In a world of round manholes, sometimes it’s hip to be square.
By Eric Carleton, P.E.
Everyone knows the saying, “You can’t fit a square peg in a round hole.” But what if you can’t fit the round peg into the round hole? When fitting circular pipes into a circular manhole this can sometimes be the case. Rather than increasing the manhole diameter, the solution may be to use a square precast structure to provide a better fit for the round peg.
Manhole structures serve many critical purposes in sewer and drainage systems. They may be the junction point where pipelines deflect to meet line and grade design conditions, where pipe diameters change, or the convergence of multiple incoming pipes with a single outlet pipe.
In the beginning
Prior to the use of precast concrete manholes, sewer and drainage pipes were laid into trenches and the manholes were constructed in the field around the pipe. Typically, this was accomplished with brick and mortar or field-cast concrete. Those structures were either built on top of, or very closely to, the outside diameter of the pipe, and relied on the strength of the vitreous clay, concrete or constructed brick arch pipe for support. Consequently, the primary consideration for the manhole size was to provide adequate room for workers to descend and ascend. Though some structures were built as small as 36 inches in diameter, most jurisdictions today have settled on 48 inches to be the minimum nominal manhole diameter.
Historically speaking, we cannot pinpoint the invention of the manhole structure. However, sometime in the early 20th century a contractor, engineer, or possibly a concrete pipe manufacturer, had a “light-bulb” moment and realized if precast concrete tubes can be installed horizontally, they can do the same vertically. That simple thought of shifting the installation axis 90 degrees brought all the benefits of precast concrete structures to sewer and drainage manholes.
A little bit of math
This new invention presented an interesting challenge. The intersection of two cylindrical shapes at 90 degrees creates an elliptical opening in the vertical cylinder rather than a simple circular opening. 1 The horizontal and vertical axes of this ellipse also need to be increased in order to provide adequate clearance for pipe connections. This means adequate annular space accounted for when grout filling if permitted, or the guidelines established by the resilient rubber pipe-to-manhole connector if specified.
National Precast Concrete Association has developed an excellent document on this subject, appropriately titled, “Manhole Sizing Recommendations.” 2 This document provides the minimum circular manhole diameter for a variety of pipe types, diameters and deflection angles. The conservative design assumptions provide for an elliptical opening 6 inches greater than the outside diameter of the pipe and a minimum inside wall spacing of 6 inches between the openings. This guide is intended for grouted connections only and not resilient rubber connectors. Resilient rubber pipe-to-structure connections require minimum rubber compression or solid steel band expansion on a curved surface. Consequently, manhole diameters may need to be increased beyond those shown on the sizing guide. Manhole sizing per the manufacturer’s recommendations will ensure a good seal for applications using resilient rubber connections.
A different way of thinking
Circular manholes are ubiquitous today, but what happens if we think outside the circle? In other words, what if we eliminate the cylindrical manhole and replace it with a square or rectangular shape? As with the example shown in Figure XX, when pipe orientations are at 90 degrees to each other, the transition from round to square (Figure YY) becomes very feasible. Now the pipe-to-structure interface is transformed from the previously elliptical configuration to a true circle; and that simplifies everything. This configuration allows improved homing ease to possibly revise the normal cast or cored opening tolerance from outside diameter +6 inches to outside diameter +4 inches. Additionally, for the resilient connector there are true and uniform walls which rubber can be solidly cast into or be used to compress against. And importantly, this shape revision makes a dynamic change to the structure size accommodating the connection. For the 72-inch round manhole design in Figure XX, the same pipe configuration could fit within the smaller 48-inch-by-48-inch box structure shown in Figure YY.
This reduction in the size of the precast structure could provide many logistical benefits, including site storage, handling and pick weight, eliminating utility conflicts and potentially reducing the overall installed costs. However, specific delivered structure costs may or may not be less expensive depending on the precaster’s type of rectangular forming system. Using fixed form or a panel-type system offers differing features and labor requirements for set up.
An important consideration when switching from a circular precast manhole to a rectangular shape is the reaction to the applied loads and the corresponding design and steel requirements to handle the loads. For the traditional circular manhole, the installed loading primarily creates a ring compression condition requiring minimal circumferential steel. A rectangular wall will need to resist the bending and shear reactions to the buried loads. Additionally, for minimum-sized structures the wall corners between two large pipe openings need to resist the vertical axial loading and may need to be evaluated more as a column rather than a wall section. For this reason, having the ability to easily vary structure wall thickness to optimize design for steel area and placement can be very beneficial.
An imperfect sewer and drainage world
Certainly, traditional circular manholes provide a great solution. However, the benefits of a flat pipe-to-manhole intersection must also be considered. Precast formwork is adaptable, and the precast industry is advanced enough to fabricate a variety of geometric configurations: triangles, hexagons and even octagons, if the need is there to offer that beneficial flat connection surface.
Perhaps the time is now to think outside the circle and consider an alternative manhole structure.
Eric Carleton, P.E., is NPCA’s director of codes and standards. He is an ASTM Award of Merit recipient and currently serves as vice-chairman of ASTM C13 on Concrete Pipe.