The Lemon Fool

We have a cesspit in our garden, it hasn't been used for > 40 years and is empty and appears to be in good condition. It is built from concrete blocks and has a depth of c. 12 feet and seems to be divided at the bottom into two chambers.

The manhole cover and top row of blocks protrude slightly above lawn level and since I am considering levelling that part of the lawn I am wondering whether it would be feasible to fill it in and remove the protruding blocks and manhole. I also have quite a large amount of old stones of varying sizes which are excess to requirements.

Does filling it up with stones and rubble and levelling the site sound like a sensible idea? Is there anything else I might need to consider?

TIA

RC

Hi,

Sadly not just a case of chucking rocks in, unless you want to be leveling again every 12 months as it settles. Just chucking stuff in leaves voids, and unless you seal the top with a layer of concrete then you will get settling. You will need to fill it carefully in layers and fill the voids as you go. You don't say how large it is, only how deep, so you might need to go inside it.

Firstly this is VERY deep - if you go in there (and you probably will need to) then you are working in a 'confined space' and the risk of getting stuck or consequence of collapse is severe. Do not work alone on this, have someone at hand to help if there is a problem. If the bottom is wet and muddy you could easily get stuck in there - chuck some OSB/sterling board in to stand on if it is soft and consider a harness and rope in case you need to be pulled out. Basic rule of thumb, anything deeper than waist height needs retaining walls and/or a taper at the top to stop collapse. You need to be absolutely sure the walls are safe, don't use them to climb out - get a long ladder.

I know this is sucking eggs, but i know someone who was injured just digging a 900mm deep chamber in a verge when it collapsed as he climbed out and trapped the leg still inside.

So.... assuming there are no pipes leading into it any more, it shouldn't be an issue. Sometimes old surface water pipes can run in and use it as a 'bund' - they should be a separate system but there are still plenty of instances where they are paired (as my neighbours found when their 4 kids returned home for lockdown and there was suddenly a lot more waste going in, then it rained hard).

I would remove the top few brick courses to a depth of 18 inches or so (and lay them in the hole). This means anything planted in the future is unlikely to find the brick edge, and the lack of an edge will soften the inevitable settling, as well as aiding uniform drainage of the surrounding areas. When you have removed the top courses taper the surrounding edge back at 45 degrees or shutter the soil back with some boarding fixed at the corners to prevent the soil collapsing in.

Depending upon how big it is, you could ask on Freecycle for spare hardcore or see if any neighbours are having building work done as you might need quite a lot of stuff to fill it properly. When filling get some sharp sand and once you've done a 'layer' chuck some in to fill the voids and compact it if possible - tamp it with a fencepost or something.. Then repeat.

Good luck.

Paul

Alternatively hire a digger and driver to dismantle it. Basically he will dig out the cesspit as far as possible and then put the rubble back in layers possibly with some other hardcore to bring it up to the level you want.

Access for the digger might be a problem depending on the site. Digger and driver prices vary around the country. In this area a 7'.5T digger and driver is £350 per day. He'd do it in a day. Safer for you, better job, less settlement.

I know it might seem a strange idea but if you can find a digger to do the job it will be worth it. If you don't know anyone try asking a local builder or groundworker they usually know who is about. Just ask around.

Don't try to hire a mini digger from a hire company the are toys on a job like this.

Good luck whatever you decide.

from your description , it sounds like a septic tank , rather than a cesspit.

cesspit's are sealed vessels that are usually emptied twice a year .

septic tanks are in two sections , waste solids settle in one part , leaving just fluids in the other , which just soakaway - may go 5 years or more before cleaning out is needed .

mine is covered by a heavy reinforced concrete slab.

it would probably be full of water in the wet months.

Do you need to water your garden in summer? If its a sealed tank, consider diverting your gutter downpipes into it to make a reservoir, and get a submersible pump when watering. As our climate changes to a more Mediterranean one, seasonal rainfall is going to be more common.

ReformedCharacter wrote:We have a cesspit in our garden, it hasn't been used for > 40 years and is empty and appears to be in good condition. It is built from concrete blocks and has a depth of c. 12 feet and seems to be divided at the bottom into two chambers.

The manhole cover and top row of blocks protrude slightly above lawn level and since I am considering levelling that part of the lawn I am wondering whether it would be feasible to fill it in and remove the protruding blocks and manhole. I also have quite a large amount of old stones of varying sizes which are excess to requirements.

Does filling it up with stones and rubble and levelling the site sound like a sensible idea? Is there anything else I might need to consider?

TIA

RC

Sports England specify 6F2 for use below their football pitches. 6F2 is, if I recall correctly 75mm down to 40mm. An aggregate. You won't have to consider fines in an enclosed space like this as the materials will simply not be able to migrate, held in place by the existing chambers. It will need compaction of some sort but nothing more than a small vibrating plate. Within 300mm of the top you could use a geotextile membrane and above that lay single size clean 6mm pipe bedding. Over that topsoil and lawn.

Alternatively you could construct a suspended concrete slab over the chamber. If it's in a garden and nothing other than pedestrians walking over it, it shouldn't need to be substantial. You'd have to have an engineer calculate it up for you which should be no more than £300. The advantage of a suspended slab is that once the concrete has set the steel allows the slab to become self supporting. Therefore you could fill the chamber with your unwanted rubbles, not compact it or have to worry about it sinking. If it does sink the suspended slab supports itself.

AiYn'U

JohnB wrote:Do you need to water your garden in summer? If its a sealed tank, consider diverting your gutter downpipes into it to make a reservoir, and get a submersible pump when watering. As our climate changes to a more Mediterranean one, seasonal rainfall is going to be more common.

What a good idea ! As mentioned, this would seem to be a septic tank. So there will be a outlet and inlet pipes to blank off. Render the sides with a sand / cement mix with waterproofing additive. 50mm of same to base.


(Didn't Sadam Hussain hide out in one of these.)

Yes, it does seem a shame to fill it in. After all, think what it would cost to create.
So, you have the makings of a small bathing pool, a largish pond, a bog garden, or indeed a storage tank. If it's 12x12x12 that's about 13,000 gallons.
Or 64 tons of something solid if you want to fill it in.

I was thinking that it's deep enough for a radio-controlled submarine, but there don't seem to be any. A gap in the market.

V8

88V8 wrote:Yes, it does seem a shame to fill it in. After all, think what it would cost to create.
So, you have the makings of a small bathing pool, a largish pond, a bog garden, or indeed a storage tank. If it's 12x12x12 that's about 13,000 gallons.
Or 64 tons of something solid if you want to fill it in.

I was thinking that it's deep enough for a radio-controlled submarine, but there don't seem to be any. A gap in the market.

V8

I'm reasonably sure radio waves don't travel through water, which would explain why there are no RC submarines.

If I'm right, that is!

I don't know if they work but they are available.

rcsubs.co.uk