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| '''Soil mechanics''' is a branch of [[engineering mechanics]] that describes the behavior of [[soil]]s. 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, liquids, and gasses and other matter.<ref name=mitchell&soga>Mitchell, J.K., and Soga, K. (2005) Fundamentals of soil behavior, Third edition, John Wiley and Sons, Inc., ISBN 978-0-471-46302-7.</ref><ref name=santamarina>{{cite book |author=Santamarina, J.C., Klein, K.A., & Fam, M.A. |year=2001 |title=Soils and Waves: Particulate Materials Behavior, Characterization and Process Monitoring |publisher=Wiley |isbn=978-0-471-49058-6}}.</ref><ref name=powrie>Powrie, W., Spon Press, 2004, ''Soil Mechanics – 2nd ed'' ISBN 0-415-31156-X</ref><ref name=bolton>A Guide to Soil Mechanics, Bolton, Malcolm,Macmillan Press, 1979. ISBN 0-333-18932-0</ref> Along with [[rock mechanics]], soil mechanics provides the theoretical basis for analysis in [[geotechnical engineering]],<ref name=fang>[http://www.tandfbuiltenvironment.com/books/Introductory-Geotechnical-Engineering-isbn9780415304023 Fang, Y., Spon Press, 2006, ''Introductory Geotechnical Engineering'']</ref> 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.<ref name=lambe&whitman>Lambe, T. William & Robert V. Whitman. ''Soil Mechanics''. Wiley, 1991; p. 29. ISBN 978-0-471-51192-2</ref> 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]].
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| [[Image:Leaning Tower of Pisa JD03092007.jpg|thumb|280px|The [[Tower of Pisa]] – an example of a problem due to deformation of soil.]]
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| This article describes the genesis and composition of soil, the distinction between ''pore water pressure'' and inter-granular ''effective stress'', capillary action of fluids in the pore spaces, ''soil classification'', ''seepage'' and ''permeability'', time dependent change of volume due to squeezing water out of tiny pore spaces, also known as ''consolidation'', ''shear strength'' and stiffness of soils. The shear strength of soils is primarily derived from friction between the particles and interlocking, which are very sensitive to the effective stress.<ref name=lambe&whitman/> The article concludes with some examples of applications of the principles of soil mechanics such as slope stability, lateral earth pressure on retaining walls, and bearing capacity of foundations.
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| [[File:FEMA - 40655 - USACOE in Valley City, ND inspecting a dike.jpg|280px|thumb|Slope instability issues for a temporary flood control levee in North Dakota, 2009]]
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| [[File:Fox-Gletscher1.jpg|280px|thumb|Fox Glacier, New Zealand: Soil produced and transported by intense weathering and erosion.]] | | |
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| ==Genesis and composition of soils== | | == Ugg Boots Antwerpen is niet Kneejerk oppositie op aardgas == |
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| 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 <ref name=mitchell&soga/><ref name=santamarina/><ref name=powrie/> 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.<ref name=powrie/>
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| 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. <br>
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| 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 silicon is more prevalent in soils is that feldspar is much more soluble than silica. <br>
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| [[Silt]], [[Sand]], and [[Gravel]] are basically little pieces of broken [[Rock (geology)|rock]]s. <br>
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| 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. <br>
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| Gravel particles are broken pieces of rock in the size range 4.75 mm to 100 mm. <br> Particles larger than gravel are called cobbles and boulders.<ref name=mitchell&soga/><ref name=santamarina/>
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| ===Transport===
| | Toen hij postte dat UNITAR studenten zouden werken in Legoland en Kidzania, was er een enorme respons van zijn volgelingen. De vraag is er. Ik denk dat mensen waarderen blootstelling. Na het updaten naar de nieuwste BIOS voor de DH77EB Desktop Board EBH7710H.86A.0071), vond ik dat ik niet meer kon opstarten in Windows. De BIOS-instellingen tussen de update niet veranderen (UEFI booting ingeschakeld, SATA AHCI-modus [http://www.boligna.be/backoffice/ckeditor/nieuws.asp?u=94-Ugg-Australia-Brussels Ugg Australia Brussels] set). De schijf, een OCZ Agility 3 SSD met de nieuwste firmware, wordt gedetecteerd in de BIOS en toont ook in de F10 opstartmenu.<br><br>Rugpijn is pijn gevoeld in de rug die meestal afkomstig uit de spieren, zenuwen, botten, gewrichten of andere structuren in de wervelkolom. Rugpijn kan een plotseling begin hebben. 11 maart 2011. FIX POGING: Ik probeerde een brute force methode, ik zet iedere bestuurder bestand van 10.4 in de game map. U kunt dit doen terwijl ze toch nog wat nieuwere Catalyst u hebt geïnstalleerd, gewoon de 10.4 installer, annuleren, opent een cmd prompt in C: \ ATI \ Support \ 104_vista64_win7_64_dd_ccc_wdm_enu \ Packages \ Drivers \ Beeldscherm \ W76A_INF \ B_98282 of iets dergelijks, dan gewoon r. C: \ pathtoyourgamefolder en voila! <br><br>Afspiegeling van de werkelijke of toekomstige waardevermindering op de baan. "In tegenstelling tot een blaastest, is dopingcontrole niet bewijzen stoornis," [http://www.campagnesurmer.be/includes/kalender.asp?p=9-Hollister-Winkels-Nederland Hollister Winkels Nederland] zei Alberta commissie directeur Marie Riddle. "Sommige drugs en alcohol gebruikers hebben een handicap. Deze financiering zal een wellidentified hiaat in dienst voor personen die de overgang van diensten in het ziekenhuis aan de gemeenschap te vullen. De Transitional Case Management posities zal zijn middelen voor onze gewaardeerde gemeenschap partners, zoals de Londen politie die behoefte hebben om toegang tot diensten te coördineren in de gemeenschap, crisis ondersteuning of om ervoor te zorgen follow-up wordt gemaakt voor verwijzingen. Deze posities [http://www.concord-remarketing.com/fr/includes/customer.asp?n=3-Air-Max-Leopard Air Max Leopard] zullen ook de druk op Emergency Rooms verlagen door het invullen van de behoefte in de gemeenschap en door steun voor de CMHA Mobile <br><br>A. C. Bennett dam. Wordt gekeken naar de MAC advies zeer zorgvuldig. Maar in het algemeen willen we werkgevers stimuleren aan te werven uit de bewoner arbeidsmarkt waar mogelijk. Hebzucht is een ziekte ENGELS en is erger dan kanker, een lange harde blik op jezelf en je bank boek! Serieus, als je als werkgever moest weer aan het werk voor iemand anders (NIET NEVER SAY) hoeveel zou je [http://www.campagnesurmer.be/includes/kalender.asp?p=39-Hollister-Bikini Hollister Bikini] moeten verdienen om bye week krijgen per week alleen maar om break? zou je part-time of full-time zijn? Als je niet weet, vraag dan uw laagst betaalde werknemer, dit wordt wakker up call, zet je HEBZUCHT ON HOLD, misschien vind uw bedrijf zal TAKE OFF!.<ul> |
| [[Image:Estructura-suelo.jpg|200px|thumb|Example soil horizons. a) top soil and colluvium b) mature residual soil c) young residual soil d) weathered rock.]]
