How to Construct Collector Wells? | Well Design | Geography

The following article will guide you about how to construct collector wells with the help of suitable diagrams.

To get large irrigation supplies from permeable alluvial aquifers with a permanent source for continuous recharge, like gravel formation from banks of lakes or perennial streams or from water-logged areas irrigated by big canal systems, radial water collectors are employed. A radial water collector is essentially a large diameter, normally 4—6 m, shallow well from which horizontal strainers protrude, radially near the bottom, into the permeable aquifers.

The central well is a vertical concrete caisson, about 4 m in diameter and a wall thickness of 45 cm. The cylindrical caisson is precast in RCC rings on the ground surface preferably with steel form work. Each ring in lowered under the weight of the next as the earth material is excavated from inside. When the required depth is reached, the bottom is sealed by pouring a thick concrete plug heavy enough to overcome the buoyancy.

When the caisson is in place, lateral pipes, fabricated from heavy steel plate 10 mm thick and slotted to have 15-18% of open area with their leading ends protected by streamlined nose cap, are driven horizontally into the water bearing formation by special hydraulic jacks installed at the bottom of the caisson through the precast port holes (left in position during casting of the caisson according to the information available from boring data) to form a radial pattern of laterals. Laterals or screens are usually available in 2.5 m lengths and the successive lengths are butt-welded till the required length is reached.

The diameters very from anything between 15 to 50 cm depending upon the local conditions and the yield required. The maximum number of radials is limited to 16 in one level and a second level of radials is possible, as may be desired if many clay lenses inhibit free vertical water movement in the vicinity of the well. The laterals need not be equally long and some may be driven with a provision for some more to be driven when growing requirements warrant an increase in yield. A common situation is a caisson on a river bank with radials only along 90° of the circumference. The maximum length of a radial so far has not exceeded 135 m.

The total length of the laterals may range from 120 to 900 m depending upon the diameter and yield required. The arrangement of laterals on river Avon at Somerdale Works in England for a yield of 3,415 m³/day is shown in Fig. 10.5. Seven laterals with slotted pipes of 20 cm diameter and total length 126.5 m were driven. Provision was made for driving three more laterals in case growing requirements should demand a greater yield.

There are different patents developed long back such as by Ranney, Fehlmann, Preussag etc. In the Ranney method, the slotted pipe itself is jacked out with a digging head with large holes in front for the removal of the aquifer material, and the slotted pipe once packed in remains as the radial collector. In the Fehlmann method, a blank casing is installed after which the perforated pipe is put inside and the blank casing withdrawn.

In both the types, the fine particles are washed out by flushing so that natural gravel packs are formed around the perforations. The water should percolate into the strainers with an entrance velocity not exceeding 6-9 mm/sec to prevent sanding and incrustation. The location of laterals near the bottom of the well ensures removal of water from the entire water bearing formation above them. This also increases the effective radius of the well or its circle of influence, resulting in higher yields.

Normally caissons operate at depths ranging from 10-25 m. In waterlogged canal irrigated tracts, this depth is not likely to exceed 15 m. The top of the caisson is raised to an elevation above the maximum flood level. The intake gate valves, operated from top, are installed both for low and high water operations. The well diameter should be large enough to accommodate the travelling cranes and hydraulic jacks for driving operations.

The initial cost of a radial water collector exceeds that of a vertical tubewell but the large yields obtained under low pumping heads and low maintenance costs lower its cost per cumec of water lifted. Over 300 such installations are already in operation in USA besides many others in France, Germany, and other parts of Europe with discharges varying from 2,000 to 60,000 lpm.

A collector well located adjacent to a surface water body like a stream or lake induces infiltration of surface water through the bed of the water body into the well and thus large supplies of good quality water (since it gets filtered through the natural river bed) are obtained for supply to municipalities, industries etc.

Recently, a collector well has been constructed in the Vaigai river bed for water supply to Madurai city. Two such wells have been constructed at Koyali near Baroda below the bed of the river Mahisagar for water supply to the Gujarat Refinery. Each well has a diameter of about 6 metres and yields about 50,000 m³/day. The quality of water is very good.

In the design of collector wells, the maximum drawdown may be calculated with reasonable accuracy assuming the radial collector well as an ordinary vertical well with an effective radius of 75 to 85% of the individual lateral lengths spaced around the entire circumference of the caisson and having equal length laterals.

Example:

A radial water collector is to be designed for extraction of 15,000 m³/day from the ground water stored in the Palar river bed which has an aquifer extending up to 13 m below the bed, with the width limited to the actual river bed itself, which is 600 m wide. A long duration pump test indicates a transmissibility of 3.6 × 10⁶ lpd/m for a saturated thickness of 12 m when the water table is lm below the river bed and a storage coefficient of 33% for the aquifer as confirmed by the laboratory tests. The water table goes down by 4.2 m over a period of 8 months (exactly 250 days) during which period of the summer rainfall amounts to 40 cm.

Solution:

Let the collector well be located in the middle of the river. The water level goes down by 4.2 m during a period of 250 days. Due to the natural recharge during this period by summer rainfall, the water table rises by 40/0.33 = 121 cm or 1.21 m. Hence, the permissible drawdown could be 4.20 + 1.21 = 5.41 m over a 250 days period. Although the well would recover from 5.41 m drawdown including 1.21 m recovery in the dry season by occasional showers, it is a good practice to design the collector well for a maximum drawdown of 4.20 m only, allowing the extreme dry periods of no rainfall at all in the dry season.

Since the river bed is a narrow stripped aquifer and the collector well is located in middle of the river bed at 300 m from both the banks which are impermeable boundaries, the drawdown due to both image wells located at 300 m beyond each bank or each at 600 m away from the collector well, has to be included.

Therefore n = 5.8, say, 6 laterals spaced uniformly at 60° around the circumference of the caisson, Fig. 10.5. The diameter of the caisson may be taken as 4 m and may be sunk up to 13 m after which the bottom is sealed by pouring a thick concrete plug. The laterals may be placed at about 1.5 m above the bottom of the aquifer to ensure proper functioning. After the well is installed, it should be properly developed usually by compressed air. Its yield should also be tested for supplying data to the pump manufacturers. Details of some radial water collectors installed in India are given in Table 10.4.