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The following article will guide you about how to construct infiltration gallery with the help of suitable diagrams.
Infiltration gallery is a horizontal perforated or porous pipe which opens joints, surrounded by a gravel filter envelope laid in a permeable aquifer with a high water table and a continuous recharge with a perennial flow. Infiltration galleries are usually laid parallel to river beds at depths of 3 to 6 m for intercepting and collecting ground water by gravity flow.
The horizontal pipes may be of vitrified clay, brick, or concrete of 0.5 to 1.5 m diameter, set in a trench across the aquifer well below the lowest permissible water table, and packed in granite chips around the concrete pipes to fine gravel against the aquifer material, Fig. 10.6, so that clear water percolates with low entrance velocities. Manholes may be provided at intervals of about 100 m for inspection and maintenance. The gallery is laid with a longitudinal slope leading to a central collecting shaft from where the water is pumped to the surface for use.
For a length of 550 m assuming 10 radial directions, the length of lateral well be ≈ 55 m, which may start buckling during driving. On the other hand, if the laterals are driven in two tiers in staggered rows, the force required for driving (a shorter lateral) will be less and also the hydraulic gradient and hence the flow efficiency will improve.
Design of Infiltration Gallery:
For the data given for the design of a radial water collector in the Palar river bed, as an alternative, an infiltration gallery is desired to be constructed for a supply of 15 × 106 lpd with a maximum permissible drawdown of 4.2 m after a 250 days period, at a distance of 3 m from the infiltration gallery. Boundary conditions need not be taken into account if the gallery is planned across the river bed as the flow is essentially two- dimensional in the vertical plane in the upstream-downstream direction. The drawdown pattern away from the gallery is given by Ferris (1950) as-
S = Qx/2T D(u) …(10.2)
u2 = x2S/4Tt …(10.3)
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Where x = distance from drain to the point of observation of drawdown and D(u) = ‘drain function’; values of D(u) for values of u2 are given in Table 10.5 and Fig. 10.7.
In the design example for x = 3 m
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u2 = 32 (0.33)/4(3.6 × 103)250 = 8.25 × 10-7
For u2 = 8.25 × 10-7, D(u) = 625 from Table 10.5.
For an allowable drawdown of 4.2 m,
4.2 = Q × 3/2(3.6 × 103) × 625
Q = 16.13 m3/day/m of gallery
For a supply of 15 x 103 m3/day,
Length of gallery required = 15 × 103 /16.13 = 940 m
But as the river bed aquifer is only 600 m wide, the yield that could be obtained
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= 600/940 (15 × 103) = 9.6 × 103 m3/day
If, however, 15 × 103 m3/day is required, two galleries should be designed. Optimum spacing of the galleries can be found, if each gallery is allowed to influence the neighbouring gallery to the extent of 0.3 m only. Then if Q is known, the distance x for s = 0.3 m can be found from Eq. 10.2.
From the above designs of radial water collector and infiltration gallery, it can be seen that the collector well is more sophisticated and expensive but has higher capacities than the infiltration gallery. Hence, choice should be made by the required yield followed by economic aspects.