Why is it preferable to have backfilling partly-completed before carrying out water test?

Water test is not intended to reassess the individual pipe performance because this should have been done in the manufacturing process. Instead, it serves to check the occurrence of faults during pipe laying process and to ensure that the pipeline can resist the internal hydrostatic service pressure it will be subjected to.

After adding water to the pipeline, 24 hours are usually specified before actual measurement. This stabilization time is required because both the escape of air trapped at pipe joints and water absorption take time to complete.

It is recommended that backfilling to pipes shall be partly-completed before the conducting of water test to cater for possible floating of pipelines in the event of accidental filling of pipe trench with water. Moreover, it is recommended to place adequate sidefill to restrain the pipeline to prevent movement of pipeline during water test. Otherwise, excessive movement of pipeline from water pressure may result in failure at joints.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.