OR/12/032 Appendix A2 - Glossary

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Hobbs, P R N, Entwisle, D C, Northmore, K J, Sumbler, M G, Jones, L D, Kemp, S, Self, S, Barron, M, and Meakin, J L. 2012. Engineering Geology of British rocks and soils - Lias Group. British Geological Survey, Nottingham, UK. (OR/12/032).

Glossary of terms
(Refer to Appendix A1 for abbreviations and units)

ALUM A potassium aluminium sulphate.
ARGILLACEOUS Containing clay. Typically applied to fine-grained sedimentary rocks composed of clay and silt- sized particles.
ATTERBERG LIMITS Consistency criteria for defining key water contents of a clay soil. They are: liquid limit, plastic limit and shrinkage limit.
BASIN A geological depression containing significant thicknesses of sediment, or in which sediment is able to accumulate. Frequently circular or elliptical in plan.
BEARING CAPACITY The ability of a material to support an applied load. Ultimate bearing capacity is the pressure at which shear failure of the supporting soil immediately below and adjacent to a foundation. A foundation is usually designed with a working load that is some proportion of the bearing capacity.
BED The smallest stratigraphic unit.
BEDDING The arrangement of sedimentary rocks in beds or layers of varying thickness or character.
BEDROCK Unweathered rock beneath a cover of soil or superficial deposits.
BERTHIERINE A green iron ore.
CALCAREOUS Carbonate-rich.
CALCITE The crystalline form of calcium carbonate, CaCO3.
CAMBERING The process whereby a brittle caprock is undermined by the lateral stress relief displacement of a weaker substrate, resulting in cantilever stresses, development of gulls, and ultimately downslope movement. Often associated with periglacial conditions.
CLAY A naturally occurring material which is a plastic material at natural water content and hardens when dried to form a brittle material. It is the only type of soil/rock susceptible to significant shrinkage and swelling. It is made up mainly, but not exclusively, of clay minerals. It is defined by its particle-size range (<0.002 mm). Clay does not have to be the dominant component of a soil in order to impart clay-like properties to it.
CLAY MINERALS A group of minerals with a layer lattice structure which occur as minute platy or fibrous crystals. These tend to have a very large surface area compared with other minerals, thus giving clays their plastic nature and the ability to support large suction forces. They have the ability to take up and retain water and to undergo base exchange.
CLAYSTONE A fine-grained sedimentary rock composed of predominately clay sized particles.
COEFFICIENT OF CONSOLIDATION A measure of the rate at which consolidation takes place.
COEFFICIENT OF VOLUME COMPRESSIBILITY A measure of the amount of compression that takes place during consolidation, measured as a change in dimension per log interval of applied stress.
COHESION Attractive force between soil particles (clay) involving a complex association of solid and water. Specifically, the shear strength of a soil at zero normal stress.
COHESIVE SOIL A soil in which particles adhere after wetting and subsequent drying and significant force is required to crumble the soil.
COMPACTION The reduction of voids (densification) of a soil mass by engineering action to produce a more stable, stronger material.
COMPRESSION INDEX The slope of the normal consolidation line with respect to the change in voids ratio over a log cycle of applied stress.
CONSOLIDATION The process in which pore water drains from a material under an applied load with a consequent reduction in volume of the material (see subsidence).
DENSITY The mass of a unit volume of a material. Often used (incorrectly) as synonym for Unit weight. Usually qualified by condition of sample (e.g. saturated, dry).
DIACHRONOUS A lithostratigraphic unit that varies in age.
DIAMICT/DIAMICTON Sediment (usually glacial) containing wide range of particle types and sizes. DIGGABILITY Measure of the ability for an excavation to be made in a material by a mechanical digger. DIP The inclination of a planar surface from horizontal. Usually applied to bedding planes.
DISCONTINUITY Any break in the continuum of a rock mass (e.g. faults, joints).
DOGGER Flattened calcareous or ferruginous concretion in a clay or sand deposit. Often stronger than the remainder of the deposit.
DRAINED Condition applied to strength tests where pore fluid is allowed to escape under an applied load. This enables an effective stress condition to develop.
