cesspit - sorry to lower the tone

[shobo]

Good afternoon.
I've had this cottage for a year now.
Years ago - don't know when - maybe mid 1980s - the outside toilet was capped off and its little hut converted to a sheddy type thing.
The garden slopes away from the cottage and around 20 feet behind the old privy is what looks to me like the top of an old cesspit which would have been perfectly positioned to take the doings. There are some decorative concrete or stone blocks surrounding what on closer inspection seems to be a massive concrete plug. Someone did well to lift that into place. Whoever did this made a good job of making the finished article look like decorative paving, and it wasn't obvious that there is a cesspit there.
Anyway, its in the way a bit because I want to create a base for a greenhouse there. Should I attempt to remove the blocks and the plug and fill it in, or is that asking for trouble ?! Should I just leave it alone. If I could have opened the top it would have been a useful place to dispose of loads of old stones that are infesting the garden. The concrete plug doesn't seem moveable - its massive and doesn't have any lifting point.
Any suggestions ?

 

[Flyfisher]

Photos would probably be helpful.

We have two septic tanks, both of which now have cast iron 'manhole' covers though previously one of them just had a large concrete slab covering the access hole to the main tank. Both tanks are very large (one is over 3m deep) and are constructed mainly from brick with the top being corbelled into a smaller footprint at ground level before being covered with a concrete slab with an access hole.

I'm not sure what you mean by a 'massive concrete plug'. Perhaps that's the overall top capping of tank, in which case I'd also expect there to be some sort of access hole. Either way, anything 'massive' doesn't sound like is intended for easy removal so I'd be looking for some other signs of a access point with a more easily removable cover.

Your plans for a greenhouse base implies that the tank is no longer in use and, presumably, no longer connected to anything, in which case if the structure appears to be solid enough (and it sounds as if it is!) then probably the easiest thing would be to just pour a new concrete base on top of it. A greenhouse would not be a heavy construction and a slab of 4-6 inches would probably be ample, though I'd probably lay some reinforcement mesh or perhaps some 'rebars' in the concrete, just in case there is any future movement underground, unlikely as that may be.

 

[worms]

Could you empty the cess pit out, seal it off, fill it with water then build the greenhouse on top? It could give you a really good heat-sink to even out temperature variations and frost-proof the greenhouse.