Excavation, Trenching, and Soil Mechanics

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Excavation, Trenching, and Soil MechanicsSubpart 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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1926.650 Scope and Application “This subpart applies to all open excavations made in the earth’s 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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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…

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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 Excavations1926.652 (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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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.

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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) InspectionsDaily 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 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.”

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Soil Tests1926 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 Test1926 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.

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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 SoilCohesive 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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Last Updated: 8th March 2018

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