DRIFT Archaic synonym for ‘superficial’ geological deposits; i.e. those overlying bedrock.
EFFECTIVE STRESS The total stress minus pore pressure. The stress transferred across the solid matter within a rock or soil.
ELASTICITY Deformation where strain is proportional to stress, and is recoverable.
EXCAVATABILITY A measure of the ability for an excavation to be made in a material by earth-moving equipment such as backacters, face shovels, scrapers, bulldozers etc. using digging, ripping and blasting as the difficulty of removing material increases.
EXPOSURE A visible part of an outcrop that is unobscured by soil or other materials.
FAULTING The displacement of blocks of strata relative to each other along planar fractures. Movement may take place in several ways, depending on the direction of the compressive or extensional forces acting on the rock mass forming normal, reverse or strike slip faults.
FAULTS Planes in the rock mass on which adjacent blocks of rock have moved relative to each other. The relative vertical displacement is termed ‘throw’. The faults may be discrete single planes but commonly consist of zones, perhaps up to several tens of metres wide, containing several fractures which have each accommodated some of the total movement. The portrayal of such faults as a single line on the geological map is therefore a generalization.
FERRUGINOUS Iron-rich. Applied to rocks or soils having a detectable iron content.
FILL Material used to make engineered earthworks such as embankments and capable of acquiring the necessary engineering properties during placement and compaction.
FISSILITY The ability of a rock (e.g. Mudstone) to be broken along closely spaced parallel planes (e.g. Shale).
FLUVIAL/FLUVIATILE Of, or pertaining to, rivers.
FORMATION The basic unit of subdivision of geological strata, and comprises strata with common, distinctive, mappable geological characteristics.
GLACIAL Of or relating to, the presence of ice or glaciers; formed as a result of glaciation.
GRADING A synonym (engineering) for particle-size analysis (see also Sorting).
GROUNDWATER Water contained in saturated soil or rock below the water-table.
GROUP A stratigraphical unit usually comprising one or more formations with similar or linking characteristics.
GULLS Tension fissures produced in a brittle caprock by cambering. Gulls may not be visible at the surface. This may be due to bridging, that is, they may not extend to the ground surface or to infilling with superficial material.
GYPSUM Mineral consisting of hydrous calcium sulphate (CaSO4.2H20), common in weathered mudstone where it is formed by the breakdown of sulphide minerals in the presence of lime-rich groundwater.
HEAD A deposit comprising material derived, transported and deposited by solifluction in periglacial regions. May include material derived also by hillwash, creep and other non-glacial slope processes. Composition is very variable and dependent on source material. Thickness is also very variable.
HOLOCENE The most recent subdivision of geologic time (RECENT) which represents the last 10 000 years.
HYDRAULIC CONTINUITY Juxtaposition of two or more permeable deposits or rock units such that fluids may pass easily from one to another.
ILLITE A 2:1 clay mineral, common in sedimentary rocks, not noted for susceptibility to shrink/swell behaviour.
INDEX TESTS Simple geotechnical laboratory tests which characterise the properties of soil (usually) in a remoulded, homogeneous form, as distinct from ‘mechanical properties’ which are specific to the conditions applied.
IRONPAN Hard layer formed by re-precipitation of iron compounds leached from overlying deposits.
JOINT A surface of fracture or parting in a rock, without displacement; commonly planar and part of a set.
JURASSIC The middle period of the Mesozoic (208.0 to 145.6 Ma).
KAOLIN A group of 2 layer, 1:1 structure clay minerals usually of low plasticity (e.g. kaolinite). The most common clay mineral.
LANDSLIDE A down slope displacement of bedrock or superficial deposits subject to gravity, over one or more shear failure surfaces. Landslides have many types and scales. Landslides may be considered both as ‘events’ and as geological deposits. Synonym of ‘landslip’.
LIGNITE Soft, brown-black earthy type of coal.
LINEAR SHRINKAGE The percentage length reduction of a prism of remoulded clay subjected to oven drying at 105°C.
LITHOLOGY The characteristics of a rock such as colour, grain size and mineralogy. The material constituting a rock.
