A CONCEPT ON TUBEWELL CONSTRUCTION
By
:- R.B.Roy
Introduction
In the state of Jharkhand more than 90% of rural population depends for their drinking water on different types of Tube wells situated in their vicinity; therefore being a DW&SD Engineer you must have good knowledge about HP. I hope all of you have seen some types of tube well and also used it.
Introduction
In the state of Jharkhand more than 90% of rural population depends for their drinking water on different types of Tube wells situated in their vicinity; therefore being a DW&SD Engineer you must have good knowledge about HP. I hope all of you have seen some types of tube well and also used it.
Tube wells
Tube well is nothing but a form of deep well as its name indicate a hole is drilled or bored into ground by driving with auger or bit by hand or by machinery and in this hole a tube or pipe is inserted. Pipes may be blind or perforated or screen. They may be constructed in soft unconsolidated soil or hard soil i.e. rocky formation.
It may be shallow or deep. It may be termed as bored well or drilled well. size may range from Their 25mm to 900mm 90 0mm in diamet diameter er.. geological condition dictate two The general type of well construction. A well that taps an aquifer of consolidated rock consists basically of a cased portion, usually extending through the loose overburden material, and an open borehole in the below.
A well that taps an aquifer of water bearing sand must necessarily be provided with a casing through the overburden materials and a well screen in the aquifer properly. To draw water from it a suitable pump is installed as per requirement and available discharge to suit our demand.
These are the distinct operation in construction of tube wells.
Site selection:-
A tube well should be so located that it is accessible for pump repair, cleaning, treatment, testing and inspection. The top of the tube well should not be within the basement. When adjacent to a building, the well should be at least 100cm outside any projection. It should be located on highest ground practicable and must be on a ground higher than nearby source of pollution. The ground surface/ platform at the tube well site should be built up so that the surface water will drain properly.
The
minimum distance from a tube well to possible source of pollution should be large enough to prevent subsurface flow or seepage of contaminated water reaching the well. Recommended minimum distance from various types of pollution sources are as below::- 30m Cesspool receiving raw sewage :- 25m Seepage pit or drain field :- 15m Septic tank or tightly joined sewer These distance are meant to be guides to good practice and may be changed as soil condition requires. They should be applied only where the soil has a filtering capacity equal to, or better than that of sand
Boring / Drilling There are numerous methods of boring/ drilling tube wells and each method has advantages related to ease of construction, cost factors, character of formations to be penetrated; well diameter and depth, sanitary protection and intended use of the well it self.
Some of the Boring / Drilling methods are as below:Percussion
method:- This is oldest and standard method of constructing tube well in hard rock, also known as cable tool method. In this method a hole is drilled by alternate raising & dropping a drill bit or auger stem or string tools either by manually or by power. While falling, the bit breaks the rock in to small pieces which is removed by means of a sand pump or bailer, after wetting the pulverized material by water.
Rotary Drilling
Rotary drilling process are of two types:Direct Circulation Drilling:- This method is suitable for drilling small diameter deep wells in soft to medium alluvial formations. In this method a heavy walled drill pipe which extends to the surface to the bit rotated by mechanical means and as the bit is turned drilling fluid usually bentonite mixed with water or suitable material is circulated under pressure which lubricates and cool the bit, carries the cutting in suspension to the surface and plaster the wall of the hole to prevent caving in. Reverse Circulation Drilling:-The reverse circulation method is same as above except water is pumped up through the drilling pipe rather than down through it.
Down-the-Hole (DTH) Hammer Drilling
This method is used for fast and economical drilling of bore up to 200mm in medium to hard rock formations. In this method down the hole pneumatic hammer is used at the lower end of the drill piped and combining the percussion and rotary actions. This method utilizes compressed air for the rapid impacting action given by the hammer to the bit; thus crushing the formation in to small chips which are flushed out through the annular space between the bore & drill pipes by the up coming air pressure. This is most commonly used method in the state.
