soil mechanics - wikipedia, the free encyclopedia

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Soil mechanics

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Soil mechanics is a branch of engineering mechanics that describes the behaviorof soils. It differs from fluid mechanics and solid mechanics in the sense thatsoils consist of a heterogeneous mixture of fluids (usually air and water) andparticles (usually clay, silt, sand, and gravel) but soil may also containorganic solids, liquids, and gasses and other matter.[1][2][3][4] Along withrock mechanics, soil mechanics provides the theoretical basis for analysis ingeotechnical engineering,[5] a subdiscipline of Civil engineering, andengineering geology, a subdiscipline of geology. Soil mechanics is used toanalyze the deformations of and flow of fluids within natural and man-madestructures that are supported on or made of soil, or structures that are buriedin soils.[6] Example applications are building and bridge foundations, retaining

walls, dams, and buried pipeline systems. Principles of soil mechanics are alsoused in related disciplines such as engineering geology, geophysicalengineering, coastal engineering, agricultural engineering, hydrology and soilphysics.


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The Tower of Pisa -- an example of a problem due to deformation of soil.This article describes the genesis and composition of soil, the distinctionbetween pore water pressure and inter-granular effective stress, capillaryaction of fluids in the pore spaces, soil classification, seepage andpermeability, time dependent change of volume due to squeezing water out of tinypore spaces, also known as consolidation, shear strength and stiffness of soils.The shear strength of soils is primarily derived from friction between theparticles and interlocking, which are very sensitive to the effective stress.[6]The article concludes with some examples of applications of the principles ofsoil mechanics such as slope stability, lateral earth pressure on retainingwalls, and bearing capacity of foundations.

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

Earthwork in Germany

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

Contents[hide] 1 Genesis and composition of soils 1.1 Genesis1.2 Transport

1.3 Soil composition 1.3.1 Soil mineralogy1.3.2 Grain size distribution 1.3.2.1 Sieve analysis1.3.2.2 Hydrometer analysis

1.3.3 Mass-volume relations

2 Effective stress and capillarity: hydrostatic conditions 2.1 Totalstress2.2 Pore water pressure 2.2.1 Hydrostatic conditions

2.2.2 Capillary action

3 Soil classification 3.1 Classification of soil grains 3.1.1Classification of sands and gravels3.1.2 Atterberg limits3.1.3 Classification of silts and clays

3.2 Indices related to soil strength 3.2.1 Liquidity index3.2.2 Relative density

4 Seepage: steady state flow of water 4.1 Darcy's law


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4.2 Typical values of permeability4.3 Flow nets4.4 Seepage forces and erosion4.5 Seepage pressures

5 Consolidation: transient flow of water6 Shear behavior: stiffness and strength 6.1 Friction, interlockingand dilation6.2 Failure criteria6.3 Structure, fabric, and chemistry6.4 Drained and undrained shear6.5 Shear tests6.6 Other factors

7 Applications 7.1 Lateral earth pressure7.2 Bearing capacity7.3 Slope stability

8 See also9 References

Genesis and composition of soils[edit]

Genesis[edit]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 intosmall particles to create soil. Weathering mechanisms are physical weathering,chemical weathering, and biological weathering [1][2][3] Human activities suchas excavation, blasting, and waste disposal, may also create soil. Over geologictime, deeply buried soils may be altered by pressure and temperature to becomemetamorphic 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 incracks, rain, wind, impact and other mechanisms. Chemical weathering includesdissolution of matter composing a rock and precipitation in the form of anothermineral. Clay minerals, for example can be formed by weathering of feldspar,which is the most common mineral present in igneous rock.The most common mineral constituent of silt and sand is quartz, also calledsilica, which has the chemical name silicon dioxide. The reason that feldspar ismost common in rocks but silicon is more prevalent in soils is that feldspar ismuch more soluble than silica.Silt, Sand, and Gravel are basically little pieces of broken rocks.According to the Unified Soil Classification System, silt particle sizes are in

the range of 0.002 mm to 0.075 mm and sand particles have sizes in the range of0.075 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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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 theirlocation. Soils that are not transported are called residual soilsthey exist atthe same location as the rock from which they were generated. Decomposed graniteis a common example of a residual soil. The common mechanisms of transport arethe actions of gravity, ice, water, and wind. Wind blown soils include dunesands and loess. Water carries particles of different size depending on thespeed of the water, thus soils transported by water are graded according totheir size. Silt and clay may settle out in a lake, and gravel and sand collectat the bottom of a river bed. Wind blown soil deposits (aeolian soils) also tendto be sorted according to their grain size. Erosion at the base of glaciers ispowerful 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 particlesizes. Gravity on its own may also carry particles down from the top of amountain to make a pile of soil and boulders at the base; soil depositstransported by gravity are called colluvium.[1][2]

The mechanism of transport also has a major effect on the particle shape. Forexample, low velocity grinding in a river bed will produce rounded particles.

