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Soil mechanicsFrom Wikipedia, the free encyclopedia

Soil mechanicsis a branch ofengineering mechanicsthat describes the behavior ofsoils.Itdiffers from fluid mechanics and solid mechanics in the sense that soils consist of a

heterogeneous mixture of fluids (usually air and water) and particles (usuallyclay,silt,sand,andgravel)but soil may also contain organic solids, liquids, and gasses and othermatter.

[1][2][3][4]Along withrock mechanics,soil mechanics provides the theoretical basis for

analysis ingeotechnical engineering,[5]

a subdiscipline ofcivil engineering,andengineeringgeology,a subdiscipline ofgeology.Soil mechanics is used to analyze the deformations of andflow of fluids within

TheTower of Pisaan example of a problem due to deformation of soil.

This article describes the genesis and composition of soil, the distinction betweenpore waterpressureand inter-granulareffective stress, capillary action of fluids in the pore spaces, soil

classification, seepageandpermeability, time dependent change of volume due to squeezingwater out of tiny pore spaces, also known as consolidation, shear strengthand stiffness of soils.The shear strength of soils is primarily derived from friction between the particles andinterlocking, which are very sensitive to the effective stress.

[6]The article concludes with some

examples of applications of the principles of soil mechanics such as slope stability, lateral earthpressure on retaining walls, and bearing capacity of foundations.

Slope instability issues for a temporary flood control levee in North Dakota, 2009

Earthwork in Germany

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Fox Glacier, New Zealand: Soil produced and transported by intense weathering and erosion.

Contents[hide]

1Genesis and composition of soils

o 1.1Genesis

o 1.2Transport

o 1.3Soil composition

1.3.1Soil mineralogy

1.3.2Grain size distribution

1.3.2.1Sieve analysis

1.3.2.2Hydrometer analysis 1.3.3Mass-volume relations

2Effective stress and capillarity: hydrostatic conditions

o 2.1Total stress

o 2.2Pore water pressure

2.2.1Hydrostatic conditions

2.2.2Capillary action

3Soil classification

o 3.1Classification of soil grains

3.1.1Classification of sands and gravels

3.1.2Atterberg limits

3.1.3Classification of silts and clays

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o 3.2Indices related to soil strength

3.2.1Liquidity index

3.2.2Relative density

4Seepage: steady state flow of water

o 4.1Darcy's law

o 4.2Typical values of permeability

o 4.3Flownets

o 4.4Seepage forces and erosion

o 4.5Seepage pressures

5Consolidation: transient flow of water

6Shear behavior: stiffness and strength

o 6.1Friction, interlocking and dilation

o 6.2Failure criteria

o 6.3Structure, fabric, and chemistry

o 6.4Drained and undrained shear

o 6.5Shear tests

o 6.6Other factors

7Applications

o 7.1Lateral earth pressure

o 7.2Bearing capacity

o 7.3Slope stability

8See also

9References

Genesis and composition of soils[edit]Genesis[edit]

The primary mechanism of soil creation is the weathering of rock. All rock types (igneousrock,metamorphic rockandsedimentary rock)may be broken down into small particles to createsoil. Weathering mechanisms are physical weathering, chemical weathering, and biologicalweathering

[1][2][3]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 temperatureto become metamorphic or sedimentary rock, and if melted and solidified again, they wouldcomplete the geologic cycle by becoming igneous rock.

[3]

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 formedby weathering offeldspar,which is the most common mineral present in igneous rock.The most common mineral constituent of silt and sand isquartz,also calledsilica,which has thechemical name silicon dioxide. The reason that feldspar is most common in rocks but silicon ismore prevalent in soils is that feldspar is much more soluble than silica.Silt,Sand,andGravelare basically little pieces of brokenrocks.According to theUnified Soil Classification System,silt particle sizes are in the range of0.002 mm to 0.075 mm and sand particles have sizes in the range of 0.075 mm to 4.75 mm.Gravel particles are broken pieces of rock in the size range 4.75 mm to 100 mm.Particles larger than gravel are called cobbles and boulders.

