A special field test has been carried out to show whether or not the soil column above the pipe (dead weight load) continuously exert loads on the pipe crown.

Case 4 (trench 2) is a pipe with a nominal stiffness of 2 kPa.  
The pipe has been installed without compacting the backfill and achieved a deflection from 6 to 10%. 

The measuring device was positioned at a spot of 9% deflection. 
Meanwhile the soil was removed layer by layer. So far no effect on pipe deflection.


No effect again on pipe deflection. Note the visibility of the original trench.

Now the crown of the pipe is free.
The effect on deflection is lessn then 0.5% recovery

The crown is now free. The effect on deflection is 1% recovery.
The pipe is fixed in the soil, and not "dead weigth" loaded.

The second trial was one at which a trench was digged next to the existing pipe. 
This pipe has a negative deflection of 1%.

No effect of digging was noticed, until the moment that the trench wall collapsed. 
Then the deflection changed from -1 to 0%.

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