Freshly fractured colluvium particles often have a very angular shape.Soil composition[edit]

Soil mineralogy[edit]

Silts, sands and gravels are classified by their size, and hence they mayconsist of a variety of minerals. Owing to the stability of quartz compared toother rock minerals, quartz is the most common constituent of sand and silt .Mica, and feldspar are other common minerals present in sands and silts.[1] Themineral constituents of gravel may be more similar to that of the parent rock.

The common clay minerals are montmorillonite or smectite, illite, and kaolinite

or kaolin. These minerals tend to form in sheet or plate like structures, withlength typically ranging between 107 m and 4x106 m and thickness typicallyranging between 109 m and 2x106 m, and they have a relatively large specificsurface area. The specific surface area (SSA) is defined as the ratio of thesurface area of particles to the mass of the particles. Clay minerals typicallyhave specific surface areas in the range of 10 to 1,000 square meters per gramof solid.[3] Due to the large surface area available for chemical,electrostatic, and van der Waals interaction, the mechanical behavior of clayminerals is very sensitive to the amount of pore fluid available and the typeand amount of dissolved ions in the pore fluid.[1]

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 99per 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 of the particles obviously has a significant effect on the soil


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behavior, the grain size and grain size distribution are used to classify soils.The grain size distribution describes the relative proportions of particles ofvarious sizes. The grain size is often visualized in a cumulative distributiongraph which, for example, plots the percentage of particles finer than a givensize as a function of size. The median grain size, , is the size for which 50%of the particle mass consists of finer particles. 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 for which 10% ofthe particle mass consists of finer particles.

Sands and gravels that possess a wide range of particle sizes with a smoothdistribution of particle sizes are called well graded soils. If the soilparticles 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 thegradation 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 areboth considered to be poorly graded. There are many methods for measuringparticle size distribution. The two traditional methods are sieve analysis andhydrometer analysis.

Sieve analysis[edit]

SieveThe size distribution of gravel and sand particles are typically measured usingsieve analysis. The formal procedure is described in ASTM D6913-04(2009).[7] Astack of sieves with accurately dimensioned holes between a mesh of wires isused 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 stackof sieves arranged from coarse to fine. The stack of sieves is shaken for astandard period of time so that the particles are sorted into size bins. Thismethod 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 notan effective method. If there are a lot of fines (silt and clay) present in thesoil it may be necessary to run water through the sieves to wash the coarseparticles and clods through.

A variety of sieve sizes are available. The boundary between sand and silt isarbitrary. According to the Unified Soil Classification System, a #4 sieve (4openings 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. Accordingto the British standard, 0.063 mm is the boundary between sand and silt, and 2mm is the boundary between sand and gravel.[3]

Hydrometer analysis[edit]

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. Ifit is important to determine the grain size distribution of fine-grained soils,the hydrometer test may be performed. In the hydrometer tests, the soilparticles are mixed with water and shaken to produce a dilute suspension in aglass cylinder, and then the cylinder is left to sit. A hydrometer is used tomeasure the density of the suspension as a function of time. Clay particles may


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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 theoreticalbasis to calculate the relationship between sedimentation velocity and particlesize. ASTM provides the detailed procedures for performing the Hydrometer test.

Clay particles can be sufficiently small that they never settle because they arekept in suspension by Brownian motion, in which case they may be classified ascolloids.

Mass-volume relations[edit]

A phase diagram of soil indicating the masses and volumes of air, solid, water,and voids.There are a variety of parameters used to describe the relative proportions ofair, water and solid in a soil. This section defines these parameters and someof their interrelationships.[2][6] The basic notation is as follows:

, , and represent the volumes of air, water and solids in a soil mixture;, , and represent the weights of air, water and solids in a soil mixture;

, , and represent the masses of air, water and solids in a soil mixture;

, , and represent the densities of the constituents (air, water and solids) ina soil mixture;

Note that the weights, W, can be obtained by multiplying the mass, M, by theacceleration due to gravity, g; e.g.,

Specific Gravity is the ratio of the density of one material compared to the

density of pure water ().Specific gravity of solids,

Note that unit weights, conventionally denoted by the symbol may be obtained bymultiplying the density ( ) of a material by the acceleration due to gravity, .

