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<br />EM 1110-2-1601 <br />1 Jul 91 <br /> <br />labor unusually cheap. Steeper side slopes can be used <br />with hand-placed riprap than with other placing methods. <br />This reduces the required volume of rock. However, the <br />greater cost of hand placement usually makes machine or <br />dumped placement methods and flatter slopes more <br />economical. Hand placement on steep slopes should be <br />considered when channel widths are constricted by <br />existing bridge openings or other SlruClUreS when <br />rights-ofway are costly. In the machine placement <br />method. sufficiently small increments of stone should be <br />released as close to their fmal positions as practical. <br />Rehandling or dragging operations to smooth the <br />revetment surface tend to result in segregation and breaJ:- <br />age of stone. Stone should not be dropped from an <br />excessive height or dumped and spread as this may result <br />in the same undesirable conditions. However, in some <br />cases. it may be economical to incre:lse the layer thick- <br />ness and stone size somewhat to offset the shortcomings <br />of this placement method. Smooth, compact riprap sec- <br />tions have resulted from compacting the placed stone sec- <br />tions with a broad-lr.1Cked bulldozer. This stone must be <br />quite resistant to abrasion. Thickness for underwater <br />placement should be increased by 50 percent to provide <br />for the uncertainties associated with this type of place- <br />ment Underwater placement is usually specified in terms <br />of weight of stone per unit area. to be distributed <br />uniformly and cona-olled by a "grid" established by shore- <br />line survey points. <br /> <br />Section V <br />Ice, Debris, and Vegetation <br /> <br />3-13. Ice and Debris <br /> <br />Ice and debris create greater stresses on riprap revetment <br />by impact and flow concenlration effects. Ice attachment <br />to the riprap also causes a decrease in stability. The Cold <br />Regions Research Engineering l..1boratory, Hanover, NH. <br />should be contacted for detailed guidance relative to ice <br />effects on riprap. One rule of thumb is that thickness <br />should be increased by 6-12 in.. accompanied by <br />appropriate increase in stone size, for riprap subject to <br />attack by large floating debris. Riprap deterioration from <br />debris impacts is usually more extensive on bank lines <br />with steep slopes. Therefore. riprapped slopes on SlreamS <br /> <br />3-10 <br /> <br />with heavy debris loads should be no steeper than I V on <br />2.5H. <br /> <br />3-14. Vegetation <br /> <br />The guidance in this chapter is based on maintaining the <br />riprap free of vegection. When sediment deposits form <br />lowfiow berms on riprap inslallations. vegetation may be <br />allowed on these berms under the foUowing conditions: <br />roots do not penelrate the riprap: failure of the riprap <br />would not jeopardize project purposes prior to repairs: and <br />the presence of the berm and vegetation does not <br />significantly reduce the discharge capacity of the project. <br />For riprap areas above the 4 or 5 percent exceedence flow <br />line, consideration may be given to overlaying the riprap <br />with soil and sod to facilitate maintenance by mowing <br />rather than by hand or defoliants. This may be <br />particularly appropriate for riprap protecting against eddy <br />action around Slructures such as gate wells and outlet <br />works in levees that are otherwise maintained by mowing. <br />Recognizing that vegection is. in most instances, inimical <br />to riprap instaUations. planned use of vegetation with <br />riprap should serve some justifiable purpose, be accounted <br />for in capacity computations. be controllable throughout <br />the project life, have a slrengthened riprap design that will <br />withstand the additional exigencies. and account for <br />increased difficulty of inspection. <br /> <br />Section VI <br />Quality Control <br /> <br />3-15. Quality Control <br /> <br />Provisions should be made in the specifICations for samp- <br />ling and testing in.place riprap as representative sections <br />of revetment are completed. Additional sample testing of <br />in-place and in-lransit riprap material at the option of the <br />Cona-acting Officer should be specified. The primary <br />concern of riprap users is that .lhe in-place riprap meets <br />specifications. Loading, lransporting, stockpiling, and <br />placing can result in deterioration of the riprap. <br />Coordination of inspection effortS by experienced staff is <br />necessary. Reference EM 1110-2-2302 for detailed <br />sampling guidance and required sample volumes for <br />in-place riprap. <br />