laboratory and field methods for evaluation of soil properties in design practice.

Charles Ladd, and Lucy Jen. Fall For more information about using these materials and the Creative Commons license, see our Terms of Use. Advanced Soil Mechanics.

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