Density, Bulk Density, or Wet Density, , are different names for the density ofthe mixture, i.e., the total mass of air, water, solids divided by the totalvolume of air water and solids (the mass of air is assumed to be zero forpractical purposes):

Dry Density, , is the mass of solids divided by the total volume of air waterand solids:

Buoyant Density, , defined as the density of the mixture minus the density ofwater is useful if the soil is submerged under water:

where is the density of water

Water Content, is the ratio of mass of water to mass of solid. It is easilymeasured by weighing a sample of the soil, drying it out in an oven and


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re-weighing. Standard procedures are described by ASTM.

Void ratio, , is the ratio of the volume of voids to the volume of solids:

Porosity, , is the ratio of volume of voids to the total volume, and is relatedto the void ratio:

Degree of saturation, , is the ratio of the volume of water to the volume ofvoids:

From the above definitions, some useful relationships can be derived by use ofbasic algebra.

Effective stress and capillarity: hydrostatic conditions[edit]

Spheres immersed in water, reducing effective stress.Main article: Effective stress

To understand the mechanics of soils it is necessary to understand how normalstresses and shear stresses are shared by the different phases. Neither gas nor

liquid provide significant resistance to shear stress. The shear resistance ofsoil is provided by friction and interlocking of the particles. The frictiondepends on the intergranular contact stresses between solid particles. Thenormal stresses, on the other hand, are shared by the fluid and the particles.Although the pore air is relatively compressible, and hence takes little normalstress in most geotechnical problems, liquid water is relatively incompressibleand if the voids are saturated with water, the pore water must be squeezed outin order to pack the particles closer together.

The principle of effective stress, introduced by Karl Terzaghi, states that theeffective stress ' (i.e., the average intergranular stress between solidparticles) may be calculated by a simple subtraction of the pore pressure fromthe total stress:

where is the total stress and u is the pore pressure. It is not practical tomeasure ' directly, so in practice the vertical effective stress is calculatedfrom the pore pressure and vertical total stress. The distinction between theterms pressure and stress is also important. By definition, pressure at a pointis equal in all directions but stresses at a point can be different in differentdirections. In soil mechanics, compressive stresses and pressures are consideredto be positive and tensile stresses are considered to be negative, which isdifferent from the solid mechanics sign convention for stress.

Total stress[edit]

For level ground conditions, the total vertical stress at a point, , on average,is the weight of everything above that point per unit area. The vertical stressbeneath a uniform surface layer with density , and thickness is for example:

where is the acceleration due to gravity, and is the unit weight of theoverlying layer. If there are multiple layers of soil or water above the pointof interest, the vertical stress may be calculated by summing the product of the


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unit weight and thickness of all of the overlying layers. Total stress increaseswith increasing depth in proportion to the density of the overlying soil.

It is not possible to calculate the horizontal total stress in this way. Lateralearth pressures are addressed elsewhere.

Pore water pressure[edit]

Main article: Pore water pressure

Hydrostatic conditions[edit]

Water is drawn into a small tube by surface tension. Water pressure, u, isnegative above and positive below the free water surfaceIf there is no pore water flow occurring in the soil, the pore water pressureswill be hydrostatic. The water table is located at the depth where the waterpressure is equal to the atmospheric pressure. For hydrostatic conditions, thewater pressure increases linearly with depth below the water table:

where is the density of water, and is the depth below the water table.

Capillary action[edit]

Water at particle contactsDue to surface tension water will rise up in a small capillary tube above a freesurface 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 completelysaturated for some distance above the water table. Above the height of capillarysaturation, the soil may be wet but the water content will decrease withelevation. If the water in the capillary zone is not moving, the water pressureobeys the equation of hydrostatic equilibrium, , but note that , is negativeabove the water table. Hence, hydrostatic water pressures are negative above thewater table. The thickness of the zone of capillary saturation depends on thepore size, but typically, the heights vary between a centimeter or so for coarsesand to tens of meters for a silt or clay.[3] In fact the pore space of soil isa 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 andD=2.7.[8]

intergranular contact force due to surface tension.The surface tension of water explains why the water does not drain out of a wetsand castle or a moist ball of clay. Negative water pressures make the water


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stick to the particles and pull the particles to each other, friction at theparticle contacts make a sand castle stable. But as soon as a wet sand castle issubmerged below a free water surface, the negative pressures are lost and thecastle collapses. Considering the effective stress equation, , if the waterpressure is negative, the effective stress may be positive, even on a freesurface (a surface where the total normal stress is zero). The negative porepressure pulls the particles together and causes compressive particle toparticle contact forces.

