Length of Restrained Pipe Calculation 3

[raydefan]

I am working with an agency that uses restrained joints in lieu of thrust blocks. The provide minumum lengths, but I would like to see how to calculate these lengths. Does anyone have a link where this calculation is explained? I am working with PVC pipe if it makes a difference.

 

[hydrae]

PVC does make a diffence

EBAA has a program that uses the parameters to calculate the lenght required, free download or web based at

http://www.ebaa.com

Hydrae

 

[wwchick]

Try

http://www.DIPRA.com

and dowload their free program. I would recommend it over EBAA.

 

[BRIS]

It is a simple friction calculation –the friction resistance per unit length of pipe is calculated from the weight of backfill times the circumference of the pipe times the coefficient of friction. The length required is determined from a simple resolution of the bend thrust into tension forces in each leg of the pipeline. The coefficient of friction is dependent on the pipe material – PVC is less than Ductile Iron but if the DI is wrapped in polyethylene sleeves there may be little difference. The spacing and shape of the spigots (bells) on spigot and socket pipe also has a major influence. You need to make sure that the strength of the PVC joint is adequate to take the thrust and you may need to consider de-rating the pressure rating of the PVC pipe to allow for longitudinal tensile stress in combination with the hoop stress – this is temperature dependent.

Agencies tend not to use restrained joints in lieu of thrust blocks because 1) any other agency opening a trench along the same alignment will remove the restraint 2) any future repairs to the restrained length of pipe must restore the restraint.

 

[hydrae]

Every Water system I work with uses restrained joints, thrust blocks are only used when the connection to the existing system results in an uncertainty on the restrained length.
We consider it a more reliable system of thrust restraint because when another agency digs up in an area of restrained joints a large area is required to compromise the system, where digging behind that lump of concrete can cause a dramatic failure with just one small hole.
Hydrae

 

[BRIS]

A couple of comments 1)most of the water mains I work with are laid in streets that are conjested with services.The problem with restrained joints is that another utility will excavate a trench parallel to the your trench removing the restraint. 2) OK if someone excavates behind your thrust block the bend drops off - repalcing the bend is a lot easier then replacing 50.0m or so of pipe that has pulled out of its trench. I know a number of authorities that install thrust blocks below the level of the pipe invert. In UK and the Middle East the use of restrained pipes is rare - also the price is 50% more. I did try to persuade the authorities in Bahrain to use restrained joints rather than massive piled thrust blocks in poor waterlogged ground but without any success - they prefer the reliabilty of thrust blocks regardlees of costs.