Excavation and Trenching Glossary

A

Accepted engineering practices

Requirements that are compatible with standards of practice required of a registered professional engineer. Subpart P uses the phrase when a design does not come from OSHA's appendices or tabulated data.

Adjacent structure stability

OSHA 1926.651(i) requires support such as shoring, bracing or underpinning when an excavation endangers nearby buildings, walls or other structures. See lesson 8.

Aluminum hydraulic shoring

A pre-engineered shoring system of aluminum hydraulic cylinders (cross braces) used with vertical rails (uprights) or horizontal rails (wales). Appendix D to Subpart P gives tabulated data for it. See shoring systems.

Angle of repose

The steepest angle at which a pile of loose soil stays put without sliding. OSHA's maximum allowable slopes in Appendix B are set to keep cut faces stable.

Appendix F

The Subpart P flowchart that walks through choosing a protective system: sloping, benching, shoring, shielding or a professional engineer's design.

B

Bell-bottom pier hole

A shaft or footing excavation whose bottom is wider than the shaft above it. Workers entering one must wear a harness with a lifeline, per 1926.651(g)(2)(ii).

Benching

Protecting workers from cave-ins by cutting the sides of an excavation into a series of horizontal steps with vertical or near-vertical faces between levels. Allowed in Type A and cohesive Type B soil, not in Type C. See sloping and benching.

Boiling

Water flowing upward into the bottom of a cut, carrying soil with it. A sign of bottom instability that can undermine a protective system.

Bottom heave

Upward movement of the floor of an excavation caused by the weight of the surrounding soil, usually in soft clays.

C

Call before you dig (811)

The free national number that connects excavators to their state's one-call center, which notifies utility owners to mark their underground lines. See utilities and 811.

Cave-in

The separation of a mass of soil or rock from the side of an excavation, or the loss of soil from under a trench shield or support system, and its sudden movement into the excavation in a quantity that could entrap, bury or injure a person.

Cemented soil

Soil whose particles are held together by a chemical agent such as calcium carbonate, so that a hand-size sample cannot be crushed into powder by finger pressure. Caliche and hardpan are examples.

Cohesive soil

Clay or soil with a high clay content that sticks together, does not crumble, can be excavated with vertical side slopes and is plastic when moist. It is hard to break up when dry.

Competent person

Someone capable of identifying existing and predictable hazards and authorized to take prompt corrective measures to eliminate them. Subpart P assigns soil classification and daily inspections to the competent person. See lesson 11.

Cross braces

Horizontal members of a shoring system installed perpendicular to the sides of the excavation, with ends bearing against uprights or wales.

D

Daily inspection

The competent person's check of the excavation, adjacent areas and protective systems before work starts, as needed during the shift, and after every rainstorm or other hazard-increasing event. Required by 1926.651(k).

Dry soil

Soil that does not show visible signs of moisture content.

E

Egress

A safe way out of an excavation. In trenches 4 feet or deeper, a stairway, ladder, ramp or other safe means of egress must be within 25 feet of lateral travel. See the egress calculator.

Excavation

Any man-made cut, cavity, trench or depression in an earth surface formed by earth removal. See lesson 1.

F

Faces (sides)

The vertical or inclined earth surfaces formed when an excavation is made.

Failure

The breakage, displacement or permanent deformation of a structural member or connection that reduces its structural integrity and supportive capability.

Fissured

Soil that tends to break along definite planes of fracture with little resistance, or that shows open cracks such as tension cracks in an exposed surface. Fissured soil cannot be classified as Type A.

G

Granular soil

Gravel, sand or silt (coarse-grained soil) with little or no clay. It has no cohesive strength, cannot be molded when moist, and crumbles easily when dry.

H

Hazardous atmosphere

An atmosphere that is explosive, flammable, poisonous, corrosive, oxidizing, irritating, oxygen-deficient, toxic or otherwise harmful and may cause death, illness or injury. See lesson 10.

Hydraulic shoring

A shoring system that uses hydraulic cylinders to press uprights or wales against the trench walls. Usually aluminum, installed and removed from outside the trench.

K

Kickout

The accidental release or failure of a cross brace.

L

Layered system

Two or more distinctly different soil or rock types arranged in layers. Layers that dip into the excavation reduce stability, and the weakest layer controls classification.

Lower flammable limit (LFL)

The lowest concentration of a gas in air that can burn. In excavations, OSHA requires precautions such as ventilation to keep flammable gas below 20 percent of the LFL.

M

Manual soil test

A hands-on test of a soil sample, such as the thumb penetration, plasticity (thread), dry strength or drying test. OSHA requires at least one manual and one visual test to classify soil. See soil tests.

Moist soil

Soil that looks and feels damp. Moist cohesive soil can easily be shaped into a ball and rolled into small threads.

P

Plastic

A property of soil that allows it to be deformed or molded without cracking or appreciable volume change.

Pocket penetrometer

A hand-held spring device pushed into a soil sample to estimate its unconfined compressive strength in tons per square foot.

Previously disturbed soil

Soil that has been dug and replaced, such as backfill over existing utilities. It cannot be Type A.

Protective system

A method of protecting workers from cave-ins, from material falling or rolling into the excavation, or from the collapse of adjacent structures. Includes support systems, sloping and benching, shields and other systems.