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| 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 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 river bed. Wind blown soil deposits ([[Aeolian processes|aeolian]] soils) also tend to be sorted according to their grain size. Erosion at the base of [[glacier]]s 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]].<ref name=mitchell&soga/><ref name=santamarina/>
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| The mechanism of transport also has a major effect on the particle shape. For example, low velocity grinding in a river bed will produce rounded particles. Freshly fractured colluvium particles often have a very angular shape.
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| ===Soil composition===
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| ====Soil mineralogy====
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| Silts, sands and gravels are classified by their size, and hence they may consist of a variety of minerals. Owing to the stability of quartz compared to other rock minerals, quartz is the most common constituent of sand and silt . Mica, and feldspar are other common minerals present in sands and silts.<ref name=mitchell&soga/> The mineral constituents of gravel may be more similar to that of the parent rock.
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| The common [[clay]] minerals are [[montmorillonite]] or [[smectite]], [[illite]], and [[kaolinite]] or kaolin. These minerals tend to form in sheet or plate like structures, with length typically ranging between 10<sup>−7</sup> m and 4x10<sup>−6</sup> m and thickness typically ranging between 10<sup>−9</sup> m and 2x10<sup>−6</sup> m, and they have a relatively large specific surface area. The specific surface area (SSA) is defined as the ratio of the surface area of particles to the mass of the particles. Clay minerals typically have specific surface areas in the range of 10 to 1,000 square meters per gram of solid.<ref name=powrie/> Due to the large surface area available for chemical, electrostatic, and [[van der Waals force|van der Waals]] interaction, the mechanical behavior of clay minerals is very sensitive to the amount of pore fluid available and the type and amount of dissolved ions in the pore fluid.<ref name=mitchell&soga/>To anticipate the effect of clay on the way a soil will behave, it is necessary to know the kinds of clays as well as the amount present. As home builders and highway engineers know all too well, soils containing certain high-activity clays make very unstable material on which to build because they swell when wet and shrink when dry. This shrink-and-swell action can easily crack foundations and cause retaining walls to collapse. These clays also become extremely sticky and difficult to work with when they are wet. In contrast, low-activity clays, formed under different conditions, can be very stable and easy to work with.
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| The minerals of soils are predominantly formed by atoms of oxygen, silicon, hydrogen, and aluminum, organized in various crystalline forms. These elements along with calcium, sodium, potassium, magnesium, and carbon constitute over 99 per cent of the solid mass of soils.<ref name=mitchell&soga/>
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| ====Grain size distribution====
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| {{main|Soil gradation}}
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| Soils consist of a mixture of particles of different size, shape and mineralogy. Because the size of the particles obviously has a significant effect on the soil behavior, the grain size and grain size distribution are used to classify soils. The grain size distribution describes the relative proportions of particles of various sizes. The grain size is often visualized in a cumulative distribution graph which, for example, plots the percentage of particles finer than a given size as a function of size. The median grain size, <math>D_{50} </math>, is the size for which 50% of the particle mass consists of finer particles. Soil behavior, especially the [[hydraulic conductivity]], tends to be dominated by the smaller particles, hence, the term "effective size", denoted by <math>D_{10} </math>, is defined as the size for which 10% of the particle mass consists of finer particles.
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| Sands and gravels that possess a wide range of particle sizes with a smooth distribution of particle sizes are called ''well graded'' soils. If the soil particles in a sample are predominantly in a relatively narrow range of sizes, the soil are called ''uniformly graded'' soils. If there are distinct gaps in the gradation curve, e.g., a mixture of gravel and fine sand, with no coarse sand, the soils may be called ''gap graded''. ''Uniformly graded'' and ''gap graded'' soils are both considered to be ''poorly graded''. There are many methods for measuring [[particle size distribution]]. The two traditional methods are sieve analysis and hydrometer analysis.
