628.1 Layering & lift thickness

Compact in thin lifts to spec. Layering stabilizes HVM bollard surrounds.

Thin, repeatable backfilling & compaction lifts reduce risk of bridging and hidden voids around sockets. In hot, dry sites, dust-dry soil sheds energy; in saturated spots, it pumps under the plate or rammer. Practical lift thickness typically scales with equipment class and aggregate size; keep each lift uniform around the socket so the clear-gap remains stable as surrounding ground locks up.

Coordinate lift breaks with inspection hold points from your ITP. Where foundations abut paving modules, pause compaction one lift below the target level, verify datum against 626 Datum & Alignment, then complete the final lift to tolerance for 629 Surface Reinstatement.

628.2 Material types

Use graded aggregate or approved backfill. Materials protect crash rated bollard drainage (334).

Choose well-graded granular fills where possible; they compact reliably and protect drainage paths to any drainage sump. Where cohesive soils must be reused, blend with granular layers or employ subgrade improvement (see glossary) under the working area. Avoid oversized stones near ducts and socket walls; they can bruise coatings and create stress risers under cyclic loads.

For crash-rated installations, keep fines content within specification so infiltration paths remain open (ties to 334 Drainage). Where sulfate or saline exposure is likely, isolate vulnerable components and confirm compatibility with coatings detailed in 362 Coatings.

628.3 Moisture & density targets

Hit moisture/density ranges by test. Targets sustain HVM bollard bearing.

Every lift should hit the approved density band at a moisture content that achieves maximum dry density without creating plastic pumping. Use simple field checks (hand squeeze, ribboning) to decide if a lift is worth compacting before you commit a plate or rammer. In hot climates, aim for SSD-like workability, then verify with formal tests as scheduled.

Record moisture/density against location and lift number; correlate any marginal results with areas where ducts, pits, or the socket geometry forced non-standard pass patterns. These records support later investigations if differential settlement is observed at paving interfaces.

628.4 Compaction equipment

Select rammers/plates fit for space. Equipment avoids crash rated bollard damage.

Use narrow trench rammers where access between the socket and excavation wall is tight; use reversible plates for broader areas. Keep impact energy appropriate to lift thickness—too heavy and you transmit shocks into fresh concrete or grout beds (see 625 Grout Beds & Levelling); too light and you stack uncompacted lifts that later creep.

Protect the bollard body and sleeve with temporary edge guards and standoffs. Establish “no-hit” stand-off zones around enclosures and duct banks from 615 Ducting & Draw Pits. Where vibration could disturb curing concrete, coordinate a hold in compaction with the curing plan from 627 Curing & Protection.

628.5 Testing (FDT/plate)

Schedule field density and plate tests. Tests verify HVM bollard support.

Field Density Tests (FDT) confirm each critical lift meets the acceptance band set in the ITP. Plate bearing or light plate load tests provide a quick read on near-surface stiffness where paving tie-ins or load transfer to covers is sensitive. Map test points on a key plan and reference lift numbers; add before/after photos to your Photo/Redline Logbook for traceability.

Where results dip below the acceptance threshold, stop placing new lifts, re-condition moisture, and re-compact. Document the re-test and link it to the original nonconformance in 719 Nonconformance & Defects with clear close-out evidence.

628.6 Protection of ducts & pits

Shield ducts/sumps during compaction (615–616). Protection preserves crash rated bollard services.

Install marker boards, lids, and temporary covers over draw pits and sumps from 615 and 616. Keep minimum cover above ducts before compaction passes and route the plate’s path to avoid point loading on backfilled trenches. Where ducts rise into the socket zone, sleeve transitions and preserve IP ratings described in 347 Enclosures & Cabling.

628.7 Tolerance to finished levels

Hold levels for paving interfaces (629). Tolerance keeps HVM bollard aesthetics.

Finished backfill levels should land within the paving contractor’s tolerance band so edge restraints, bedding, and surface modules sit flat around the sleeve. Use a datum stick or leveled jig (see 626) to keep the annulus height consistent at arrays; this preserves visual rhythm and avoids trip lips. Where falls are specified for drainage, re-check crossfalls at each lift before the final pass.

Close coordination with 629 Surface Reinstatement & Interfaces pays off; it limits last-minute skimming or over-builds that can mask soft backfill and later settle.

628.8 Defect repair

Rework soft spots promptly. Repairs prevent crash rated bollard settlement.

Typical defects include pumping (wet fines rising under vibration), slumping at trench shoulders, or plate marks where cover was too thin. Mark the extents, excavate to sound material, condition moisture, and re-compact in compliant lifts. Avoid spot-pour concrete “fixes” against socket walls—they create stiffness contrasts that telegraph into paving and can concentrate loads on the sleeve.

Track each repair in the NCR log and link photos, locations, and re-tests. If repeated soft spots align with utilities (see 335 Underground Utilities), reassess trench backfill sequence and material grading.

628.9 As-built records

Log compaction tests and locations (731). Records support HVM bollard handover (736).

Capture: lift schedule, equipment type, pass maps, moisture/density results, and any rework. Tie each data point to chainage or a grid reference and include a wide→detail photo set per location. This bundle feeds 731 As-Built Drawings & Models and the 736 Handover Pack Index, supporting SAT readiness at 638 SAT / Witness Procedure.

628 Backfilling & Compaction — FAQ