The maximum steepness of an armored slope is often determined by the nature of the 

 armoring material. A small stone riprap of rounded cobbles may not be stable on a slope 

 steeper than about 1 on 2.5, whereas with increasing size and angularity the steepness may 

 be increased to a maximum of about 1 on 1.5 provided the cut slope will stand on this slope 

 until the armor is placed. Extremes of wave action and current flows may determine the 

 minimum stone size of a riprap slope. These possible extremes (sometimes occurring in an 

 off-season when the boats have been removed from their berths) should be determined in 

 project planning and the proposed armoring materials checked for size against appHcable 

 formulas for seawall and river revetment design. 



Because of the strong pumping action of waves and eddy currents, armor stone ripraps 

 should always have a thickness of twice the average stone dimension, and a filter blanket 

 under this armor layer is a vital requirement. If an appreciable amount of the underlying 

 material should be pumped from beneath the riprap at any point, the armor stones wiU drop 

 down into the resulting scour pockets, leaving the native soil exposed. Progressive erosion 

 may then destroy large sections of the revetment before the damage can be repaired. 

 Deposition of properly graded filter gravel of adequate thickness (0.5- to 1-foot thick 

 depending on riprap stone size) may prevent such a -failure. However, the danger of 

 displacement of this filter material during construction requires careful supervision during 

 riprap laying operations. An alternate filtering device that is gaining wide acceptance is the 

 continuous cloth filter, which ensures continuity and complete filtering action throughout 

 the revetted area (Barrett, 1966). Riprap cross sections specifying these two types of filters 



are shown in Figure 44. 



A slope paved with precast concrete blocks also requires a filter layer as the underlying 

 soil may be pumped through the joints between the blocks. Using filter cloth alone for this 

 purpose may provide inadequate reUef for hydrostatic uplift pressures, and may result in 

 individual blocks being hfted out of the section. Unless the underlying soil is exceptionally 

 porous, a gravel layer between the cloth and concrete blocks is recommended to allow quick 

 lateral distribution of locally high pressures to points of relief. The ease with which an entire 

 face of the block paving can slide down a slope dictates a need for an adequate toe-thrust 

 resistor. This usually takes the form of a sheet-pile cutoff wall, or if it can be built in the 

 dry, a poured wall or edge beam. 



If the entire slope can be unwatered or can be armored at the extreme low water level, 

 concrete slope paving may be the best solution (Fig. 45). However, a soils engineer should 

 ensure that the soil is not subject to heaving and shrinking, and crack the concrete. 

 Expansion joints should be provided at intervals of 50 to 100 feet depending on the extreme 

 range of temperature and, within the concrete, temperature steel reinforcing bars should be 

 spaced not more than 12 inches apart both laterally and longitudinally. Weep holes 

 connected by French drains should be spaced at about 10-foot intervals down and along the 



87 