Negative pore pressures in clayey soil can be much more powerful than those insand. Negative pore pressures explain why clay soils shrink when they dry andswell as they are wetted. The swelling and shrinkage can cause major distress,especially to light structures and roads.[9]

Shrinkage caused by dryingLater sections of this article address the pore water pressures for seepage andconsolidation problems.

Soil classification[edit]

Geotechnical engineers classify the soil particle types by performing tests ondisturbed (dried, passed through sieves, and remolded) samples of the soil. Thisprovides information about the characteristics of the soil grains themselves. Itshould be noted that classification of the types of grains present in a soildoes not account for important effects of the structure or fabric of the soil,terms that describe compactness of the particles and patterns in the arrangementof particles in a load carrying framework as well as the pore size and porefluid distributions. Engineering geologists also classify soils based on theirgenesis and depositional history.

Classification of soil grains[edit]

In the US and other countries, the Unified Soil Classification System (USCS) isoften used for soil classification. Other classification systems include theBritish Standard BS5390 and the AASHTO soil classification system.[3]

Classification of sands and gravels[edit]

In the USCS, gravels (given the symbol G) and sands (given the symbol S) areclassified according to their grain size distribution. For the USCS, gravels maybe 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 largeamount 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%) maybe given a dual classification such as SW-SC.

Atterberg limits[edit]

Clays and Silts, often called 'fine-grained soils', are classified according totheir Atterberg limits; the most commonly used Atterberg limits are the Liquid


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limit (denoted by LL or ), Plastic Limit (denoted by PL or ), and Shrinkagelimit (denoted by SL). The shrinkage limit corresponds to a water content belowwhich the soil will not shrink as it dries.

The liquid limit and plastic limit are arbitrary limits determined by traditionand convention. The liquid limit is determined by measuring the water contentfor which a groove closes after 25 blows in a standard test.[10] Alternatively,a fall cone test apparatus may be use to measure the liquid limit. The undrainedshear strength of remolded soil at the liquid limit is approximately 2kPa.[4][11] The plastic limit is the water content below which it is notpossible to roll by hand the soil into 3 mm diameter cylinders. The soil cracksor breaks up as it is rolled down to this diameter. Remolded soil at the plasticlimit is quite stiff, having an undrained shear strength of the order of about200 kPa.[4][11]

The Plasticity index of a particular soil specimen is defined as the differencebetween the Liquid limit and the Plastic limit of the specimen; it is anindicator of how much water the soil particles in the specimen can absorb. Theplasticity index is the difference in water contents between states when thesoil is relatively soft and the soil is relatively brittle when molded by hand.

Classification of silts and clays[edit]

According to the Unified Soil Classification System (USCS), silts and clays areclassified by plotting the values of their plasticity index and liquid limit ona plasticity chart. The A-Line on the chart separates clays (given the USCSsymbol C) from silts (given the symbol M). LL=50% separates high plasticitysoils (given the modifier symbol H) from low plasticity soils (given themodifier symbol L). A soil that plots above the A-line and has LL>50% would, forexample, be classified as CH. Other possible classifications of silts and claysare ML, CL and MH. If the Atterberg limits plot in the"hatched" region on thegraph near the origin, the soils are given the dual classification 'CL-ML'.

Indices related to soil strength[edit]

Liquidity index[edit]

The effects of the water content on the strength of saturated remolded soils canbe quantified by the use of the liquidity index, LI:

When the LI is 1, remolded soil is at the liquid limit and it has an undrainedshear strength of about 2 kPa. When the soil is at the plastic limit, the LI is0 and the undrained shear strength is about 200 kPa.[4][12]

Relative density[edit]

The density of sands (cohesionless soils) is often characterized by the relativedensity,

where: is the "maximum void ratio" corresponding to a very loose state, is the"minimum void ratio" corresponding to a very dense state and is the in situvoid ratio. Methods used to calculate relative density are defined in ASTMD4254-00(2006).[13]


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Thus if the sand or gravel is very dense, and if the soil is extremely looseand unstable.

