A utility mark, record drawing, or geophysical depth estimate can be useful for planning. It may not be enough to support a final design decision, a tight excavation, or a proposed utility crossing. When a project depends on knowing where a utility actually is and how it is configured, a planned utility test hole can provide the physical verification that surface-based methods cannot.
Quality Level A (QL-A) utility locating uses physical exposure and measurement to obtain detailed information at a specific point. Vacuum excavation is a common non-destructive excavation method for creating these test holes. It allows a qualified field team to carefully remove soil around a known or suspected utility so the utility can be observed, measured, and documented.
This does not mean every marked utility needs to be daylighted. It means the project team should identify the assumptions that carry the most design, safety, schedule, or construction risk—and verify those assumptions before they become expensive field problems.
Why a utility mark may not answer the design question
Utility designation methods, including electromagnetic locating and ground penetrating radar (GPR), can provide valuable evidence of a subsurface feature. They can help a team trace a signal, identify an interpreted route, and develop a utility investigation plan. However, they do not physically reveal the utility.
A marked line may not establish the pipe or conduit’s actual outside diameter, the number of conduits in a bank, casing details, the elevation at a critical crossing, or whether an apparent line is active, abandoned, or one of several nearby facilities. Depth readings from locating equipment are estimates affected by signal behavior, coupling, congestion, soil conditions, and other field variables.
Physical utility exposure can answer more specific questions at the test-hole location. Depending on access and safe exposure conditions, the field team may observe:
- Actual horizontal position at the exposed point
- Depth below the existing surface or another documented reference point
- Top, centerline, or invert measurements, as appropriate to the facility and project need
- Pipe or conduit outside diameter
- Visible material, such as metal, plastic, concrete, clay, or duct bank components
- Number and arrangement of conduits where they can be safely observed
- Casing, encasement, tracer wire, warning tape, or protective features
- Vertical separation and configuration where utilities cross
- Visible condition concerns, such as damaged coating, deformation, voids, or prior repair evidence
Observation at one test hole does not prove that conditions remain identical along the entire route. Utilities can change depth, direction, material, or configuration. The test-hole plan should therefore focus on locations where the information will meaningfully affect design or construction decisions.
How Quality Level A relates to Quality Level B
Subsurface Utility Engineering (SUE) investigations often combine multiple methods. In general terms, Quality Level B information involves geophysical designation: interpreting utility evidence from methods such as electromagnetic locating and GPR. Quality Level A information involves exposing the utility and obtaining precise measurements and documentation at the point of exposure.
The distinction matters. QL-B helps establish where to investigate. QL-A can resolve the critical unknowns that remain when a proposed improvement approaches, crosses, or depends on an existing utility.
ASCE 38-22 describes quality levels and investigation practices for utility information. The ASCE standard overview and the Federal Highway Administration’s SUE guidance both distinguish geophysical designation from physical verification. A vacuum excavation test hole can support a QL-A investigation when it is planned, measured, and documented to the applicable project scope and standard. Not every pothole automatically satisfies every Quality Level A requirement, and not every vacuum excavation assignment is a complete SUE investigation.
For projects that need coordinated designation and test-hole work, Visionary Subsurface Solutions provides Subsurface Utility Engineering and QL-B services alongside physical verification.
When engineers should specify utility test holes
Utility test holes are most useful where uncertainty could change a design, method, or construction sequence. Rather than treating them as a routine add-on, specify them around clear decision points.
At proposed crossings and utility conflicts
A plan view can show two utilities appearing to cross, but it cannot establish vertical separation by itself. If a proposed storm line, water service, electrical duct bank, wall, structure, or bore path will pass near an existing facility, daylighting may confirm whether there is enough clearance—or whether the design must change.
The investigation should identify what needs to be measured. For example, a utility crossing verification may require the top of an existing pipe, its outside diameter, and a surveyed elevation. For a gravity line, the project team may also need invert elevation and flow direction. A generic note to “pothole utility” may not collect the information the designer actually needs.
Before finalizing profiles, grades, and drainage design
Roadway reconstruction, site grading, and drainage improvements often rely on vertical design. Existing utility records may show an approximate depth, but the installed facility may differ due to field changes, pavement overlays, settlement, earlier repairs, or incomplete recordkeeping.
Test holes at critical profile points can help a designer determine whether the proposed grade creates a cover issue, whether a drainage system can clear an existing facility, or whether a relocation or protection strategy should be considered before bid or construction.
Before directional drilling or other trenchless work
Horizontal directional drilling requires a bore plan that accounts for existing underground infrastructure. Locating is an important starting point, but physical exposure is often appropriate at high-risk crossings, entry and exit areas, and locations with limited vertical separation. The purpose is not simply to find a utility; it is to understand the verified geometry that affects the proposed bore path.
Where utility records conflict or are incomplete
Records are valuable reference documents, not a substitute for field verification. A facility may have been installed differently from its drawing, altered during prior work, omitted from records, or shown at a scale that does not support construction-level decisions. A test hole can compare an important field condition with the available plans before the project proceeds on an untested assumption.
