How to Construct Tube Wells? | Groundwater | Water Engineering

In this article we will discuss about how to construct tube wells.

Construction of Shallow Tube Wells:

Shallow tube wells are constructed by boring, driving and jetting. The wells constructed by these methods are designated as bored wells, driven wells and jetted wells.

These methods are briefly described below:

(i) Bored Wells:

Where, water table exists at a shallow depth in an unconsolidated aquifer, wells may be constructed by boring with hand operated or power driven augers. Hand operated augers are available in different shapes and sizes but in general they have cutting blades at the bottom that bore into the ground as the auger is rotated. When the blades are full of loose earth, the auger is removed from the hole and the operation is repeated until the desired depth is reached.

The hand operated augers can bore wells only upto 200 mm in diameter and 15 m in depth. However, the power driven augers can bore wells upto lm in diameter and depths exceeding 30 m. A power driven auger consists of a cylindrical steel bucket with a cutting edge projecting from a slot in the bottom, which cuts into the soil and fills it in the bucket as the auger is rotated. When full, the auger is hoisted to the surface and the excavated material is removed through hinged openings on the side or bottom of the bucket. Reamers, attached to the top of the bucket, can enlarge holes to diameters exceeding the auger size.

A continuous-flight power driven auger has a spiral extending from the bottom of the hole to the surface. In this case cuttings are carried to the surface as on screw conveyor.

Augers work best in formations that do not cave in. However, where loose sand and gravel are encountered in a large-diameter hole, or when the boring reaches the water table a concrete or metal casing pipe is introduced into the hole and the boring is continued inside the casing.

After the boring is done upto the desired depth the well pipe together with strainer is introduced centrally in the hole and the casing pipe is removed. The annular space between the well pipe and the surrounding strata is backfilled with gravel.

(ii) Driven Wells:

A driven well is constructed in an unconsolidated formation by the use of a well point. A well point consists of a perforated pipe wrapped over by a fine wire mesh (or screen), with a conical steel shoe fixed at the bottom. The conical shoe is known as drive point and it is provided to facilitate the driving of the well point into the soil and also to protect its screened section during driving.

The well point is connected to a pipe, and the well point along with the pipe is driven by repeated impacts into the ground. Additional lengths of pipe are connected by threaded couplings and the driving is continued until the desired depth is reached when the entire well point penetrates the aquifer sufficiently below the water table.

The impact is provided at the top of the pipe by the falling weight of a drop hammer. A drive cap is provided at the top of the pipe to protect the pipe and its threads against the possible damage due to the impact of the falling weight.

The diameter of the driven wells are equal to the diameters of the well points used for their construction. Since from practical considerations the diameters of the well points seldom exceed 100 mm, the diameters of the driven wells are small and are mostly in the range of 30 to 100 mm.

Also the depths of the driven wells are generally limited to 15 m though in a few cases the depths may exceed 20 m. As such because of the limitations on the size and depths the yields from the driven wells are small, with discharges of about 4 to 10 m³/hour.

The driven wells may be constructed only in those unconsolidated formations which do not contain large gravel or rock that may damage the drive point. However, the important advantages of the driven wells are that they can be constructed in a short time, at minimum cost, and even by one person.

(iii) Jetted Wells:

A jetted well is constructed by the cutting action of a high velocity jet of water. In this method a casing pipe of diameter slightly larger than that of the well pipe is first introduced in a suitable hole dug at the surface. A jetting pipe (or drill pipe) is lowered in the casing pipe and a high velocity jet of water issuing from the jetting pipe is directed to the bottom of the hole.

The soil in the hole is thus loosened and it is carried out of the hole in the form of a slurry by the water returning in the upward direction through the annular space between the casing pipe and the jetting pipe.

The hole is thus deepened and the casing pipe is lowered. During the jetting operation the jetting pipe is turned slowly to ensure a straight hole. In penetrating hard clayey soils a jetting drill bit is attached to the end of jetting pipe which is raised and dropped, thus causing the bit to disintegrate the soil.

When the casing pipe has penetrated the aquifer to a sufficient depth below the water table, the well pipe with a strainer at the bottom is introduced in the casing pipe. The casing pipe is then pulled out and gravel is inserted in the annular space between the well pipe and the surrounding strata.

By this method generally the wells of small diameters in the range of 30 to 100 mm are constructed but their depths may be more than 15 m. However, the well of larger diameters upto 300 mm or more may also be constructed by this method. Further the yields from the jetted wells are also small and these wells may also be constructed only in unconsolidated formations consisting of gravel, sand, clay or other soft deposits.

Alternatively a jetted well may also be constructed by using a self jetting well point. In this method casing pipe is not used. Also no separate jetting pipe is used but the well pipe itself is used as a jetting pipe. For this a perforated brass pipe wrapped over by a fine wire mesh (or screen) and ending in a nozzle is screwed to the well pipe.

