Excavation Safety

Purpose

The purpose of this guideline is to reduce the risk of injury or death for those individuals who enter trenches or excavations.

Scope and Applicability

This guide shall apply to all staff and faculty from the Knoxville campus of the University of Tennessee. It does not apply to contractors and their employees who are conducting work onsite. UT students and minors who are not enrolled, but engaged in University sponsored activities on campus are prohibited from entering excavations exceeding four feet in depth. This guideline applies to trenches and excavations onsite and offsite.

Abbreviations and Definitions

Abbreviations

ACGIH – American Conference of Governmental Industrial Hygienists

EHS – campus Environmental Health and Safety

PEL – Permissible Exposure Limit

TLV – Threshold Limit Value

Definitions

Aluminum hydraulic shoring: an engineered shoring system comprised of aluminum hydraulic cylinders (cross braces), used in conjunction with vertical rails (uprights) or horizontal rails (walers). Such a system is designed specifically to support the sidewalls of an excavation and prevent cave-ins.

Benching: a method of protecting employees from cave-ins by constructing the sides of an excavation to form one or a series of horizontal levels of steps, usually with vertical or near-vertical surfaces between levels.

Cave-in: the separation of a mass of soil or rock material from the side of an excavation, or the loss of soil from under a trench shield or support system, and its sudden movement into the excavating, either by falling or sliding, in sufficient quantity so that it could entrap, bury, or otherwise injure and immobilize a person.

Competent person: one who is capable of identifying existing and predictable hazards in the surroundings, or working conditions that are unsanitary, hazardous, or dangerous to employees, and who has authorization to take prompt corrective measures to eliminate them. A competent person should have and be able to demonstrate the following:

• Training, experience, and knowledge of:
  • Soil analysis.
  • Use of protective systems.
  • Requirements of 29 CFR 1926 Subpart P.
• Ability to detect:
  • Conditions that could result in cave-ins.
  • Failures in protective systems.
  • Hazardous atmospheres.
  • Other hazards including those associated with confined spaces.
• Authority to take prompt corrective measures to eliminate existing and predictable hazards and to stop work when required.

Excavation: any man-made cut, cavity, trench, or depression in an earth surface, formed by earth removal.

Registered professional engineer: a person who is registered as a professional engineer.

Shield (shield system): a structure that is able to withstand the forces imposed on it by a cave-in and thereby protect employees within the structure. Shields can be a permanent structure or can be designed to be portable and moved along as work progresses. Also known as trench box or trench shield.

Shoring (shoring system): a structure such as a metal hydraulic, mechanical or timber shoring system that supports the sides of an excavation and which is designed to prevent cave-ins.

Sloping (sloping system): a method of protecting employees from cave-ins by excavating to form sides of an excavation that are inclined away from the excavation so as to prevent cave-ins. The angle of incline varies with differences in such factors as the soil type, environmental conditions of exposure, and application of surcharge loads.

Soil Types

• Type A – Most stable: clay, silty clay, and hardpan (resists penetration). No soil is Type A if it is fissured, is subject to vibration of any type, has been previously been disturbed, or has seeping water.
• Type B – Medium stability: silt, sandy loam, medium clay and unstable dry rock; previously disturbed soils unless otherwise classified as Type C; soils that meet the requirements of Type A soil but are fissured or subject to vibration.
• Type C – Least stable: gravel, loamy sand, soft clay, submerged soil or dense, heavy unstable rock, and soil from which water is freely seeping.
• Layered geological strata (where soils are configured in layers) – The soil must be classified on the basis of the soil classification of the weakest soil layer. Each layer may be classified individually if a more stable layer lies below a less stable layer, i.e. where a Type C soil rests on the top of stable rock.

Trench (trench excavation): a narrow excavation (in relation to its length) made below the surface of the ground. In general, the depth is greater than the width, but the width of a trench is not greater than fifteen (15) feet. If forms or other structures are installed or constructed in an excavation as to reduce the dimension measured from the forms or structure to the side the excavation to one hundred fifty four (154) feet or less, the excavation is also considered to be a trench.

Roles and Responsibilities

Environmental Health and Safety shall:

• Assist supervisors with employee training upon request
• Review and revise this procedures
• Provide interpretations of this procedure where the intent is not clear
• Investigate complaints involving trenching and shoring activities under the control of UTK.
• Conduct safety assessment upon request of projects involving trenching and shoring
• Recommend controls and protective measures regarding trenches and excavations
• Investigate any accidents in trenches or excavations
• Conduct air monitoring for hazardous gases upon request in trenches and excavations

Departments who have staff, faculty or students who enter trenches or excavations shall:

• Ensure the individuals are properly trained and follow this procedure
• Provide any required safety equipment
• Assign a competent person to each excavation

Competent persons shall:

• Prohibit unauthorized individuals from entering trenches and excavations
• Evaluate soils conditions
• Conduct and document inspections of trenches and excavations
• Ensure no utilities are likely to be damaged prior to excavation

Staff or faculty who enter trenches and excavations shall:

• Prohibit students under their supervision from entering trenches that are more than four feet in depth.
• Participate in training
• Stop work in the event of imminent danger or when ordered by a competent person
• Follow the requirements established during training
• Report any problems, accidents, near misses or other hazards associated with trenching and excavation to their supervisor.

