Manholes are access chambers built into sewer and drainage networks so workers can enter the system for inspection, cleaning, and repair. A typical manhole links the underground pipe run to the surface through a shaft, a working chamber, and manhole covers and frames that close the opening securely against traffic and debris. These structures appear wherever gravity sewers change direction, grade, or diameter, and they must handle the same flows as the pipes they serve while remaining safe to enter.
Common practice spaces manholes every 90 to 100 meters on straight runs, with closer spacing at junctions and direction changes, so every reach of pipe can be rodded or inspected without excavating the road. The chamber itself is a small civil structure: a brick, concrete, or plastic box buried at pipe depth, sized to give a worker room to stand and move, and capped with a traffic-bearing cover at the finished surface.
What Is a Manhole and Why Are Manholes Built?
A manhole is a vertical structure that connects a branch sewer to a main sewer and gives workers physical access to the flow. Manholes are placed at intervals along a sewer line, at junctions, at changes in direction or slope, and at grade changes, so that every reach of pipe can be reached without excavating the road. The chamber also ventilates the sewer, releasing gases that build up in a sealed line.
Purpose of Manholes
- Inspection of pipe condition and flow behavior
- Cleaning and rodding of blockages
- Ventilation of gases from the sewer
- Junction points where branch sewers join the main line
- Access for testing new sections before handover
The walls of the chamber are commonly built in brick masonry, and the manhole and sewer bricks used for the job must be dense, well burned, and resistant to the moisture and gases inside the sewer. A good brick gives the plaster a sound backing, while a soft or under-burned brick spalls within a few years and lets groundwater seep through the wall.
Where Manholes Are Placed
A sewer run is divided into reaches by its manholes. The spacing depends on the pipe diameter and the cleaning method: 90 to 100 meters is common for small sewers cleaned by rodding, while large sewers that can be walked through allow longer spacing. A manhole is also required at every junction, change of direction, change of pipe size, and change of gradient, and at the head of each branch sewer.
Types of Manholes
Manholes are classified in two broad ways: by the depth of the chamber and by the material used for the structure. Depth decides whether a working chamber is needed, and material decides how the unit is built and how long it lasts in the ground.
Classification by Depth
Shallow, Normal, and Deep Manholes
Shallow manholes are built at the head of a branch or where the sewer runs close to the surface, and they are just deep enough for inspection of the inlet pipe. Normal manholes are the everyday chambers of a gravity system, deep enough for a worker to stand inside. Deep manholes are used where the sewer lies far below the surface, and they need a full working chamber, steps, and heavier covers.
| Type | Depth range | Typical features |
|---|---|---|
| Shallow manhole | Up to about 0.75 to 1.0 m | Inspection only, no working chamber |
| Normal manhole | 1.0 to 1.5 m | Standard access for cleaning and inspection |
| Deep manhole | More than 1.5 m | Working chamber, steps or ladder, heavier cover |
Classification by Construction Material
- Brick masonry manholes, built in situ with cement mortar and plastered inside
- Precast concrete manholes, factory cast in rings and lowered into the excavation
- Plastic manholes, lightweight units for small-diameter drainage and light loads
- Fibreglass manholes, corrosion resistant and suited to aggressive effluents
Precast concrete rings speed up construction and give consistent quality, though engineers still check reinforcement in precast concrete manhole units carefully, because rings that are unreinforced or lightly reinforced can crack during handling, backfilling, and ground movement. Plastic and fibreglass units weigh a fraction of the concrete equivalent, which cuts crane time, but they need careful bedding to resist buoyancy where the water table is high.
Manhole Construction Step by Step
A brick manhole is built in a defined sequence, and each stage affects the next. The steps below follow standard practice for a normal-depth manhole on a firm subgrade.
