Question

The formation consists of soft sandstone and clay (compressive strength <15 MPa). Which pressure series of DTH tools should be selected: low, medium, or high?

The low air pressure series (working pressure 0.8-1.5 MPa) is recommended. Soft formations require lower impact energy for effective rock breaking. Low-pressure DTH hammers ensure sufficient penetration efficiency, reduced energy consumption, and slower tool wear. Medium- or high-pressure tools may cause excessive impact, increased cuttings pulverization, and higher risk of hole collapse, resulting in higher overall construction costs.

For large-diameter pile holes with diameters greater than 800 mm, should the large-diameter series be selected regardless of the formation?

The large-diameter series is the preferred option. This series is engineered for large-diameter hole making and stable hole formation. Structural features include thicker tool walls for improved torsional strength and wider discharge channels to prevent cuttings blockage. Compatibility with high-torque output from pile-foundation drilling rigs ensures efficient, stable operation. Standard low- or medium-pressure tools may deform under load or suffer from inadequate cuttings discharge.

A drilling rig operates at a rated working pressure of 1.8-2.2 MPa. The target formation is moderately weathered granite with a compressive strength of 15-30 MPa. Which pressure series should be selected?

The medium air pressure series (working pressure 1.5-2.5 MPa) is suitable. Medium-pressure DTH tools deliver proper impact energy for medium-hard rock formations while matching the 1.8-2.2 MPa rig pressure range. Low-pressure tools provide insufficient impact, leading to slow penetration. High-pressure tools exceed the rig’s pressure capacity and cannot achieve normal performance.

How should DTH bits be selected? For example, within the same medium-pressure tool series, are different DTH bits required for limestone and sand-gravel formations?

Correct bit selection is essential.

Limestone (dense, hard rock): Use spherical-button DTH bits, which provide high wear resistance and concentrated impact points for efficient rock breaking.
Sand-gravel formations (loose formations with pebbles): Use flat-edge ballistic bits, which offer wider discharge grooves for rapid removal of gravel and reduced bit blockage.

Bit interfaces must match the selected DTH hammer connection standard, for example API REG threads, to ensure stable sealing and power transmission.

A composite formation contains an upper sand layer and a lower hard-rock layer. Is frequent replacement of DTH tools necessary, or can one toolset handle both?

A medium air pressure DTH hammers paired with a composite-formation DTH bit such as a spherical-button can meet both layers’ requirements. Medium air pressure tools support efficient drilling in both sand layers and hard-rock layers. DTH bits with wear-resistant ballistic buttons and wide discharge grooves enhance cuttings removal in sand layers and improve rock penetration in hard rock.

When the lower hard-rock compressive strength exceeds 30 MPa, only the DTH hammer may be replaced with a high-pressure model while other components remain unchanged, reducing replacement cost.

DTH Hammers (High Air Pressure)