GLOSSARY OF COMMON GEOLOGY TERMS
Base Coarse - The layer of select material used in a pavement system that is located directly above the subbase or subgrade. It may help to distribute loads, provide drainage, and to minimize frost action throughout the pavement system.
Bearing Wall - The structural wall which supports part of the load from above and transfers the load down to the lower floor or foundation.
Bedrock- Rock of relatively great thickness and extent in its native location. May be overlain by soil or exposed to the surface.
Bedding Plane - The surface that separates a layer of stratified rock, such as shale, from an overlying or underlying layer.
Bench - A relatively level step, excavated into a slope on which fill is to be placed. Its purpose is to provide a firm stable contact between the existing material and the new fill which is to be placed.
Bentonitic Clay - A clay with a high content of the mineral montmorillonite, characterized by high swelling or wetting. Commonly used as drilling mud for rotary wash drilling.
Berm - A narrow, low mound (usually at the top of a slope) used to divert water. Typically, it is four to five inches wide and twelve to eighteen inches high.
Boulder - A rock fragment, usually rounded by weathering or abrasion, with minimum dimensions of twelve inches or greater.
Bridging - The transfer of stress from a yielding part of a soil mass to adjoining less yielding or restrained parts of the mass. Bridging is usually considered when designing slot cut widths or arching between soldier piles.
Buttress - A man-made fill designed and built to support a weak or unstable slope. A buttress is usually provided with a "keyway" at the toe of slope which is excavated into stable material. Compacted fill is benched into stable material as the fill slope is constructed. The width and thickness of the buttress are dictated by design engineering.
Caisson - A cylindrical shaft drilled into competent material, at which depth the bottom of the shaft is sometimes reamed, or belled, into a conical shape to provide a larger base for foundation support. The shaft is reinforced with steel and filled with concrete.
Capillary Action - The rise or movement of water int the interstices of a soil due to capillary forces.
Casing - Special steel or fiberglass tubing welded or screwed together and lowered into a borehole to prevent entry of loose rock, gas or liquid into the borehole. Casing may also be used to help prevent loss of circulation fluid into porous, cavernous or crevassed ground.
Certification - A written engineering or geologic opinion concerning the progress and completion of related work.
Circulating Fluid - A fluid pumped into a borehole through the drill stem, the flow of which cools the bit, washes away cuttings from the bit, and transports the cuttings out of the borehole.
Clay - A fine grained soil or the fine grained portion of a soil that can be made to exhibit plasticity within a range of water contents, and which exhibits considerable strength when air dry. Clay particles are smaller than 0.074mm (#200 sieve) and must plot above the A- line on the plasticity chart (ASTM D2487).
Clean Soil - Less than 5% of the soil passing the #200 sieve.
Cobble - A rock fragment, usually rounded or semi-rounded, with a minimum dimension of 3 to 12 inches.
Cohesion - All of the shear strength of a soil not due to friction. Cohesion can be pictures as an inherent tensile force that arises from the attraction that exists between very small particles.
Cohesionless Soil - A soil that when confined has little to no strength when air dried, and has little or no strength submerged (such as clean sand).
Cohesive Soil - Soil that when unconfined has strength when air dried, and has significant cohesion when submerged (such as a clay).
Compaction - The densification of a soil by means of mechanical manipulation.
Compaction Curve - The curve showing the relationship between the dry unit weight (density) and the moisture content of a soil for a given compactive effort.
Compaction Test - A laboratory compacting procedure whereby a soil at a known moisture content is placed in a specified manner into a mold of given dimensions, subjected to a compactive effort of controlled magnitude, and the resulting unit weight determined. The procedure is repeated at various moisture contents to establish a relation between the moisture content and the unit weight.
Compressibility - Property of a soil pertaining to a decrease in the volume of a soil mass resulting from the expulsion of only pore water when subjected to load.
Consistency - The relative ease with which a soil can be deformed.
