Risk Assessments · Mandatory · Pre-Construction

RAMS — Drainage and Sewer Works

A specialist RAMS for drainage and sewer construction covering trench collapse, toxic gas from live sewers (H2S, methane, CO2), confined space entry to inspection chambers, heavy pipe manual handling, dewatering, and CCTV testing. Distinct from utility diversions RAMS (B38). Inspection chambers are confined spaces requiring atmospheric testing before entry. Covers foul drainage, surface water drainage, and adoptable sewers under S104.

Last reviewed: 2 April 2026 — This guide reflects UK law as of this date. MHSWR 1999 / CDM 2015 / Confined Spaces Regs 1997 remains current with no amendments enacted as of 2 April 2026. Next scheduled review: 2 April 2027.

1. A Distinct Activity from Utility Diversions

Drainage and sewer works are on the critical path of almost every construction project. The activity involves deep trenches in saturated ground, working adjacent to live sewers, manually lifting 900 mm concrete pipes from 3 m trenches, and entering inspection chambers where toxic gases may be present. These are not hazards that can be adequately addressed by a generic groundworks RAMS or by a utility diversions RAMS — drainage and sewer works carry a distinct combination of risks that require their own specific assessment.

The critical distinction is one of exposure profile. Groundworks RAMS (B17) covers earthworks: bulk excavation, reduced-level dig, trenching for foundations. It does not address the toxic gas hazards associated with live sewers, the confined space entry risks of inspection chambers, or the manual handling demands of heavy concrete pipe sections. Utility diversion RAMS addresses the relocation of third-party apparatus — gas, electric, water, telecoms — not the installation of new foul and surface water drainage systems. Drainage and sewer works sit between the two and must be assessed on their own terms.

Inspection chambers and manholes are confined spaces

Never enter an inspection chamber or manhole without atmospheric testing, rescue equipment, and a permit to work. Hydrogen sulphide (H&sub2;S) above 50 ppm causes rapid incapacitation; above 500 ppm it causes near-instantaneous loss of consciousness. Workers have died entering chambers that appeared safe visually. Atmospheric conditions inside a chamber can change within minutes — continuous gas monitoring is essential for the duration of any entry.

2. Key Content

The following table sets out the key content areas that a drainage and sewer works RAMS must address, and the specific information required in each section.

Section	Content required
Works description	Full scope of drainage and sewer works: pipe runs, diameters, materials, depths, gradients, inspection chambers, manholes, connections to existing sewers, attenuation systems, pumping stations. Include drawings and layout references.
Site investigation	Ground conditions, water table level, soil type, contamination history, existing drainage and sewer records, CCTV survey of existing sewers, trial hole data, presence of made ground or obstructions.
Underground services (STATS)	Statutory utility searches (STATS), record drawings from all utility providers, LSBUD searches. Identify all services that cross or run parallel to proposed drainage routes.
CAT scan	Cable avoidance tool survey of all trench routes before excavation. CAT and Genny scan to detect metallic services. Note limitations — CAT cannot detect plastic services.
Trial holes	Physical exposure of underground services at crossing points and areas of uncertainty. Hand-dig within 500 mm of known services. Record depth, position, size, and material of all services exposed.
Permit to dig (E4)	Permit to dig system in place before any excavation. Permit references STATS results, CAT scan data, trial hole findings, and any exclusion zones. Renewed for each new trench section or change in conditions.
Trench support	Trench support system specified for all excavations exceeding 1.2 m depth. Type of support (sheet piling, trench boxes, hydraulic shores, timber waling). Design by competent person. Inspection regime under CDM Schedule 4.
Live sewer proximity	Assessment of risks from working adjacent to or connecting into live sewers. Gas monitoring (H&sub2;S, CH&sub4;, CO, O&sub2;). Biological hazards. Structural stability of existing sewer during adjacent excavation. Isolation and flow management plan.
Confined space — Atmospheric test	Pre-entry atmospheric testing of all inspection chambers and manholes using a calibrated 4-gas monitor (H&sub2;S, CH&sub4;, CO, O&sub2;). Continuous monitoring during entry. Action levels and alarm set-points defined.
Confined space — Permit to Work (E3)	Confined space entry permit required for all chamber and manhole entries. Permit specifies atmospheric test results, ventilation arrangements, rescue plan, communication method, entry duration, and authorised personnel.
Confined space — Rescue equipment	Rescue equipment immediately available at the point of entry: rescue harness and tripod/davit system, self-contained breathing apparatus (SCBA) for rescuer, first aid equipment, means of raising an incapacitated person vertically from the chamber.
Manual handling	Assessment of manual handling tasks: lifting concrete pipes (900 mm pipes weigh ≥200 kg per unit), manoeuvring pipe sections in confined trench spaces, lifting manhole rings and covers. Mechanical aids specified: mini excavator with pipe grab, chain blocks, pipe rollers.
Dewatering	Dewatering method for saturated ground: well-pointing, sump pumping, header drain. Discharge consent from the Environment Agency or water company. Monitoring of adjacent structures for settlement during dewatering.
Pipe bedding & testing — CCTV	Post-installation CCTV survey of all completed pipe runs to check for joint displacement, cracking, debris, and gradient compliance. CCTV survey report referenced in handover documentation.
Pipe bedding & testing — Water test	Water test to BS EN 1610 for gravity sewers. Test section isolated, filled with water, and monitored for leakage over the specified test period. Pass/fail criteria recorded.
Pipe bedding & testing — Air test	Air test as alternative to water test where appropriate. Pressure applied and monitored for the specified duration. Results recorded against pass/fail criteria in accordance with BS EN 1610.
Emergency procedures	Emergency procedures for: trench collapse, flooding, gas release from live sewer, confined space rescue, service strike, injury in excavation. Emergency contacts, muster points, and rescue equipment locations specified.

3. Common Mistakes

Treating drainage as covered by the groundworks RAMS

B17 covers earthworks — bulk excavation, reduced-level dig, trenching for foundations. It does not address the toxic gas hazards from live sewers, confined space entry into inspection chambers, or the manual handling demands of heavy concrete pipe sections. Drainage and sewer works carry a distinct risk profile that requires a dedicated RAMS. Relying on a generic groundworks RAMS leaves critical hazards unassessed and uncontrolled.

Not testing atmosphere before entering inspection chambers, even new ones

New chambers that have recently been grouted or had joints sealed can contain elevated levels of CO&sub2; from cement curing. Any chamber connected to or adjacent to a live sewer may contain H&sub2;S, CH&sub4;, or depleted oxygen. The fact that a chamber is newly constructed does not mean it is safe to enter without atmospheric testing. Always test before entry — a 4-gas monitor takes seconds to provide a reading and can prevent a fatality.

4. Frequently Asked Questions

Does a confined space permit apply to new inspection chambers during construction?▾

It depends on depth and configuration. Deep chambers (greater than 1.2 m) that have been recently grouted may contain elevated CO&sub2; from cement curing. Any chamber connected to or adjacent to a live sewer is within scope of the Confined Spaces Regulations regardless of whether it is ‘new’ or ‘existing.’ As a practical rule of thumb, any chamber deep enough to require a ladder for access should be treated as a potential confined space until atmospheric testing confirms that it is safe. A permit-to-work system provides the documented assurance that testing has been carried out, rescue arrangements are in place, and entry is authorised.

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