Firm Up Your Company?s Excavation Safety Plan

Sept. 1, 1999
How, and why, do deadly cave-ins occur during excavation? A construction safety expert reviews facts about soil mechanics and applicable OSHA standards.

by Michael W. Hayslip, Esq., P.E.

Excavations are serious hazards in the construction industry. Up to 100 of our fellow citizens will die this year in earth failures. Most of these individuals will have suffocated. Because of serious hazards that excavation presents, the Occupational Safety and Health Administration (OSHA) continues to have excavation safety as a national emphasis program.

Unsupported excavations will not remain open forever, although many of us behave as though we believe they could. When dirt is excavated, it has a natural tendency to "heal" itself.

Did you know that a single cubic yard of dirt weighs 3,000 pounds and could reach 4,000 pounds, based on varying types of dirt and moisture content? Therefore, 1 cubic yard of soil weighs more than many small pickup trucks.

This fact has some serious consequences when coupled with the fact that excavations may fail in waves. These failures are called "secondary failures" and often cause a more serious injury than the first, or primary, failure. Once a supporting mass is removed, there is little to hold up the remaining portion of soil. Imagine having a pickup truck on your legs as the secondary failure surrounds you!

Once a failure occurs, it is usually unwise to panic, jump in the excavation and attempt a rescue, only to die alongside a trapped co-worker (See rescue techniques at the end of this article).

There is no predetermined manner in which excavations fail. Excavations can fail from the bottom as in a rotational failure; in the middle, by bulging, or from a sand pocket; and at the top, through tension cracks. Dirt is not always homogeneous: Differing soil types and methods of failures may exist within the same trench.

"Rats running down the hill" is a term commonly referring to the occurrence of small rocks or clods of dirt rolling down a sloped embankment. This can be an early warning sign. When the "rats" start to run, it means the embankment is moving, but so slowly that the average worker does not notice the movement. Keep this in mind as we next discuss soil mechanics.

Soil Mechanics

The limited length of this article dictates offering only basic and somewhat oversimplified principles of soil mechanics to the reader. This section shall put forth the "why" and "how" of lateral earth failures in a question-and-answer format:

Question: Why does an excavation cave in?

Answer: An excavation caves in due to the loss of shear strength in the soil.

Question: What is shear strength?

Answer: Shear strength is the ability to resist a shearing force, which tends to slide one particle past another. This is contrasted with compression (a direct push) or tension failure (a direct pull). Soils have limited resistance to tension. In fact, the roots of plants and trees in the soil generally offer much greater resistance to tension than does the soil itself.

Question: How is shear strength developed?

Answer: There are two primary factors that develop soil shear strength. They are cohesion and friction.

Basically, cohesion is the stickiness of a soil. (For example, clay is sticky or cohesive and sand is not.). When sand seems to stick to itself, as in a sand castle, the particles actually are held together by the water between them and not by a particle-to-particle attraction. This is called "apparent" cohesion.

The friction of a soil measures how easily particles slide past each other. Soil friction is dictated by particle shape. For example, round particles slide past each other more easily than rough or angular ones.

Question: What lowers shear strength and, thus, should be controlled?

Answer: There are many factors that lower soil shear strength. This is because they affect the cohesion or friction of a soil. The two most dangerous factors reducing shear strength are water and vibration. Other negative influences exist, however, such as surcharge loads, previously disturbed conditions and freeze thaw cycles.

Overview of Federal Excavation Standards

OSHAs regulations for excavation safety in construction may be found in 29 CFR 1926 Subpart P. Individual states are permitted to pass regulations that go beyond federal mandates. Check to see whether you are in a "state-plan" state.

Before the elements of the excavation standard are highlighted, we must discuss a term of art that is cardinal to the proper administration of the regulations. It is the term "competent person."

A competent person is one who can satisfy a two-prong test: A competent person is (1) someone capable of identifying existing or predictable hazards, including unsanitary working conditions, and who also (2) has authority to take prompt corrective action to eliminate them. OSHA does not certify anyone as being competent because of the second part of the test involving the granting of authority. That must flow from an individual business entity. [See 29 CFR 1926.651(b)].

This section highlights construction excavation standards:

Underground installations must be located [29 CFR 1926.651(b)].

Ladders, ramps or stairs shall be spaced no more than 50 feet on center when excavation exceeds 4 feet in depth [29 CFR 1926.651(c)].

