Underground Sewers

Lore & Background

Historically, sanitary manholes were built in one piece from bricks. A large circular concrete base was built at the bottom to form smooth channels for sewage to pass through, and a brick wall was laid to raise the structure closer to ground level. In this section, the wall diameters could be gradually smaller. Near the top, the brick wall diameters were rapidly reduced, forming a cone shape to meet a smaller diameter cover. A ladder of iron rungs was built onto the wall. Modern sanitary manholes have five main sections from bottom to top: the base section (where channels are located), the barrel section (a straight chamber sitting on the base), the cone section (reducing from chamber size to cover size), the chimney section (a straight, narrow section below the cover), and the cover section. In the 19th century, manholes in the United States were built with bricks and mortar or stone. In the mid-20th century, precast concrete was introduced, and in the late-20th century, cast-in-place concrete was used. Newer materials such as HDPE, concrete with corrosion-reducing additives, and plastics are now employed. Currently, precast concrete is the most common material for new manholes, though bricks and concrete blocks are still used in special situations, such as small work sites unsuitable for precast concrete lifting operations or for chemical waste lines.

In Their Own Story

A common method to add a new lateral pipe to an existing sewer line without any service interruption is to use a doghouse manhole. A special precast concrete structure has two upside-down U-shaped openings at the bottom directly opposite each other, and a hole at a higher point for a new pipe. To install it, the doghouse manhole is lowered so the two openings slide over the existing pipe. Concrete is poured to form the base with slopes from the precast concrete wall down to the half point of the existing pipe, creating a half bench for the manhole base. The top half of that existing pipe is then cut open to form a channel, and a new pipe is inserted through the wall hole. This allows wastewater from the new pipe to drop onto the bench and slide down to the channel. The upper portion of the manhole is built on top of that section. Sanitary manholes located in wetlands, salt marshes, or floodplains are susceptible to inflow issues, where outside water enters the sewers through manhole covers. Floodplain manholes should be elevated so the rim of the top frame is above the maximum flood level to prevent inflow. When that is not possible, any manhole covers subject to standing water should be completely sealed; pick holes should be plugged with small balls, and plastic or stainless steel bowls can be inserted between the cover and the frame to prevent inflow from reaching the chamber.

Reader's Guide

Sanitary manholes should be constructed at locations where there is a change from a simple straight sewer line, including all junctions that combine multiple lines into one or split from one, bends, changes in elevation, changes in pipe size, and changes in pipe type. For straight sewer lines, there is no universal agreement on spacing requirements. Historically, in business cores of urban areas in the United States, manholes may be placed at each street block, 300 to 400 feet (91 to 122 m) apart. A more modern U.S. guideline suggests having them every 400 feet (120 m) for small diameter sewers and every 500 feet (150 m) for larger sewer mains. Similar spacing of 120 m and 150 m is seen in Canada with specificities on sewer diameters. England sets maximum spacing at 90 metres (300 ft), with an exception for sewers with diameters larger than human heights, which can have spacing up to 200 metres (660 ft). Maximum spacing of 100 metres (330 ft) is used in Scotland, 120 metres (390 ft) in Brazil, and 150 metres (490 ft) in South Africa. A main component of the base section is a channel that provides a smooth conduit for wastewater to flow from inlet pipe to outlet pipe. The channel is in a U shape formed by a "half pipe" as if a pipe is cut in half, leaving the bottom half. The width of the channel should match the inside diameter of the inlet and outlet pipes. The lowest point of the channel is called the invert, which should have a slope to allow wastewater to pass through easily. The surface area from the manhole wall to the top of the channel is a bench, which should have a slope to direct water back down to the channel. The bench can be a half bench (if the depth of the channel from the lowest point of the bench to the invert is half the channel's width) or a full bench (if the depth is the channel's width).

Did You Know?

• Sanitary manholes are sometimes used as vents to prevent the buildup of pressurized sewage gas.
• When underground sewer lines are laid at depths of less than 0.75 metres (2.5 ft) or 2 metres (6.6 ft), an inspection chamber is used instead of a manhole.
• In the 19th century, manholes in the United States were built with bricks and mortar or stone; precast concrete was introduced in the mid-20th century.
• A doghouse manhole allows adding a new lateral pipe to an existing sewer line without any service interruption.
• Floodplain manholes should be elevated so the rim of the top frame is above the maximum flood level to prevent inflow of outside water.