Facilities operations at the United States’ Amundsen-Scott South Pole Station present many challenges. The extremely low temperatures, associated wind chills, darkness during the long austral night, and drifting snow all hamper normal operations. For over 40 years the concept of tunnel construction for utilities and personnel in polar regions has been tried with mixed results. In 1991, the National Science Foundation (NSF) tasked the US Army Cold Regions Research and Engineering Laboratory (CRREL) to develop a tunnelling system for use at the new South Pole Station. The South Pole Tunnelling System, currently in use in Antarctica, is the result of this work.

Background

Tunnels of various forms and construction are neither new nor uncommon in extremely cold regions. The US Army first investigated tunnelling concepts in ice and snow in the 1950s and 1960s while deeply involved in research in northern Greenland. Work at Camp Tuto, Camp Century, and the Distant Early Warning ice cap radar stations all involved tunnels.

At Camp Tuto, near Thule AFB, tunnels were bored into the edge of the ice cap in 1957 using modified hard-rock mining equipment and explosives. These tunnels were constructed primarily in ice. They were unlined and, for the most part, unbraced. A small electric mining train ran through the tunnels, used for removing the mining debris. Work was labour intensive despite the use of heavy machinery (Abel, 1961).

Camp Century, built in 1960 by the US Army about 150km north east of Thule AFB, was a subsurface base constructed primarily of cut-and-cover tunnels (Clark 1965). These tunnels are formed by machining a trench, usually with a large snow miller, placing a corrugated metal arch above the trench, and backfilling over the arch. The metal arch supports the tunnel roof beneath the hardened machined and blown snow. For such near-surface applications two types of steel arches are used. The chordal arch is a large-radius, short-span (2m to 4m) arch that spans the top of the trench and depends on the overlying sintered snow for structural strength. In some cases, these arches are removed after the snow has hardened, resulting in a totally unlined tunnel. The larger Wonder Arch is semicircular and forms the top and sides of the tunnel. These are designed to be more permanent structures and, due to their size, are not easily removed once buried. Wonder Arches can exceed 19m in span.

Constructing stations in Antarctica after the International Geophysical Year (1956), techniques developed in Greenland were employed for subsurface structures. Unfortunately, problems associated with partially lined tunnels in Greenland were also carried over. The primary problem is the settlement and subsequent crushing of the hard structures.

As an alternative to the lined tunnel concept, a means of creating unlined tunnels was investigated once again. In 1963, CRREL developed the Russell Miner, a tunnel boring machine (TBM) specifically designed for tunnelling in snow and ice. Although somewhat effective at machining snow, the two-phase pneumatic spoils transport system, incorporating a series of vane-axial fans, was prone to freeze-up and breakdown. After tests in Greenland at Camp Century in June 1965 the programme was dropped. However, very valuable lessons were learned that were directly applicable to the current South Pole tunnelling concept.

The South Pole Tunnelling System

The CRREL South Pole Tunnelling System is a turnkey system composed primarily of five major subsystems. These are the tunnelling machine, the chip disposal system, the drill rig, the generator set (genset) module, and the workshop module. The tunneller is designed to process firn, a strong, highly compacted form of snow (Walsh 1999). The system as currently deployed and described here is a prototype and is under continuous development.

System overview

The tunnelling system is designed to operate at a maximum depth of 16m. The operating temperature in the tunnel is -50°C and the target production rate for generating a 2m x 3m tunnel is 6m³/h. The anticipated crew size per shift is four: a tunnelling machine operator, a down-hole assistant/operator, a surface equipment operator, and a foreman or engineer. Because of the short operations season (November to January), two 10 hour shifts with a one hour overlap were anticipated.

In operation the chip disposal fan (blower) and access hole drill rig are used on the surface while the tunneller and chip conveyance ducting are below the surface. As the tunneller machines the face of the tunnel, the chips accumulate at the base of the face. The snowblower, extensibly attached to the front of the tunneller, is fed into the chip pile, directing the chips into the airstream of the ejector pipe going over the tunneller. Behind the tunnelling machine, a series of telescoping duct assemblies direct the chips to the transition sled that is also attached to a series of fixed-length vertical pipes. At the surface, the vertical pipes are attached via a flexible hose and variable-length horizontal duct to the blower, which powers the chip conveyance system and blows the chips clear of the operations area. The rig drills the holes used for the vertical chip conveyance tubing as well as the power cord that supplies electricity from the sled-mounted generator module on the surface to the tunneller. The generator also provides power to the centrifugal fan and workshop on the surface. The workshop is the operations base and a facility for making minor repairs to the equipment.

The tunnelling machine

The tunnelling machine is loosely based on a transverse rotational continuous miner. The basis for the tunneller is a small commercial hydraulic excavator with an electrohydraulic powerpack mated to the existing diesel-hydraulic system. The machine is fully electric for subsurface operations and can be run on diesel power on the surface. The plough, originally mounted to the undercarriage, has been replaced with a snowblower for debris mucking. The dipperstick at the end of the boom is replaced by a hydraulically powered cutting drum.