[1][2]

Transport[edit]

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org/wiki/Soil_mechanics#Lateral_earth_pressurehttp://en.wikipedia.org/wiki/Soil_mechanics#Lateral_earth_pressurehttp://en.wikipedia.org/wiki/Soil_mechanics#Bearing_capacityhttp://en.wikipedia.org/wiki/Soil_mechanics#Bearing_capacityhttp://en.wikipedia.org/wiki/Soil_mechanics#Bearing_capacityhttp://en.wikipedia.org/wiki/Soil_mechanics#Bearing_capacityhttp://en.wikipedia.org/wiki/Soil_mechanics#Slope_stabilityhttp://en.wikipedia.org/wiki/Soil_mechanics#Slope_stabilityhttp://en.wikipedia.org/wiki/Soil_mechanics#Slope_stabilityhttp://en.wikipedia.org/wiki/Soil_mechanics#Slope_stabilityhttp://en.wikipedia.org/wiki/Soil_mechanics#See_alsohttp://en.wikipedia.org/wiki/Soil_mechanics#See_alsohttp://en.wikipedia.org/wiki/Soil_mechanics#See_alsohttp://en.wikipedia.org/wiki/Soil_mechanics#See_alsohttp://en.wikipedia.org/wiki/Soil_mechanics#Referenceshttp://en.wikipedia.org/wiki/Soil_mechanics#Referenceshttp://en.wikipedia.org/wiki/Soil_mechanics#Referenceshttp://en.wikipedia.org/wiki/Soil_mechanics#Referenceshttp://en.wikipedia.org/w/index.php?title=Soil_mechanics&action=edit&section=1http://en.wikipedia.org/w/index.php?title=Soil_mechanics&action=edit&section=1http://en.wikipedia.org/w/index.php?title=Soil_mechanics&action=edit&section=1http://en.wikipedia.org/w/index.php?title=Soil_mechanics&action=edit&section=2http://en.wikipedia.org/w/index.php?title=Soil_mechanics&action=edit&section=2http://en.wikipedia.org/w/index.php?title=Soil_mechanics&action=edit&section=2http://en.wikipedia.org/wiki/Igneous_rockhttp://en.wikipedia.org/wiki/Igneous_rockhttp://en.wikipedia.org/wiki/Igneous_rockhttp://en.wikipedia.org/wiki/Igneous_rockhttp://en.wikipedia.org/wiki/Metamorphic_rockhttp://en.wikipedia.org/wiki/Metamorphic_rockhttp://en.wikipedia.org/wiki/Metamorphic_rockhttp://en.wikipedia.org/wiki/Sedimentary_rockhttp://en.wikipedia.org/wiki/Sedimentary_rockhttp://en.wikipedia.org/wiki/Soil_mechanics#cite_note-mitchell.26soga-1http://en.wikipedia.org/wiki/Soil_mechanics#cite_note-mitchell.26soga-1http://en.wikipedia.org/wiki/Soil_mechanics#cite_note-powrie-3http://en.wikipedia.org/wiki/Soil_mechanics#cite_note-powrie-3http://en.wikipedia.org/wiki/Soil_mechanics#cite_note-powrie-3http://en.wikipedia.org/wiki/Soil_mechanics#cite_note-powrie-3http://en.wikipedia.org/wiki/Soil_mechanics#cite_note-powrie-3http://en.wikipedia.org/wiki/Feldsparhttp://en.wikipedia.org/wiki/Feldsparhttp://en.wikipedia.org/wiki/Feldsparhttp://en.wikipedia.org/wiki/Quartzhttp://en.wikipedia.org/wiki/Quartzhttp://en.wikipedia.org/wiki/Quartzhttp://en.wikipedia.org/wiki/Silicahttp://en.wikipedia.org/wiki/Silicahttp://en.wikipedia.org/wiki/Silicahttp://en.wikipedia.org/wiki/Silthttp://en.wikipedia.org/wiki/Silthttp://en.wikipedia.org/wiki/Sandhttp://en.wikipedia.org/wiki/Sandhttp://en.wikipedia.org/wiki/Sandhttp://en.wikipedia.org/wiki/Gravelhttp://en.wikipedia.org/wiki/Gravelhttp://en.wikipedia.org/wiki/Gravelhttp://en.wikipedia.org/wiki/Rock_(geology)http://en.wikipedia.org/wiki/Rock_(geology)http://en.wikipedia.org/wiki/Rock_(geology)http://en.wikipedia.org/wiki/Unified_Soil_Classification_Systemhttp://en.wikipedia.org/wiki/Unified_Soil_Classification_Systemhttp://en.wikipedia.org/wiki/Unified_Soil_Classification_Systemhttp://en.wikipedia.org/wiki/Soil_mechanics#cite_note-mitchell.26soga-1http://en.wikipedia.org/wiki/Soil_mechanics#cite_note-mitchell.26soga-1http://en.wikipedia.org/wiki/Soil_mechanics#cite_note-mitchell.26soga-1http://en.wikipedia.org/w/index.php?title=Soil_mechanics&action=edit&section=3http://en.wikipedia.org/w/index.php?title=Soil_mechanics&action=edit&section=3http://en.wikipedia.org/w/index.php?title=Soil_mechanics&action=edit&section=3http://en.wikipedia.org/w/index.php?title=Soil_mechanics&action=edit&section=3http://en.wikipedia.org/wiki/Soil_mechanics#cite_note-mitchell.26soga-1http://en.wikipedia.org/wiki/Soil_mechanics#cite_note-mitchell.26soga-1http://en.wikipedia.org/wiki/Unified_Soil_Classification_Systemhttp://en.wikipedia.org/wiki/Rock_(geology)http://en.wikipedia.org/wiki/Gravelhttp://en.wikipedia.org/wiki/Sandhttp://en.wikipedia.org/wiki/Silthttp://en.wikipedia.org/wiki/Silicahttp://en.wikipedia.org/wiki/Quartzhttp://en.wikipedia.org/wiki/Feldsparhttp://en.wikipedia.org/wiki/Soil_mechanics#cite_note-powrie-3http://en.wikipedia.org/wiki/Soil_mechanics#cite_note-powrie-3http://en.wikipedia.org/wiki/Soil_mechanics#cite_note-mitchell.26soga-1http://en.wikipedia.org/wiki/Soil_mechanics#cite_note-mitchell.26soga-1http://en.wikipedia.org/wiki/Sedimentary_rockhttp://en.wikipedia.org/wiki/Metamorphic_rockhttp://en.wikipedia.org/wiki/Igneous_rockhttp://en.wikipedia.org/wiki/Igneous_rockhttp://en.wikipedia.org/w/index.php?title=Soil_mechanics&action=edit&section=2http://en.wikipedia.org/w/index.php?title=Soil_mechanics&action=edit&section=1http://en.wikipedia.org/wiki/Soil_mechanics#Referenceshttp://en.wikipedia.org/wiki/Soil_mechanics#See_alsohttp://en.wikipedia.org/wiki/Soil_mechanics#Slope_stabilityhttp://en.wikipedia.org/wiki/Soil_mechanics#Bearing_capacityhttp://en.wikipedia.org/wiki/Soil_mechanics#Lateral_earth_pressurehttp://en.wikipedia.org/wiki/Soil_mechanics#Applicationshttp://en.wikipedia.org/wiki/Soil_mechanics#Other_factorshttp://en.wikipedia.org/wiki/Soil_mechanics#Shear_testshttp://en.wikipedia.org/wiki/Soil_mechanics#Drained_and_undrained_shearhttp://en.wikipedia.org/wiki/Soil_mechanics#Structure.2C_fabric.2C_and_chemistryhttp://en.wikipedia.org/wiki/Soil_mechanics#Failure_criteriahttp://en.wikipedia.org/wiki/Soil_mechanics#Friction.2C_interlocking_and_dilationhttp://en.wikipedia.org/wiki/Soil_mechanics#Shear_behavior:_stiffness_and_strengthhttp://en.wikipedia.org/wiki/Soil_mechanics#Consolidation:_transient_flow_of_waterhttp://en.wikipedia.org/wiki/Soil_mechanics#Seepage_pressureshttp://en.wikipedia.org/wiki/Soil_mechanics#Seepage_forces_and_erosionhttp://en.wikipedia.org/wiki/Soil_mechanics#Flownetshttp://en.wikipedia.org/wiki/Soil_mechanics#Typical_values_of_permeabilityhttp://en.wikipedia.org/wiki/Soil_mechanics#Darcy.27s_lawhttp://en.wikipedia.org/wiki/Soil_mechanics#Seepage:_steady_state_flow_of_waterhttp://en.wikipedia.org/wiki/Soil_mechanics#Relative_densityhttp://en.wikipedia.org/wiki/Soil_mechanics#Liquidity_indexhttp://en.wikipedia.org/wiki/Soil_mechanics#Indices_related_to_soil_strength