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| ===== Sieve analysis =====
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| [[File:Laboratory sieves BMK.jpg|thumb|Sieve]]
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| The size distribution of gravel and sand particles are typically measured using sieve analysis. The formal procedure is described in ASTM D6913-04(2009).<ref>ASTM Standard Test Methods of Particle-Size Distribution (Gradation) of Soils using Sieve Analysis. http://www.astm.org/Standards/D6913.htm</ref> A stack of sieves with accurately dimensioned holes between a mesh of wires is used to separate the particles into size bins. A known volume of dried soil, with clods broken down to individual particles, is put into the top of a stack of sieves arranged from coarse to fine. The stack of sieves is shaken for a standard period of time so that the particles are sorted into size bins. This method works reasonably well for particles in the sand and gravel size range. Fine particles tend to stick to each other, and hence the sieving process is not an effective method. If there are a lot of fines (silt and clay) present in the soil it may be necessary to run water through the sieves to wash the coarse particles and clods through.
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| A variety of sieve sizes are available. The boundary between sand and silt is arbitrary. According to the [[Unified Soil Classification System]], a #4 sieve (4 openings per inch) having 4.75mm opening size separates sand from gravel and a #200 sieve with an 0.075 mm opening separates sand from silt and clay. According to the British standard, 0.063 mm is the boundary between sand and silt, and 2 mm is the boundary between sand and gravel.<ref name=powrie/>
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| =====Hydrometer analysis=====
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| The classification of fine-grained soils, i.e., soils that are finer than sand, is determined primarily by their [[Atterberg limits]], not by their grain size. If it is important to determine the grain size distribution of fine-grained soils, the hydrometer test may be performed. In the hydrometer tests, the soil particles are mixed with water and shaken to produce a dilute suspension in a glass cylinder, and then the cylinder is left to sit. A [[hydrometer]] is used to measure the density of the suspension as a function of time. Clay particles may take several hours to settle past the depth of measurement of the hydrometer. Sand particles may take less than a second. [[Stoke's law]] provides the theoretical basis to calculate the relationship between sedimentation velocity and particle size. ASTM provides the detailed procedures for performing the Hydrometer test.
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| Clay particles can be sufficiently small that they never settle because they are kept in suspension by [[Brownian motion]], in which case they may be classified as [[colloid]]s.
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| ====Mass-volume relations====
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| [[Image:soil-phase-diagram.svg|thumb|A phase diagram of soil indicating the masses and volumes of air, solid, water, and voids.]]
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| There are a variety of parameters used to describe the relative proportions of air, water and solid in a soil. This section defines these parameters and some of their interrelationships.<ref name=santamarina/><ref name=lambe&whitman/> The basic notation is as follows:
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| <math>V_a</math>, <math>V_w</math>, and <math>V_s</math> represent the volumes of air, water and solids in a soil mixture;
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| <math>W_a</math>, <math>W_w</math>, and <math>W_s</math> represent the weights of air, water and solids in a soil mixture;
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| <math>M_a</math>, <math>M_w</math>, and <math>Ms</math> represent the masses of air, water and solids in a soil mixture;
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| <math>\rho_a</math>, <math>\rho_w</math>, and <math>\rho_s</math> represent the densities of the constituents (air, water and solids) in a soil mixture;
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| Note that the weights, W, can be obtained by multiplying the mass, M, by the acceleration due to gravity, g; e.g., <math>W_s = M_s g</math>
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| [[Specific Gravity]] is the ratio of the density of one material compared to the density of pure water (<math>\rho_w = 1 g/cm^3</math>).
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| '''Specific gravity of solids''', <math>G_s = \frac{\rho_s} {\rho_w}</math>
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| Note that [[unit weight]]s, conventionally denoted by the symbol <math>\gamma</math> may be obtained by multiplying the [[density]] ( <math>\rho</math> ) of a material by the acceleration due to gravity, <math>g</math>.
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| '''[[Density]]''', '''Bulk Density''', or '''Wet Density''', <math>\rho</math>, are different names for the density of the mixture, i.e., the total mass of air, water, solids divided by the total volume of air water and solids (the mass of air is assumed to be zero for practical purposes):
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| :<math>\rho = \frac{M_s + M_w}{V_s + V_w + V_a}= \frac{M_t}{V_t}</math>
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| '''Dry Density''', <math>\rho _d</math>, is the mass of solids divided by the total volume of air water and solids:
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| :<math>\rho_d = \frac{M_s}{V_s + V_w + V_a}= \frac{M_s}{V_t}</math>
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| '''Buoyant Density''', <math>\rho '</math>, defined as the density of the mixture minus the density of water is useful if the soil is submerged under water:
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| :<math>\rho ' = \rho\ - \rho _w</math>
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| where <math>\rho _w</math> is the density of water
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| '''Water Content''', <math>w</math> is the ratio of mass of water to mass of solid. It is easily measured by weighing a sample of the soil, drying it out in an oven and re-weighing. Standard procedures are described by ASTM.
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| :<math>w = \frac{M_w}{M_s} = \frac{W_w}{W_s}</math> | |
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| '''Void ratio''', <math>e</math>, is the ratio of the volume of voids to the volume of solids:
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| :<math>e = \frac{V_V}{V_S} = \frac{V_V}{V_T - V_V} = \frac{n}{1 - n}</math>
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| '''[[Porosity]]''', <math>n</math>, is the ratio of volume of voids to the total volume, and is related to the void ratio:
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| :<math>n = \frac{V_v}{V_t} = \frac{V_v}{V_s + V_v}= \frac{e}{1 + e}</math>
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| '''Degree of saturation''', <math>S</math>, is the ratio of the volume of water to the volume of voids:
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| :<math>S = \frac{V_w}{V_v} </math>
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| From the above definitions, some useful relationships can be derived by use of basic algebra.
