You're right Harv ...
By the way, primary mechanism of support for a culvert is a phenomenon known as 'soil arching'. Pipe strength itself is not supposed to be the controlling factor - it's how the load is distributed over and around the pipe - same concept as an old stone bridge. I believe that the correct figure is 75-80% of the load is handled by arching - and the remaining 20% by the pipe itself.
Now, when you have a small diameter, relatively thick-walled, plastic pipe you would be surprised at the load it can handle. The killers for plastic pipe are vibration, poor backing (not compacted or full of sharp rocks) and temperature. So, if Bird has small diameter, thick walled pipe in a fairly warm climate, laid on well compacted clay with hard clay above it then his comments don't surprise me!
Concrete pipe is also well known for its strength in compression and has the unique ability to carry large 'hoop' stresses. The really tricky stuff is large diameter, thin walled plastic pipe. I've attached a link below to some information that anyone thinking of working with corrugated plastic pipe should find handy - download the pdf file - it's an interesting read ...
<A target="_blank" HREF=http://www.cppa-info.org/install/index.html>Corrugated Plastic Piping Association</A>
... as always, the supplier of slightly off-topic information and corrupter of threads,
Patrick
By the way, primary mechanism of support for a culvert is a phenomenon known as 'soil arching'. Pipe strength itself is not supposed to be the controlling factor - it's how the load is distributed over and around the pipe - same concept as an old stone bridge. I believe that the correct figure is 75-80% of the load is handled by arching - and the remaining 20% by the pipe itself.
Now, when you have a small diameter, relatively thick-walled, plastic pipe you would be surprised at the load it can handle. The killers for plastic pipe are vibration, poor backing (not compacted or full of sharp rocks) and temperature. So, if Bird has small diameter, thick walled pipe in a fairly warm climate, laid on well compacted clay with hard clay above it then his comments don't surprise me!
Concrete pipe is also well known for its strength in compression and has the unique ability to carry large 'hoop' stresses. The really tricky stuff is large diameter, thin walled plastic pipe. I've attached a link below to some information that anyone thinking of working with corrugated plastic pipe should find handy - download the pdf file - it's an interesting read ...
<A target="_blank" HREF=http://www.cppa-info.org/install/index.html>Corrugated Plastic Piping Association</A>
... as always, the supplier of slightly off-topic information and corrupter of threads,
Patrick