# LWCE-402 L#06 (b).pdf

28 de May de 2023
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### LWCE-402 L#06 (b).pdf

• 1. Faculty of Agricultural Engineering & Technology Department of Land & Water Conservation Engineering Soil Mechanics LWCE-402 Permeability & Seepage
• 2. Quick Sand Condition Quicksand is a condition and not a soil type. This condition is created in saturated thick layers of loose fine sandy soils when disturbed either due to vibration, such as, from pile driving in the neighborhood, or due to pressure of flowing water (at the time of heavy pumping in excavation). Quicksand forms in saturated loose sand when the sand is suddenly agitated. When water in the sand cannot escape, it creates a liquefied soil that loses strength and cannot support weight. Quicksand can form in standing water or in upwards flowing water (as from an artesian spring).
• 3. Flow Net The flow of water through a soil can be represented graphically by a flow net, a form of curvilinear net made up of a set of flow lines intersected by a set of equipotential lines. Flow Line The paths which water particles follow in the course of seepage are known as flow lines. Water flows from points of high to points of low head, Equipotential lines As the water moves along the flow line it experiences a continuous loss of head. If we can obtain the head causing flow at points along a flow line, then by joining up points of equal potential we obtain a second set of lines known as equipotential lines.
• 4. Flow Net Hydraulic gradient The potential drop between two adjacent equipotentials divided by the distance between them is known as the hydraulic gradient.
• 5. Calculation of seepage quantities Nd = number of potential drops Nf = number of flow channels h = total head loss q = total quantity of unit flow.
• 6. Flow Net Construction
• 7. Flow Net Construction
• 8. Flow Net Construction
• 9. Flow Net Construction
• 10. A river bed connsists of a layer of sand 8.25 m thick overlying impermeable rock; the depth of water is 2.5 m. A long coffer dam 5.50 m wide is formed by driving two lines of sheet piling to a depth of 6 m below the level of the river bed, and excavation to a depth of 2 m below bed the level is carried out within the cofferdam. The water level within the cofferdam is kept at excavation level by pumping. If the flow of water into the cofferdam is 0.25 m3/hr per unit length, what is the coefficient of permeability of the sand? What is the hydraulic gradient immediately below the excavated surface?
• 13. Design of Soil Filter As seen above, water seeping out of the soil can lead to piping and therefore drainage should be provided in such situations to ensure ground stability. To prevent soil particles being washed into the drainage system, soil filters can be provided as the interface between base material and drain. The design procedure for a filter is largely empirical, but it must comprise granular material fine enough to prevent soil particles being washed through it and yet coarse enough to allow the passage of water.  D15 filter > 5 × D15 of base material  D15 filter < 5 × D85 of base material
• 14. Design of Soil Filter The formulae used in the specification of the filter material are  D15 filter > 5 × D15 of base material  D15 filter < 5 × D85 of base material The first equation ensures that the filter layer has a permeability several times higher than that of the soil it is designed to protect. The requirement of the second equation is to prevent piping within the filter. The ratio D15 (filter)/D85 (base) is known as the piping ratio.
• 15. Design of Soil Filter Determine the approximate limits for a filter material suitable for the material shown in Fig. 2.13.
• 16. Design of Soil Filter Solution: From the particle size distribution curve: D15 = 0.01 mm; D85 = 0.2 mm UsingTerzaghi’s method: Maximum size of D15 for filter =5×D85 of base =5×0.2=1.0 mm Minimum size of D15 for filter =5×D15 of base = 5×0.01 = 0.5 mm This method gives two points on the 15% summation line. Two lines can be drawn through these points roughly parallel to the grading curve of the soil, and the space between them is the range of material suitable as a filter (Fig. 2.13).