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Since the water level drops fast during the first one or two hours of the test, readings should be taken at short intervals which are gradually increased as the pumping continues. Fairly accurate measurements of water level can be made by using an electric sounder, by the wetted tape method (the steel tape is coloured with chalk and lowered inside the well; the reading to the wetted portion is read) or by the air line method, Fig. 13.4. The instant of each reading is noted by a stop watch.
Sometimes, the yield test is conducted by using an air compressor. It involves an air line inside an educator pipe lowered into the well, Fig. 13.11. The compressor should be capable of developing a pressure of 7-10 kg/cm2 depending on the depth from which water is to be pumped. For determining the compressor capacity the thumb rule is to provide 6 litres of free air for each litre of water at the anticipated pumping rate. The air lift produces best results when the submergence of the air line is about 60%, i.e., when 60% of the total length of the air line is under water.
1. Slug Tests:
Carrying out pump tests becomes problematic in hard rock borewells due to high drawdown and small diameters and the compressor yields not being reliable due to inadequate submergence or capacity. A preliminary evaluation of yield may be made by slug tests. Slug test on a borewell involves sudden injection of a known quantity of water, say a 10 litres slug, and taking a rapid series of water level measurements to define the shape of the head dissipation hydrograph, i.e., residual head versus elapsed time since slugging. Similarly a known quantity of water can instantaneously be bailed out and the recovery trend of the water level can be analysed by the method outlined by Skibitzke (1958). Similar tests have been described by Houtkamp and Jack (1972).
Slug tests were carried out on 16 borewells tapped in weathered or fractured granitic gneisses. The transmissibility values were obtained by the Cooper method and were correlated with the compressor yields. For hard rocks, T of the order of 100 m3/day/m may be considered good. The dissipation of head is very rapid for T values of over 50 m3/day/m and is very difficult to measure when it is beyond 100 m3/day/m. For ready use by the uninformed well driller, head dissipation hydrographs as shown in Fig. 13.5 can be prepared for the size of the hole his rig is capable of drilling.
A constant slug of 10 litres will facilitate comparison of results from different borewells. The slug test transmissibility value is a better indicator of the intrinsic yield capacity of the well rather than the compressor yield. Slug tests may also prove useful in finding out the water yielding capacities of rocks at different depths during drilling. If tests are carried out at different depths, vertical variation in permeability can be determined, which can throw light on the potential water yielding horizon.
2. Bailing:
For small yields, water can be bailed out from the well at a constant rate till the water level in the well stabilises (constant drawdown). The total number of bailing and the total time taken are recorded. Knowing the volume of the bailer and the average number of bailings per unit time, the flow rate can be determined. The observed drawdown may not be equal to the actual drawdown when the well is pumped at the same rate, since the bailing is intermittent. A bailer test, however, gives an idea of the productivity of the well.
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The yield test is usually run for 10-15 hours and the sustained yield of the well for a constant drawdown is noted. The specific capacity of the well is its yield per unit drawdown and is true for the given pumping period.
The efficiency of the well may be determined by the step drawdown test. Thus the two characteristics of a well, i.e., the specific capacity and efficiency, are determined.
Thus, the operation of testing for yield consists of:
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(i) Measuring the static water level (before pumping begins), i.e., the depth below a measuring point (MP) or ground level.
(ii) Pumping at a maximum until the water level in the well stabilises i.e., the well yields a constant discharge (sustained yield) after pumping for 10-15 hours and the drawdown becomes constant.
(iii) Noting the drawdown by measuring the depth of the stabilised level of water in the well.
(iv) Measuring the discharge from the well, i.e., the sustained yield under a constant drawdown after a long duration (10-15 hours) pumping.
The results of the yield test, i.e., a sustained yield under a constant drawdown after pumping for 10-15 hours, well help in the selection of a suitable pumping set to be installed on the well and the data has to be furnished to the pump manufacturers.