Seepage: steady state flow of water[edit]

A cross section showing the water table varying with surface topography as wellas a perched water table.If fluid pressures in a soil deposit are uniformly increasing with depthaccording to then hydrostatic conditions will prevail and the fluids will notbe flowing through the soil. is the depth below the water table. However, ifthe water table is sloping or there is a perched water table as indicated in theaccompanying sketch, then seepage will occur. For steady state seepage, theseepage velocities are not varying with time. If the water tables are changinglevels with time, or if the soil is in the process of consolidation, then steadystate conditions do not apply.

Darcy's law[edit]

Darcy's law states that the volume of flow of the pore fluid through a porousmedium per unit time is proportional to the rate of change of excess fluidpressure with distance. The constant of proportionality includes the viscosityof the fluid and the intrinsic permeability of the soil. For the simple case ofa horizontal tube filled with soil

Diagram showing definitions and directions for Darcy's law.The total discharge, (having units of volume per time, e.g., ft/s or m/s), is

proportional to the intrinsic permeability, , the cross sectional area, , andrate of pore pressure change with distance, , and inversely proportional to thedynamic viscosity of the fluid, . The negative sign is needed because fluidsflow from high pressure to low pressure. So if the change in pressure isnegative (in the -direction) then the flow will be positive (in the -direction).The above equation works well for a horizontal tube, but if the tube wasinclined so that point b was a different elevation than point a, the equationwould not work. The effect of elevation is accounted for by replacing the porepressure by excess pore pressure, defined as:

where is the depth measured from an arbitrary elevation reference (datum).Replacing by we obtain a more general equation for flow:

Dividing both sides of the equation by , and expressing the rate of change ofexcess pore pressure as a derivative, we obtain a more general equation for theapparent velocity in the x-direction:

where has units of velocity and is called the Darcy velocity (or the specificdischarge, filtration velocity, or superficial velocity). The pore orinterstitial velocity is the average velocity of fluid molecules in the pores;


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it is related to the Darcy velocity and the porosity through theDupuit-Forchheimer relationship

(Some authors use the term seepage velocity to mean the Darcy velocity,[14]while others use it to mean the pore velocity.[15])

Civil engineers predominantly work on problems that involve water andpredominantly work on problems on earth (in earth's gravity). For this class ofproblems, civil engineers will often write Darcy's law in a much simplerform:[4][6][9]

where is the hydraulic conductivity, defined as , and is the hydraulicgradient. The hydraulic gradient is the rate of change of total head withdistance. The total head, at a point is defined as the height (measuredrelative 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:

and the is zero if the datum for head measurement is chosen at the sameelevation as the origin for the depth, z used to calculate .

Typical values of permeability[edit]

Values of the permeability, , can vary by many orders of magnitude depending onthe soil type. Clays may have permeability as small as about , gravels may have

permeability up to about . Layering and heterogeneity and disturbance during thesampling and testing process make the accurate measurement of soil permeabilitya very difficult problem.[4]

Flow nets[edit]

Darcy's Law applies in one, two or three dimensions.[3] In two or threedimensions, steady state seepage is described by Laplace's equation. Computerprograms are available to solve this equation. But traditionally two-dimensionalseepage problems were solved using and a graphical procedure known called flownets.[3][9][16] One set of lines in the flow net are in the direction of the

water flow (flow lines), and the other set of lines are in the direction ofconstant total head (equipotential lines). Flow nets may be used for example toestimate the quantity of seepage under dams and sheet piling.

A plan flow net to estimate flow of water from a stream to a discharging wellSeepage forces and erosion[edit]

When the seepage velocity is great enough, erosion can occur because of thefrictional 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 adam or levee. Erosion of the soil, known as "soil piping", can lead to failureof the structure and to sinkhole formation. Seeping water removes soil, startingfrom the exit point of the seepage, and erosion advances upgradient.[17] Theterm "sand boil" is used to describe the appearance of the discharging end of anactive soil pipe.[18]


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Seepage pressures[edit]

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 verticalstress at that point, the effective stress is zero and the soil has nofrictional resistance to deformation. For a surface layer, the verticaleffective stress becomes zero within the layer when the upward hydraulicgradient is equal to the critical gradient.[9] At zero effective stress soil hasvery little strength and layers of relatively impermeable soil may heave up dueto the underlying water pressures. The loss in strength due to upward seepage isa common contributor to levee failures. The condition of zero effective stressassociated with upward seepage is also called liquefaction, quicksand, or aboiling condition. Quicksand was so named because the soil particles move aroundand 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. Onthe contrary, you would float with about half your body out of the water.)[19]

