Excavation, Trenching, and Soil Mechanics

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Excavation, Trenching, and Soil Mechanics

• Subpart P

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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!

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DANGER!

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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.

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• 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.

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1926.650Scope and Application

• This subpart applies to all open excavations
  made in the earths surface. Excavations are
  defined to include trenches.

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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.

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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!
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Mechanics of a Cave-In

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Diagram of a cave in
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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.

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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

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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.

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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.

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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)

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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.

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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.

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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.

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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.

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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

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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.

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SOLID ROCK

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

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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.

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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

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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

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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

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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

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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.)
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Sloping in Type A Soil
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Sloping in Type B Soil
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Sloping in Type C Soil
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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.

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Trench Boxes or Shield
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Is this allowable?

• YES! If the Trench boxes are designed to be
  stacked and are used in accordance with their
  tabulated data.

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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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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.

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Hydraulic Shoring
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Stacked Hydraulic Shores
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Whaler System
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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

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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

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Locates

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

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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.

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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.