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| :<math>\rho = \frac{(G_s+Se)\rho_w}{1+e}</math> | |
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| :<math>\rho = \frac{(1+w)G_s\rho_w}{1+e}</math>
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| :<math>w = \frac{S e}{G_s}</math>
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| ==Effective stress and capillarity: hydrostatic conditions==
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| [[Image:Effstress2.jpg|thumb|right|Spheres immersed in water, reducing effective stress.]]
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| {{main|Effective stress}}
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| To understand the mechanics of soils it is necessary to understand how normal stresses and shear stresses are shared by the different phases. Neither gas nor liquid provide significant resistance to [[shear stress]]. The shear resistance of soil is provided by friction and interlocking of the particles. The friction depends on the intergranular contact stresses between solid particles. The normal stresses, on the other hand, are shared by the fluid and the particles. Although the pore air is relatively compressible, and hence takes little normal stress in most geotechnical problems, liquid water is relatively incompressible and if the voids are saturated with water, the pore water must be squeezed out in order to pack the particles closer together.
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| The principle of effective stress, introduced by [[Karl Terzaghi]], states that the effective stress ''σ''' (i.e., the average intergranular stress between solid particles) may be calculated by a simple subtraction of the pore pressure from the total stress:
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| :<math>\sigma' = \sigma - u\,</math>
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| where ''σ'' is the total stress and ''u'' is the pore pressure. It is not practical to measure ''σ''' directly, so in practice the vertical effective stress is calculated from the pore pressure and vertical total stress. The distinction between the terms pressure and stress is also important. By definition, [[pressure]] at a point is equal in all directions but [[Stress (mechanics)|stresses]] at a point can be different in different directions. In soil mechanics, compressive stresses and pressures are considered to be positive and tensile stresses are considered to be negative, which is different from the solid mechanics sign convention for stress.
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| ===Total stress===
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| For level ground conditions, the total vertical stress at a point, <math>\sigma_v</math>, on average, is the weight of everything above that point per unit area. The vertical stress beneath a uniform surface layer with density <math>\rho</math>, and thickness <math>H</math> is for example:
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| :<math>\sigma_v = \rho g H = \gamma H</math>
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| where <math>g</math> is the acceleration due to gravity, and <math>\gamma</math> is the unit weight of the overlying layer. If there are multiple layers of soil or water above the point of interest, the vertical stress may be calculated by summing the product of the unit weight and thickness of all of the overlying layers. Total stress increases with increasing depth in proportion to the density of the overlying soil.
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| It is not possible to calculate the horizontal total stress in this way. [[Lateral earth pressure]]s are addressed elsewhere.
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| ===Pore water pressure===
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| {{main|Pore water pressure}}
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| ====Hydrostatic conditions====
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| [[Image:CapillaryTube.jpg|thumb|240px|Water is drawn into a small tube by surface tension. Water pressure, u, is negative above and positive below the free water surface]]If there is no pore water flow occurring in the soil, the pore water pressures will be [[hydrostatic]]. The [[water table]] is located at the depth where the water pressure is equal to the atmospheric pressure. For hydrostatic conditions, the water pressure increases linearly with depth below the water table:
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| :<math>u = \rho_w g z_w </math> | |
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| where <math>\rho_w</math> is the density of water, and <math>z_w</math> is the depth below the water table.
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| ====Capillary action====
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| [[Image:WetParticles.jpg|thumb|120px|Water at particle contacts]]
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| Due to [[surface tension]] water will rise up in a small capillary tube above a free surface of water. Likewise, water will rise up above the water table into the small pore spaces around the soil particles. In fact the soil may be completely saturated for some distance above the water table. Above the height of capillary saturation, the soil may be wet but the water content will decrease with elevation. If the water in the capillary zone is not moving, the water pressure obeys the equation of hydrostatic equilibrium, <math>u = \rho_w g z_w </math>, but note that <math>z_w </math>, is negative above the water table. Hence, hydrostatic water pressures are negative above the water table. The thickness of the zone of capillary saturation depends on the pore size, but typically, the heights vary between a centimeter or so for coarse sand to tens of meters for a silt or clay.<ref name=powrie/> In fact the pore space of soil is a uniform fractal e.g. a set of uniformly distributed D-dimensional fractals of average linear size L. For the clay soil is has been found that L=0.15 mm and D=2.7.<ref>Ozhovan M.I., Dmitriev I.E., Batyukhnova O.G. Fractal structure of pores of clay soil. Atomic Energy, 74, 241–243 (1993).</ref>
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| [[Image:WetParticlesFBD.jpg|thumb|120px|intergranular contact force due to surface tension.]]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. Considering the effective stress equation, <math>\sigma' = \sigma - u,</math>, if the water pressure is negative, the effective stress may be positive, even on a free surface (a surface where the total normal stress is zero). The negative pore pressure pulls the particles together and causes compressive particle to particle contact forces.
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| 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.<ref name=Holtz&Kovacs>Holtz, R.D, and Kovacs, W.D., 1981. An Introduction to Geotechnical Engineering. Prentice-Hall, Inc. page 448</ref>
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| [[File:Drymud-IMG 2920.JPG|thumb|120px|Shrinkage caused by drying]]
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| Later sections of this article address the pore water pressures for [[Soil_mechanics#Seepage:_steady_state_flow_of_water|seepage]] and [[Soil_mechanics#Consolidation: transient flow of water|consolidation]] problems.
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| ==Soil classification==
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| Geotechnical engineers classify the soil particle types by performing tests on disturbed (dried, passed through sieves, and remolded) samples of the soil. This provides information about the characteristics of the soil grains themselves. It should be noted that classification of the types of grains present in a soil does not account for important effects of the ''structure'' or ''fabric'' of the soil, terms that describe compactness of the particles and patterns in the arrangement of particles in a load carrying framework as well as the pore size and pore fluid distributions. Engineering geologists also classify soils based on their genesis and depositional history.
