Rock Riprap

The following elements of rock riprap should be considered:
  • size – Rock riprap consists of loose rock that is dumped on the slope and distributed. The size of the rock should be large enough that it withstands the forces of wind and water directed at the slope.
  • placement – The rock should be placed on a bedding of sand, engineering fabric pinned to the slope, or both a bedding of sand and engineering fabric pinned to the slope. Bedding is primarily for the purpose of keeping the embankment material in place as the embankment is saturated and drained.
  • keyed rock riprap – An effective rock riprap variation is keyed riprap. Keyed riprap is rock that has been placed and distributed on bedding upon the slope and then slammed with a very heavy plate to set the rock riprap in place (i.e., to key the rock together). Rock riprap is considered a rough slope when computing wave runup on the slope.
Once the wind effects are known, the weight of the median stone and the total thickness of the riprap blanket can be established using the following equations:
EquationObject332399
Equation 12-1.
EquationObject333400
Equation 12-2.
where:
  • W
    50
    = weight of the median sized stone (lbs.)
  • γ
    s
     = specific unit weight of the stone
  • H
     = design wave height (ft.)
  • K
    D
     = riprap stability coefficient, 4.37 is appropriate for TxDOT
  • α = slope angle from the horizontal in degrees
  • G = specific gravity of the stone material
  • W
    max
    = weight of the maximum sized stone (lbs.)
  • W
    min
    = weight of the minimum sized stone (lbs.)
  • T
    = thickness of the riprap layer (in.)
  • ç = number of layers of W
    50
     (typically taken as 2)
  • KÄ = layer thickness coefficient (typically taken as 1)