**A**

**Active earth pressure**

The minimum horizontal stress exerted by a mass of soil on a retaining wall as the wall moves away from the soil.

**Active zone**

The area behind a retaining structure that is above the failure plane.

**Activity**

The ratio of plasticity index to percent by weight of clay. This property is correlated with the type of clay.

**Adhesion**

The shear resistance between soil and a structure (e.g. steel, concrete or timber piles; along a retaining wall).

**Air-voids ratio**

The ratio of the volume of air to the total volume of a mass of soil.

**Allowable bearing capacity**

The bearing pressure that can be allowed on a foundation soil, usually to limit settlements.

**American Association of State Highway Transportation Officials' Classification System**

A system of soil classification based on grain size, liquid limit and plasticity of soils.

**Angle of internal friction**

For a given soil, the angle on the graph of the shear stress and normal effective stresses at which shear failure occurs.

**Angle of repose**

The maximum angle, just before failure, of a slope composed of granular material .

**Angle of shearing resistance**

The ratio of effective shear and normal stresses mobilized at any state prior to failure.

**Angle of slip plane**

The angle referred to horizontal of a plane or other surface along which a discontinuous slip or rupture may occur.

**Angle of wall friction**

The angle of friction between soil and the surface of a retaining wall or bottom side of a foundation.

**Angular distortion**

The ratio between the relative deflection between two points in a foundation and the distance between them.

**Anisotropic**

A mass of soil having different properties in different directions. Primarily refers to permeability or stress-strain characteristics.

**aquifer**

A stratum of soil with relatively high permeability; a water-bearing stratum of rock or soil.

**Artesian**

A condition that exists when the water table piezometric surface lies above the ground level.

**At-rest earth pressure**

The horizontal stress developed in a mass of soil loaded in conditions of zero horizontal strain. The retaining wall neither moves away or towards the soil.

**Atterberg Limits**

The water contents of a soil mass corresponding to the transition between a solid, semi-solid, plastic solid or liquid. Laboratory test used to distinguish the plasticity of clay and silt particles.

**Axial strain**

Direct strain measured along an axis of a triaxial test sample.

**Axial stress**

Total or effective stress (Both confining and vertical stresses combined) acting along an axis of a triaxial test sample.

**
**

**Bearing Capacity**

The ability of the underlying soil to support the foundation loads without shear failure.

**Bearing capacity factors**

Empirically derived factors used in a bearing capacity equation that usually correlates with the angle of internal friction of the soil.

**Bearing pressure**

The total stress transferred from the bottom of a foundation to the soil below the foundation.

**Bedrock**

Strong rock underlying surface deposits of soil and weathered rock.

**Boulders**

Soil particles over 12 inches (305 mm) in size.

**Boussinesq equation**

An equation used to determine the increase in vertical pressure at a particular depth that is caused by an application of a point load at a given surface.

**Braced excavation**

The use of bracing for temporary trenches or cuts.

**British Soil Classification System**

A system of soil classification based on size, consistency and structure.

**Bulk density**

The total mass of water and soil particles contained in a unit volume of soil.

**Bulk unit weight**

The total weight of water and soil particles contained in a unit volume of soil.

**Buoyant Density**

Also, submerged density. Difference between the total density and the density of water. Buoyant density = Soil density - Density of water.

**C**

**Caisson**

A component of a particular foundation system.

**California Bearing Ratio (CBR)**

A laboratory test that is used to determine the suitability of of a soil for use as a subbase in a pavement section.

**Capillary rise**

The height to which water will rise above the water table due to negative pore water pressure or capillary action of the soil.

**Capillary stresses**

Pore water pressures less than atmospheric ＆#118alues produced by surface tension of pore water acting on the meniscus formed in void spaces between soil particles.

**Clay**

Soil particles which are finer (smaller) than 0.002 mm in size.

**coarse-grained soils**

Soils with more than 50% by weight of grains retained on the #200 sieve (0.075mm).

**Cobbles**

Soil particles between 3 inches (76 mm) and 12 inches (305 mm) in size.

**Coefficient of active earth pressure**

The ratio of the minimum horizontal effective stress of a soil to the vertical effective stress at a single point in a soil mass retained by a retaining wall as the wall moves away from the soil.

