( . n linn irrni fflnnrnnj|^ •imnnnt' •iiiiiiiiinm! ■iimiiii||]Ti« .mrnrmn' iiillTTnnniii dliTTnnnn' <nnn nnni c mnmnni rtf'* (nn mn < •iT Tmrnrn nmiiiiiini mnnnnp .mranmr*. nmniiuTni itflmrmnil It nnrrnnni mTTTnnm lUllinTnm* iniT Inn ■II l mTTi X lilTIlliii|l nnrl hit '•uiuin* < W Tm < nnrnnr ii|f||iuuni» uiiiiini nuiiun tmiiiinr Z .^Appropriate exploitation of groundwater forms sui important practicstl aispect of any hydrogeological study in the delineated Deccan basaltic aqiiifer(s). The subsequent management of the available water forms another important aspect of such study. The shallow unconfined Deccan basaltic aquifer(s) (upto a depth of 15 metres b.g.l.) are usually tapped by means of large diameter shallow open dug wells. This water is utilised for domestic and agricviltural purposes. In recent times, partic\ilarly after the 1972 drought, bore wells have been drilled to tap the deeper confined aquifers as aui additional source suid subsequently as an independent source of water. Nowadays there is an increasing tendency to drill borewells to augment the water resources even in the agricultural sector. The inception of the 'Down The Hole Hammer' (DTH) technique has resulted in a tendency to prefer borewells to dugweUs particularly in the agricultural and industrial sectors. The following points advocate the choice of drilling of borewells over digging of open dug wells. 1) The borewells occupy negligible space on surface (0.105 metrer to 0.15 metre) because of its very small diameter as compared to open dug wells ( 8 metre to 10 metres). 2) The excavated material in case of a borewell is much less as compared to the large amount of debris derived from the excavation of dugwells. Further, the excavated debris is piled up auround the well aund occupies a lot of space. 3) The total time for drilling a borewell to an average depth of 50 metres is very less (maximum 1 day) as compared to that for an average dug well of 5 metre diameter and 12 metre depth (approximately 60 days). 4) A borewell, apart from the insertion of the casing pipe, does not require any further construction or civil work, while a dugwell usually requires lining especially in caise of areais where thick over burden is present. 5) DugweUs usually tap the shallow unconfined aquifers while borewells usually tap the deeper confined aqviifers along with the shallow unconfined aquifers. 6) The shallow unconfined Deccan basaltic aquifers, in general, have a low Trauasmissivity au:id consequently a slow well recuperation. Therefore, the large diameter dug wells are preferred because of their large storage capacity. Earlier it was thought that borewells may not be suitable for exploitation of the low transmissive Deccan basaltic aquifers especially for agriculture and industrial purposes. However, a properly exploited and planned bore well can yield a sustainable quamtity of groundwater for agriculture auid industrial purposes from the deeper Deccan basaltic aquifers. 7) A fully operational dug well (15 metres deep and 5 metre in diameter), without a pump, costs Rs.52,000 (Khutavakar,1984). On the other hand, a boreweU 150 mm in diameter and 50 metres deep, in the same area, costs only Rs.12,000. The cost of a lifting device from the well would approximately be Rs.20,000 in comparison to Rs.25,000 in case of borewells. METHODS OF DRILLING Drilling of borewells to augment the existing supply of water for agricultural, industrial or domestic supply is commonly done in the Deccan basaltic terrain. The compact nature of the terrain makes drilling conditions in this area rather tough. In earlier days, rotary rigs using rock roller or core bits were used, but presently with the advent of the DTH technique, a majority of the borewells are drilled with the DTH rigs. The DTH drilling system consists of three basic units made up of the air compressor the drill stem and the drilling hammer. The compressor, forms the heaurt of the whole system, as compressed air is the main driving force. Compressed air is supplied to the hammer through the drill stem (the hammer is attached to the lower end of the drill stem) at a working pressure of approximately 7 kg/cm2 or more. This air facilitates the rotation of the drill stem at a speed of about 10-30 rpm and the hammering action of the bit. The hammer which is made up of aUoy steel has a bit attached at the base which rapidly strikes the rock while the drill stem is slowly rotated.. This is similar to the blows delivered by a cable tool bit. The bit is cQso made from cdloy steel and consists of buttons of tungsten caurbide embedded in it (Photo 8). It is these buttons that actually provide the cutting or chipping surfaces. The rotation of the bit assures an even penetration . The cuttings are removed continuously by the air which is used to drive the hammer. Unlike a conventional cable tool bit which constantly strikes previously broken fragments, the DTH bit (or buttons on the hammer) always strike a fresh surface, thereby making the DTH technique more efficient (DriscoU, 198<). The advantages of DTH drilling method are- 1) Cutting removal is extremely rapid. 