Consolidation - The gradual reduction in volume of a soil mass due to an increase in compressive strength.
Consolidation Test - A test in which a soil sample is laterally confined in a ring and is compressed between two porous plates. The vertical movement (consolidation) is recorded over time by reading a vertical deflection dial.
Core Sample - A cylindrical sample of rock or soil recovered from a borehole.
Core Barrel - A length of steel tube designed to receive the core sample as it is being cut by the drill bit. It then retains the core sample until it can be raised to the surface.
Creep - The slow downslope movement of rock, debris, or soil, usually imperceptible except to observations of long duration.
Daylight Line - The surface contact between cut and fill.
Density (Unit Weight) - Weight of a material per unit volume.
Density Test - A field test used to determine the "in place" unit weight of a soil. Types of density test devices include the sand cone and nuclear density gauge.
Desiccated - Expansive soils (usually silts and clays) which have developed cracks due to shrinkage from a loss of moisture.
Diatomaceous Soil - A mixture of soil and the external fossil skeletons of microscopic plants.
Direct Shear Test - A test in which soil under an applied load is stressed to failure by moving one section of the soil container (shear box) relative to the other section.
Drill Mud - Water mixed with clay (usually bentonite) and sometimes other material such as barite, oil, etc. Drilling mud is used as a circulation medium to help keep the drill bit cool and to bring the cuttings to the surface.
Drive Sampler - A thick walled tube that is forced into the soil (without rotation) by hydraulic pressure or a drive hammer. Typically the hammer is 140 pounds and is dropped 30 inches.
Elasticity - The property of a soil which causes it to recover its original shape and size when it is deformed and the deforming forces are removed.
Erosion - The wearing away of soil and rock as a result of the movement of wind, water and/or ice.
Excavation - The mechanical removal of earth material which results in a pit or trench below a reference ground surface. It can be a large pit (such as a basement) or a small trench ( such as a footing).
Expansive Soil - A soil, usually with clay or silt, which changes volume with changes in moisture content. As the moisture of the soil increases, the soil swells or expands, as the moisture content decreases, the soil shrinks.
Fault - A fracture in the earth's crust across which there has been relative movement.
Fill - Man-made deposits of soil and/or waste material
Filter - (Permeable filter) A layer or combination of layers of permeable material (typically sand and gravel) designed and placed in such a manner as to provide drainage, yet prevent the movement of soil particles due to flowing water.
Fines - Portion of the soil smaller (finer) that the #200 sieve (0.074mm).
Finish Grade - The final elevation of the ground following grading.
Floc - Loose, open-structured mass formed in a suspension by the aggregation of minute particles.
Footing - The portion of the foundation of a structure that transmits loads directly to the soil.
Formation - A similar structure or geologic arrangement of a soil or rock mass in certain regions.
Foundation - The lower part of a structure that transmits the load to the earth.
Friable - Easily broken or crumbled, often used in describing sandstones.
Free Water - Water that is free to move through a soil mass under the influences of gravity.
Glacial Till - Material deposited by glaciation, usually composed of a wide range of particle sizes, which has not been subjected to the sorting action of water.
Gradation - (Grain Size Distribution) - Amount of material by weight of each grain or particle size present in a given soil.
Grade - The vertical location of the ground surface.
Granular - Soil or portion of soil that is larger that the #200 sieve (0.074mm).
Grain Size Analysis (Sieve analysis) - The process of determining gradation.
Granitic - Made of granite or other light colored plutonic rock with a composition approaching granite.
Gravel - Rounded to angular particles of rock that will pass a 3" sieve (75mm) and are retained on a #4 sieve (4.17mm).
Grouting - The process of pumping a slurry of cement or a mixture of cement and fine sand into crevices or voids in a rock or soil to densify the subsurface material. This process if often referred to as "compaction grouting".
