GATE CE key terms

The amount of dissolved oxygen consumed by microorganisms while decomposing organic matter in a water sample over a stated period, usually five days at 20 degrees Celsius, expressed in milligrams per litre.

A measure of the oxygen equivalent of organic and oxidisable inorganic matter in a water sample, determined by chemical oxidation with a strong oxidant such as dichromate, expressed in milligrams per litre.

The concentration of oxygen gas dissolved in water, expressed in milligrams per litre, which sustains aquatic life and indicates the degree of organic pollution in a water body.

The longitudinal profile of dissolved oxygen deficit downstream of a pollution discharge, showing an initial fall, a minimum where reaeration balances oxygen demand, and gradual recovery toward saturation.

The natural ability of a water body to reduce pollutant concentration and restore its quality through dilution, sedimentation, reaeration, and biological breakdown of organic matter.

The optical property of water that causes light to be scattered rather than transmitted, arising from suspended and colloidal particles, and commonly measured in nephelometric turbidity units.

A water treatment process in which chemicals such as alum neutralise particle surface charge and destabilise colloids, then gentle mixing promotes collisions that grow small particles into settleable flocs.

A treatment tank designed to slow water flow so that suspended particles settle under gravity, commonly characterised by surface overflow rate, detention time, and weir loading.

The quantity of chlorine consumed by oxidisable impurities in water before a residual concentration remains, after which additional dosing produces free available chlorine for disinfection.

The dosing practice in which chlorine is added until combined chlorine compounds are fully oxidised and the residual chlorine curve turns upward, leaving free chlorine to maintain disinfection.

The intergranular stress carried by the soil skeleton, equal to total stress minus pore water pressure, which controls compression, shear strength, and volume change in saturated soils.

The time-dependent reduction in volume of a saturated soil caused by expulsion of pore water under sustained load, described by compression index and coefficient of consolidation.

The maximum vertical effective stress to which a soil has been subjected in its history, determined from the void ratio versus log pressure curve obtained in an oedometer test.

Loss of shear strength in saturated loose cohesionless soil when cyclic loading raises pore water pressure until effective stress approaches zero, allowing the soil to behave as a fluid.

The ratio of available shear strength along a potential slip surface to the shear stress required for equilibrium, computed by methods such as the method of slices.

The maximum pressure a foundation soil can sustain without excessive settlement or shear failure, commonly estimated using Terzaghi or Meyerhof expressions for shallow footings.

A laboratory compaction test in which soil is compacted in a mould with a standard rammer energy to determine the optimum moisture content and maximum dry unit weight.

The water content at which a given compaction effort produces the maximum dry unit weight of a soil, reflecting the balance between lubrication and trapped air voids.

The water content at which a fine-grained soil transitions from a plastic state to a semi-solid state, determined by rolling a thread to three millimetres diameter.

The water content at which a fine-grained soil changes from a liquid to a plastic state, determined with the Casagrande apparatus or cone penetrometer.

The numerical difference between the liquid limit and plastic limit of a soil, indicating the range of water content over which the soil behaves plastically.

A workability test for fresh concrete in which a standard cone is filled, lifted, and the vertical settlement of the concrete is measured, giving a slump value.

The ratio of the mass of water to the mass of cement in a concrete mix, which primarily governs strength and durability; lower values generally give higher strength.

The ratio of the mass of concrete actually filling a standard mould after free fall to the mass of fully compacted concrete in the same mould, used as a workability measure for dry mixes.

The upward migration of mixing water to the surface of freshly placed concrete as solid particles settle, which can weaken the top layer and reduce bond with subsequent lifts.

The separation of the constituents of a concrete mix so that coarse aggregate, mortar, and paste are no longer uniformly distributed, often caused by excessive vibration or long drops.

The ratio of the elastic modulus of steel reinforcement to that of concrete, used to transform a reinforced section into an equivalent homogeneous concrete section for stress analysis.

A design approach in which permissible stresses are obtained by dividing material strengths by factors of safety, and elastic analysis of the transformed section is used for service loads.

A design philosophy that checks a structure against specified limit states, such as collapse and serviceability, using partial safety factors applied to loads and material strengths.

The ability of a structural member or material to undergo large inelastic deformation without losing load-carrying capacity, providing warning of failure and energy dissipation.

The line in a beam cross-section where longitudinal bending stress is zero, passing through the centroid for homogeneous elastic sections and shifting with cracking in reinforced concrete.

The second moment of area of a cross-section about a specified axis, representing its resistance to bending and appearing in the flexure formula relating stress to bending moment.

The ratio of the moment of inertia of a cross-section to the distance from the neutral axis to the extreme fibre, used to compute maximum bending stress directly from bending moment.

A diagram showing how a particular response, such as shear or bending moment at a section, varies as a unit load moves across a structure.

The value of an influence line at a given point, used with moving load positions to compute maximum response through superposition and ordinate products.

The number of redundant reactions or internal forces beyond those needed for static equilibrium, computed from member, joint, and reaction counts minus available equilibrium equations.

The number of independent joint displacements, including rotations and translations, that must be restrained to make a structure kinematically determinate, used in stiffness and moment distribution methods.

A rapid transition of flow from supercritical to subcritical state, producing turbulence, energy dissipation, and a rise in water surface elevation.

A dimensionless parameter equal to flow velocity divided by the square root of gravitational acceleration times hydraulic depth, indicating subcritical, critical, or supercritical flow.

The total energy head of flowing water per unit weight measured relative to the channel bed, equal to depth plus velocity head, and used to analyse critical flow and transitions.

An empirical formula expressing uniform open-channel velocity as the reciprocal of roughness coefficient times hydraulic radius to the two-thirds power times bed slope to the one-half power.

The ratio of the cross-sectional area of flow to the wetted perimeter, used in Manning and Chezy equations for uniform flow in channels and pipes.

The direct runoff hydrograph resulting from one unit of effective rainfall applied uniformly over a catchment during a specified duration, used to derive other storm hydrographs.

The time required for runoff from the most remote part of a catchment to reach the outlet, used to determine critical storm duration in rational and unit hydrograph methods.

The downward entry of water into the soil surface, expressed as a rate that typically decreases during a storm and is estimated by Horton or Green-Ampt models for runoff calculation.

A classification of bituminous binder based on the depth in tenths of a millimetre that a standard needle penetrates a sample under specified conditions, indicating hardness.

The maximum load a compacted bituminous mix specimen resists at 60 degrees Celsius in the Marshall apparatus, used with flow value to select optimum binder content.

The ratio of the load required to penetrate a soil or pavement specimen to that required for a standard crushed stone, expressed as a percentage and used in flexible pavement design.

The transverse slope provided on a road surface or cross-section to drain rainwater quickly toward the edges, also called cross fall.

The transverse slope of the carriageway at a horizontal curve, provided to counteract centrifugal force and improve vehicle stability and comfort at design speed.

The ratio of pressure applied to a soil subgrade to the resulting deflection, used as the foundation stiffness parameter in rigid pavement analysis.