3. Comparison of large and small diameter wells

Using the expression given above, the yield of a hypothetical 15 cm diameter well might be compared with that of a 150 cm diameter well, assuming that the cone of depression extends 25 m beyond the periphery of each well and that the depths of penetration are the same. Under these conditions the large diameter well will yield approximately 1.6 times as much water as the small diameter well. However, the volume of material excavated in making the larger well would be 100 times that of the small well. By contrast, increasing H from 2 m to 3 m while holding the drawdown constant at 0. 5 m would make the yield 2.3 times greater for a well of a given diameter. These examples, while oversimplified, help to point out that increasing depth is frequently a more efficient way of increasing the yield of a well than is increasing the diameter.

In this paper a large diameter well is defined as one which is large enough for workers to enter for construction or maintenance. A small diameter well, on the other hand, is one which is too small to enter and is constructed from the ground surface using special tools. Table 1 lists some advantages and disadvantages of the two types of wells.

Table 2 compares various methods for drilling small diameter wells. Summing these different drilling techniques with the various methods and materials available for large diameter well construction gives a large total of options for self-help well projects. These options are compounded by the possibility of combining various techniques and/or materials in a single well.

The final choice will depend on many factors including:

- the geology of the location
- materials available and their cost
- skills available and their cost
- end use of the well.

In some cases, the most desirable type of well or construction technique may be determined only after some trial and error.

In considering whether to have a large or small diameter well, it is important to consider also whether water storage is needed, particularly in the poorly permeable rocks where wells cannot obtain large flows even in the best conditions. If storage is needed then cost of a surface reservoir or tank should be included in the small well option to compare fairly with the cost of large wells.

Table 1. RELATIVE ADVANTAGES AND DISADVANTAGES OF SMALL AND LARGE DIAMETER WELLS

SMALL

LARGE

Equipment Required:

(1) For Construction

(1) Specialized equipment such as augers and bailing buckets required.

(1) Little specialized equipment is absolutely necessary.

(2) For Raising Water

(2) Specialized equipment such as pumps or small diameter well buckets are necessary.

(2) Ropes and buckets are frequently used.

Cost of Construction

Lower, because relatively little material is required.

Higher, because much more material is required.

Sanitation

Potentially good, especially when a hand pump is used.

Poor, since top of well is open. Buckets and ropes which may be dirty are used in the well.

Safety

Danger during construction and use negligible.

Construction: Danger of cave-in may be eliminated by proper construction. Danger of something dropping on worker in well always possible. Use: Proper construction of top of well can minimize danger of people falling in.

Maximum Number of People Able to Use the Well Concurrently

One

Three or Four

Rate of Discharge Possible

Potentially better since well can be made almost any depth below static, water level. Good possibility of putting perforated part of casing in material of high permeability.

Depth to which well may be excavated below static water level is limited by equipment. Therefore, rate of discharge is limited

Skill Required:

(1) Well Construction

(1) Somewhat more, since tools are special and work can't be seen.

(1) Somewhat less.

(2) Water Raising Equipment

(2) More, must be able to maintain and repair pump and/or small dia. well buckets.

(2) Little required.

Reliability:

(1) Well

(1) Excellent

(1) Good only if certain precautions are taken in constructing the bottom of the well.

(2) Water Raising

(2) Frequently a problem under village use (requires trained maintenance personnel).

(2) Good.

Ability to Store Water For Hours of Peak Demand (Of. Possible Importance When the Permeability of the Aquifer is Very Low).

Negligible.

May be increased by increasing the diameter and depth of well.

Limitations on When Well May Be Constructed.

None

Should be done at the time of year when the water level is at its lowest.

TABLE 2. A summary of. methods for drilling small diameter wells

METHOD

HOW PENETRATION IS ACCOMPLISHED

MINIMUM EQUIPMENT REQUIRED

REMOVAL OF MATERIAL FROM HOLE

ADVANTAGES/DISADVANTAGES,, LIMITATIONS

AUGERED OR BORED

CUTTING LIPS OF A ROTATING AUGER SHAVE OR CUT MATERIAL LOOSE FROM THE BOTTOM OF THE HOLE.

AUGER, DETACHABLE TUBULAR EXTENSIONS, AND A HANDLE FOR ROTATING.

