الفهرس 
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Abstract
Regulating structures on rivers and large canals with sand beds are usually provided with revetments on the downstream side consisting of large concrete blocks to protect the bed against the erosive action of the water issuing with high velocity from the sluices. The blocks are laid, with open joints between them, on a layer of broken stone or coarse gravel. The latter
acts as a filter which prevents the fine sands of the bed from being entrained water rising from beneath the structure with a relatively high gradient. In the design of each structure’ the sub-floor is usually assumed to a totally free exit under the block revetment. In other words, the flow resultant uplift pressure distribution on the underside of the are assumed to be the same as if the block revetment did not exist. This assumption, however, cannot be justified under all circumstances. Any constriction in the effective exit area for the sub-flow must lead to an increase tin the loss of head through both the filter bed and the sand bed above what it been if the constriction did not exist. This effect is illustrated Depending on the relative extent t ~~\oc:k zo~e. 00 0.0’0°,00,0 c.uooo ~l\~r , ” t) , t) 0 & ( a.’” et., c ” rb v.!tt’. • I) C- 0n/o o. c.Ao.~ .. o~ aonl!’: . .’. l:f ” .. : 0.;[4.. ” Sa.n4
of the constriction in the block zone, certain areas in both the filter zone and the sand bed will be rendered inactive and the pressure Fig. (1) gradient in the active areas will be increased. A similar effect is met with in the case of flow into covered pipe drains embedded in a gravel filter. The exit area 01- l-l.OW l.UL- ,”,ue •.•..•. ”” .•.•• -0- --- is confined to the open gaps between the pipe units. This area constitutes only a small fraction of the total perimetric area of the pipeline. The effect on the sub-flow of constriction in the exit area is equiva¬lent to a lengthening of the solid floor. Such lengthening is an advantage as far as the reduction in the risk of piping is concerned, but it means that the uplift pressure on the underside of the solid floor is increased. Now, one of the safety conditions in the design of such regulating struc¬tures is that the thickness of the solid floor in the extension beyond the piers shall be everywhere sufficient to counteract any uplift pressure by sheer dead weight so as to avoid bending stresses in the floor. ”Hence it is desirable to find out how far the uplift pressure distribution is affected by constriction in the exit area beyond the solid floor. The free space between the concrete blocks in the D. S. revetments is always a small fraction of the total exit area, especially when it is filled with sand or broken stone. The existence of the broken stone filter under the blocks may be an amelinating factor in this respect but it cannot be taken for granted that it will totally eliminate the effect of the reduced flow area between the blocks. Obviously, this question can be settled only by experi¬ment.