DETERMINATION OF UPLIFT PRESSURE
Object:- To determine uplift pressure on foundation of hydraulic structure.
Apparatus:- Permeability tank.
Theory:-
Whenever a structure (dam, weir, retaining wall, etc) is built on permeable material and separates two water pools with different elevations of water level, seepage occurs underneath and the water exerts pressure on the structure along its whole submerged perimeter. That part of the pressure that acts upward and tries to lift the structure is called the uplift pressure and is extremely important in the stability analysis. As explained earlier, the submerged perimeter of a structure represents a flow line and the pressure at any point can be determined from the flow net. However, it can also be measured directly by means of standpipes.
The objective of the present experiment is to demonstrate how such direct pressure measurements are made, using the horizontal bottom of a submerged structure as the basis for experiment. In this special case all the pressure is considered as uplift pressure.
The objective of the present experiment is to demonstrate how such direct pressure measurements are made, using the horizontal bottom of a submerged structure as the basis for experiment. In the special case all the pressure is uplift pressure.
Experimental
Setup:-
The setup is basically the same as in Experiment # 04 with an additional element consisting of a simulated foundation of a structure. The “foundation” is simulated by means of a Plexiglas plate, about 0.25 inch thick, 1.5 feet long and 6 inches wide, with both long edges covered with the elastic rubber packing similar to that of the Plexiglas screen supplied with the tank. Simulated foundation has about 5 standpipes fitted along the centerline as shown in figure.
The “foundation”, with standpipes in place, is positioned horizontally immediately downstream of the vertical impermeable screen and on top of the sand. The contact between foundation and screen is sealed. The contact between the rubber edges of the foundation and the tank walls should also be sealed by grease. In order to prevent the foundation from being lifted some weights can be put on it. Finally, water is poured in. The uplift pressure profile is shown by the rise of water in the standpipes.
Observations:-
Uplift pressure at the various point of the foundation is recorded as shown in following table.
No. of Stand Pipe | 1 | 2 | 3 | 4 | 5 |
Distance (inch) | |||||
Uplift Pressure (inch) |
Graph: From the observed values Uplift pressure profile is drawn as shown.
[Attach the uplift profile here]
Result:- Uplift pressure in stand pipes is visualized and confirmed that it decreases towards downstream side.
Conclusion:-