Excavation, Trenching,
and Soil Mechanics
OSHA Regulations
• 1926.650: Scope and Application
• 1926.651: Specific Excavation
Requirements
• 1926.652: Requirements for Protective
Systems
• 1926 Subpart P Appendix A: Soil
Classification
• Other Appendix and Tabulated Data
DANGER!
Most Frequently Cited Serious Violations
in Construction – FY00
Standard & Subpart - 1926.
Fall protection - Unprotected sides & edges
1468
5 0 1 (b )(1 ) - M
Head protection
994
1 0 0 (a ) - E
Scaffolds - Fall protection
948
4 5 1 (g )(1 ) - L
Excavations - Protection of employees
785
2 1 (b )(2 ) - C
Scaffolds - Safe access
783
4 5 1 (e )(1 ) - L
Employee training
755
6 5 2 (a )(1 )- P
Scaffolds - Platform construction
660
4 5 1 (b )(1 ) - L
Fall hazards training program
5 0 3 (a )1 )- M
2 0 (b )(2 ) - C
1 0 5 3 (b )(1 ) - X
609
Inspections by competent persons 5 9 5
Portable ladder 3 feet above landing
surface
522
Number of Serious Violations - FY 00
2
Subpart P - Excavations
(1926.650 - 652)
Employee protection in excavations - Protective system use
6 5 2 (a )(1 )
755
Standard - 1926.
Egress from trench excavations
6 5 1 (c )(2 )
351
Inspections by competent person
6 5 1 (k )(1 )
345
Protection from falling/rolling materials/equipment
6 5 1 (j)(2 )
263
Competent person inspection - Employees removed from hazard
6 5 1 (k )(2 )
83
Number of Serious Violations - FY 00
16
Fatal Facts!
• An employee was installing a small
diameter pipe in a trench 3’ wide, 12-15’
deep and 90 feet long. The trench was not
sloped or shored nor was there a box or
shield to protect the employee. Further,
there was evidence of a previous cave-in.
The employee apparently reentered the
trench, and a second cave-in occurred,
burying him. He was found face down in
the bottom of the trench.
Fatal Facts!
• Four employees of a mechanical
contractor were laying a lateral
sewer line at a building site. The
foreman, a plumber by trade, and
a laborer were laying an eightinch, 20-foot long plastic sewer
pipe in the bottom of a trench 36
inches wide, nine feet deep, and
approximately 50 feet long. The
trench was neither sloped nor
shored, and there was water
entering it along a shale seam
near the bottom. The west side of
the trench caved in near the
bottom, burying one employee to
his chest and completely covering
the other. Rescue operations took
two and five hours - too late to
save the men.
Scope and Application
1926.650
• “This subpart applies to all open
excavations made in the earth’s
surface. Excavations are defined to
include trenches.”
What Is Soil?
• A soil is a mixture of rock, water, air and
a variety of other substances.
• Soil is made up of rock in the form of
small particles and spaces called voids.
Normally some part of these voids is
filled with water.
Weighing in on Soil…
Solid Rock weighs about 167 lbs. Per cubic
foot. Since a cubic foot of soil is about ½ rock
it will weigh about 83 lbs. Add in water and
the weight begins to rise to around 114 lbs. (if
saturated).
Do the math and a simple cubic yard of soil
can weigh over 3000 lbs! That is a ton and a
½!
Mechanics of a Cave-In
Diagram of a Cave-In
Diagram of a Cave-In
Protecting Employees in
Excavations
• 1926.652 (a)(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 in entirely stable rock; or
• (ii) Excavations are less than 5’ in depth and
examination of the ground by a competent person
provides no indication of a potential cave-in.”
Protective Systems
• “a method of protecting employees from
cave-ins, from material that could fall or
roll from an excavation face or into an
excavation, or 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”
So…We Have Options
• Slope, or Bench according to allowable
configurations…
• Use a Registered Professional Engineer to
design a sloping or benching system.
• Use support systems in accordance with
requirements set forth in the standard.
• Purchase an engineered system, (example..a
trench box or shield.)
• Have a PE design a shielding or shoring
system specific to your task.
29 CFR 1926.652(b)(4)(i)
• Design by a registered professional
engineer…
• Any system not found within the guidelines
set forth in the standard, or within the
tabulated data tables in the standard requires
use of a Professional Engineer.
• No where in the standard or tabulated data
does it allow for excavations deeper than 20
feet.
Competent Person
• “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
the authorization to take prompt
corrective measures to eliminate them”
• (Knowledge & Authority)
Inspections
1926.651(k)(1)
• Daily Inspection of excavations shall be conducted
by a ‘competent person’ prior to the start of work and
as needed throughout the shift.
• Inspections shall also be conducted after every
rainstorm or other hazard increasing occurrence.
Access and Egress
1926.651 (c)(2)
• “A stairway, ladder, ramp or other safe
means of egress shall be located in a
trench excavations that are 4’ or more in
depth so as to require no more than 25’
of lateral travel for employees.”