**Coefficient of compressibility**

The ratio of void ratio difference to the effective pressure difference of two different loadings during primary consolidation.

**Coefficient of consolidation**

The rate of change of volume during primary consolidation. Units: ft?day, m?s.

**Coefficient of curvature**

Also curvature coefficient. A measure of the shape parameter obtained from a grain size distribution curve.

C_{D}=(D_{30})^{2}/D_{10}D_{60}.

**Coefficient of earth pressure at rest**

The ratio of horizontal effective stress of a soil to the vertical effective stress at a specific point in a soil mass behind a retaining wall when the wall does not move (loaded in conditions of zero horizontal strain).

**Coefficient of friction**

The ratio between the tangential force required to cause a body, such as a foundation surface or a retaining wall, to slide along a plane and the normal force between the body and the plane.

**Coefficient of passive earth pressure**

The ratio of the maximum horizontal effective stress of a soil to the vertical effective stress at a specific point in a soil mass behind a retaining wall as the wall moves toward the soil.

**Coefficient of permeability**

See hydraulic conductivity.

**Coefficient of secondary consolidation**

The change in volumetric strain per a logarithmic cycle of time after primary consolidation is complete.

**Coefficient of uniformity**

Also uniformity coefficient. A measure of the slope of a grain size distribution curve, and therefore the uniformity of the soil.

C_{u}=D_{60}/D_{10}.

**Cofferdam**

A temporary structure used to enclose a construction area, and prevent soil or water from entering the construction area.

**Cohesionless soils**

Granular soils (sand and gravel type) with ＆#118alues of cohesion close to zero.

**Cohesive soils**

Clay type soils with angles of internal friction close to zero. Cohesion is the force that holds together molecules or like particles within a substance.

**Compaction**

Volume change in soils which air is expelled from the voids, but with the water content remaining constant. Compaction may occur due to vibration, and self-weight. In construction, compaction is achieved by rolling, tamping or vibrating fill soils.

**Compression index**

(Also, compressibility index) The logarithmic slope of the primary consolidation curve. The slope of the normal compression line and critical state line of the Casagrande Method of consolidation curves.

**Cone resistance**

The resistance force divided by the end area of the cone tip, measured during the cone penetration test.

**Cone penetration test**

A penetration test in which a cone that has a 60?point is pushed into the ground at a continuous rate. Resistance is measured by correlating the depth penetrated with the force applied.

**Confined aquifer**

An aquifer that is contained between two stratifications of low permeability soil or rock.

**Consistency**

Water content of clay relative to the Atterberg limits.

**Consistency index**

A measure of the relationship between the current water content and the consistency limits.

**Consistency limits**

See Atterberg limits.

**Consolidation**

Volume change due to dissipation of excess pore pressure from static loads.

**Consolidation (settlement)**

The settlement of a foundation due to sqeezing out of water from the pores as the soil comes to equilibrium with the applied loads.

**Coulomb earth pressure theory**

An earth pressure theory that includes friction between the soil and retaining structure, and assumes that failure occurs along a flat plane behind the retaining structure at an angle that is in part derived from the angle of internal friction.

**Coulomb's equation**

Named after Charles Augustin Coulomb, (1736-1806) An equation relating the shear strength of soil to the normal effective stress on the failure plane.

**Counterfort walls**

Type of retaining wall that depends on tension ribs between the stem and the heel in order to resist flexure and overturning.

**Creep**

Time dependent deformations which occurs in soil at constant effective stress without changes in volume and pore water pressure.

**Critical circle**

In a slope stability analyses, the slip circle that corresponds to the lowest factor of safety.

**Critical ground slope angle**

The angle of the ground slope that corresponds to a factor of safety of 1.0 relative to the slope stability.

**critical height**

The height of a slope that corresponds to a factor of safety of 1.0 relative to slope failure.

**Critical hydraulic gradient**

The hydraulic gradient at which effective stresses becomes zero. Upward seepage.

**Curvature coefficient**

See coefficient of curvature.

**Cyclic stress ratio**

A numerical rating of the potential for liquefaction in sands.

