Mike,

No experience with that but when has lack of experience stopped an old man from giving advice...

I suggest you go to the library and view a copy of "The complete concrete, masonry, and brick handbook" by JT Adams. ISBN 0-668--04340-7

In my copy, a short Chapter 13 (of course) is dedicated to underwater pouring using three different methods.

The book has a nice drawing of the tremie method and the book will provide a lot of information on cement.

Method one is the the Tremie method where you make a form and then use a large pipe with a funnel to fill the form from the bottom upward in one smooth pour.

Method two is the bucket method where a bucket with a drop bottom is used. This is for deep situations.

Method three is the sack method. Used for emergency to slow and perhaps stop a large leak. A 1 cu. ft. jute sack is filled 2/3 with wet cement and are placed on the leak in a layer followed by a second criss cross layer and so on.

The book makes notes that current needs to be less than 10 ft. / minute for best results.

For a sack method, I would use a "hot" cement mixture of a half bag of portland and four five gallon pails of small grain clean sand. Use very little water to get the mixture stiff but plyable. This will be a strong mortar that should be very moldable and should allow you to pack the sack diretly on and around the crack. When dry, it is a water proof mortar.

Of course the Tremie method with forms is best and you can use a standard concrete mixture with gravel with this method.

Building forms for an angle pour is tough so I
would try the sack method to slow the leak down to a point where benonite could be used. I would use small bags (cut a gunny sack to fit your need). Perhaps even a sock or nylon hose for a very small crack?

I would clean the existing cement and crack with a wire brush and try to pour a bucket of cement directly into the crack pack as much mortar into the crack by hand. I would then quickly follow with a bag of mortar directly onto the crack.

If the flow is reduced enough, I would followup with benonite. If the flow is still too high for benenite, try the sack method again.

It should only take four hours of total time to try the quick fix method.

If the quick fix does not work, try building a metal form and use sandbags around the form to hold it in place then use the TREMIE method to fill the form. I suggest using tin, wood strips and screws to build the form. You may have to be creative and make the forms with flaps that you can load with sand bags to help keep the contrapion in place.

Regardless of the method used, after you stop the leak, you may want to consider excavating around the drain pipe after everything drys out. You may need to pour a anti-seep collar or cement "ring" around the stand pipe to prevent a straight shot for water to flow along side the stand pipe.

WARNING: There may be some large voids in the dam so be very careful moving anything heavy across the dam. We almost turned a back hole over and into a pond when a fresh beaver den collapse while driving across my dam. The beaver escaped and I was carrying a pistol, I guess I was too busy praying to get a good shot off.

A post hole digger may be a safe way to determine if there are any large voids.

Please let this site know what you do and how it works out. I am sure there are a lot of leaky overflow pipes.

Dennis