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| ===Classification of soil grains===
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| In the US and other countries, the [[Unified Soil Classification System]] (USCS) is often used for soil classification. Other classification systems include the British Standard BS5390 and the [[AASHTO]] soil classification system.<ref name=powrie/>
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| ====Classification of sands and gravels====
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| In the USCS, gravels (given the symbol ''G'') and sands (given the symbol ''S'') are classified according to their grain size distribution. For the USCS, gravels may be given the classification symbol ''GW'' (well-graded gravel), ''GP'' (poorly graded gravel), ''GM'' (gravel with a large amount of silt), or ''GC'' (gravel with a large amount of clay). Likewise sands may be classified as being ''SW'', ''SP'', ''SM'' or ''SC''. Sands and gravels with a small but non-negligible amount of fines (5–12%) may be given a dual classification such as ''SW-SC''.
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| ====Atterberg limits====
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| Clays and Silts, often called 'fine-grained soils', are classified according to their [[Atterberg limits]]; the most commonly used Atterberg limits are the [[Liquid limit]] (denoted by ''LL'' or <math>w_l</math>), [[Plastic Limit]] (denoted by ''PL'' or <math>w_p</math>), and [[Shrinkage limit]] (denoted by ''SL''). The shrinkage limit corresponds to a water content below which the soil will not shrink as it dries.
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| The liquid limit and plastic limit are arbitrary limits determined by tradition and convention. The [[liquid limit]] is determined by measuring the water content for which a groove closes after 25 blows in a standard test.<ref name=ASTMclass>{{Cite journal|publisher=American Society for Testing and Materials |year=1985 |series=D 2487-83 |title=Classification of Soils for Engineering Purposes: Annual Book of ASTM Standards |volume=04.08 |pages=395–408 |url=http://www.astm.org/Standards/D2487.htm|postscript=<!-- Bot inserted parameter. Either remove it; or change its value to "." for the cite to end in a ".", as necessary. -->}}</ref> Alternatively, a [[fall cone test]] apparatus may be use to measure the liquid limit. The undrained shear strength of remolded soil at the liquid limit is approximately 2 kPa.<ref name=bolton/><ref name=wood>Wood, David Muir, Soil Behavior and Critical State Soil Mechanics, Cambridge University Press, 1990, ISBN 0-521-33249-4</ref> The [[plastic limit]] is the water content below which it is not possible to roll by hand the soil into 3 mm diameter cylinders. The soil cracks or breaks up as it is rolled down to this diameter. Remolded soil at the plastic limit is quite stiff, having an undrained shear strength of the order of about 200 kPa.<ref name=bolton/><ref name=wood/>
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| The [[Plasticity index]] of a particular soil specimen is defined as the difference between the [[Liquid limit]] and the [[Plastic limit]] of the specimen; it is an indicator of how much water the soil particles in the specimen can absorb. The plasticity index is the difference in water contents between states when the soil is relatively soft and the soil is relatively brittle when molded by hand.
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| ====Classification of silts and clays====
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| According to the [[Unified Soil Classification System]] (USCS), silts and clays are classified by plotting the values of their [[plasticity index]] and [[liquid limit]] on a plasticity chart. The A-Line on the chart separates clays (given the USCS symbol ''C'') from silts (given the symbol ''M''). LL=50% separates high plasticity soils (given the modifier symbol ''H'') from low plasticity soils (given the modifier symbol ''L''). A soil that plots above the A-line and has LL>50% would, for example, be classified as ''CH''. Other possible classifications of silts and clays are ''ML'', ''CL'' and ''MH''. If the Atterberg limits plot in the"hatched" region on the graph near the origin, the soils are given the dual classification 'CL-ML'.
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| ===Indices related to soil strength===
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| ====Liquidity index====
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| The effects of the water content on the strength of saturated remolded soils can be quantified by the use of the ''liquidity index'', ''LI'':
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| :<math> LI = \frac{w-PL}{LL-PL}</math>
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| When the LI is 1, remolded soil is at the [[liquid limit]] and it has an undrained shear strength of about 2 kPa. When the soil is at the [[plastic limit]], the LI is 0 and the undrained shear strength is about 200 kPa.<ref name=bolton/><ref name=schofield>Disturbed soil properties and geotechnical design, Schofield, Andrew N.,Thomas Telford, 2006. ISBN 0-7277-2982-9</ref>
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| ====Relative density====
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| The density of sands (cohesionless soils) is often characterized by the relative density, <math> D_r</math>
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| :<math> D_r= \frac{e_{max} - e}{e_{max} - e_{min}} 100% </math>
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| where: <math>e_{max}</math> is the "maximum void ratio" corresponding to a very loose state, <math>e_{min}</math> is the "minimum void ratio" corresponding to a very dense state and <math>e</math> is the ''in situ'' void ratio. Methods used to calculate relative density are defined in ASTM D4254-00(2006).<ref>ASTM Standard Test Methods for Minimum Index Density and Unit Weight of Soils and Calculation of Relative Density. http://www.astm.org/Standards/D4254.htm</ref>
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| Thus if <math> D_r = 100% </math> the sand or gravel is very dense, and if <math> D_r = 0% </math> the soil is extremely loose and unstable.
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| ==Seepage: steady state flow of water==
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| {{redirect|Seepage|the Tech N9ne EP|Seepage (EP)}}
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| [[Image:WaterTable.gif|right|thumb|300px|A cross section showing the water table varying with surface topography as well as a perched water table.]]
