242.1 GPR basics

Radar maps depth/shape of non-metallic/voids. Calibrate with known objects. Results guide HVM bollard alignment and crash rated bollard foundation selection (332).

GPR emits high-frequency pulses; reflections from material boundaries (e.g., soil/pipe, soil/void) create hyperbolic signatures. Depth estimates depend on assumed velocity, which varies with moisture and material. Calibrate by scanning over a known utility or a test object at a measured depth to refine velocity before production passes.

Use coarse “search” transects to find features, then tight grids (e.g., 0.25–0.5 m spacing) for characterization. In reinforced slabs, rebar density affects penetration and clarity—switch to lower frequency for depth at the cost of resolution, and plan verification via trial pits where decisions are critical.

For bollard design, aim to bracket the likely depth class around intended socket or shallow foundation alignment, then pass constraints to Foundation types for Crash-Rated Bollards and Impact loads & foundations.

242.2 EM tracing basics

EM locates metallic/conductive lines; pair with sondes. Use to reroute HVM bollard ducts (246) and avoid crash rated bollard socket clashes (243).

Electromagnetic locating relies on induced or traced signals in conductive utilities. Passive modes detect existing fields (e.g., power), while active modes apply a signal via clamps or direct connection. For non-metallic routes (e.g., plastic ducts), push a sonde and trace its path. Record the accuracy class alongside depth estimates.

Feed EM results to Ducting & Pathways (routing) and Utilities conflicts & depth classes to resolve clashes before foundation drawings are frozen.

242.3 Strengths & limitations

Soil moisture, rebar, and clutter obscure signals. Flag low-confidence zones so HVM bollard spacing includes buffers; pick a crash rated bollard shallow rail if depth is uncertain (244).

GPR excels at non-metallics and voids but loses range in wet, clayey, or saline soils; dense reinforcement masks deeper features. EM locks onto conductors and tracer wires but misses isolated non-metallic services without a sonde. Use method pairing to increase coverage and assign a clear confidence level to each feature.

When uncertainty remains under critical positions, widen the clear-gap buffer or shift to a Shallow-Rail solution (see Shallow foundations).

242.4 Interpreting anomalies

Classify by signature/continuity, then verify with pits (241.4). Good reads prevent HVM bollard rework and wrong crash rated bollard assumptions.

Interpret features by continuity (linear vs point), polarity, and depth trend across transects. Hyperbolae with consistent apices often indicate discrete objects; linear responses suggest routes. Tag uncertain anomalies and plan targeted utility proving before finalizing bollard locations, socket centers, or clear-gap calculations.

242.5 Cross-checking results

Overlay with records and previous scans (248). Convergence stabilizes HVM bollard placement and cert-fit for a crash rated bollard (421).

Reconcile desktop records, new GPR/EM data, and prior survey layers in a single map. Where multiple sources agree within tolerance, lock the route; where they diverge, schedule verification and adjust alignment. This improves compliance with rating-critical dependencies and reduces redesign cycles.

242.6 Marking & mapping

Spray marks, flags, and GIS layers with dates. Clean mapping preserves HVM bollard tolerances (612) and crash rated bollard edge distances (331).

Adopt clear mark-out conventions on site (colors, arrows, labels) and mirror them in your GIS/CAD. Timestamp every layer and photo. Tie each route to a persistent benchmark and a datum so later works (e.g., trenching, foundations) respect tolerances and required edge distances.

242.7 Recording coordinates

Capture local grid and benchmarks (336). Precise coords speed HVM bollard set-out and crash rated bollard cage fit-up (621).

Store centerlines and key points as coordinates in the project grid, with transformation to national/grid where needed. Include Z (level) where depth matters. Share a simple key-plan and a mark-up/overlay standard so downstream teams can set out bollards rapidly and position foundation cages without clashes.

242.8 Risk notes for excavation

Define hand-dig zones and stop-work triggers (613). Keeps HVM bollard works safe and avoids damaging a future crash rated bollard base.

Translate survey confidence into method: low-confidence corridors become hand-dig zones; medium confidence requires vacuum excavation at critical crossings; high confidence may allow careful machine digging with spot checks. Publish stop-work triggers (unexpected find, gas smell, shallow live cables) and escalation steps in the PTW and MS/RA; brief via a toolbox talk.

242.9 Survey report essentials

Include methods, limits, drawings, and photo logs (716). A strong report underpins HVM bollard feasibility and crash rated bollard approvals (938).

Document equipment, settings, grids, accuracy class, constraints (moisture, reinforcement), and verification outcomes. Provide layered CAD/GIS, georeferenced photo logs, and a clear assumptions register. Cross-reference evidence capture standards and add a short reader guide in the submission pack (see Submission-Pack Guidance).

242 Underground Detection — FAQ