Excavation Safety

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Florida Department of Labor and Employment Security

Summary Statement

A presentation on the excavation standard, overview of soil mechanics and recognition of excavation hazards. Part of a collection.
June 3, 1996

These materials were produced by the Division of Safety, Florida Department of Labor and Employment Security, as part of a special OSHA program to reduce work-related deaths in construction in California, Florida, and Texas. Because Florida's funding for the program - CARE, or Construction Accident Reduction Emphasis - ended in June 2000, the materials will not be updated. The materials should be reviewed yearly for any needed changes.


To provide students with:

  • An introduction to 29 CFR 1926,
  • Subpart P-Excavation Standard
  • An overview of soil mechanics
  • An introduction to trenching and excavation hazard recognition

29 CFR 1926, Subpart P


  • Scope, application, and definitions applicable to this subpart
  • General requirements
  • Requirements for protective systems

1926.650 Scope, Application, and Definitions

  • Scope and application
  • Definitions
  • Competent Person
  • Accepted engineering practices
  • Aluminum hydraulic shoring
  • Bell- bottom pier
  • Benching
  • Cave- in
  • Competent person
  • Cross braces
  • Kick- out
  • Protective systems
  • Ramp
  • Registered professional engineer
  • Sheeting
  • Shield
  • Shoring
  • Excavation
  • Faces or sides
  • Failure
  • Hazardous atmospheres`
  • Sloping
  • Stable rock
  • Structural ramp
  • Trench.


"Excavation" means any:

  1. Man- made cut
  2. Cavity
  3. Trench
  4. Depression in an earth surface, formed by earth removal.

.Confined space is a space that, by design and/ or configuration has:

  1. Limited openings for entry and exit
  2. Unfavorable natural ventilation
  3. May contain or produce hazardous substances
  4. Is not intended for continuous employee occupancy.

"Trench (Trench excavation)" means a narrow excavation (in relation to its length) made below the surface of the ground.

  1. In general, the depth is greater than the width, but the width of a trench (measured at the bottom) is not greater than 15 feet (4.6 m).

    figure 19-9

  2. If forms or other structures are installed or constructed in an excavation so as to reduce the dimension measured from the forms or structure to the side of the excavation to 15 feet (4.6 m) or less (measured at the bottom of the excavation), the excavation is also considered to be a trench.


Accepted engineering practices are procedures that are compatible with the standard practice required of a registered professional engineer.

.Adjacent structure stability refers to the stability of the foundation( s) of adjacent structures whose location may create surcharges, changes in soil conditions, or other disruptions that have the potential to extend into the failure of the excavation or trench.

A competent persons is one who is capable of identifying existing and predictable hazards in the surroundings, or working conditions which are unsanitary, hazardous, or dangerous to employees, and who has authorization to eliminate them.

A competent person must have specific training in, and be knowledgeable about,

  • soils analysis
  • the use of protective systems
  • requirements of this standard

Protective systems refers to a method of protecting employees from cave- ins, material that could fall or roll from an excavation face into an excavation, and from the collapse of adjacent structures.
Protective systems include support systems, sloping and benching systems, shield systems, and other systems that provide the necessary protection.

 figure 19-3

figure 19-28

1926.651 - General requirements

  • Surface encumbrances
  • Underground installations
  • Access and egress
  • Exposure to vehicular traffic
  • Exposure to falling loads
  • Warning system for mobile equipment
  • Stability of adjacent structures
  • Protection of employees from loose rock or soil
  • Inspections
  • Fall protection
  • Hazardous atmospheres
  • Protection from hazards associated with water accumulation

1926.651( c)( 2) Means of egress
Means of egress from trench excavations: .

  1. A stairway, ladder, ramp or other safe means of egress shall be located in trench excavations that are 4 feet (1.22 mm) or more in depth.
  2. A means of egress should require no more than 25 feet (7.62 m) of lateral travel for any employee to reach.

1926.651( k)- Inspections

Daily inspections of excavations, the adjacent areas, and protective systems shall be made by a competent person for evidence of a situation that could result in:

  1. Possible cave- ins
  2. Indications of failure of protective systems
  3. Hazardous atmospheres
  4. Other hazardous conditions.