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| If fluid pressures in a soil deposit are uniformly increasing with depth according to <math>u = \rho_w g z_w</math>
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| then hydrostatic conditions will prevail and the fluids will not be flowing through the soil. <math>z_w</math> is the depth below the water table. However, if the water table is sloping or there is a perched water table as indicated in the accompanying sketch, then '''seepage''' will occur. For steady state seepage, the seepage velocities are not varying with time. If the water tables are changing levels with time, or if the soil is in the process of consolidation, then steady state conditions do not apply.
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| ===Darcy's law===
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| [[Darcy's law]] states that the volume of flow of the pore fluid through a porous medium per unit time is proportional to the rate of change of excess fluid pressure with distance. The constant of proportionality includes the viscosity of the fluid and the intrinsic permeability of the soil. For the simple case of a horizontal tube filled with soil [[Image:Darcy's Law.png|thumb|right|300px|Diagram showing definitions and directions for Darcy's law.]]
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| :<math>Q=\frac{-K A}{\mu}\frac{(u_b-u_a)}{L}</math>
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| The total discharge, <math>Q</math> (having units of volume per time, e.g., ft³/s or m³/s), is proportional to the [[Permeability (fluid)|intrinsic permeability]], <math>K</math>, the cross sectional area, <math>A</math>, and rate of pore pressure change with distance, <math>\frac{u_b-u_a}{L}</math>, and inversely proportional to the [[dynamic viscosity]] of the fluid, <math>\mu</math>. The negative sign is needed because fluids flow from high pressure to low pressure. So if the change in pressure is negative (in the <math>x</math>-direction) then the flow will be positive (in the <math>x</math>-direction). The above equation works well for a horizontal tube, but if the tube was inclined so that point b was a different elevation than point a, the equation would not work. The effect of elevation is accounted for by replacing the pore pressure by ''excess pore pressure'', <math>u_e</math> defined as:
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| <math>u_e = u - \rho_w g z</math>
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| where <math>z</math> is the depth measured from an arbitrary elevation reference ([[datum (geodesy)|datum]]). Replacing <math>u</math> by <math>u_e</math> we obtain a more general equation for flow:
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| :<math>Q=\frac{-K A}{\mu}\frac{(u_{e,b}-u_{e,a})}{L}</math> | |
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| Dividing both sides of the equation by <math>A</math>, and expressing the rate of change of excess pore pressure as a [[derivative]], we obtain a more general equation for the apparent velocity in the x-direction:
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| :<math>v_x=\frac{-K }{\mu}\frac{d u_e}{d x}</math>
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| where <math>v_x = Q/A</math> has units of velocity and is called the ''Darcy velocity'' (or the ''specific discharge'', ''filtration velocity'', or ''superficial velocity''). The ''pore'' or ''interstitial velocity'' <math>v_{px}</math> is the average velocity of fluid molecules in the pores; it is related to the Darcy velocity and the porosity <math>n</math> through the ''Dupuit-Forchheimer relationship''
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| :<math>v_{px}=\frac{v_x}{n}</math> | |
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| (Some authors use the term ''seepage velocity'' to mean the Darcy velocity,<ref>Smith, I. (2013) Smith's elements of soil mechanics, 8th edition, John Wiley and Sons, Inc., ISBN 978-1-405-13370-8.</ref> while others use it to mean the pore velocity.<ref>Delleur, Jacques W. (2007) The handbook of groundwater engineering, Taylor & Francis, ISBN 978-0-849-34316-2.</ref>)
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| [[Civil engineer]]s predominantly work on problems that involve water and predominantly work on problems on earth (in earth's gravity). For this class of problems, civil engineers will often write Darcy's law in a much simpler form:<ref name=bolton/><ref name=lambe&whitman/><ref name=Holtz&Kovacs/>
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| :<math>v = k i</math>
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| where <math>k</math> is the [[hydraulic conductivity]], defined as <math>k = \frac {K \rho_w g} {\mu_w}</math>, and <math>i</math> is the [[hydraulic gradient]]. The hydraulic gradient is the rate of change of [[total head]] with distance. The total head, <math>h</math> at a point is defined as the height (measured relative to the datum) to which water would rise in a [[piezometer]] at that point. The total head is related to the excess water pressure by:
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| :<math>u_e = \rho_w g h + Constant </math>
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| and the <math> Constant </math> is zero if the datum for head measurement is chosen at the same elevation as the origin for the depth, z used to calculate <math> u_e </math>.
