Excavation, Trenching, and Soil Mechanics

12/09/18 www.oshainfo.gatech.edu 1
Excavation,
Trenching, and
Soil Mechanics
Subpart P
Jim Howry

Points To Be Covered:
 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!

4www.oshainfo.gatech.edu12/09/18
Number of Serious Violations - FY 00 2
1468
994
948
785
783
755
660
609
595
522
501(b)(1) - M
100(a) - E
451(g)(1) - L
21(b)(2) - C
451(e)(1) - L
652(a)(1)- P
451(b)(1) - L
503(a)1)- M
20(b)(2) - C
1053(b)(1) - X
Most Frequently Cited SeriousViolations
in Construction – FY00
Employee t raining
Head prot ect ion
Scaffolds- Plat form const ruct ion
Excavat ions- Prot ect ion of employees
Inspect ions by compet ent persons
Scaffolds- Fall prot ect ion
Fall prot ect ion- Unprot ect ed sides & edges
Port able ladder 3 feet above landing surface
Fall hazards t raining program
Scaffolds- Safe access
Standard & Subpart
-1926.

Number of Serious Violations - FY 00 16
Subpart P - Excavations
(1926.650 - 652)
83
263
345
351
755652(a)(1)
651(c)(2)
651(k)(1)
651(j)(2)
651(k)(2)
Standard - 1926.
Employee prot ect ion in excavat ions- Prot ect ive syst em use
Inspect ions by compet ent person
Prot ect ion from falling/ rolling mat erials/ equipment
Egress from t rench excavat ions
Compet ent person inspect ion- Employees removed from hazard

Fatal Facts!
 An employee was installing a small
diameter pipe in a trench 3’ wide, 12-
15’ deep and 90 feel 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.

 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 eight-
inch, 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.

1926.650
Scope and Application
 “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.

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!
Weighing in on Soil…

Mechanics 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 a 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 and 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)

1926.651(k)(1)
Inspections
•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 at 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 cave-ins.”

Hydraulic Shoring

Stacked Hydraulic
Shores

Whaler System

Surface Encumbrances
 Adjacent
Structures
 Roads and/ or
Sidewalks
 Curb and Gutter
 Light poles
 Utilities
 Mailboxes
 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”

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 gasses 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.

Excavation, Trenching, and Soil Mechanics

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