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Excavations
Introduction
"Construction contractors must dig trenches not graves". The Occupational Safety and Health Administration (OSHA) has used this comment many times in press releases involving citations under a national emphasis program that focuses on excavations. Too often workers have entered trenches only to lose their lives in a cave in. In fact, over 200 workers are killed annually in trench cave in accidents.


OSHA Activity
The following are some examples of OSHA inspections and citations for Excavation inspections.

  • OSHA cited a PA contractor with proposed penalties of $47,000 after a fatal cave-in. The contractor was hit with one willful penalty of $42,000 for failure to provide adequate protection. The company was also cited for failure to have a competent person in charge to classify the soil and select protection measures and failure to train employees to recognize cave-in hazards.

  • Two contractors were penalized with over $218,000 in proposed penalties when a second inspection revealed that workers were working in unprotected nine feet deep trenches. Two employees were buried up to their waists at an excavation just four days before the second inspection.

  • Two contractors were cited by OSHA for exposing employees to the hazards of an unprotected trench that was 134 feet in length and over six feet deep. There were no ladders in the trench to provide safe egress for the workers.

These examples are but a few of the inspections that OSHA has conducted to ensure that excavations are safe for workers. An OSHA inspector can perform an excavation inspection anytime. Contractors must comply with the rules to protect employees and to prevent OSHA citations.


Hazard Control
Hazard control for excavations involves pre-planning, selection of proper control measures and daily inspections to identify any changes.

Pre-Planning
The OSHA Standard for Excavations Subpart P (1926.650) provides detailed information to plan an excavation. A matrix that is found in Appendix F of the standard summarizes the requirements for excavations twenty feet or less in depth.

Initially, it must be determined whether or not the excavation will be greater than five feet in depth. If it will be five feet or less, excavation may be made with vertical sides, unless there is a potential for cave in. If the excavation is greater than five feet it may be made with vertical sides only if the excavation is entirely in stable rock. Stable rock is defined as natural solid mineral matter. If the excavation will not be in stable rock, the excavation must be slopped, shored or shielded.


Proper Control Measures

Sloping
Sloping can be used without soil classification only if a slope of 1 ½ horizontal to 1 vertical can be maintained. Oftentimes this slope is not feasible so soil classification must be done.

Soil types are classified as follows:

Soil Type A – Most Stable: clay, silty clay, and hardpan (resists penetration)

Soil Type B – Medium stability: silt, sandy loam, medium clay and unstable dry rock.

Soil Type C – Least stable: gravel, loamy sand, soft clay, submerged soil or dense, heavy unstable rock

A competent person must determine soil classification using a visual method and one mechanical method.


Visual Testing

  • Soil particle size and type.

  • Does the soil clump when dug? It could be clay or silt.

  • Cracks in walls and spalling can mean soil types B or C.

  • Layered systems with adjacent hazardous areas – buildings, roads and vibrating machinery may require a professional engineer for classification.

  • Standing water or seeping water through trench walls automatically means type C.


Manual Testing

  • Sedimentation Test – Determines the amount of silt and clay that are in sandy soil.

  • Wet Shaking Test – another way to determine the amount of sand versus clay and silt in a soil sample.

  • Thread Test – determines cohesion.

  • Ribbon Test – determines cohesion and is used as a backup for the Thread Test.

These tests are detailed in Appendix A of the Excavation Standards.

The maximum allowable slopes for each soil class when the trench is less than 20 feet is as follows:

Soil Type Height/Depth Ratio

- Stable Rock Vertical

- Type A ¾:1

- Type B 1:1

- Type C 1 and1/2:1

For example a 10 foot-deep trench in Type B soil would have to be sloped to a 45-degree angle, or sloped back 10 feet in both directions. In Type C soil, the trench would be sloped at a 34-degree angle, or 15 feet back in both directions.

Excavations or trenches 20 feet deep or greater must have a protective system designed by a registered professional engineer.


Benching
There are two types of benching, single and multiple, which can be used in conjunction with sloping. In type B soil the vertical height of the benches must not exceed 4 feet. Benches must be below the maximum allowable slope for that soil type. A 10-foot deep trench in Type B soil must be benched back 10 feet in each direction with a maximum of a 45-degree angle. Benching is not allowed in Type C soil.