Consolidation: transient flow of water[edit]

Main article: Consolidation (soil)

Consolidation analogy. The piston is supported by water underneath and aspring. When a load is applied to the piston, water pressure increases tosupport the load. As the water slowly leaks through the small hole, the load istransferred from the water pressure to the spring force.Consolidation is a process by which soils decrease in volume. It occurs whenstress is applied to a soil that causes the soil particles to pack together moretightly, 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 squeezethe water out of a thick deposit of clayey soil layer might be years. For alayer of sand, the water may be squeezed out in a matter of seconds. A buildingfoundation or construction of a new embankment will cause the soil below toconsolidate and this will cause settlement which in turn may cause distress tothe building or embankment. Karl Terzaghi developed the theory of consolidationwhich enables prediction of the amount of settlement and the time required forthe settlement to occur.[20] Soils are tested with an oedometer test todetermine their compression index and coefficient of consolidation.

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 theconsolidation process. If the stress is reapplied, the soil will re-consolidateagain along a recompression curve, defined by the recompression index. Soil thathas been consolidated to a large pressure and has been subsequently unloaded isconsidered to be overconsolidated. The maximum past vertical effective stress istermed the preconsolidation stress. A soil which is currently experiencing themaximum past vertical effective stress is said to be normally consolidated. Theoverconsolidation ratio, (OCR) is the ratio of the maximum past vertical


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effective stress to the current vertical effective stress. The OCR issignificant for two reasons: firstly, because the compressibility of normallyconsolidated soil is significantly larger than that for overconsolidated soil,and secondly, the shear behavior and dilatancy of clayey soil are related to theOCR through critical state soil mechanics; highly overconsolidated clayey soilsare dilatant, while normally consolidated soils tend to be contractive.[2][3][4]

Shear behavior: stiffness and strength[edit]

Main article: shear strength (soil)

Typical stress strain curve for a drained dilatant soilThe shear strength and stiffness of soil determines whether or not soil will bestable or how much it will deform. Knowledge of the strength is necessary todetermine if a slope will be stable, if a building or bridge might settle toofar into the ground, and the limiting pressures on a retaining wall. It isimportant to distinguish between failure of a soil element and the failure of ageotechnical 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 point" for a soil element from a stress-strain curve. One may define thepeak shear strength as the peak of a stress strain curve, or the shear strengthat critical state as the value after large strains when the shear resistancelevels off. If the stress-strain curve does not stabilize before the end ofshear strength test, the "strength" is sometimes considered to be the shearresistance at 15% to 20% strain.[9] The shear strength of soil depends on manyfactors including the effective stress and the void ratio.

The shear stiffness is important, for example, for evaluation of the magnitudeof deformations of foundations and slopes prior to failure and because it isrelated 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 theshear modulus

Friction, interlocking and dilation[edit]

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 stronglycemented together by chemical bonds. The shear strength of soil is primarily dueto interparticle friction and therefore, the shear resistance on a plane isapproximately proportional to the effective normal stress on that plane.[3] Butsoil also derives significant shear resistance from interlocking of grains. If

the grains are densely packed, the grains tend to spread apart from each otheras they are subject to shear strain. The expansion of the particle matrix due toshearing was called dilatancy by Osborne Reynolds.[12] If one considers theenergy required to shear an assembly of particles there is energy input by theshear force, T, moving a distance, x and there is also energy input by thenormal force, N, as the sample expands a distance, y.[12] Due to the extraenergy required for the particles to dilate against the confining pressures,dilatant soils have a greater peak strength than contractive soils. Furthermore,


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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 contractiondecreases until they reach a critical void ratio.

A critical state line separates the dilatant and contractive states for soilThe tendency for a soil to dilate or contract depends primarily on the confiningpressure and the void ratio of the soil. The rate of dilation is high if theconfining pressure is small and the void ratio is small. The rate of contractionis high if the confining pressure is large and the void ratio is large. As afirst approximation, the regions of contraction and dilation are separated bythe critical state line.

