Civil Engineering: Soil Mechanics and Hydraulics Vocabulary

A complete set of vocabulary flashcards covering basic soil properties, Atterberg limits, soil classification systems, permeability, consolidation, shear strength, lateral earth pressure, and foundation engineering as described in the lecture notes.

Void Ratio

The ratio of the volume of void space to the volume of solid substance.

Porosity

The ratio of the volume of voids to the volume of the soil sample or specimen; it represents the open space between soil grains.

Degree of Saturation

The ratio of the volume of water in the void spaces to the volume of the voids, expressed as a percentage ranging from $0$ to $100$.

Moisture Content

The ratio of weight of water to the weight of solids in a given volume of soil.

Unit Weight

The weight of soil per unit volume.

Dry Unit Weight

The weight per unit volume of soil, excluding water.

Saturated Unit Weight

The bulk unit weight of a soil when it is $100$ percent saturated.

Bulk unit weight

Also known as total, wet or moist unit weight; it is the total weight divided by the total volume.

Critical hydraulic gradient

The hydraulic gradient that brings a soil (essentially, Coarse-grained soils) to static liquefaction.

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.

Dry density

The ratio of the mass of the solids (soil grains) to the total unit volume of soil.

Submerged Density

Also known as buoyant density; the difference between the total density and the density of water.

Density index

Also known as relative density; the density of a granular soil relative to the minimum and maximum densities achieved for that particular soil.

Consistency

A term used to describe the degree of firmness of soil.

Cohesion

The attraction of one water molecule to another resulting from hydrogen bonding (water-water bond).

Adhesion

The attraction of a water molecule to a non-water molecule (water-solid bond).

Stickiness

The capacity of soil to adhere to other objects, estimated at moisture content displaying maximum adherence between thumb and forefinger.

Rupture Resistance

A field measure of the ability of the soil to withstand an applied stress or pressure as applied using the thumb and forefinger.

Atterberg’s Limit

The limits of water content used to define soil behavior.

Liquid Limit

The moisture content in percent required to close a distance of $12.7\,\text{mm}$ along the bottom of the groove after $25$ blows; the point where soil begins to behave as a liquid and flow.

Cup Method

A laboratory method to determine Liquid Limit using a device consisting of a brass cup and hard rubber where the cup is dropped by a cam operated by a crank.

Plastic Limit

The moisture content at which the transition from Semi-Solid to Plastic state occurs.

Shrinkage Limit

The moisture content at which the transition from Solid to Semi-Solid takes place, where no further volume change occurs with further reduction in moisture content.

Liquidity Index

A ratio which signifies the relative consistency of a cohesive soil in the natural state.

Plasticity

The degree a soil can be molded or reworked causing permanent deformation without rupturing.

Consistency Index

The ratio of the difference between the liquid limit and water content to the difference between the liquid limit and the plasticity index.

Plasticity index

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

Shrinkage index

The difference between the plastic and shrinkage limits.

Residual Soil

Soils formed by the weathered products at their place of origin.

Lacustrine Soil

Soils formed by deposition of quiet lakes.

Alluvial Soil

Soils transported by running water and deposited along streams.

Glacial Soil

Soils formed by the transportation and deposition of glaciers.

Marine Soil

Soils formed by deposition in the seas.

Gravel Soil

Soil with occasional particles of quartz, feldspar and other minerals.

Aelian Soil

Soils transported and deposited by wind.

Activity (Skempton)

The ratio of the plasticity index to the percent of clay size fraction, by weight.

Sieve Analysis

Used to determine the grain size distribution of coarse-grained soil.

Hydrometer Analysis

Used to determine the grain size distribution of soil passing the No. $200$ sieve.

Effective Size

The diameter in the particle size distribution curve corresponding to $10\%$ finer ($D_{10}$); used to measure hydraulic conductivity and drainage.

Coefficient of Uniformity

The ratio between the grain diameter corresponding to $60\%$ passing on the curve ($D_{60}$) divided by the diameter of the $10\%$ passing ($D_{10}$).

Coefficient Gradation

The ratio between the square of the grain diameter corresponding to $30\%$ passing ($D_{30}$) divided by the product of $D_{60}$ and $D_{10}$.

Coefficient of curvature

Also known as curvature coefficient; a measure of the shape parameter obtained from a grain size distribution curve calculated as $C_D = \frac{(D_{30})^2}{D_{10} \times D_{60}}$.

Poorly Graded

A type of soil where most soil grains are the same size.

AASHTO System

A soil classification system consisting of eight major groups, A-1 through A-8, based on grain size distribution and Atterberg limits.

USCS System

Unified Soil Classification System used in engineering and geology to describe texture and grain size, represented by a two-letter symbol.

Coarse-grained soils

Soils with more than $50\%$ by weight of grains retained on the No. $200$ sieve ($0.075\,\text{mm}$).

