In 1987 we purchased our first Air Core system. From the first job we had major problems with drilling clays and found we couldn’t drill more than 200mm without the bit and inner tubes blocking up. We removed the end surfaces of the bit face with a gas torch and noted an immediate improvement in penetration. However, the inner tube still continually blocked up when drilling clays. The onsite solution was to coat the internal surfaces of the bottom 40mm of the inner tube with brass braising, reducing the internal diameter which immediately reduced inner tube blockages.
We realised from the first air core rod drilled that;
- Clay drilling requires the hole sample to be swaged to fit into the inner-tube for recovery to the surface as clay core. Swagging was required for clay, maintains the volume but reduces the diameter, while increasing the length of sample recovered.
- This process is the only efficient method to drill plastic materials.
- A sizing/swaging ring was required in the bottom of the inner tube to allow the sample of clay to float without touching the inner tube anywhere, apart from the bottom 40mm.
Without a sizing ring; the greater the length of clay core that is in the inner tube, the greater the contact area and suction increases until the plastic modulus of the clay is exceeded and the core becomes compressed tightly in the inner tube. This then requires a rod pull and the clay to be drilled out of the inner tubes. With the Bostech system this doesn’t happen. All that is required is a momentary pull back to snap the core, which when passed through the sizing ring allows the core to be blown to the surface.
Bostech Air core bits are maximised for versatility, reliability and production. Bostech can drill almost all weathered ground types using one standard bit type. This is possible using combinations of bit face, air pressure, venturi assistance, different inner tube configurations (high pressure and low pressure) and combinations of trumpet lengths.