Sanitary Sewer Manholes - Round vs. T-Base

I'll freely admit that when it comes to sewers I don't know my @ss from a T-base hole in the ground.

It's easier to dig a round hole.

For the material a cylindrical shape will handle greater forces.

And you can't drop the cover down the hole. Ever try to lift one of those things. I wouldn't want to have to carry it back to the surface.

Now I Googled T-Base manholes and didn't get a hit so you want to provide some info.

Here, we only use round structures.

The components come pre-fabricated to standard sizes regardless of manufacturer and are readily transported.

We seldom if ever use them for person entry and if there is entry, the manhole is isolated from flow before access, so the presence or absence of benching would not be an issue.

Our systems only handle sewer (no stormwater) so the pipes interconnecting the manholes are not as large as might be used where mixed flow systems are in use. We use a lot of remote control cameras, so no bench would provide some difficulty in orientation of teh cameras for inspection.

I suspect that no bench would also leave "dead spots" to accumulate debris that could lead to clogging isues if dislodged in a single bolus.

Benching means that the pipe invert and thus the flow velocity is maintained for transport of solids through the system. Again, detained solids could lead to fermentation and evolution of gas. This could increase the incidence of Hydrogen Sulfide attack to concrete portions of the system and also increased risk to operators, especially if they have to disturb accumulated material while inside the structure.

We are fortunate in that we only have 20 that are deeper than 10m in a network of 10,000 structures. Our average depth is 1.5m, so most are readily serviced by operators without entry.

I know even less, but one snippet I quite like is that you can't drop a round manhole cover into the hole whereas a square (or two triangles forming a square) cover can be.

I quite like that bit in Terry Gilliam's Brazil where Central Services attempt to plug the hole in the ceiling, only to find that their plug drops straight through - "they've gone metric again!"

So the round manhole cover referred to above is the one intended for that purpose because of course you can drop a different round manhole cover into the hole. Some people like to be picky, I'm just trying to cover bases.

Just to stop this from going any further, I want to clarify. This discussion only concerns the bases/inverts of the manholes. The tops of the manholes are standard. We only use round manhole ring and covers.

Here in New York State, we predominately use precast concrete structures, and for good reason. They're stronger (to resist highway wheel impact loads and lateral earth pressures) and more resistant to our harsh winter conditions, namely deep frost penetrations into the surrounding ground. Most are round. personally, I prefer round over square or rectangular structures, as it's much easier to properly align the pipes perpendicular to the wall face, as well as ease of setting up the pipe laying laser and transit for horizontal alignments. I design and spec square or rectangular structures if the pipes are large, but they area real bear to structurally design. Also, round structures are much easier to install, and IMPO much stronger because the entire round wall structure is optimized to act as a compression ring to resist lateral earth and hydrostatic groundwater pressures. Then there is the uplift/bouancy issue to contend with when you use fiberglass T-Base structures and the absolute need to properly anchor then with a concrete base & anchor bolts.....I just don't see the cost savings of the T-Base manhole installation vs. precast concrete structure installation there. Oh yeah, they're lighter (most of the time) and easier to move around for the pipe-laying crews, but when they have a concrete wall ring cast around them (usually for strength purposes) and the need to anchor them, I just don't see the advantages on many different levels. Basically, I think that the Contractors wanting to install T-base structures are getting a little bit carried away with their damn laziness in regard to the ease of swinging them around off flatbed trucks and dropping them down into the hole.

I have never used pre-fabricated fiberglass T-Base sanitary (or storm) manholes in my entire engineering career. As far as I know, the NYSDOT has never allowed them to be used on their projects, and that is okay with me. When I was Resident Engineer with the USACOE down south we never permitted the use of T-Base manholes because of a host of reasons. They may permit their use now, but not back then 20+ years ago.

I also don't know how you can effectively align the pipes, both in vertical and horizontal, with pre-fabbed pipe portals, especially IF there is even the slightest error by the Contractor in installing the pipes or the manhole structure. If that's the case, and it usually is because no one can construct a sanitary or storm sewer absolutely perfect, then you're screwed. How to correct those errors with a fiberglass manhole structure where you cannot core a new pipe portal in the wall. I can imagine that there would be some high degree of wall stress issues with that sort of field change......the necessary extra fiberglass reinforcement around the cored portal would be nearly non-existent.

