Manholes should be of durable structure, provide easy access to sewers for maintenance, and cause minimum interference to sewage flow. Manholes for small sewers are usually about 1.2 m (4 ft) in diameter. Sewers larger than 60 mm (24 in) in diameter should have larger manhole bases although a 1.2-m barrel can still be used.
Manholes should be located at the end of the line (called terminal cleanout), at sewer intersections, and at changes in grade and alignment except in curved sewers as shown in Figure 7.11.1. The maximum spacing of manholes is 90-180 m (300-600 ft) depending on the size of the sewer
Figure 7.11.1 shows a plan and profile of a sanitary sewer and its laterals with enlarged sections of sewer trenches and manholes.
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