LITHOSTRATIGRAPHIC UNIT A rock unit defined in terms of lithology and not fossil content (Biostratigraphic unit).
LIQUID LIMIT The moisture content at the point between the liquid and the plastic state of a clay. An Atterberg limit.
LOWER LIAS Equivalent to those deposits corresponding broadly with the Redcar Mudstone Formation of the Cleveland Basin, and Scunthorpe Mudstone/Blue Lias Formation plus Charmouth Mudstone Formation elsewhere.
MARL A calcareous mudstone, sensu-strictu having >30% carbonate content.
MASSIVE Applied to a rock mass containing no visible internal structure.
MEDIAN The 50th percentile of a distribution; that is, the value above and below which 50% of the distribution lies.
MEMBER A distinctive, defined unit of strata within a formation characterised by relatively few and distinctive rock types and associations (for example, sandstones, marls, coal seams).
MICACEOUS Containing mica, a sheet silica mineral.
MIDDLE LIAS Corresponds broadly to the Staithes Sandstone plus Cleveland Ironstone formations in the Cleveland Bsin, and Dyrham plus Marlstone Rock formations elsewhere.
MINERAL A naturally occurring chemical compound (or element) with a crystalline structure and a composition which may be defined as a single ratio of elements or a ratio which varies within defined end members.
MOISTURE CONDITION VALUE (MCV) Test to determine suitability of soil as compacted fill. The test measures the minimum compactive effort required to produce a state of near-full compaction.
MOISTURE CONTENT See Water content.
MUDROCK A term used by engineers, synonymous with mudstone.
MUDSTONE A fine-grained, non-fissile, sedimentary rock composed of predominately clay and silt-sized particles.
NATURAL WATER CONTENT The water content of a geological or engineering material in its natural or ‘as found’ state.
NORMALLY CONSOLIDATED (NC) A deposit, such as clay, that is compacted by exactly the amount to be expected from the pressure exerted by the overburden. This deposit has never been subject to an overburden greater than its present overburden.
OCHREOUS Containing hydrated iron oxides, usually reddish or orange in colour.
OEDOMETER Laboratory apparatus for measuring consolidation properties of a soil.
OOLITE Rock (limestone) consisting of, or containing, carbonate or iron-coated grains (ooids).
OUTCROP The area over which a particular rock unit occurs at the surface.
OUTLIER A deposit or an outcrop of rock surrounded by the outcrops of older deposits or rocks and separated from the main body by erosion.
OVERBURDEN Material, or stress applied by material, overlying a particular stratum. Unwanted material requiring removal (quarrying).
OVER-CONSOLIDATED (OC) Deposit, such as clay, that in previous geological times was loaded more heavily than now and consequently has a tendency to expand if it has access to water and is subject to progressive shear failure. The moisture content is less than that for an equivalent material which has been normally consolidated.
PARTICLE-SIZE ANALYSIS (PSA) The measurement of the range of sizes of particles in a dis-aggregated soil sample. The tests follow standard procedures with sieves being used for coarser sizes and various sedimentation, laser or X-ray methods for the finer sizes usually contained within a suspension.
PARTICLE-SIZE DISTRIBUTION (PSD) The result of a particle-size analysis. It is shown as a ‘grading’ curve, usually in terms of % by weight passing particular sizes. The terms ‘clay’, ‘silt’, ‘sand’ and ‘gravel’ are defined by their particle sizes.
PERCHED GROUND WATER Unconfined groundwater separated from an underlying main body of groundwater by an unsaturated zone.
PERIGLACIAL An environment beyond the periphery of an ice sheet influenced by severe cold, where permafrost and freeze-thaw conditions are widespread. Fossil periglacial features may persist to the present day or may have been removed by subsequent glaciation or erosion.
PERMEABILITY The property or capacity of a rock, sediment or soil for transmitting a fluid; frequently used as a synonym for ‘hydraulic conductivity’ (engineering). The property may be measured in the field or in the laboratory using various direct or indirect methods.
