In 1987 we purchased our first Air Core system. From the first job we had major problems with drilling clays. We discovered that 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 and reduce the diameter of the bottom 40mm of the inner tube with braising rod reducing the internal diameter and thus reducing inner tube blockages.
We realised from the first air core rod drilled that;
- Clay drilling requires the whole sample to be swaged to fit into the inner tube for recovery to the surface as clay core. Swagging was required for clay, not drilling. Swagging maintains the volume but reduces the diameter whilst increasing the length of sample recovered.
- This process is the only efficient method to drill plastic materials.
- A sizing/swagging ring was required in the bottom of the inner tube to allow the sample of clay to float without touching the inner tube anywhere other than the bottom 40mm.
Without the use of a sizing ring it was found that a greater length of clay core in the inner tube resulted in a greater contact area leading to an increase in stiction (Force required to overcome static friction of the clay in the inner-tube and initiate movement). This would then lead exceed plastic modulus of the clay until the core becomes tightly compressed within the inner tube, leading to numerous delays requiring the removal of the rod from the ground so that clay could be drilled out of the inner tubes. The Bostech Air Core System significantly reduces the frequency of these delays. All that is required is a momentary pull back to snap the core, which when passed through the sizing ring allows the now free 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 a combination of bit face, air pressure, venturi assistance, different inner tube configurations (high pressure and low pressure) and combinations of trumpet lengths.