On June 14, 2002, while working on a sewer pipe in a trench 10 feet deep, a worker was buried alive under a rush of collapsing muck and mud. A husky plumber's apprentice, barely 22 years old, he clawed for the surface as thousands of pounds of dirt pressed on his chest.
Until that very moment, it had been a just a normal day in the trenches...another day at work.
If you earn your paycheck in the water or wastewater business, and it's a pretty sure bet that you do if you're reading this article...then it is important that you understand the dangers that come with working in and around an excavation site and the safety precautions you are required to take.
According to the Occupational Safety and Health Administration (OSHA), between the years 1992 and 2001, 542 workers lost their lives in trenching and excavation related accidents making it one of the most hazardous of all construction operations.
Before you, or someone working for you, turns that first shovel-full of dirt, take the time to make absolutely certain that everyone involved in the process, from the "competent person" all the way down to the guy behind the shovel has been properly trained. This relatively simple act could keep your employees from accidentally digging their own graves.
A cave-in can trap you within seconds and kill you with in a matter or minutes. Remember, a mere two cubic yards of soil weigh about 6,000 pounds—roughly the equivalent of a full-size SUV! In addition to the
dangers posed by cave-ins, lack of oxygen, toxic fumes, explosive gasses, and buried power lines may also create potential dangers.
In this issue of the Safe Drinking Water Trust eBulletin we will take a look at how to safely get down to where much of your system's infrastructure is located and ways to make certain that your pipes and lines are all that end up getting buried.
Additional Resources
Occupational Safety and Health Administration Excavation Guide
www.osha.gov/Publications/osha2226.pdf [1]
Trench Vs. Excavation: Is There a Hole Lot of Difference?
Dig a hole in the ground and you've made an excavation. It's just that simple. Whether that hole is wide, narrow, deep or shallow, a hole by any other name is still an excavation.
Tip: Be sure to check out the Trenching and Excavation Glossary at the end of this article if you run across a term you don't know.
Now, if that hole just happens to be of the narrow variety, but not more than 15 feet wide at the bottom and is deeper than it is wide, then you my friend have yourself a trench.
If you or your employees will be working in an excavation that is more than five feet deep, then according to OSHA standards, you must take steps to protect the workers from a cave-in.
The best way to do this is by planning ahead, educating your staff and yourself, and insisting that all state and federal guidelines are followed. Before the dirt starts flying you should always take into consideration:
• Surface debris near the excavation site that could create a hazard.
• Weather condition
• Stability of the soil at the excavation site
• Location of underground utility lines
• Overhead power lines
• Vehicle traffic near the excavation site
• Stability of structures adjacent to the excavation site
• How employees will get in and out of the excavation
• Vehicles and other mobile equipment that will operate near the excavation
• Possibility of water accumulation in the excavation
• Possibility of atmospheric accumulation in the excavation (gasses, fumes, etc.)
• How to respond in an emergency
• How to protect people from falling into the excavation
By taking these few simple points into consideration, you will greatly reduce the chance that something might go wrong.
Additional Resources
National Institute for Occupational Safety and Health (NIOSH) Trench Safety Online Training
www.cdc.gov/niosh/docs/2006-133D/flash/index.html [2]
How Competent is Your Person?
When it comes to digging holes, especially deep ones...a little competence is a great thing to have on your side. However, when it comes to trenching and excavating, it is a requirement.
A designated "Competent Person" is one who has training in soil analysis, protective systems, and excavation requirements and according to OSHA, must be on site to complete these tasks and inspect the protective system once it has been installed.
The term "Competent Person" appears in many OSHA standards and documents. As a general rule, the term is not specifically defined. In a broad sense, an OSHA competent person "is an individual who, by way of training and/or experience, is knowledgeable of applicable standards, is capable of identifying workplace hazards relating to the specific operation, is designated by the employer, and has authority to take appropriate actions."
It is very important that the excavation's competent person has the authority to immediately correct and identified hazards and order workers to leave the excavation until those hazards have been corrected. Remember, an employee who is trained and can identify excavation hazards but doesn't have the authority to correct them is not a competent person.
Additional Resources
OSHA Technical Manual: Section V: Chapter 2: Excavations: Hazard Recognition in Trenching and Shoring
www.osha.gov/dts/osta/otm/otm_v/otm_v_2.html [3]
Testing…1…2…Dirt?
No matter how many trenching, shoring, and backfilling jobs you may have done in the past, it is important to approach each new job with the utmost care and preparation.
Cave-ins are one of the biggest concerns whenever you are working in or around an excavation. A cave-in is the movement of soil or rock into the excavation, or the loss of soil from under a trench shield or support system, in amounts large enough to trap, bury, or injure and immobilize a person.
Undisturbed soil stays in place because of opposing horizontal and vertical forces. When you create an excavation, you are removing the very soil that provides that horizontal support...Soil will eventually move downward into an excavation. Therefore, the longer the face (a side of the excavation) remains unsupported, the more likely it is to cave in.