PERMAFROST Permanently frozen ground, may be continuous (never thaws), discontinuous (with unfrozen patches, especially in summer) or sporadic (unfrozen areas exceed frozen areas). The surface layer subject to seasonal thaw is the ‘active layer’.
pH Measure of acidity/alkalinity on a scale of 1 to 14 (<7 is acid, >7 is alkaline).
PLASTICITY INDEX The difference between the liquid and plastic limits. It shows the range of water contents for which the clay can be said to behave plastically. It is often used as a guide to shrink/swell behaviour, compressibility, strength and other geotechnical properties.
PLASTIC LIMIT The water content at the lower limit of the plastic state of a clay. It is the minimum water content at which a soil can be rolled into a thread 3 mm in diameter without crumbling. The plastic limit is an Atterberg limit.
PLEISTOCENE The first epoch of the Quaternary Period prior to the Holocene from about 2 million years to 10 000 years ago.
POINT LOAD A simple test to determine the strength of a strong rock, the result of which is a Point Load Index (Is). It may be used on drill core or random lumps.
POISSON’S RATIO The ratio of the strain parallel to an applied stress to that perpendicular to it [rock mechanics].
PORES The microscopic voids within a soil or rock. The non-solid component of a soil or rock. May be filled with liquid or gas.
PORE PRESSURE The pressure of the water (or air) in the pore spaces of a soil or rock. It equals total stress minus effective stress. The pore pressure may be negative.
PYRITE The most widespread sulphide mineral, FeS2 (iron pyrites).
QUARTZ The most common silica mineral (SiO2).
QUARTZITE A sandstone composed (almost) entirely of cemented quartz (silica) grains.
QUATERNARY A sub-era that covers the time from the end of the Tertiary to the present, approximately the last 2.0 Ma, and includes the Pleistocene and Holocene.
RESIDUAL SHEAR STRENGTH The strength along a shear surface (clay) which has previously failed or has undergone significant displacement. Generally the minimum shear strength. Tends to be constant for a given soil.
ROCKHEAD The upper surface of bedrock at surface (or its position) or below a cover of superficial deposits.
RUNNING SAND Fluidisation of sand and flow into an excavation below the water table or into a perched water table, under the influence of water flow into an excavation.
SAND A soil with a particle-size range 0.06 to 2.0 mm. Typically consists of quartz particles in a loose state.
SANDSTONE Sandstones are clastic rocks of mainly sand-sized particles (0.06–2.0 mm diameter), generally with quartz being the dominant component. Sandstones exhibit some form of cementation.
SATURATION The extent to which the pores within a soil or rock are filled with water (or other liquid).
SEDIMENTARY ROCKS Rocks which formed from sediments deposited under the action of gravity through a fluid medium and were subsequently lithified. Commonly: mudstone, siltstone, sandstone and conglomerate.
SETTLEMENT The lowering of the ground surface due to an applied load (see consolidation).
SHALE A fissile mudstone.
SHEAR PLANES/SURFACES A series of closely spaced, parallel surfaces along which differential movement has taken place. Usually associated with landslides or stress-relief. May be polished (slickensides).
SHEAR STRENGTH The maximum stress that a soil or rock can withstand before failing catastrophically or being subject to large unrecoverable deformations.
SHRINKAGE The volume reduction of a clay (or clay-rich soil or rock) resulting from reduction of water content. Shrinkage may cause subsidence of shallow foundations.
SHRINKAGE LIMIT The water content below which little or no further volume decrease occurs during drying of a clay (or clay-rich soil or rock). The laboratory tests which measure shrinkage limit have largely fallen into disuse in the UK. An Atterberg limit.
SIDERITE Carbonate mineral of iron (FeCO3).
SILT A soil with a particle-size range 0.002 to 0.06 mm (between clay and sand).
SILTSTONE A sedimentary rock intermediate in grain size between sandstone and mudstone.
SLAKE DURABILITY A measure of the ability of a rock to resist degradation by the combined action of wetting/drying cycles and mechanical abrasion.
SLICKENSIDES See shear planes.
SMECTITE A group of 2:1 clay minerals noted for their high plasticity and susceptibility to shrink/swell behaviour (e.g. montmorillonite).