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| ===Typical values of permeability===
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| Values of the permeability, <math>k</math>, can vary by many orders of magnitude depending on the soil type. Clays may have permeability as small as about <math>10^{-12}\frac{m}{s}</math>, gravels may have permeability up to about <math>10^{-1}\frac{m}{s}</math>. Layering and heterogeneity and disturbance during the sampling and testing process make the accurate measurement of soil permeability a very difficult problem.<ref name=bolton/>
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| ===Flownets===
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| {{main|Flownet}}
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| Darcy's Law applies in one, two or three dimensions.<ref name=powrie/> In two or three dimensions, steady state seepage is described by [[Laplace's equation]]. Computer programs are available to solve this equation. But traditionally two-dimensional seepage problems were solved using and a graphical procedure known called [[flownet]].<ref name=powrie/><ref name=Holtz&Kovacs/><ref name=cedergren>Cedergren, Harry R. (1977), ''Seepage, Drainage, and Flow Nets'', Wiley. ISBN 0-471-14179-8</ref> One set of lines in the flownet are in the direction of the water flow (flow lines), and the other set of lines are in the direction of constant total head (equipotential lines). Flownets may be used to estimate the quantity of seepage under [[dam]]s and [[sheet piling]]. [[Image:flownet pumping well.png|thumb|right|200px|A plan flow net to estimate flow of water from a stream to a discharging well]]
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| ===Seepage forces and erosion===
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| {{anchor|Seepage forces and erosion}}
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| When the seepage velocity is great enough, [[erosion]] can occur because of the frictional drag exerted on the soil particles. Vertically upwards seepage is a source of danger on the downstream side of sheet piling and beneath the toe of a dam or levee. Erosion of the soil, known as "soil piping", can lead to failure of the structure and to [[sinkhole]] formation. Seeping water removes soil, starting from the exit point of the seepage, and erosion advances upgradient.<ref>{{cite journal | last = Jones | first = J. A. A. | title = Soil piping and stream channel initiation | journal = Water Resources Research | volume = 7 | issue = 3 | pages = 602–610 | year = 1976 | doi = 10.1029/WR007i003p00602 | bibcode=1971WRR.....7..602J}}</ref> The term "sand boil" is used to describe the appearance of the discharging end of an active soil pipe.<ref>{{cite web |last=Dooley |first=Alan |title=Sandboils 101: Corps has experience dealing with common flood danger | work = Engineer Update |publisher=US Army Corps of Engineers |date=June 2006 |url=http://www.hq.usace.army.mil/cepa/pubs/jun06/story8.htm |accessdate = 2006-08-29 |archiveurl = http://web.archive.org/web/20060727161844/http://www.hq.usace.army.mil/cepa/pubs/jun06/story8.htm <!-- Bot retrieved archive --> |archivedate = 2006-07-27}}</ref>
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| ===Seepage pressures===
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| Seepage in an upward direction reduces the effective stress within the soil. When the water pressure at a point in the soil is equal to the total vertical stress at that point, the effective stress is zero and the soil has no frictional resistance to deformation. For a surface layer, the vertical effective stress becomes zero within the layer when the upward hydraulic gradient is equal to the critical gradient.<ref name=Holtz&Kovacs/> At zero effective stress soil has very little strength and layers of relatively impermeable soil may heave up due to the underlying water pressures. The loss in strength due to upward seepage is a common contributor to levee failures. The condition of zero effective stress associated with upward seepage is also called [[soil liquefaction|liquefaction]], [[quicksand]], or a boiling condition. Quicksand was so named because the soil particles move around and appear to be 'alive' (the biblical meaning of 'quick' – as opposed to 'dead'). (Note that it is not possible to be 'sucked down' into quicksand. On the contrary, you would float with about half your body out of the water.)<ref name=terzaghiPeckMesri>Terzaghi, K., Peck, R.B., and Mesri, G. 1996. Soil Mechanics in Engineering Practice. Third Edition, John Wiley & Sons, Inc. Article 18, page 135.</ref>
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| ==Consolidation: transient flow of water==
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| {{main|Consolidation (soil)}}
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| [[Image:Consolidation spring analogy.jpg|600px|thumb| Consolidation analogy. The piston is supported by water underneath and a spring. When a load is applied to the piston, water pressure increases to support the load. As the water slowly leaks through the small hole, the load is transferred from the water pressure to the spring force. |center]]
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| Consolidation is a process by which [[soil]]s decrease in volume. It occurs when [[Stress (physics)|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.<ref name=terzaghi>Terzaghi, K., 1943, ''Theoretical Soil Mechanics'', John Wiley and Sons, New York</ref> Soils are tested with an [[geotechnical investigation|oedometer test]] to determine their compression index and coefficient of consolidation.
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| 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.<ref name=santamarina/><ref name=powrie/><ref name=bolton/>
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| ==Shear behavior: stiffness and strength==
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| {{main|shear strength (soil)}}
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| [[File:StressStrainPeakCrit.JPG|200px|thumb|Typical stress strain curve for a drained dilatant soil]]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.g., a building foundation, slope or retaining wall); some soil elements may reach their peak strength prior to failure of the structure. Different criteria can be used to define the "shear strength" and the "[[yield (engineering)|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. If the stress-strain curve does not stabilize before the end of shear strength test, the "strength" is sometimes considered to be the shear resistance at 15% to 20% strain.<ref name=Holtz&Kovacs/> The shear strength of soil depends on many factors including the [[effective stress]] and the void ratio.
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| 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]]
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| ===Friction, interlocking and dilation===
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| 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.<ref name=powrie/> But soil also 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]].<ref name="schofield"/> If one considers the energy required to shear an assembly of particles there is energy input by the shear force, T, moving a distance, x and there is also energy input by the normal force, N, as the sample expands a distance, y.<ref name=schofield/> Due to the extra energy required for the particles to dilate against the confining pressures, dilatant soils have a greater peak strength than contractive soils. 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.
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| [[File:E logP criticalStateLine.JPG|200px|thumb|A critical state line separates the dilatant and contractive states for soil]]
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| 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.
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| ===Failure criteria===
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| After a soil reaches the critical state, it is no longer contracting or dilating and the shear stress on the failure plane <math> \tau_{crit}</math>
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| is determined by the effective normal stress on the failure plane <math> \sigma_n ' </math>
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| and critical state friction angle <math> \phi_{crit} '\ </math>:
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| :<math> \tau_{crit} = \sigma_n ' \tan \phi_{crit} '\ </math> | |
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| The peak strength of the soil may be greater, however, due to the interlocking (dilatancy) contribution.