An inspection shall be conducted by the competent person:

  1. Prior to the start of work and as needed throughout the shift.
  2. After every rainstorm or other hazard increasing occurrence.
  3. These inspections are only required when employee exposure can be reasonably anticipated.

1926.652 - Requirements for protective systems

  • Protection of employees in excavations
  • Design of sloping and benching systems
  • Design of support systems, shield systems, and other protective systems
  • Materials and equipment
  • Installation and removal

1926.652 ( a)- Protection of employees in excavations

.(1) Each employee in an excavation shall be protected from cave- ins by an adequate protective system designed in accordance with paragraph (b) or (c) of this section except when:

  • (i) Excavations are made entirely in stable rock; or
  • (ii) Excavations are less than 5 feet (1.52 m) in depth and examination of the ground by a competent person provides no indication of a potential cave- in.

 figure 19-3
figure 19-1

1926.652 (d)- Materials and equipment

.(1) Materials and equipment used for protective systems shall be free from damage or defects that might impair their proper function.

(3) When material or equipment that is used for protective systems is damaged, a competent
person shall examine the material or equipment and evaluate its suitability for continued use.

If the competent person cannot assure the material or equipment is able to support the intended loads or is otherwise suitable for safe use.

  1. Such material or equipment shall be removed from service.
  2. Such material or equipment shall be evaluated and approved by a registered professional engineer before being returned to service.

Worker Protection Systems

Appendix A

  • Soil Classification
Appendix B
  • Sloping & Benching
Appendix C
  • Timber Shoring
Appendix D
  • Aluminum Hydraulic Shoring

figure 19-9

Soil Testing


Provide the student with:

  • A brief overview of 29 CFR 1926 Subpart P Excavation Standard
  • A Brief Introduction into Soil mechanics
  • A Brief overview of tests they can use in determining soil conditions

Overview: Soil Mechanics

Soil Mechanics

A number of stresses and deformities can occur in an open cut or trench.

For example, increases and decreases in moisture content can adversely affect the stability of a trench or excavation.

Following are some of the more frequently identified causes of trench failure.

Tension Cracks: Usually form at a horizontal distance of .5 to .75 times the depth of the trench, measured from the top of the vertical face of the trench.

Sliding or Sluffing: May occur as a result of tension cracks.

Toppling: In addition to sliding, tension cracks can cause toppling.

  1. Toppling occurs when the trench’s vertical face shears along the tension crack line and topples into the excavation.

Subsidence and Bulging:

  1. An unsupported excavation can create unbalanced stress in the soil, which in turn, causes subsidence at the surface and bulging of the vertical face of the trench.
  2. If uncorrected, this condition can cause face failure and entrapment of workers in the trench.
Heaving or Squeezing:

Bottom heaving or squeezing is caused by the downward pressure created by the weight of adjoining soil. This pressure causes a bulge in the bottom of the cut.

Heaving and squeezing can occur even when shoring or shielding has been properly installed.

Boiling is evidenced by an upward water flow into the bottom of the cut.

  1. A high water table is one cause of boiling.
  2. Boiling produces a “quick” condition in the bottom of the cut, and occur even when shoring or trench boxes are used.

Soil - Composition

What is Soil ?

Soils - Types


  • larger than 2 millimeters
  • Smaller than 2 millimeters but larger than 0.075 millimeters
  • Smaller than 0.075 millimeters but larger than 0.002 millimeters
  • Smaller than 0.002 millimeters

Determination of Soil Type

OSHA categorizes soil and rock deposits into four types. Each type is briefly described below.

Stable rock is natural solid mineral matter that can be excavated with vertical sides and remain intact while exposed.

Black Pine Mine Slide - Idaho

Type A soils are cohesive soils with an unconfined compressive strength of 1.5 tons per square foot or greater.

Examples of type A soils are: clay, silty clay, sandy clay, clay loam, and in some cases silty clay loam and sandy clay loam.

No soil is type A if it is fissured, is subject to vibration of any type, has previously been disturbed, is part of a sloped, layered system where the layers dip into the excavation on a slope of 4H to 1V or greater, or has seeping water.

Type B soils are cohesive soils with an unconfined compressive strength greater than 0.5 tons per square foot, but less than 1.5 tons per square foot.

