The Need for Air Control in Down the Hole Hammer Casing Advancement Drilling-Paper

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The paper examines how compressed-air exhaust from down-the-hole (DTH) hammers can disturb surrounding ground during casing advancement, potentially causing settlement and damage to nearby foundations. DTH drilling is useful for penetrating difficult conditions such as boulders, cobbles, and glacial till, but air escaping forward from the drill bit can migrate through permeable soils, create cavities, and affect existing piles. Reported subsidence near Hong Kong construction works highlights the risks and growing regulatory concern.<br /><br />Computational fluid dynamics (CFD) simulations compared three pilot-bit designs: two that exhaust air forward and one that directs it radially. At the modeled air flow and soil permeabilities, the radial-exhaust design substantially reduced air penetration into the soil. For example, at a permeability of 10⁻⁵ cm², the proportion of inlet air entering the soil was 8.8% for the radial design, compared with 44.5–54% for the forward-exhaust designs. The radial design also produced much lower pressure and axial force at the soil interface. The study suggests that CFD results, combined with load-cell testing, could help assess bit designs and establish project-specific limits on ground-directed airflow.<br /><br />A Helsinki case study describes construction of a watertight pipe-pile wall beside existing buildings. The contractor used a bit designed to avoid forward air exhaust. No foundation settlement was measured, and the wall was completed straight; planned jet-grout protection between wall sections was discontinued after the initial section proved successful.<br /><br />The paper concludes that DTH casing advancement can be an efficient, predictable foundation method, provided exhaust air is controlled. Radial air discharge appears less disruptive than forward flushing, although performance depends on soil permeability.

Keywords

down-the-hole hammer

compressed air exhaust

casing advancement

ground settlement

foundation damage

computational fluid dynamics

radial exhaust design

soil permeability

pipe-pile wall

Helsinki case study