Would a keyway reduce the hydrostatic pressure of my pond, and, hence reduce or eliminate the leaks I'm having in the berm of my pond? Does anyone have a picture or drawing of what a keyway should look like?

This should explain everything.
http://www.cjnetworks.com/~sccdistrict/pond.htm

welcome to PB auburnbob, good to see another northern californian here.

a keyway will not reduce hydrostatic pressure. that is solely dictated by the volume of water in pond and the force it exerts on the substrate of pond.

nice link JBL.

auburnbob, think of a keyway as a core trench dug into the native surface (below the dam or berm that is about to be built)in which dam materials are compacted in lifts, and continue to be brought up above the core trench to form the dam or berm creating one continuous barrier that is effectively jointed into the ground.

it will help reduce seepage below (underneath) the dam. if yer seepage is from within the berm itself, you probably need to groom and repack the berms.

A good keyway will reduce the pressure. You can email me at mikeotto@ottosdirtservice.com and perhaps we can help you futher.

mr. otto, please educate me on this one....how does a keway reduce hydrostatic pressure?

thanks,

Educate me too. Also the rest of the forum. Please.

Here is a question for the engineers and geologists. If a certain hydrostatic pressure ( water weight) is spread out over a larger surface area (bigger dam face or dam plus keyway/core) does it reduce the pressure ( as in force needed to cause failure) on the dam?

Hmmm... Pressure we measure in pounds/square inch. Spread the weight of the same number of pounds of water out over a bigger area - psi goes down.

Hydrostatic pressure is based on the height of a column of liquid and acts equally in all directions. You could say that a dam keyway would help the dam to resist or hold a higher hydrostatic pressure. The only way to reduce the pressure at any given point under water is to reduce the amount of water above it. The height of the water column is the only variable here. Hydrostatic pressure always acts normal to a surface. Decreasing the slope on the face of a dam changes the direction of the force the water exerts on it. A shallower slope directs the force more downward. However, the area of the face of the dam also increases as the slope decreases, so that the total sum of the force is greater. Since the change in surface area is based on the sine of the dam face angle, the net result is that there is always the same amount of force trying to push the dam away from the pond, no matter what the angle. It's the product of the pressure across the depth x the cross sectional area of the valley being dammed. Looking forward to some rain in TN this weekend.

great post hillbilly.

the way i see it theo, the (dam) surface area in contact with water is the same whether or not there is a keyway. since (an ideal) keyway is buried below the interface with the pond water, there is no net change of force on that part of the dam in contact with the water. it makes the dam stronger, but does not affect the force exerted by the water.

you guys are right that if the surface area is increased the pressure is decreased, but all things being equal (same size dam plus or minus keway) a keyway itself does not affect hydraulic pressure on pond side of dam.

unless i'm missin something, which is ALWAYS possible.

This is one I like;
The volume in water makes no difference in preasure, only the height of the water.
If a 1 in. dia. pipe is right beside a 100 ft. dia. water tower and both are full to the same height the preasure will be equal.

If surface area is increased the pressure decreases, because the water will be shallower.

The way I understand it, the weight of the dam is what holds the water back. The deeper the water, the more force you have against the dam.

The slope of the dam is to increase the weight of the dam to hold back the water. You need so much freeboard above the water level to add more weight to the dam to keep it in place.

Without a keyway, the dam that is built on top of the soil can slide or move over time. Having the keyway locks the dam into the soil and holds in in place. In theory, you wouldn't need a keyway if the dam was so much taller that there would be no way of it ever moving, but that could make the dam twice or three times bigger. the keyway allows us to build a dam with less dirt.

Eddie

I'm gonna crawl into the pigpen all the way.

I agree with TNHB & D.I.E.D. that the amount (height) of water determines the pressure on the dam. But it seems like a keyway increases the vertical cross-sectional area of the dam (by going deeper than the bottom of the pond), thus lowering the ... let's say strain ... on an individual area of dirt in the dam.

