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EM 1110-2-1601 1 Jul 91 Y, = saturated surface dry specific or unit weight of stone. pef Plate 31 presents relations between spherical diameter and weight for severn! values of specific or unit weight. Design procedures for determining the stone size required to resist the erosive forces of channel tlow are presented in paragraph 3-4 below. c. Unit weight. Unit weight of stone Y, generally varies from 150 to 175 pef. Riprap sizing relations are relatively sensitive to unit weight of stone, and Y, should be determined as accurately as possible. In many cases. the unit weight of SlOne is not known because the quarry is selected from a list of approved riprap sources after the consa-uction contract is awarded. Riprap coming from the various quarries will not be of the same unit weight. Under these circumstances, a unit weight of stone close to the minimum of the available riprap sources can be used in design. Contract options covering specific weight ranges of 5 or 10 pef should be offered when sufficient savings warrant. d. GratlaliDn (1) The gradation of stones in riprap revea-nent affects the riprap's resistance to erosion. Stone should be reasonably well graded throughout the in-place layer thickness. Specifications should provide for two limiting ~radation curves. and any stone gradation as determined from quarry process. stockpile, and in-place field test s:lJllples that lies within these limits should be acceptable. Riprap sizes and weights are frequently used such as D3O<min), DIOO(max), Wso(min), etc. The D or W refers to size or weigh!. respectively. The number is the percent finer by weight as discussed in b above. The (max) or (min) refers to the upper or lower limit gradation curves. respectively. Engineer Form 4794-R is a standard form for plotting riprap gradation curves (plate 32). The gradation limits should not be so restric- tive that production costs would be excessive. The choice of limits also depends on the underlying bank soils and filter requirements if a graded stone filter is used. FIlters may be required under riprap revetments. Guidance for filter requirements is given in EM 1110-2-1901. FIlter design is the responsibility of the Geotechnical Branch in each District. (2) Standardized gradations having a relatively nar- row range in sizes (DgsID1S of 1.4-2.2) are shown in Table 3-1. Other gradations c:m be used and often have a wider range of allowable sizes than those given in Table 3-1. One example is the Lower Mississippi Valley 3-2 Division (LMVD) Standardized Gradations presented in Appendix F. The LMYD gradations are similar to the gradations listed in Table 3-1 except the LMVD W so<max) and W lS{max) weights are larger, which can make the LMVD gradations easier to produce. Most graded ripraps have ratios of DsslD1S less than 3. Uni- form riprap (DsslD15 < 1.4) has been used at sites in the US Army Engineer Division. Missouri River, for reasons of economy and quality conlrOl of sizes and placement. (3) Rather than a relatively expensive graded riprap, a greater thickness of a quarry-run stone may be consid- ered. Some designers consider the quarry-run SlOne to have another advantage; its gravel- and sand-size compo- nents serve as a filter. The gravel and sand sizes should be less by volume than the voids among the larger stone. This concept has resulted in considerable cost savings on large projects such as the Arlcansas and Red River Navi- gation Projects. Not all quarry-run stone can be used as riprap; stone that is gap graded or has a large range in maximum to minimum size is probably unsuitable. Quarry-run stone for riprap should be limited to DsslDlS :S 7. (4) Determining optimum gradations is also an economics problem that includes the following factors: (a) Rock quality (dUJ1lbility under service conditions) (h) Cost per ton at the quarry (including capability of quarry to produce a particular size) (c) Number of tons required (d) Miles lransported (e) Cost of lransportation per ton-mile (I) Cost per ton for placement (g) Need for and cost of ruter (h) Quality conlrOl during consa-uction (it is easier to ensure even coverage with a narrow gradation than with a wide gradation) (i) Number of different gradations required. Some- times cost savings can be realized by using fewer gradations. See EM 1110-2-2302 for further discussion of these factors.