Safe Digging

Safe Digging: Sticky Tolerance Zones

Sometimes tolerance zones can be tricky. In the example below a hospital is adding a drop-off area to the front of the building (indicated by the purple box). It is located in a major intersection with many underground facilities.

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Here, the facility locators have marked the communications and electric lines. The gray area represents your work site, which you have white lined. What you notice is the communications (orange) line runs through your work site and the electric line is outside your worksite. 

Here, the facility locators have marked the communications and electric lines. The gray area represents your work site, which you have white lined. What you notice is the communications (orange) line runs through your work site and the electric line is outside your worksite. 


The entire tolerance zone for the communications line is within your work site. Remember, the tolerance zone is 24 inches on either side of the underground facility. Because the electric line is outside your work site, can you assume that beyond the communications line you can dig freely?

The entire tolerance zone for the communications line is within your work site. Remember, the tolerance zone is 24 inches on either side of the underground facility. Because the electric line is outside your work site, can you assume that beyond the communications line you can dig freely?


NO! When you figure the tolerance zone for the electric line, you see that it runs within your work site. If you dig beyond the communication's tolerance zone without first verifying where the electric line runs, you run the risk of damaging the electric line.

NO! When you figure the tolerance zone for the electric line, you see that it runs within your work site. If you dig beyond the communication's tolerance zone without first verifying where the electric line runs, you run the risk of damaging the electric line.

Safe Digging: Measuring the Tolerance Zone

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Every underground facility has a tolerance zone. The tolerance zone helps protect facilities from damage. It offers a margin of error for those times when the underground facility is not directly underneath the locate marks.

In Florida, the tolerance zone for an underground facility is the width of the line, cable or pipe plus 24 inches on either side. 

Jobsite Inspection: Checking the Marks

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The marks at your jobsite, and how you interpret them, can make or break your safety. The most important thing you can do is compare the marks to your positive response codes. There can be some obvious signs of something wrong.

Do the Marks Match What You See?

Let's say you arrive at the work site and see marks for communications, but from positive response you learned there should also be marks for electric.

Why would this happen?

  • The locator could have misunderstood the marking instructions and marked the wrong place within your work site.
  • A completely different work site was marked.
  • It could also mean that the marks were destroyed before you got to the work site.
  • Maybe kids or vandals pulled up the flags.

These are all possibilities and you shouldn't begin until you've done some checking. In the situation mentioned in the first paragraph, the best thing to do is contact the electric company.


Identifying Properly Placed Marks

When flags and paint are used together, flags are usually placed within the borders of the paint mark. If you find flags set away from the paint mark, they may have been tampered with. Once again, if something doesn't look right to you, STOP and check it out. A call to the facility owner may be necessary.

The following pictures show proper paint and flag placement.

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NEVER assume that "no marks" at the work site means "no lines."

Others have, particularly when other facility marks are visible within the work site. This incorrect interpretation can result in a damage or worse, injuries and death.

In the left photo below there are no locate marks. The excavator assumed that meant no lines were present. It was the wrong assumption to make. Make sure you check the positive response system and verify, verify, verify the lines that are present.

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Safe Digging: Depth

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Flags and painted lines on the ground tell you the horizontal direction the facility is running. They do not tell you its depth. If you are going to cross or work near an existing facility, you may want to first verify its depth by hand digging.

Yes, there are standards for installing underground facilities at a certain depth. But that depth can change over years.

In the example above, the power line was buried at a depth of three feet. Later, the area was developed and landscaped. At the berm the line is now five feet underground, but at the swale it is only one foot below the surface. Accidentally discovering this situation could literally be a heart stopping experience.

The best way to know how deep the line is buried it to expose it using the safe digging methods mentioned in another module.

Jobsite Inspection: When Marks are Missing

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It's always a good idea to take a look around your work site before digging. Make sure that what you see is reflected in the Positive Response System codes. If something doesn't look right to you, STOP and check it out. You can never be too safe.

During site inspection, you should also look for depressions in the ground that are not marked. They may be signs of an old facility. Any utility contractor can tell stories of discovering things that are not marked or don't show up on any map. Use your detective skills.

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If you're working near a gas pipeline, the following situations could indicate a gas leak. Contact the gas pipeline owner. Digging into this could create a life-threatening condition.