Before mechanical excavation enters a tight utility area
When a planned trench, foundation excavation, pole base, or sawcut area approaches marked utilities, daylighting can establish a known location before heavier equipment works nearby. Vacuum excavation is intended to reduce the risk associated with conventional mechanical digging around known or suspected utilities, but it is still active excavation. Safe work practices, site controls, utility-owner requirements, and qualified operators remain essential.
What vacuum excavation adds to a utility investigation
Vacuum excavation removes loosened soil with a high-powered vacuum system. With hydro excavation, controlled water is used to break up soil before it is vacuumed into a debris tank. With air excavation, compressed air loosens soil and the vacuum removes the spoil.
The best method depends on the site. Hydro excavation can be effective in many compacted soils, but it creates slurry that must be managed and disposed of appropriately. Air excavation can produce drier spoil that may be easier to handle in some settings, but it may be slower or less effective in certain hard, frozen, or cohesive soils. Access, groundwater, restoration needs, weather, local disposal requirements, utility type, and work-zone constraints should all be considered.
For utility test hole services, the important outcome is controlled exposure—not the equipment alone. A planned process typically includes review of available records, utility designation where appropriate, test-hole layout, safe excavation, field measurements, photographs or sketches if required, and coordination with a surveyor when precise coordinates or elevations are needed.
Depth, measured depth, and surveyed elevation are not the same
Project teams often use these terms interchangeably, which can create avoidable confusion.
- Estimated depth: A depth suggested by an electromagnetic locator, GPR interpretation, or record information. It is useful for planning but remains an estimate.
- Measured depth after exposure: A field measurement from a documented surface reference to an observed point on the utility, such as the top of pipe or conduit. The report should state the reference point and measured feature.
- Surveyed elevation: A horizontal and vertical coordinate collected by survey methods from the exposed utility feature. This is generally needed when the design requires precise placement in a profile, base map, or construction model.
Vacuum excavation provides access to the utility. It does not automatically create a surveyed elevation. If elevation accuracy is critical, the test-hole scope should define the survey control, feature to be collected, coordinate system, datum, and required deliverable.
A practical test-hole request for engineers and contractors
A well-defined request helps the field crew expose the right feature and helps the design team receive usable information. Before requesting utility potholing services, identify:
- The decision to be made. Is the goal to clear a proposed crossing, confirm cover, verify a duct bank, investigate a conflict, or support a bore plan?
- The target utility and location. Provide plan sheets, available records, proposed work limits, utility marks, and station/offset information when available.
- The features to document. Specify whether the team needs top of utility, centerline, invert, outside diameter, material, conduit count, casing details, or vertical separation.
- The required accuracy and survey needs. State whether a tape measurement is sufficient or whether survey-grade horizontal and vertical data are required.
- Access and work-zone conditions. Address traffic control, pavement removal, restoration, site access, permits, railroad or utility-owner coordination, and property restrictions.
- Material handling. Consider slurry or spoil containment, disposal, groundwater, and any site-specific environmental requirements.
Common mistakes that reduce the value of daylighting
Using random holes instead of investigating critical points
More holes do not necessarily produce better information. Test-hole locations should be tied to design conflicts, profile changes, proposed crossings, congested corridors, and other areas where a verified condition affects the project.
Collecting measurements without defining the reference
A note that a utility is “five feet deep” is incomplete without identifying whether that measurement is to the top of pipe, centerline, or invert, and whether it is from existing grade, pavement, or another reference. Clear references make the information usable by surveyors, designers, and field crews.
Assuming an exposed utility answers every question
A test hole is a point observation. It may not reveal conditions beyond the hole, hidden parallel utilities, downstream changes in pipe condition, or the complete route of an abandoned line. Additional locating, test holes, CCTV inspection, survey, or engineering review may still be needed.
Waiting until construction to resolve predictable conflicts
Some field discoveries are unavoidable. Others are foreseeable from early utility records, conceptual design, and designation work. Resolving high-consequence unknowns during design can provide more options than discovering them after crews, equipment, and schedules are committed.
FAQ: Quality Level A utility test holes
Can vacuum excavation confirm utility material and size?
Physical exposure can allow visible characteristics such as pipe or conduit material, outside diameter, conduit count, casing configuration, and protective features to be observed and documented. The exposed area may be limited, so conclusions should be limited to what was safely visible at that location.
Does utility daylighting guarantee that no other utilities are present?
No. A daylighted utility confirms conditions at the exposure point. It does not guarantee that every utility on a site has been identified. A coordinated investigation using records, designation, site observations, and appropriately placed test holes provides a stronger basis for planning.
How many utility test holes does a project need?
The number depends on the proposed work, congestion, utility uncertainty, and consequences of an incorrect assumption. Critical crossings, profile constraints, bore paths, connection points, and areas where records conflict are common priorities.
Turn critical utility assumptions into usable field data
Vacuum excavation for utilities is most effective when it is part of a focused utility verification plan. It can give project teams observed information about a utility’s position, depth, elevation reference, size, material, and configuration at the points that matter most. That information can support more informed design, utility coordination, construction planning, and excavation decisions.
Visionary Subsurface Solutions provides vacuum excavation and QL-A utility verification services for projects throughout Pennsylvania, New Jersey, Delaware, Maryland, New York, and the Washington, D.C. to New York City corridor. Contact our team to discuss the utility questions, access conditions, and documentation needs for your project.
