hydrodynamic suction

huangyhg

hydrodynamic suction
according to indian code, the hydrodynamic pressure is same, whatever the direction - positive pressure or suction.
should not the suction be limited to that of vacuum? how does water apply suction or tension on the face of a retaining wall or dam?
thanks.
er, it is limited to the vacuum pressure.  you can get lower pressures in soils via soil suction, but that's a whole different topic...
the way i have accounted for the "suction" in the past is that it actually reduces the hydrostatic load by the "suction" amount.  truly, you will always have a load on the   
soil suction is defined as air pressure minus pore water pressure (ua - uw).  this is called matric suction.  we get suction when the air pressure in the soil is greater than the pore water pressure.  this generally occurs above the water table.  below the water table there is no air pressure so we are left with just pore water pressure as the constitutive variable and above the water table we have both pressures, thanks to capillary rise, ground surface conditions, environmental conditions, etc.  
for such short questions they are very complex.  i have other questions you may want to ask.  
- how high might the lateral pressures be against the retaining wall upon wetting of the backfill?  how much lateral movement might be anticipated as a result of the backfill becoming saturated?
- what are the magnitudes of active and passive earth pressures for the soil above the water table?
- would the lateral pressures be related to the "swelling pressure" of the soil?
- how would infiltration along the top of your dam change your soil suction values and thus the stability of your dam?
these questions are very complicated even to the most senior geotechnical engineer.  i would recommend that you talk to a local geotech engineer in your area for assistance.  sorry if i just complicated your questions even further.  good luck.
hi flame,
the hydrodynamic forces are for fluid in motion over a spillway for example. if your crest is not designed correctly you will get uplift on your spillway or crest.
consequently the ogee profile is calculated to ensure you do not get uplift at the design flood. at lower flows you will get pressure and at higher flows "suction".
this uplift occurs when you try and change the direction of the water flow quicker than gravity allows. should the negative pressures become too high you get problems with cavitation