The cutting drum has a series of 26 arms oriented at 15° increments. Each arm, running the diameter of the drum, has opposing cutters with flat, 1mm wide teeth, each with an 11° clearance angle and a 24° top rake. The cutters vary in width from 50mm to 76mm, and the arms are oriented so that the linear engagement of each row is between 406mm and 432mm. The drum is driven by a low-speed, high-torque axial vane hydraulic motor with a 7:1 gear reducer. A low-temperature chain connects the 2m long drum shaft to the gear motor through 21- and 25-tooth sprockets.

Spoil at the face of the cut is gathered with a modified 1.8m wide, two-stage snowblower mounted on an extendable frame that can be raised and lowered. The auger centres the spoil and feeds it to the impeller, which in turn injects it into the airstream of the chip disposal system. From the injection point, the debris travels through a 250mm rigid 90° elbow to an extensible 250mm ejector tube to the rear of the machine where it connects to the main disposal system.

The electrohydraulic powerpack consists of a 37kW through-shaft electric motor with tandem gear pumps mounted on each end. One set of pumps powers the tunnelling machine controls, traction, and all lifting cylinders. The other set powers the snowblower and drum. The drum pump output is rated for 16kW at 17MPa. The torque at the drum is 1,000Nm at 100 rev/min with theoretical system inefficiencies. Due to the extreme cold, actual inefficiencies reduce the effective power available at the cutter to closer to the 6kW required to disaggregate the material (Mellor 1977). The pump outboard of the drum circuit pump drives the snowblower. This pump is operated at 13.8MPa and 22.7 litre/s and drives a direct-coupled rotary-abutment-type low-speed, high-torque hydraulic motor. A tandem set of gear pumps powers the circuitry built into the original excavator. All circuits are protected by pressure relief valves.

Debris disposal system

Mucking out the debris from the mining process has always been the choke point in previous snow tunnelling systems. The South Pole Tunnelling System returned to the pneumatic method of transporting chips. Unlike the Russell Miner, however, the motive force is applied at the end of the disposal line and the system is belt-driven rather than direct-drive. A large centrifugal fan, powered by a 37kW electric motor, creates the necessary suction to transport the debris a minimum of 19m vertically and 52m horizontally.

Horizontal ducting consists of nested tubes sealed with a silicon rubber annular gasket. These tubes are supported on adjustable-height sleds or trucks that allow movement relative to the tunnelling machine. Three long expansion units in the tunnel have two nested tubes for a 6m range. One single-nested tube unit is located behind the tunneller and one in front of the surface fan. These have a 1.5m range. Wire rope is used to transfer tension loads through the ducting system and to keep the tube assemblies from pulling apart. V-shaped band clamps hold the tube end connectors together.

At the downstream end of the tunnel, a transition sled connects the horizontal ducts to the vertical ducts and anchors the horizontal duct system. A short section with an adjustable nested tube (1m throw) connects the fixed-length vertical tubes at the transition sled. V-shaped band clamps are once again used to join the tube sections. A fixed flange on the uppermost section supports the assembled vertical tubes, and a section of 250mm flexible steel hose connects the vertical tubing to the nested tube assembly attached to the centrifugal fan inlet. Tube diameters for the disposal system range from 236mm to 273mm.

Support systems

Power for the system is supplied by a 205kVA turbo-diesel generator set. Due to the altitude and thin air at the Pole, the engine is de-rated to 180kVA. Power connections to the operating equipment are made through 60A breakers on the 300A panel located on the end of the genset module. Power is transmitted through 23m lengths of four-conductor Type-W cord with locking end connectors. Distribution includes the tunneller, the centrifugal fan, a warm-up shelter, and the workshop.

The workshop module contains tools and equipment, sufficient to conduct normal repairs. The building is heated and is used as a surface base of operations, holding spare parts and communications equipment.

The drill rig is used to drill access holes for both the power cables to the tunneller and a warm-up shelter in the tunnel, as well as the vertical ducting for the spoils transport system. Hole locations are surveyed prior to tunneller penetration. When the tunnel face passes the survey location for the hole, the tunneller is backed up and the hole drilled with spoil dropping into the tunnel. The tunneller advances again, cleaning up the spoil that have fallen to the floor. This procedure can be accomplished in 15 minutes by an adept crew.

Deployments

The South Pole Tunnelling System was first deployed to Antarctica in late January 1996. A team from CRREL assembled and operated the equipment at the Amundsen-Scott South Pole Station. After surface testing, the system was brought into a trench to test in operation. A faulty blower motor and a broken pump shaft halted operations after a short advance, and with the end of the season only a few days away, operations were halted for the year.