Failure criteria[edit]

After a soil reaches the critical state, it is no longer contracting or dilating

and the shear stress on the failure plane is determined by the effective normalstress on the failure plane and critical state friction angle :

The peak strength of the soil may be greater, however, due to the interlocking(dilatancy) contribution. This may be stated:

Where . However, use of a friction angle greater than the critical state valuefor design requires care. The peak strength will not be mobilized everywhere atthe same time in a practical problem such as a foundation, slope or retainingwall. The critical state friction angle is not nearly as variable as the peakfriction angle and hence it can be relied upon with confidence.[3][4][12]

Not recognizing the significance of dilatancy, Coulomb proposed that the shearstrength of soil may be expressed as a combination of adhesion and frictioncomponents:[12]

It is now known that the and parameters in the last equation are notfundamental soil properties.[3][6][12][21] In particular, and are differentdepending on the magnitude of effective stress.[6][21] According to Schofield(2006),[12] the longstanding use of in practice has led many engineers towrongly believe that is a fundamental parameter. This assumption that and areconstant can lead to overestimation of peak strengths.[3][21]

Structure, fabric, and chemistry[edit]

In addition to the friction and interlocking (dilatancy) components of strength,the structure and fabric also play a significant role in the soil behavior. Thestructure and fabric include factors such as the spacing and arrangement of thesolid particles or the amount and spatial distribution of pore water; in somecases cementitious material accumulates at particle-particle contacts.Mechanical behavior of soil is affected by the density of the particles andtheir structure or arrangement of the particles as well as the amount andspatial distribution of fluids present (e.g., water and air voids). Other


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factors include the electrical charge of the particles, chemistry of pore water,chemical bonds (i.e. cementation -particles connected through a solid substancesuch as recrystallized calcium carbonate) [1][21]

Drained and undrained shear[edit]

The presence of nearly incompressible fluids such as water in the pore spacesaffects the ability for the pores to dilate or contract.

If the pores are saturated with water, water must be sucked into the dilatingpore spaces to fill the expanding pores (this phenomenon is visible at the beachwhen apparently dry spots form around feet that press into the wet sand).

Foot pressing in soil causes soil to dilate, drawing water from the surfaceinto the poresSimilarly, for contractive soil, water must be squeezed out of the pore spacesto allow contraction to take place.

Dilation of the voids causes negative water pressures that draw fluid into thepores, and contraction of the voids causes positive pore pressures to push thewater out of the pores. If the rate of shearing is very large compared to therate that water can be sucked into or squeezed out of the dilating orcontracting pore spaces, then the shearing is called undrained shear, if theshearing is slow enough that the water pressures are negligible, the shearing iscalled drained shear. During undrained shear, the water pressure u changesdepending on volume change tendencies. From the effective stress equation, thechange in u directly effects the effective stress by the equation:

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 drainedshear strength depending upon whether the soil is contractive or dilative.

Shear tests[edit]

Strength parameters can be measured in the laboratory using direct shear test,triaxial shear test, simple shear test, fall cone test and vane shear test;there are numerous other devices and variations on these devices used inpractice today. Tests conducted to characterize the strength and stiffness ofthe soils in the ground include the Cone penetration test and the Standardpenetration test.

Other factors[edit]

The stress-strain relationship of soils, and therefore the shearing strength, isaffected by:[22]soil composition (basic soil material): mineralogy, grain size and grain sizedistribution, shape of particles, pore fluid type and content, ions on grainand in pore fluid.state (initial): Define by the initial void ratio, effective normal stress and


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embankments, excavated slopes, and natural slopes in soil and soft rock.[23]

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 simple2-D circular analysis package...[24] A primary difficulty with analysis islocating the most-probable slip plane for any given situation.[25] Manylandslides have been analyzed only after the fact.

See also[edit]Critical state soil mechanicsEarthquake engineeringEngineering geologyGeotechnical engineeringGeotechnicsRock mechanicsSlope stability analysisHydrogeology, aquifer characteristics closely related to soil characteristicsWiktionary seepage

References[edit]

^ a b c d e f g h Mitchell, J.K., and Soga, K. (2005) Fundamentals of soilbehavior, Third edition, John Wiley and Sons, Inc., ISBN 978-0-471-46302-7.^ a b c d e f Santamarina, J.C., Klein, K.A., & Fam, M.A. (2001). Soils and

Waves: Particulate Materials Behavior, Characterization and ProcessMonitoring. Wiley. ISBN 978-0-471-49058-6. .^ a b c d e f g h i j k l m n Powrie, W., Spon Press, 2004, Soil Mechanics -2nd ed ISBN 0-415-31156-X^ a b c d e f g h A Guide to Soil Mechanics, Bolton, Malcolm,Macmillan Press,1979. ISBN 0-333-18932-0^ Fang, Y., Spon Press, 2006, Introductory Geotechnical Engineering^ a b c d e f Lambe, T. William & Robert V. Whitman. Soil Mechanics. Wiley,1991; p. 29. ISBN 978-0-471-51192-2^ ASTM Standard Test Methods of Particle-Size Distribution (Gradation) ofSoils using Sieve Analysis. http://www.astm.org/Standards/D6913.htm^ Ozhovan M.I., Dmitriev I.E., Batyukhnova O.G. Fractal structure of pores ofclay soil. Atomic Energy, 74, 241-243 (1993).