**D**

**Darcy's law**

(After H.P.G. Darcy, 1856) The formula used for laminar flow of water through porous saturated soils. Velocity of flow = hydraulic conductivity X hydraulic gradient.

**Degree of consolidation**

The proportion of consolidation that has occurred after a given elapsed time.

**Degree of saturation**

The proportion of the volume of water to the total volume of voids of a given mass of soil.

**Density**

The ratio of the total mass to the total volume of a unit of soil. Usually expressed as a unit weight where weight is interchanged with mass.

Units: lbm/ft? kg/m?

**Density index**

Also, relative density. The density of a granular soil relative to the minimum and maximum densities achieved for that particular soil.

**Density of soil grains**

The average density of the mineral or rock of which the soil particles are composed.

**Density of water**

The density of water will vary with temperature and pressure. ＆#118alues used for soils analysis are 62.4 lbm/ft?or 1000 kg/m? A ＆#118alue of 64.0 lbm/ft?is the ＆#118alue for sea water.

**Depth factor**

One of the terms in the bearing capacity equation that relates to depth of the foundation. The ratio between the depth of a slip circle below the top of a slope and the height of the slope.

**Desiccation**

The process of shrinkage or consolidation of the fine-grained soil produced by increase of effective stresses in the grain skeleton accompanying the development of capillary stresses in the pore water.

**Deviator stress**

The difference between the axial and radial stresses of a triaxial test sample.

**Differential settlement**

The vertical displacement due to settlement of one point in a foundation with respect to another point of the foundation.

**Direct shear test**

Laboratory test used to determine the relationship of shear strength to consolidation stress

**Direct strain**

The ratio of the change in length to the original length of a soil mass.

**Downdrag**

Negative skin friction. Forces induced on deep foundations resulting from downward movement of adjacent soil relative to the foundation element.

**Drained loading**

Loading which is slow enough for the water to drain from the soil as the total stresses increase. Pore pressure will not change, and volume will with loading.

**Drawdown**

The magnitude of the lowering of a water table, usually near a well being pumped.

**Dry density**

The ratio of the mass of the solids (soil grains) to the total unit volume of soil. Units: lbm/ft? kg/m?

**Dry unit weight**

The weight of solids (soil grains) to the total unit volume of soil. Units lbm/ft? kN/m?

**Dynamic compaction**

The use of high-energy impact to densify loose granular soils.

**E**

**Earth pressure**

The force per unit area exerted by soil on a retaining wall.

**Earth pressure coefficients**

Coefficients used in determining earth pressure. Components may include angle of internal friction, friction between the soil and wall face, angle of the wall face, and angle of the sloping backfill.

**Effective stress**

The portion of the total stress that is supported through grain-to-grain contact of the soil. The stress in a soil mass that is effective in causing volume changes and in mobilizing the shear strength arising from friction. The difference between the total stress and the pore water pressure. Effective stress = Total stress - Pore water pressure.

**Efficiency of a pile**

Also, pile efficiency. For a given pile in a group of piles, the ratio of the average ultimate load in the group to the individual ultimate load on the given pile.

**Elastic deformation**

Deformation caused in a soil due to a change in loading, and the soil recovers completely when the load is removed.

**Engineering properties**

Engineering parameters of a soil such as permeability, shear strength and consolidation. Different from index properties.

**Equipotential**

For a flow net, lines connecting points of equal total head. Equipotential lines are usually drawn so that the interval, or equipotential drop, is constant. Equipotentials intersect flow lines and impermeable boundaries at right angles.

**Equivalent fluid pressure**

Horizontal pressures of soil, or a combination of soil and water, which increase linearly with depth and are equivalent to those that would be produced by a heavy fluid of a selected unit weight.

**Excess pore pressure**

That increment of pore water pressures greater than hydro-static ＆#118alues, produced by consolidation stresses in compressible materials or by shear strain; excess pore pressure is dissipated during consolidation.

**Exit gradient**

The hydraulic gradient near an exposed surface through which seepage is moving.

**F**

**Factor of safety**

The ratio of a limiting ＆#118alue of a quantity to the design ＆#118alue of that quantity.

**Failure envelope**

For a given soil, the graph of the shear stress and normal effective stresses at which shear failure occurs.

**Fault**

A shear fracture in a rock mass along which movement has taken place.

