Manhole calculations

Published by Strada Imaging on

Manhole is a multi-purpose access point to underground utilities, like sewers and water supply, hence is an integral part of civil infrastructure. Depending on the underground material transport or flow design, the elevation of pipes and channels will vary throughout the network. Hence, at the access points where manholes are supposed to be construction the elevation of pipes will be specified by the designer. Usually the bottom level of a pipe section is specified and is known as its invert level. Similarly, the final ground surface level for any point within the construction area will also be approximately specified. However, the exact ground level will be determined by project realities and site conditions. As manholes connect the ground surface to the underground pipe network, their dimension will depend on the ground level and the invert level specified. As the ground level can vary, as explained earlier, the vertical dimension of the manhole will also change.

Figure 1. A manhole cover

The height of the manholes is predominantly determined by the height of its ring, which is made of concrete. Despite best design efforts, site personnel alone invariably decide the height of the ring, using the existing ground level (also known as cover level) and the invert level, as shown below.

Figure 2. Cross sectional details of a manhole

The calculation, even though straightforward, involves a number of parameters, as explained in Figure 2, thus is error-prone. In addition there are different types of manholes, like standard and catchpit. GroundApp helps site personnel with these calculations via an easy to use interface.