Utility installation work generates consistent core drilling requirements at the point where underground or overhead infrastructure enters a structure. Whether it is a telecom duct entering a building, a water main connecting to a header chamber, or a power cable penetrating through a basement wall, the correct hole size, scan protocol, and compliance approach is determined by the duct or pipe OD, the construction material, and the site context — particularly whether work is on or near a public highway.
Utility Contractor Core Drilling at a Glance
- Telecom duct (100mm OD): 107–120mm core hole through external masonry walls
- Multi-duct entries (150–200mm OD bundle): 160–220mm core hole through external wall or chamber
- Highway slab coring: always CAT+Genny scan and PAS 128 utility survey before drilling
- Post-tensioned highway structures (bridges, underpasses): GPR scan mandatory
- Near buried services: 300mm exclusion zone without positive location of service position and depth
- NRSWA S50 licence required for new apparatus in the public highway; S55 notice for works on existing apparatus
- M-Class dust extraction required for concrete coring in enclosed or semi-enclosed spaces
Typical Utility Contractor Core Drilling Applications
Building Duct Entry Points
The most common utility contractor core drilling task is creating a duct entry through an external wall or chamber wall for incoming services. Telecoms, power, water, and gas all require a clean circular opening through the building envelope at the point of entry. The opening size is determined by the duct or pipe OD and the entry fitting or duct frame specification.
Chamber Wall Penetrations
Utility chambers, manholes, and substation buildings require core drilling through precast concrete or in-situ reinforced concrete walls for duct entries. Precast chamber rings (Hepworth, Marshalls, and equivalent) have standard inlet positions designed to accept rubber sealing boots; bespoke chamber walls may require custom-diameter drilling to match the duct frame. Always drill on the manufacturer's specified inlet positions where available to avoid reducing the structural reserve of the ring.
Highway Slab Coring
Infrastructure connections that require drilling through a road slab — for a new duct crossing, a service reinstatement, or a connection to a buried chamber — involve a concrete pavement or base slab layer that must be cored before excavation. Highway slab thicknesses vary from 150mm (light-duty estate roads) to 300mm+ (trunk roads and motorways). Core sizes are typically 100–150mm for service duct crossings.
Standard Hole Sizes for Utility Duct Entries
| Duct / Pipe Type | Nominal Size | Typical OD | Core Hole Size |
|---|---|---|---|
| Telecom duct (single) | 100mm | 110mm | 120mm |
| Power cable conduit | 100mm | 110mm | 120mm |
| Water service (MDPE) | 32–63mm | 32–63mm OD | OD + 10–15mm clearance |
| Gas service (yellow MDPE) | 32–63mm | 32–63mm OD | OD + 10–15mm clearance |
| Multi-duct bundle / frame | 150mm frame | 160–180mm | 175–200mm |
| Drainage or sewer | 150mm pipe | 165mm OD | 180mm |
Always confirm the duct frame or compression fitting specification before drilling — manufacturers' installation instructions define the required core opening. An undersized hole that damages the fitting or prevents correct seating is the most common avoidable error on utility duct entries. For full bit size reference: core drill bit sizes chart.
Highway Slab Coring and Street Works
Pre-Drill Scanning Requirement
Highway slab coring must always be preceded by a buried service scan. The recommended standard is PAS 128 (specification for underground utility detection) using CAT and Genny equipment and a GPR survey where the utility record density is high. A 300mm exclusion zone from a positively located buried service applies under HSG47 (Avoiding Danger from Underground Services); in highway environments, the density of installed services can make this zone continuous in some locations.
Where positive location of all services cannot be achieved by scanning alone (for example, in areas of high electromagnetic interference or near older metallic services without tracer wire), vacuum excavation alongside the planned core position may be required to expose and verify service depths before drilling begins. See: GPR scanning before core drilling.
NRSWA Licences and Permits
Work in the public highway (road, footpath, verge) requires authority under the New Roads and Street Works Act 1990 (NRSWA). Utility contractors require a Section 50 licence from the highway authority for new apparatus in the highway. Works on existing apparatus require advance notice under Section 55, with a minimum three working days' notice for non-emergency planned works. Operatives undertaking highway excavation must hold NRSWA Unit 2 accreditation.