**Field density test**

Also In-place density test. Field test that determines density of compacted fill to verify that it meets specifications. Types of tests include sand cone, rubber balloon or nuclear densiometer.

**Fine-grained soils**

Silt and clay soils. Soils containing particles smaller than No. 200 sieve or 0.075 mm in size according to the Unified Soil Classification System.

**Fines fraction**

Soil grains smaller than No. 200 sieve (0.075 mm), e.g. Clay and Silt.

**Fissured clay**

A clay having an internal network of narrow cracks or separations, in which the width and depth tends to increase upon drying.

**Fissures**

Small cracks.

**Flow line**

The path water will follow travelling from high head to low head in a seepage flow analysis.

**Flow net**

A graphical analysis of seepage flow in a mass of soil to estimate flow quantities and pore pressures.

**Flow quantity**

The total volume of water flowing in a seepage analysis.

**Flow rate**

The ratio of total volume of water flowing to a particular unit of time.

**Flow slide**

Shear failure in which a soil mass moves over a relatively long distance in a fluid-like manner, occuring rapidly on flat slopes in loose, saturated, uniform sands, or in highly sensitive clays.

**Flow velocity**

The velocity of water flow through a soil.

**Footing**

An enlargement at the base of a foundation that is designed to transmit forces to the soil.

**Foundation**

A component of an engineered structure that transmits the structure's forces into the soil or rock that supports it.

**Founding depth**

The depth below the ground surface where the base of a foundation is located.

**Friction angle**

See angle of internal friction.

**Friction pile**

A pile that derives the majority of its loadbearing ability from the skin friction between the soil and the pile.

**Frost jacking**

A pile forced upward due to freezing ground moving upward or repeated freeze-thaw events.

**
**

**Geotextiles**

A synthetic fabric used to stabilize soils, retain soils, prevent the mixing of dissimilar soils, provide a filtering function, pavement support, subgrade reinforcement, drainage, erosion control and silt containment.

**Grading curve**

See grain size distribution curve.

**Grain size distribution**

See particle size distribution.

**Grain size distribution curve**

A curve drawn on a log scale to represent the distribution of particle sizes in a soil.

**Gravity walls**

Retaining walls which depend upon their self weight to provide stability against overturning and sliding; usually made of a high bulk structure.

**H**

**Historical maximum stress**

See Pre-consolidation load.

**Homogenous earth dam**

An earth dam whose embankment is formed of one soil type without a systematic zoning of fill materials.

**Homogenous soils**

A mass of soil where the soil is of one characteristic having the same engineering and index properties.

**Horizontal strain**

Strain measured in a horizontal direction.

**Horizontal stress**

Total or effective stress acting in a horizontal direction.

**Hydraulic conductivity**

Also, coefficient of permeability. The constant average discharge velocity of water passing through soil when the hydraulic gradient is equal to 1.0. Clays are considered relatively impervious, while sands and gravels are considered pervious.

**Hydraulic gradient**

Between two points in a hydraulic flow: the difference in total head (piezometric levels) divided by the length of the flow path (distance between the two points).

**hydrometer test**

Laboratory test used to determine the finer particles of soil (Soils that pass the No. 200 sieve, or 0.075 mm).

**Hydrostatic pore pressure**

Pore water pressures exerted under conditions of no groundwater flow where the magnitude of pore pressures increase linearly with depth below the ground surface.

**I**

**Immediate settlement**

The settlement of a foundation occurring immediately upon loading.

**Index properties**

Attributes of a soil such as moisture content, void ratio, specific gravity, Atterberg limits and grain size distribution. Different from index properties.

**In-place density test**

See field density test.

**Isotropic**

A soil mass having essentially the same properties in all directions. Primarily refers to permeability and stress-strain characteristics.

**J**

**K**

**L**

**Liquefaction**

The sudden, large decrease of shear strength of cohesionless soil caused by collapse of the soil structure, produced by small shear strains associated with sudden but temporary increase of pore water pressure.

**Liquid limit**

The water content above which the soil will flow like a liquid, but below which it will have a plastic consistency.

**Liquidity index**

A measure of the relationship between the current water content of a soil and its consistency limits.

