ᐅ Cistern – Flat Tank or Cylindrical Tank – Groundwater

Created on: 22 May 2022 21:34
M
Matthias_1212
Dear forum community,

During our garden planning, we came across our soil report and the information regarding the water conditions. If we understand the report correctly, there is already groundwater at a depth of 1.9 m (6.2 ft)?
Does this mean that a standard plastic round tank could potentially float?
Am I interpreting the report correctly?

Thank you very much for your assessment.
Matthias_1212

Groundwater conditions: HGW 0.5 m, MHGW 1.5 m below surface; possibility of perched water.
Tolentino24 May 2022 13:11
Deliverer schrieb:

And it's only about the part below the 1.9-meter (6.2-foot) depth
In my case (4m³ round tank), that would be about 50cm (20 inches). The top 80cm (31 inches) is the shaft, so the rest is also covered by soil. I wouldn’t worry too much about it either. Even empty, it weighs at least 120kg (265 lbs).
W
WilderSueden
24 May 2022 13:50
From the report, I see not only 1.9m (6.2 ft) but also that you can frequently expect 1.5m (4.9 ft), and in extreme conditions even up to 0.5m (1.6 ft). I wouldn’t design the entire plan based on the extreme value, but planning with 1.9m (6.2 ft) as the maximum is definitely wrong. Presumably, even under extreme conditions, the cistern will be filled, which adds relief (or perhaps actually more load?).
MaxiFrett24 May 2022 14:28
We have similar conditions.

I have read about "submersible check valves" that are installed inside the tank beforehand.
They allow water to flow through the valve in only one direction.

Maybe this is what you are looking for.
M
Matthias_1212
24 May 2022 20:33
Specifically, a 5000-liter (1320-gallon) tank is to be installed. It would either be 2.5 m (8.2 ft) high plus a 50 cm (20 inch) deep excavation, so a total of 3.0 m (9.8 ft) excavation depth. Alternatively, a flat tank with a height of 1.5 m (4.9 ft) and an excavation of 2.0 m (6.6 ft) could be used. Could the 3.0 m (9.8 ft) excavation pose a problem with the groundwater level?
W
WilderSueden
24 May 2022 21:00
There will most likely be water flowing into the excavation. To determine when this becomes critical and what kind of protection is needed, it’s best to ask the foundation contractor. If you plan to do it yourself with a rented mini excavator, I would consider that too risky.
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dab_dab
25 May 2022 09:35
As far as I know, good cisterns are also approved for installation in groundwater layers when made of plastic (depending on the manufacturer and tank geometry up to the tank shoulder).