Then there's the issue of providing the requisite concrete trough and benches, especially in sanitary sewer manhole structures. Sometimes these are also required in storm manholes, but that requirement is usually based on hydraulic and sediment load concerns. Personally, I don't like them in storm sewer structures because I want the sediment to settle-out in the manhole and catch basin sumps rather then in the storm sewer pipes themselves.

After design sanitary and storm sewers for nearly 35 years, you can count me out specifying T-Base manhole structures. I just don't like the problems that they present during construction. And don't forget the large Contractor Change Orders that would result in their use!

===Signed,

CaptMoosie, PhD, NYS P.E.

Civil, Structural and Environmental Engineer

The Tindall T-Base manholes are pre-cast concrete. They do not have to be anchored and buoyancy is less of a problem than round, because of smaller volume compared to empty space.

Sorry, I don't have any knowledge regarding the Tindall T-Base manhole structures. Therefore I cannot comment further regarding pre-fabricated fiberglass manholes and their usage. My previous statements about them still stand, especially in regard to field corrections.

Regardless of what structure is used, a buoyancy calculation should be performed to insure that the requisite Factor of Safety is obtained, especially in high groundwater areas. All too man times I have seen manholes that have "popped-up" out of the ground due to high groundwater levels or as a result of flooding.

I'd be a tad worried about a small interior size, as it affords very little working room inside of the manhole for sewer dept. personnel. Here, the minimum acceptable manhole ID is 48" for small diameter sewers.

Yes, agreed that only use of round manhole covers and frames. It's virtually impossible to drop a round cover down into a manhole or catch basin, unless of course it's been beaten to death and broken with a vandal's sledgehammer! Anyhow, most of the time vandals/thieves will instead steal the damn things to scrap metal yards for a quick buck!

There is plenty of room to move around in the manhole. The risers are standard diameter. It sounds as thought you are not familiar with the product.

I interned at a pre-cast manufacturing plant and better than 95% of the manholes we made were round. We made a few box bottom manholes, and we might have produced 1 or 2 T's in the time I was there.

From a manufacturing point of view, the rounds are easier to produce, and you can provide more flexibility to the end user as far as the clocking and heights of the inlets and the outlet. This same flexibility is seen in the box bottom, but these were larger units for special projects. The T's were fairly difficult to produce - they required 2 separate casting processes.

As far as use in the field goes, I would guess that round manholes would be allow more flexibility and be cheaper. Ultimately I would say it's the engineer's preference.

The round manholes are cheaper to produce in small sizes, there is no argument. However, I am discussing 48-inch diameter gravity sewer mains, entering 12-foot diameter manholes. The cost to produce round manholes is twice the cost of a T-Base, plus the invert can not be cast in place, because of DOT weight restrictions. Also, we require the holes in manholes to be cast in or cored at factory. If the design is correct, there shouldn't be any concerns with "adjusting" things in the field.

Precast concrete manufacturer's can also produce square and rectangular sanitary & storm manholes/catch basins. They are not limited to just the round ones.

Typically, the maximum required pipe O.D. = 0.707 precast concrete manhole/catch basin ID, but structural and fit considerations must be taken into account, especially in regard to close pipe "clock" locations. Of course you have to take into account the cast pipe portal diameter for field adjustment and "wiggle room".

IMO, that 12-foot structure dimension seems a bit of overkill just for a 48-inch nominal pipe size, even if it's HDPE or RCP, unless there are a multitude of pipes entering and leaving the structure and they are in close proximity to one another.

And you are correct, I am not familiar with the manufacturer of the T-Base structures you mention. I did state this previously.

i'm not familar with t-shaped man holes, but most men are are basically round. i don't see the advantage of a t-shaped hole unless the opening was designed for a purpose other than being an access for workers. a t-shaped "manhole" may be designed to be used for specialized equipment?