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Infinite and finite length of blanket
A natural impervious blanket of large areal extent if available may be considered as a balnket of infinite length
For a blanket of infinite length, the solution of equation (iv) is
In this case for the convenience, the point x = 0 is taken at the downstream end of the blanket and hence h0 is the total loss of head through the blanket upto the end of the blanket.
As a measure of the efficiency of a blanket of any length x (where x may be infinite or a finite length) a length Xr is considered which is known as equivalent resistance of the foundation and is defined below.
It may be defined as the length of a prism of the foundation soil of thickness Zf and coefficient of permeability kf which under the loss of head h would carry flow equivalent to the flow which passes the blanket system under the same loss of head. Thus
(2) Solution for finite length of blanket :
For a blanket of uniform thickness and finite length of blanket ihe solution equation (iv) is
which h is the loss of head through the blanket upto any point at a distance and hn is a constant for computing h. hn depends on the total head loss of the system of which the blanket is a part and on the ratio of the blanket to the remainder of system. *
From equation (viii) at x - 0, h = 0, and hence in this case the point x = 0 is taken at the upstream end of the blanket.
Differentiating both sides of equation (viii) with respect to x, we get