**Log-spiral earth pressure theory**

An earth pressure theory that assumes that the failure surface is curved behind a retaining structure.

**Long-term conditions**

Conditions in the ground where full consolidation has taken place and the soils are fully-drained.

**M**

**Machine foundation**

A foundation that receives regular or irregular vibratory loads that are generated from rotating or impact machinery.

**Maximum dry density**

A soil property obtained in the laboratory from a Proctor test. Density of soil at 100% compaction.

**Mean normal stress**

The mean ＆#118alue of the three orthogonal stresses.

**Micropile**

Piles with relatively small diameters.

**Modified Proctor**

Laboratory test used to determine maximum dry density and optimum moisture content of soils. Test results are used to specify compaction in the field. Soils are compacted in the laboratory in five layers with a 10 pound hammer falling 18 inches.

**Modulus of subgrade reaction**

Also, subgrade modulus. The ratio between the bearing pressure of a foundation and the corresponding settlement at a given point. The slope of the line in the loading range encountered by the soil in a plate bearing ＆#118alue test.

**Mohr's circle**

A circle constructed in the triaxial test using the measured stresses (principal stresses) in order to determine the stresses on the failure plane.

**Moisture content**

See Water content. The ratio between the mass of water and the mass of soil solids.

w = (wet weight - dry weight) / dry weight.

**
**

**N-＆#118alue**

Also, standard penetration resistence. The number of blows required to drive a split-spoon sampler during a standard penetration test a distance of 12 inches (0.305 m) after the initial penetration of 6 inches (0.15 m).

**Negative skin friction**

. Forces induced on deep foundations resulting from downward movement of adjacent soil relative to the foundation element.

**Nominal bearing pressure**

Allowable bearing pressure for spread foundations on various soil types, derived from experience which provides safety against shear failure or excessive settlements.

**Normal compression line**

The relationship between void ratio and the normal effective stress for soil loaded beyond the current yield stress in an isotropic compression.

**Normal force**

Force acting normal to the plane of reference. Units: lb, N.

**Normalization**

Analysis of soil test data to compensate for different states of samples.

**Normally consolidated soil**

Soil having a current state which lies on the normal compression line.

**O**

**One-dimensional compression**

Compression taking place with zero radial and horizontal strain.

**One-dimensional modulus**

The ratio of the change in vertical effective stress to the change in vertical strain, when there is zero horizontal strain.

**Open layer**

A layer or stratum of soil from which porewater may drain both upward and downward into overlying and underlying permeable layers, thus enabling two-way drainage.

**Optimum moisture content**

The water content at which the maximum dry density of a soil is obtained using a specific effort of compaction.

**Organic soils**

Earth comprised of organic material, peat, muskeg.

**Overburden Soil**

Overlying soil of a desirable soil stratum.

**Overburden pressure**

(Also overburden stress) The stress at a given depth due to the weight of overlying soil or rock.

**Overconsolidated soil**

A clayey soil carrying a higher load in the past. Soil having a current state which lies inside the normal compression line

**Overconsolidation ratio**

The ratio of maximum past pressure (preconsolidation pressure) to the current effective stress.

**P**

**Particle size characteristics**

Used in particle size analyses and grading specifications: stated as the maximum particle size of a specified percentage (smallest) of the total content of a soil graded by mass.

d_{10} = maximum size of the smallest 10% of the particle content (also referred to as effective size)

d_{60} = maximum size of the smallest 60% of the particle content

**Particle size distribution**

Soil particle sizes that are determined from a representative sample of soil that is passed through a set of sieves of consecutively smaller openings.

**Passive earth pressure**

The maximum horizontal stress exerted by a mass of soil on a retaining surface as the surface moves toward the soil.

**Peak shear strength**

The maximum shear strength of a soil at a given normal effective stress and water content.

**Permeability**

A measure of continuous voids in a soil. The property which allows the flow of water through a soil. See also coefficient of permeability.

**pH ＆#118alue**

A measure of acidity or alkalinity of groundwater or soil water extract based on the hydrogen ion content.

**Pier**

A deep foundation component that is usually cast-in-place, instead of driven, drilled or jetted as a pile.

**Piezometer**

An instrument used to measure in-situ pore water pressures.