To use a firearms analogy, a blowback action uses only the mass of the bolt to hold the cartridge being fired in place, thus limiting pistol-sized bolts to relatively weak cartridges (9mm Makarov about the maximum power) or requiring massive bolts for more powerful cartridges (like .45 ACP in a Thompson submachinegun). This is analogous to (as Eddie stated) a normal-sized unkeyed dam holding back a shallow pond, or a inordinately massive dam being needed for a deep pond.

But if we add a keyway to the dam (or a locking mechanism to the bolt) the same sized dam (or bolt) can hold back a deeper pond (or fire a more powerful cartridge - how .45 acp can be safely fired in a gun the size of a 1911 instead of a Thompson).

If water leaks through the dam, this has the effect of weakening the dam - lowering it's shear point. This is like using soft, weak steel on a bolt's locking ribs. A good keyway also helps waterproof the dam to prevent this.

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I think most misunderstandings (which may appear to be disagreements) we can have on this issue are the result of not correctly understanding terms like pressure and force wrt ponds and dams. I'm pretty sure I'm personally screwed up some on these words.

I see Theo picked up on my point. I agree that hydrostatic pressure is just that. You can think of it as the force exerted downward by the water in the Miss. River that keeps New Orleans from sinking in the mud. But the question assumes at least 2 types of pressure.

Hydrostatic pressure which has been well defined in the posts above. But in the context of the original question there is inferred a second type of pressure. Theo called it Shear pressure , I used dam failure pressure. I think Otto was talking about a keyway/core strengthening the dam and thus reducing the risk of a second type of pressure event i.e. reaching the breaking point pressure of the dam. Better stated as -- a core/keyway will reduce the chance of shear pressure being reached and exerted on the dam or it will reduce the pressure (chance of a leak/break) on the dam.

Shall we all agree that the core does not reduce the pressure, but reduces the chance of detrimental action of the same pressure on the base of the dam?

excellent discussions theo and ewest.

semantically though it still appears in my thick skull like the (ideal) keyway strengthens the ability of the dam to hold back water (increases resistive force), but by itself does nothing to change the force of the water on the dam which is a function of how much water is there and the surface area on which it acts.

so perhaps we are speaking/defining net force....if the resistive force of dam is increased (via having a keyway) the effective or acting hydraulic pressure is reduced even though the absolute hydraulic pressure remains the same....how about that?

sorry, just felt like beating a horse this morning did we scare off auburnbob?

...way to go, guys; we lost another one.

I'd say auburnbob got his money's worth on that one. I'd like to know his ip address.

If you dont want to know the answer looked at as a thousand points of light, in 10 different dialects and/or languages. Dont ask.

Keyway good, dam failure bad.

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oh brettski....i'm glad i had a glass of water nearby, darn near joined him, death by laughter.

i sure hope you have a good sense of humor mr. auburnbob....we really are nice gentle people...

edited post.....we are....arent we?

Ahh, that's the way I like it. Just give me the bottom line!

Brettski, what are you doing with my cat?

Auburnbob, Auburnbob, I sincerely apologize. My friends just have 'cabin fever'.

Someone sent me that picture with the caption - Last known photo of Saddam's cat.

Now we have the cold hard proof. Brettski I knew all along that you were the low down pirate that stole the Furhead Wheel Chock from Dwight. But did you have to send a good old American Calico Cat to Saddam. I guess ole Furhead got her revenge by jerking the chair out from under him and swatting at his feet. He should have never tried to force Furhead to eat those sand rats.

ROFLOL !!!!

Auburnbob,

I sincerely appologize for my fellow forums members. All you did was post a well forumed question with the expectation that you would receive an informative reply. And what happened? Your post was hijacked.

Please come back Bob we need more California members.

As for the rest of you consider yourselves reprimanded for callously hijacking Bob's post. There are a few members here like myself that NEVER hijack posts with doctored photos or off hand comments. You should feel shame and discomfort.

Speaking of discomfort that reminds me of a joke..

Two Redeared Sunfish, a Nun and a monkey go into a bar...

Thanks Jeff.

It does appear that my posting was hijacked.

