It is common practice to observe that only DTH rigs are
operating in Hard rock areas, Direct Rotary drilling machines in unconsolidated
or alluvial formations and Percussion drilling machines in the areas where
boulders, gravel and karst topography(limestone solution cavities) are
prevalent. Percussion drilling also can be taken up in consolidated and
unconsolidated formations but the drilling rate is slow and financially not
viable compared to DTH and Direct rotary drilling methods. But the problem
arises when alluvial formations or unconsolidated formations encountered in
predominantly dominated Hard rock areas.Similarly,several problems occur when
the local boring methods are being used instead of Direct Rotary drilling
method in unconsolidated formations,particularly when the depth of
Tubewells increases from 25 feet to more than 100 feet due to increase in
utility of groundwater.
Common problems in Hard Rock Areas
1.
Problems
due to occurrence of unconsolidated formations
The alluvial formations occur in hard areas in patches due to
buried river channels, along stream courses, floodplains, palaeo-channels etc
or lake beds which are encroached for building construction. As only DTH rigs
are available in hard rock areas,(as Direct Rotary drilling or local hand
boring methods are not available )drilling for bore wells in this area taken up
by DTH rigs have to be abruptly stopped due to collapse of unconsolidated
formations and as such drilling could not be proceed beyond 12 to 15 meters.
And the owners of the buildings who have no other option except to satisfy with
these shallow bore wells having very limited quantity of water. But later few
years, the residents of the building face water scarcity as the shallow bore well
goes dry. Direct Rotary drilling rigs or combination Rigs(combination of DTH
and Rotary drilling) are not available in these areas to take up
drilling under such conditions.
2.
Problems due to occurrence of Limestone Karsts topography:
In Singanamala mandal of Anantpur district, drilling of bore well
came to an abrupt end at 100 feet as the DTH rig could not proceed further as
continuation of granite rock is missing. This area is transition zone for
Archaean Granite Gneisses and Kadapa rock formations. In fact Limestone
formation having solution cavities (Karst limestone) struck at 100 feet. In
Karst topography, Percussion drilling rigs could be able to drill bore wells
but such rigs are not available in this area.
3.
Distance criteria not followed in urban areas:
In urban areas like Hyderabad large number of bore wells are
drilled for drinking water
purposes. Due to increase in number of Apartments, the
prescribed distance criteria between two Bore wells are not being followed.
With the result, certain problems cropped up in the performance of Bore wells.
Some of the important problems are -lowering of water table, reduction in
discharge of bore wells, and hampering of drilling progress. In the areas where
bore wells are located close to each other I:e distance between two bore wells
is about 20 meters and less-when drilling is under progress for new bore well,
the Air that is emanating from Air Compressor- instead of adding to the
progress of drilling, Air get leaked through adjacent bore well/Dug well and
hinder the progress of new bore well. The drilling process has to be stopped at
this stage and make a retreat.
4. Effect of thick clay layer on the success of Bore well:
In one of the villages of Srikakulam district, it was observed
that a bore well was drilled up to 150 feet which was located in
highly favorable location turned out to be a unsuccessful well.
Nearby river is flowing by the Bore well location has good catchment area and
weathered rock existing up to the depth of 90 feet. The reasons for failure of
Bore well could not be traced out easily. After a detailed investigation
of the area, it was observed that one meter thickness of Clay is overlying
the weathered rock and not allowing surface water to percolate to sub surface. There
is one big Lake located in the village having abundant water. It was reported by
villagers that throughout year, water exists in full capacity of the lake as
underlying clay material will not allow any percolation to subsurface water. There
is only evaporation of lake water and no recharge to groundwater. Thick clayey
material is common in hard rock areas particularly in Kadapa formations, Deccan
trap areas and in Achaeans.
5. Highly weathered formation: In few locations, highly weathered (loose soil) formations from
80 to 100 feet struck in few bore wells. Generally Rig operators lower casing
pipe up to 25 to 40 feet and complete the construction of bore well. In such
cases, after few days of pumping it will be noticed that discharge of bore well
has decreased. The reason for reduction of discharge is collapse of bore well
from 40 feet onwards till the hard rock strikes. Casing pipe should cover the
entire thickness of highly weathered rock formation.
