Infiltration Galleries and its Yield | Ground Water | Water Engineering

In this article we will discuss about the infiltration galleries and calculation of its yield.

Infiltration Galleries:

Ground water travels towards lakes, rivers or streams. This water which is travelling can be intercepted by digging a trench or by constructing a tunnel with holes on sides at right angle to the direction of flow of underground water. These underground tunnel used for tapping underground water near rivers, lakes or streams are called ‘Infiltration Galleries’.

Underground water may be allowed to enter these infiltration galleries from both sides or one side as desired. The yield from these galleries may be as much as 1.5 × 10⁴ litres/day/metre length of the infiltration gallery. Sometimes these are also known as horizontal walls.

For maximum yield the galleries should be placed at the full depth of the aquifer. Infiltration galleries may be constructed with masonry or concrete with weep-holes of 5 cm x 10 cm. Infiltration galleries are surrounded on sides and top with gravel or pebble stones to increase their intake capacity. Longitudinal slope is given to the galleries and at the end sump well is constructed, from where water is pumped out.

Yield of an Infiltration Gallery:

Following assumptions are made while determining, the yield of an infiltration gallery:

(a) The soil is isotropic and incompressible.

(b) The tangent to the water table line or its slope is equal to its sine.

(c) The rate of flow of the water into the gallery is uniform and remains horizontal throughout the depth of the aquifer.

Ref. to Fig. 4.25, let

q = yield of the infiltration gallery per unit of its length

= ky.dy/dx

Where, k = coefficient of permeability

Y = the effective height of the ground water table above the soil of the gallery at point x.

Area of unit width through which the water enters the gallery

= y × 1

Slope of the ground water table at point x = dy/dx

.^.. q. dx = hy. dy

Integrating it q.x. = hy²/2 + C

Putting x = L = length of the gallery

y = H

when

The yield of the infiltration gallery

Where

k = coefficient of permeability

H = depth of the permeable stratum above the bottom of the infiltration gallery.

h = height of the water surface inside the infiltration gallery.

L = distance through which seepage takes place.

Example 1:

Determine the yield of an Artesian well with the following data:

(a) Depth of water before pumping = 50 m.

(b) Thickness of the aquifer = 25.5 m

(c) Depth of water during pumping = 42.0 m

(d) Radius of the circle of influence = 92.0 m

(e) Radius of the well = 10 cm.

(f) Value of constant k’ = 4 x 10^-3

Solution:

Using formula 4.15

Example 2:

Determine the yield of an infiltration gallery from the following data:

(i) Coefficient of permeability = 1.5 x 10 ³

(ii) Height of water in the infiltration gallery = 1.10 m.

(iii) Depth of permeable stratum above the bed level of gallery = 7.5 m.

(iv) Distance through which seepage takes place = 13.5 m.

Solution:

Using formula 4.16, the yield of the infiltration gallery.