# Geo-technical Engineering Terms

Absorbed water

Water held mechanically (by surface tension) in a soil mass

Activity

Ratio of the plasticity index to the clay fraction

Shearing resistance between soil and another material under zero externally applied pressure

Water in a soil mass, held by physio-chemical forces, having physical properties are substantially different from absorbed or free water or chemically combined water at the same temperature and pressure

Air void ratio

The ratio of the volume of the air space to the volume of solids, in a soil mass

Allowable bearing pressure (Gross)

The maximum allowable gross loading intensity on the ground on any given case, taking into account the maximum safe bearing capacity (gross), the amount and the kind of settlement expected, and the ability of the given structure to take up this settlement. It is , therefore, a combined function of both the site conditions and characteristics of the particular structure it is proposed to erect thereon.

Allowable bearing pressure (Nett)

The allowable bearing pressure (gross) minus the surcharge

The load which may be safely applied to a pile after taking into account its ultimate bearing resistance, pile spacing, overall bearing capacity of the ground below the piles and allowable settlement.

Angle of internal friction

Angle between the abscissa and tangent to the curve representing the relationship between the shearing resistances at failure to the normal stress acting within a soil

Angle of repose

Angle between the horizontal and the maximum slope that a soil assumes through natural processes. For granular soils, the effect of height of slope is negligible; for cohesive soils, the effect of height of slope is so great that the angle of repose is meaningless.

Angle of wall friction

Angle between the abscissa and the tangent of the curve representing the relationship of shearing resistance to normal stress acting between soil and surface of another material

Area ratio of a sampling spoon

The area ratio is an indication of the volume of soil displaced by the sampler in proportion to the volume of soil displaced by the sampler in proportion to the volume of the sample, calculated as follows:

Ar = [(De2 – Di2) / Di2]*100

Where,

Ar = Area ratio of a sampling spoon

De = Maximum external diameter of the sampling spoon

Di = Minimum internal diameter of the sampling spoon at the cutting edge

Bearing capacity factor

Non dimensional factors for the computation of bearing capacity

Bearing capacity, maximum safe

The maximum intensity of loading that the soil will safely carry with a factor of safety against shear failure irrespective of any settlement that may result.

Black cotton soil

Black cotton soils are inorganic clays of medium to high compressibility and forms a major soil group in India. They are predominantly montmorillonitic in structure and black or blackish grey in colour. They are characterized by high shrinkage and swelling properties.

Boulder

A more or less rounded block or fragment of rock and of average dimension 300 mm or greater. Usually boulders are rounded by being carried or rolled along by water or ice; sometimes also by weathering in place in which case they ae known as boulders of weathering, disintegration or exploration.

Bulking

It is the increase in volume of a material due to handling. Rock bulk upon excavation; damp sand bulk if loosely deposited, as by dumping, because the apparent cohesion prevents movement of the soil particles to form a reduced volume.

California bearing ratio

The ratio of the force per unit area required to penetrate a soil mass with a circular piston of 50 mm diameter at the rate of 1.25 mm/min to that required for the corresponding penetration of a standard material. The ratio is usually determined for penetrations of 2.5 mm and 5 mm. where the ratio at 5 mm is consistently higher than that at 2.5 mm, the ratio at 5 mm is used.

Capillary water

Water subject to the influence of capillary action.

Clay

An aggregate of microscopic and sub microscopic particles derived from the chemical decomposition and disintegration of rock constituents. It is plastic within a moderate to wide range of water content.

Clay size

That portion of the soil finer than 0.002 mm.

Cobble

A rock fragment, usually rounded or semi-rounded, with an average dimension between 80 and 300 mm.

Coefficient of compressibility

The secant slope, for a given pressure increment of the effective pressure

Coefficient of consolidation

A coefficient utilized in the theory of consolidation, containing the physical constants of a soil affecting its rate of volume change

Coefficient of curvature (Cc)

It is used to classify soil.