One of the key factors in predicting and preventing situations where a cave-in may occur is the type of soil that you are digging into. There are generally three soil types that you will encounter:
Type A soil is generally very stable. Clay is common example.
Type B soil is less stable than type A soil. Crushed rock, silt, and soils that contain an equal mixture of sand and silt are examples.
Type C soil is less stable than type B soil. Gravel and sand are prime examples.
There are other qualities of soil that can affect its stability. These include granularity, saturation, cohesiveness, and unconfined compressive strength. Granularity refers to the size of the soil grains; the larger the grains, the less stable the soil. Saturation refers to how much water soil will absorb. Cohesiveness means how well soil holds together (clay is a cohesive soil). Unconfined compressive strength is determined by a simple test that shows how much pressure it takes to collapse a soil sample
A job sites' competent person must conduct visual and manual soil tests before anyone enters an excavation. These tests are a critical part of determining the type of protective system that will be used.
Visual tests may include the following:
•Observing the soil as it is excavated. Soil that remains in large clumps when excavated may be cohesive. Soil that breaks up easily is generally granular.
•Examine the particle sizes of excavated soil to determine how they hold together.
•Look for cracks or fissures in the faces of the excavation.
•Look for layers of different soil types and the angle of the layers in the face of the excavation as they may indicate instability.
•Look for water seeping from the sides of the excavation.
•Look for signs of previously disturbed soil from other construction or excavation work.
•Consider vibrations from construction activities or highway traffic that may affect the stability of the excavation.
Manual testing involves evaluating a sample of soil from an excavation to determine qualities such as cohesiveness, granularity, and the unconfined compressive strength. Soil can be tested either on or off site, but should be tested as soon as possible to preserve its natural moisture.
Manual tests can include:
Plasticity Test: Shape a sample of moist soil into a ball and try to roll it into threads about one-eighth of an inch in diameter. Cohesive soil will roll into one-eighth of an inch threads without crumbling.
Dry Strength Test: Hold a dry soil sample in your hand. If the soil is dry and crumbles on its own or with moderate pressure into individual grains of fine powder, it’s granular. If the soil breaks into clumps that are hard to break into smaller clumps, it may be clay combined with gravel, sand, or silt.
Thumb Penetration Test: This test roughly estimates the unconfined compressive strength of a sample. Press your thumb into the soil sample. If the sample resists hard pressure it may be Type A soil. If it’s easy to penetrate, the sample may be type C.
Pocket Penetrometers offer a more accurate estimate of unconfined compressive strength. When pushed into the sample, an indicator sleeve displays an estimate in tons per square foot or kilograms per square centimeter.
Additional Resources
California Department of Transportation’s Excavation Soil Classification Guide
www.dot.ca.gov/hq/esc/construction/Manuals/TrenchingandShoring/ch3_soil.pdf#search=%22excavation%20soil%20testing%20guide%22 [4]
Oklahoma State University Trenching Safety Manual
www.pp.okstate.edu/ehs/manuals/Trench.htm [5]
An Ounce of Prevention
Once the soil type and a variety of other soil qualities have been inspected it is time for the competent person to determine the best way to protect the workers that will be working in and around the excavation. The basic systems used for protecting folks working in excavations from cave-ins are sloping, benching, shoring, and shielding.
The best system for a particular situation depends on factors such as soil type, water content, excavation depth and width, the nature of the work, and nearby activities. The competent person has the responsibility for considering these factors and for selecting the appropriate protective system. It is no small responsibility!
For excavations deeper than 20 feet, a registered professional engineer is generally required to design the appropriate protective systems.
Sloping and benching provide protection by removing material from the faces of an excavation at an angle to its floor. The flatter the angle, the greater the protection. A slope not steeper than 1 ½ feet back for every foot of depth is safe for nearly any soil type.
Benches are cuts in the slope that give it a stair-step appearance. There are two types of benches: simple and multiple. Benched slopes are permitted only in type A and B soils.
Shoring and shielding prevent cave-ins in excavations with or without sloped or benched faces. Shores are vertical or horizontal supports that prevent the faces of an excavation from collapsing. Vertical shores are called uprights and are often used in stable soil or in shallow excavations that have parallel faces. Horizontal shores are called walers and are often used when unstable soil makes sloping or benching impractical and when sheeting is necessary to prevent soil from sliding into the excavation. Shields provide those working in an excavation a safe work area by protecting them from collapsing soil.
Additional Resources
Center for Disease Control's Electronic Library of Construction Occupational Safety and Health’s Excavation Safety Guide
www.cdc.gov/eLCOSH/docs/d0300/d000381/d000381.html [6]
OSHA's Sloping and Benching Standards
www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&p_id=10932 [7]
A Pound of Cure
When you work for a water or wastewater system you are responsible for more than just protecting your customer's safety, you must also consider yourself and your employees safety. That is why it is extremely important to follow the rules and not take any chances when it comes to excavations and trenches.