Fine-grained soils

Silt and clay soils containing particles smaller than the No. $200$ sieve ($0.075\,\text{mm}$).

Cobbles

According to the USCS, a soil particle whose size is greater than $75\,\text{mm}$.

USDA System

Also called the Textural classification system.

Clay

Soil particles which are finer (smaller) than $0.002\,\text{mm}$ in size.

Compaction

Volume change in soils where air is expelled from voids while water content remains constant; achieved via rolling, tamping, or vibrating.

Optimum moisture content

The moisture content at which the maximum dry unit weight is attained.

Proctor Compaction Test

The laboratory test used to obtain the maximum dry unit weight of compaction and the optimum moisture content.

Permeability

The property of soil which permits the flow of water or other liquids through it.

Maximum dry density

A soil property obtained from a Proctor test representing the density of soil at $100\%$ compaction.

Coefficient of Permeability

A factor that indicates the ease or difficulty with which water moves through soil; also known as hydraulic conductivity.

Hydraulic conductivity

The constant average discharge velocity of water passing through soil when the hydraulic gradient is equal to $1.0$.

Transmissivity

The ability of an aquifer to transmit water through its entire thickness.

Discharge velocity

The quantity of water flowing in unit time through a unit gross cross-sectional area of soil at right angles to the direction of flow.

Constant head test

A test used to determine the coefficient of permeability of coarse-grained soil.

Falling Head Test

A test used to determine the coefficient of permeability of fine-grained soil.

Drawdown

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

Effective Stress

The sum of the vertical components of the forces developed at the points of contact of the solid particles per unit cross-sectional area.

Pore water pressure

Stress induced by water pressure.

Intergranular Stress

Stress resulting from particle-to-particle contact of soil.

Total Stress

The sum of effective stress and neutral (pore water) stress.

Capillary rise

The height to which water rises above the water table due to negative pore water pressure or capillary action.

Capillary stresses

Pore water pressures less than atmospheric values produced by surface tension acting on the meniscus in void spaces.

Piezometer

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

Equipotential line

A line along which the potential head at all points is equal.

Flow Line

A line along which a water particle travels from upstream to downstream in a permeable medium.

Flow Net

A combination of a number of flow lines and equipotential lines.

Boussinesq equation

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

Primary consolidation settlement

Volume change in saturated cohesive soils due to the expulsion of water from void spaces.

Normally Consolidated

Type of clay whose present effective overburden pressure is the maximum pressure the soil was subjected to in the past.

Over Consolidated

Type of clay whose present effective overburden pressure is less than the maximum pressure the soil was subjected to in the past.

Secondary consolidation settlement

The result of the plastic adjustment of soil fabrics.

Immediate settlement

Settlement caused by elastic deformation without any change in moisture content.

Compression index

The logarithmic slope of the primary consolidation curve; the slope of the normal compression line.

Swell Index

An index smaller in magnitude than the compression index.

Coefficient of Consolidation

A value that generally decreases as the liquid limit of soil increases.

Overconsolidation ratio

The ratio of preconsolidation pressure to present effective overburden pressure.

Direct Shear Test

The oldest and simplest shear test using a metal shear box to estimate cohesion and angle of internal friction.

Triaxial Shear Test

One of the most reliable methods for determines shear strength parameters, widely used for research and conventional testing.

Confined aquifer

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

Unconfined Compressive Strength Test

A laboratory test on plastic soils where undrained shear strength is calculated as half of the unconfined compressive strength.

Undrained shear strength

The shear strength of saturated soil under loading conditions where no drainage of pore water takes place, independent of applied stresses if void ratio is constant.

Coulomb's equation

An equation relating the shear strength of soil to the normal effective stress on the failure plane.

Deviator stress

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

Artesian

A condition where the water table piezometric surface lies above the ground level.

Angle of shearing resistance

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

Earth pressure

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

Active earth pressure coefficient

The ratio between lateral and vertical principal effective stresses when a structure moves away from a retained soil.

Passive earth pressure coefficient

The ratio between lateral and vertical principal effective stresses when a structure is forced against a soil mass.

Coulomb earth pressure theory

A theory including friction between soil and structure, assuming failure along a flat plane behind the retaining structure.

Tension crack depth

The depth from the ground surface to where the horizontal effective stress is zero.

Overturning

Failure resulting from excessive lateral earth pressures causing a retaining wall to topple or rotate.

Shallow foundation

A foundation where the ratio of embedment depth to minimum plan width ($D_i / B$) is roughly less than or equal to $2.5$.

Ultimate bearing capacity

The maximum pressure that soil can support causing a shear failure.

Allowable bearing capacity

The working pressure ensuring a margin of safety against collapse from shear failure; also called safe bearing capacity.