Sewage plant installation question

[RJHumphrey]

Hello,

just wondering about others thoughts on my current predicament. I have installed a Marsh Ensign 6 sewage plant and poured concrete up to the outlet level, as per installation advice, but now slightly confused on my next steps. In the installation sheet supplied it states if the ground is free draining, pea shingle can be used to backfill around the tank, but I have some concerns, obviously above the outlet level the tank is not provided support from the water inside, I also plan to use the supplied manhole riser which could add up to 1m of additional weight on the sewage plant from backfill to reach the cover height. Am I worrying about nothing? I did contact the manufacturer for advice, but felt a little fobbed off with their response, ‘as they can’t comment on individual sites as the ground conditions all differ’

I have visions of adding the backfill and the tank imploding, any advice gratefully received.

thanks, 

Ryan

[SuperJohnG]

It'll be fine. They are meant to be buried so can take it, backfill with peashingke or just earth which would be much cheaper! 

[ToughButterCup]

11 hours ago, RJHumphrey said:

...

Am I worrying about nothing?

ground conditions all differ’ 

I have visions of adding the backfill and the tank imploding, any advice gratefully received.

...

 

No, you aren't worrying about nothing.  You are understandably concerned.

 

First, backfill: the pressure of the backfill is designed in to the product: think about it for a moment - if it weren't , the tank - ie. not just yours , but all tanks - would implode.  And thats a sh1t situation to be in as a manufacturer.

 

The manufacturer is correct ground conditions do differ: but theirs was a mealy-mouthed response. They could have asked you what your ground conditions are - the one thing to be concerned about - slightly - is the water table. A high water table means a higher likelihood of a buoyant tank.  Yes, improperly anchored, they do pop out of the ground. Spectacular when you see it happen.

 

IF thats the case for you ( high water table) , then  simply anchor the tank with (say) a cubic meter of concrete , and top off the rest with normal backfill  . We did, and for good measure threw in some  concrete fence posts at the bottom of the pit, chained them to the periphery  of the tank, a bit like an emergency anchor for a boat.  Our poo tank is going nowhere in a hurry. 

 

Makes for a really secure feeling that when seated on the throne.

[Russell griffiths]

I backfilled with gravel as per manufactures details, I looked at it for a week then took 300mm out and capped it off with 150mm of concrete, mine will never be driven over, but I liked the security the concrete provided. 

Edited April 20, 2022 by Russell griffiths

[joe90]

I backfilled up to outlet with concrete as I have a high water table, topped off with gravel to stop flooding with heavy rain.

[RJHumphrey]

Thanks for your replies, I would have been more comfortable if I wasn’t adding the chamber riser (supplied by the manufacturer) so approx 1.5 metre of  backfill on top of the tank. The tank isn’t going anywhere as enough concrete has been poured to secure it in place, up to the same level as you Joe90, where there is a route for any groundwater/ rainwater to naturally flow away, just concerned about the additional weight required to bring the ground level up to chamber level. I have phoned the supplier who is putting me in contact with the rep, but can’t see him knowing much about installation, may be proved wrong. Did think about adding lintels across the tank to support the weight, but is this overkill, and are these designed to be in the wet ground for years, if they were to fail it would be game over for the tank.

[joe90]

22 minutes ago, RJHumphrey said:

Did think about adding lintels across the tank to support the weight, but is this overkill,

I would say yes, if they were required then the installing instructions would say so surely, as long as you follow the manufacturers guidelines then you are ok, if they are unclear demand they confirm how it should be installed.

[Russell griffiths]

I thought this was the best way to spread the load of additional soil. 

[RJHumphrey]

Yes Russell Griffiths, that removed the worry of additional weight on the tank. Did you pour this in stages.

[Russell griffiths]

9 hours ago, RJHumphrey said:

Yes Russell Griffiths, that removed the worry of additional weight on the tank. Did you pour this in stages.

Nope just one pour. 

[RJHumphrey]

2 hours ago, Russell griffiths said:

Nope just one pour. 

What Sewage treatment plant did you install?

[Russell griffiths]

2 hours ago, RJHumphrey said:

What Sewage treatment plant did you install?

Graff. 

[martynjl]

On 21/04/2022 at 11:01, RJHumphrey said:

What Sewage treatment plant did you install?

Hiya mate, I'm in the same exact situation, with the exact same tank. filled up the outlet level with concrete. Did you back fill with soil after?

Edited June 8 by martynjl

[Kelvin]

Graf here. We don’t have a high water table and it’s in a place where it won’t ever be driven over so it’s back filled with shingle then earth. 

[flanagaj]

I’m installing one shortly and as we are not in a high water table area, I’ll be backfilling with pea gravel.  The concrete backfill is a terminal operation and cannot be undone.

[Russell griffiths]

2 hours ago, flanagaj said:

I’m installing one shortly and as we are not in a high water table area, I’ll be backfilling with pea gravel.  The concrete backfill is a terminal operation and cannot be undone.

If when you dig your hole it fills with water, you then want to concrete it in. 

[saveasteading]

The tank will only ever have a risk of floating when empty combined with high water table.

After installation it will always be full, except on the few occasions of pumping it out.

On 20/04/2022 at 10:36, ToughButterCup said:

Spectacular when you see it happen.

Do tell.

[flanagaj]

10 hours ago, Russell griffiths said:

If when you dig your hole it fills with water, you then want to concrete it in. 

A non extraction well next door has the water table depth at > 20 metres, so should be ok.  But I take your point.

[saveasteading]

Handy hint.

Of course keep the excavated hole as tight as possible to minimise costly backfill and reduce earth  settlement in future.

But if you can't avoid a bigger hole, esp sloping sides, then you can minimise concrete and gravel backfill with the use of a board or a cladding sheet. 

Gravel against the tank, earth behind, pull up and repeat.

 

[ProDave]

1 hour ago, saveasteading said:

The tank will only ever have a risk of floating when empty combined with high water table.

After installation it will always be full, except on the few occasions of pumping it out.

Do tell.

I was badly advised on our first build and the ST was just backfilled with pea gravel.  I worked around that situation by only ever getting it pumped out in the summer after a long dry spell when I was reasonably sure the water table was low. And then immediately running a hose to re fill it with fresh water rather than waiting for it to fill gradually.

 

But I did see one float out of the ground.  It was a treatment plant I don't know the make but it's design seemed to be the water level in the system was quite low and after heavy rain one morning it just pushed it's way up out of the ground.

[saveasteading]

If it is on a concrete slab then the tank can be pinned or lashed to that.

Some tanks have the bases designed to ease the interlinking of base concrete with the tank. Gaps allow bars to be pushed through or cables over the top.

 

I'd like to hear other users experiences of pumping out. Hoe ling have you sared keavd it? Any ptobitms?

My personal feeling is that it can be left for years.

The biggest problem would be toilet stuff that should not be flushed. I guess that floats for ever in the first chamber.