Forces Acting On Dams

FORCES ACTING ON DAMS

The design of any dam, the forces generally considered as acting on the structure may consist of the following.

·        Ice pressure

·        Wave pressure

·        Weight of dam

·        Water thrust

·        Uplift pressures at the Bottom of the dam

·        Silt pressure

·        Earth-quake forces

·        Reaction of the foundation

·        Atmospheric pressure

Ice pressure

Formation of a thick sheet of ice occurs in winter in cold countries where the air current temperatures go as low as to 10Â o F. In such case, ice sheet forms on the top surface of the sheet will be at the temperature of blowing wind well below the freezing point, while the temperature varies from below the freezing point at the top to +32Â o F at the bottom surface. If the sides of the dam provide restraint, then the pressure exerted will be comparatively more, than when the sides do not provide any constraint.

Where the dam is provided with an overflow spill way, the spill way crest is usually some distance below the maximum water level. The maximum level occurs only at times of maximum flow and a solidly frozen sheet at that time is highly improbable. It is usual to assume that the worst ice condition will occur only with water at the spill way lip.

Nothing is known of the action of ice during an earth quake, and its earth-quake effect on the stability of the dam is ignored.

The thrust exerted by expanding Ice sheet varies with the thickness of Ice sheet, air temperature rise, and the restraint.

Wave Pressure

The upper portions of dams are subject to the wave action; the dimensions of the waves depend upon the extent of the water (Fetch) surface and the velocity of wind.

Aknowledge of the wave heights is important, if over-topping by waves is to be avoided.

MOLITER,         STEVENSON’S  FORMULA  FOR  WAVE  HEIGHT

If V is the velocity of the wind in KM/hour and F is the fetch in kilometers, then the height of the wave is given by

hw =0.032 Rt(VF) + 0.763  0-.271 F^1/4

And when F > 32 km, then hw = 0.032 Rt(VF)

Where is‘hw’theheight of the wave, from the trough to the crest, and 2/3 ‘h of the crest of the wave above the still water level. Free board for the dam should be at least 1.5 times the wave height ‘hw’.     When   theinst thewavevertical forcehitsofthe dam,agait rides up the face of the dam to a height of       1/3 ’hw’above the still water level as shown in the figure.  for   earth dams having flat slopes, it is assumed that the waves ride up the slope, a vertical distance above the still waterto level,1.4‘hw’,equalthetoforheightthe1smoothest.4‘hw’va slope.

The maximum wave pressure to Pw occurs at 0.125 hw above the still water level and is given by the equation

Pw = 2.4w ´hw  or   Pw = 24 hw   (in Kilopascals)

The total wave force is given by Pw = 2 ´w ´hw² or Pw = 20 hw² (n kilo Newtons) and acts

at 0.375 hw   above   still   water   level,   where   ‘w’   is   t

And hw is height of wave in Metres.