Practice of hole drilling for soil nails

Soil nail construction involves the formation of holes followed by insertion of steel reinforcement, grouting and construction of soil-nail head typically in a recess on the slope surface.

In Hong Kong, drilling of a soil-nail hole is commonly carried out by pneumatic percussive rigs (Plate 1) with air as the flushing medium. Temporary casing is normally not required in the typical saprolitic soils encountered in Hong Kong. In special circumstances, a more robust drilling rig, for example, the Odex type drilling equipment, is used for driving temporary casing into a hole concurrent with drilling to prevent the hole from collapsing in difficult ground conditions. Soil nails are typically designed to incline downwards at 10° to 20° to the horizontal. The common practice to achieve the hole inclination is by setting the inclination of the drilling rig and drill rods using a protractor prior to drilling (Plate 2). To set the bearing of a soil nail as normal to the slope toe, the usual practice is by aligning the drilling rig to an approximately normal direction to the slope face. Typical lengths of soil nails range from less than 10 m to 20 m, although some may reach more than 20 m occas.

Quick and reliable technologies were essential for World’s projects.Self drilling soil nails were beneficial because they were drilled and grouted in one operation and also offered significant cost savings.

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