Infiltration well or sunk well is only the inter position of a masonry or concrete barrel into such a formation so as to intercept as large a quantity of water as possible. Infiltration wells are generally proposed in the river bank, riverbed or lakebed to tap water from the unconfined aquifer. Infiltration wells are preferred where the minimum saturated thickness of aquifer is at least 5m. Generally, this will provide a very good supply of water throughout the year. Even if the river dries up during times of little rain, water will be available from the underground. Water from the stream passes through the sand and silt in the river bank and impurities are removed by natural filtration. The degree of purification will depend on the extent of contamination of the stream and on the soil type. In many cases, the purification process in the sub soirsystem will be sufficient, and no further treatment is necessary.
Diameter of the infiltration wells
The yield of the well is dependent on the rate of flow of groundwater and the hydraulic gradient due to the depression of water level in the well rather than the diameter of the well. The effect of the diameter of the well on the yield is very small and therefore the diameter of the infiltration well should not be very large. Clogging of pores of the well steining with fine sand could be avoided, by adopting large diameter of well and maintaining very small entrance velocity of flow through the well steining.
The diameter of the infiltration well shall be determined based on the required yield limiting the entrance velocity of flow into the well in the range of 3 to 10 cm/s. Generally the diameter of the well shall be in the range of 3.0 to 5.0 m.
Depth of the well
The depth of the well shall be decided based on the saturated thickness of the aquifer. If the saturated thickness of the aquifer is more than 8 m, the bottom of the well shall be sunk to a level in such a way that there is a clearance of half the diameter of the well between the bottom of the curb and the bottom of the aquifer. If the saturated thickness of the aquifer is less than 8 m, the well shall be sunk up to the bottom of the aquifer or rock level and anchored to the rock. The water from the aquifer is abstracted through the well steining through either weep holes, or porous concrete steining.
Weep holes are provided in the well steining at suitable interval both horizontally and vertically. These weep holes could be formed of cut lengths of 75 or 100 mm diameter GI pipes fixed into the well steining with wire gauze fixed at the outer end to prevent fine sand entering into the well. The velocity of flow through weep holes should not be more than 6 cm/s.
Porous Concrete Steining
If the number of weep holes required is very large, the part of well steining may be made porous with pores sufficiently smaller to eliminate the finer particles of the aquifer, to abstract sand free water. The pores shall be designed in such a way to limit the entrance velocity not more than 6cm/ s. Porous concrete panels of the well steining shall be made within the saturated thickness of the aquifer.
To render the concrete porous, fine aggregate is omitted and the concrete is made up of stone and cement only. Because of the omission of the fine aggregate, the concrete is often referred to as no- fine concrete. In normal concrete the fine aggregate fills the interstices between the stones and makes the concrete denser, while in porous concrete the interstices between the stones are left as voids. However carefully the concrete is mixed, placed and tamped, moisture is entrapped in the pores. When the moisture eventually evaporates, minute hollows and channels are formed through which percolation of water takes place.
Following precautions are to be observed in the construction of porous concrete.
1.The porous concrete should not be placed within 3.0 m depth from the bed level of the river to prevent entry of the polluted water.
2.The porous concrete should be placed sufficiently below the level of draw down expected when the design yield is pumped from the well. Porous concrete should never be exposed above water level.
3.The stone sizes should be carefully graded fixing upper and lower limits for sizes of the graded stones based on the effective size and uniformity coefficient of the aquifer materials, which shall be determined from the sieve analysis of the samples of soil collected during the trial boring at the site.
4.Usual specification of stone aggregate for porous concrete in medium sand as well as coarse sand aquifers shall be that all stones should pass through 20 mm mesh but none should pass through 10 mm sieve. The broken stones obtained from the quarry should be sieved with 10 mm and 20 mm mesh sieves to eliminate the smaller and larger particles.
5.For the aquifer made up of fine sand, the specification for stone aggregates for the porous concrete shall be all stones should pass through 12 mm sieve and none pass through 6 mm sieve.
6.The stones must be absolutely clean and free from any silt, clay or loam. This is ensured by proper washing of the aggregates.
A stone cement ratio of 4:1 should be adopted.
7.The mixture of concrete should not be too wet; the minimum water content consistent with the workability must be aimed for.
8.The tamping of concrete should not be excessive.
9.Moulds should not be removed before the completion of 48 hrs.
10.The curing should be thorough for at least for one week, preferably for two weeks; the concrete must be kept wet and shielded from sun.
Construction of Infiltration Well
The steining of infiltration well shall be made up of either brickwork or panels of concrete with RCC pilasters and tie-beams.