A small hole is dug at the surface and a high velocity jet issuing from the nozzle is directed to the bottom of the hole. As the jetting action progresses, the well pipe goes on sinking. An annular space around the well pipe is automatically created due to the upward motion of water carrying the loosened soil. When the well pipe has been sunk to the desired depth, jetting is stopped and the annular space is packed with gravel.

Construction of Deep Tube Wells:

Deep tube wells are constructed by drilling.

The various methods of drilling which are commonly used for constructing deep tube wells are as follows:

(i) Cable tool method

(ii) Hydraulic rotary method

(iii) Reverse rotary method

(i) Cable Tool Method:

The cable tool method is also known as the percussion method or standard method. This method is capable of drilling wells of 80 to 600 mm diameter through consolidated rock materials to depths of 1500 m. However, in unconsolidated sand and gravel this method is least effective.

The method consists of sinking a casing pipe usually slightly larger in diameter than the size of the tube well required with the help of a rig. The function of the casing pipe is to support the hole and prevent it from caving in while the drilling is in progress.

The drilling rig for the cable tool method consists of a mast, a multiline hoist, a walking beam and an engine all mounted on a truck for ready portability. Drilling is accomplished by regular lifting and dropping of a string of tools. The string of tools consists of swivel socket, a set of jars, a drill stem, and a drilling bit. These tools are made of steel and their total weight may amount to several thousand kilograms.

The most important part of the string of tools is the drilling bit which does the actual drilling. The drilling bit is at the lower end of the string of tools and when it is lifted and dropped then with its relatively sharp chisel edge breaks the rock by impact. The drilling bits are manufactured in lengths of 1 to 3 m and weigh upto 1500 kg.

The drill stem is a long steel bar that adds weight and length to the drill so that it may cut rapidly and vertically. A set of jars consists of a pair of narrow connecting links and their purpose is only to loosen the tools if they stick in the hole. The swivel socket attaches the drilling cable to the string of tools.

A pit of about 2 to 3 m diameter 5 to 8 m deep is dug at the site where the well is to be built. A casing pipe with a cutter shoe screwed to its bottom is introduced into the pit. The string of tools is introduced into the casing pipe and the drilling is started by alternate lifting and dropping mechanically the string of tools so that the drilling bit produces a hammering action on the formation resulting in its disintegration. During drilling the tools make 20 to 40 strokes per minute, ranging from 0.4 to 1 m in length.

The drilling line is rotated so that the drilling bit forms a round hole. For drilling operation water is necessary and it must be added to the hole until it is encountered in the hole itself. Water forms a paste with the cuttings, thereby reducing friction of the drilling bit as it falls. After the drilling bit has cut 1 to 2 m through the formation the string of tools is taken out and a bailer is introduced into the hole to remove the cuttings.

A bailer consists of a section of pipe with a flap valve at the bottom. When the bailer is introduced into the hole, the valve is automatically opened due to an upward pressure exerted on it by the paste of cuttings and water and the same enters the bailer.

The valve, however, prevents the paste from moving out of the bailer during lifting. When the bailer is full, it is lifted to the surface and emptied. The bailers are available in a range of diameters with their lengths varying from 3 to 8 m and capacities upto 0.25 m³. After the cuttings are taken out from the hole, the string of tools is again introduced into the hole and the drilling is continued until the required depth is reached.

The casing pipe is required only in unconsolidated formations, in which case as the drilling progresses and the hole is deepened the casing pipe must also be driven down and maintained almost upto the bottom of the hole to avoid caving in of the wall of the hole.

The casing pipe is driven down by means of drive clamps fastened to the drill stem; the up and down motion of the tools striking the top of the casing pipe, protected by a drive head, sinks the casing pipe. The casing pipe is usually in 3 m lengths which are flush jointed by screwing one length on the other. Thus when the casing pipe has been sunk 3 m another pipe length is screwed on the top.

A record of the material collected by bailer during drilling is kept. A bore log is then plotted to know the depths of various formations. A well pipe assembly comprising strainers and blind pipes is then introduced into the casing pipe in such a way that strainers are placed against the aquifers or water bearing strata which are to be tapped and the blind pipes against aquicludes or the strata which are not to be tapped.

The cable tool method is not suitable for drilling tube wells in unconsolidated formations comprising sand and gravel. In the case of such formations the tube well may be drilled only with the help of a bailer (also known as sand pump or sledger). The bailer in this case consists of a steel pipe 2 to 4 m long having a flap valve at the bottom and a cutting shoe of hard steel riveted to its bottom. A casing pipe is introduced in a pit dug at the well site. Water is added to the casing pipe to facilitate drilling.

