Ground Anchor Drill Rig

Overview

A ground anchor drill rig installs the tensioned anchors that hold back retaining walls, stabilize slopes, and resist uplift. An anchor is a steel tendon grouted into competent ground at the back of a hole and tensioned at the face, so it pulls the wall or slope against the stable ground behind it. The rig drills the inclined hole, often through soil into rock, then places and grouts the anchor. The same machine drills soil nails, rock bolts and micro-scale tiebacks, which makes it a core tool for excavation support and slope work.

The rig is a tracked drill built to reach anchor positions at almost any angle. The Base Carrier carries and steadies it, the Drill Mast aims it, the Rotary Percussion Head drills, the Drill String reaches depth, the Flushing System clears the hole, and the Anchor Grout Unit bonds the anchor.

How an anchor is installed

The operator positions the Drill Mast at the design angle and drills the hole to depth. Once the hole is complete, the steel tendon is inserted, either a bundle of strands or a threaded bar, with a free length near the face and a bond length at the back. The Anchor Grout Unit fills the bond zone with cement grout that locks the back of the tendon into the ground. After the grout cures, the tendon is tensioned against a bearing plate at the wall and locked off, so the anchor carries a permanent load that holds the structure.

In self-drilling anchors, the Drill String itself is left in the hole as the tendon, with grout pumped through the hollow bar during or after drilling. This is common in loose or caving soils where a separate hole would collapse before the tendon could be placed.

Drilling through soil and rock

Anchors usually pass through soil into stronger rock, so the Rotary Percussion Head both rotates and hammers. The Rotation Gearbox turns the Drill Rod while the Percussion Unit, a hydraulic top hammer, strikes the rod thousands of times a minute. Each blow from the Hammer Piston sends a stress wave down the string to the Drill Bit, whose carbide buttons chip the rock. Rotation indexes the bit between blows so it strikes fresh rock. The Shank Adapter passes percussion from the hammer into the string, and the Accumulator smooths the hydraulic pulses that drive the piston.

Aiming the mast

Anchors fan out across a wall at many angles, so the Drill Mast is highly articulated. The Positioning Boom reaches the mast out to each position, the Angle Cylinder sets the inclination from near-horizontal to steeply down, and the Swing Cylinder swings it to the next hole. The Rod Guide keeps the rod aligned as it enters the ground so the hole starts straight. On benched and sloped sites the Base Carrier and its Outrigger stabilizers hold the rig steady while it drills at angle.

Feed and string handling

The Feed System pushes the head down the hole and pulls it back. The Feed Cylinder drives the head along the Feed Beam, and the Feed Chain extends the stroke to the full beam length. As the hole deepens, the Rod Clamp holds the string while a new Drill Rod is threaded on through a Rod Coupling, then the clamp grips again to break the joint when the string is withdrawn.

Flushing and dust

Drilling makes cuttings that must leave the hole, and rock dust that must not reach the crew. The Flushing System handles both. The Air Compressor blows air down the rod to lift cuttings in dry drilling, the Flush Water Pump supplies water flush where dust must be suppressed, and the Dust Collector captures dry dust at the collar. Flush enters the turning string through the Flush Swivel.

Grouting the bond zone

The Anchor Grout Unit makes and places the cement that gives the anchor its capacity. The Grout Mixer prepares the grout and the Grout Pump, a progressive-cavity pump, injects it down the hole or hollow bar to fill the bond length around the tendon. A Pressure Sensor tracks injection so the bond zone is filled completely, since a void there would cut the anchor capacity.

Control and use

The Drill Controller in the Operator Cab balances feed, rotation and percussion for fast clean drilling and logs hole depth and penetration for each anchor. Ground anchor rigs are used for tieback retaining walls around deep excavations, soil nailing and rock bolting of slopes and cuttings, tower and dam uplift anchors, and underpinning, anywhere a tensioned steel element must be drilled and grouted into the ground at a controlled angle and depth.