AUGER MUST BE REMOVED FROM THE HOLE WHENEVER IT IS FULL OF CUTTINGS. THIS NECESSITATES UNCOUPLING EXTENSIONS.

EQUIPMENT IS SIMPLE AND CAN USUALLY BE FABRICATED OR ADAPTED LOCALLY. CANNOT PENETRATE HARD FORMATIONS. UNCOUPLING EXTENSIONS SLOWS WORK AT GREATER DEPTHS. USUALLY CANNOT BE USED BELOW THE WATER TABLE.

DRIVEN

A POINT ON THE LOWER END OF A STRING OF PIPE ALLOWS THE PIPE TO PENETRATE AS IT IS DRIVEN ON THE UPPER END.

DRIVE POINT WHICH USUALLY ALSO INCLUDES A WELL SCREEN ABOVE IT, SPECIAL DRIVE PIPE WITH COUPLINGS, DRIVE CAP, AND DRIVER.

MATERIAL IS NOT REMOVED FROM THE HOLE, BUT IS FORCED OUT LATERALLY AS THE DRIVE POINT IS FORCED THROUGH IT.

FAST AND SIMPLE.
SPECIAL well POINTS AND HEAVY DRIVE PIPE MAY NOT BE AVAILABLE LOCALLY. HARD FORMATIONS CANNOT BE PENETRATED. LIMITED TO SMALL DIAMETERS, BUT MULTIPLE well POINTS MAY BE CONNECTED TO A COMMON PUMP.

JETTED

A HIGH VELOCITY STREAM OF water COMING OUT OF THE BOTTOM OF A VERTICAL PIPE WASHES AWAY MATERIAL AHEAD OF IT AS IT IS LOWERED.

PIPE EQUIPPED WITH JETTING ORIFICE (S) AT LOWER END, COUPLINGS, SUITABLE PUMP (HAND. OR MOTOR POWERED), FLEXIBLE CONNECTION BETWEEN PUMP AND PIPE, AND SUPPLY OF water.

THE water USED FOR DRILLING RETURNS TO THE GROUND SURFACE BY HAY OF THE ANNULAR SPACE AROUND THE JETTING PIPE CARRYING THE MATERIAL REMOVED WITH IT.

FAST.
CANNOT PENETRATE HARD FORMATIONS. DIFFICULTY IN BRINGING LARGE GRAVEL OR STONE TO THE SURFACE. DRILLING EQUIPMENT CAN BE FABRICATED LOCALLY, BUT A PUMP AND A SOURCE OF water ARE REQUIRED.

HYDRAULIC PERCUSSION

THE HOLE IS KEPT FULL OF MATER. THE ALTERNATE RAISING AND DROPPING OF A STRING OF PIPE EQUIPPED WITH A CUTTING BIT AT THE BOTTOM ALLOWS PENETRATION BY A COMBINATION OF MECHANICAL AND HYDRAULIC ACTION.

HOLLOW DRILL BIT WITH WATER INLETS AND A CHECK VALVE, STRING OF PIPE, DEVICES TO AID RAISING AND DROPPING. A MAN'S HAND OVER THE TOP OF THE DRILL PIPE MAY BE SUBSTITUTED FOR THE CHECK VALVE.

THE RAISING AND DROPPING ACTION IN CONJUNCTION WITH THE CHECK VALVE CAUSES water TO BE PUMPED UP THE INSIDE OF THE DRILL PIPE CARRYING THE CUTTINGS WITH IT.

EQUIPMENT CAN BE FABRICATED LOCALLY OR PURCHASED. water REQUIRED. TRADITIONALLY USED IN SOME AREAS, THUS UNDERSTOOD BY LOCAL well DRILLERS. HARD FORMATIONS CANNOT BE PENETRATED. DIFFICULTY IN BRINGING LARGE GRAVEL OR STONES TO THE SURFACE.

CABLE TOOL PERCUSSION

A HEAVY CYLINDRICAL HEIGHT EQUIPPED WITH A CUTTING EDGE AT THE BOTTOM AND WITH A ROPE OR CABLE ATTACHED TO THE UPPER END IS ALTERNATELY RAISED AND DROPPED. IMPACT PULVERIZED MATERIAL AT THE BOTTOM OF THE HOLE.

