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-   -   Sinking septic tank? (https://www.contractortalk.com/f62/sinking-septic-tank-200929/)

bordercollie050 06-06-2015 12:33 AM

Sinking septic tank?
 
Got a call from someone a few days ago who said their septic tank has sunk down a few inches after being installed by someone else and they wanted to know what their options were.

Anyone know how this can be prevented, the water level here is kind of high now so soil conditions are definitely a factor

thanks!

Morning Wood 06-06-2015 09:18 PM

Get it inspected and make sure the piping is still intact.

dayexco 06-06-2015 11:57 PM

Tank itself settled, or soil on top of it?

dayexco 06-06-2015 11:58 PM

High water table, empty septic tank = submarine

rino1494 06-07-2015 11:06 AM

Quite possibly the previous installer over-excavated the hole and backfilled with soil that wasn't suitable or compacted causing settlement. This is just a theory. If correct, then you will have to dig down along side and set-up a sump to pump from. Dig around the tank. Have it pumped out. Have the manufacturer lift the tank back out with the boom truck. Dig out unsuitable material and replace with structural fill and compact. Replace tank and plumbing.

GettingBy 06-07-2015 11:11 AM

In principle,
you could pour a slab on the soil surface with the tank in the middle and fasten the tank walls to the slab. Then the downward force is spread out over the slab.

Upchuck 06-07-2015 11:51 AM

We always set tanks on 6" stone base. Sometimes the ground on the side of the tank will settle after a heavy rain. That might be what people are seeing and thinking everything settled. Opening tank and shooting some grades compared to as built plan or exposing inlet and outlet pipes is where I would start.

JDEERE 06-07-2015 06:56 PM

Quote:

Originally Posted by Upchuck (Post 3102770)
We always set tanks on 6" stone base. Sometimes the ground on the side of the tank will settle after a heavy rain. That might be what people are seeing and thinking everything settled. Opening tank and shooting some grades compared to as built plan or exposing inlet and outlet pipes is where I would start.

I agree. I have installed over 1,000 septic tanks and I can't ever remember one sinking. I have had a few float up, but never down.

GettingBy 06-07-2015 07:41 PM

Quote:

Originally Posted by JDEERE (Post 3104018)
I have had a few float up

Which means that the weight of the tank plus contents
was less than the weight of the soil/water it displaced.

I'm not a civil engineer but I'd think these things should be routinely anchored somehow
such that a varying water table wouldn't raise or lower it, even over years.

muskoka guy 06-07-2015 07:46 PM

Why don't they call whoever installed the tank. Why would they call someone who doesn't do septic tanks or have any knowledge about such things.

JDEERE 06-07-2015 07:48 PM

Quote:

Originally Posted by GettingBy (Post 3104274)
Which means that the weight of the tank plus contents
was less than the weight of the soil/water it displaced.

I'm not a civil engineer but I'd think these things should be routinely anchored somehow
such that a varying water table wouldn't raise or lower it, even over years.

When they are used (full) they never float up, only when empty and usually when they haven't been completely backfilled.

Upchuck 06-08-2015 06:17 PM

I've only seen one pop up when I was a kid. Put it in one day. Next day it was up 6". Crazy wet site.

rino1494 06-09-2015 03:43 PM

I had a 8,000lb tank pop up on me. Supplier set it in the ground, backfilled it. It was late in the day. That night we got huge rains and the tank lifted about 6". Had to dig around it and the supplier came back and lifted it back out for us. Had to scoop out the muck and place stone. After that, we never leave the job without having at least half the tank full of water.

markandrews 09-08-2015 11:12 AM

Quote:

Originally Posted by JDEERE (Post 3104018)
I agree. I have installed over 1,000 septic tanks and I can't ever remember one sinking. I have had a few float up, but never down.

I've not done installations myself, but have seen quite a few. I've also only seen them float...

Fouthgeneration 09-12-2015 11:22 AM

Over dig = settlement =broken or up hill sewer/septic lines.

Plastic septic easily float even large concrete tanks can float, especially in liquidized soil at 120+lbs/cubic foot.

Fill the tank the same day as it is covered, no worries. If you ever pump clean a septic tank, refill ASAP with water....

Document the lack of rock under the tank for the sue fest.:whistling

Use rock fill compacted everywhere soil is remove under the plumbing and tanks.

I recommend to install a field tile as close as legally possible to ensure the septic field and the cover over and around the system doesn't ever become supersaturated.

Soon it will be unheard of to pay tens of thousands of dollars for something without photographic records.... Check invoices for tons/yards of fill rock....

Septic tanks are ALMOST idiot proof.:laugh


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