Drilling a tube-well inside a wide-diameter well (preliminary trials)

Transcription

1 Drilling a tube-well inside a wide-diameter well (preliminary trials) Peter Morgan 2016

2 Introduction In an earlier manual, the method of connecting a PVC pipe from an underground water chamber to the headworks of a well was described. The underground chamber gathered water from deep in the ground and a bailer-bucket was used to raise water. The combination of a bailer-bucket working inside a tube-well improves water quality compared to extracting water from a wider diameter wells with a bucket. Similarly the construction of a good headworks built on top of a wider diameter well (apron, water run-off and raised collar and lid) improves the quality of water raised with a bucket compared to water raised from poorly protected wells. Well hygiene is important if water quality is to be improved. Water derived from narrow diameter tube-wells tends to be of higher quality from that extracted from wider diameter wells, because there is a higher rate of turnover of the water in the underground chamber, whether it be a tube or well chamber. The wider diameter well (about one metre diameter) is the most user friendly of simple water sources, since it can be deepened with local equipment and lined with locally available fired bricks. However it is known that locally available fired bricks of poor quality can be used to line wells and these tend to disintegrate when placed under-water. So care must be taken in choosing the bricks. Also there is a limit to how far beneath the water table hand digging can penetrate. Wells are best dug or deepened at the time of year when the water table is at its lowest, which in Zimbabwe is during early to mid-december. One of the advantages of tube-wells drilled by hand is that they can penetrate far deeper beneath the water table compared to hand dug wells and this can be an advantage if the level of water tables is unpredictable. However to perform this task, a hand drilling rig must be available, on site. And in Zimbabwe, they are few and far between. This manual describes early trials in deepening a wide diameter well with a hand auger from within the well itself. In this case the tripod and related gear associated with hand drilling has been dispensed with. The tool used to drill the tube-well inside the well is a hand auger, with extensions and a cross bar. This combination is more portable than the equipment which uses a tripod and also much of it can be made locally, although in this case the auger itself forms part of the VonderRig made by V&W Engineering, Harare. In this case the cross bar used to turn the auger is one metre long. This manual shows that a tube-well can be drilled by two people working inside a wider diameter well if the excavation is 1.2m in diameter in the region where the auger is being operated. The reason for attempting this is simple. The tube well can be drilled much deeper into the aquifer than the wide diameter well, and is therefore less susceptible to drying out. The method is best used in areas where the soil is relatively easy to penetrate with a hand auger. The main well can be lined with bricks or unlined if the soil is moderately firm. But a strong ring beam must be built around the head of the well, and the well head raised and fitted with a strong concrete well cover, a raised collar around the upper end of the PVC casing and an apron and water run-off. Once again, a simple-bailer bucket can be used to raise water, as this is simple and easy to make and spares can be kept within the home. It is also important to fit a cap to cover the upper end of the PVC and desirable to make it lockable, as mischief makers like adventurous children can enjoy listening to the plop of stones being thrown down the tube-well. Such adventures are more difficult to carry out, when a hand pump is fitted to the tube well. This manual describes a preliminary trial carried out in the writer s garden in February 2016 Peter Morgan Harare, February,

3 Stages of construction Whilst this technique has not (to my knowledge) been field tested in this country, it is assumed that if successful the method could be used to deepening existing wider diameter wells or could be used on new wells that are dug down to just above the water table and then deepened several metres below the water level. It is near impossible to deepening wide diameter wells to several meters below the water table because the inflow of water into the well chamber is too great. This is possible however if a tube well is drilled. Hence an attempt is made to combine both methods in one installation. The method does however require a PVC casing and an auger system and the use of a bailer bucket as the simplest method of water extraction. It s another option which may be of use in some situations. The photos and description below were made on an experimental unit, designed to pre-test the concept. Stage 1 dig the well and make a ring beam Dig down the well to required depth. In this case to a suitable depth to test the concept. The well diameter is widened to 1.2m at the point where the auger will be used. A ring beam of bricks is then laid in a radial pattern to form a sound foundation. This is laid within a cutting made in the soil at the head of well excavation. Further extensions of the well head are made by laying bricks above the ring beam. The well cover is laid on top of these circles of bricks which are laid using a corbelling technique when each course of bricks is stepped inwards. The Auger The auger itself is part of the VonderRig made by V&W engineering. This is attached to 32mm steel pipe which fits into a steel box tube. The box tube is cut into convenient sections which can be fitted together. A crossbar fits through a component which is held around the box tube, making the tube and auger rotate by the action of drillers on the cross bar. 3

