DESIGNING STONE STRUCTURES

Angular, quarried stone, often called riprap, is one of the most common materials used in bed and bank stabilization. Design variables for stone properties include size gradation, specific weight, and the geometry of the structure. For example, a stone blanket revetment design must include the angle from the horizontal of the face of the blanket, and the thickness of the blanket. Construction techniques (such as whether the stone is placed underwater or "in the dry") affect the selection of design parameters, as does the level of turbulence expected under design conditions. Larger stone is required for spur dikes, revetments subjected to impinging flow, or locations downstream from weirs than that required for structures where flow is essentially parallel to the bank face and turbulence levels are low.

Complete designs for stone blanket revetments should include specification of filter layers or filter fabric, if needed, and design of upper bank protection, upstream and downstream revetment edges, and toe details.

The size of stone used for a bed or bank protection structure is a key design variable. Stone size is governed by the anticipated hydraulic loading, which may be specified as as shear stress or velocity for riverine conditions and a wave height for sites subjected to wave wash (See Special Topic: Revetments to Resist Wave Wash). Stone size is usually specified by a gradation curve (a size frequency distribution). The Federal Highway Administration, U.S. Army Corps of Engineers, and some state Departments of Transportation (DOTs) have developed their own methods of sizing riprap. Most states have their own specifications for classifying riprap size and gradation. A computer program developed by the Corps of Engineers named Chanlpro¹ is available for sizing riprap for trapezoidal and natural channels as well as designing gabion mattresses and estimating scour depth in erodible channels. Details are provided in the user’s guide.

¹ Note: Netscape users may need to access this file directly from the folder on the CD. (HTML > WorksCited > CHANLPRO >CHANLPRO.exe)