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| This may be stated:
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| :<math> \tau_{peak} = \sigma_n ' \tan \phi_{peak} '\ </math>
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| Where <math> \phi_{peak}' > \phi_{crit}' </math>. 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.<ref name=powrie/><ref name=bolton/><ref name=schofield/>
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| 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:<ref name=schofield/>
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| :<math> \tau_f = c' + \sigma_f ' \tan \phi '\,</math>
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| It is now known that the <math> c ' </math>
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| and <math> \phi'</math> parameters in the last equation are not fundamental soil properties.<ref name=powrie/><ref name=lambe&whitman/><ref name=schofield/><ref name=terzaghipeck&mesri>Terzaghi, K., Peck, R.B., Mesri, G. (1996) Soil mechanics in Engineering Practice, Third Edition, John Wiley & Sons, Inc.,ISBN0-471-08658-4</ref> In particular, <math> c '</math> and <math> \phi '</math> are different depending on the magnitude of effective stress.<ref name=lambe&whitman/><ref name=terzaghipeck&mesri/> According to Schofield (2006),<ref name=schofield/> the longstanding use of <math> c' </math> in practice has led many engineers to wrongly believe that <math> c'</math> is a fundamental parameter. This assumption that <math> c '</math> and <math> \phi '</math> are constant can lead to overestimation of peak strengths.<ref name=powrie/><ref name=terzaghipeck&mesri/>
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| ===Structure, fabric, and chemistry===
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| 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.g., water and air voids). Other factors include the electrical charge of the particles, chemistry of pore water, chemical bonds (i.e. cementation -particles connected through a solid substance such as recrystallized calcium carbonate) <ref name=mitchell&soga/><ref name=terzaghipeck&mesri/>
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| ===Drained and undrained shear===
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| The presence of nearly [[incompressible]] fluids such as water in the pore spaces affects the ability for the pores to dilate or contract.
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| 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). [[File:DilatingBeachSandClose.JPG|200px|thumb|Foot pressing in soil causes soil to dilate, drawing water from the surface into the pores]]
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| Similarly, for contractive soil, water must be squeezed out of the pore spaces to allow contraction to take place.
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| 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. 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:
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| :<math>\sigma' = \sigma - u\,</math> | |
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| and the strength is very sensitive to the effective stress. 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.
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| ===Shear tests===
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| 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]].
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| ===Other factors===
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| The stress-strain relationship of soils, and therefore the shearing strength, is affected by:<ref>Poulos, S. J. 1989. Advance Dam Engineering for Design, Construction, and Rehabilitation: Liquefaction Related Phenomena. Ed. Jansen, R.B, Van Nostrand Reinhold, pages 292–297.</ref>
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| # ''soil composition'' (basic soil material): mineralogy, grain size and grain size distribution, shape of particles, pore fluid type and content, ions on grain and in pore fluid.
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| # ''state'' (initial): Define by the initial [[void ratio]], effective normal stress and shear stress (stress history). State can be describe by terms such as: loose, dense, overconsolidated, normally consolidated, stiff, soft, contractive, dilative, etc.
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| # ''structure'': Refers to the arrangement of particles within the soil mass; the manner in which the particles are packed or distributed. Features such as layers, joints, fissures, slickensides, voids, pockets, cementation, etc., are part of the structure. Structure of soils is described by terms such as: undisturbed, disturbed, remolded, compacted, cemented; flocculent, honey-combed, single-grained; flocculated, deflocculated; stratified, layered, laminated; isotropic and anisotropic.
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| # ''Loading conditions'': Effective [[stress path]] -drained, undrained, and type of loading -magnitude, rate (static, dynamic), and time history (monotonic, cyclic).
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| ==Applications==
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| ===Lateral earth pressure===
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| {{main|Lateral earth pressure}}
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| 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]]. A lateral earth stress coefficient, K, is defined as the ratio of lateral (horizontal) effective stress to vertical effective stress for cohesionless soils (K=σ'<sub>h</sub>/σ'<sub>v</sub>). 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.
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| ===Bearing capacity===
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| {{main|Bearing capacity}}
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| The bearing capacity of soil is the average contact [[Stress (physics)|stress]] between a [[foundation (architecture)|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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| ===Slope stability===
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| [[Image:Slopslump2.jpg|thumb|right|Simple slope slip section]]
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| {{main|Slope stability}}
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| 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.<ref>[http://www.usace.army.mil/publications/eng-manuals/em1110-2-1902/entire.pdf US Army Corps of Engineers Manual on Slope Stability]</ref>
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| 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...<ref>{{cite web |url=http://www.wise-uranium.org/cssth.html |title=Slope Stability Calculator |accessdate=2006-12-14 |work= }}</ref> A primary difficulty with analysis is locating the most-probable slip plane for any given situation.<ref>{{Cite journal|last1=Chugh|first=Ashok|authorlink=|year=2002|title=A method for locating critical slip surfaces in slope stability analysis|journal=NRC Research Press|volume=|issue=|pages=|url=http://article.pubs.nrc-cnrc.gc.ca/ppv/RPViewDoc?_handler_=HandleInitialGet&journal=cgj&volume=39&calyLang=eng&articleFile=t02-042.pdf|postscript=<!--None-->}}</ref> Many landslides have been analyzed only after the fact.
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| ==See also==
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| * [[Critical state soil mechanics]]
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| *[http://soilmechanics.us/dssm Dynamical Systems based Soil Mechanics--a short, self-study course]
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| * [[Earthquake engineering]]
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| * [[Engineering geology]]
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| * [[Geotechnical engineering]]
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| * [[Geotechnics]]
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| * [[Rock mechanics]]
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| * [[Slope stability analysis]]
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| * [[Hydrogeology]], aquifer characteristics closely related to soil characteristics
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| * [[International Society for Soil Mechanics and Geotechnical Engineering]]
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| * Wiktionary [[Wikt:seepage|seepage]]
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| ==References==
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| {{reflist|30em}}
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| </div> | |
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| {{Geotechnical engineering}}
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| {{DEFAULTSORT:Soil Mechanics}}
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| [[Category:Geotechnical engineering]]
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| [[Category:Articles with inconsistent citation formats]]
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| [[Category:Soil mechanics| ]]
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