EMBANKMENT LECTURE 11

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Upstream Impervious Blanket: A horizontal blanket of impervious soil may be provided on the river bed on the U/S side of the earth dam to reduce the quantity of seepage through the pervious foundation Under the earth dam. The impervious blanket increases the length of the part of seepage Under the dam and thus reduce the velocity and quantity of seepage. The impervious blanket should be connected to the impervious core of the dam. The soil used for impervious blanket should have far less permeability than that of the foundation soil. The necessary thickness and length of the blanket depend on the permeability of the soil of the blanket, thickness of the impervious foundation and the maximum depth of water in the reservoir. The blanket thickness varies from 0.6 to 2 m.

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Upstream Impervious Blanket::

Upstream Impervious Blanket: 1/23/2014 1 PREPARED BY V.H.KHOKHANI, ASSISTANT PROFESSOR, DIET.

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A horizontal blanket of impervious soil may be provided on the river bed on the U/S side of the earth dam to reduce the quantity of seepage through the pervious foundation U nder the earth dam. The impervious blanket increases the length of the part of seepage Under the dam and thus reduce the velocity and quantity of seepage. The impervious blanket should be connected to the impervious core of the dam. 1/23/2014 2 PREPARED BY V.H.KHOKHANI, ASSISTANT PROFESSOR, DIET.

PowerPoint Presentation:

1/23/2014 3 PREPARED BY V.H.KHOKHANI, ASSISTANT PROFESSOR, DIET.

PowerPoint Presentation:

The soil used for impervious blanket should have far less permeability than that of the foundation soil. The necessary thickness and length of the blanket depend on the permeability of the soil of the blanket, thickness of the impervious foundation and the maximum depth of water in the reservoir. The blanket thickness varies from 0.6 to 2 m. 1/23/2014 4 PREPARED BY V.H.KHOKHANI, ASSISTANT PROFESSOR, DIET.

Bennett's analysis ::

Bennett's analysis : Bennett gave the mathematical solution for the performance of the u/s impervious blanket. At any point X, under the blanket, the horizontal flow q f through the foundation is equal to the flow through the blanket itself ( q b ) upstream from the point plus the inflow q 0 under the u/s end of the blanket. 1/23/2014 5 PREPARED BY V.H.KHOKHANI, ASSISTANT PROFESSOR, DIET.

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Thus where dq b is the vertical flow through a small element of the blanket of width dx and thickness Z b . The value of dq b varies directly with the head loss h at x, and inversly with the blanket thickness Z b . If kb is the permeability coefficient of the blanket, we have 1/23/2014 6 PREPARED BY V.H.KHOKHANI, ASSISTANT PROFESSOR, DIET.

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differentiating both sides w.r.t . x, we get 1/23/2014 7 PREPARED BY V.H.KHOKHANI, ASSISTANT PROFESSOR, DIET.

Since q0 is independent of x,:

Since q 0 is independent of x, 1 = 1 1/23/2014 8 PREPARED BY V.H.KHOKHANI, ASSISTANT PROFESSOR, DIET.

The value of qf is also obtained by Darcy's law as:

The value of qf is also obtained by Darcy's law as differentiating both sides, we get 2 1/23/2014 9 PREPARED BY V.H.KHOKHANI, ASSISTANT PROFESSOR, DIET.

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equating ( i ) and (ii), we get Equation (iii) is the differential equation for the pressure head dissipated through blanket. The solution of this differential equation is obtained in two cases : 3 1/23/2014 10 PREPARED BY V.H.KHOKHANI, ASSISTANT PROFESSOR, DIET.

(a) blanket of uniform thickness (b) blanket of variable thickness:

(a) blanket of uniform thickness (b) blanket of variable thickness 1/23/2014 11 PREPARED BY V.H.KHOKHANI, ASSISTANT PROFESSOR, DIET.

Blanket of uniform thickness:

Blanket of uniform thickness For a blanket of uniform thickness is a constant. Let this constant be represented by a^2 1/23/2014 12 PREPARED BY V.H.KHOKHANI, ASSISTANT PROFESSOR, DIET.

The solution of this equation has been obtained for the following two cases ::

The solution of this equation has been obtained for the following two cases : 1/23/2014 13 PREPARED BY V.H.KHOKHANI, ASSISTANT PROFESSOR, DIET.

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