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 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.