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Example soil horizons. a) top soil and colluvium b) mature residual soil c) young residual soil d) weathered rock.

Soil deposits are affected by the mechanism of transport and deposition to their location. Soils

that are not transported are calledresidual soils

they exist at the same location as the rock fromwhich they were generated.Decomposed graniteis a common example of a residual soil. Thecommon mechanisms of transport are the actions of gravity, ice, water, and wind. Wind blownsoils include dune sands andloess.Water carries particles of different size depending on thespeed of the water, thus soils transported by water are graded according to their size. Silt andclay may settle out in a lake, and gravel and sand collect at the bottom of a river bed. Wind blownsoil deposits (aeoliansoils) also tend to be sorted according to their grain size. Erosion at thebase ofglaciersis powerful enough to pick up large rocks and boulders as well as soil; soilsdropped by melting ice can be a well graded mixture of widely varying particle sizes. Gravity onits own may also carry particles down from the top of a mountain to make a pile of soil andboulders at the base; soil deposits transported by gravity are calledcolluvium.

[1][2]

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 colluviumparticles often have a very angular shape.

Soil composition[edit]

Soil mineralogy[edit]

Silts, sands and gravels are classified by their size, and hence they may consist of a variety ofminerals. Owing to the stability of quartz compared to other rock minerals, quartz is the mostcommon constituent of sand and silt . Mica, and feldspar are other common minerals present insands and silts.

[1]The mineral constituents of gravel may be more similar to that of the parent

rock.

The commonclayminerals aremontmorilloniteorsmectite,illite,andkaoliniteor kaolin. These

minerals tend to form in sheet or plate like structures, with length typically ranging between107

m and 4x106

m and thickness typically ranging between 109

m and 2x106

m, and theyhave a relatively large specific surface area. The specific surface area (SSA) is defined as theratio of the surface area of particles to the mass of the particles. Clay minerals typically havespecific surface areas in the range of 10 to 1,000 square meters per gram of solid.

[3]Due to the

large surface area available for chemical, electrostatic, andvan der Waalsinteraction, themechanical behavior of clay minerals is very sensitive to the amount of pore fluid available andthe type and amount of dissolved ions in the pore fluid.

[1]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-activityclays make very unstable material on which to build because they swell when wet and shrinkwhen dry. This shrink-and-swell action can easily crack foundations and cause retaining walls tocollapse. 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 towork with.

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The minerals of soils are predominantly formed by atoms of oxygen, silicon, hydrogen, andaluminum, 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.

[1]

Grain size distribution[edit]Main article:Soil gradation

Soils consist of a mixture of particles of different size, shape and mineralogy. Because the size ofthe particles obviously has a significant effect on the soil behavior, the grain size and grain sizedistribution are used to classify soils. The grain size distribution describes the relative proportionsof particles of various sizes. The grain size is often visualized in a cumulative distribution graphwhich, for example, plots the percentage of particles finer than a given size as a function of size.

The median grain size, , is the size for which 50% of the particle mass consists of finerparticles. Soil behavior, especially thehydraulic conductivity,tends to be dominated by the

smaller particles, hence, the term "effective size", denoted by , is defined as the size forwhich 10% of the particle mass consists of finer particles.

Sands and gravels that possess a wide range of particle sizes with a smooth distribution ofparticle sizes are called well gradedsoils. If the soil particles in a sample are predominantly in a

relatively narrow range of sizes, the sample is uniformly graded. If a soil sample has distinct gapsin the gradation curve, e.g., a mixture of gravel and fine sand, with no coarse sand, the samplemay be gap graded. Uniformly gradedand gap gradedsoils are both considered to bepoorlygraded. There are many methods for measuringparticle size distribution.The two traditionalmethods are sieve analysis and hydrometer analysis.

Sieve analysis[edit]

Sieve

The size distribution of gravel and sand particles are typically measured using sieve analysis.The formal procedure is described in ASTM D6913-04(2009).

[7]A stack of sieves with accurately

dimensioned holes between a mesh of wires is used to separate the particles into size bins. Aknown 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 periodof time so that the particles are sorted into size bins. This method works reasonably well forparticles in the sand and gravel size range. Fine particles tend to stick to each other, and hencethe sieving process is not an effective method. If there are a lot of fines (silt and clay) present inthe soil it may be necessary to run water through the sieves to wash the coarse particles andclods through.

A variety of sieve sizes are available. The boundary between sand and silt is arbitrary. Accordingto theUnified Soil Classification System,a #4 sieve (4 openings per inch) having 4.75mmopening size separates sand from gravel and a #200 sieve with an 0.075 mm opening separatessand from silt and clay. According to the British standard, 0.063 mm is the boundary betweensand and silt, and 2 mm is the boundary between sand and gravel .

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