Hardness Scale - (Moh's Scale) Units by which the scratch hardness of a mineral is determined. The units of hardness are expressed in numbers ranging from 1 to 10, each of which is represented by a mineral having a lower ranking number. The minerals are ranked from softest, as follows : (1) Talc, (2) Gypsum, (3) Calcite, (4) Fluorite, (5) apatite, (6) Orthoclase, (7) Quartz, (8) Topaz, (9) Corundum, (10) Diamond.
Hardpan - A layer of extremely dense soil
Heave - Aun upward movement of soil caused by expansion or displacement resulting from reasons such as : moisture absorption, removal of overburden, driving of piles, and frost action.
Hydrometer - An instrument for determining the specific gravity of a liquid. A hydrometer may be used in the grain size analysis of the soil smaller than 0.074mm (#200 sieve).
Igneous Rock - A rock that has formed by solidifying from a molten state.
Import Material - Fill material acquired from an offsite location for use in grading at another site.
In-Situ - In place and undisturbed.
Jetting - A method of settling soil by injecting water under pressure into the soil mass by use of a small diameter pipe or hose.
Key - A trench excavation made below the toe of a fill slope, excavated into firm, competent material. It's purpose is to establish a strong bond between the existing soil and the proposed fill slope. The keyway is filled with compacted fill.
Landslide - The failure of a slope bank in which the movement of the soil or rock mass takes place along a plane of sliding (slide plane).
Lath - The surveyor's stakes used to mark specific locations and elevations on a grading job.
Leaching - The removal of soluble soil material by percolating water.
Lift - A horizontal layer of soil placed prior to compaction.
Liquefaction - The sudden large decrease of the shearing resistence of a cohesionless soil. It is caused by a collapse of structure by shock or other types of strain and is associated with a sudden, but temporary, increase of the porefluid pressure. It involves a temporary transformation of the material into a fluid-like mass.
Liquid Limit - The water content corresponding to the arbitrary limit between the liquid and plastic states of consistency of a soil. It is also the water content at which a pat of soil, cut by a groove of standard dimensions, will flow together for a distance of ½ inch under the impact of 25 blows of a standard liquid limit device.
Live Load - The load imposed on a structure for temporary or occasional forces, such as wind, earthquakes, people, moveable machinery, etc.
Lost Circulation - A condition that occurs when drilling fluid escapes into cracks and crevices or porous sidewalls of a borehole and does not return to the top of the borehole.
Maximum Density - The dry unit weight defined by the peak of a compaction curve.
Metamorphic Rock - A rock that has formed from another rock (without melting) in response to changes in temperature, pressure and chemical environment, generally well below the surface of the earth.
Micaceous - A soil or rock type which contains an appreciable amount of the mineral mica. The thin flexible and elastic flakes distinguish it from most other minerals.
Mineral - An inorganic substance occurring in nature, such as quartz or mica, having a definite and unique chemical composition and usually of definite crystal structure.
Moisture Content - The ratio, as expressed in percentage, of the weight of water in a given soil mass to the weight of solid particles.
Mud - A mixture of soil and water in a fluid or semi-solid state.
Mud jacking - See pressure grouting
Natural Ground - The soil or rock deposited by nature, such as weathering, erosion, etc. Soils which have not been placed by man.
Optimum Moisture - The moisture content at which a soil can be compacted to the maximum dry unit weight by a given compaction effort.
Organic Soil - Soil with a high organic content, such as plant or animal material. Peat is an organic soil.
Overburden - See Uppermost soils
Over-Consolidation - A soil deposit that has been subjected to an effective pressure greater than the present overburden pressure.
Packer - A device lowered into a borehole, which automatically swells or can be made to swell at the correct time to produce a water tight joint against the side of the borehole or casing.
Parent Material - Material from which a soil or fill has been derived.
Peat - A highly organic material, its USCS symbol is Pt.
Pedology - The science that pertains to soil, including the nature, properties, formation, behavior and response to use and management.
Percent Compaction - The ratio, expressed as a percentage, of the dry density of a soil (obtained by a field density test), and the maximum density (obtained in a laboratory compaction test).
