Locate Underground Utilities: GPR, Marking, Stand-Offs, Protection

Definition: Locate underground utilities for construction teams using records, electromagnetic locating, and GPR to identify, mark, stand-off, and protect existing services while explicitly excluding new utility construction activities.

Review records, scan with GPR/EM, and reconcile discrepancies.
Mark services with depth notes and set plant stand-off limits.
Protect shallow assets with mats, barriers, and controlled excavation.
Interactive, commentable checklist with export and QR code verification.

Start Interactive
Checklist

Locate underground utilities is the first safeguard against costly strikes, delays, and service outages. This checklist guides teams through utility detection using records review, electromagnetic locating, and ground-penetrating radar, then marking services, establishing stand-offs, and implementing protection. It focuses on avoidance and verification only—no new utility construction is included. You will plan from as-builts and GIS, validate with GPR and EM methods, reconcile conflicts, and confirm critical depths with safe potholing or vacuum excavation. The workflow aligns with subsurface utility engineering practices to improve accuracy and reduce excavation risk. Acceptance cues, tolerances, and photo evidence are specified to ensure consistent results and defensible documentation across shifts and contractors. Use this interactive tool on live projects to brief crews, control plant movement, and maintain exclusion zones as conditions change. Start in interactive mode to tick items, add comments, and export as PDF/Excel with a QR-secured record.

Cut strike risk by combining records, electromagnetic locating, and GPR in a structured workflow. The checklist enforces coverage targets, cross-checks between methods, and photographic evidence, so supervisors can confidently approve excavation windows while maintaining auditable compliance with project specifications and authority requirements.
Set clear, measurable acceptance criteria for each step: document record currency, log EM/GPR settings and tracks, tag marked utilities with depth and date, and verify critical depths by potholing. Evidence bundles—photos, readings, and signatures—enable traceability and fast dispute resolution across stakeholders.
Implement practical protection: stand-off barriers for plant, exclusion corridors on site plans, load-spreading mats over shallow assets, and vibration monitoring near sensitive lines. These measures maintain service continuity, minimize rework, and allow safe progress when utilities cannot be relocated or de-energized.
Interactive online checklist with tick, comment, and export features secured by QR code.

Pre-Planning & Records Review

1. Retrieve latest utility records (as-builts, GIS, one-call responses) for the work area; confirm issue dates within 3 months; upload PDFs to the checklist as evidence.
2. Compile utility owner contacts and emergency numbers; record ticket/reference numbers and validity periods; screenshot confirmations for the file.
3. Conduct a walkover to identify surface clues (covers, valves, pedestals, markers); geotag at least 10 photos covering the work zone edges and centreline.
4. Draft a utility risk plan defining high-risk corridors, proposed stand-offs, and confirmation points; obtain supervisor approval; attach signed plan.

Notifications & Permits

5. Notify relevant utility owners of planned works and request locating assistance where required; log responses and conditions; attach email thread or portal receipts.
6. Obtain ground penetration/excavation permits covering the exact grid extents and dates; record permit numbers; attach scans before fieldwork.
7. Brief crew with a toolbox talk on utility hazards, GPR/EM safety, and exclusion zones; capture attendance signatures and photos of briefing board.

GPR & Locating Survey

8. Calibrate GPR on a known target to set time-zero; record antenna model and centre frequency; photograph calibration screen with timestamp.
9. Scan a grid at 0.5 m transects in orthogonal directions; log GNSS/total station tracks; achieve ≥95% coverage over the work footprint; export track map.
10. Use EM locator in passive (50/60 Hz, radio) and active modes; apply peak–null method; record signal strengths and frequencies; store screenshots for each line.
11. Sweep with a ferrous magnetometer around suspected steel pipes/cables; flag anomalies; log coordinates with horizontal accuracy ≤0.20 m.
12. Estimate depths from GPR hyperbola fits using measured dielectric; note depth to nearest 0.1 m; mark confidence level; validate at test holes where feasible.
13. Cross-check GPR/EM results against records; highlight conflicts or unknowns on a layered plan; add comments tagging responsible reviewers.

Marking & Stand-Offs

14. Mark surface alignments with durable paint and flagged pins per approved project specifications and authority requirements; ensure marks are visible from 10 m; photograph every 5 m.
15. Stencil or tag each mark with utility owner, service type, depth (m), date, and initials; include a scale in photos (0.1 m increments).
16. Install stand-off barriers at a minimum 1.5 m from marked alignments for heavy plant; verify offsets with tape to ±0.05 m; tag barriers with date.
17. Update site layout to show no-go corridors at least 1.0 m either side of marked services; issue to operators; obtain acknowledgement signatures.

Protection Measures & Controls

18. Place load-spreading mats or timber road over utilities with cover <1.0 m where plant traffic is unavoidable; document mat type and bearing capacity; photo before/after.
19. Schedule vacuum excavation or hand-dig test holes at critical crossings to confirm depth and type; record actual depth to 0.01 m; restore per approved specifications.
20. Implement vibration/compaction monitoring near sensitive utilities; set alert limits per approved project specifications; log PPV or compaction readings with location and time.

Documentation & Handover

21. Compile a utility avoidance plan with coordinates (E/N in metres), depths, confidence ratings, and photos; export GIS/PDF; include revision number and date.
22. Complete digital sign-off by supervisor and, where applicable, utility representative; export PDF/Excel with QR authentication; archive to project document control.