4 Stage 2. Attempt the drilling with auger inside the well The auger is introduced into the experimental test well chamber and the writer and his assistant gardener, Oswald, prepare to test to see if a hole can be drilled with a 1m long cross bar within a 1.2m diameter well chamber. Working within a 1.2m diameter well excavation and using a 1m long crossbar it was possible to drill down into the base of the well. Stage 3. Lower the casing into the inner tube well Once drilled to a satisfactory length for the test, the 110mm class 10 PVC casing is fitted into the drilling. Gravel filter (used on working installations) In practice 300mm length of 110mm PVC pipe is cut and drilled with a large number of small holes. One end of the pipe is sealed. Washed fine gravel chips are then placed inside the tube and held in place with a stainless steel screen. This section is attached to the lower end of the PVC pipe casing. In this current experiment, no gravel filter was used, but has been described in earlier works by Aquamor. 4

5 Stage 4. Adding a gravel pack A prepared length of 110mm PVC casing was lowered into the drilled tube well and the annular space filled with small granite ships. This will be about twice the length of the gravel filter in practice. The chips are followed with well washed river sand to the top of the drilling and above. Stage 5. Building up the headworks The brickwork laid on top of the primary ring beam using a corbelling technique. Stage 6. Adding the cover slab A 1m diameter slab is fitted on the corbelled brickwork. In fact it is an old toilet slab with squat and vent holes. There are a few lying about in the writer s garden. The smaller hole was used to make observations within the test well. 5

6 Stage 7. Making the raised collar which will fit around the PVC casing on the slab This is made with a strong mix of concrete (3 parts river sand to 1 part Portland cement). It is made to sit on the cover slab around the exposed part of the PVC casing pipe. The mould consists of two bricks and 2 cut ends of a plastic bucket. A 110mm PVC casing is placed in a suitable position within the mould and surround by a thin sheet of cardboard to make the hole for the casing slightly larger. It is left to cure for 2 or 3 days before mounting in the concrete slab. Stage 8. Continue with the headworks The construction of the headworks continues with building a brick rim around the concrete cover slab which becomes the apron. Also building a water run off channel and suitable drainage area. In the demonstration unit, the water run-off channel is short, but should be at least 2 or 3m long. The bricks are laid loosely at first to assemble the headworks 6

7 Cement mortar is then prepared to fill the spaces between the bricks. The concrete raised collar is then placed around the protruding PVC casing pipe, which is cut off at the same height as the collar. A plug has been made to fit over the second hole in the slab (only for the demonstration unit). This becomes a viewing hole for what lies beneath. This construction has been made for testing and demonstration only. The bricks are then plastered to improve appearance. The surface of the apron, rim, and water run-off are plastered and sloped so water runs into the soakaway area. The PVC casing has a cap with a handle placed on it. This can also be made with a strong mix of cement and river sand. If necessary this can be locked in place with a chain if links are embedded in the collar. 7

8 Observation hole The second hole in the slab is not fitted to a working unit and is only used here as a demonstration to view what is beneath the slab. When the sun is almost overhead the light passing down the PVC tube will illuminate the interior of the well. A used toilet role tube can be used to view through the opening and observe the PVC and drilled hole inside. This opens the eyes iris and a reasonable view can be seen of the casing entering the drilling (far better than the photo below). Planting something in the seepage area Making observations into the test well A shrub can be planted in the seepage area to absorb water. In this case a mulberry tree lies close by and this will absorb a lot of waste water from the seepage area. Raising water In this demonstration unit, the casing does not descend into the water table. To demonstrate the operation of the bailer-bucket water lifting device, the bottom of the casing has been sealed off so that it will hold about 20 litres of water. The use of the simple bailer-bucket can then be demonstrated. As the bailer-bucket enters the water, it fills with water through the non-return valve, which opens. When the bailer-bucket is raised, the valve closes and water is retained within the tube. Demonstrating the use of the bailer-bucket 8

9 The Bailer-bucket This consists of a 500mm length of 90mm PVC pipe with a brass non return valve embedded in a plug of concrete at the base and a handle at the top. It is raised and lowered into the casing with a thin rope. This water lifting device is very easy to make in the homestead if the parts are available. A spare can even be made and stored as a back-up. Each bailer-bucket holds 2.5 litres of water if made at this length. Interesting comparisons A similar looking headworks has been built outside the kitchen of the writer, but below the ground components are quite different. In this case a large underground brick water storage tank has been built with a high delivery Blair Pump fitted to extract water. This stored water is delivered from a borehole or a hygienic plastic sheet rain-catcher. The borehole is the main source of water. The water is of very high quality and used for drinking etc. The tank forms a valuable reservoir in which high quality water can be stored and used at the user s convenience.. The writer pumping pure water from the underground tank into a bucket 9