What is utility daylighting?
Utility daylighting means digging down to a buried line until you can see it, using hand tools or a hydrovac instead of a machine bucket. It is done because locate marks are approximate: under Missouri law the line can sit up to two feet to either side of the mark.
The crew confirms the line's position, depth and condition, records it, and then the main dig goes ahead. In the field, daylighting and potholing mean the same thing.
How many holes, and how long?
| Scope | Potholes | Typical time |
|---|---|---|
| One utility crossing | One | 1 to 2 hours, setup to cleanup |
| Longer trench run | One per crossing, plus one every 50 to 100 feet | Set by footage and crossings |
| 200-foot trench, four crossings | Six to ten | Half a day to a full day |
Jefferson County clay digs slower than sandy or loamy soil.
Why aren't locate marks enough?
Calling 811 is required by Missouri law before any digging, and a crew marks the lines with flags and paint. Those marks show a band, not a point.
The band: Missouri defines a marked line's approximate location as its width plus two feet on either side. For a single line, that is a strip more than four feet wide. Locators also estimate depth rather than measure it.
For what 811, a private locator and potholing each tell you, read utility locating versus potholing.
Can daylighting find missed lines?
Yes. Non-conductive lines without tracer wire, lines abandoned without documentation, and services rerouted without updated drawings can all go undetected by a standard locate. Daylighting finds them before the main excavation does.
Locators trace metal lines by signal, so plastic water lines and conduit only show up if their tracer wire is there and intact. J1S finds misplaced utilities during daylighting on a regular basis, such as a gas line 18 inches off its mark, or a water main at 4 feet where records said 2.5 feet.
Is daylighting required in Missouri?
Missouri law defines a marked utility's approximate location as the width of the line plus two feet on either side, and Missouri 811 treats that strip as a hand-dig zone. Daylighting meets that standard by using a hydrovac to expose the utility without striking it. Many contractors go further and daylight any crossing within 5 feet of a marked line.
Hand-dig zone means expose the line with hand tools or vacuum excavation before a machine works there. OSHA also requires that the exact location of a buried utility be found by safe means as a dig approaches it. Experienced crews also daylight:
- Before directional boring, to confirm the bore path is clear
- Before deep trenching
- On sites with old lines or incomplete records
- Before utility trenching in a crowded utility corridor
How does the process work?
- Call 811 and wait for the marks. The law requires notice at least two working days before digging.
- Review the marks and find where lines cross the planned trench or dig.
- Set up the hydrovac. Its boom reaches about 15 to 20 feet, so the truck needs a spot that close.
- Dig beside the mark. The operator works in from the side with water and vacuum, with pressure set for the soil.
- Expose the line until it is plainly visible.
- Measure and record depth from grade, offset from the mark, type and condition.
- Start the main dig with those facts in hand.
On bigger jobs, daylighting runs as its own visit ahead of the trench crew, so the crew arrives knowing every depth and offset.
Why use a hydrovac?
A shovel meets the rule but is slow in Missouri clay: one 5-foot pothole by hand is a half-day job for two people. A hydrovac does the same hole in 1 to 2 hours, including setup. Water loosens the soil and the vacuum removes it, so nothing hard strikes the line. The wand can stop within an inch of a gas line without risk of striking it. More in how hydroexcavation works.
What does a gas line strike cost?
A gas strike clears the area, typically a 300-foot radius, and can cut service to dozens of homes on that main. The job shuts down, OSHA can investigate, and the contractor faces possible citations, fines and liability for damages. The delay alone easily reaches five figures on a typical job. Against 1 to 2 hours of hydrovac time per crossing, it is not a close call.
Sources
- Missouri Revised Statutes, Section 319.015: the approximate location of a marked line is defined as the width of the facility plus two feet on either side.
- Missouri Revised Statutes, Section 319.026: an excavator must notify the notification center at least two working days before starting an excavation.
- OSHA, 29 CFR 1926.651, Specific Excavation Requirements: when an excavation approaches the estimated location of an underground utility, its exact location must be determined by safe and acceptable means.