Manhole Buoyancy

[JDWright]

I am specifying some relatively deep 5' diameter precast concrete manholes (25'-35' deep) which are located in a floodplain (will be fully submerged during floods) and thus they will have water tight lids with MH rims at existing ground level. I have ran the standard buoyancy calculations and extended the MH footings base lip by 6" to provide for more total weight (concrete and soil weight) to offset flotation with a FS of 2.

My more theoretical question is why does the upper portion of the manhole not float up from the base? For all the precast details I have seen, there is never any mention of needing to ensure that there is not separation from the base section with the upper sections due to the water tight lid. Sr. engineers around here have no idea. All the models for analysis seem to show manholes that are assumed to be open at the top and tightly sealed at the base with the uplift buoyancy forces acting on the underside of the footing. Would a water tight upper portion of a MH float up independent of the base causing separation of the precast sections (or at least causing separation of the shallowest top section to relieve the pressure?

 

[nackra]

I had a project where we used SS straps with Hilti (Redhead) anchors to bolt sections together.

other references can be found here:

http://www.concretepipe.org/pipe-box-resources/design/design-data/

 

[cvg]

the uplift goes through the centroid of the structure which should put it through the middle of the base slab. so the upper rings would not float.

 

[ashtree]

How many sewer systems are so tight that during a flood the manhole will be empty? Even if you put a flood proof manhole cover on the manhole and the precast concrete joints are totally sealed there is likely to be enough ingress points elsewhere to ensure that the manhole will have a considerable amount of water inside. This may not totally answer your question but it certainly contributes to the lack of buoyancy.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[bimr]

You are over thinking this. Unless the manhole sits in the flood for a very long time and the entire 25 feet liquifies, the soil friction alone will hold the manhole. Have you ever tried to pull a driven stake out of the ground? Difficult because the soil friction prevents it from being pulled.

See the link for a discussion:

http://www.concretepipe.org/wp-content/uploads/2014/09/DD_41.pdf

 

[cvg]

photos of manhole and tank which have floated due to hydrostatic uplift pressure.

 

[ashtree]

I may be wrong but is that in a location close to the sea? No mention of the depth of the manhole or the connecting sewers?

In some places soak-age or infiltration from the sea can be problematic when the whole area is sand causing empty septic tanks to become buoyant. But a septic tank which might be 5 foot in the ground is a very different matter to a 25 foot deep manhole.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[bimr]

Sure, great pictures of structure floatation. The gravity sewer or force main pipe in the first picture also seems to have floated which indicates it is probably very shallow.

However, these are different arrangements than a 25' deep manhole.

Regardless, the concrete pipe formulas covers these applications as well.

 

[JDWright]

After much exploration and even building a scale proof of concept model, I have made some conclusions.

The important key here is the joints of the precast sections should be water tight as necessary for the depth (about 15 PSI for a 35' deep submerged manhole excluding air pressure). If the manhole experiences failure of a join seal, there is a potential for the mechanism of buoyancy to move from acting externally on the manhole to working internally on the column of air. The manhole acts as if there were no bottom to it. When that happens, if the lid of the manhole is watertight, there will be internal upward lift acting on the manhole. This internal force could lift the manhole, breaking it apart vertically and the precast sections, if weight and soil friction are overcome.

Most 'watertight' manhole lids will release internal air at pressures great enough to lift the lid. However, if the lid is both locked and watertight, the rate of release will be greatly reduced, if it happens at all.

For my specific design application, based on total weight, side friction, factors of safety, the pressure rating of the rubber joint material, and the buoyancy forces, I am not concerned with internally acting uplift. However, there are situations when this internal acting buoyancy force mechanism could be an issue and I am surprised that more design manuals and guidelines don't at least touch on the subject as it pertains to deep manhole applications.

This are my thoughts on the matter. Your guys insight is welcome. Is there any other major thing that I might be missing in the mechanisms of buoyancy in this application when looked at from a physics standpoint?

 

[ashtree]

With 15 psi acting vertically on the horizontal surfaces(basically the lid and its surrounds approximately 20 ft2) you only have about 300 pounds of up lift force. I doubt if that is enough force to break the manhole apart given the mass of most of the components and the sealing compounds applied.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[ashtree]

The other thing is I doubt that manholes are in practice that air tight, otherwise every time the flow was high in the system and the level in the manhole started to rise it would pressurise and prevent further increases in depth in the manhole. This does not happen in reality. Manholes that are well constructed are generally water tight and are probably somewhat air tight but i have doubts about your supposition.

As bimr said above you are overthinking this. Sewer manholes are a very mature technology and the basic design and installation principles well known and proven in the field. See bimr's link above.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[JDWright]

Thanks for the responses guys.

I am over thinking things...as an engineer its my job! And its also a fun thought experiment. I don't ever just *accept* what I am told without fully understanding to the best of my ability. My questions dont apply to basic design and installation principles, Im asking and thinking about the overall physical forces in basic and complex design cases.

