Hydraulically powered unit is the first to cut rectangular tunnels in hard rock.
Circular, full-face tunnel boring machines (TBMs) are well-engineered, proven pieces of equipment, and they’re considered the most efficient machines for mechanical excavation of tunnels in rock. But they are not often used in underground mining.
That’s because mines generally require a flat floor, so rubber-tired vehicles can readily transport materials and personnel. Driving a tunnel with a circular full-face TBM requires secondary operations to create the flat roadway, such as leveling the bottom slopes by drill and blast or by pouring a concrete floor. Such methods are time consuming and add expense. As a result, most mines excavate mining drifts and access tunnels through slow and arduous drill-and-blast techniques.
Robbins, headquartered in Solon, Ohio, has now developed a non-circular tunnel boring machine, the MDM5000 (Mine Development Machine), to address these shortcomings. The MDM5000 produces a rectangular cross section 5 m wide and 4.5 m high with a flat tunnel invert. (The invert is the lowest section of a tunnel: the floor on a square-bottom configuration or approximately the bottom 90° arc of a circular tunnel.)
If additional cross-sectional area is not needed for ventilation flow or other purposes, then the rectangular shape is most efficient. Compared with a circular profile, the rectangular tunnel can have the same useable roadway width with a smaller cross-sectional area, which means substantially less rock to excavate and haul away.
While other machines have been developed for cutting relatively soft material, the MDM5000 represents the first successful foray into rectangular hard-rock tunneling, according to Robbins officials. It’s designed for use in rock up to 200 MPa UCS (uniaxial compressive strength).
The MDM relies on much the same technology as in more traditional TBMs, said Chief Engineer Dennis Ofiara, including 30 double-disc cutters that each run in the same track during a boring cycle. Nominal individual load per cutter is 267 kN (60,000 lbf).
During boring, front and rear hydraulic grippers extend against the tunnel walls and react against the machine’s forward thrust, as in standard TBMs. A unique floating gripper system pushes on the sidewalls and locks in place while propel cylinders extend, allowing the MDM to advance. Rear gripper and torque cylinders provide steering, much like on circular TBMs.
Hydraulic thrust cylinders extend and push the cutters into the rock. The transfer of this high thrust through the rolling disc cutters fractures the rock, causing chips to break away from the tunnel face. Because the rock is mechanically fractured, no secondary crushing is required and the broken rock is well-suited for conveyor haulage.
In addition, continuous ground support is placed immediately behind the cutterhead in a mine-standard pattern. Installation of ground support and utilities such as piping, ventilation, and lighting are done simultaneously to boring.
The cutting geometry, however, is completely different. Unlike other TBMs with rotating heads that bore a circular cross-section tunnel, the MDM5000 excavates with a reciprocating and swinging cutterhead motion to generate the rectangular cross-section tunnel with a flat invert.
Hydraulic cylinders swing the cutterhead up and down about a horizontal axis, and the cutters sweep across the rock face with nearly constant penetration to provide the most effective cutting action.
Muck (excavated ore and rock) removal in the MDM also differs from a standard TBM. Each down-sweep of the cutterhead pushes muck rearwards onto an apron, where twin loading wheels on each side move it to a center hopper and onto a chain conveyor that carries it to the rear. A Robbins extensible tunnel conveyor system then hauls away the muck. On a standard TBM, in contrast, buckets empty onto a belt conveyor as the cutterhead rotates.
Like other TBMs, the MDM is equipped with a hydraulically operated roof shield that protects the front gripper area. Special high-tensile wire mesh for mine support and rock bolts are installed just behind the roof shield. Hydraulic percussive-type drills supplied by Fletcher install the bolts across the width of the roof.
To manage the major functions, sizeable hydraulic components and systems abound on the MDM5000. According to Ofiara, these include:
Two cutterhead swing cylinders with 430 mm bore, 260 mm rod diameter and 2,680 mm stroke. The swing cylinders provide 113˚ of operation in either direction from horizontal and generate 3,500 kNm maximum torque.
Two cutterhead thrust cylinders with 850 mm bore, 750 mm rod diameter and 225 mm stroke. The cutterhead cylinders provide maximum thrust of 10,876 kN.
A double-ended front gripper cylinder with 750 mm bore, 550 mm rod diameter and 2 × 550 mm stroke. Front gripper maximum force is 14,594 kN.
Four 150 mm bore cylinders handle the hydraulic roof supports.
Seven high-pressure pumps for cutterhead extension and swing functions are rated at 317 bar (4,600 psi) maximum, said Ofiara. Other circuits use lower pressures. The cutterhead units are variable-displacement Oilgear piston pumps with constant-horsepower control. They’re driven by 112 kW electric motors on soft start controllers. Various smaller pumps supply accessory circuits.
The main power unit module holds six of the 112 kW motors and has installed power of 950 kW. This unit measures 1,100 mm wide × 1,800 mm high × 6,400 mm long. The main 8,100 l (2,140 gal) reservoir mounts above this pump module and measures 1,600 × 1,100 × 5,400 mm.
In addition, four muck gathering wheels collect the cut rock and feed it to the central chain conveyor. Two wheels on each side are connected by power-transmission chain driven by a single hydraulic motor. The chain conveyor, driven by a 1,800 cc/rev piston-type motor from SAI Hydraulics, is run by a separate 75 kW power unit. Capacity is 300 tons per hour. Likewise, the roof bolting system has its own HPU with a 93 kW and two 55 kW modules.
All power equipment, including three smaller auxiliary HPUs, mounts on skid type gantries towed behind the MDM. Like other TBMs, the MDM tows a back-up system composed of seven gantries, plus three storage gantries for supplies. The back-up gantries provide space to mount the power and control equipment, dust scrubber and ventilation system, muck removal and other systems. This is, in essence, a tunneling plant that produces a finished, well-supported roadway tunnel with life-of-mine piping installed.
The hydraulic system uses mineral oil fluid, standard for the Fresnillo silver mine in Zacatecas, Mexico where the machine operates. It also has an Ansul type fire-protection system. To control fluid contamination each pump has a suction separator and a pressure filter on the outlet side. A kidney-loop circulation system runs constantly and provides additional filtration and desiccant moisture removal.
All large cylinders have induction-hardened rods and hard chrome plating, which Robbins engineers deemed to offer the best protection for this environment (abrasive materials but no acidic or aggressive liquids, only water). Special heavy-duty rod wipers are used.
Fluid connections include welded pipe where feasible and hose protectors where possible. The cutterhead extension cylinders have a hood-type shield and an external wiper. Neither a telescopic hood shield, nor a boot-type protector, could be used on the cutterhead swing cylinders due to limited space, particularly when the rod ends pass “over center” through the mounting clevis on the cutterhead rotation drum. The swing cylinders have a water flush system on the rods to help flush away debris from the rock cutting action.
Large cartridge-type logic valves control the major cylinders, along with large counterbalance valves on cutterhead swing cylinders to control the overhung load. A PLC system controls all the valves. The machine operator sits in an air-conditioned cab with HMI and computer screens for operation, machine guidance, mine conveyor control, and control of accessories. CCTV provides the operator with necessary monitoring of the working areas.
MDM in operation
The machine is currently cutting an access tunnel at the Fresnillo mine. The MDM was assembled as completely as possible on the surface and transported to the -695 m level of the mine in three main modules: the cutterhead, front main frame, and rear gripper section. Final assembly was completed in a launch cavern only slightly larger than the 5 m x 4.5 m dimensions of the MDM cut.
The MDM5000 is particularly suited for long access tunnels and development drifts or passageways, such as mine access tunnels, ventilation tunnels as well as ore haulage tunnels. The MDM5000 is currently boring in andesite and shale with quartz intrusions, with advance rates up to 52 m in one week and 191 m in one month.
Thus, MDM tunneling has advance rates roughly twice that of a drill-and-blast heading, is safer, and results in smooth tunnel walls and less overbreak. Higher advance rates are partly due to the machine’s continuous progress, unlike drill-and-blast operations where crews must exit the tunnel during blasting for safety. In addition, simultaneous ground support installation further increases overall advance rates compared with drill-and-blast operations that must install ground support sequentially. The technology is not only useful for the mining industry, but for many applications in civil tunneling as well.