Underground Shotcrete Sprayer

Overview

An underground shotcrete sprayer applies a layer of concrete onto the rock surface of a mine tunnel to support it. After a heading is mined and scaled, the exposed rock needs sealing and reinforcement before crews work under it. Sprayed concrete, called shotcrete, does this: it bonds to the rock, fills cracks, and forms a shell that holds small blocks in place and slows weathering. This machine carries the whole spraying process on one self-propelled chassis so an operator can coat a face in minutes without a separate pump, hose crew, and manual nozzle.

The rig uses the wet-mix method. Ready-mixed concrete is loaded into the machine, a piston pump pushes it through a pipe to a boom-mounted nozzle, and at the nozzle compressed air and a liquid accelerator are injected so the concrete sprays as a fast jet that sets the moment it lands. Five systems make this work: the Carrier Chassis moves and plants the rig, the Concrete Piston Pump moves the concrete, the Spray Manipulator Boom aims the nozzle, the Accelerator Dosing System meters the accelerator, and the Air Supply supply atomizes the stream. A Hydraulic Power Pack pack and a Control Station station tie them together.

Carrier and chassis

The Carrier Chassis is an articulated wheeled frame, a Front Frame Section and a Rear Frame Section joined at a Articulation Joint so the rig steers by bending in the middle and turns inside tight drifts. A Diesel Engine of around 55 kW drives two Drive Axle units and four Wheel & Tire assemblies on foam-filled tires, and a Exhaust Catalytic Scrubber cleans the exhaust for the confined air. When the rig reaches the face it lowers four Outrigger Leg legs so the boom and pump reactions do not move it during spraying.

Concrete pump

The Concrete Piston Pump is the heart of the machine. Concrete is tipped into a Concrete Hopper fitted with a Remixer Agitator that keeps the mix turning so the aggregate does not settle out. Two Material Delivery Cylinder bores draw concrete in and push it out, driven by a pair of Hydraulic Drive Cylinder hydraulic rams. A S-Tube Swing Valve swing valve switches the outlet from one delivery cylinder to the other on each stroke, so while one fills the other pumps and the flow to the nozzle stays nearly continuous. The S-tube wipes against a Wear Plate & Cutting Ring cutting ring that takes the abrasion of the sand and seals the joint. Concrete leaves through a Delivery Pipe & Hose to the boom, with output of 10 to 30 m³/h at up to 70 bar, watched by Pressure Sensor units that flag a blockage.

Spray manipulator

The Spray Manipulator Boom holds the nozzle and sweeps it across the surface so spraying is even and the operator stays clear of rebound. It is a telescoping boom: a Boom Base Section and a Telescoping Boom Section section, swung by a Slew Ring Bearing, raised by a Lift Cylinder, and extended by a Extension Cylinder. At the tip the Spray Nozzle Head carries the Nozzle Tip and a Nozzle Rotator that oscillates the nozzle in small circles for a uniform layer. The Air/Accelerator Mixing Ring inside the head is where air and accelerator enter the concrete just before it leaves.

Accelerator and air

Concrete sprayed onto a wall will sag and fall off unless it stiffens fast, so the Accelerator Dosing System system meters a liquid set accelerator into the stream at the nozzle. An Additive Tank holds the accelerator, a Peristaltic Dosing Pump feeds it, and a Accelerator Flow Meter keeps the dose locked to the concrete rate, usually 3 to 8 percent of cement weight. The Air Supply system supplies the energy that turns a slow extrusion into a spray. A Rotary Screw Compressor feeds an Air Receiver Tank, and 7 to 12 m³/min of air at 6 to 7 bar enters the Air/Accelerator Mixing Ring to throw the concrete onto the rock hard enough to compact it.

Hydraulics and control

The Hydraulic Power Pack power pack drives the pump rams, the boom, and the outriggers. A Variable Displacement Pump feeds a Proportional Valve Bank that meters oil to each function, with a Oil Cooler handling the heat of continuous pumping. The Control Station station runs the rig: a Machine Control Unit links pump rate to accelerator dose, and a tethered Tethered Remote Control lets the operator stand at the best viewing angle while watching a Operator Display under four LED Work Light lamps.

Use and variants

A spraying cycle runs like this: load concrete into the hopper, set the accelerator dose for the mix, plant the outriggers, and walk the nozzle across the surface in overlapping passes until the design thickness is built up, often 50 to 150 mm. Wet-mix rigs like this one dominate underground work because output is high and the rebound and dust are low. Dry-mix machines, which add water at the nozzle instead, suit small repair jobs and long pumping distances. Larger rigs carry a bigger Concrete Piston Pump and a longer Spray Manipulator Boom for big civil tunnels, while compact units fit narrow-vein mines, all built around the same wet-mix spraying process.