Manholes are typically tested during construction for water tightness (fill them and they cant leak water after some specified time) or air tightness (pressurize to 5 psi for some specified time). Over time, the integrity of the manhole can be expected to diminish. Additionally, most your typical manholes do not have water tight locking lids, thus are the issues I am thinking about dont really apply in most situations. However, my basic questions remain...

I am not sure I agree that the uplift forces are only 15 psi (this is hydrostatic pressure at 35' deep)...in what plain do the buoyancy forces act upon (~45kip for 5' 25' deep manhole)?

 

[bimr]

The buoyant force is equal to the weight of water displaced by the manhole structure, not 15 psi. This force is defined as the density of water multiplied by the volume of water displaced by the structure.

Don't understand why you are not using the concrete pipe manhole floatation methodology. What is the objection?

If you think the manhole sections are susceptible to separating, an inexpensive solution is to strap and bolt the sections together.

 

[JDWright]

Bimr, I am using standard methodology and have no objections to it at all, I am just curious as to how the forces are acting on the structure in various "theoretical" situations. Everyone mentions buoyancy forces, but all I am asking is if a structure has no bottom (say, due to a joint failure of a lower section of the manhole), and is just sitting in the water as a tube of air trapped in a column, are the buoyancy forces then acting through the top of the structure (on the manhole cover)? If that theoretical structure had a safety valve (a ring and cover), how much pressure would it experience? I believe that there could be situations when the manhole could separate, and it is that phenomenon that I am trying to explore. The standard methods don't talk about this at all. Its also somewhat applicable in real world situations, because manholes are tested at 5 psi, but could easily experience hydrostatic forces 2-3-4 times higher then that if deep enough during a flood event. The standard rubber gaskets used are rated at 15 psi and thus could leak at deeper sections of the manhole. If the leak is significant enough with enough infiltration, the buoyancy forces could then start acting internally on the manhole. Id love to know when straps are necessary and why, if at all. Instead of just guessing.

 

[cvg]

manhole lids are typically vented with a pick hole. your column of trapped air is unlikely to happen. leakage through a joint would happen quite slowly (unless caused by earthquake or other force or impact) so I just dont see a realistic failure mode. a 35 foot deep manhole, fully submerged could only see a maximum hydrostatic of 15 psi, so your gaskets are adequate. suggest if you were truly worried, provide better gaskets

 

[JDWright]

CVG, I agree with your analysis. The situation I'm exploring would have water tight lids. These are good to about 20 psi. Failure of a joint would be slow and rare.

Again, this is a mental exercise and exploration..

I created a little model:

https://youtu.be/IM0gyPWD1jE

 

[ashtree]

There is also the soil friction discussed earlier acting against the buoyancy effect that will help prevent the sections separating.

I am not going to say that it has never happened but i would suggest that it is a non issue.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[bimr]

The typical concrete manhole is not suitable to be installed in water. For that application, one would have to use other materials and methods.

Your model does not include the sliding resistance nor the weight of soil sitting on the bottom lip.

If you bother to do the calculation, you will see that the manhole sliding resistance alone is so large there is no concern with floatation.

 

[JDWright]

Lets try this again. lol.

Assume, theoretically, that a deep, precast concrete manhole with a water tight lid is designed based on all standard methods and criteria (following DD-41:

http://www.concretepipe.org/wp-content/uploads/2014/09/DD_41.pdf).

A lower lip on the base is even added to be extra safe.

But, the assumption I am trying to explore here is that, based on standard calculations, the buoyancy force is assumed to always act EXTERNALLY on the manhole. What if the forces acted INTERNALLY(as I assume they would if the manhole had an open bottom).

My model shows that it is possible for the mechanism of buoyancy can act internally if there is a joint failure (such as when the gaskets experience greater than 15PSI pressure, or age, or whatever). Once that failure takes place and water leaks into the manhole filling it up to that failure point, the buoyancy force of the ENTIRE remaining internal column of air above that failure location will be acting vertically on the TOP section (cone or flat). This means that the top section(s) could float off independent of the resisting weight (concrete and soil), and side friction of anything below. If this were to happen, all the standard design guides are mute. However, this type of analysis (even to say that it is not possible or something only to worry about when X happens) is no where to be found in the literature.

I am looking for physical calculations and explanations why this could, couldn't, or doesn't happen. Maybe I am understanding the physics wrong?

This is a thought exercise. One where I am exploring the possibilities based on physics.

 

[bimr]

Over two millennia ago, Archimedes of Syracuse developed the Archimedes Principle, which states, “any object, wholly or partially immersed in a fluid, is buoyed up by
a force equal to the weight of the fluid displaced by the object.”

A buried tank can fail due to the buoyancy force, when the groundwater exerts more pressure upward on the underside of the base slab.

A closed bottom tank would have more buoyancy force than an open bottom tank because the air and water are separated.