Cc = D302 / (D10 x D30)

Where

D10, D30 and D60 are the diameters of particles corresponding to 10, 30 and 60% respectively

Coefficient of earth pressure

The ratio between the lateral effective pressure and vertical effective pressure at any point in the soil mass

Coefficient of earth pressure, Active

The smallest value of coefficient of earth pressure resulting when the soil expands laterally till failure

Coefficient of earth pressure, at Rest

Value of coefficient of earth pressure when the soil is permitted neither to expand nor compress laterally

Coefficient of earth pressure, passive

The maximum value of coefficient of earth pressure when the soil is compressed laterally there by inducing failure in the soil

Coefficient of permeability

The rate of flow of water under laminar flow conditions through a unit cross sectional area of porous medium under a unit hydraulic gradient and standard temperature conditions

Ratio of load per unit area (applied through a centrally loaded rigid body) of horizontal surface of a mass of soil to corresponding settlement of the surface. It is determined as the slope of the secant draw between the point corresponding to zero settlement and the point of 1.25 mm settlement, of a load-settlement curve obtained from a plate load test on a soil using a 75 cm or greater loading plate.

Coefficient of uniformity, Cu

It is mathematically expressed as D60/D10, where D60 is the particle diameter corresponding to 60% finer on the grain size curve and D10 is the particle diameter corresponding to 10% finer on grain size curve.

Coefficient of volume compressibility or modulus of volume change, mv

The compression of a soil layer per unit of original thickness due to a given unit increase in pressure. It is numerically equal to the coefficient of compressibility divided by one plus the original void ratio, or

mv = av / (1 + e)

Cohesion, c

The portion of the shear strength of a soil indicated by the term c, in coulomb’s equation,

Cohesion, apparent

Cohesion in granular soil due to capillary forces

Cohesionless soils

A soil that, when unconfined, has little or no strength when air dried and that has little or no cohesion when submerged

Cohesive soils

A soil that, when unconfined, has considerable strength when air dried and that has significant cohesion when submerged

Compaction

The densification of a soil by means of mechanical manipulation

Compaction curve

The curve showing the relationship between the dry unit weight and the water content of a soil for a given compaction effort

Compaction test

A laboratory compaction test procedure where by a soil at a known water content is placed in a specified manner into a mould of given dimensions, subjected to a compaction effort of controlled magnitude and the resulting unit weight determined. The procedure is repeated for various water contents sufficient to establish a relation between water content and dry unit weight.

Compressibility

Property of a soil pertaining to its susceptibility to decrease in volume, when subjected to load

Compression index

The slope of the linear portion of pressure-void ratio curve on a semi-log plot, with the pressure on the log scale

Consistency

The degree of resistance offered by a fine grained soil to deformation

Consistency index

Ratio of the liquid limit minus the natural moisture content to the plasticity index of a soil

Consolidated drained test (Slow shear test)

A soil test in which a soil specimen is first allowed to consolidate fully under an applied stress and shear stresses are then applied in such a manner that there is full dissipation of excess pore water pressure developed during shear.

Consolidated-Undrained test (Quick test)

In this case, the soil is consolidated under applied normal loads but no drainage of water from the soil is permitted to take place during shear

Consolidation

The gradual reduction in volume of a soil mass partly or fully saturated resulting from an increase in and continued application of compressive stress and is due to the expulsion of water from the pores.

Initial consolidation

A comparatively sudden reduction in volume of a soil mass under an applied load due principally to expulsion and compression of air in soil voids preceding primary consolidation

Primary consolidation

The reduction in a volume of a soil mass caused by the application of a sustained load to the mass. This is due principally to squeezing out of water from the void paces of the mass and accompanied by a transfer of the load from the soil water to the soil solids.

Secondary consolidation

The reduction in volume of soil mass caused by the application of a sustained load to the mass. This is due principally to the adjustment to the internal structure of the soil mass after most of the load has been transferred from soil water to soil solids.

Consolidation ratio

The ratio of the amount of consolidation at a given point within the subsoil and at a given time to the total amount of consolidation obtainable at that point under a given stress increment

Contact pressure

The soil reaction per unit area at the surface of contact between the foundation and the underlying soil-mass produced by self eight of the foundation and all forces acting on it.

Controlled strain test

A test in which the load is so applied that a controlled rate of strain result (a test in which a specified rate of deformation is applied and the reaction to this deformation is measured)

Creep

The time dependent deformation behavior of soil under constant compressive stress

Critical circle (critical surface)

The sliding surface assumed in a theoretical analysis of the stability of a soil mass for which the factor of safety is a minimum.

