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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />For a typical drop structure where weep drains are provided at the toe and the supercritical occurs <br />at the toe flow, these pressure fluctuations should not be of a great concern. However, when <br />submergence causes the jump to move upstream and the drains discharge to the jump zone, pressure <br />fluctuations could be transferred to areas under supercritical flow and produce a net uplift condition <br />of concern. <br /> <br />OVERALL FORCE ANALYSIS. SECTION XI <br />The overall force analysis should consider specific points along the drop and the overall drop structure <br />stability including geotechnical and structural stability, The horizontal components are generally <br />small (less than I psi) and capable of being resisted by the strength of the grouted rock or reinforced <br />concrete, When problems occur, they are generally a result of net vertical instability at Point 3 and/or <br />seepage uplift pressure problems, It has been concluded that a weep drain system and/or a large toe <br />drain is generally needed with evaluation of the hydraulic jump and submergence problems discussed <br />previously, <br /> <br />The critical design factors are seepage cutoff and relief, and pressure fluctuations associated with the <br />hydraulic jump. Safety factors usually have some comparison of stresses occurring versus stress <br />allowable. With the denominator fixed, various safety factors that have been reported are comparable, <br />In the 1968 MWE report, a safety factor approach was suggested based upon the sum or forces down <br />divided by the sum of forces up. Since the forces up can vary from situation to situation, the resultant <br />safety factors are inconsistent. After further study, a more meaningful approach has been developed <br />whereby a safety allowance is required at any point and under all flow conditions up to and including <br />the 100 year flood, Generally a 30 pound net downward safety allowance should be provided, and 60 <br />pounds is much preferred, <br /> <br />For grouted rock and concrete, this method will generally reduce the thickness requirements indicated <br />in the 1986 MWE report, This is particularly true for higher discharges and depths, Therefore, Figure <br />XI-2 should be replaced with the simpler 30 pound net force guideline, <br /> <br />GROUTED SLOPING BOULDER DROPS. SECTION XII <br />Table 1-2 presents the minimum guidelines for grouted rock layer thickness based upon the <br />application of the above safety allowance approach, <br /> <br />1-6 <br />