R

Ramp

An inclined walking or working surface used to get from one level to another. Structural ramps used only by employees must be designed by a competent person.

Registered professional engineer (PE)

An engineer registered in the state where the work is done. A PE must design protective systems for excavations deeper than 20 feet, and approves tabulated data.

S

Saturated soil

Soil whose voids are filled with water. Soil from which water is freely seeping is classified as Type C.

Sheet piling

Interlocking steel sheets driven into the ground before excavation to retain soil, often used for deep or wet excavations and designed by an engineer.

Sheeting

Members of a shoring system that retain the earth in place and are in turn supported by other members, such as uprights or wales. Plywood sheeting is common with hydraulic shores.

Shield (trench box)

A structure able to withstand the forces of a cave-in and protect workers inside it. Shields can be permanent or portable and moved along as work progresses. See trench boxes.

Shoring

A structure, such as a hydraulic, mechanical or timber system, that supports the sides of an excavation and is designed to prevent cave-ins.

Short-term exposure

In Appendix B, a period of 24 hours or less that an excavation is open. A steeper ½:1 slope is allowed in Type A soil for short-term excavations 12 feet deep or less.

Slide rail system

A modular shoring system of steel panels that slide down vertical posts (rails) as the excavation deepens, used for deep or long open excavations and pits.

Sloping

Protecting workers from cave-ins by cutting the sides of an excavation back at an angle away from the excavation. See the slope calculator.

Soil classification system

The method in Appendix A of categorizing soil and rock as Stable Rock, Type A, Type B or Type C, in decreasing order of stability.

Spalling

Chunks or slabs of soil breaking off a trench face, often as the soil dries.

Spoil pile

Soil removed from an excavation and placed beside it. OSHA requires it, and other materials and equipment, to be kept at least 2 feet from the edge or held back by retaining devices.

Stable rock

Natural solid mineral matter that can be excavated with vertical sides and remain intact while exposed.

Submerged soil

Soil that is underwater or free-seeping. It is classified as Type C.

Support system

A structure such as underpinning, bracing or shoring that provides support to an adjacent structure, underground installation or the sides of an excavation.

Surcharge load

Extra weight near the edge of an excavation, such as spoil, stored material, equipment, vehicles or nearby buildings, that adds pressure on the trench wall.

Surface encumbrance

Anything at the surface near an excavation that could create a hazard, such as utility poles, trees, signs, sidewalks or boulders. OSHA requires them to be removed or supported.

T

Tabulated data

Tables and charts approved by a registered professional engineer and used to design and construct a protective system. Manufacturer's tabulated data for trench boxes and shores must be kept at the jobsite.

Tension crack

A crack that forms at the surface parallel to the trench edge, usually at a distance of about 0.5 to 0.75 times the trench depth. A warning sign of potential collapse.

Thumb penetration test

A manual test pressing the thumb into a fresh soil sample. Type A soil can be indented by the thumb but penetrated only with very great effort; Type C soil can easily be penetrated several inches.

Timber shoring

Shoring built from wood uprights, wales and cross braces, designed from Appendix C tables or by an engineer.

Tolerance zone

The width on either side of a marked underground utility in which excavators must use hand tools or other non-mechanical methods. Typically 18 to 24 inches, set by state law.

Trench

A narrow excavation made below the surface of the ground, generally deeper than it is wide, with a bottom width of no more than 15 feet.

Type A soil

Cohesive soil with an unconfined compressive strength of 1.5 tons per square foot or more, such as clay, silty clay and hardpan, that is not fissured, disturbed or subject to vibration. Max slope ¾:1 (53°).

Type B soil

Cohesive soil with an unconfined compressive strength above 0.5 but below 1.5 tsf, angular gravel, silt, silt loam, previously disturbed soil, and Type A soil that is fissured or subject to vibration. Max slope 1:1 (45°).

Type C soil

Cohesive soil with an unconfined compressive strength of 0.5 tsf or less, granular soil such as gravel, sand and loamy sand, submerged soil and soil with freely seeping water. Max slope 1½:1 (34°).

U

Unconfined compressive strength

The load per unit area at which a soil fails in compression, in tons per square foot (tsf). It is the main measure behind Type A, B and C classification.

Underpinning

Supporting the foundation of an existing structure, for example by extending it deeper, so an excavation next to it does not undermine it.

Uprights

The vertical members of a trench shoring system placed against the earth, usually so that individual members do not contact each other.

V

Visual soil test

Observing the soil in the excavation and the spoil for particle size, cohesion, fissures, layers, water, previous disturbance and vibration sources. OSHA requires at least one visual test.

W

Wales

Horizontal members of a shoring system placed parallel to the excavation face, whose sides bear against the vertical members or the earth.

Walkway

A crossing over an excavation for workers or equipment. Walkways 6 feet or more above lower levels need guardrails.

Wet soil

Soil that contains significantly more moisture than moist soil, in a range where cohesive material slumps or begins to flow when vibrated.

Last reviewed: October 25, 2026 · Free general education based on OSHA 29 CFR 1926 Subpart P. It does not replace the site-specific training and competent person your employer must provide, and it does not issue a certificate. Nothing you enter on this page is saved or sent.