There is some good information about force vectors, etc. from TN Hillbilly ... and, ewest about the value of a keyway/core trench (i.e. reducing the chance of sheer pressure being reached ... and chance of leak/break.

My leaks are through the dam (built without a keyway by the previous property owner). At this point, I think my options are: a) drain the pond and rebuild the dam with a keyway (expensive); b) line the pond with a pond liner material (expensive ... over $30K!); c) keep adding french drains to vector the leaks back to the spillway creek (moderately expensive - $5-10K); or d) do nothing and hope the leaks through the porous dam eventually seal up.

- Bob

Bob :

If you can id where the front side leaks are you could do a front side patch with good clay and some bentonite added in to the clay. This could be done with the pond lowered but not drained all the way. If so you may be able to save the fish. But it depends on iding the location of the leak. Since we high jacked the thread the least I could do is find a few similar threads below.

Fix it or forget it? http://www.pondboss.com/cgi-bin/ubbcgi/ultimatebb.cgi?ubb=get_topic;f=2;t=000105;p=1

Soil tests in excavated pond http://www.pondboss.com/ubb/ultimatebb.php?ubb=get_topic;f=2;t=000140#000001

Pelletized Bentonite vs. granular? http://www.pondboss.com/cgi-bin/ubbcgi/ultimatebb.cgi?ubb=get_topic;f=3;t=000050

Repairing Dam leaks from the backside http://www.pondboss.com/cgi-bin/ubbcgi/ultimatebb.cgi?ubb=get_topic;f=3;t=000079

Water leaking under Dam. Pics http://www.pondboss.com/ubb/ultimatebb.php?ubb=get_topic;f=3;t=000081#000003


Agacultural Lime to stop leaking? http://www.pondboss.com/ubb/ultimatebb.php?ubb=get_topic;f=20;t=001716#000003

Pond Leaking should I worry? http://www.pondboss.com/ubb/ultimatebb.php?ubb=get_topic;f=20;t=001522#000001

I think I am gonna jump off my dam - sprung a leak http://www.pondboss.com/ubb/ultimatebb.php?ubb=get_topic;f=20;t=001709

Auburnbob,
As the head buffoon, charged with master-minding the hi-jack, please allow me to offer a little help with additional core input and some pix. I started this thread when I ran into some issues with core application and execution. Core (keyway) thread

I think that draining and putting in a good core could be your least expensive of the three choices you mention.

is the base of dam/berm leaking at all, or are all leaks emanating at various points up on the berm?

as i mentioned in my first post, if the leaks are up in the berm (and not beneath it), you have another less costly option of simply repacking and grooming the inside side of dam face. non keywayed dams/berms can work fine in many situations depending on the geology etc..

also, i've seen several non keywayed dams that seep slowly at the base, but not enough to cause fatal summer drawdowns, particularly if you have supplemental summer water (i.e. small creek, spring, well etc.).

Thanks to Eldorado dave and ewest for some good info.

I believe the leaks are through the dam/berm ... at about a level five to six feet down from the top of the dam/berm. When I lower the pond level by five or six feet, the leaks slow way down and/or stop.

When I visited the website (http://www.cjnetworks.com/~sccdistrict/pond.htm)that JBL directed me on Jan 9th, I noticed that Ref. line #3 states that lack of a core trench (keyway) is the major reason why ponds leak. However, ewest's comments make alot of sense about the purpose/value of a keyway ... namely, to tie the dam to the soil substructure and prevent shear pressure from causing a dam failure.

I've already had the dam's surface packed with bentonite (one ton's worth) ... and, I added french drains at the points where the leaks emerge from the back side of the dam/berm. Now the leaks are occuring closer to the foundation of my house ... as we all know, water (under tons of hydrostatic pressure) will find a path of least resistance.

I've stopped working on the leaks for the winter. If I bring the pond level back up to full for the summer (so I draw water from it to irrigate my pastures) and if the leaks are still there, I may add more french drains near the foundation of the house to vector the water back to the spillway creek.

Later,

- Bob