**Piezometric surface**

An imaginary surface corresponding to the hydrostatic water level of a confined body of groundwater.

**Pile**

A slender member of a deep foundation system that is driven (hammered), drilled or jetted into the ground. Piles are usually constructed of timber, steel or prestressed reinforced concrete.

**Pile Driving Analyzer**

A method to compute average pile force and velocity by using wave equation analysis with electonic measurements.

**Pile efficiency**

See efficiency of a pile.

**Pile spacing**

The distance from center to center of piles.

**Piping**

The movement of soil particles as a result of unbalanced seepage forces produced by percolating water. Piping leads to the development of boils or erosion channels.

**Plane strain**

A two-dimensional state of stress, where the out-of-plane strain (i.e. the strain normal to the plane being considered) is zero. An example of a plane strain situation would be on a cross-section through a long structure being loaded in the x-y plane, such as an embankment dam.

**Plastic deformation**

The distortion of soil resulting in a permanent and irrecoverable change in shape or volume.

**Plastic limit**

The moisture content in which a soil will have a plastic consistency.

**Plastic strain**

Deformation of soil that is not recovered upon unloading.

**Plasticity**

The property of a soil which allows it to deform continuously, usually a mass of clay size particles.

**Plasticity index**

The difference between the liquid limit and plastic limit of a soil mass.

**Plate bearing ＆#118alue**

A field test that is performed on compacted soil that provides an indication of shear strength pavement components.

**Point-bearing capacity**

Also, tip resistance or point capacity. The bearing capacity at the bottom tip of one member of a deep foundation system.

**Point-bearing pile**

A pile that derives the majority of its loadbearin, , g ability from the support of the soil layer beneath the tip of the pile.

**Poisson's ratio**

(After Simeon Poisson 1781-1840) The ratio of the change in strain perpendicular to the direction of loading to the change in strain caused in the same direction.

**Pore air pressure**

The pressure of air within the void space of a partially saturated soil.

**Pore pressure**

Also, hydrostatic pressure. The pressure exerted by the fluid within the pores or voids in a porous material; in saturated soil the pore pressure is the pore water pressure.

**Pore pressure coefficient**

The ratio of the change in pore pressure to the change in deviator stress.

**Pore pressure coefficient**

The ratio of the change in pore pressure to the change in isotropic stress in undrained loading.

**Pore pressure ratio**

At a given depth of soil, the ratio of the porewater pressure to the vertical overburden pressure.

**Pore space**

See porosity.

**Pore water pressure**

See pore pressure.

**Porosity**

Also, pore space. The ratio of the volume of voids to the total volume. Expressed in %, or unitless.

**Pre-consolidation load**

The maximum load ever imposed on a particular soil mass in its geological history.

**Preconsolidation pressure**

The maximum past pressure of a soil.

**Pressure head**

The height of a column of water required to develop a given pressure at a given point.

**Pressure in tension crack**

The horizontal pressure exerted in a slope or against a retaining wall due to hydrostatic water pressure in tension cracks.

**Primary consolidation**

The long-term consolidation of a clay from the loss of water from the voids due to a high pressure.

**Principal strains**

The strains occurring in the directions of the principal axes of strain.

**Principal stresses**

Normal stresses acting in the direction of principal axes of stress.

**Proctor test**

Laboratory test used to determine maximum dry density and optimum moisture content of soils. Test results are used to specify compaction in the field.

**Q**

**Quick sand**

The effective stresses within a mass of sand is zero.

**R**

**Radial stress**

In a triaxial sample, the total or effective stress acting perpendicular to the longitudinal axis.

**Radius of influence**

The outer radius from a well that has a depression of water pressure or water table produced by pumping of a well.

**Raft Foundation**

A type of foundation that combines the footing and floor slab, and usually covers the entire area beneath a building supporting all walls and columns.

**Rankine earth pressure theory**

Earth pressure analysis that disregards friction between the wall and soil, and assumes that failure occurs along a flat plane behind the retaining structure at an angle that is derived from the angle of internal friction.

**Recompression index**

The logarithmic slope recompression (reloading) line.

**Relative deflection**

The deflection in a foundation due to settlement.