Percent Saturation - The ratio, expressed as a percentage, of the volume of water in a given soil mass, to the total volume of intergranular space (voids).
Perched Water Table - A water table usually of limited area maintained above the normal free water elevation by the presence of an intervening impermeable confining strata.
Percolation - The movement of gravitational water through soil.
Permafrost - Permanently frozen ground, or ground that remains below freezing temperature for two or more years.
Permeability - The ease or difficulty with which water will flow or pass through the pores of soil or rock.
Permeability Test - A procedure usually followed to determine the water tightness of a soil or rock formation. It is typically used before the construction of dams. The test is performed by placing packer assemblies in boreholes to segregate each successive strata or each five foot depth, with water injected into the borehole space between the packers with a high pressure pump. The volume of water escaping into the rock or soil is a measure of permeability.
Piezometer - An instrument for measuring water pressure head.
Pile - (Friction or driven) - A relatively slender structural element which is driven, drilled or otherwise introduced into the soil or rock. Support is provided by skin friction between the pile and soil.
Plastic Limit - The water content at which a soil will just begin to crumble when rolled into a thread approximately 1/8 of an inch in diameter. It is also the water content corresponding to an arbitrary limit between the plastic and semisolid states of consistency of a soil.
Plasticity - The property of a soil which allows it to be deformed beyond the point of recovery without cracking or appreciable volume change.
Plasticity Index - The numerical difference between the liquid limit and the plastic limit.
Pre-Saturation - Usually refers to the moisture conditioning (above optimum moisture) of a pad and/or footing excavation prior to the pouring of concrete. Pre-saturation is usually done to expansive soils to reduce the future swelling of the soil, which can cause concrete slabs and footings to crack and separate.
Pressure Grouting - A method of stabilizing or improving the density of a soil mass by filling in voids through injecting a mixture of cement, sand and soil under high pressure.
Porosity - The ratio, usually expressed as a percentage, of the volume of voids of a give n soil mass, to the total volume of the soil mass.
Porous Soil - Soil with numerous small voids or holes. Porous soils are usually susceptible to collapse when subjected to moisture or loading.
Quick Condition - (Quick sand) - Condition in which water is flowing upwards with sufficient velocity to reduce significantly the bearing capacity of the soil through a decrease in intergranular pressure.
Recovery - (Percent Recovery) - The amount of soil or rock sample obtained from a borehole by use of any soil sampling device. May be expressed as the ratio of sample length obtained to the length of sample attempted.
Relative Density - The ratio of the difference between the void ratio of a cohesionless soil in the loosest state and ay given void ratio to the difference between its void ratios in the loosest and densest states.
Remolded Soil - Soil that has had its natural structure modified by manipulation.
Rip-Rap - Broken rock used for the protection of embankments, cut slopes, etc. against agents of erosion.
Sand - Particles of rock that will pass the #4 sieve (4.76mm) and be retained by the #200 sieve (0.074mm).
Sand Cone - A device used for determining the "in place"density of a soil. ASTM designation D 1556.
Sandstone - A sedimentary rock composed mostly of sand size grains.
Sedimentary Rock - A rock formed by the accumulation and cementation of mineral grains or rock fragments deposited by agents of erosion.
Seepage - The slow movement of gravitational water through soil and rock.
Settlement - The reduction of surface elevation due to the compressibility of the underlying soils.
Shale - A sedimentary rock composed of silt and clay size particles. Shales tend to break and separate into thin layers.
Shear Failure - Failure in which movement caused by shearing stresses in a oil mass is of sufficient magnitude to destroy its structure.
Shear Strength - The maximum resistence of a soil to shear stress.
Shear Stress - The action, resulting from applied forces, that tends to cause soil masses to slide adversely in a direction parallel to their plane of contact.
Shoring - Means by which temporary excavations are supported. Shoring can include braces and piles.
Sieve - A wire mesh container with square openings used for separating sizes of soil particles.
