VISUALIZATION OF FLOW LINES
Object:-To visualize the flow lines.
Apparatus:-Permeability tank.
Theory:- Seepage underneath a sheet pile wall is one of the seepage problems that are most common in practice. Sheet pile walls are used to reduce seepage under all types of dams, sea walls, dividing walls, coffer dams, and similar structures. They are also used to reduce leakage from canals, rivers, and subsoil surrounding an excavation and the lake.
The streamlines represent the paths along which the water flows through the subsoil. Every particle entering the soil at a given point upstream of the work will trace out its path and will represent a streamline. The first streamline follows the bottom contour of the work and is the same as Bligh’s path of creep. The remaining streamlines follow smooth curves transiting slowly from the outline of the foundation to a semi-elipse.
Experimental
Setup:-
- Prepare about 1 pint of the fluorescent solution by slowly adding the chemical into the water until the solution becomes a semi-transparent opal-like liquid of orange-greenish color.
- Fill the tank with pure sand to a level of about 1 foot above the bottom of the tank.
- Adjust the upstream overflow so that its top is about 4 inches below the top of the tank and the downstream overflow so that its top is about 1 inch above the surface of the sand bed.
- Adjust the impermeable screen in the middle of the tank. Leave about 5 to 6 inches of clearance between the screen and the bottom of the tank. Seal the contact between the screen and tank walls with grease or other easily removable seal. Apply the seal on the downstream side to prevent leakage in case the screen moves under the final pressure difference.
- Pour water slowly into the tank. Start with the downstream pool and transfer the input into the upstream after the lower pool is full. After overflow occurs both upstream and downstream reduce water input to the minimum needed to maintain a constant water level in the upstream pool (in this case there will be a small continuous overflow from the upper pool). Smooth out any sharp irregularities of the sand bed that may have formed while filling the pool.
- Fix the bottle with dye on the stand in such a position that the liquid level is approximately at the same elevation as the water level in the upper pool. Then, depending on the desired number of flow lines (3 to 4 recommended), insert the corresponding number of dye-injection needles vertically about 0.25 inches into the sand along one of the transparent walls of the upper pool. To obtain an approximately “square” flow net, the spacing between the needles should progressively increase in the upstream direction from the impermeable screen. The suggested needle distances from the screen, for 4 flow lines, are approximately 2, 4.5, 9, and 15 inches. Tape the needles (or the tubes connecting them with the bottle) to the tank wall just above the water.
- After the dye-injection needles have been fixed, lift the bottle with dye and position it so that the liquid level is about 0.5 inches above the water level in the upper pool. The position of the bottle should be adjusted according to the appearance of the flow lines. If the lines are wide, the bottle is too high. If no dye appears or its flow is irregular, the bottle is too low. The formation of flow lines may require several minutes to an hour or two, depending on the permeability of the sand used.
- To stop the experiment, shut off the dye input by lowering the dye container until the dye surface is about 2 inches below the water level in the pool. Let the flow lines wash away. Don’t take out the needles before the dye input has been shut off as indicated. Otherwise, dye will get into the water in the pool while the needles are being removed.
Observations:-
[Draw the observed streamlines and attach them here.]
Results:-Flow lines are visualized and drawn as shown above.
Conclusions:-