Drainage in manhole when watertable is high? 1

[Redacted]

Hi there,

I am working on a manhole that is below the water table.

The client is asking if we can install some drainage inside the manhole but I am thinking that if we put a drainage hole at the bottom of the manhole, there will be water ingress in the manhole up to the water table level.

How is drainage in this case normally dealt with?

I could potentially just tell the client to make it water tight but I would want some reasoning behind that statement.

The manhole is being used to transition subsea fiber cables to land. If I make it watertight, I would also need to make the hole around the cables watertight as well. I guess an additional question whilst I'm at it. Would these be adequately watertight if I just have them cast in the concrete or would there need to be some form of water tight gasket seal or something?

Any advice would be appreciated.

 

[canstruct12]

Hi, I have worked with underground chambers that required to be dry, we installed exterior drainage to a rock filled sump pit with a 4" pipe with a back flow preventer (water could only flow out). This functionally did not work as the water table was higher than the sump, and some of the back flow preventers malfunctioned and the chambers filled.

We used "linkseal" gaskets to seal all other penetrations entering the chamber. These work very well if they are installed correctly (I believe each link needs to be hand tightened (i.e. no impact gun)).

 

[Ron]

Make the manhole watertight. Have dealt with that before in my area where the water table is only a few feet below grade. Coat the exterior of the manhole with a good waterproofing mastic and make sure all penetrations are watertight. Yes, you will likely need gasket seals.

 

[Redacted]

Thanks Canstruct12, your comment is very helpful.

When you say you installed exterior drainage, do you mean with a mechanical pump, pumping the water to the sump pit?

My structure's foot print is quite small but I am guessing a similar issue to what you encountered.

See below

The link seal gaskets look interesting.

So, are you saying that you tried to originally install drainage with your structure but that didn't work, so you made it water tight?

If my 4x4" pipes were cast into the concrete when the manhole walls are getting constructed would the surrounding concrete provide a water tight fit, or would I need a product like what you mentioned (linkseal)?

 

[Redacted]

@Ron thanks you posted the same time as me

Yes, I estimate the water table is around 4' below grade. The location for this manhole is about 15' or so from the ocean. Do you have any recommendations for waterproofing mastic. Is this just to be applied on the walls? What is normally done for the underside of the footing?

On another note, this has me thinking now. What is the general practice for constructing a small structure like this within a water table?

The manhole is only 8'x8'. How is this normally done, would the contractor need to put sheet piles down, pour a concrete plug and dewater? Is there a cheaper/easier way?

 

[SlideRuleEra]

1) The client is asking if we can install some drainage inside the manhole...

2) ...I estimate the water table is around 4' below grade.

3) The location for this manhole is about 15' or so from the ocean.

4) If my 4x4" pipes were cast into the concrete when the manhole walls...

5) What is the general practice for constructing a small structure like this within a water table?

1) When I first read that, I thought your Client was "nuts"... based on what you have revealed since, maybe not.

2) More important is how much it varies (i.e. higher).

3) Relative elevation of the manhole invert and mean high tide is more useful.

4) Are these 4" conduits (PVC or metal), with the fiber optic cables inside the conduits? Don't fool yourself, if ground water is high enough, ground water can flood the manhole by flowing inside the conduit... happens at our generating stations on a regular basis.

5) Part of a Contractor's "ways & means", a cofferdam is the most expensive and almost certainly unnecessary for what you describe. "Best" way will depend on answers to factors above.

http://www.SlideRuleEra.net

 

[Redacted]

@SlideRuleEra Hi SRE, thanks for your comment, I'm always happy to see your comments on my posts , they are helpful and appreciated.

2+3) I agree, knowing the variations would be helpful, although as you probably know by now most of my projects that I get given hardly have enough information with them...

4) fiberoptic cables will be inside rigid PVC conduit sleeves

5) For discussion's sake, let's assume that the low tide still has the water table above the manhole location, so the contractor won't be able to wait for low tide to form the rebar cage and pour. What would be the construction approach for this? I agree, the cofferdam approach seems complete overkill but I am not too familiar with any other way of doing construction in water. Would you provide some guidance? (Although I am not the one coming up with the construction methodology, I want to make sure it can work and have an idea of how it would be constructed for all designs that I do before issuing to a client).

**The manholes will be installed in 3 locations. Only 1 is in the water table. For the locations that are not in the water table, I can add a form of simple drainage. Would a grated hole in the center of the manhole with the floor pitched to the drainage hole be fine for an application like this (no borehole or anything)? Essentially water would just drain back into the earth. What is a typical detail for this, or where can I look to find one?

 

[canstruct12]

Hi Redacted

Sorry for the confusion, hopefully my explanation below clarifies.

The chamber was installed with a small 8" deep depression (sump) in one corner of the concrete base. The chamber floor sloped towards this depression to collect any water in the chamber.

In the depression in the base, a 4" pipe with a backflow preventer penetrated the wall (just above the bottom of the depression) with a linkseal gasket. This 4" pipe lead to an adjacent rock filled pit lower than the base of the chamber.

