Cable tool drilling, also known as percussion drilling, is one of the oldest and most reliable methods for creating boreholes, particularly in hard rock formations and unconsolidated soils. This technique uses repeated impacts from a heavy drill bit suspended on a cable to crush and break the ground, making it ideal for water wells, geotechnical investigations, and mineral exploration in challenging terrains like those found across Kenya.
Originating over 4,000 years ago in ancient China, where bamboo tools reached depths of 3,000 feet, cable tool drilling evolved into modern rigs powered by walking beams or hydraulic winches. In East Africa, including Mombasa’s coastal geology with its mix of coral limestone and overburden soils, Bestcare Borehole Drilling employs this method for its precision and cost-effectiveness in areas inaccessible to larger rotary rigs.
How It Works
The core of cable tool drilling involves a percussion cycle powered by a walking beam or mast-mounted winch that lifts and drops a heavy bit assembly—typically 1,200 pounds—15 to 60 times per minute. The bit, often a carbide-tipped chisel or button type, penetrates rock by crushing it into fine fragments while simultaneously reaming the borehole walls for stability.
Water is added periodically to form a slurry with the cuttings, which softens the material and aids removal. A bailer—a cylindrical bucket with a foot valve—is lowered to scoop out the debris after every few feet of progress. Casing, usually steel tubes of 6 to 12 inches in diameter, is driven in as drilling advances to prevent collapse, especially in loose sands or clays common in Kenyan coastal regions.
Drilling progresses to depths of 30-60 meters, with stroke length adjusted for formation hardness: shorter for soft soils, longer for hard rock. Standard Penetration Tests (SPT) can be integrated for soil strength data, and tools like clay-cutters or shells handle cohesive or non-cohesive materials respectively.
Equipment and Setup
A typical cable tool rig includes a tripod or mast for stability, a diesel or electric winch for cable control, and support gear like a small truck for welding and water supply. Modern versions, such as tracked rigs for restricted access, allow deployment on slopes or over water, minimizing site disturbance.
At Bestcare, we use compact, hydraulically operated rigs with emergency stops for safety, ensuring tidy operations where spoil is contained on-site. Noise from the winch and casing drive is present but manageable, and boreholes are backfilled post-completion.
Advantages and Applications
Cable tool excels in superficial deposits, weak rocks, and obstructed ground, providing high-quality, undisturbed samples for lab analysis—crucial for geotechnical work. It’s slower than rotary methods (4-10 feet per hour) but cheaper in remote areas, with low water needs and no drilling fluids that could contaminate aquifers.
In Kenya, it’s perfect for rural boreholes in hardpan or fractured basalt, enabling monitoring wells or water abstraction. Unlike rotary, it handles boulders without deviation and allows real-time strata logging.
Limitations and Best Practices
Penetration rates drop in very soft or extremely hard formations, and depths beyond 100 meters favor rotary. Safety risks from falling tools require strict protocols, including constant cable tension.
Bestcare recommends it for projects up to 60m in stable soils. We combine it with geophysical surveys for optimal siting, ensuring reliable yields. Contact Bestcare Borehole Drilling for expert cable tool services tailored to your needs—delivering sustainable water solutions across Mombasa and beyond.