This is a big responsibility that encompasses a wide range of activities from testing the soil to making sure that there is a safe means for entering and exiting an excavation within 25 feet of employees working in an excavation.
While simply reading this fascinating article doesn't mean that you are now ready to step forward and serve as your system's Competent Person the next time a trench is necessary, we hope that it will help remind you of the importance of safety and preparation when it comes to excavations.
Trenching and Excavation Glossary
Accepted Engineering Practices means the standards of practice required by a registered professional engineer.
Adjacent Structure Stability refers to the stability of the foundation of adjacent structures whose location may create surcharges, changes in soil conditions, or other disruptions that have the potential to extend into the failure zone of the excavation.
Aluminum Hydraulic Shoring means a manufactured shoring system consisting of aluminum hydraulic cylinders (cross braces) used with vertical rails (uprights) or horizontal rails (wales). Such system is designed to support the sidewalls of an excavation and prevent cave-ins.
Bell-bottom Pier Hole means a type of shaft or footing excavation, the bottom of which is made larger than the cross section above to form a belled shape.
Benching or Benching System is a method of protecting employees from cave-ins by excavating the sides of an excavation to form one or more horizontal steps, usually with vertical or near-vertical surfaces between levels.
Cave-in means the movement of soil or rock into an excavation, or the loss of soil from under a trench shield or support system, in amounts large enough to trap, bury, or injure and immobilize a person.
Competent Person means one who has been trained to identify hazards in the workplace, or working conditions that are unsafe for employees, and who has the authority to have these hazards eliminated or controlled.
Cross Braces mean the horizontal members of a shoring system installed from side to side of the excavation. The cross braces bear against either uprights or wales.
Excavation means any man-made cut, cavity, trench, or depression in an earth surface formed by earth removal.
Faces or Sides mean the vertical or inclined earth surfaces formed as a result of excavation work.
Failure means the movement or damage of a structural member or connection that makes it unable to support loads.
Hazardous Atmosphere means an atmosphere that is explosive, flammable, poisonous, corrosive, oxidizing, irritating, oxygen deficient, toxic, or otherwise harmful, that may cause death, illness, or injury.
Ingress and Egress mean "entry" and "exit" respectively, and refer to the safe means for employees to enter or exit.
Kickout means the accidental movement or failure of a cross brace.
Protective System means a method of protecting employees from cave-ins, from material that could fall or roll from an excavation face into an 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.
Ramp means an inclined walking or working surface that is used to gain access to one point from another. A ramp may be constructed from earth or from structural materials such as steel or wood.
Registered Professional Engineer means a person who is registered as a professional engineer in the state where they are working.
Sheeting means the members of a shoring system that retain the earth in position and in turn are supported by other members of the shoring system.
Shield or Shield System means a structure used in an excavation to withstand cave-ins and which will protect employees working within the shield system. Shields can be permanent structures or portable units moved along as work progresses.
Shoring or Shoring System means a structure that is built or put in place to support the sides of an excavation to prevent cave-ins.
Sides See "Faces."
Sloping or Sloping System means sloping the sides of the excavation away from the excavation to protect employees from cave-ins. The required slope will vary with soil type, weather, and surface or near surface loads that may affect the soil in the area of the trench (such as adjacent buildings, vehicles near the edge of the trench and so forth).
Stable Rock means natural solid mineral material that can be excavated with vertical sides that will remain intact while exposed.
Structural Ramp means a ramp built of steel or wood, usually used for vehicle access.
Ramps made of soil or rock are not considered structural ramps.
Support System means a structure such as underpinning, bracing, or shoring, which provides support to an adjacent structure, underground installation, or the sides of an excavation.
Surface Encumbrances include underground utilities, foundations, streams, water tables, transformer vaults, and geologic anomalies.
Surcharge means an excessive vertical load or weight caused by spoil, overburden, vehicles, equipment, or activities that may affect stability.
Tabulated Data means tables and charts approved by a registered professional engineer and used to design and construct a protective system.
Trench means a narrow excavation (in relation to its length) made below the surface of the ground.
Trench Box or Shield See "Shield".
Unconfined Compressive Strength is the load per unit area at which soil will fail in compression.
Underground Installations include, but are not limited to, utilities, tunnels, shafts, vaults, foundations, and other underground fixtures or equipment that may be encountered during excavation work.
Uprights mean the vertical members of a trench shoring system placed in contact with the earth and usually positioned so that individual members do not contact each other. Uprights placed so that individual members are closely spaced, in contact with or interconnected to each other, are often called "sheeting."
Wales are horizontal members of a shoring system placed in the direction of the excavation face whose sides bear against the vertical members of the shoring system or earth (the uprights or sheeting).