The intent was that any water that collected in the chamber would outflow to the adjacent rock filled pit outside the chamber.

Unfortunately the water table was higher that the rock filled pit, which made the gravity flow outflow useless (similar to the situation you have I believe)

The chamber was also intended to be water tight, but it was installed over winter so the peel and stick sealant around the chamber joints (precast chamber) did not work as intended (it was too cold and brittle to effectively compress and seal)
As well the backflow preventers in some of the chambers malfunctioned allowing water to enter that way (was told later that they were cheap ones).
Finally, the contractor tried to grout in the linkseal gaskets instead of tightening them to seal around the penetrations.

We explored the idea of putting a mechanical pump, but in our case it was not feasible due to power availability and the client felt maintenance would be an issue (township with limited resources).

In the end, the backflow preventers were repaired and all joints were sealed by drilling holes at the joints and injecting a expanding sealing foam (can't remember the product right now).
The water tightness was verified by completing infiltration/exfiltration testing. you may want to consider doing this, it is a good way to ensure the contractor delivers a water tight construction if that is what you have specified ( in our case there was an allowable infiltration rate based on the specification we referenced - something like 0.3% of the chamber volume in a 24hr period)

Casting a pipe into your manhole may work. The only concern I would have would be ground settlement or movement due to freeze/thaw (not sure where you are located, always a concern in canada). I believe the benefit of the linkseal is that they can be re-tightened.

I hope this helps.

 

[SlideRuleEra]

....contractor won't be able to wait for low tide to form the rebar cage and pour.
...cofferdam approach seems complete overkill but I am not too familiar with any other way of doing construction in water.

In open water, a cofferdam is the only way I know of... but for construction on land with high water table there are options.

As a former bridge contractor, the first option I would consider for a "shallow" excavation, such as this one, is to start excavating. Use diaphragm pumps (mud hogs) to attempt to control water inflow. If this works, shape the excavation so that the pumps can use a "low spot", just outside the work area as a sump. For this manhole, 15' from the ocean, doubt this will be successful - inflow probably too great.

Second option would be well points, but if the excavation/pump attempt did not work, well points probably will not either - just can't handle large inflow.

Third option... retract my statement about a cofferdam not being necessary and design/construct a (ridiculous) 3+' deep (at low tide) cofferdam. The cost (relative to the cost of manhole) will be astronomical.

There are other ways to work below the water table, deep wells with submersible pumps, bentonite slurry cut-off wall & ground freezing, but those don't apply here.

About your question on drains for manholes above the water table... I can't answer that. For all of my work, manholes were partially or completely in the water table.

Time for some "Dutch Uncle" advice... IMHO, this project, the way proposed/described, is absolutely absurd:

A cast-in-place manhole, at edge of the ocean, below low tide elevation.
Water table elevation, unknown... just a WAG presented in this thread.
Invert elevation of the manhole, unknown.
Elevation of mean high tide, unknown.
Tidal range, unknown.

These questions need to be answered before proceeding.

http://www.SlideRuleEra.net

 

[Redacted]

@Canstruct12 Thanks that does make it a bit clearer. Did you do something like this (see sketch below)? If so, how would you maintain the rock-filled pit in case it got clogged up

@SRE thanks for this information. The construction will be on land with a high water table, although the land is close to the ocean.

I agree that the cost of a cofferdam would be significant. There is the option to potentially put the manhole further up the hill, which will reduce the water table height. I will recommend that the client do this.

Ok, so all of the manholes that you have designed/built were water tight? Do you have any advice/tips on common mistakes to avoid when trying to have a system waterproofed.

If the structure will be cast concrete 10" thick, what would the purpose of a mastic exterior layer be?

As for the dutch uncle comments

- Yes the water table elevation is pretty much a WAG. The client had done some trenching in the area a while ago and they have photos showing the water table in a 4' deep trench, see below.

- When you say invert elevation are you referring to the cable invert elevation?

- You are correct, high water and low water elevations are unknown. When I designed the manhole, I designed it so that the water table was the full height of the tank (essentially submerged). I sized the manhole for this buoyancy force. This is quite conservative but as highlighted in the thread, there is a lack of information. Are there any other considerations that I should be taking in mind in a situation like this?

Would you also be able to provide some insight into water ingress from the rigid PVC sleeves? How did you solve this issue?

 

[SlideRuleEra]

1) ...manholes that you have designed/built were water tight?
2) There is the option to potentially put the manhole further up the hill...
3) When you say invert elevation are you referring to the cable invert elevation?

1) None were water tight, nor was any special attempt made to make it so. The manholes remain filled with ground water that is pumped out when access is needed. Water table at the generating station I have in mind is assumed to be at the surface of the ground... which is exactly right several times a year, it's a one square mile site at the edge of a swamp.

2) A hill? If the manhole can be moved away (vertically) from the ocean, then simple excavation / mud hog pump dewatering is a real possibility.

3) The invert of a manhole is the lowest spot on the floor of the manhole.

Other considerations? What happens to the manhole and it's contents during hurricane storm surge (I know from your other thread that Cat 5 are possible)... which could put several feet of water over the top of the manhole? Maybe not a problem, but something to address.

