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10 2.9 FLOW PATH 29-1 Flow Path is the approximate line or path droplet or molecule of water takes whe moving tlucugh a material. These comparisons are intended to rank or order the materials in terms of how much area or how many pathways are available for water to flow through. A larger void area reduces the soil's ability to resist the flow of water. 2.8 WATER PRESSURE An understanding of two aspects of water behavior is important in understanding why water tends to flow through a soil. These are: 1. The pressure exerted at any point in a body of water increases with increased depth below the water surface. 2. The water pressure exerted at a point can act in any direction. Figure 2.8-1 shows a container with three holes of the same size along one side. Water flowing from the bot- tom hole moves much further horizontaIly because of the increased pressure at that exit point. FIGURE 2.11-1 WATER PRESSURE VARIES WITH DEPTH Figure 2.8-2 shows several smaIl pipes originating at approximately the same point. The water flows in the various directions. This illustrates that water pressure at any point can cause flow in any direction if a path is available. FIGURE 2.11-2 WATER PRESSURE AT ANY POINT ACTS IN ALL DIRECTIONS '\ A . . , ~-......; . .. ~.. ~.' 4'w ''..-.:~;'~'-~:,.~ ". .: .. . ~ .... . ...... . .,.f . . ~ . .....: . . '''.,. B . . FIGURE 2.!H FLOW PATH OF WATER MOVING THROUGH SAND- FILLED CONTAINER. In the above figure water is introduced into a sand-filled container at the top and exits through the pipe in the side. The path water . takes through the container is caIIed the flow path. A droplet of water moves along a flow path from an initial contact point A with a material or combination of materials to some exit point B from those materials. Point A, Figure 2.9-1, is where the water droplet contacts the sand material and Point B is where that same droplet exits the material and begins moving under the influence of gravity alone. 2.10 LENGTHENING THE FLOW PATH In the experiment shown here the quantity of flow through a compacted sand material contained in a j's- inch diameter tube was reduced about 5y\% for each I-inch unit of increased flow path when subjected to a constant water pressure. llJis illustrates that lengthen- ing of the flow path can help red'Jce flow through the material. FIGURE 2.10.1 REDUCED AMOUNT OF FLOW CAUSED BY LENGTHENING THE FLOW PATH By increasing the length of the path through the same material, tbe amount of flow is reduced. Another example of increased flow path is shown here. Two identicaIIy sized tanks are made. One of the tanks is equipped with a series of alternating baftIes which are waterproof. Both tanks are filled with sand. Each tank is