The circular tube is inherently better than the box because it is structurally more efficient.  The tube becomes a tension structure which is naturally stable.  The box will require thicker steel plate to carry the bending moments induced in the side walls.  The tube would tend to flex and distort under its own weight ( as would the box but perhaps to a lesser degree depending on plate thickness etc).  If the tube is buried then the ground will support the tube.

Pipes of the size you are considering will be constructed by rolling plate steel into a circle and welding the seam where the ends of the plate meet.  Extruding is not really an option.

Andy Machon

 
 

Thanks for your comments, Andy.  Very helpful.  I would still like to know more about large diameter extrusion technology.  A college saw a Popular Science (or similar) issue some time ago that touted the potential ( for economy, cost, strength, etc) in extrusion.

Dear Carr

Glad I could be of help.  I would suggest contacting CORUS or look on their web site to see if they can help you with regard to pipe extrusion.  I've not seen anything on extrusion of such large diameter pipes.  I assume you are talking about steel?  Usually plastics, aluminium, copper or other materials with high ductility are suitable for extrusion.  CORUS doo specialise in several metals so they are my best guess for advice.

Regards

Andy Machon