Shoring/Shielding
Shoring or shielding is used when the location and depth of the cut makes sloping impractical. Shoring involves setting up a structure that supports the sides of excavations to prevent cave ins. There are two types of shoring: aluminum hydraulic and timber. Shielding involves the use of a structure that is able to withstand the forces imposed on it by a cave-in and thereby protect employees. This equipment ics commonly referred to as a trench box. Soil classification is required when shoring or shielding is used. OSHA allows shoring or shielding using one of four options:

Option 1: Appendices A and C of OSHA Standard 1926.650 outlines the procedures and specifications for Timber Shoring. Appendix A outlines the requirements for soil classification. Appendix C provides Tables that provide specifications for the cross braces, wales and uprights based on the soil type and trench depth.

Option 2: Manufacturer’s data can be used to when using hydraulic shoring, trench jacks, air shores and shields. This information must be kept at the job site while the protective system is being constructed. After that time, the information can be kept off site, but must be made available upon request by an OSHA inspector. Any deviations from the manufacturer data can only be made with the written permission of the manufacturer.

Option 3: The use of other tabulated data from tables prepared by a professional engineer. The tables and other data must be kept at the job site while the protective system is being constructed. After that time, the information can be kept off site, but must be made available upon request by an OSHA inspector.

Option 4: Design of the excavation by a professional engineer. The design must be in written form and shall include a plan describing the materials and equipment to be used and the identity of the professional engineer who designed the plan.


Other Hazards and Controls
While cave in is the hazard most often associated with excavations, there are other hazards that must be considered.

Hazardous Atmospheres
If there is any possibility that the trench or excavation could contain a hazardous atmosphere, air testing must be done prior to entry. Conditions that might warrant testing include if the excavation takes place in a landfill area or if it is in the vicinity of pipelines containing a hazardous chemical. Testing should be done periodically to ensure that conditions have not changed, especially when equipment is operated in the trench or when welding, brazing or cutting is done.


Standing Water and Water Accumulation
Some control measures include:

  • Use of special support or shield systems designed by a registered professional engineer.

  • Water removal equipment, such as well pointing, used and monitored by a competent person.

  • Safety harnesses and lifelines used.

  • Employees removed from the trench during rainstorms

  • Trenches carefully inspected by a competent person after each rain and before employees are permitted to re-enter the trench.


Spoil
Temporary spoil shall be placed no closer than 2 feet from the surface edge of the excavation, measured from the nearest base of the spoil to the cut. Place spoil so that it will channel rainwater and other run-off water away from the excavation. Remove permanent spoil further away from the excavation


Surface Crossing of Trenches
Surface crossings should not be made unless absolutely essential. Vehicle crossings must be designed by and installed by a professional engineer. Walkways or bridges must:

  • Have a minimum clear width of 20 inches,

  • Be fitted with standard rails, and

  • Extend a minimum of 24 inches past the surface edge of the trench


Entry and Exit
Trenches 4 feet or more in depth must have a fixed means of egress. A worker must be able to access a ladder from a distance of 25 feet at most from any part of the excavation. Ladders must be secured and extend a minimum of 36 inches above the landing.


Exposure to Vehicles
Employees exposed the vehicle traffic must wear reflective vests or other garments. Trained flag persons, signs, signals and barricades must be used when needed.


Exposure to Falling Loads
Workers in trenches should wear hardhats. Employees must not be permitted to work under raised loads or other loads being lifted or moved by heavy equipment.


Mobile Equipment
Warning systems, hand or mechanical signals or stop logs must be used as necessary to prevent vehicles from falling into the trench.


Competent Person and Inspections
A competent person for the purposes of excavations is one who is capable of identifying existing and predictable hazards in the surroundings or working conditions and who has the authorization to eliminate them. The competent person must have specific training in soil analysis, the use of protective systems and the OSHA Standards.

Inspections are required:

  • Daily and before the start of each shift.

  • As dictated by the work being done in the trench

  • After every rainstorm

  • After other weather events that could increase hazards

  • When fissures, tension cracks, sloughing, undercutting, water seepage, bulging at the bottom or other similar conditions occur

  • When there is a change in the size, location or placement of the spoil pile

  • When there is any indication of a change or movement in adjacent structures.


Conclusion
Excavations pose significant safety challenges. Pre-planning is a vital part of the OSHA requirements. Soil analysis followed by the selection of the most effective cave-in control measure will ensure that workers are properly protected. The competent person oversees the day to day hazard control program. Constant attention to changing conditions and the status of hazard control measures will ensure the ongoing safety of workers.

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