Small Diameter DTH Drilling in Foundation

DTH Bits and Ring Bits

Down-the-hole (DTH) bits and ring bits represent essential drilling consumables in small-diameter foundation drilling operations, particularly for constructing ground walls, cutoff curtains, and related deep foundation elements. These specialized cutting tools are integral to modern geotechnical engineering, enabling efficient and cost-effective drilling in foundation engineering applications requiring precise bore geometry and consistent performance across varying ground conditions. DTH bits function as percussion drilling tools coupled with DTH hammer units, delivering impact energy directly at the drill bit face to fragment rock and competent soil strata. Ring bits, characterized by their annular cutting surface with hardened steel or tungsten carbide segments, are specifically engineered for diamond-core or open-hole drilling applications. In the context of foundation engineering, these bits are primarily employed in constructing diaphragm walls and cutoff curtains—continuous subsurface barriers used for groundwater control, seepage prevention, and lateral load support. Their application extends to secant pile construction, where overlapping, precision-drilled boreholes create load-bearing structural elements, and sheet pile wall installation requiring pilot hole drilling. Specialized foundation applications also include jet grouting pilot holes and soil-mixing column pre-drilling, where DTH bits provide the accuracy and penetration rate necessary for ground improvement techniques. The operational principle depends on bit type and ground conditions. DTH hammers deliver rapid, controlled percussion strikes—typically 1,500–2,500 blows per minute—transferring kinetic energy through the bit to the borehole bottom. Ring bits rely on the rotational force and percussion action to create a grinding and fracturing action at the formation boundary, with the ring geometry allowing material evacuation through the hole's central core. The cuttings removal system varies: air flush circulation evacuates cuttings in air rotary drilling, while mud circulation (for ring bits) transports fragmented material to surface, enabling continuous drilling documentation and formation logging. Equipment types within this category include standard DTH percussion bits with various flushing hole configurations (typically 6 to 16 holes) sized for specific hammer models; button bits incorporating tungsten carbide inserts in matrix bodies for enhanced wear resistance in abrasive strata; and ring bits in both open-diameter and barrel configurations for controlled-diameter drilling. Bit diameters range from 75 to 200 millimeters for small-diameter foundation work, matching foundation design specifications and DTH hammer bore sizes. Selection criteria for DTH and ring bits include ground classification (soil, weathered rock, competent rock, and heterogeneous strata), required drilling rate, borehole stability requirements, and casing configuration. Contractors assess bit life economics based on expected penetration rates in anticipated geology, percussion energy rating of available hammer units, and pressure requirements. Environmental factors—groundwater aggressiveness, temperature extremes in cold-climate applications—influence material selection, with cobalt-enriched matrices and specialized resin systems providing enhanced performance in corrosive or low-temperature conditions. Relevant standards guiding DTH drilling practice include ISO 13143 (percussion drilling for piles), DIN 4098 (diaphragm wall construction), and EN 1538 (diaphragm walls executed in the ground). API standards for drilling fluids and mud systems (API 65) ensure circulation medium compatibility, while manufacturer specifications define operating parameters for hammer-bit compatibility and performance optimization, ensuring consistent borehole quality across foundation engineering projects.