Hydrodynamic pressure

Hydrodynamic pressure

The heel of a structure is sunk into permeable subsoil, which allows free water flow below the structure – see figure. The unit weight of soil lifted by uplift pressure γ_su is modified to account for flow pressure. These modifications then depend on the direction of water flow.

Action of hydrodynamic pressure

When computing the earth pressure in the area of descending flow the program introduces the following value of the unit weight of soil:

and in the area of ascending flow the following value:

where:	γsu	-	unit weight of submerged soil
	Δγ	-	alteration of unit weight of soil
	i	-	an average seepage gradient
	γw	-	unit weight of water

An average hydraulic slope is given:

where:	i	-	an average seepage gradient
	hw	-	water tables difference
	dd	-	seepage path downwards
	du	-	seepage path upwards

If the change of unit weight of soil Δγ provided by:

where:	i	-	an average seepage gradient
	γw	-	unit weight of water

is greater than the unit weight of saturated soil γ_su, then the leaching appears in front of structure - as a consequence of water flow the soil behaves as weightless and thus cannot transmit any loading. The program then prompts a warning message and further assumes the value of γ = 0. The result therefore no longer corresponds to the original input – is safer.