Examples are: angular gravel, silt, silt loam, previously disturbed soils unless otherwise classified as type C soil.

Soils that meet the unconfined compressive strength or cementation requirements of type A soils but are fissured or subject to vibration; dry unstable rock; layered systems sloping into the trench at a slope less than 4H to 1V ( only if the material would be classified as a type B soil).

Type C soils are cohesive soils with an unconfined compressive strength of 0.5 tons per square foot or less and include granular soils such as gravel, sand and loamy sand, submerged soil, soil from which water is freely seeping, and submerged rock that is not stable.

Type C soils also include in this classification material in a sloped, layered system where the layers dip into the excavation or have a slope of 4H to 1V or greater.

1 cubic ft.  
rock = 167 lbs.
water = 62.4 lbs
Specific Gravity
The ratio of the weight of
an object, to the weight of
an equal volume of water.

Soils - Volume

Soils - Cubic Yard Weight

 Average soil


 Physical characteristics of sand and silt
 Soil sample
 Physical characteristics of clay

Field Tests

  • Documenting field tests
  • Sedimentation
  • Ribbon
  • Torvane
  • Pocket Penetrometer
  • Thumb imprint

Documenting Field Test

  • All Tests

    • Representative sample
    • Spoil pile
    • Inside the trench
    • Diagram
    • Site map
    • Record reading
    • Soil rating

Soil Sample
  • Ribbon test
  • Wet shaking
  • Sedimentation Test

Mix it - Quarter it

Mix it - Quarter it

Mix it - Quarter it

 Test # 1 - Field Sedimentation Test

  • Determines sand content
  • Used only on sandy soils
  • Sample taken from the spoil pile
  • Representative of soil in the excavation
  • Fill glass jar
  • 5 inches of water on top of soil
  • 1 1/2 inches of soil
  • Flat bottom container - at least 7 inches high

  • Place lid on jar and shake
    • Set jar down
    • Rotate slightly
    • Larger particles settle out immediately
    • Wait 30 seconds
    • Mark jar
    • Silt after several minutes
    • Fine clays in an hour
    • Make second mark

The Ribbon Test

  • Run only on that part of the soil which passes # 40 sieve
  • Test shows clay material
  • Run on disturbed soil
  • Representative sample from spoil pile
  • Mix soil + water to make into plastic mass
  • Roll mass into cylindrical shape 1/2 to 3/4 inch diameter
  • Lay across palm of hand
  • Press between thumb and second joint of index finger
  • Pass through thumb
  • Squeeze until it takes the shape of a 1/8 to 1/4 inch thick strip
  • Allow to hang freely from hand
  • Clay loam will barely ribbon and break easily
  • Clay = relatively long ribbon 6 to 8 inches or more
  • More clay = longer and stronger ribbon Silt has tendency to produce short ribbon with broken appearance


Test # 3 Torvane Shear Test

Torvane Shear

  • Designed to be used on saturated cohesive clay soil
  • Vanes are inserted into soil
  • Twist and shear soil at base and around circumference of vanes
  • Select fresh clod or block of undisturbed soil from spoil pile
  • Cut a smooth surface on the clod
  • Insert vanes of device into the soil
  • Retract vanes to show foot imprint
  • Set indicator at zero
  • Hold device firmly against soil and twist in clockwise manner until soil fails in shear

Consistency Term Shear Strength, psf Unconfined Compressive Strength, psf Soil Type
very soft
Type C
Type B
stiff stiff
very stiff
Type A

Pocket Penetrometer
  • Read the unconfined compressive strength at bottom of the red slip ring
  • The reading may be 2.0 tons per square foot shear strength
  • Which indicate the boundary between stiff and very stiff
  • Device is designed to work on saturated clay soil
  • Measures unconfined compressive strength of soil
  • Twice the value of shear strength of same soil
  • Note machine ring about a quarter of an inch
  • Push red ring on the barrel all the way toward the handle
  • Push shaft into the soil up to the red ring
  • Hold barrel so as to not to interfere with the spring inside the barrel
  • NOTE slip ring moved on the barrel as barrel was pushed back into the handle

Consistency Term Shear Strength, psf Unconfined Compressive Strength, psf Soil Type
very soft
Type C
Type B
stiff stiff
very stiff
Type A

Hazard Recognition Slides