underground concrete tanks

[rittz]

I saw a drawing for an underground concrete water storage tank which was approx 20' x 30' x 8' high, The lop of the roof slab was 12" or so above grade. Located 4 or 6 ft to the side of the tank was a 12" dia pipe which extended vertically from a foot above grade down to the level of the bottom base slab (no closure on the bottom of the pipe). It was essentially a deep sump (9ft). What was the purpose of that sump anyone? The tank was in use as a fire pump test facility.
I saw a drawing for an underground concrete water storage tank which was approx 20' x 30' x 8' high, The lop of the roof slab was 12" or so above grade. Located 4 or 6 ft to the side of the tank was a 12" dia pipe which extended vertically from a foot above grade down to the level of the bottom base slab (no closure on the bottom of the pipe). It was essentially a deep sump (9ft). What was the purpose of that sump anyone? The tank was in use as a fire pump test facility.

 

[BAretired]

I could not open your three links.

I am guessing that the purpose of the standpipe was to measure groundwater level. If groundwater level is high and the reservoir is drawn down, the floor can fail from excess upward pressure.

BA

 

[hokie66]

That's a reasonable guess.

 

[msquared48]

I would guess that it was an easy way to measure how much head or water remained in the tank for the fire drills.

The tank may have been filled manually via a tanker truck and not have a float for automatic filling, particularly if this was at a remote site.

Mike McCann
MMC Engineering

 

[hokie66]

But the OP said the bottom of the pipe was not closed, which I think he meant was not connected to the tank.

 

[msquared48]

True... Under that condition, it would be illogical to connect it to the tank as the tank would drain to the level of the connection invert.

That being said, assuming the pipe is to measure the groundwater level, it appears to me that the tank was not properly degigned, OR, the groundwater level changed due to development after the fact, necessitating the standpipe addition. However, that does not explain why the standpipe was on the drawings to begin with, assuming this was the initial construction drawing and not an addition...

Mike McCann
MMC Engineering

 

[msquared48]

Another thought here - some rough calcs on the tank shows me that with the interior of the tank drawn down, and the outside water table high, with a 5 to 6 foot difference, the tank will want to float. Now, with the drawing showing the sump, which would be a groundwater elevation monitor for this, why would the original design engineer not correct the problem so that the monitoring was not needed? This makes no sense to me.

Mike McCann
MMC Engineering

 

[Ron]

This question was originally posted in the "Wood Engineering and Design" forum...yes, "wood". I replied much in the same manner as BAretired and suggested to the OP that it be posted here or in a more relevant forum.

As for why the design didn't accommodate the uplift...well, it did. Apparently the standpipe is there for that purpose and as long as someone remembers to look before they draw down the tank, it will work. Not good design, but somewhat functional. My guess as to why it was not accommodated in the placement and mass of the tank is that they probably couldn't afford it, although if the top of the tank is already 12 inches above grade, why not raise it another couple of feet to get it out of the water table influence?....

 

[JedClampett]

As someone who has designed numerous buried tanks, the first preference is to provide enough mass to prevent groundwater uplift. Other methods to anchor tanks such as piles or soil nails are also considered.
But sometimes you get a cheapskate of a client who tells you that they don't want to pay for a 30 inch thick (or whatever) mat. They'll tell you they'll monitor the groundwater and not drain the tank when it's high. And what can you do? Call them liars? Tell them that while you might do it, will the next guy? You point out all the weaknesses, but they're the boss. It's not really violating any code. And if you don't do it, the next guy will.

 

[BAretired]

It is not just groundwater which can cause uplift issues with underground tanks. If the tank is backfilled with granular material and the native soil is impermeable clay, then uplift will occur if the area is flooded, the backfill becomes saturated and the reservoir is fully drawn down.

I saw a vivid example of this in a nearby town about thirty years ago. An underground cylindrical water reservoir with a diameter of about sixty feet sat on a twelve inch layer of drain rock which rested on impermeable clay. The walls were backfilled with granular fill. It had been a dry summer and the demand for water had been high, so the tank was drawn down to dangerously low levels. Then, one day, a sudden rainstorm flooded the area. For some reason, the drainage system did not function properly and pressure built up around the walls and under the floor.

If the floor had been capable of resisting the uplift pressure, the entire reservoir would presumably have floated. Instead, the floor slab ruptured and water gushed in through large gaps in a minimally reinforced four or five inch thick floor slab.

It was an expensive lesson for that town.

BA

 

[msquared48]

Jed et al...

OK I guess, but it would never fly here, or float for that matter.

Mike McCann
MMC Engineering