Buried Pipes and Concrete

Hello AvatarIndo,

You are almost a 'new' members so welcome you. Enjoy and do not be afraid to ask again and again until you get the advice you want.

Can you tell me what sizes concrete pipe you refer to, and is it 'sealed' and joined with other lengths. Is it concrete or 'glassed drainage? What is the pipe used for?.

If you can tell me why you need to know also please?

Take care.

You can refer any text book or hand book to get the density of concrete. You can also check with your concrete pipe supplier

Hello AvatarIndo,

You say you have been working with "Guidelines of the design of buried steel pipe u.g 6.0 ASCE series".

http://www.rinkerpipe.com/TechnicalInformation/InfoBriefs_Series/IS%20402%20Pipe%20Buoyancy.pdf

However your pipe is concrete. With ref' to the pipe type I have listed below the address of a site about concrete pipe. It's weight, compared to plastic and steel and other substances are in a Table there. You should always work out buoyancy assuming the pipe is empty.

The sites gives other details including the weight of different sized pipe, and a Formulae to work out 'buoyancy'.

Get back to me and tell me how you got on please?

Good luck.

Dear Avatar,

Are you trying to work out the weight of Concrete Weight Coating as applied to steel linepipe, for service in subsea or immersed conditions, which includes being under the natural water table. This is commonplace thus preventing the pipeline from 'floating' its way out of the trench, or putting additional stresses on the pipeline. What diameter is the pipe, how long is it, what are the products passing through, do they pass through continuously or intermittently thus changing the bouyancy.

You see there are many variables in this question, however my own experiences have given me (an empirical) 'rule of thumb', this being, < 10" dia. 3" (75 mm) of concrete weight coating required, between 10" up to 24" dia, 4" (100 mm) of concrete weight coating required, >24" dia (up to 42" where my personal experience finishes) 5" (125 mm ) is used.

I hope this helps you, I must repeat that these are for Additional 'Weight Coating' applied to steel linepipe, and can be supplied with the weight coating pre-applied at the pipe coating works rather than site application.

Regards,

George,

Why worry about bouyancy? Assuming that the water table is a few feet below the surface and the subject pipe is buried in a trench and backfilled with soil or soil/rock, there will be ample mass in the backfill to resist any bouyancy forces. In order to install the pipe, the trench area will have to be dewatered and remain so until backfilling is complete. If you are discussing the installation of pipe underwater, then you would have to have sufficient ballasting with either quarried rock as cover or a system of regular anchoring.

I agree. Bouyancy of the pipe is most likely not an issue if the diameters are nominal and the pipe is buried. If it is a large diameter pipe, then I disagree with you. Also, bouyancy of manholes and inlets is definitely an issue. I have seen them pop out of the ground because some engineer did not take ground water into account.

If you did observe this, the structures were made of HDPE or some other light composite product. There is no way that precast concrete structures will float out of their positions, assuming the surrounding soil backfill has been properly compacted. For example, a 4' diameter manhole, 10' deep will displace about 12,500# of water, and it will weigh about 12,000#. When you allow for negative skin friction on the perimeter, the bouyancy needed to raise an in situ manhole out of the ground would be far greater than the weight of the water displaced by it.

In regard to large diameter pipes, unless they have less than a couple of feet of cover and unless they are HDPE, PVC, CMP, or any of the other relatively lightweight materials, they will not float either. What does happen occasionally, is that storm drainage that is built into fairly steep profiles can suffer from dislodgement on account of flowing at capacity and, therefore under hydraulic pressure that can mine enough soil from around the pipe to allow it to move and be forced out of the ground from the pressure and lack of confinement. If there are manholes and inlets associated with this situation, they could be dislodged as well. but they do not float.

This appears to be a correct result. The buoyancy force should be negative relative to the downward pointing vector of gravity as the empty pipes will indeed attempt to float out of the cement as it is poured.

Filling the pipes with water for the pour will help reduce this potential problem.

Anchoring may still be required.

I know about 1600mm DI pipes that floated out of their trenches (while waiting to be backfilled) after the rain, although the dewatering was applied the whole time. I suppose the capacity of the dewatering pumps was not sufficient for all of the water.

Hello drift,

Of course the local 'Water-Table' has quite a lot to do with whether the water 'lays' in a dug trench anyway?

Take care.