Soil Tests
• 1926 Subpart P Appendix A, Soil
Classification (c)(3).
• In order to classify a soil A, B, or C at
least one visual and one manual test
must be performed by a competent
person.
Visual Test
• “Visual analysis is conducted to
determine the qualitative information
regarding the excavation site in general,
the soil adjacent to the excavation, the
soil forming the sides of the open
excavation, and the soil taken as
samples from the excavated materials.”
Manual Tests
• “Manual analysis of soil samples is
conducted to determine quantitative as
well as qualitative properties of soil to
provide more information in order to
classify soil properly.”
• Examples: Thumb penetration, Pocket
penetrometers, Drying tests,
Sedimentation…
Thumb Penetration Test
• 1926 Subpart P Appendix A(2)(iii)
• …type A soils with an unconfined
compressive strength of 1.5 tsf can be readily
indented by the thumb; however, they can
only be penetrated by the thumb only with
great effort…Type C soils with an unconfined
compressive strength of .5 tsf can be easily
penetrated several inches by the thumb, and
can be molded by light finger pressure.
Solid Rock
• “Natural solid mineral matter that can be
excavated with vertical sides and
remain intact while exposed”
Type A Soil
• Cohesive soil with an unconfined,
compressive strength of 1.5 Tsf.
• Examples of this type of soil are:
• Clays, silty clay, sandy clays, and clay
loam.
A Soil is NOT type ‘A’ If:
•
•
•
•
The soil is fissured.
The soil is subject to vibration.
The soil has been previously disturbed.
The material is subject to other factors
that would require it to be classified as a
less stable material.
• Water is present.
Sources of Vibration
•
•
•
•
•
•
Traffic
Railroad Operations
Heavy Equipment Operations
Jack Hammer Operations
Tamping Machine Operations
Thumping Car Stereos that make you
want to pull your hair out and to…never
mind…
Type ‘B’ Soil
• Cohesive soil with an unconfined
compressive strength greater than .5 Tsf
• Granular cohesionless soils including; silt, silt
loam, sandy loam, and some sandy clay loam
• Previously disturbed soils except those which
would otherwise be classified as Type ‘C’ soil
Type ‘C’ Soil
• Cohesive soils with an unconfined
compressive strength of 0.5 Tsf or less
• Granular soils including gravel, sand,
and loamy sand
• Submerged soil, or soil from which
water is freely seeping
Sloping Requirements
by Soil Type
Soil or Rock Type
Maximum
Allowable Slope for
Excavations less
than 20’
Stable Rock
Vertical (90 Deg.)
Type A
¾ to 1 (53 Deg.)
Type B
1 to 1 (45 Deg.)
Type C
1 ½ to 1 (34 Deg.)
Sloping in Type ‘A’ Soil
Sloping in Type ‘B’ Soil
Sloping in Type ‘C’ Soil
Trench Boxes or Shield
Systems
• “A structure that is able to withstand the
forces imposed on it by a cave-in and
thereby protect employees.”
Trench Boxes or Shield
Is This Allowable?
YES! If the Trench boxes are designed to be
stacked and are used in accordance with their
tabulated data.
Trench Boxes and Sloping or
Benching
• Trench boxes are generally used in
open areas, but they may also be used
in combination with sloping and
benching. The box should extend at
least 18 inches above the surrounding
area if there is sloping toward the
excavation.
Shoring
(Shoring System)
• “A structure such as metal hydraulic,
mechanical or timber shoring system
that supports the sides of an excavation
and which is designed to prevent caveins.”
Hydraulic Shoring
Stacked Hydraulic Shores
Whaler System
Surface Encumbrances
• 1926.651 (a)
• “All surface
encumbrances that are
located so as to create
a hazard to employees
shall be removed or
supported as
necessary to
safeguard employees”
• Adjacent
Structures
• Roads and/ or
Sidewalks
• Curb and Gutter
• Light poles
• Utilities
• Mailboxes
Utilities
• 1926.651(b)
• The estimated location of utility
installations, such as sewer, telephone,
fuel, electric, …that reasonably may be
expected to be encountered during
excavation work, shall be determined
prior to opening an excavation…
Locates
•
Red = Power
Blue = Water
Orange = Communications
Yellow = Gas
Can an Excavation be a
Confined Space…?
• Oxygen deficient atmospheres: less
than 19.5% oxygen.
• Potential for other gases to be present
include but not be limited to:
• Natural Gas from potential leaks or cut
lines.
• Methane from decayed matter.
Summary
• Call before you dig.
• Use at least one visual and one manual soil test to
determine soil type.
• A ladder is required for access and egress at 4’.
• The ladder must be within 25’ lateral travel at all times.
• At 5’ depth some type of ‘protective system is
required’.
• Treat all soil as Type ‘C’ and slope at 1 ½ : 1 and you
are covered.
• If not: Use some type of shoring or shielding to
protect your employees.
• Excavations over 20’ require the use of a P.E.