Tank Floor Slab Floatation & Rock Anchors

[llamallama]

I am currently reviewing the design for several circular wastewater treatment tanks in an area with an unpredicatble water table. The tanks are 18' deep and 95' & 60' in diameter. The design calls for rock anchors to resist uplift. The opinion of our staff is that a drainage system under the slab and around the tank would eliminate the need for the anchors. I tend to lean this way as well. Is the design just being conservative?

 

[JedClampett]

For a WWTP where the tanks are normally filled, I would prefer a drainage system. Can the slab be made thick enough to resist the upward pressure? How about pop up PRV's?

 

[dcarr82775]

It will need to be a robust drainage system to keep the water table below the tanks. The drainage effectively needs to be a permanent dewatering system.

 

[llamallama]

Thanks for the input. The proposed tankage supplements an existing treatment train, so the tanks will be drained for service, and it could be for an extended period of time. I want to stay away from PRV's because with the watertable they would most likely be open most of the time.

You are correct the dewatering system would be permanent. The site permits gravity flow to an open channel downslope. The question I am being asked is the relative risk of dewatering vs. rock anchors.

 

[PSlem]

Is it in an area where freeze is possible?

 

[BigH]

I would say the risk lies in the necessity that the drainage system will be able to handle all expected inflows from a high water table under normal operations (since you indicated gravity flow) and that there would be no chance of blockage of the system that might cause hydrostatic build-up. You might want to look at the cost of the two systems - rock anchors can be installed relatively cheaply once the rig is there.

 

[JedClampett]

As far as risk, there's some. If you have electric sump pumps, they'll be useless during a power outage. You can have them connected to emergency power, but that increases the demand on the emergency generators. The drains could clog, the pumps could fail, there's several potential issues. We've installed sumps and monitoring wells. Alarm the sump levels to the main control room, so if something goes haywire, they get a warning. The monitoring wells should be checked to make sure that they're at the same level as the sump. If you do a complete job, you've removed as much risk as possible.

 

[dcarr82775]

I would keep the hold down anchors in the design. Too many chances for something to go wrong with the dewatering system

 

[oldestguy]

Sewage in my experience is very corrosive. Any anchors should be made of stuff that can resist that. Stainless seems to be common, but ask the anchor guys about this. That may raise the price considerably.

 

[paddingtongreen]

In the main, drainage is more likely to go wrong than the anchors, therefore, use the anchors unless there is a huge cost difference. My guess is that the present value of the present and future costs of the drainage system will be exceed that of the anchors.


Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[llamallama]

Thanks everyone. It seems the decision is to go with both. After review it seemed the anchors were the best choice, however the drainage is being installed as a monitoring system against tank failure. Due to the size of the tanks and the contents (this is industrial wastewater) monitoring was also a concern.