HEAVY DRILL BIT, ROPE OR GABLE, DEVICES TO AID RAISING AND DROPPING.

THE PULVERIZED CUTTINGS ARE MIXED INTO A SLURRY WITH WATER DURING DRILLING. THESE ARE REMOVED USING A BAILER.

ALL FORMATIONS CAN BE PENETRATED AT VARYING RATES. SOME water REQUIRED. COMMERCIALLY BUILT RIG IS EXPENSIVE AND REQUIRES CONSIDERABLE SKILL TO OPERATE, BUT A SIMPLE SET OF TOOLS CAN BE FABRICATED LOCALLY AND ADAPTED TO MAN OR MOTOR POWER.

BAIL DOWN

A LONG, CYLINDRICAL BUCKET WITH A CHECK VALVE AT THE BOTTOM AND A ROPE OR CABLE ATTACHED TO THE TOP IS ALTERNATELY RAISED AND DROPPED IN A HOLE PARTIALLY FILLED WITH WATER. PENETRATION IS ACCOMPLISHED BY A COMBINATION OF HYDRAULIC AND MECHANICAL ACTION.

BAILER, ROPE, DEVICES TO AID RAISING AND DROPPING.

SLURRY OF CUTTINGS AND water ENTER THE BAILER AS IT IS REPEATEDLY DROPPED. THESE ARE PREVENTED FROM LEAVING THE BUCKET BY THE CHECK VALVE. THE BUCKET IS RAISED TO THE SURFACE FOR EMPTYING.

EQUIPMENT CAN BE FABRICATED LOCALLY. FREQUENTLY USED IN CONJUNCTION WITH OTHER METHODS, SUCH AS PERCUSSION. HARD FORMATIONS CANNOT BE PENETRATED BY THE BAILER ALONE.

HYDRAULIC ROTARY

A HOLLOW DRILL BIT WITH EITHER A FIXED CUTTING EDGE OR TOOTHED ROLLERS IS ROTATED AT THE BOTTOM END OF A STRING OF PIPE. MATERIAL IS SCRAPED, ABRADED OR CHIPPED AWAY BY MECHANICAL ACTION.

DRILL BIT, DRILL PIPE, CIRCULATING PUMP, DEVICE FOR ROTATING DRILL PIPE.

water OR "MUD" IS PUMPED DOWN THE HOLLOW DRILL STEM TO LUBRICATE THE BIT AND TO CARRY THE CUTTINGS UP TO THE SURFACE THROUGH THE ANNULAR SPACE AROUND THE DRILL PIPE. CIRCULATION MAY ALSO BE IN THE REVERSE DIRECTION.

COMMERCIALLY BUILT RIG IS EXPENSIVE AND REQUIRES CONSIDERABLE SKILL TO OPERATE. HOWEVER, SMALL ADAPTATIONS USING EITHER MAN POWER OR SMALL ENGINES HAVE BEEN DEVISED. A WATER SUPPLY IS NECESSARY. IT IS DIFFICULT TO DRILL IN LOOSE FORMATIONS.

	SMALL	LARGE
Equipment Required:
(1) For Construction	(1) Specialized equipment such as augers and bailing buckets required.	(1) Little specialized equipment is absolutely necessary.
(2) For Raising Water	(2) Specialized equipment such as pumps or small diameter well buckets are necessary.	(2) Ropes and buckets are frequently used.
Cost of Construction	Lower, because relatively little material is required.	Higher, because much more material is required.
Sanitation	Potentially good, especially when a hand pump is used.	Poor, since top of well is open. Buckets and ropes which may be dirty are used in the well.
Safety	Danger during construction and use negligible.	Construction: Danger of cave-in may be eliminated by proper construction. Danger of something dropping on worker in well always possible. Use: Proper construction of top of well can minimize danger of people falling in.
Maximum Number of People Able to Use the Well Concurrently	One	Three or Four
Rate of Discharge Possible	Potentially better since well can be made almost any depth below static, water level. Good possibility of putting perforated part of casing in material of high permeability.	Depth to which well may be excavated below static water level is limited by equipment. Therefore, rate of discharge is limited
Skill Required:
(1) Well Construction	(1) Somewhat more, since tools are special and work can't be seen.	(1) Somewhat less.
(2) Water Raising Equipment	(2) More, must be able to maintain and repair pump and/or small dia. well buckets.	(2) Little required.
Reliability:
(1) Well	(1) Excellent	(1) Good only if certain precautions are taken in constructing the bottom of the well.
(2) Water Raising	(2) Frequently a problem under village use (requires trained maintenance personnel).	(2) Good.
Ability to Store Water For Hours of Peak Demand (Of. Possible Importance When the Permeability of the Aquifer is Very Low).	Negligible.	May be increased by increasing the diameter and depth of well.
Limitations on When Well May Be Constructed.	None	Should be done at the time of year when the water level is at its lowest.