Critical density

The unit weight of a saturated granular material below which it will lose strength and above which it will gain strength when subjected to rapid deformation.

Critical height

The maximum height at which a vertical or slopped bank of soil will stand unsupported under a given set of conditions

Critical slope

The maximum angle with the horizontal at which a slopped bank of soil of given height will stand unsupported under a given set of conditions

Critical void ratio

Void ratio prior to the process of shear in which net volume change at failure is zero

Deflocculating agent (dispersing agent)

An agent that prevents fine soil particles in suspension from coalescing to form flocs

Degree of compaction

The ratio of dry density of compacted material in the field to the laboratory standard maximum dry density of the material multiplied by 100

Degree of consolidation

The ratio expressed as a percentage of the amount of consolidation at a given time, within a soil mass to the total amount of consolidation obtainable under a given stress condition.

Degree of saturation

The ratio, expressed as a percentage, of the volume of water in a given soil mass to the total volume of voids

Density index (relative density) ID

It is defined as the ratio of the difference between the void ratio of a cohesionless soil in the loosest state and any given void ratio, to the difference between its void ratios in the loosest and in the densest state.

ID= [(emax – e) / (emax – emin)] x 100

emax = Void ratio in loosest state

emin = void ratio in densest state

e = void ratio in natural state

Deviator stress

The difference between the major and minor principle stresses in a triaxial test

Dilatancy

The expansion of cohesionless soils when subjected to shear deformation

Direct shear test

A shear test in which soil under an applied normal load is stressed to failure by moving one section of the soil container (shear box) relative to other section

Discharge velocity

Rate of discharge of water through a porous medium per unit of total area perpendicular to the direction of flow

Dry density

The weight of oven dried soil per unit volume of soil mass

Active earth pressure

It is the minimum value of earth pressure. This condition will exist when a soil mass is permitted to yield sufficiently to cause its internal shearing resistance along with a potential failure surface to be completely mobilized.

Earth pressure at rest

It is the value of earth pressure when the soil mass is in its natural state without having been permitted to yield or without having been compressed.

Passive earth pressure

It is the maximum value of earth pressure. This condition exists when a soil mass is compressed sufficiently to cause its internal shearing resistance along a potential failure surface to be completely mobilized.

Effective diameter, D10

Particle diameter corresponding to 10 % finer on the grain size curve

Effective force

The force transmitted through a soil mass by inter granular pressure

Exchange capacity

The capacity to exchange ions as measured by the quantity of exchangeable ions in a soil

Fines

Portion of a soil finer than 75 micron

Floc

Loose, open structured mass formed in a suspension by the aggregation of minute particles

Flow curve

The locus of points obtained from a standard liquid limit test using the mechanical device plotted on a graph representing water content as ordinate on an arithmetic scale and the number of drops as abscissa on a logarithmic scale

Flow failure

Failure in which a soil mass moves over relatively long distances in a fluid-like manner

Flow index

The slope of the flow curve obtained from a liquid limit test using the mechanical devices

Flow line

The path that a particle of water follows in its course of seepage under laminar flow conditions

Flow net

A graphical representation of flow lines and equipotential lines used in the study of seepage phenomena

Footing

A spread construction in brick work, masonry or concrete under the base of a wall or column for the purpose of distribution of load over a larger area

Foundation

That part of the structure which is in direct contact with and transmits load to the ground

Frost action

Freezing and thawing of moisture in materials and the resultant effects on these materials and on the structures of which they are a part or with which they are in contact

Proportion of material of each grain size present in a given soil

Grain size analysis

Gravel

Angular, rounded or semi rounded particles of rock or soil of particle size between 4.75 mm and 80 mm

Heave

Upward movement of soil caused by expansion or displacement resulting from phenomena, such as moisture absorption, removal of overburden, driving of piles, frost action and hydrostatic pressure

The difference or drop of hydraulic head per unit distance of flow

Hydraulic gradient at which the inter granular pressure in a mass of cohesionless soil is reduced to zero by the upward flow of water

Hydraulic pressure

The pressure in a liquid under static condition; the product of the unit weight of the liquid and the difference in elevation between the given point and the free water elevation

Excess hydrostatic pressure

The pressure that exerts in pore water in excess of the hydrostatic pressure

Linear expansion

The increase in one dimension of soil mass, expressed as a percentage of that dimension at the shrinkage limit, when the water content is increased from the shrinkage limit to a given water content.

