Damage prevention
Potholing and vacuum excavation
A pothole is the one step in a bore plan where a utility’s position is measured rather than inferred. When the standards require one, the pressure and standoff limits that stop the exposure doing its own damage, how much clearance survives reaming, and what has to be recorded for the hole to be worth anything in the profile.
The terms
Potholing — also daylighting, or a test hole — is digging a small vertical hole from the surface to a buried utility to see it. The Gas Technology Institute defines it as “exposure of a facility by safe excavation practices used to visually ascertain the precise horizontal and vertical position of underground lines or facilities”.
Vacuum excavation is the usual way to do it: in GTI’s words, “a means of soil extraction through vacuum when using pressurized water or air for breaking ground”. A wand cuts or fluidises the soil, a blower lifts the spoil up a tube into a debris tank, and no blade or tooth ever reaches the pipe. GTI notes it is “commonly accepted as being equivalent or safer than hand digging within the ‘tolerance zone’” — the tolerance zone being the strip of ground around a marked utility where digging is limited to careful methods such as hand digging and vacuum excavation.
When exposure is required
Three separate things can require it, and they stack.
Statute. California Government Code §4216.4(a)(1) is a strict example: inside the tolerance zone the excavator must determine the exact location of the conflicting installation with hand tools before using any power-driven excavation or boring equipment. Vacuum excavation may be substituted only where that intent was declared when the ticket was obtained and the facility operator agreed; the statute’s other exceptions cover breaking out pavement with no installation in it, and equipment the state’s Dig Safe Board approves by regulation. CGA Best Practice 5-20 (Best Practices Guide, Version 22.0) is the consensus version, listing pot holing, hand digging, soft digging, vacuum excavation and pneumatic hand tools among the methods to consider inside the zone.
Geometry of the crossing. GTI’s Trenchless Best Practices for Damage Prevention (Version 17, 3 May 2016) sets an explicit rule. Positively identify by potholing all crossed utilities expected to be above and within 5 ft of the proposed vertical alignment, below and within 3 ft of it, and additionally as requested by the owner of the right of way or the owner or operator of the utilities being crossed. GTI does not say why the limit is larger above the bore than below it.
Running parallel. The same document sets a separate schedule for drilling alongside a line rather than across it:
| Parallel within 3 ft | Utility exposed at least every 25 ft; drill head tracked at least every 5 ft |
|---|---|
| Parallel within 5 ft | Utility exposed at least every 100 ft; drill head tracked at least every 10 ft |
Alongside those, GTI requires that trenchless tooling “must not be operated within the tolerance zone of other unexposed underground utility infrastructure”, that backreamer size be considered when choosing where the pilot bore runs, that the drill head be tracked every half to full drill rod, and that drilling stop any time the ability to track is lost or hampered. Every crossing is visually observed during both pilot bore and pullback, even under pavement, where coring and potholing may be needed, and “if visual observation is not possible, another path should be taken”.
CGA Best Practice 5-29 adds the design side: the trenchless excavation operator “confirms and maintains the path and minimum clearances established by the project owner and design engineer by tracking and recording the path of the trenchless excavation until complete”. The clearance number is the engineer’s, not a rule of thumb, and the field job is to prove it was held.
Exposing without causing the damage
A pressurised water jet can cut into a pipe’s coating as well as the soil around it, and a damaged coating is damage under GTI’s definition — “any impact or contact with an underground facility, its appurtenances or its protective coating, or any weakening of the support for the facility or protective housing which requires repair”. GTI’s Vacuum Excavation Best Practice & Guideline (January 2012) puts numbers on the limits:
| Straight tip nozzle | Maximum 2,500 psi during excavation in public roads or easements; below a depth of 18 in, reduce to a maximum of 1,500 psi |
|---|---|
| Spinning tip nozzle | Maximum 3,000 psi. The rotor disperses the jet into a cone of not less than 20°, so the same flow arrives spread over area rather than concentrated on a point |
| Heated water | Temperature and pressure never to exceed 115 °F and 2,500 psi respectively |
| Standoff | 8 in between the end of the wand nozzle and the facility or the subsoil; the nozzle is never inserted into the subsoil inside the tolerance zone |
| Wand handling | Never motionless during excavation, and never aimed directly at the facility |
Pressure is read off a gauge mounted on the machine, not assumed, and a device capable of stopping excavation on demand stays at the point of excavation. If a facility or its coating is damaged anyway, the excavator contacts the facility owner — a reporting obligation, not a judgement call.
Air versus water is not cosmetic. Water digs faster in most soils and copes better with frozen or hard ground; air can leave the spoil dry enough to go back in as backfill, where the owner allows it, and air is not a conductor, which is why trade guidance recommends it for digging close to electric lines. GTI refers hydrovac work near electric distribution plant to a separate guide, the E&USA Excavating with Hydrovacs in the Vicinity of Underground Electrical Plant. Facility owners set their own rules: GTI is explicit that “some underground facility owners/operators accept vacuum excavation as equivalent to hand excavation for exposing their facilities and others have restrictions on its use”. Ask before the crew mobilises.
How much clearance is actually left
The easy mistake is measuring clearance to the pilot rod. The rod is the smallest thing that will ever occupy that path.
The Plastics Pipe Institute’s handbook chapter on directional drilling gives the reaming ratio: “Typical over-sizing may be from 1.2 to 1.5 times the diameter of the product pipe.” Work it through. A 12 in product pipe reamed at 1.5× leaves an 18 in hole, whose wall sits 9 in from the pilot rod’s centreline. Plan the rod 12 in clear of a gas service and the finished bore passes 3 in from it — before any tracking error, before any locate error. At the lower 1.2× ratio the hole is 14.4 in, the wall is 7.2 in out, and 4.8 in is left.
