Methods of Drilling Boreholes

In many developing countries, water is obtained from handpumps installed above shallow (less than 60m deep) boreholes. It can be expensive to drill the borehole, however, if traditional machined drilling rigs are used. This Technical Brief outlines simple, low-cost drilling methods which may be used in various situations. Each can be used and maintained easily.

Drilling constraints

Whatever drilling method is used, there are several considerations which must be taken into account:
The amount of energy required to drill is governed by the rock type. Unconsolidated formations such as sand, silt or clay are weak and much easier to drill than consolidated rocks such as granite, basalt or slate which are hard, strong and dense. For hard rocks, cutting tools will need cooling and lubrication. Rock cuttings and debris must be removed. Unconsolidated formations will require support to prevent the hole from collapsing.

Hand-auger drilling

Drilling methods

The following low-cost, appropriate drilling methods are described and illustrated on the following pages:

1. Percussion drilling
2. Hand-auger drilling
3. Jetting
4. Sludging
5. Rotary-percussion drilling
6. Rotary drilling with flush

Once a suitable site has been selected and borehole drilling decided on, the proper drilling method must be chosen. Another primary consideration in project planning is the availability of existing water sources and water points. There may be completed dug wells and boreholes already in the area. Are they in use? If not, can they be rehabilitated to augment water availability or to reduce the cost of the program? Drilling equipment, such as compressors, can be used to bring disused boreholes back into use; the question of rehabilitation will be addressed in this guideline. This section outlines the factors that must be considered when choosing a drilling method.

Common Drilling Methods
Essentially, a drilling machine consists of a mast from which the drilling string components (tools plus drill pipes or cable) are suspended and, in most cases, driven. Modern systems are powered rotary-driven, but it is probably worth a short digression to describe some methods of manual drilling for water. Simple, low-cost methods include:

– Hand-auger drilling

Hand-auger drilling

Method: The cutting tool (known as the auger head) is rotated to cut into the ground, and then withdrawn to remove excavated material. The procedure is repeated until the required depth is reached. Note: This method is only suitable for unconsolidated deposits.

Auger drills, which are rotated by hand, cut into the soil with blades and pass the cut material up a continuous screw or into a ‘bucket’ (bucket auger). Excavated material must be removed and the augering continued until the required depth has been reached. Auger drilling by hand is slow and limited to a depth of about 10 meters (maximum 20 meters) in unconsolidated deposits (not coarser than sand, but it is a cheap and simple process.

Advantages of hand-auger drilling:
• Inexpensive.
• Simple to operate and maintain.

Disadvantages of hand-auger drilling:
• Slow, compared with other methods.
• Equipment can be heavy.
• Problems can occur with unstable rock formations.
• Water is needed for dry holes.

– Jetting

Jetting

Method: Water is pumped down the centre of the drill-rods, emerging as a jet. It then returns up the borehole or drill-pipe bringing with it cuttings and debris. The washing and cutting of the formation is helped by rotation, and by the up-and-down motion of the drill-string. A foot-powered treadle pump or a small internal-combustion pump are equally suitable.

A method whereby water is pumped down a string of rods from which it emerges as a jet that cuts into the formation. Drilling may be aided by rotating the jet or by moving it up and down in the hole. Cuttings are washed out of the borehole by the circulating water. Again, jetting is useful only in unconsolidated formations and
only down to relatively shallow depths, and would have to be halted if a boulder is encountered.

Advantages of jetting:
• The equipment is simple to use.
• Possible above and below the water-table.

Disadvantages of jetting:
• Water is required for pumping.
• Suitable for unconsolidated rocks only (e.g. sand, silt, clay)
• Boulders can prevent further drilling.

– Sludging

Sludging

Method: This method has been developed and used extensively in Bangladesh. A hollow pipe of bamboo or steel is moved up and down in the borehole while a one-way valve – your hand can be used to improvise successfully – provides a pumping action. Water flows down the borehole annulus (ring) and back up the drillpipe, bringing debris with it. A small reservoir is needed at the top of the borehole for recirculation. Simple teeth at the bottom of the drill-pipe, preferably made of metal, help cutting efficiency.

