In the City of Calgary’s bedding standards for both rigid and flexible
pipe, there are no minimum trench widths specified for either type of pipe,
and Class B bedding (bedding up to spring- line) has not been recognized in
Calgary for concrete pipe. Therefore, when fill heights exceed what Class 5
pipe can handle, based on Class C minimum bedding factors, then in Calgary
you have to use Class A concrete bedding.
One of the problems with this is that most contractors use an open graded 40mm
material up to springline that the City accepts as compacted to 95% in a dumped
condition (i.e. contractor’s have been installing Class B bedding but
concrete pipe has only been getting credit for Class C in the design.)
Click
here to view a pdf file used for explaining the benefits of using Standard
Installations for concrete pipe bedding in Calgary.
This is one of the reasons
why the Lafarge pipe producer in Calgary has been working with the City
to approve a new bedding standard – because
contractors have been installing better than Class C for some time.
Performance Issues
There are two technical issues in the design of buried drainage
pipe systems that engineers and technologists must understand clearly to make
the right choice for the project at hand. These are pipe deflection and hydraulics.
Most jurisdictions now limit deflection to 5% for small diameter pipe, and
less for larger diameter pipes. For whatever value you use, that is the deflection
you will get, so use a lower value for the safety factor.
Click here to learn
more about deflection and hydraulics of plastic pipe.
California has learned to avoid this deflection
loophole in the ASTM standards. As currently written, ASTM limits on deflection
allow for tolerances in the calculation of mandrel sizes. This loophole allows
an additional 2% deflection. On a 36-inch pipe with a 7.5% max deflection limit
you could actually be getting 3 ½ inches of deflection and still be
within specification. Designers must question if they can tolerate this amount
of settlement, or loss of crown in city streets.