METHOD	HOW PENETRATION IS ACCOMPLISHED	MINIMUM EQUIPMENT REQUIRED	REMOVAL OF MATERIAL FROM HOLE	ADVANTAGES/DISADVANTAGES,, LIMITATIONS
AUGERED OR BORED	CUTTING LIPS OF A ROTATING AUGER SHAVE OR CUT MATERIAL LOOSE FROM THE BOTTOM OF THE HOLE.	AUGER, DETACHABLE TUBULAR EXTENSIONS, AND A HANDLE FOR ROTATING.	AUGER MUST BE REMOVED FROM THE HOLE WHENEVER IT IS FULL OF CUTTINGS. THIS NECESSITATES UNCOUPLING EXTENSIONS.	EQUIPMENT IS SIMPLE AND CAN USUALLY BE FABRICATED OR ADAPTED LOCALLY. CANNOT PENETRATE HARD FORMATIONS. UNCOUPLING EXTENSIONS SLOWS WORK AT GREATER DEPTHS. USUALLY CANNOT BE USED BELOW THE WATER TABLE.
DRIVEN	A POINT ON THE LOWER END OF A STRING OF PIPE ALLOWS THE PIPE TO PENETRATE AS IT IS DRIVEN ON THE UPPER END.	DRIVE POINT WHICH USUALLY ALSO INCLUDES A WELL SCREEN ABOVE IT, SPECIAL DRIVE PIPE WITH COUPLINGS, DRIVE CAP, AND DRIVER.	MATERIAL IS NOT REMOVED FROM THE HOLE, BUT IS FORCED OUT LATERALLY AS THE DRIVE POINT IS FORCED THROUGH IT.	FAST AND SIMPLE. SPECIAL well POINTS AND HEAVY DRIVE PIPE MAY NOT BE AVAILABLE LOCALLY. HARD FORMATIONS CANNOT BE PENETRATED. LIMITED TO SMALL DIAMETERS, BUT MULTIPLE well POINTS MAY BE CONNECTED TO A COMMON PUMP.
JETTED	A HIGH VELOCITY STREAM OF water COMING OUT OF THE BOTTOM OF A VERTICAL PIPE WASHES AWAY MATERIAL AHEAD OF IT AS IT IS LOWERED.	PIPE EQUIPPED WITH JETTING ORIFICE (S) AT LOWER END, COUPLINGS, SUITABLE PUMP (HAND. OR MOTOR POWERED), FLEXIBLE CONNECTION BETWEEN PUMP AND PIPE, AND SUPPLY OF water.	THE water USED FOR DRILLING RETURNS TO THE GROUND SURFACE BY HAY OF THE ANNULAR SPACE AROUND THE JETTING PIPE CARRYING THE MATERIAL REMOVED WITH IT.	FAST. CANNOT PENETRATE HARD FORMATIONS. DIFFICULTY IN BRINGING LARGE GRAVEL OR STONE TO THE SURFACE. DRILLING EQUIPMENT CAN BE FABRICATED LOCALLY, BUT A PUMP AND A SOURCE OF water ARE REQUIRED.
HYDRAULIC PERCUSSION	THE HOLE IS KEPT FULL OF MATER. THE ALTERNATE RAISING AND DROPPING OF A STRING OF PIPE EQUIPPED WITH A CUTTING BIT AT THE BOTTOM ALLOWS PENETRATION BY A COMBINATION OF MECHANICAL AND HYDRAULIC ACTION.	HOLLOW DRILL BIT WITH WATER INLETS AND A CHECK VALVE, STRING OF PIPE, DEVICES TO AID RAISING AND DROPPING. A MAN'S HAND OVER THE TOP OF THE DRILL PIPE MAY BE SUBSTITUTED FOR THE CHECK VALVE.	THE RAISING AND DROPPING ACTION IN CONJUNCTION WITH THE CHECK VALVE CAUSES water TO BE PUMPED UP THE INSIDE OF THE DRILL PIPE CARRYING THE CUTTINGS WITH IT.	EQUIPMENT CAN BE FABRICATED LOCALLY OR PURCHASED. water REQUIRED. TRADITIONALLY USED IN SOME AREAS, THUS UNDERSTOOD BY LOCAL well DRILLERS. HARD FORMATIONS CANNOT BE PENETRATED. DIFFICULTY IN BRINGING LARGE GRAVEL OR STONES TO THE SURFACE.
