Drainage Design Manual - the City of San Luis Obispo

GRADING OF ROCK SLOPE PROTECTION PERCENTAGE LARGER THANSTANDARDRock SIZEor Rock MASSor Rock WEIGHTRSP-Classes [A]Method A Placement: Larger RSPMethod B Placement: smaller RSPRSP-Classes other than BackingBacking No.25 lb 11 kg 95-10090-100 25-750-55 lb 2.2 kg 90-10025-751 lb 0.4 kg 90-100[A][B]US customary names (units) of RSP-Classes listed above SI names, Example US is “2 ton” metric is “2 T”.“Facing” has same gradation as “Backing No. 1". To conserve space “Facing” is not shown.Example for determining RSP-Class of outside layer. By using Equation 1, if the calculated W=135 kg (minimum stablerock size):1. Enter table at left and select closest value of STANDARD Rock SIZE which is greater than calculated W, in thiscase 220 kg2. Trace to right and locate “50-100" entry3. Trace upward and read column heading “1/4 T”, then 1/4 T is first trial RSP-Class.6.7.5 Determine the Required Layers of RSPInexperienced designers sometimes use Table 6-3 and specify all the RSP-Classes between the"final selection" outside RSP layer and Backing No. 3. To avoid this pitfall, use Table 6-4,California Layered RSP. Standard designs include RSP-fabric, Backing Class, Inner, andOutside layers of RSP as shown. In Table 6-4, in conformance with filtration theory, from thebank to the stream, each layer was designed progressively larger, so an inner layer will not passthrough voids of the next layer. The thickness of the entire cross section is reduced and lesscostly when RSP-fabric replaces Backing No. 3. Do not arbitrarily eliminate RSP-fabric. If youdo not use RSP-fabric, then 230 mm of Backing No. 3 is normally required as the initial "filterseparator"layer, and it is placed directly on the bank to be protected. If Backing No. 3 isrounded, river-run material, then the steepest allowable slope angle should be 1V:2.5H, contraryto the recommended 1V:2H of the Caltrans Standard Specifications.An example using RSP-fabric is: Type B RSP-fabric is placed directly on the bank as the initial"filter-separator" material, the inner layer is Light, and the outside layer is 1T. Notice that inTable 6-4, when the outside layer is 1T or larger there is more than one possible design for innerlayers. Each design satisfies filtration theory, that is, underlying layers are retained. Rockavailability and/or cost of producing one design versus another may determine which RSPclassesare selected as inner layers. Or on another part of a project there may already be aspecified inner RSP-class, and rather than introducing another inner RSP-class, use the one thatis already specified.SLO Creek Drainage Design Manual 81 February 2003