Silt - Material passing the #200 sieve (0.074mm) that has little or no plasticity and low to no dry strength when air dried.
Skin Friction - The frictional resistence developed between soil or rock and a structure.
Soil - Sediments and other accumulations of solid particles produced by the chemical and physical disintegration of rocks, and which may or may not contain organic matter.
Soil Horizon - One of the layers of the soil profile, distinguished principally by its texture, color, structure and chemical properties.
"A" Horizon - the uppermost layer of the soil profile from which inorganic colloids and other soluble materials have been leached. It usually contains remnants of organic life.
"B" Horizon - The layer in a soil profile in which material leached from the overlying "A" horizon is accumulated.
"C" Horizon - Undisturbed parent material from which the overlying soil profile has been developed.
Soil Profile - A vertical section of a soil mass, showing the nature and sequence of various layers, as developed by natural or mechanical means.
Soil Stabilization - The chemical or mechanical treatment of soil designed to increase or maintain the stability of a mass of soil or otherwise improve its engineering properties.
Specific Gravity - The ratio of the weight of air in a given volume of soil solids at a stated temperature to the weight of air in an equal volume of distilled water at a stated temperature.
Spring - Location where the water table seeps out at the ground surface in a more or less continuous fashion.
Standard Penetration Test - A soil sampling procedure to determine the relative density of the underlying soil. The number of blows by a 140 pound drive hammer, freely falling a distance of 30 inches per blow is counted to drive a split spoon sampler a depth of one foot.
Strain - The change in length per nit of length in a given direction.
Stratum - A single layer of homogeneous or gradational lithology deposited parallel to the original dip of the formation. It is separated from adjacent strata or cross strata by surface erosion, non-deposition, or abrupt changes in character. Stratum is not synonymous with the terms "bed" or "lamination", but includes both. "Bed" or "lamination" carry definite connotations of thickness.
Stress - The force per unit area acting within the soil mass.
Subbase - A layer used in a pavement system between the subgrade and the base coarse material.
Subgrade - The soil prepared or compacted to support a structure or a pavement system.
Subdrain - A drainage system placed below the surface to drain subsurface water. Typically consists of a filter material (3/4" gravel) and/or a specified type and size (4-inch PVC) perforated drain pipe.
Swell - An increase in volume of a soil due to the absorption of water into the intergranular pore space.
Terrace - A relatively level step constructed in the face of a slope, for drainage and maintenance purposes.
Toe of Slope - The contact between the base of the slope and a level horizontal ground surface.
Topsoil - Surface soil usually containing organic matter.
Triaxial Shear Test - A test in which a cylindrical sample of soil encased in an impervious membrane is subjected to a confining pressure and then loaded axially to failure.
Unconsolidated Soil - A deposit of soil that is not fully consolidated under the existing overburden pressure.
Underpinning - A footing introduced beneath an existing footing for the purpose of transferring the foundation load to a lower depth, and more suitable material.
Undisturbed Samples - A soil sample that has been obtained by methods in which every precaution has been taken to recover the sample in its natural, in-situ state.
Uniform Building Code (UBC) - A standard set of specifications for safe construction. Codes are published in a book form and updated every four years.
Unified Soil Classification System (USCS) - A widely used system for identifying and classifying soils. This system was adopted by the Army Corps of Engineers in 1952 and is now standardized by ASTM.
Unit Dry Weight - The weight of soil solids per unit total volume of the soil mass.
Unit Weight of Water - The weight per unit volume of water, normally equal to 62.4 pounds per cubic foot.
Varved Soil - Alternating layers of silt or fine sand and clay formed by variations in sedimentation during various seasons of the year. Often exhibiting contrasting colors when dry.
Viscosity - The property of a fluid to resist internal flow.
Void - The space in a soil mass not 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.
Weathered - The term used to describe rock that has been partially decomposed or disintegration due to the forces of nature, such as erosion, chemical reactions, etc.