For all modern  pipe-laying    techniques, whether   in rehabilitation   work   or    the  construction   of  new pipelines,   HDPE   pipes    usually   provide   the simplest,  most  economic  solution.   Indeed, radically   new   rehabilitation    techniques  have been developed (like Cosmoplast’s COSMOLOK™ sewer    rehabilitation   system )   which   rely   on polyethylene’s   unique    properties.
When polyethylene pipe  is  laid, like any other pipe it  may  suffer  surface  damage  if  not  properly handled.

Precautions should be taken to reduce this, although surface scoring  up  to  10%  of  the pipe  wall  thickness will not  affect  the  overall  performance  of  the  pipe. Generally, at least 3 pipe lengths of ground should be excavated ahead of mains laying to expose any obstructions which may necessitate deviation from the planned route.

GENERAL POINTS ON TRENCH EXCAVATION

Installation of HDPE systems requires a minimal trench width, therefore considerable saving can be made in terms both of reduced labour costs and less soil to  be moved.  Additionally, reinstatement costs are cut and where needed smaller  quantities   of  imported  backfill are required.

 

The dimensions of a trench line opening are normally governed by the pipe diameter, method of jointing and site conditions.  Normal minimum depth of cover for mains  should be 900mm from ground level to the crown of the pipe.  Trench  width  should  be  as narrow as possible, but never less than the  outside  diameter   of  the   pipe  plus  250mm  to  allow  for correct compaction of side fill.

     

In some instances it may be acceptable  to  lay  HDPE  pipe  directly  on  the bottom of the trench - but only where the soil is uniform, relatively soft and fine-grained without large flints, stones and other hard objects.  The trench  bottom  should  be  brought  to  an  even finish, providing consistent support for pipes along their whole length.

 

In other  cases,  the  trench  should  be  cut to  a  depth  which  will  allow for the necessary thickness of selected bedding material  below  the  bottom  of  the  pipe.  If  soil  from  the excavation is unsuitable, granular  material  should  be  imported.  Gravel  or  broken  stone 

Recommended Trench Composition

graded between five and ten millimetres in size provides suitable bedding since it needs little compaction.  Coarse sand, a sand and gravel  mix,  or  gravel  smaller  than  20mm are also acceptable straight from the quarry.

 

Excavators with narrow buckets are best suited for conventional trenching  methods.  Pipes are then positioned by being lifted and placed into the correct location.  After installation, the ground can be backfilled as noted above and properly consolidated.

BAC K FILLING

Unless   special   procedures   apply,   such    as    local    agreements   for    carriageway reinstatement,  appropriate  excavated  material   may   be   returned   to  the  trench   and compacted  in  layers   of   a   thickness  specified  by   the   appropriate   utility.    Heavy  compaction   equipment  should  not  be  used  until the fill over the crown of the pipe is at least 300mm.