So the clearance a plan actually delivers at a crossing is
clearance = measured separation − reamer radius − survey uncertainty − tracking uncertainty
and each term needs a source. That is what potholing buys. ASCE 38-22, the standard guideline for investigating and documenting existing utilities, calls an exposed and surveyed utility Quality Level A, with conventional accuracies of 0.1 ft (30 mm) vertically and 0.2 ft (60 mm) horizontally, tied to the project survey datum. Quality Level B is surface geophysics — the electromagnetic and radar methods a locator uses, with the marks surveyed — and the standard is explicit that depth is not a component of QL-B data. FHWA’s sample scope of work for subsurface utility engineering asks for QL-B marks within ±1.5 ft (450 mm) horizontally. Quality Level C is surveyed surface features correlated to records; Quality Level D is records and recollection.
Horizontally, that is a factor of seven and a half between a QL-B mark and a test hole. Vertically there is no factor at all: QL-B carries no depth, and at a crossing a depth is what the reamer radius has to come off.
Cover on the new bore pulls the other way. The PPI handbook says pipe subject to vehicular loading should sit at least 18 in or one pipe diameter below the road surface, whichever is greater, and that HDD installations typically go deeper still to prevent inadvertent returns, or frac-outs — drilling fluid finding a path to the surface instead of returning along the bore. Clearance below the utility and cover below the surface compete for the same vertical space, which is what makes the profile a design problem rather than a depth setting.
Turning a test hole into a constraint
A pothole backfilled without being surveyed has produced nothing that survives to the next shift. What the profile needs from each one:
- horizontal position and elevation on the same datum as the bore plan, not a chalk mark on the kerb;
- depth to the top of the facility plus its outside diameter, so both surfaces are known: a bore passing over it has to clear the top, one passing under it the bottom;
- material and size — steel and polyethylene of the same size fail differently when hit, and only the steel carries a locate signal without a tracer wire;
- how it was measured and how good that measurement was, carried forward as a number rather than dropped;
- a photograph before backfill. GTI also calls for a photo of the locate marks and an as-built map, and requires a camera inspection of sewer lines afterwards where not all of them could be verified and located — the cross-bore check.
Then the profile has to be checked along its whole length, not only at the test holes. At a crossing the pothole sits on the point that governs. Alongside a parallel line the tightest clearance can fall between exposures, and GTI’s parallel schedule caps how far apart they can be. Crossing angle decides how long the bore spends near a utility: the length of bore inside a tolerance zone grows as 1/sin of the crossing angle, so a crossing at 20° spends about three times as long inside it as one at 90°, and a shallow enough crossing starts to behave like the parallel case.
One GTI requirement is easy to skip and expensive to skip: the operator and tracker “must not vary installed path from planned path (direction or depth) without re-evaluating and exposing all underground utilities in the new path”. Under that rule, a steer correction that moves the bore off the planned path is a new bore plan.
Whether it pays
FHWA commissioned Purdue University in 1996 to measure exactly this. Seventy-one projects across the Virginia, North Carolina, Texas and Ohio DOTs, with total construction costs above one billion dollars, were studied. Obtaining QL-B and QL-A data cost less than 0.5% of total construction cost and produced construction savings of 1.9% against projects relying on QL-C and QL-D information — $4.62 saved for every $1.00 spent. Three of the seventy-one projects returned less than they cost.
Carrying it into the plan
BorePlanner Coming soon is our Android and Windows tool for bore path work: depth profile, utility clearance callouts, bend-radius checks and strike-risk warnings, with USGS lidar elevation cached for offline use at the jobsite. Each utility goes in by station, depth to top and size, and the clearance check runs to the wall of the reamed hole — taken as 1.5 times the product diameter — rather than to the pilot rod, against a required clearance you set. On Android, entry, exit and surface points along the line can be surveyed from the phone’s own GNSS receiver or through RTK corrections, either from a correction network or from your own base station over local WiFi, which needs no cell service. Points taken with the GPS buttons are tagged with how they were measured: plain GNSS, RTK float or RTK fixed.
See also: why utility strikes still happen — what a locate mark measures, and the published error figures for electromagnetic locating near bends and parallel runs.
Sources
Gas Technology Institute, Trenchless Best Practices for Damage Prevention, Version 17 Final, 3 May 2016. Gas Technology Institute, Vacuum Excavation Best Practice & Guideline, Final, January 2012. CGA Best Practices Guide Version 22.0, March 2026, practices 5-20 and 5-29. California Government Code §4216.4. ASCE/UESI/CI 38-22, Standard Guideline for Investigating and Documenting Existing Utilities; its quality-level wording as reproduced in L. Arcand and J. Cowan, 4Sight Inc., “Subsurface Utility Engineering (SUE) Deliverable – Does it Meet ASCE 38-22”, October 2023. FHWA, Sample SUE & Utility Coordination Scope of Work for Consultant Services, and Cost Savings on Highway Projects Utilizing Subsurface Utility Engineering, Purdue University, December 1999. Plastics Pipe Institute, Handbook of Polyethylene Pipe, Chapter 12, Horizontal Directional Drilling, revised May 2026. Air and water excavation compared: T. Rawn, “Hydrovac or Airvac? What’s Better?”, Trenchless Technology, September 2021; N. Bruhn, “Highlighting the Differences Between Air and Hydro Vacuum Excavation”, Damage Prevention Professional, Summer 2018, reprinted by Joe Johnson Equipment.