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Excavation, Trenching, and Soil Mechanics

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Title: Excavation, Trenching, and Soil Mechanics


1
Excavation, Trenching, and Soil Mechanics
  • Subpart P

2
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!

3
DANGER!

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6
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.

7
  • 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.

8
1926.650Scope and Application
  • This subpart applies to all open excavations
    made in the earths surface. Excavations are
    defined to include trenches.

9
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.

10
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!
11
Mechanics of a Cave-In

12
Diagram of a cave in
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14
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.

15
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

16
SoWe 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.

17
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.

18
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)

19
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.

20
Access and Egress1926.651 (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.

21
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.

22
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.

23
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

24
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 thumbhowever they can only be penetrated by
    the thumb only with great effortType 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.

25
SOLID ROCK
  • Natural solid mineral matter that can be
    excavated with vertical sides and remain intact
    while exposed

26
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.

27
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

28
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 tonever mind

29
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

30
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

31
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.)
32
Sloping in Type A Soil
33
Sloping in Type B Soil
34
Sloping in Type C Soil
35
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.

36
Trench Boxes or Shield
37
Is this allowable?
  • YES! If the Trench boxes are designed to be
    stacked and are used in accordance with their
    tabulated data.

38
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.

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42
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.

43
Hydraulic Shoring
44
Stacked Hydraulic Shores
45
Whaler System
46
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

47
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

48
Locates
  • Red Power
  • Blue Water
  • Orange Communications
  • Yellow Gas

49
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.

50
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.
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