Reduce Uplift / Flotation by ground drainage 3

[renko]

Hello,

I've designed a Water Tank which is mostly below ground. When considering uplift 3 options were developed:

1. Increased Tank weight with extra concrete.
2. Provide toe around the tank to mobilise weight of backfill.
3. Reduce uplift by providing ground drainage around the tank to a local canal.

Having visited the site I opted for No.3 as there is already a drainage network in place close to the tank which could be extended. The existing pipelines are draining well and the canal level is much lower than the tank. I then highlighted that a maintenance regime would be necessary to ensure the pipelines are kept clear and the drainage is not compromised.

The Client has now objected to this approach not willing to accept the risk that a poor maintenance regime could cause a problem in the future.

My argument is that this method is accepted by the code (British Standard BS8007) and the tank will very rarely be empty, and that the pipelines should be subject to a maintenance regime anyway.

Anyone got any advice on this?

 

[Ussuri]

Were the options discussed with the Client at an earlier stage? Options 1 and 2 may have cost a little more but would ensure resistance to flotation without any external requirements. Option 3 is a higher risk option as you have pointed out and was possibly the cheapest. If you discussed and agreed with the client then fair enough.

If not, your design choice has now put a possible long term requirement on your client, in that they now have a new financial cost checking this drain for the full service life of the tank. Drains get blocked, fact of life.

In addition, is it possible that a flash flood/storm might exceed short term capacity of the pipe causing the area to surcharge.

Form your post it sounds like you expect the risk to be pretty small however you have commited your client to an additional maintenance programme which could have been avoided using options 1 or 2.

Its a moot point whether they should or not. And tanks are emptied periodically for inspection and cleaning, during which time they may be left empty for a number of days.

Can you design your drainage system such that it has multiple redundancies? I.E. three drains each with sufficient hydraulic capacity to carry the flows associated with the rising ground water?

Is the ground strata permeable and therefore groundflow adequate to prevent a rising water table?

Have you assessed how high the ground water needs to rise before you have a buoyancy problem and then checked to see if it ever reaches that level?

 

[registeredpe]

Greetings:

A fourth option would be to introduce "pile"(sometimes referred to as "pole")type footings of the proper diameter and depth below said tank, this would provide resistance to vertical movement by means of skin friction on the piles.

Best regards, registeredpe in AZ

 

[JStephen]

After facilities have been in service quite a few years, it is not uncommon to find maintenance problems of various kinds. People will rotate through the facility, old drawings and files will be lost, etc. Thirty years from now, it's quite likely that nobody would even know if the tank had a ground drainage system or not, much less maintaining it.

 

[renko]

Thanks everyone -
Ussuri: you've got it all pretty spot on. Options 1 & 2 were my first design submission. But we're appointed to the Contractor so we eventually worked on option 3 due to the reduced cost.
And yes, the Clients objection seems to be based on the points you've made above.

'Can you design your drainage system such that it has multiple redundancies? I.E. three drains each with sufficient hydraulic capacity to carry the flows associated with the rising ground water?' - I've suggested this and am awaiting feedback.

'Is the ground strata permeable and therefore groundflow adequate to prevent a rising water table?' - no, the ground is CLAY over ROCK but we are specifying a granular free draining backfill material.

'Have you assessed how high the ground water needs to rise before you have a buoyancy problem and then checked to see if it ever reaches that level?' - Yes, the ground water does reach a level which would cause a buoyancy problem.

registeredpe: the tank sits on rock so piles aren't necessary. I suggested rock anchors but the price again was a problem.

JStephen: very good points, but as designers aren't we entitled to expect a certain level of future competence, particularly when we identify a critical issue. (expect the answer is - as designers shouldn't we design for the removal of critical issues).

I do expect the risk to be minimal and will also propose the installation of a permanent standpipe, so that ground water levels can be checked prior to emptying the tank for inspections / maintenance.


PS apologise for the looong post!

 

[Ussuri]

Sounds like a design and build.

I have been in this situation before. What is the contractors opinion as it was pressure them that 'forced' you down that design route?

The problem is that the contractor is essentially trying to move the upfront cost of materials, which he pays, to a maintenance cost, which the client pays.

In the past for large, deep tanks we have taken advantage of the skin friction to give a bit extra resistance.

 

[oneintheeye]

we has similar problem again ended up with so much concrete that the local supplier entire daily output was going on the mass concrete pour. Not much you can do really if thats what the wants. Could point out the cost savings. though drains will probably get blocked.

 

[miecz]

Agree with JStephen. I wouldn't count on maintenance of a drainage system. I would investigate option 2 further. You can mobilize a surprising amount of resistance if you take into account a soil pyramid acting on the toe to resist uplift. See NAVFAC DM 7.2-171.

 

[renko]

Ussuri,
The Contractors opinion is that we are contracted to design to the code (BS8007) and that providing effective drainage is accepted 'as far as local conditions permit'. I suppose the long term local conditions are the arguing point, which is why I'm pushing for the extra drainage pipelines as you suggested.

miecz: thanks for the reference. I've been looking for a good example for mobilizing a wedge of soil over the toe, as opposed to just what's directly above it.

 

[Ussuri]

How about a really wide french drain between your tank and the canal?

 

[miecz]

For more on option 2, see thread507-154895.

 

[msquared48]

Professionally, I would not rely on the drainage system to lower the water table, as they do eventually get plugged or restricted, plus freak storms do occur.

Mike McCann
McCann Engineering

 

[hokie66]

You could also consider hydrostatic relief valves to allow the ground water to drain into the tank in an emergency. Whether or not that approach is acceptable to the client would be his call.

Ultimately, the cost of the tank should be borne by the party who benefits, i.e. the owner. If the owner insists on a higher standard than that which you determine is professionally defensible, he has to pay for it.

 

[oneintheeye]

I looked into pressure relief valves, again noone would be confident that these are effective long term as they would block. Plus you have the ground water entering the system, this may not be acceptable. In general i found that it would be hard for anybody to accept the risk.

 

[miecz]

I came to the same conclusion with the pressure relief valves. Too many stories of them freezing up. Wish I knew if the stories were true.

 

[renko]

Pressure relief valves aren't accpetable as this is part of a potable water supply system.

miecz: apparently the stories of them freezing up are true, according to a colleague of mine. But whenever we specify them we put in quite a few to allow some redundancy.

hokie66: 'Ultimately, the cost of the tank should be borne by the party who benefits, i.e. the owner. If the owner insists on a higher standard than that which you determine is professionally defensible, he has to pay for it.' - this is pretty much the approach the Contractor (and by association, me) will be taking.

It's in a formal process now so I'll post with the outcome.

Thanks All.