The Construction Sequence
- Excavation: dig to the plan depth and dimension, with extra width so workers can move around the chamber
- Bed concrete: lay a 1:4:8 concrete bed 20 to 30 cm thick; manholes deeper than 4.25 m may need a special foundation where soil is loose
- Brick masonry: raise the walls in cement mortar to the required thickness, keeping the inside face plumb
- Plaster and pointing: plaster the inside face with waterproof mortar and finish the external joints
- Channel and benching: form the half-channel through the invert and slope the benching toward it so solids do not settle
- Footrests: fix steps into the wall at regular spacing for safe descent
- Covers and frames: bed the frame on mortar at the finished road level and fit the cover
Excavation is the stage that sets the risk profile of the job. The pit must be shored or battered where there is any chance of collapse, and dewatering keeps the base dry while the bed is placed. Field crews follow the same sequence shown in the how to construct a manhole reference drawings, which detail the dimensions, mortar mix, and benching slopes for each manhole type.
The invert is the most important part of the finished chamber. A half-channel in the bottom carries the flow straight through, and the benching on either side slopes toward it at about 1 in 10 to 1 in 12, so solids and debris are swept along instead of lodging in the corners. Rough benching causes grit to accumulate, which blocks the downstream pipe over time.
Manhole Components and Sewer Appurtenances
A manhole is more than a hole in the ground. It is assembled from components that each do a specific job, and it sits within a family of sewer appurtenances that keep the network working.
Main Components of a Manhole
- Access shaft: the vertical opening from the surface down to the working area
- Working chamber: the enlarged space where a worker stands
- Base and sidewalls: the load-bearing walls of the chamber
- Bottom or invert: the shaped channel that carries the flow through the chamber
- Steps or ladder: the fixed climbing aids in the shaft
- Cover and frame: the traffic-bearing closure at ground level
Covers are classified by the load they carry. A cover in a road or parking area is rated for vehicle traffic, often class D400, while covers in gardens and footpaths can be lighter. The frame is bedded in mortar or concrete so the cover sits flush with the finished surface, and a locking or key-operated mechanism stops unauthorized opening and keeps the cover from rattling under traffic.
Sewer Appurtenances
Appurtenances are the auxiliary structures that keep a sewerage system operating. Besides manholes, the list includes lamp holes for inspection of small sewers, cleanouts for rodding access at the head of a run, catch basins that trap grit before it enters the pipe, inverted siphons that carry flow under obstacles, storm regulators that divert excess flow, flushing tanks that scour flat sections, and gullies that collect surface water. Each one is sized for the flows it handles and placed where the network needs it.
Before laying out the appurtenances, it helps to review the manhole purpose, types, and construction, since each structure exists to solve a specific hydraulic or maintenance problem, and a network that mixes the wrong appurtenances ends up with more cleaning work than it should.
Drop Manholes, Hydraulic Design, and Site Safety
How Drop Manholes Work
A drop manhole connects an inlet pipe that enters well above the outlet invert, so the flow falls through a vertical pipe before joining the main sewer. The main sewer flow is routed outside the chamber, and the vertical pipe that passes through the barrel is called an inside drop, while one fixed to the exterior wall is an outside drop. Drop manholes have a larger sloping gradient than standard chambers, which makes this type of construction less economical, so they are specified only where the ground or the pipe layout leaves no alternative.
Manhole Losses in Hydraulic Design
Every manhole adds a small energy loss to the sewer because of the change in direction, the contraction at the inlet, and the turbulence in the chamber. Hydraulic models include these losses when sizing pipes, and manhole losses in gravity pipeline design can change pipe diameters and invert levels on long runs. Ignoring them leaves a system that runs deeper or steeper than it needs to, which pushes up excavation and pumping costs.
Construction Site Safety
Manhole construction happens in deep, confined excavations next to live traffic and sometimes live sewers. Shoring or battering is required in unstable soil, gas testing is done before entry, and a second worker stays at the surface while anyone is inside the chamber. Covers must be secured as soon as the chamber is complete, and open pits are fenced and signed overnight.
On road projects the hazard extends beyond the crew inside the pit, because passing plant and vehicles can strike workers or drop loads into open shafts. The manhole and underground fixture safety practices used on road construction sites protect both the public and the workforce around every open excavation, and they belong in the method statement before ground is broken.