**Relative compaction**

A minimum density specification usually designated as a percentage of the maximum dry density.

**Relative density**

Also, density index. The density of a granular soil relative to the minimum and maximum densities achieved for that particular soil.

**Response spectrum**

Information regarding frequency-dependent energy distribution of an earthquake derived from the Fourier analysis.

**Resultant force**

The single force or pressure that will produce the same effect as several forces or pressures acting along a plane. Used for easier calculations, such as analyzing earth pressures.

**S**

**Sand**

Particles that pass through a #4 sieve (4.75 mm), and retained on a #200 sieve (0.075 mm).

**Sand cone test**

Apparatus used for measuring density of soil in the field.

**Secondary compression index**

The logarithmic slope of the secondary compression line.

**Secondary consolidation**

The considerably decreased rate of consolidation following the completion of primary consolidation.

**Seepage**

The flow of water through soil.

**Seepage force**

The force transmitted to a mass of soil due to the seepage of groundwater.

**Seepage pressure**

The seepage force per unit volume.

**Seepage velocity**

The average velocity at which groundwater flows through the pores of a soil. The ratio of the volume flow rate to the average area of voids in a soil cross-section.

**Sensitivity**

A measure of the change in ultimate strength of clays between undisturbed and disturbed samples.

**Settlement**

The downward movement of soil, or the downward movement of a foundation.

**Shallow Foundation**

Refers to a foundation system that has a shallow founding depth relative to the foundation width. These foundations include spread footings, continuous footings and mats.

**Shape factors**

Factors used in a general bearing capacity equation which provides an adjustment relating to the footing geometry.

**Shear modulus**

The ratio of the change in shear stress to the resulting change in shear strain.

**Shear strain**

The angular distortion or change in shape of a mass of soil.

**Shear strength**

The maximum shear stress which a soil can sustain under a given set of conditions. For clay, shear strength = cohesion. For sand, shear strength = the product of effective stress and the tangent of the angle of internal friction.

**Shear stress**

The force per unit area acting tangentially to a given plane or surface.

**Sheet pile**

Steel section panels that are driven into the ground to provide lateral support.

**Shrinkage index**

The difference between the plastic and shrinkage limits.

**Shrinkage limit**

The water content corresponding to the transition between a brittle solid and a semi-solid. The water content below which further reduction in water content causes no further reduction in volume.

**Sieve analysis test**

Soil particle sizes that are determined from a representative sample of soil that is passed through a set of sieves of consecutively smaller openings.

**Skin-friction capacity**

Also, skin resistance or side resistance. The bearing capacity for the shaft of one member of a deep foundation system.

**Skin friction stress**

The shear stress on the shaft of a pile, caisson or cone penetrometer.

**Skin resistance**

See skin-friction capacity.

**Soil classification**

Standardized classification schemes that delineates soil characteristics that are important in determining soil behavior.

**Soil Nailing**

Slope stabilization method that involves installing and usually gouting closely spaced rebar in the soil or rock face.

**Soil suction**

See capillary rise.

**Specific gravity**

The ratio of the density of a body or a substance to the mass of an equal volume of water. Unitless.

**Spread footing**

Also, isolated footing. A footing designed to support a single column.

**SPT**

See Standard Penetration Test.

**Standard Penetration Resistance**

See N-＆#118alue.

**Standard Penetration Test**

Also, SPT. Measures resistance of the soil to the penetration of a standard split-spoon sampler that is driven be a 140-pound (63.5 kg) hammer dropped from a height of 30 inches (0.76 m).

**Standard Proctor**

See Proctor test.

**Steady state pore pressure**

The pore water pressure at equilibrium when all excess pore pressures have fully dissipated.

**Stiffness**

Susceptibility to distortion or volume change under an applied load.

**Strain**

A measure of the change in size or shape of a mass of soil relative to its original size or shape.

**Stress**

The intensity of force per unit area; normal stress is applied perpendicularly to a surface or plane, shear stress is applied tangentially to a surface or plane.

**Stress history**

The past history of loading and unloading of a soil mass.

**Subgrade modulus**

See Modulus of subgrade reaction.

**Submerged Density**

Also, buoyant density. Difference between the total density and the density of water. Submerged density = Soil density - Density of water.