Safety vests are required when individuals are exposed to public vehicular traffic [29 CFR 1926.651(d)].

Employers shall prevent exposure to hazardous atmospheres [29 CFR 1926.651(g)].

Spoils shall be kept a minimum of 2 feet from the edge of the cut [29 CFR 1926.651(j)(2)].

A competent person shall conduct visual and manual tests daily (see Appendix A) before individuals may begin work in an excavation. Competent-person inspections shall be performed after a hazardous occurrence, as well. Examples of a hazardous occurrence could be rainstorms or damage to a structural member retaining the force of lateral earth pressures, such as a shore or trench box [29 CFR 1926.651(k)].

To protect people working in them, excavations of 5 feet in depth or with an indication of a potential for cave-in need to be protected [29 CFR 1926.652(a)(1)(ii)].

Support systems shall be immediately backfilled and removed from the bottom up [29 CFR 1926.652(e)(1)(v)].

The proper protective system (shoring, shielding, sloping or benching) is based on the integrity of the soil, as classified by the competent person. Through the regulations, OSHA recognizes soils types designated as A, B, C or Rock, rather than clay, silt or sand (Appendix A). Note: Trench-box manufacturers commonly use a "C60" designation in their designs. This is a subclassification of the Type C soil.

A professional engineer shall design protective systems greater than 20 feet in depth (Appendix F).

Note: This outline is not exhaustive and is intended to serve only as a brief overview. Its not to replace the need to read, understand and comply with the entire standard and those other regulations that affect this work, such as Subpart M on Fall Protection.

Rescue Techniques

This is a short list of advisory rescue techniques. Please preplan for an excavation hazard by freely distributing, then discussing, these items with co-workers. These items are not in any particular sequence:

Get exposed individuals out of the excavation.

Remember to breathe, then focus and evaluate the situation.

Account for everyone.

Appoint someone to gather and retain critical facts for rescue personnel.

Find out if the failure damaged a utility line. If so, take appropriate action.

Call 911, then send someone out to meet the rescue vehicle(s). Many developments are new, and street names may not be on maps yet.

Follow up the arrival of rescue crews by cooperatively working with these rescue professionals. It is likely they are seeing your job for the first time and may be anxious. There is a great probability that they have never experienced a real-life excavation rescue. Remember that they are there to help. Work with them.

Designate one person to communicate with the media and family members.

Span tension cracks with a plank or sheet of plywood.

Cut off all equipment to eliminate harmful vibration.

Begin or continue removing seeping ground water.

When looking into a collapsed trench, do so from the short end, rather than along the trench. Generally, the short end is more stable.

Keep from unreasonably entering a failed excavation.

Consider tossing a digging tool tied by a rope to a victim so they he may dig himself out without having another person enter the excavation.

Do not remove hand tools, personal protective equipment or other material that may locate a victim.

Do not pull a partially buried victim out by a rope or belt to avoid the static soil load and skin friction that will cut him in half or pull out limbs from the body.

Set up four controlled access zones at approximately 100-foot intervals from the edge of the site and permit access within the zones only to designated personnel:

Zone 1 (closest to the failure) rescue and first aid professionals.

Zone 2 (100 feet removed from the site) rest station for rescue workers and staging area of materials, supplies and equipment.

Zone 3 (200 feet removed from the site) immediate family members and owner representatives.

Zone 4 (300 feet from the site and beyond) media, general public and those not directly involved in the rescue. It is wise to signify these areas with caution tape and to appoint an individual to ensure unauthorized individuals do not cross into the zone.

Debrief co-workers as reasonably necessary and practical.

Mike Hayslip is the corporate safety and health director for Lithko Contracting, Inc. He has more than 15 years of hands-on construction experience from his work with the Jones Group (Heavy and Marine Division), Wilcon Corp. (design/build firm) and Baker Concrete Construction. Hayslip has served as carpenter, surveyor, draftsman, office engineer, project engineer, project manager, marketing director and now safety director. He is an attorney and registered civil engineer. Mike recently received the prestigious American Society of Safety Engineers award for Safety Professional of the Year from the construction division from Region VII, plus the All-Ohio and Kitty Hawk chapter Safety Professional of the Year awards.

Voice your opinion!

To join the conversation, and become an exclusive member of EHS Today, create an account today!