Procedures

All excavations shall be made in accordance with the rules, regulations, requirements, and guidelines set forth in 29 CFR 1926.650, 651, and .652; the Occupational Safety and Health Administration’s standard on Excavations, except where otherwise noted below.

A. Procedures

1. A competent person shall be placed in charge of all excavations and shall be onsite at all times. Note that more than one person onsite may meet the qualifications for a competent person.
2. Underground utilities must be located and marked before excavation begins.
3. Employees are not allowed in the excavation or trench while heavy equipment is digging.
4. Provisions (e.g. fence, barricade) shall be made to prevent unauthorized individuals from accessing trenches or excavations more than four feet in depth.

B. Inspections

The competent person shall conduct inspections:

1. Daily and before the start of each shift.
2. As dictated by the work being done in the trench.
3. After every rainstorm.
4. After other events that could increase hazards, such as snowstorm, windstorm, thaw, earthquake, dramatic change in weather, etc.
5. When fissures, tension cracks, sloughing, undercutting, water seepage, bulging at the bottom of the trench or excavation or other similar conditions occur.
6. When there is a change in the size, location, or placement of the spoil pile.
7. When there is any indication of change or movement in adjacent structures.

For excavations and trenches four (4) feet or greater in depth, a trench inspection form shall be filled out for each inspection.  See Appendix B (trench inspection form)

C. Testing Methods

The competent person in charge of the excavation shall be responsible for determining whether the soil is Type B or C. The competent person shall use a visual test coupled with one or more manual tests.

D. Visual test

1. In addition to checking the items on the trench inspection form, the competent person should perform a visual test to evaluate the conditions around the site. In a visual test, the entire excavation site is being observed, including the soil adjacent to the site and the soil being excavated. The competent person also checks for any signs of vibration.
2. During the visual test, the competent person should check for crack-line openings along the failure zone that would indicate tension cracks, look for existing utilities that indicate that the soil has been previously disturbed, and observe the open side of the excavation for indications of layered geologic structuring.
3. This person should also look for signs of bulging, boiling, or sloughing, as well as for signs of surface water seeping from the sides of the excavation or from the water table.
4. In addition, the area adjacent to the excavation should be checked for signs of foundations or other intrusions into the failure zone, and the evaluator should check for surcharging and the spoil distance from the edge of the excavation.

E. Manual tests

1. Thumb penetration test. Attempt to press the thumb firmly into the soil in question. If the thumb penetrates no further than the length of the nail, it is probably type B soil. If the thumb penetrates the full length of the thumb, it is Type C. It should be noted that the thumb penetration test is the least accurate testing method.
2. Dry strength test. Take a sample of dry soil. If it crumbles freely or with moderate pressure into individual grains, it is considered granular (Type C). Dry soil that falls into clumps that subsequently break into smaller clumps) and the smaller clumps can only be broke with difficulty) is probably clay in combination with gravel, sand, or silt (Type B).
3. Plasticity or Wet Thread Test. Take a moist sample of soil. Mold it into a ball and then attempt to roll it into a thin thread approximately 1/8 inch in diameter by two (2) inches in length. If the soil sample does not break when held by one end, it might be considered Type B.

A pocket penetrometer, shearvane, or torvane may also be used to determine the unconfined compression strength of soils.

F. Imminent Danger

Work in a trench with vertical walls and a depth exceeding seven feet is considered imminent danger. No employee shall enter a trench with vertical walls unless the trench is in solid rock or unless protection is provided.

G. Additional Program Requirements

The following subjects relative to trenching and excavations are covered in Appendix A

1. Spoil Piles
2. Crossing Trenches
3. Ingress and Egress
4. Exposure to Vehicles
5. Exposure to Falling Loads
6. Warning Systems for Mobile Equipment
7. Hazardous Atmospheres and Confined Spaces
8. Standing Water and Water Accumulation
9. Benching, Shoring, Sloping and Shielding Requirements

Recordkeeping

The following records shall be maintained in accordance with Records Retention for Safety, Health and Environmental Compliance (Procedure GS 43) in the Safety Manual.

• Employee training
• Trench Inspection Form (Appendix B)

Training and Information

Personnel who perform work in excavations shall comply with the requirements of these guidelines and shall receive appropriate training that shall include at a minimum:

• Safe work practices that must be followed during work in excavations;
• The use of personal protective equipment (PPE) that will typically be required during work in excavations, including but not limited to safety shoes, hardhats, and fall protection devices;
• Procedures to be followed if a hazardous atmosphere exists or could reasonably be expected to develop during work in an excavation; and
• Emergency and non-entry rescue methods, and procedures for calling rescue services.

Training shall be provided for individuals who are removing soil, enter the trench or excavation and the competent person.  The training shall be documented and a record maintained.

Refresher training shall be conducted every three years or when any one of the following occurs:

• The employee is not following correct excavation procedures
• Equipment or processes change
• Regulations changes

References

29 CFR 1926.650, 651, and .652

 

 

Disclaimer

The information provided in these guidelines is designed for educational use only and is not a substitute for specific training or experience.

The University of Tennessee Knoxville and the authors of these guidelines assume no liability for any individual’s use of or reliance upon any material contained or referenced herein. The material contained in these guidelines may not be the most current.

This material may be freely distributed for nonprofit educational use. However, if included in publications, written or electronic, attributions must be made to the author. Commercial use of this material is prohibited without express written permission from the author.