This method, which may be described as reverse jetting, involves a pipe (bamboo has been successfully employed) being lowered into the hole and moved up and down, perhaps by a lever arm. A oneway valve (such as someone’s hand at the top of the pipe) provides pumping action as water is fed into the hole and returns (with debris) up the drill pipe. There may be simple metal teeth at the cutting end of the pipe, and a small reservoir is required at the top of the hole for recirculation. The limitations of sludging are similar to those of the previous two methods.

Sludging

Advantages of sludging:
• The equipment can be made from local,
low-cost materials, and is simple to use.

Disadvantages of sludging:
• Water is required for pumping.
• Suitable for unconsolidated rocks only.
• Boulders can prevent further drilling.

– Percussion drilling

Method: The lifting and dropping of a heavy (50kg+) cutting tool will chip and excavate material from a hole. This drilling method has been used in China for over 3000 years. The tool can be fixed to rigid drill-rods, or to a rope or cable. With a mechanical winch, depths of hundreds of metres can be reached.

Drilling by percussion is done by simply dropping a heavy cutting tool, of 50 kilograms or more, repeatedly in the hole. The drilling tools are normally suspended by a rope or cable; and – depending on the weight of the drill string, which, for manual operation, is obviously limited – it is possible to drill to considerable depths in both soft and hard formations. Basic percussion drilling systems are still widely used in Abudwaq and Galgadgob districts of Somalia to drill shallow boreholes. They consist of a strong steel tripod, cable and power winch, percussion tools, and a baler.

Percussion drilling

These systems are seriously hindered when the ground is hard, and can accidentally change direction along weaker zones, causing boreholes to become crooked or tools to jam. Unconsolidated materials, although easy to drill with cable tool, become very obstructive when boulders are present. Sticky shales and clays are also difficult to penetrate with cable tool rigs, and loose sand tends to collapse into the hole almost as fast as it can be bailed. These manual shallow drilling techniques might be used as low-cost alternatives in groundwater investigations for dug well sites, particularly if geophysical surveys prove to be ineffective, unavailable or impracticable because of ground conditions. In such instances, when the drilling is done solely for the purpose of prospecting, only small holes are drilled, rapidly

Advantages of percussion drilling:
• Simple to operate and maintain.
• Suitable for a wide variety of rocks.
• Operation is possible above and below the water-table.
• It is possible to drill to considerable depths.

Disadvantages of percussion drilling:
• Slow, compared with other methods.
• Equipment can be heavy.
• Problems can occur with unstable rock formations.
• Water is needed for dry holes to help remove cuttings.

– Rotary drilling

Most borehole applications in the field will require rotary drilling. True rotary drilling techniques allow much deeper boreholes to be constructed, and use circulating fluids to cool and lubricate the cutting tools and to remove debris from the hole. Circulating fluids usually take the form of compressed air or of pumped water with additives, such as commercial drilling muds or foams.

Rotary-percussion drilling

Method: In very hard rocks, such as granite, the only way to drill a hole is to pulverize the rock, using a rapid-action pneumatic hammer, often known as a ‘down-the-hole hammer’ (DTH). Compressed air is needed to drive this tool. The air also flushes the cuttings and dust from the borehole. Rotation of 10-30 rpm ensures that the borehole is straight, and circular in cross-section.

Advantages of rotarypercussion drilling:
• Drills hard rocks.
• Possible to penetrate gravel.
• Fast.
• Operation is possible above and below the water-table.

Disadvantages of rotary-percussion drilling:
• Higher tool cost than other tools illustrated here.
• Air compressor required.
• Requires experience to operate and maintain.

Rotary drilling with flush

Method: A drill-pipe and bit are rotated to cut the rock. Air, water, or drilling mud is pumped down the drill-pipe to flush out the debris. The velocity of the flush in the borehole annulus must be sufficient to lift the cuttings.

Advantages of rotary drilling (with flush):
• Most rock formations can be drilled.
• Water and mud supports unstable formations.
• Fast.
• Operation is possible above and below the water-table.
• Possible to drill to depths of over 40 metres.
• Possible to use compressedair flush.

Disadvantages of rotary drilling (with flush):
• Requires capital expenditure in equipment.
• Water is required for pumping.
• There can be problems with boulders.
• Rig requires careful operation and maintenance.