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Therefore

Excess head loss through coarse silt:

, 7.8 x 10"6 x 4 h = —5—r-r—^— = 0.78 m 4 x 10~5

Excess head loss through fine sand:

, 7.8 x 10-6 x 4 h = —-——j— = 0.16 m 2 x 10"4

Excess head at interface between fine and coarse silt

= 2.5 — 1.56 = 0.94m Excess head at interface between fine sand and coarse silt = 0.94 - 0.78 = 0.16 m

2.25 Seepage through soils of different permeability

When water seeps from a soil of permeability kt into a soil of permeability k2 the principle of the square flow net is no longer valid. If we consider a flow net in which the head drop across each figure, Ah, is a constant then, as has been shown, the flow through each figure is given by the expression:

If Aq is to remain the same when k is varied, then b/1 must also vary. As an illustration of this effect consider the case of two soils with kj = k2/3. Then

Fig. 2.31 Effect of variation of permeability on a flow net.

If the portion of the flow net in the soil of permeability k, is square, then:

12 3 °r 12 " k2 The effect on a flow net is illustrated in Fig. 2.31.

2.26 Refraction of flow lines at interfaces

An interface is the surface or boundary between two soils. If the flow lines across an interface are normal to it, then there will be no refraction and the Fig. 2.32 Flow across an interface when the flow lines are at an angle to it.

Fig. 2.32 Flow across an interface when the flow lines are at an angle to it.

flow net appears as shown in Fig. 2.31. When the flow lines meet the interface at some acute angle to the normal, then the lines are bent as they pass into the second soil.

In Fig. 2.32 let RR be the interface of two soils of permeabilities kj, and k2. Consider two flow lines, fi and f2, making angles to the normal of a\ and oti in soils 1 and 2 respectively.

Let hi and h2 be the equipotentials passing through A and B respectively and let the head drop between them be Ah.

With uniform flow conditions the flow into the interface will equal the flow out. Consider flow normal to the interface.