Question for person with concrete knowledge

Ginger83

Hi there's a concrete lid on our septic tank made up of 6 sections. Each section is 7 foot by 2 foot by 4 inches. 2 of these sections have a 4 inch hole i presume as vents.

Would it be safe to make 1 of the 4 inch holes bigger to 9 inches to allow a pipe to be dropped in to empty it?

Calahonda52

I don't think so
I presume holes are in middle
1: you may come across some rebar in the slab, when cut, will weaken the slab further.
2: (24-9)/2 = 7.5 inches either side of a 9" hole: too little in my book

Ginger83

Calahonda52 wrote: »

I don't think so
I presume holes are in middle
1: you may come across some rebar in the slab, when cut, will weaken the slab further.
2: (24-9)/2 = 7.5 inches either side of a 9" hole: too little in my book

Should i get 2 sections cast together, eg 7 foot by 4 foot by 4 inches with a 9 inch hole?

Calahonda52

That will weigh over a tonne .

Metric Tensor

When I have seen tanks like this emptied previously the end slab has been lifted up and either propped or turned over. I cannot guarantee they are structurally capable of handling this type of twisting though, nor can anyone without knowing what reinforcement is within them.

Also the four inch hole may be connected to a pipe underneath and to the T-piece of the tank so whacking away at it might cause more damage than you would think.

Ginger83

Calahonda52 wrote: »

That will weigh over a tonne .

Would you have any other ideas?

Metric Tensor

PS - each of your slabs is a little over 300kg for what it's worth.

fepper

You can get another slab made with a 9 inch hole cast in it,you could ask them to insert a bolt and steel plate that will cover the hole when not being used,maybe make this 7x4 so you replace existing 2 single slabs and put at end of tank

Ginger83

fepper wrote: »

You can get another slab made with a 9 inch hole cast in it,you could ask them to insert a bolt and steel plate that will cover the hole when not being used,maybe make this 7x4 so you replace existing 2 single slabs and put at end of tank

I was planning to put a 9 inch pipe and cover so i could cover the lid with clay and seed over for a robot mower.

Lumen

The lids are designed to allow inspection and access. If you cover with soil and grass you won't have any access. Then when you have a septic problem you'll need to dig it all up.

Also, the grass won't grow properly because the soil is shallow and badly drained.

I would install a new lid with an access panel and then clad the access panel in wood or artificial grass, and then go ahead and grass over the rest, but do it all properly so that the grass has 4 inches of topsoil over 1 inch of pea gravel for drainage.

Consult a profession to make sure what you're planning will give them the access they need and is compliant with whatever regs govern such things.

Ginger83

Lumen wrote: »

The lids are designed to allow inspection and access. If you cover with soil and grass you won't have any access. Then when you have a septic problem you'll need to dig it all up.

Also, the grass won't grow properly because the soil is shallow and badly drained.

I would install a new lid with an access panel and then clad the access panel in wood or artificial grass, and then go ahead and grass over the rest, but do it all properly so that the grass has 4 inches of topsoil over 1 inch of pea gravel for drainage.

Consult a profession to make sure what you're planning will give them the access they need and is compliant with whatever regs govern such things.

Woulds the 9 inch pipe allow access to empty it?

the_pen_turner

you should be able to put a bigger hole in it.
if you cant you could get a new lid made .
if you do put the hole in it i would buy some concrete lintels and put them across to give it more strength. then a layer of cement of concrete to stop clay washing into it.

put an AJ riser and lid on and it should be ok.

make sure you do something to stop anyone driving over it. i once saw a tacter trailer stuck in a tank . it was lightly covered over and the driver put the trailer wheels on one side into it. it took a track machine to lift it out

Metric Tensor

OP - given your plans I think your best bet might be to get a new lid or lids cast for the tank. Lids to contain two four inch vents and a separate manhole opening for each chamber. I'd imagine a local engineer will design it for you without too much cost.

You could then build up from the manhole covers to put two covers at ground level and extend the vents too. Then soil and seed the remainder to give you a flat finish with two manhole covers and two vents that the robotic lawnmore can drive over or around.

It's obviously a little more expensive than anything else discussed but it is a permanent job that should last the test of time.

fepper

Ginger83 wrote: »

I was planning to put a 9 inch pipe and cover so i could cover the lid with clay and seed over for a robot mower.

You must be mad about lawns if you intend to cover it with grass,I thought you just you wanted it done to make it easier emptying it, less hassle and safer than opening back slab.....

Ginger83

fepper wrote: »

You must be mad about lawns if you intend to cover it with grass,I thought you just you wanted it done to make it easier emptying it, less hassle and safer than opening back slab.....

We are investing 3k in a robot mower to cut down on labour so i'd rather cut it than not if you know what i mean.

Ginger83

Metric Tensor wrote: »

OP - given your plans I think your best bet might be to get a new lid or lids cast for the tank. Lids to contain two four inch vents and a separate manhole opening for each chamber. I'd imagine a local engineer will design it for you without too much cost.

You could then build up from the manhole covers to put two covers at ground level and extend the vents too. Then soil and seed the remainder to give you a flat finish with two manhole covers and two vents that the robotic lawnmore can drive over or around.

It's obviously a little more expensive than anything else discussed but it is a permanent job that should last the test of time.

Its an old tank so only 1 chamber.

Metric Tensor

Is there are argument for a new properly designed, two-chamber, precast tank with appropriately designed lid? Might save on a lot of pricking around in the long term.

Ginger83

Metric Tensor wrote: »

Is there are argument for a new properly designed, two-chamber, precast tank with appropriately designed lid? Might save on a lot of pricking around in the long term.

It does work fine and is emptied every 2 years

Lumen

Ginger83 wrote: »

Woulds the 9 inch pipe allow access to empty it?

I think you're misunderstanding what I mean by access. Last year my T piece broke off, percolation area clogged up and my tank overflowed. Getting that repaired through a 9 inch pipe, including fitting new T piece and jetting the percolation area would be like doing keyhole surgery.

Through a manhole cover its an easy job.

As it happened the cover was covered by 6 inches of mulch so it took a while to find, but at least I didn't have to dig up a lawn.

Metric Tensor

Ginger83 wrote: »

It does work fine and is emptied every 2 years

I'm not saying it doesn't work the way it was meant to work. (Which is a slightly different thing than "works fine" but that's not the discussion we're having!)

I'm just saying if you are in the process of upgrading everything else would it be worth buying a new one to replace the existing one rather than trying to do some sort of homemade DIY surgery on the existing and winding up with a poor solution rather than a purpose made one.