^ a b c d e Holtz, R.D, and Kovacs, W.D., 1981. An Introduction toGeotechnical Engineering. Prentice-Hall, Inc. page 448^ Classification of Soils for Engineering Purposes: Annual Book of ASTMStandards. D 2487-83. 04.08. American Society for Testing and Materials. 1985.

pp. 395408^ a b Wood, David Muir, Soil Behavior and Critical State Soil Mechanics,Cambridge University Press, 1990, ISBN 0-521-33249-4^ a b c d e f g Disturbed soil properties and geotechnical design, Schofield,Andrew N.,Thomas Telford, 2006. ISBN 0-7277-2982-9^ ASTM Standard Test Methods for Minimum Index Density and Unit Weight ofSoils and Calculation of Relative Density.http://www.astm.org/Standards/D4254.htm

^ Smith, I. (2013) Smith's elements of soil mechanics, 8th edition, John Wileyand Sons, Inc., ISBN 978-1-405-13370-8.^ Delleur, Jacques W. (2007) The handbook of groundwater engineering, Taylor &

Francis, ISBN 978-0-849-34316-2.^ Cedergren, Harry R. (1977), Seepage, Drainage, and Flow Nets, Wiley. ISBN0-471-14179-8^ Jones, J. A. A. (1976). "Soil piping and stream channel initiation". WaterResources Research 7 (3): 602610. Bibcode:1971WRR.....7..602J.


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doi:10.1029/WR007i003p00602.^ Dooley, Alan (June 2006). "Sandboils 101: Corps has experience dealing withcommon flood danger". Engineer Update. US Army Corps of Engineers. Archivedfrom the original on 2006-07-27. Retrieved 2006-08-29.^ Terzaghi, K., Peck, R.B., and Mesri, G. 1996. Soil Mechanics in EngineeringPractice. Third Edition, John Wiley & Sons, Inc. Article 18, page 135.^ Terzaghi, K., 1943, Theoretical Soil Mechanics, John Wiley and Sons, New

York^ a b c d Terzaghi, K., Peck, R.B., Mesri, G. (1996) Soil mechanics inEngineering Practice, Third Edition, John Wiley & Sons, Inc.,ISBN0-471-08658-4^ Poulos, S. J. 1989. Advance Dam Engineering for Design, Construction, andRehabilitation: Liquefaction Related Phenomena. Ed. Jansen, R.B, Van NostrandReinhold, pages 292-297.^ US Army Corps of Engineers Manual on Slope Stability^ "Slope Stability Calculator". Retrieved 2006-12-14.^ Chugh, Ashok (2002). "A method for locating critical slip surfaces in slopestability analysis". NRC Research Press

[hide]vte

Topics in geotechnical engineering

Soils

ClaySiltSandGravelPeatLoam

Loess

Soil properties

Hydraulic conductivityWater contentVoid ratioBulk densityThixotropyReynolds' dilatancyAngle of repose

CohesionPorosityPermeabilitySpecific storage

Soil mechanics

Effective stress


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Pore water pressureShear strengthOverburden pressureConsolidationSoil compactionSoil classificationShear waveLateral earth pressure

Geotechnical investigation

Cone penetration testStandard penetration testExploration geophysicsMonitoring wellBorehole

Laboratory tests

Atterberg limits

California bearing ratioDirect shear testHydrometerProctor compaction testR-valueSieve analysisTriaxial shear testHydraulic conductivity testsWater content tests

Field tests

Crosshole sonic loggingNuclear densometer test

Foundations

Bearing capacityShallow foundationDeep foundationDynamic load testingPile integrity test

Wave equation analysisStatnamic load test

Retaining walls

Mechanically stabilized earthSoil nailingTieback


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GabionSlurry wall

Slope stability

Mass wastingLandslideSlope stability analysis

Earthquakes

Soil liquefactionResponse spectrumSeismic hazardGround-structure interaction

Geosynthetics

GeotextileGeomembranesGeosynthetic clay linerCellular confinement

Instrumentation for Stability Monitoring

Deformation monitoringAutomated Deformation Monitoring

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