Batter Protection Road Construction

Embankment Protection by Pitching

Embankment protection by pitching  (hand  placed  rip  rap)  consists  of  large  stones  placed  by  hand  on  the  batter. Smaller rock fragments are placed in the voids between the larger rocks.
The pitching  may also  be cement grouted, in which case a sand cement grout (usually  3:1)  is broomed or rodded into the voids between the stone. The finished product is rigid but has very little  strength.  Pitching  requires  adequate,  firm  support  from  the  embankment  and  must  be protected from undermining at the toe and ends. Some joints should be left ungrouted to prevent uplift from water pressure.

The quality of stone used should be similar to that used for rip rap except that the stone needs  to  be  selected  to  obtain  more  uniformly  sized  and  cubically  shaped  pieces  to  facilitate hand placing.

During  construction  the  shape  of  the  batter  is  controlled  from  string  lines  stretched between timber profile boards set to the batter profile.

Pitching  has  been  used  extensively  in  the  past  as  a  form  of  embankment  protection, however its construction is labor intense. It is more rigid than dumped rip rap and consequently does  not  have  an  equal  capacity  to  adjust  to  any  movement  of  the  supporting  material,  or  to arrest an undermining scour.

Batter Protection Road Construction

Embankment Protection by Rip Rap

The  role  of   embankment  protection by rip rap  is  to  minimize  scouring  and  maintain  the embankment in a structurally stable condition during inundation.

It is usually not economically justifiable to provide floodway protection that will prevent scouring  at  all  flow  conditions,  but  such  protection  must  be  able  to  arrest the  scour  before  it seriously affects the stability of the embankment.

The types of protection used include grassing, rip rap (dumped rock fill), stone pitching, stone filled wire baskets or mattresses and concrete slope paving.

Rip Rap

Rip  rap  is  coarse,  graded  stone  which  is  dumped  on  the  prepared  batter  to  provide protection against scour. It is placed by tipping from trucks or by means of a loader or
other mechanical equipment. Manual labor is required only for moving the occasional stone so that  the  rip  rap  profile  conforms  to  the  required  lines,  and  to  correct  gross  segregation  in localized areas. where the embankment is composed of fine material, a filter blanket of gravel (filter  material)  is  required  between  the  rip  rap  and  the  embankment to  prevent  the  soil  fines from the embankment being washed out through the rip rap.

Details of the size and quality of stone to be used are given in the job specifications or special provisions.

It  has  been  found  from  experience  that  well  graded  rip  rap  is  superior  to  a  mass  of uniformly  large stone, since the  latter has  large voids through which the  filter  material can  be drawn  by  the  action  of  water.  On  site,  the  gradation  of  the  rip  rap  is  normally  controlled  by visual  inspection.  To  aid  the  supervisor’s  judgment,  a  sample  of  about  5  tonnes  may  be prepared by sorting, weighing and remixing in the proper proportions. This is kept for reference on site or in the quarry.

The depth of rip rap specified should be at least equal to the maximum sized stone. The rip rap blankets should extend below stream bed level in the form of a cut off wall. Alternatively a stone toe should be provided to supply stone to fall into scour hole and thus extend the blanket.

The stone used should be hard, durable, angular in shape, and resistant to weathering and the action of water. It should be free of overburden soil etc. The least dimension of individual stones should be not less than one third their maximum dimension. Rounded stone is usually not acceptable.

Rip rap should be placed to the full course thickness in one operation without displacing the underlying material. The larger stones should be well distributed within the mass of rip rap.

Placing in layers, dumping into chutes or any other method likely to cause segregation should be avoided. The filter material and the rip rap protection should be placed in conjunction with the construction of the embankment, so that the minimum amount of embankment is left unprotected at any one time. Timber profile boards may be used to control line and level.

The  advantages  of  rip  rap  batter  protection  are  that  it  is  flexible  and  able  to  adjust  to scoured conditions, is free draining (which limits the water pressure which may act upon it), and requires  a  minimum  of  labor  for  placing.  The  major  disadvantage  of  rip  rap  is  that  a  ready supply of good quality quarried rock is required.