Water Jet Boring
A water jet is used for boring the well which implies that a drilling bit with nozzels is attached to the drill pipes at the bottom through which water is pumped at high pressure. The water on its return flows through the annular space between the bore and the drill pipe and brings the cutting along with it to the surface. Casing pipe is simultaneously used to avoid caving in. This method is useful in sandy formation.
OTHER METHOD OF DRILLING Drilling by combination rig Auger Drilling Hand boring by Dheki process Hand drilling using force & lift pump combined with reverse circulation Core drilling
Installing/ inserting casing and / or Housing and/ or screen pipes
General:- Well casing is used to prevent caving in of surrounding soil in to the well or bore hole. The upper portion of the casing used for housing the pumping equipment, is called housing pipe. The casing may be temporary and is removed on completion of the well (drive pipe) or it may be a permanent part of the structure. The length and diameter of the casing pipe is selected on the basis of static water level, drawdown, discharge expected from the pump and the size of the pump to be installed.
Casing Pipe
Casing pipes may be of MS tubes with plain end or screwed (confirming to IS 4270 : 1992) where tubes are required to be galvanized the zinc coating on the tubes shall be in accordance with IS 4736 : 1986/ 1239 : 1990(for GI), or UPVC tubes (confirming to IS 12818 : 1992). MS black tubes shall be externally coated with a bituminous solution or any other protective anticorrosive coating as per standard specification. All the pipes should be marked with following details:Nominal size, length of tube, manufacturers trade mark, inspecting agency inspection mark with other marks as per direction of supply order.
All the UPVC casing pipe shall be marked with CM or CS (to denote medium well casing & shallow well casing pipe), nominal diameter in mm, inspecting agency inspection mark with other marks as per direction of supply order. Diameter and wall thickness is selected as per requirement. Screen pipes:MS, UPVC, Brass, Fiber glass copper wire, galvanized steel wire and stainless steel various types of screen pipes are available in the market to suit our specific requirement to suit the strata condition.
Present Scenario in our department
In our department normally two types of tube wells are constructed as spot water source now a days known as Drilled tube well in rocky area & Gravel packed tube well in alluvial soil area. However in some area old TARA Pump may also be found with Ordinary Shallow HP & Special HP tube well. Drilled Tube well:- It is drilled with DTH mounted rig machine and we use UPVC casing pipe of CM class having nominal bore diameter 125mm confirming to IS 12818 : 1992 (latest revision). Its average depth is 50 to 55m and average casing consumption is 12 to 15m. Some times GI casing of 125mm nominal bore diameter is also in use, where insertion of casing is required hammering.
DRILLING OPERATION
The drilling operation for construction of bores for tube wells should be carried out by the DTH Mounted Rig Machine as mentioned below The boring in the over burden should be continued through the rock at least up to 25 cm, so that casing pipes can be properly embedded in the rock. Boring through rocks shall be 115mm diameter and total depth from the ground level of the bore shall be average 55/50 meter on completion of drilling. The bore shall be flushed till it gives sufficient clear water. The yield should not be less than 1000 liters per hour for acceptance as successful bore. The diameter of the bores in the over burden shall be sufficient for insertion of 125 mm dia casing pipes with sockets leaving sufficient annular space for grouting the casing pipes.
If even after a depth of 60 mtr from the ground level bore remains dry or if the yield is found to be less than 1000 liters per hour, the bore will be declared unsuccessful, but if it is detected that after increasing the depth beyond 60m the bore became success extra depth may be allowed subject to condition that bore must be made success and extra depth does not cause increase in average sanctioned estimated depth beyond limit. If the boring in the over burden is done up to the depth of 30 meters and further boring is not possible due to adverse and unsuitable geological condition the bore shall be declared unsuccessful.
After completion of the boring the bore should be developed & washed by means of air compressor for a minimum period of 30 minutes to give clean potable water. The contractor should maintain strata chart in the prescribed format for each tube wells, and submit “Strata Chart” at the time of preparation of bill to the Engineer-in-charge; otherwise payment will not be made.
