Round manholes are the most widely used maintenance utility structures to provide access to buried pipelines for inspection and cleanout. Manholes are typically used for connecting two or more converging storm or sanitary sewer lines, or pipes, which may be of different diameter and may enter the manhole at various angles and elevations. It is important, therefore, that sizing design ensures structural integrity for the required number of pipe openings for a given junction. Manhole sizing design includes not just the outer diameter or dimensions of the structure itself, but also the thickness of its walls and the distance, or leg, between pipe openings.

Manhole design must be flexible enough to permit pipe-size changes, accommodate abrupt changes in alignment or grade, and allow for direct surface-flow interception. The largest impact on the design size of these structures is the diameter and angle of entrance of the intersecting pipe. This article provides a minimum guideline for sizing round manhole structures for various sizes and angles of incoming pipe.

ASTM standards and design considerations
The two main design criteria in designing manholes are:

  1. Manholes must be large enough to accommodate the maximum intersecting pipe size.
  2. The minimum leg, or structural length, between adjacent pipe openings in a manhole must be maintained.

ASTM C 478 does not specify the minimum leg distance between pipe openings, but proof-of-design testing methods are addressed in ASTM C 497.1 For this reason, industry design tolerance guidelines based on accepted test methods are useful.

For manholes, the minimum pipe opening is assumed to be the pipe’s inside diameter plus its wall thickness. The minimum structural leg, as a practical industry guideline, is taken as 6 in. (150 mm). Anything less than a 6-in. minimum leg between pipe openings may reduce manhole structural strength and allow cracking to occur that can lead to leakage, structural distress or durability concerns. Some state departments of transportation and other regional agencies may specify a greater minimum distance between entering pipes in manholes.

In designing manhole pipe blockouts, an additional oversizing of no more than 4 in. (100 mm) greater than the outside pipe diameter is typically used for the cutout. The final cutout or blockout opening, therefore, includes all of the aforementioned design provisions with respect to all other pipe openings, relative pipe elevations and vertical clearances. In complying with the two main manhole design criteria, these tolerances are guidelines:

  1. As a general rule, the minimum concrete leg between openings for adjacent pipes in manholes is 6 in. (150 mm), based on the outside diameter of the intersecting pipe.
  2. Blockout or cutout diameters are based on the outside diameter of the pipe plus a distance not to exceed 4 in. (100 mm).

Additional caveats
The type of pipe entering the structure, and the connection method (boot, compression or mortar) must be known to accurately determine the required hole size and, consequently, the manhole size (See Table 1 below). When possible, avoid pipes entering into manhole structure joints and, for square manholes, avoid pipe entry at corners; this intersecting geometry may compromise the structural integrity and watertightness of the manhole.

It should be noted, however, that pipe entry at structure joints and corners may be unavoidable for certain installations; in these cases, design considerations should be left to the discretion of an experienced precast concrete manufacturer. Always consult your local precast concrete manufacturer and connector supplier for recommended design requirements and product specifications.

Design method

a. Maximum pipe size for a given manhole diameter is based on normal pipe alignment (no deflection) to a deflection angle of 45° deflection
b. Maximum pipe size for a given manhole diameter is based deflection angles of 45° to 90°, without any additional pipe connections

The manhole sizing recommendations in this article provide a quick and easy resource for estimating the optimal size for manholes, depending on the size and number of incoming pipe and type of connections being used. Ultimately the manhole design is the responsibility of the engineer, who must ensure the angles of deflection are correct. The contractor must also measure and confirm the as-built pipe deflection angles in the field. If the pipe is installed at deflection angles other than normal or 90°, new calculations must be made.

Claude Goguen, P.E., LEED AP, is NPCA’s director of Technical Services.

1 ASTM C 478 “Standard Specification for Precast Reinforced Concrete Manhole Sections” provides requirements for precast manhole manufacturing and purchasing. ASTM C 497 “Standard Test Methods for Concrete Pipe, Manhole Sections, or Tile” describes product testing and acceptance methods.