**Surcharge**

An additional force applied at the exposed upper surface of a restrained soil.

**Swell**

Increase in soil volume; volumetric expansion of particular soils due to changes in water content.

**Swelling index**

The slope of the swelling (unloading) line.

**
**

**Tension crack**

Cracks appearing at the surface of a soil mass, often adjacent to a retaining wall or top of a failing slope.

**Tension crack depth**

The depth of a tension crack from the ground surface to a depth at which the horizontal effective stress is zero.

**Tension pile**

Piles that are designed to resist upward forces.

**Time factor**

A dimensionless quantity dependent on the degree of consolidation that is used in primary consolidation analyses.

**Tip resistance**

See point-bearing capacity.

**Tolerable vibration**

The level of vibration magnitude that a structure is designed. Structure vibrations range from unnoticeable to persons to structural danger.

**Total head**

The height of the free water surface above a given datum.

**Total stress**

The stress acting on or within a soil mass due to surcharges, overlying weight, etc. Total stress = effective stress + pore water pressure

**Triaxial shear strain**

A strain parameter used in the interpretation of triaxial stress test results.

**Triaxial stress test**

Laboratory test for determining the strength of soils.

**U**

**Ultimate bearing capacity**

The bearing stress which would cause shear failure in the soil below a foundation; dependent upon the shear strength of the soil, applied loads and on the shape and depth of the foundation.

**Undrained shear strength**

(Also undrained strength) The shear strength of a saturated soil at a given water content (or voids ratio, or specific volume) under loading conditions where no drainage of porewater can take place. The undrained shear strength of soil is independent of applied stresses and therefore can be measured at any level of stress, provided the void ratio remains constant. The undrained Mohr-Coulomb envelope will be horizontal.

**Unified Soil Classification System**

Known as USCS. A system of soil classification based on grain size, liquid limit and plasticity of soils.

**Uniformity Coefficient**

See coefficient of uniformity.

**Unit weight**

The ratio of the total weight of soil to the total volume of a unit of soil.

Units: lb/ft? kN/m?

**Unit weight of water**

The weight of a unit volume of water;

62.4 lb/ft? 9.81 kN/m?

**USCS**

See the Unified Soil Classification System.

**V**

**Vane shear test**

A field test used to measure the shear strength of a soil that is low-strength, homogeneous and cohesive.

**Varved clays**

Clayes that are layered with fine and coarse varieties.

**Vertical stress**

The total or effective stress acting vertically in a soil mass at a given depth caused by the soil's own weight.

**Vibrodensification**

The compaction (densification) of cohesionless soils by imparting wave energy to the soil mass so as to rearrange soil particles relulting in less voids in the overall mass.

**Vibroflot**

A crane suspended cylindrical penetrator with an opened water jet at the tip and acts in conjunction with vibrations to dig a hole.

**Vibroflotation**

A method to densify granular soils using a vibroflot to dig a hole and then backfilled with sand or gravel that is dumped in from the surface and densified.

**Void ratio**

The ratio of the volume of voids to the volume of solids (soil grains). Unitless

**Volume of solids**

Volume of soil grains in a total soil volume.

**Volume of voids**

Volume of air space in a total soil volume.

**Volume of water**

Volume of water in a total soil volume.

**Volumetric strain**

The ratio of the change in soil volume to the original soil volume.

**W**

**Water content**

See Moisture content. The ratio between the mass of water and the mass of soil solids.

w = (wet weight - dry weight) / dry weight.

**Water table**

The level in a body of soil at which the hydrostatic water pressure is zero.

**Weight of soil grains**

The dry weight of soil grains in a mass of soil.

**Weight of water**

The weight of water contained in the void space of a body of soil.

**X**

**Y**

**Yield point**

The point at which the soil loading behavior changes from elastic to inelastic.

**Yield stress**

The stress at which yielding takes place in soils. The stress at which the swelling-recompression line joins the normal compression line.

**Z**

**Zero air voids curve**

The curve created by plotting dry densities of soils corresponding to saturation at each water content.

**Zoned earth dam**

An earth dam embankment zoned by the systematic distribution of soil types according to their strength and permeability characteristics, usually with a centrally impervious core and shells of coarser materials.