Hollow Bar Micropiles have emerged since the early to mid 1990’s as an economical system for installation primarily in soils. This type of micropile is easily installed in low headroom or restricted access applications and exhibits high unit load capacities coupled with superior stiffness performance compared to conventional drilled and grouted ground anchors without the need for temporary or permanent casing.
Hollow Bar Micropiles are fully bonded, drilled and grouted micropiles which consist of segments of load carrying hollow threaded bars, couplers and a sacrificial drilling bit that are bonded to soil or rock by a neat water-cement grout. They are installed by drilling with the hollow bar assembly and flushing the hole with the micropile grout. Due to the installation procedures used, hollow bar micropiles are especially advantageous in subsurface conditions where casing might otherwise need to be employed with conventional micropile configurations. The manner in which hollow bar micropiles are installed results in higher grout to ground bond stresses and greater axial stiffness than conventional solid anchors or uncased micropiles that are drilled and subsequently gravity grouted. Hollow Bar Micropiles are also known as Injection Bore Micropiles or Self Drilling Micropiles.
A variety of types and diameters of sacrificial cutting head / drill bits are available for installation in varying subsurface conditions. The grout to ground bond stress is directly proportional to the drill bit diameter, so proper bit selection is critical both for installation and for optimizing hollow bar micropile capacity.
The Sinorock hollow-bar micropile is a composite monolithic structure of micro alloy steel and high-strength cement grout. The hollow steel bar has a continuous external thread running the entire length of the bar and is advanced under rotation and possibly with some hammering action in very stiff or hard soils. As the bar is advanced,the grout is injected through the hollow bar and out through the sacrificial cutting head.
Since the outside diameter of the cutting head is typically 1 to 4 times larger than the outside diameter of the bar, the grout (under pressure) will flush the drill cuttings to the surface through the cavity along the outside of the bar and the wall of the excavation created by the cutting head.