Tank will be at least 40 years old, may be a lot older. The outflow chamber is connected to a 110' long outflow pipe laid in 1" gravel parallel to, but not connecting with, a ditch. The pipes are about 1' long, laid end to end but not sealed or cemented together in any way. They are grooved on the outside, and look as if they are porous rather than glazed, so presumably the waste water is meant to pass through their walls as well as down them, into the gravel and thence into the ground and eventually the ditch.
Rodding indicated a blockage a about 60 feet. We were enormously lucky to find the far end of the pipe when we were digging rather randomly where we thought the end might be, and rodding upwards indicated a blockage at exactly the same place, so we knew where to dig. What we found strange was that the far end of the outflow just seemed to end in the ground, with no pipe work across to the ditch or anything like that, and not even any trace of a soakaway. The hole we dug to expose it has filled with water and remained filled.
Digging at the site of the blockage we have managed to expose the pipe and lift one section. Again, very luckily, we managed to open up the channel above the blockage so at least the water can get away no, and seem to be soaking away into the ground very easily. Our next task is to try and sort out the blockage.
Is it possible that if the pipe was originally porous it may have lost its porosity because it has been sludged up for perhaps 20 years +? And if so, is there anything we can do about it (e.g. send a chimney brush down on the rods)? Do we need to do anything about it? We're planning to put an inspection chamber where we've opened the pipe to make it easier to rod if it gets blocked again. Should we dig a new soakaway at the bottom, and if so, how big? Are there any other observations people might have as to how to get a more satisfactory result from rather an antiquated system?