2) Aquifers are not plugged with drilling fluids as air is the 'drilling fluid'. 3) Absence of mud pumps, 4) Bit life is longer as compared to that in cable tool drilling because the bit always strikes a fresh surface thereby making it more efficient. 5) Extreme weather conditions do not hamper the drilling work. 6) Penetration rates are high even in highly resistant and compact rocks like basalt. 7) An estimate of the yield can be made during drilling by the air-lift method using the same compressed air which is the drilling fluid. The disadvantages of the DTH drilling method are- 1) It is restricted to semi-consoKdated and well consolidated material/formations. 2) Initial costs and maintenance costs of the large compressor are high, 3) The litholog prepared from the lithological samples of the DTH drilling is rather approximate as compared to the one by core drilling. The borewells drilled using the DTH technique can be broadly classified into two basic types, on the basis of their diameter. 1) The 105 mm diameter borewell and 2) The 150 mm diameter borewell. Conventionally, borewells with the above mentioned diameters are more commonly drilled for agricultural, industrial and domestic purposes even though it is possible to drill borewells with diameters otherwise. Further, there is a vast difference between the DTH rigs drilling the 105 mm diameter borewells and the 150 mm diameter borewells. Some of the relevant differences are listed below in Table-3.1. DRILL-TIME OBSERVATIONS The drilling period in the case of borewells drilled by the DTH rigs is rather short (rounging approximately from 4 hours to 24 hours). However, this drilling period offers an unique opportunity to acquire first hand information about Deccan basaltic aquifers. Certain observations can be made so as to obtain a better understanding of the subsurface Deccan basaltic aquifers tapped by the borewells and also about the approximate yields, depths and the thicknesses of the different aquifers tapped. Some details of the Drill-time observations have been described below. COLLECTION OF LITHOLOGICAL SAMPLES Use of DTH rigs for the drilling of borewells in Deccan basalts causes the pulverising of basaltic rocks into smaller pieces. These can be broadly categorised into powder, fragments or chips, and the larger flat pieces. These pieces are available for observation when they are brought to the surface by the drilling flmd, which in this case is compressed air, or if the aqiufer has been tapped, along with the mixture of air and water . The lithologic samples are collected at regxilau: intervals of approximately 4.5 metres by the rig operating crew. The usual practice is to coUect the pieces flushed out and lying around the drillstem at the time when the drilling is temporarily stopped to attach a new drillstem. The approximate interval of 4.5 metres corresponds to the length of one drillstem in csise of the 150 mm diameter borewells auid to 3 drillstems in the caise of the smaller 105 mm diameter borewells. This has been a standard methodology used by drilling agencies. Even the data supplied to Government organisations Kke the Groundwater Survey and Development Agency by its contractors is of a similsir nature. Table 3.1 : Drilling by 150 mm and 105 mm rigs- a comparison 150 MM nRILLU15_^LiaS__EQE__IQ5.....^ 1) The weight of the rig is 1) The weight of the rig considerable and is always truck excluding the Compressor is less mounted. The rig cannot be (approximately 400 kg.). These dismantled; only the mast can be are portable and can be raised and lowered. assembled and dismantled on the site itself. 2) Higher operating pressures 2) Works on a lower pressure require larger capacity of the and hence requires a smaller ccjmpressors, thereby increasing capacity compressor, thereby the diesel consumption. reducing the diesel consumption. 3) Depth is no restriction- 3) The rig normally have a borewells can eeisily be drilled limitation of a maximum depth of upto depths of 200 metres. 45 metres. 4) Drilling rate is faster. 4)Due to the smaller capacity of the compressor the rate of drilling is comparatively slower. 5) Borewells can be drilled only 5) It is possible to drill in places which are readily boreweUs in confined accessible to a truck. spaces,aswell as in existing dugweUs. 6) Drilling platfom is fixed to 6) The driUing platform is the drilling rig which is movable and is usually mounted on a truck along with connected to the compressor by the cortpressor.