Laserfiche WebLink
<br />e <br /> <br />e <br /> <br />e <br /> <br />DRAINAGE CRITERIA MANUAt. (V. 2) <br /> <br />HYDRAULIC STRUCTURES <br /> <br />10. Concrete walls and footer dimensions should be determined by conventional structural methods. <br />Cutoffs and underdrain requirements should be determined by seepage analysis discussed <br />earlier in this chapter. <br /> <br />11. The hydraulic impact forces on the baffles should be determined to allow the structural engineer <br />to size adequate reinforcing steel. Figure HS-12d may be used as a guideline. The structural <br />engineer should apply a conservative safety factor. <br /> <br />2.6 SeeDaae Control <br /> <br />2.6.1 SeeDaae Analysis Methods. Seepage analysis through manual and computerized flow net <br />analyses is preferred as a deterministic methodology that can directly quantify flows, pressures, and <br />critical gradients. These methods can deal with multiple media strata and other complicated situations. <br />The user is referred to Cedergren (1967), USBR (1987) and Taylor (1967) for more extensive information <br />and analysis techniques. <br /> <br />At a minimum, Lane's Weighted Creep Method can be used to identify probable seepage problems, <br />evaluate the need for control measures, and roughly estimate uplift. This technique was originally <br />proposed by EW. Lane in 1935. The method has been deleted in the 1987 revision of Design of Small <br />Dams (USBR 1987), possibly indicating greater use of flow net and computer modeling methods. <br />Although Lane's method is relatively well founded, it should be used as a guideline, and when marginal <br />conditions or complicated geological conditions exist, more sophisticated analysis should be used. The <br />essential elements of Lane's method are as follows: <br /> <br />1. The weighted-creep distance through a cross section of a structure is the sum of the vertical <br />creep distances, L, (along contact surfaces steeper than 45 degrees), plus one-third of the <br />horizontal creep distances, LH (along contact surfaces less than 45 degrees). <br /> <br />2. The weighted-creep head ratio is defined as: <br /> <br />c J~+Lv) <br />w H <br />s <br /> <br />(HS-13) <br /> <br />in which: <br /> <br />Cw = creep ratio <br /> <br />Hs = differential head between analysis points (ft) <br /> <br />3. Reverse filter drains, weep holes, and pipe drains help to reduce seepage probiems, and <br />recommended creep head ratios may be reduced as much as 10% if they are used. <br /> <br />06/2001 <br />Urban Drainage & Flood Control District <br /> <br />HS-33 <br />