Problems confronted in unconsolidated
formations:
1. About 2 decades back, in Deltaic areas, Coastal areas, along
stream courses and sometimes in command areas of Water bodies, filter points
and shallow Tube wells depth up to 25 to 35 feet used to be ground water abstraction
structures. Of late dependence on groundwater has increased in these
areas, consequently, the depth of filter points and shallow tube wells in these
areas has increased to 100 feet and some places the depth is more. But in these
areas, though the depth of shallow tube wells increased, the same old
methods of hand boring methods are still being practiced. It is a simple way of
constructing shallow tube wells in alluvial formations without lowering slotted
pipes or gravel filling as per the subsurface geological
conditions. For shallow depths, these hand boring methods were working
satisfactorily due to abundant groundwater. But for the deep tube wells (above
100 feet) certain problems encountered during the operation of these tube wells
drilled by local boring methods. Main problems are, low discharge, muddy water
and collapse of bore well etc. When number of tube wells are increased and
depth of wells increased, it is necessary to follow proven technical methods
while taking up the construction of wells. But the cost of construction of Tube
wells on proven technical methods is high and the Direct rotary drilling
methods with mud circulation are so far employed by government agencies only. The
drilling process includes delineation of subsurface litho logical data, selection
of gravel packing material as per geological formations, selection of slotted
pipe, blank pipe, and development of Tube well until the water become free from
clay, silt and Sand. In few locations, farmers constructed the tube wells
lowering the casing up to the bottom of tube well without using slotted pipe
and gravel packing etc. In such cases wherever sand zones are struck, the
drilling will be stopped and lower the casing up to bottom. In such cases full
potential of groundwater sources could not be exploited due to inadequate
application of required technical methods.
2. Inadequate knowledge while encountering saline water. The
quality of groundwater problem occurs along the coastal belt and in deltaic
areas. The salinity problem is very complex and there is no uniformity in the
pattern of occurrence of saline water. In the coastal areas of Andhra Pradesh
and Tamilnadu states generally, fresh water occurs on upper shallow zone and
saline Water encounter below the fresh water. In cities like Chennai and small
towns located along the coastal area, utmost care is to be taken to pump out
fresh water overlying the saline water. Any overexploitation or overdraft in
this area will yield saline water particularly during summer season. The
situation is so delicate in Chennai city, in the western portion of the city
fresh water occurs up to 70 feet below ground level and towards
east(towards Sea)in intermittent zone, fresh water is available up to 50 feet
and near coast fresh water is available up to 20 to 25 feet only. More
new number of wells and more exploitation of water lead to discharge of saline
water. Thanks to the efforts of state government which made Rain
water Harvesting structures compulsory in all the Apartments of Chennai city
which makes the situation better, as far as the availability of fresh water is
concerned.
In rural areas of
coastal area, agricultural activities and fish pond culture has increased
the exploitation of fresh water which caused yielding of saline water in
few locations. This has resulted severe drinking water problem along the
coastal area. In such areas, local authorities supplying drinking water through
tankers.
In deltaic areas of
Godavari, Krishna and Cauvery rivers, shallow tube wells are generally drilled
up to 70 to 100 feet which gives high yield of potable groundwater. In
addition, along River courses, along canals, potable groundwater is available
in shallow tube wells.
The situation is quite different in west Bengal where in
few major stretches like Diamond Harbour-Namkhana track , Saline water occur on
upper zones and fresh water in lower zones. The same situation continues up to
major part of Orissa state but near Puri Town, the position changes where
fresh water occur on upper zone and saline water underlying it. In coastal Gujarat
both the conditions exists, i.e some places saline water occurs on upper zones
and in lower zones fresh water and in other areas, reverse positon is existing.
In the areas where groundwater quality problem exists,
the drilling units should be capable of carrying out Packer tests and cement
sealing tests so as to seal the saline water and tap only fresh ground water. These
techniques are so far used by government agencies and ordinary farmers should
be able to follow these techniques. Government agencies or NGOs involved in such
activities should guide the farmers when they go for Tube wells in their areas.
The problems which encounter in hard rock areas due to occurrence
of unconsolidated formations, thick clayey layers and construction of Tube wells
by local boring methods are prevalent in large part of Southern India and
Deccan trap areas of Maharashtra, parts of M.P and Gujarat. In remaining parts
of the country also similar problems occur though extent of hard rock areas is
limited. There should be some agencies which should take care of such problems
by taking extra efforts as the cases reported are few in number from Taluka
and district level. Or some system should be evolved to register the cases from
different areas and chalk out a plan to tackle the situation.
C Satyanarayana
Retd.Dy.Chief(Hydrogeology)
REC.