Plan from Records and Site Intelligence

Effective utility avoidance starts with a disciplined review of available data. Source the most recent as-builts, GIS layers, and one-call responses, then verify their currency and coverage against the actual work footprint. A walkover identifies surface features—valve boxes, handholes, pedestals, and marker posts—that often reveal undocumented spurs or changes in alignment. Converting these inputs into a risk-based plan defines where to expect high-density corridors, where stand-offs must be conservative, and which unknowns demand field verification. Documenting assumptions and attaching evidence (maps, photos, approvals) builds a trail that withstands audits and site challenges. Acceptance cues include date-stamped records, geotagged walkover photos at logical intervals, and a signed plan that assigns responsibilities for locating, marking, and protecting assets. When gaps appear—missing records or mismatched features—flag them early so field methods can focus on those areas with tighter grids and planned test holes to close uncertainties before excavation.

Use the latest records; verify dates and coverage limits.
Capture geotagged photos of surface clues and boundaries.
Write a risk plan assigning roles and confirmation points.
Flag unknowns early and plan tighter survey grids.
Attach all evidence and approvals to the checklist.

Field Locating with GPR and Electromagnetic Methods

Ground-penetrating radar and electromagnetic locating provide complementary detection. GPR reveals non-conductive and conductive utilities by interpreting hyperbolas, while EM detects conductive lines in passive or active modes. A tight, orthogonal grid with logged tracks ensures coverage; calibration on a known target sets confidence in depth conversions. Magnetometers can help identify ferrous pipelines or fittings that GPR or EM alone might miss. Cross-checking between methods and records reduces false positives and confirms true alignments. Acceptance cues include documented equipment models and settings, track coverage ≥95%, depth estimates noted to the nearest 0.1 m, and annotated screenshots for significant finds. Conflicts or unknowns should be escalated for targeted potholing, keeping excavation controlled and evidence-rich. Soil conditions, moisture, and reinforcement can limit GPR; in such cases, EM and selective test holes become the primary tools to maintain reliable outcomes without over-reliance on a single method.

Run orthogonal grids with logged GNSS or total station tracks.
Record equipment models, frequencies, and calibration evidence.
Estimate depths from GPR fits; state confidence levels.
Cross-check findings and escalate conflicts for potholing.

Marking, Stand-Offs, and Protection on Site

Clear, durable markings translate findings into actionable site controls. Each mark should include service type, owner, depth, and date, photographed with a scale for traceability. Stand-off barriers and no-go corridors keep heavy plant offset from utilities, while mats protect shallow services under unavoidable traffic. Where excavation must cross suspected lines, pre-plan vacuum excavation or hand-digging to expose and measure actual depth, then restore per approved requirements. Sensitive assets may also need vibration limits or temporary supports. Acceptance cues include visible markings from 10 m, verified barrier offsets to ±0.05 m, documented mat specifications, and signed briefings for operators. Refresh markings after rain, resurfacing, or heavy traffic, and re-verify alignment if site controls shift. The goal is a living control system that adapts as conditions change while maintaining evidence strong enough to support audits, client queries, or incident investigations without ambiguity.

Mark utilities with depth, date, and initials.
Maintain stand-offs and publish exclusion corridors.
Use mats over shallow assets under plant routes.
Confirm critical crossings with vacuum excavation.

How to Use This Interactive Utility Locating Checklist

Preparation: Gather latest records (as-builts, GIS, one-call), calibrate GPR/EM locators, confirm permits, and brief the crew on hazards, stand-offs, and documentation requirements.
Preparation: Set up the project in the app, define the survey grid boundaries, invite stakeholders, and enable location services for geotagged photos and track logging.
Using the Interactive Checklist: Start interactive mode, tick items as completed, and attach photos, PDFs, and screenshots as evidence at each step.
Using the Interactive Checklist: Use comments to flag conflicts or unknown utilities, tag responsible parties, and assign follow-up actions with due dates.
Using the Interactive Checklist: Monitor completion status, verify acceptance criteria (coverage, depths, stand-offs), and adjust the plan based on findings.
Sign-Off: Generate an export as PDF/Excel with embedded photos, maps, and readings; enable QR authentication to lock revisions.
Sign-Off: Collect digital signatures from the supervisor and relevant utility representatives, distribute the export, and archive in project document control.

Start Interactive Checklist

Locate Underground Utilities Checklist

Call to Action

Santiago Torres Santiago Torres

Nov 11, 2025

113

FAQ

Question: How accurate is GPR for locating and depth estimation?

GPR accuracy depends on soil conditions, moisture, reinforcement, and antenna frequency. With proper calibration and orthogonal grids, horizontal position typically falls within a few decimetres, and depth estimates within about ±10%. Always record dielectric assumptions, note confidence levels, and confirm critical depths by vacuum excavation or hand-digging before mechanical excavation proceeds.

Question: When should vacuum excavation be used during utility locating?

Use vacuum excavation when utilities intersect planned excavation, when records conflict with GPR/EM results, or when depths are uncertain near high-risk assets. Potholing confirms actual depth and type with minimal damage risk. Log measurements to the nearest 0.01 m, photo the exposure with a scale, and backfill and reinstate per approved project specifications and authority requirements.

Question: What stand-off distances are appropriate for plant near marked utilities?

Establish risk-based stand-offs per approved project specifications and authority requirements, considering utility type, depth, and plant size. As a conservative starting control, maintain wide exclusion corridors and install barriers measured to a documented tolerance. Validate stand-offs during the daily briefing, adjust for changing conditions, and re-measure after any layout shift or remarking.

Question: How often should utility markings be refreshed on site?

Refresh markings after rainfall, resurfacing, mechanical cleaning, or heavy traffic, and at least before each shift in high-traffic zones. Reconfirm alignment when controls move or when new evidence appears. Photograph refreshed marks with a scale, update depths and dates, and reissue the site plan so operators always see current, legible guidance.