Hollow Bar Anchor System Installation

The Williams / MAI Hollow Bar Anchor System is optimally used in poor or very poor ground conditions ranging from inconsistent fill, boulders, rubble and weathered rock to sand and gravel. Generally the system is installed with rotary percussive drilling and continuous grouting. The thickness of grout can be varied depending on susceptibility of the borehole to collapse, however a grout mixture of 0.4 water to cement ratio is recommended in poor, collapsible soils to ensure a high enough density to support the annulus. In all cases, a final grout mixture of 0.40 w/c should be used for adequate in-situ structural capacity. Williams / MAI recommends mixing the grout in a colloidal (shear type) mixer, so once pumped, the fine grout particles are fully able to disperse into the small voids of the surrounding soil. This well mixed grout exits the side ports of the drill bit under pressure to flush and remove the softer parts of the soil while penetrating into the firmer material for increased bond capacity. Also the percussive action of the drill acts as a “poker” to further consolidate the grout. Williams / MAI recommends to partially withdraw each fully drilled section up the drill mast prior to attaching new sections, this way the drilling can begin in a plunging type action to even further improve grout penetration. Utilizing proper drilling and grouting techniques is important as the system would generally fail between the soil/anchor interface, not the grout/anchor interface.

In all cases the hollow bar system should be drilled slow enough to ensure rotation through the soil as opposed to excessive percussion and feed pressure with limited rotation. Such practice will provide the formation of a true borehole with consistent grout cover. Grouting pressure should be sufficient to maintain circulation at all times with a small amount of grout return visible at the mouth of the borehole. Normal drilling rotation is in the range of 40 and 100 RPM.

• Keep the number of top components to a minimum.
• Apply a locking torque to the striker bar end. Horse hair and Loc-Tite are available from Williams to facilitate in preventing release. Williams / MAI can also provide the striker bar end of the rotojec shank or coupling box to be machined to a tighter than normal thread tolerance so that a permanent, locking torque can be applied in the field.
• Liberally grease the top end of each hollow bar going into the rotojec shank or coupling box.
• To release the installer should grip the hollow bar and apply percussion combined with feed pressure in a clockwise rotation. Caution: Care should be taken not to allow more than two turns of clockwise rotation as further rotation would detach the drill bit or couplers within the borehole.