The bailer is introduced into the casing pipe and it is lifted and dropped manually by means of a rope the other end of which passes over a pulley fixed centrally to a tripod. As the bailer is lifted and dropped it cuts through the formation and a paste of the cuttings and water formed. The flap valve is automatically opened due to an upward pressure exerted on it by the paste and the same moves into the bailer. When the bailer is full with the paste it is taken out and emptied.

The process is repeated till the required depth is reached. As the drilling progresses and the hole is depended the casing pipe must also be driven to always keep it upto the bottom of the hole to avoid caving in of the wall of the hole.

For driving the casing pipe a platform is attached to the upper end of the casing pipe which when loaded by sand bags, rails, etc., drives the casing pipe slowly into the hole. After the drilling is carried out upto the required depth the well pipe is introduced into the casing, the well is shrouded and the casing pipe is taken out.

(ii) Hydraulic Rotary Method:

The hydraulic rotary method (or rotary method) is a rapid method of drilling tube wells which is suitable only for unconsolidated strata. Wells upto 0.45 m diameter (upto 1.5 m diameter with a reamer) and depths over 1500 m have been constructed by this method.

The drilling rig for a hydraulic rotary method consists of a derrick or mast, a rotating table, a pump for the drilling mud, a hoist and the engine. In this method the drilling is done with the help of a hollow drilling bit attached to the lower end of a drill pipe which extends to the ground surface. The upper end of the drill pipe is attached to a square section known as Kelly. A rotating table that fits closely around the Kelly rotates the drill pipe and allows the drill pipe to slide downward as the hole deepens.

During drilling a mixture of clay and water, known as drilling mud, is continuously pumped into the hole through the drill pipe. The mud leaves the drill pipe through one or more openings of the drilling bit where it cools and lubricates the cutting surface of the drilling bit. The mud flows out of the hole through the annular space between the drill pipe and the walls of the hole.

The material loosened by the drilling bit is carried to the surface by the mud flowing out of the hole. In this method no casing pipe is ordinarily required during drilling because the mud forms a clay lining on the wall of the hole and thus prevents caving in of the wall of the hole. The mud flowing out of the hole is taken to a settling basin where the cuttings settle down and the mud is again used for pumping into the hole. To maintain the required consistency of the mud, clay and water are added to the circulation mud from time to time.

A complete boring record is maintained to know the type of formations at various depths. When the required depth is reached the drill pipe and the drilling bit are pulled out from the hole and a well pipe containing strainers and blind pipes at appropriate locations is introduced in the hole. Since in this case the wall of the hole is coated with clay, it should be thoroughly washed to obtain more yield from the well.

(iii) Reverse Rotary Method:

The reverse rotary method is also suitable for drilling tube wells only in unconsolidated formations. It is probably the most rapid method of drilling and hence it has become increasingly popular. The drilling rig for this method is similar to that for the hydraulic rotary method. The drilling in this method is also done by a hollow drilling bit attached to the lower end of a drill pipe as in the case of the hydraulic rotary method.

However, by this method usually large diameter holes with a minimum diameter of about 0.4 m may be drilled. The drilling bits for this method range in diameter from 0.4 to 1.8 m. Further in this method also no casing pipe is ordinarily used.

Instead during drilling the drilled hole is always kept filled with water upto the ground surface, so that the water pressure exerted on the formation prevents the caving in of the wall of the hole. Moreover, the fine-grained soil particles held suspended in the column of water also help in stabilizing the wall of the hole.

The material loosened by the drilling bit is carried to the surface by the water which is pumped out of the hole through the drilling bit and the drill pipe. As such in order to maintain the water level in the hole upto the ground surface, water is continuously supplied to the drilled hole from the top in the annular space outside the drill pipe. Moreover, the water pumped out of the hole is taken to a settling basin where the cuttings settle down and the clear water is again supplied to the hole.

In this method due to the use of clear water during drilling, a considerable quantity of water seeps into the aquifer and hence a large quantity of water is required. However, the amount of water taken by the aquifers during drilling is an indication of the yield of the well after completion. Thus better aquifers need more quantity of water during drilling.

To prevent the caving in of the wall of the hole the pressure head of the water in the hole must be lager than that of the water in the aquifer. As such in general this method is applicable when the water table is about 3 to 4 m below the ground surface so that sufficient pressure head is developed to prevent the caving in of the wall of the hole. However, by using a casing pipe this method can also be used to drill tube wells when the water table is either too high or too low.

Where the water table is close to the surface, by using a casing pipe extending above the ground surface the necessary pressure head may be developed by filling water upto the desired level in the casing pipe. On the other hand, where the water table is deep, a large amount of water filled in the hole may be lost by seepage into the aquifers, which may be minimized by introducing a casing pipe into the hole.

Samples of the formation are collected from the water as it comes up through the drill pipe. When the required depth is reached the drill pipe and the drilling bit are pulled out from the hole. A well pipe assembly comprising strainers and blind pipes at appropriate places is then introduced into the hole and the well is shrouded.