CABLE TOOL PERCUSSION	A HEAVY CYLINDRICAL HEIGHT EQUIPPED WITH A CUTTING EDGE AT THE BOTTOM AND WITH A ROPE OR CABLE ATTACHED TO THE UPPER END IS ALTERNATELY RAISED AND DROPPED. IMPACT PULVERIZED MATERIAL AT THE BOTTOM OF THE HOLE.	HEAVY DRILL BIT, ROPE OR GABLE, DEVICES TO AID RAISING AND DROPPING.	THE PULVERIZED CUTTINGS ARE MIXED INTO A SLURRY WITH WATER DURING DRILLING. THESE ARE REMOVED USING A BAILER.	ALL FORMATIONS CAN BE PENETRATED AT VARYING RATES. SOME water REQUIRED. COMMERCIALLY BUILT RIG IS EXPENSIVE AND REQUIRES CONSIDERABLE SKILL TO OPERATE, BUT A SIMPLE SET OF TOOLS CAN BE FABRICATED LOCALLY AND ADAPTED TO MAN OR MOTOR POWER.
BAIL DOWN	A LONG, CYLINDRICAL BUCKET WITH A CHECK VALVE AT THE BOTTOM AND A ROPE OR CABLE ATTACHED TO THE TOP IS ALTERNATELY RAISED AND DROPPED IN A HOLE PARTIALLY FILLED WITH WATER. PENETRATION IS ACCOMPLISHED BY A COMBINATION OF HYDRAULIC AND MECHANICAL ACTION.	BAILER, ROPE, DEVICES TO AID RAISING AND DROPPING.	SLURRY OF CUTTINGS AND water ENTER THE BAILER AS IT IS REPEATEDLY DROPPED. THESE ARE PREVENTED FROM LEAVING THE BUCKET BY THE CHECK VALVE. THE BUCKET IS RAISED TO THE SURFACE FOR EMPTYING.	EQUIPMENT CAN BE FABRICATED LOCALLY. FREQUENTLY USED IN CONJUNCTION WITH OTHER METHODS, SUCH AS PERCUSSION. HARD FORMATIONS CANNOT BE PENETRATED BY THE BAILER ALONE.
HYDRAULIC ROTARY	A HOLLOW DRILL BIT WITH EITHER A FIXED CUTTING EDGE OR TOOTHED ROLLERS IS ROTATED AT THE BOTTOM END OF A STRING OF PIPE. MATERIAL IS SCRAPED, ABRADED OR CHIPPED AWAY BY MECHANICAL ACTION.	DRILL BIT, DRILL PIPE, CIRCULATING PUMP, DEVICE FOR ROTATING DRILL PIPE.	water OR "MUD" IS PUMPED DOWN THE HOLLOW DRILL STEM TO LUBRICATE THE BIT AND TO CARRY THE CUTTINGS UP TO THE SURFACE THROUGH THE ANNULAR SPACE AROUND THE DRILL PIPE. CIRCULATION MAY ALSO BE IN THE REVERSE DIRECTION.	COMMERCIALLY BUILT RIG IS EXPENSIVE AND REQUIRES CONSIDERABLE SKILL TO OPERATE. HOWEVER, SMALL ADAPTATIONS USING EITHER MAN POWER OR SMALL ENGINES HAVE BEEN DEVISED. A WATER SUPPLY IS NECESSARY. IT IS DIFFICULT TO DRILL IN LOOSE FORMATIONS.