Highway Reinstatement Standard
Highway slab reinstatement after core drilling must comply with the Specification for the Reinstatement of Openings in Highways (SROH). A core cut through a highway slab leaves a circular void requiring filling to the reinstatement specification by layer: capping, binder course, and surface course matched to the surrounding surfacing type and road category.
Post-Tensioned Highway Structures
Bridges, underpasses, multi-storey car parks, and elevated highway decks constructed post-1970 may use post-tensioned structural concrete. Post-tensioned tendons in bridge decks run longitudinally at high tension — severing a tendon can cause catastrophic loss of prestress and structural failure. GPR scanning by a specialist operator familiar with PT construction is mandatory before any core drilling in these structures.
Unlike building slabs where PT tendons run at relatively consistent depths, bridge deck PT tendons may follow a draping profile that varies in depth along the span. A GPR scan must cover the full planned core depth along the drilling axis, not just a surface sweep. See: core drilling through concrete: contractor's guide and drilling through rebar: technique guide.
Material Considerations
Pavement and Highway Concrete
Highway and airfield concrete is typically a high-strength, low water/cement ratio mix (C40–C50) with maximum aggregate sizes of 20–40mm. This concrete is more abrasive and harder than standard structural concrete. A soft-bond wet diamond core bit is the correct specification; medium-bond bits will glaze quickly on high-strength pavement concrete. Steel mesh reinforcement is common in highway slabs — use a rebar-rated wet bit where mesh is confirmed or suspected.
Chamber Walls and Precast Units
Precast concrete chamber rings and manhole segments are factory-produced to consistent compressive strengths (typically C40–C50). Drilling through these is straightforward with a rebar-rated wet bit if reinforcement is present, or a standard wet bit if the precast unit is unreinforced. Core the specified inlet positions to preserve the ring's structural integrity.
Dust and Noise Management in Urban Environments
Urban utility drilling carries specific environmental obligations:
- Dust control: Wet coring is preferred to dry coring near pedestrians and in urban environments. Where dry coring is used in an enclosed or semi-enclosed space (inside a building, chamber, or plant room), M-Class dust extraction is required under COSHH. See: core drill dust extraction guide.
- Noise: Diamond core drilling is significantly quieter than percussive road breaking — typically 75–85 dB(A) at 1m compared to 110+ dB(A) for hydraulic breakers. This is a material advantage in time-restricted working zones where noise permits apply.
- Vibration: Diamond core drilling operates rotation-only; hand-arm vibration exposure is substantially lower than from percussive demolition tools. See: low-vibration drilling methods and HAVS.
Utility Contractor Core Drilling: Common Questions
What size core hole do I need for a 100mm telecom duct through an external brick wall?
A 120mm core hole gives 10mm annular clearance around a 100mm OD duct — sufficient for a standard duct entry frame or compression fitting. Some manufacturers specify a 107mm or 110mm hole for their specific entry fitting; always check the fitting installation instructions before drilling. A dry diamond core bit at 300–500 RPM on a dedicated core drill machine (rotation-only mode) is correct for standard cavity brick and blockwork external walls. For concrete or reinforced concrete walls, use a wet core bit on a machine with a water inlet.
Do I need to scan for buried services before core drilling in the highway?
Yes — scanning for buried services is mandatory before any excavation or drilling in the public highway under HSG47 (Avoiding Danger from Underground Services). The minimum is a CAT and Genny scan to detect metallic and active services; in areas of high service density, a PAS 128-compliant GPR survey should also be carried out. All positively located services should be plotted before drilling. A 300mm exclusion zone applies unless the position and depth of the service is positively known. Where doubt remains, hand-dig or vacuum-excavate to expose the service before commencing core drilling.
Can I core drill through a highway slab without a NRSWA licence?
No — any work on or under the public highway (road, footpath, verge) that constitutes excavation, including core drilling through a highway slab, requires appropriate NRSWA authority. Utility contractors installing new apparatus require a Section 50 licence from the highway authority. Works on existing apparatus require advance notice under Section 55. Operatives must hold NRSWA Unit 2 accreditation. Carrying out highway works without NRSWA authority is a criminal offence under the New Roads and Street Works Act 1990.