Installation of a Clenviro STP In a High Water Table

[Arrenite]

Hi folks has anyone got any experience in installing a Clenviro Sewage treatment plant in a high water table? If so what thickness and type of concrete did you use please? Thanks Dave

[Russell griffiths]

There’s a couple of us who have had to deal with a high water table, you need to state how high as this could really change your method of installation. 
concrete fill will be determined by your hole size, if you end up with a large hole due to the sides collapsing from water problems you will end up using far more concrete than you planned. 
 

you really need a good plan if you have a seriously high water table. 

[Arrenite]

Cheers Russell. Really appreciate your post. Can you give me an example of a “really good plan” ? My main concern is lateral hydrostatic loads, and how to prevent them. Cheers, Dave

[Russell griffiths]

 

side loads can be controlled with good shuttering, pumps are essential, plus a spare pump, and a spare pump for the spare pump. 
I actually had two submersible pumps running as we filled with concrete,both these pumps are still in the ground buried under 4 tonne of concrete. 
we started digging without the box shuttering and had to stop work and re think and come back to the job with some proper kit. 

[Arrenite]

@Russell griffiths that’s impressive ! What type of concrete did you use for the “box”? I forgot to say, water table is almost up to ground level at wet period in winter cheers, Dave

Edited Sunday at 19:45 by Arrenite

[Russell griffiths]

Concrete depends on if you are filling a wet hole or dry hole, I still had water coming in that was being pumped out at the same time, if I turned the pumps off it would be 1m deep in an hour. 
so concrete was bone dry mix out of a truck that mixes on site, like a kerb mix dry just binding together, I filled the hole burying the pumps and kept them running, I expected them to stall out or blow a fuse, they just kept pumping and I watched the outlet hose, when it slowed down and looked like I was pumping concrete water I turned them off and cut the hose and leads of and carried on filling with dry mix. 

[Alan Ambrose]

>>> Can you give me an example of a “really good plan” ? My main concern is lateral hydrostatic loads, and how to prevent them.

 

Suggest you get an SE to do a quick check/calc. Main variables are the max ground water level, tank geometry, pipe invert levels. If you vow never to empty the tank in winter when the ground water is high and your tank doesn’t need to be buried too deep for invert levels purposes (thus having a buoyant void below the ground water level) then a smallish amount of stone / concrete will work fine. Archimedes principal - you can do the calcs yourself if you are confident. Then there’s a bit of understanding about how the dirt / stone / concrete loads down the tank. That’s it. The loads are in the multiple tons though so best not to guess.

 

A couple of people here, me included, used stakes and concrete below the tank to provide holding down load too.

 

I’ve found that 300mm perf twinwall down to the bottom level of the tank and next to it and sitting in shingle will hold a submersible pump and that works well for temporary dewatering. Alos allows you to easily monitor the ground water level.

[Arrenite]

@Russell griffiths morning. Did you dig a hole, insert the treatment plant and then encase it in concrete? Or build a concrete box to put the plant in, then place reg tank in that, and then fill that with concrete? Thanks a lot, Dave

[Russell griffiths]

Dig hole, put a semi dry mix of concrete in the bottom, place tank, wiggle into position, fill tank with water, hold tank down with straps or props or whatever you think is appropriate, backfill with wet/ dry concrete as needed. 
 

al the above steps will have periods of swearing, wishing you had not started the job,wanting to smash it all up, and thinking the bloke up stairs hates you. 
 

I’ve built my entire house save for a couple of bits, and my tank was the singular hardest part of the entire house build. 
probably all down to water. Dig a test hole to see what you are dealing with. 
I called on the experience of a man that fits these all day everyday and he gave me some pointers. 
your water level will indicate exactly how you need to do this job. 
 

if I had of had solid clay to put it in I think I could have got the hole dug in a day and filled the next day. 
 

the water and extra equipment took another 5 days of endless frustration. 
 

but I was quoted £17,000 to do it and in the end it cost me about £7 ish so I saved a grand a day for all the pain and suffering. 
 

I even slept in the shed the day the tank went in to keep the pumps filled up with diesel, until the concrete arrived in the morning. 

[saveasteading]

I don't like the idea of a massive concrete surround for a tank.

Cost of corse and that it is not usually necessary.

Beautiful work in the pic above btw.

 

Tying it down to a slab will need reference to the tank chosen but should be easy and controllable.

4 hours ago, Alan Ambrose said:

If you vow never to empty the tank in winter when the ground water is high 

This is key. There is no reason why you can't put a notice up on your tank explaining this. And another one under the extract cover. If it's tied down anyway, this shouldn't matter, but better to be safe.

In reality a good tank doesn't often need emptying so wait for a dry summer.

[ProDave]

We have seasonally high water table.  When i dug  test pits for the SE to check soil conditions I left one open and observed anything from a dry 2m deep hole, to water almost up to ground level.

 

I have mentioned before at our previous house I was badly advised and the guy that installed the tank just set it in pea gravel.  That is still okay as long as you only empty it in the middle of summer after a long dry spell.  It would probably end badly if emptied in the middle of winter.  I made sure they buyer of that house was aware of the situation.

 

Even now with the tank in this house concreted it, I choose only to empty it in the summer.

[saveasteading]

It appears to be a requirement in owning an outdated but legal treatment system, that you explain it in basically a house maintenance manual.

 

It makes sense to do the same for any quirky design decision like this one.

 

 This chapter is then displayed in a fuse cupboard so that it doesn't go missing.

 So @ProDave could do this re the tank and rest easy for any future owner or contractor being fully aware.

 

[Arrenite]

@ProDave hi prodave,which STP did you choose for your site, cheers Dave

[Russell griffiths]

 

 

this is the highest it got probably about 750 below ground level  

timberbracing on top of the tank to hold it down and keep it all level while concrete was put in  

[Arrenite]

@Russell griffiths thanks a lot for the photos Russell ! Am I right in thinking that you created a concrete box around the tank and then filled that with concrete? Cheers Dave

[Russell griffiths]

2 hours ago, Arrenite said:

@Russell griffiths thanks a lot for the photos Russell ! Am I right in thinking that you created a concrete box around the tank and then filled that with concrete? Cheers Dave

No. The tank was placed inside that  blue trench box

the trench box was lined with plastic sheets to prevent the concrete making a mess of the trench box and me having to power wash it all off. 
the concrete was placed inside the trench box and vigorously stamped down around the tank. 
the next day I dismantled the box and pulled it out of the ground. 
then I back filled the gap the box took up and used a wacker plate to firm it all up. 

[Russell griffiths]

DIG BIG HOLE. 

 

 

 

Edited 12 hours ago by Russell griffiths

[Russell griffiths]

PUT BOX IN HOLE. 

[Russell griffiths]

PUT TANK INSIDE TRENCH BOX AND FILL WITH DRY CONCRETE. 

Edited 12 hours ago by Russell griffiths

[Russell griffiths]

 

PULL BOX OUT AND BACKFILL. 

[Russell griffiths]

 

CONCRETE CAP BECAUSE IM A LUNATIC. 

[Arrenite]

@Russell griffiths thank you very much for all the information, much appreciated. What type of concrete did you use please? The reason why I ask is that I used semi-dry concrete on another job, and it was permeable and just let the water in around the tank I had put in. Cheers Dave