Protecting abutments and piers at bridges is beneficial in limiting the effects of scour. The use of stone protection is recommended over concrete riprap due to its flexible nature. Concrete riprap, due to its rigidity, masks problems. Consequently, voids can form under them and eventually undermine the pavement or approach slab.

Stone protection needs to be designed for the conditions that exist at the bridge. The recommended methodology for the design of stone protection is to use HEC – 23 Bridge Scour and Stream Instability Countermeasures: Experience, Selection, and Guidance (see Volume 2 for guidance on design). Upon completing the design, the appropriate D50 will be determined. This value should then be compared to the tables in Item 432 – Riprap to determine the appropriate size of the stone protection. Once the appropriate size of the stone protection has been identified then the appropriate thickness of the stone protection needs to be determined. The thickness is a function of the conditions where the stone protection is being used. However, a general rule of thumb is that the thickness needs to be equal to or larger than 1.5 times the size of the stone protection listed in Item 432. This is consistent with the procedures outlined in HEC-23 for the thickness of the stone protection, where it is typically taken as the larger of either the 100% size (i.e. maximum size) or 2 times the D50 size. If one compares the range of the D50 values for the various sizes listed in table 2 of Item 432 and then multiplies by these by 2 one will obtain approximately 1.5 times the size of the stone protection listed. In the plans Stone Protection should be specified as follows:

Riprap (Stone Protection) XX in. (where XX is the size in inches)
Thickness = YY in. (where YY is the appropriate thickness)