Linear shrinkage

Decrease in one dimension of a soil mass, expressed as a percentage of the original dimension, when water content is reduced from a given value to the shrinkage limit

Liquidity index

The ratio expressed as a percentage of the natural water content of soil minus its plastic limt to its plasticity index

Liquid limit

It is the amount of water content at which the soil is just starts to flow but have a little shear strength

Mohr circle

A graphical representation of the stresses acting on the various planes at a given point

Mohr envelope

The envelope of a series of mohr circles representing stress conditions at failure for a given material. According to Mohr rupture hypothesis, a rupture envelope is the locus of points the co ordinates of which represents the combination of normal and shearing stresses that will cause a given material to fail

Moisture content

The ratio expressed as percentage of the weight of water in a given soil mass to the weight of solid particles under a specified testing condition

Normally consolidated soil deposit

A soil deposit that has never been subjected to an effective pressure greater than the existing effective overburden pressure

Optimum moisture content

The water content at which a soil can be compacted to the maximum dry unit weight by a given compaction effort

Over consolidated soil deposit

A soil deposit that has been subjected to an effective pressure, which is greater than the present effective over burden pressure

Permeability

The property of soil which permits percolation

Phreatic line

The upper free water surface of the zone of seepage

Phreatic surface

Free water surface of the zone of seepage

Pile

Relatively slender structural element which is driven, otherwise introduced, into the soil, usually for the purpose of providing vertical or lateral support

Piping

The movement of soil particles by percolation water leading to internal erosion and the development of channels in the soil mass

Plasticity

The property of soil which allows it to be deformed beyond the point of recovery without cracking or appreciable volume change

Plasticity index

Numerical difference between the liquid limit and the plastic limit

Plastic limit

The water content expressed as a percentage of weight of oven dry soil, at the boundary between the plastic and semi-solid states of consistency of the soil

Pore pressure coefficient

The change in pore pressure due to change in applied stresses is expressed in terms of empirical coefficients known as pore pressure coefficient

Porosity

The ratio, usually expressed as a percentage, of the volume of voids of a given soil mass, to the total volume of the soil mass.

Pre-consolidation pressure

The maximum effective pressure to which a soil has been subjected

Pressure bulb

The zone in a loaded soil mass bounded by an arbitrarily selected isobar of stress

Principal plane

Each of three mutually perpendicular planes through a point in a soil mass on which the shearing stress is zero

Quick condition or Quick sand

Condition in which water is flowing upwards with sufficient velocity to reduce the shear resistance of the soil through a decrease in inter-granular pressure

Relative density

It is defined as the ratio of the difference between the void ratio of a cohesionless soil in the loosest state and any given void ratio, to the difference between its void ratios in the loosest and in the densest state.

ID= [(emax – e) / (emax – emin)] x 100

emax = Void ratio in loosest state

emin = void ratio in densest state

e = void ratio in natural state

Remoulded

Soil that has had its natural structure modified by manipulation

Rock

Natural solid mineral matter connected by strong and permanent cohesive forces, occurring in large masses of fragments

Sand

Cohesionless aggregates of angular, sub rounded, rounded, flaky or flat fragments of more or less unaltered rocks or minerals of size between 4.75 mm and 75 microns

Seepage (Percolation)

Slow movement of gravitational water through the soil

Sensitivity

The ratio of unconfined compressive strength of an undisturbed specimen of the soil mass to the unconfined compressive strength of specimen of the same soil after remoulding at unaltered water content.

Shear failure

Failure in which movement caused by shearing stresses in a soil mass is of sufficient magnitude to destroy or seriously endanger a structure

General shear failure

Failure in which the ultimate strength of the soil is mobilized along the entire potential surface of sliding

Local shear failure

Failure in which the ultimate shearing strength of the soil is mobilized only locally along the potential surface of sliding

Shear strength

The maximum resistance of a soil to shearing stress

Shrinkage index

The numerical difference between the plastic and shrinkage limit

Shrinkage limit

The maximum water content expressed as percentage of oven dry weight at which any further reduction in water content will not cause a decrease in volume of the soil mass, the soil mass being initially of soil in its undisturbed state.