SOLID A term used in geology to indicate mappable bedrock (see also Superficial).
SOLIFLUCTION The slow, viscous, down slope flow of waterlogged surface material, especially over frozen ground.
SORTING A descriptive term to express the range and distribution of particle sizes in a sediment or sedimentary rock, which has implications regarding the environment of deposition. Well-sorted (= poorly graded of engineering geology terminology) indicates a small range of particle sizes, poorly sorted (= well-graded) indicates a larger range.
STANDARD PENETRATION TEST (SPT) A long-established in-situ test for soil where the number of blows (N) with a standard weight falling through a standard distance to drive a standard cone or sample tube a set distance is counted. Used as an indication of lithology and bearing capacity of a soil.
STIFFNESS The ability of a material to resist deformation.
STRAIN A measure of deformation resulting from application of stress.
STRATIGRAPHY The study of the sequence of deposition of rock units through time and space.
STRESS The force per unit area to which it is applied. Frequently used as synonym for pressure.
SUBCROP The area over which a particular rock unit or deposit occurs immediately beneath another deposit, e.g. the Solid unit lying below Superficial Deposits (i.e. at rockhead).
SUBSIDENCE The settling of the ground or a building in response to physical changes in the subsurface such as under ground mining, clay shrinkage or drained response to overburden (consolidation).
SUCTION The force exerted when fluid within pores in a soil or rock is subjected to reduced atmospheric (or other environmental) pressure.
SUPERFICIAL DEPOSITS A general term for usually unlithified deposits of Quaternary age overlying bedrock; formerly called ‘drift’.
SWELLING The volume increase of a clay (or clay-rich soil or rock) resulting from an increase in water content. Swelling behaviour may cause heave of shallow foundations.
SWELLING INDEX The rebound (unloading) equivalent of the Compression index.
THAUMASITE Sulphate mineral produced by pyrite oxidation. It is extremely aggressive to concrete foundations (‘thaumasite attack’, TSA).
TILL An unsorted mixture which may contain any combination of clay, sand, silt, gravel, cobbles and boulders (diamict) deposited by glacial action without subsequent reworking by meltwater.
TRAFFICABILITY The capacity of a soil to support vehicle movement. This is influenced by soil shear strength, water content, and surface friction, ground pressure and vehicle wheel or track configuration.
TRIAXIAL TEST A laboratory test designed to measure the stress required to deform a sample until it fails, or until a constant rate of deformation is obtained.
UNCONFORMITY A break in the sedimentary record indicating cessation of deposition.
UNCONSOLIDATED See Consolidation.
UNDRAINED Condition applied to strength tests where pore fluid is prevented from escaping under an applied load. This does not enable an effective stress condition to develop.
UNIAXIAL COMPRESSIVE STRENGTH The strength of a rock sample (usually a cylinder) subjected to an axial stress causing failure (usually in an undrained condition) in the laboratory.
UNIT WEIGHT The weight of a unit volume of a material. Often used (incorrectly) as synonym for Density. Usually qualified by condition of sample (e.g. saturated, dry).
UPPER LIAS Corresponds broadly to the Whitby Mudstone Formation plus Blea Wyke Sandstone Formation in Cleveland Basin, and to Whitby Mudstone Formation plus Bridport Sand Formation elsewhere.
VALLEY BULGING The folding and displacement of incompetent beds in a valley floor due to stress relief (lateral forces exceeding vertical forces). Usually associated with periglacial conditions.
WATER CONTENT In a geotechnical context: the mass of water in a soil/rock as a % of the dry mass (usually dried at 105°C). Synonymous with moisture content.
WATER TABLE The level in the rocks at which the pore water pressure is at atmospheric, and below which all voids are water filled; it generally follows the surface topography, but with less relief, and meets the ground surface at lakes and most rivers. Water can occur above a water table.
WEATHERING The physical and chemical processes leading to the breakdown of rock materials (e.g. due to water, wind, temperature).
YOUNG’S MODULUS A measure of linear stiffness. The slope of the stress-strain graph for elastic deformation [rock mechanics].