LOWERING OF CASING PIPE
Casing pipes should be properly socketed or welded to ensure continuous length (leak proof joint) and lower through the over burden to reach at least 25 cm in side the rock. The length of casing pipe should be such that at least 30cm should remain projected above the ground level after completion of drilling work at site. The top of the casing pipe shall have to be capped by 125 mm dia GI cap till the hand pump is installed over the bore. The casing shall have to be lowered in such a manner that it remains vertical so as to ensure installation of deep well hand pump and accessories properly and without difficulty.
GROUTING
After fixing or lowering the casing pipe in proper position the grouting around the casing pipe is done with Cement & Bentonite solution for sanitary sealing of bore well. If it is not done then the bore may be polluted through seepage from any nearby pollution source if any.
GPT
Gravel Packed Tube Well:- It is either constructed with Reverse Circulatory Rig Machine or Manually or by Combination Rig Machine suitable for drilling in Rocky formation as well as alluvial formation and 125mm UPVC CM class casing pipe and screen pipes is used. Its average depth is 38m except in some area of Ranchi, Gumla & Lohardagga district, where depth goes up to 60m.
GPT
Normally GPT is constructed in Alluvial formation where water yielding capacity of aquifer is less. Boring Operation :- The boring operation for construction of gravel packed T/W should be carried out by means of Reverse Rotary Rig Machine or Water Jet Plant satisfying the following conditions: The diameter of the bore shall be minimum 250mm. The bore shall be vertical. The soil sample shall be collected and observed carefully at regular intervals of each 3 meters and whenever any change in strata is encountered, the sample should be collected and its corresponding depth from the ground level shall be noted.
GPT cont.
Once the water yielding strata is encountered the sample should be carefully observed and its depth from G.L. shall be recorded carefully and after boring in water bearing formation up to a depth of 7/10 meter the boring process shall be stopped. Bentonite solution should be used if necessary to avoid caving.
Lowering Of Casing Pipes And Screen Pipe
After completion of boring the lowering chart should be designed according to strata condition encountered. Lowering chart should be designed in such a way that the ribbed screen pipe must be placed in the water bearing formation. The ribbed screen pipe should be placed in the bottommost portion of casing pipes or 1 to 3 m above blank pipe for settling of sand) with its open end suitably plugged with well plug. Five center guides is also placed at regular intervals spanning the whole length of the pipe and ribbed screen pipe with one guide each at the top and the bottommost ends of the pipes, ensuring that the pipe is placed at the center of the bore.
Lowering Of Casing Pipes And Screen Pipe cont.
The annular space of bore must be filled immediately with approved quality Pea gravel (3 to 6mm) up to 10 M below ground level after lowering of the pipe assembly and from G.L. up to 10 m Below ground level by sticky clay to prevent contamination from surface. . To disinfect the bore a little bleaching powder should be mixed with P gravel. After surrounding the bore with gravel developing should be started with the help of water jet and air compressor till clear water discharge takes place in adequate quantity. „
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Development Of Bore After of completion of boring, lowering of pipes, ribbed screen pipe and surrounding the tube well with Pea gravel the bore should be developed and washed by means of air compressor or water jet or combination of both to give sand/ silt free clear adequate drinking water. The discharge of bore should not be less than 1000 liter per hour. If water is not found clean and discharge is found less than 1000 liter, the bore shall be declared un successful. The plain & screen pipes are used of UPVC of 125mm nominal diameter.
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Testing for maximum yield
After completion of boring and developing the bore shall be tested for the maximum yield and drawdown with the help of 900 free flowing V notch method and such “
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record should be kept for office record
8 15
5/ 2
cd 2 g tan / 2( H )
Where 90
cd 0.62
0
Installation of suitable pump. At present IM II Deep well Hand Pumps are used in the department for both types of Tube wells i.e. DT & GPT. However so many types of Pumps are available in market. The Deep well IMII HP must be confirming to IS : 9301 - 1984 specification latest amendment.
Pump Installation