Silt

Fine grained soil which exhibits a little or no plasticity and has a little or no strength when air dried

Silt size

The portion of soil finer than 75 micron IS sieve and coarser than 0.002mm.

Skin friction

The frictional resistance developed between soil and structure

Soil (Earth)

Sediments or other unconsolidated accumulations of solid particles produced by physical and chemical disintegration of rocks, and which may or may not contain organic matter

Soil mechanics

That branch of engineering which deals with the application of soil science, the static and dynamic laws and principals of mechanics and hydraulics to engineering problems dealing with soil as structural material

Soil profile

Vertical section of soil, showing the natural and sequence of various layers, as developed by deposition or weathering, or both

Soil stabilization

Chemical or mechanical treatment design to increase or maintain the stability of a mass of soil or otherwise to improve its engineering properties

Soil structure

Arrangement of soil particles in soil mass

Flocculent structure

An arrangement composed of flocs of soil particles instead of individual soil particles

Honeycomb structure

An arrangement composed of soil particles having a comparatively loose, stable structure resembling a honey comb

Single grained structure

An arrangement composed of individual soil particles, characteristic structure of coarse grained soils

Stability number

A pure number used in the analysis of the stability of a soil embankment, defined by the following equation

Ns = cd/(γcHc)

Cd = mobilized cohesion,

γc = effective unit weight, and

Hc = critical height of the sloped bank

Stability factor

It is the reciprocal of the stability number

Standard penetration resistance

Numbers of blows required for 30 cm penetration of a standard sampling spoon with 65 kg hammer falling freely through a height of 75 cm

The soil prepared and compacted to support the pavement system

Time factor

Dimensionless factor utilized in the theory of consolidation containing the physical constants of a soil stratum influencing its time rate of consolidation

Toughness index

The ratio of the plasticity index to the flow index

Tri axial shear test

A test in which a cylindrical specimen of soil encased in an impervious membrane is subjected to a confining pressure and then loaded axially to failure

Ultimate bearing capacity (Gross)

The gross intensity of loading at the base of a foundation which causes shear failure of the soil support

Ultimate bearing capacity (Nett)

The nett intensity of loading at the base of a foundation which causes shear failure of the soil support, in excess of that at the same level due to the surrounding surcharge, that is ultimate bearing capacity (gross) minus the surcharge

Unconfined compressive strength

The load per unit area at when unconfined prismatic or cylindrical specimen of standard dimensions of soil will fail in a simple compression test

Unconsolidated-undrained test (Quick test)

A soil test in which the water content of the test specimen remains practically unchanged during the application of the confining pressure and the additional axial force

Under consolidated soil deposit

A deposit that is not fully consolidated under the existing effective overburden pressure

Undisturbed sample

A soil sample that has been obtained by methods in which every precaution has been taken to minimize disturbance to the sample

Unit weight

Weight per unit volume of a soil mass

Dry unit weight

The weight of oven dry soil per unit volume of soil mass

Effective unit weight

That unit weight of a soil which, when multiplied by the height of the overlying column of soil, yields the effective pressure due to the weight of the overburden

Saturated unit weight

The unit weight of soil mass when saturated

Submerged unit weight

The weight of the solids in air minus the weight of water displaced by the solids per unit volume of soil mass; the saturated unit weight minus the unit weight of water

Wet unit weight

The weight per unit total volume of soil mass, irrespective of the degree of saturation

Void

Space in a soil mass not occupied by solid mineral matter. This space may be occupied by air, water or other gaseous or liquid material

Void ratio

The ratio of the volume of void space to the volume of solid particles in a given soil mass

Critical void ratio

The void ratio prior to the process of shear in which the net volume change at failure is zero

Wall friction

Frictional resistance mobilized between a wall and the soil in contact with the wall

Wall holding capacity

The smallest value to which the water content of a soil can be reduced by gravity drainage

Zero air void curve

Dry density moisture content curve for 100 percent saturation.

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