Also, accelerated corrosion (all the time, not just after storms) from being near the ocean. Think, rebar cover and any parts, equipment, or structure made of steel.

http://www.SlideRuleEra.net

 

[Redacted]

Hi @SRE

1) Ok, that is an option as well. Just leaving it partially submerged constantly. I'll run that by the client as an option to see if they would agree to that. Only issue I am thinking of is mainly an environmental one. I don't know if that would be a breeding ground for mosquitos etc?

2) Well kind of a hill, it's on a road that is on an incline.

3) Ok thanks that is helpful. The invert would just be the height of the manhole 6~ and road surfacing on top so, maximum 7~.

That's an interesting question and one that I was wondering as well, as in, does storm surge cause any effects outside of flooding(or elevate the effects of flooding>). I think because I sized the manhole with the water table at the top, as in displaced volume is the full volume of the manhole, storm surge shouldn't be an issue. My understanding of buoyancy is that if you design for the water table at the top, that is the worst condition as buoyancy force wouldn't increase past this point? I could be wrong there though so if anyone can chime in on that comment, it would be helpful.

Yes, accelerated corrosion is definitely an issue, so much so that it is stipulated in the local code that all rebar used is to be galvanized.

 

[SlideRuleEra]

Redacted - If the manhole can be raised higher in or above the water table I would waterproof it. We had no reason to because of our site conditions and the high reliability needed for a generating station. In our case, if "something" (manhole flooding) can happen, assume it will.

Slope of the "hill" is not important. Additional elevation of the manhole invert above sea level is what helps.

Speaking of invert. The invert is not a "measurement", it's the elevation of a specific "spot".
I would rephrase your sentence to something like: "Top of the manhole is 6 feet above the manhole invert; road surfacing is 7 feet above the manhole invert." Invert elevations are very important for design of gravity pipelines. If there was one drop of water, (theoretically) the invert is where it would be found.

Here is a paper on manhole flotation by the American Concrete Pipe Association. I have not read it recently, but it has useful info.

Some tips I used for flotation calcs:

Rely only on the dead weight of concrete, no covering soil weight or soil friction. You have no control about future soil covering and even if you did, hurricane surge can remove it in an instant.

Of course use the appropriate safety factor for flotation.

For buoyancy calcs, use 145 lb/ft[sup]3[/sup] (or less) for the weight of concrete.
The commonly use weight of 150 lb/ft[sup]3[/sup] is conservative for calcs like dead load. Buoyancy calcs are the opposite. You are relying on the weight of concrete to prevent flotation. Weight of 145 lb/ft[sup]3[/sup] is more realistic.
Note: I prefer 144 lb/ft[sup]3[/sup], it's divisible by 12... easy to make a mental sanity check on calcs, 4" thick concrete weighs 48 lb/ square foot.

http://www.SlideRuleEra.net

 

[Redacted]

@SRE thanks for that document it is very helpful and I've added it to my resource library. I had done the tank design using the PCA methodology. For buoyancy that does use 145lb/ft3.

I note in your linked document : "American Concrete Pipe Association" that they say you can use a factor of safety of 1, when considering only the dead load from the concrete weight.

I tend to agree with this.

On another note, I've always tried to wrap my head around applying safety factors to worst-case scenarios.

For example, as stated above, I calculated the displaced water volume as the full volume of the tank (worst case). I don't think it can get worse than that. So in situations like that, what would be the point of applying safety factor greater than 1?

Also noted with thanks about the invert terminology.

 

[SlideRuleEra]

I calculated the displaced water volume as the full volume of the tank (worst case).

Depends on what you mean by "full volume of the tank".

Talking about only the buoyancy safety factor based on the tank's dead weight.

If you mean the total volume of all the concrete plus the total volume of all the air in the tank, that's right.

Take a simple example, a 3' x 3' x 3' cube of concrete that is hollow - an air-filled 1' x 1' x 1' hole inside the concrete cube.

26 ft[sup]3[/sup] of concrete that displaces 27 ft[sup]3[/sup] of water when submerged.

The buoyancy safety factor is weight of the 26 ft[sup]3[/sup] of concrete at 145 lb/ft[sup]3[/sup] divided by weight of the 27 ft[sup]3[/sup] of displaced water at either 62.4 lb/ft[sup]3[/sup] (fresh water) or 64.0 lb/ft[sup]3[/sup] (sea water). For your project, suggest you use 64.0 lb/ft[sup]3[/sup]

If my math is right, I get a buoyancy safety factor of 2.18

It's just Archimedes' principle.

http://www.SlideRuleEra.net

 

[Redacted]

@SRE

Yes, we are on the same page.

When calculating the buoyancy, I included the full volume of the concrete and the full volume of the hollow portion within the tank.

Weight of the tank is 29570 lb and buoyancy force was 27588 lb.

This results in a safety factor of 1.07 > 1 therefore OK.

The above calculation neglects any favourable action from the soil (friction and deadweight etc).

 

[SlideRuleEra]

Sounds good.

http://www.SlideRuleEra.net