It may also be a well or a cesspool. Our house was built in the 1940s in suburban Philadelphia. I did not know that this system existed until the end of last fall (2009), when we first noticed the erosion.

We noticed that one of the cornerstones of our flagstone patio was loose. Upon lifting the piece, we shockingly noticed that there was a large cavity beneath it, extending into neighboring ground space. Removing the corner piece revealed a neighboring vertical concrete pipe that goes six feet into the ground. The pipe's top entrance appears as though it had been broken off, indicating that the pipe might have, at one time, extended upward beyond the surface. Instead, the pipe was covered by a 12"x12"X3" solid cinder block. Upon removing the cinder cover and inspecting the pipe's interior, we noticed that a cavity existed about halfway down the pipe. The cavity seems to be what is enabling the erosion.

The pipe travels about 6 feet into the ground. Photos indicate that the pipe may actually be two three foot pipes stacked on top of one another. The hole is 12' deep. The pipe is 9 1/4" in diameter and its sides are 3/4" thick, which makes its inner diameter 7 3/4". It is made of concrete and has a really thin shiny copper paint layer encasing the pipe (it does not appear shiny in the photos due to being dirty -- I rinsed off a broken piece which is shiny). The walls of the pipe are solid and nonporous. The whole appears to be filled with air only. The pipe is over 15' away from the foundation.

My guess is that this system is inactive and merely deteriorating.

In terms of fixing the problem, my thoughts are:

A) replace 6' pipe
B) somehow patch existing 6' pipe
C) fill entire hole
D) a better idea

But really I have no idea how to remedy this. Photos follow:

http://lh4.ggpht.com/_MbnCG0pH0wc/S5Lap5BGG7I/AAAAAAAAEhI/ZWfdIG5w6mU/s144/IMG_0116.JPG

http://lh3.ggpht.com/_MbnCG0pH0wc/S5LaaTB3HtI/AAAAAAAAEgo/_mTAOI2Ie5M/s144/IMG_0109.JPG

http://lh3.ggpht.com/_MbnCG0pH0wc/S5La8ODEUjI/AAAAAAAAEh4/NcbPem17gOI/s144/IMG_0125.JPG

http://lh6.ggpht.com/_MbnCG0pH0wc/S5La1Jafa4I/AAAAAAAAEhk/Wq-0Ahe6MkM/s144/IMG_0121.JPG
ABOVE: reaching down the pipe with my hand, about 1' into the pipe

http://lh3.ggpht.com/_MbnCG0pH0wc/S5La6J0QRfI/AAAAAAAAEh0/ZcOP5Qt2vRc/s144/IMG_0124.JPG
ABOVE: reaching down the pipe with my hand, about 2' into the pipe

http://lh5.ggpht.com/_MbnCG0pH0wc/S5La27QnOGI/AAAAAAAAEho/RxJUInhEEEU/s144/IMG_0122.JPG
ABOVE: reaching down the pipe with my hand, about 3' into the pipe. I basically reached my arm as deep as it could go in order to get this shot. Note that the cavity is on the left, although clearly there are cracks around almost the entire interior perimeter of the pipe.

More photos here (http://picasaweb.google.com/superjack5000/Hole)

Thanks for considering this problem!
John


The photos I included above are tiny, but you can easily see much larger versions of the same photos over at this gallery: Picasa Web Albums - John - hole (http://picasaweb.google.com/superjack5000/Hole)

Photos indicate that the pipe may actually be two three foot pipes stacked on top of one another By "stacked" do you mean end to end?
If so rhen that's how we used to run drainfields. I don't recall using concrete pipes back then, we used clay (Terra-Cotta) pipes in 3 foot lengths leaving a small space between them for the liquid to disperse into the soil.
I can't say for sure what you have but It's been abandoned and the cavity should be filled in.
Newer codes mandate that when you convert to city sewer that the septic tank or dry well be filled up and covered over.
Good luck, Tom