United Utilities Developer Services: Manhole Construction Details

Introduction

United Utilities Developer Services provides comprehensive guidelines for the construction of manholes within our network. These details ensure standardization, safety, and longevity of underground utility access points across our service area. Proper manhole construction is critical for maintenance operations, network monitoring, and emergency access to the water and wastewater infrastructure.

This document outlines the technical specifications, materials, construction methods, and quality assurance processes required for manhole installation projects. All developers and contractors working within our jurisdiction must adhere to these standards to ensure the integrity of our utility network.

Manhole Types and Applications

United Utilities recognizes several standard manhole types, each designed for specific applications:

• Standard Access Manholes: Located along sewer and water mains to provide maintenance access at regular intervals.
• Junction Manholes: Installed at pipe intersections, directional changes, or where pipe sizes change.
• Riser Manholes: Used where depth to the pipe exceeds standard reach, providing intermediate access levels.
• Terminal Manholes: Placed at the end of sewer/water branches for dead-end points in the network.
• Special Purpose Manholes: Designed for specific operational needs such as flow monitoring or specialized equipment installation.

Design Specifications

Dimensions

• Manhole Diameter: Standard sizes include 900mm, 1050mm, and 1200mm internal diameter. Larger diameters may be required for special applications.
• Depth: Determined based on the invert level of the associated utility pipes, with a minimum working space requirement of 1.0m within the manhole.
• Wall Thickness: Minimum 150mm reinforced concrete for standard applications, increasing based on depth/soil conditions.
• Base Slab: Minimum 200mm thickness for depths up to 3m, with structural engineering required for deeper installations.

Cover and Frame Requirements

• Covers must comply with BS EN 124:2015 standards.
• Load rating: Minimum D400 for roadways, B125 for pedestrian areas.
• All covers must be watertight and secure in their frames.
• Covers must not protrude above the finished surface level.

Construction Materials

All materials used in manhole construction must meet the following specifications:

Concrete

• Minimum C40 compressive strength at 28 days for manhole walls and base.
• All concrete must be properly cured and protected from adverse weather conditions during placement.
• Water/cement ratio must not exceed 0.45.

Reinforcement

• High yield deformed steel bars conforming to BS 4449.
• Minimum cover to reinforcement: 50mm in soil, 40mm internally.
• All reinforcement must be securely tied and adequately supported before concrete placement.

Brickwork

• Engineering bricks to BS EN 771-1 with minimum water absorption of 4.5%.
• Mortar mix ratio 1:3 using cement and sand complying with relevant British Standards.
• All joints must be properly filled and struck smooth.

Pipelining and Channels

• UC channels half-round or full-round in manhole base, properly shaped to accommodate flow.
• Benching provided with 1:12 gradient toward channel.
• All surfaces finished smooth to minimize debris accumulation.

Construction Methodology

Excavation

1. Excavate to required dimensions, allowing for working space and formwork.
2. Implement appropriate temporary works and shoring as required by health and safety regulations.
3. Remove all water from excavation before concrete placement.
4. Prepare formation level to receive concrete blinding.

Base Construction

1. Place blinding concrete to a minimum thickness of 50mm.
2. Install reinforcement according to design specifications.
3. Construct formwork for base slab ensuring correct dimensions.
4. Place concrete in layers not exceeding 300mm thickness.
5. Compact concrete thoroughly to eliminate voids.
6. Create channels and benching while concrete is still workable.

Shaft Construction

1. Erect formwork for manhole walls, ensuring verticality and correct dimensions.
2. Install reinforcement as specified, including ladders or step irons.
3. Place concrete in controlled lifts, compacting between lifts.
4. For brick construction, build courses in accordance with good practice, with proper bonding and jointing.
5. Install pipe connections at appropriate levels, making sure they are properly sealed against the manhole structure.

Backfilling

1. Ensure manhole has cured sufficiently before backfilling begins (minimum 3 days, subject to conditions).
2. Remove all temporary works equipment.
3. Backfill in layers not exceeding 150mm compacted thickness.
4. Use suitable backfill material avoiding large stones or debris adjacent to the manhole.
5. Compact each layer to 95% maximum dry density.

Safety Considerations

Manhole construction involves significant safety considerations that must be addressed throughout the construction process:

Site Safety

• Comprehensive risk assessment must be undertaken prior to commencing works.
• Appropriate barriers, signage, and lighting must be provided around the excavation.
• All personnel must wear appropriate personal protective equipment.
• Vehicles must be kept at safe distance from excavation edge.

Confined Spaces

• Manhole construction involves working in confined spaces and must comply with the Confined Spaces Regulations 1997.
• Adequate ventilation must be provided at all times.
• Gas monitoring must be conducted before and during work in the manhole.
• Rescue equipment and procedures must be established before work commences.

Temporary Works

• Trench support systems must be designed by competent persons.
• All temporary works must be inspected daily and following adverse weather conditions.
• Equipment must be suitable for the ground conditions and loadings.

Testing and Inspection

Inspection Stages

The following stages of construction require inspection and approval by United Utilities:

1. Excavation and formation level
2. Reinforcement placement before concreting
3. Completed concrete work or brickwork
4. Pipe connections to manhole
5. Completed installation before backfilling
6. Finished street level restoration

Testing Requirements

• All manholes must be tested for water tightness before acceptance.
• Air or water testing may be conducted using approved methods.
• Manholes must withstand a head of water 1.0m above the invert for 24 hours without significant leakage.
• Any leaks identified must be rectified before final approval.

Compliance and Standards

All manhole construction must comply with the following standards and regulations:

• Water Industry Act 1991
• Building Regulations Part H (Drainage and Waste Disposal)
• BS EN 752: Drain and sewer systems outside buildings
• Sewers for Adoption (7th Edition)
• Health and Safety at Work Act 1974
• Construction (Design and Management) Regulations 2015

Where this document conflicts with the above standards, the more stringent requirement shall apply. Developers should maintain up-to-date knowledge of any changes to these standards.

Maintenance Considerations

Proper manhole design and construction facilitate long-term maintenance operations:

• Step irons or ladders must be corrosion-resistant and securely fixed.
• Cover and frame assemblies should permit future lifting with minimal disruption.
• Internal surfaces should be smooth to resist adhesion of deposits.
• Adequate lighting provision must be accommodated in the design.
• Location markers must be visible from ground level.

United Utilities Developer Services reserves the right to refuse acceptance of any manhole that does not facilitate future maintenance operations effectively.

Conclusion

These construction details have been formulated to ensure that manholes within the United Utilities network provide safe, reliable, and maintainable access to our underground infrastructure. Developers should ensure that all staff involved in manhole construction are familiar with these specifications and have received appropriate training in the construction methods described.

Should any clarification or additional information be required regarding these construction details, developers should contact United Utilities Developer Services during the planning stages of their project. Early engagement can prevent costly delays and ensure compliance with all relevant standards.