Backfilling the trench

 

Before testing the pipeline, the ditch is backfilled. Sometimes the excavated soil is used to fill

the trench and sometimes other selected backfill is used. The trench is backfilled using a

backfilling; no foreign materials are permitted in the trench.

 Backfilling requires special attention in the following areas:

- in steep slopes (where the soil is generally rocky), the backfill material used to prepare

the pipeline bed and which is in contact with the pipe may be easily washed away.

Indeed, the trench is working like a drain and over the years the backfill will tend to

slip down the hill. To avoid this, special arrangements are made to stabilize the backfill in place (sand bag walls for instance) and to evacuate the water under control (drain pipe, diverters).

- when crossing underground structures or in any case when the minimum cover cannot

be achieved, special measures should be taken to provide the same level of protection:

backfilling with concrete, concrete slabs, etc… More than in normal cases, the backfill around the pipe should be properly compacted to provide the necessary strength to the line against static and dynamic pressure from loads crossing the trench.

- The above also applies when for some reasons the normal cover could not be achieved

due to construction problems: collapsing of the trench during the lowering in operation or river crossings (when the crossing is in open trench, the backfill of this trench shall be stabilized against erosion from the river).

 

Testing

 

After backfilling, the pipeline is hydrostatically tested following applicable regulations.

Actually, pipeline testing is more than just leak detection and resistance checking. Pipelines

are tested even when there is no applicable regulation. Indeed, the purposes of this “testing” operation are multiple:

- remove all debris from the inside; 

- verify that there is no obstruction or diameter reduction;

- make sure the pipeline will not come out of the ground under pressure (Bourdon

effect);

- make sure the additional weight of the test water can be supported by the trench;

- check for leaks with maximum accuracy,

- check for resistance, verify the strength of the pipe and the weld,

- empty totally the test water (even this little tiny pocket under this river crossing)

- clean and dry (mainly for gas pipelines) the internal surface of the pipeline before the

client introduces valuable product into it,

- close the line at both ends to avoid any introduction of foreign materials and water.

- avoid corrosion;

- avoid polluting the environment with the huge quantities of rusty test water;