The second trial took place in November 1996 under the direction of one of the co-authors, Michael Walsh. The original objective of developing a 33m proof-of-concept tunnel was expanded by NSF to encompass a 120m utilidor that is critical to the overwinter operation of the station. The equipment was modified to avoid some of the problems encountered earlier. Operations began on 22 November, with two days for equipment shake-down and a day of training. Tunnelling was completed on 3 December.

On the third deployment in November 1999, the equipment was operated by a crew under the direction of co-author John Wright with remote support from CRREL in the US. The objective was to develop a 600m long tunnel to be used as a utilidor for the new South Pole Station. Work started on 29 November. Problems with the snowblower positioning system, repeated failures of drum hydraulic system components, and resupply and logistics problems resulted in premature termination on 15 January 2000. A total of 290m of tunnel was achieved. However a concerted effort to maintain grade and direction with the tunnelling machine was quite successful.

The 2000-2001 deployment started on 15 December after the arrival of critical parts and a shakedown run. The effort was once again headed up by John Wright with a new crew. The objective was to complete the tunnel started in the previous season as well as to develop an additional 65m spur off the main tunnel. Reliability problems with the tunnelling machine hydraulics and snowblower lift mechanism continued to inhibit effective operation of the equipment, although much progress has been made. Refinements in both the equipment and operating procedures continue to improve the effectiveness of the system, although much work needs to be done before it can actually be used as a production machine.

System operation

Tunnelling begins at a face prepared by dozing a trench to the requisite depth at the end of a surveyed route. In the initial stage, the blower is located in the trench behind the tunneller until the face has been advanced sufficiently to redeploy it to the surface, a distance of at least 15m. At this point, an access hole is drilled from the surface to the tunnel and the vertical discharge line installed. The transition sled and the small duct truck assembly are then installed and tunnelling resumes. As the tunneller advances, large duct assemblies are added until the face has advanced 33m. At this point, the disposal system is dismantled and the centrifugal fan and vertical ducts moved to the next access hole. The power cable can then be fed through the former access hole, or a second hole is drilled near the new access hole. The transition sled and duct trucks are narrow enough to pass each other in the tunnel, facilitating setup and operations. The small duct assembly experiences the back-and-forth motions by the excavator during normal sumping and clean-up operations. Advances draw out the nested tubes of the larger duct trucks through the wire rope tying together the in-tunnel disposal system.

The cutter drum operates at a fixed speed. Sumping in is done by advancing the excavator on its tracks. Face cuts are made by raising or lowering the boom that holds the drum. The tunneller was originally equipped with an electronic protractor to provide the operator with feedback on the drum position. This was replaced with a mechanical device in 1999. Directional and grade control are maintained with a laser alignment system that displays a spot within the operator’s view. When the laser beam hits the drum drive chain during facing, the operator is on the tunnel centreline. The laser is also inclined to the tunnel grade. Every 1.5m a measurement is made from the laser spot on the tunnel face to the tunnel floor to verify grade. A cross-axis digital protractor on the frame of the tunneller indicates the current pitch and roll of the tunnel floor. Parameters can also be verified with a spirit level.

The debris mucking operation at the face of the tunnel requires a concerted effort on the part of the operator. Although some of the debris is ingested by the disposal system during the facing operation, a significant pile accumulates at the base of the face. The snowblower mounted to the front of the machine can be fed into the pile to muck out the debris. Care must be taken not to overwhelm the suction system, and attention must be paid to an in-cab vacuum gauge to ensure that the disposal system does not stall. It is the responsibility of the in-tunnel assistant to clean up any extraneous debris after the passage of the tunneller and to ensure the ducts are working properly. The surface assistant is responsible for monitoring the blower and resetting it if it stalls and trips a breaker. With the foreman, he also bores the access shafts with the drill.

At appropriate locations a shaft to the tunnel is drilled for emergency egress. A 300mm shaft is first drilled to a cut in the side of the tunnel and a 1m backboring bit assembled to the end of a drill string to ream out the pilot bore.

Evaluation

The South Pole Tunnelling System has been demonstrated as a viable concept. However, the production rate and reliability are not sufficient for the system, as it stands, to be used for production. The goal of the system as originally designed was for a production rate of 18m³/h and an uptime target of around 75%. Over the course of the three tunnelling deployments, the production rate is nearer 6m³/h with an uptime of around 50%. Most of the problems can be attributed to the harsh conditions and remoteness in which the machine operates. However, specific issues need addressing.

Foremost among the tunneller’s shortcomings are the hydraulics. Electric drives for both the drum and snowblower need to be considered. Frequent failure of pressure relief valves has led to shaft and motor failures on the drum circuit and blown hoses. Directional control valves are also problematic, not just on the tunneller but also on other heavy equipment at the Pole. Although the material being machined is theoretically snow, it is of great strength and high density. Frequent stalling during sumping operations due to the strength of the material stresses the hydraulics, contributing to component failure.

The other component needing consideration is the snowblower. The impeller is located in the centre of the unit and is underpowered. An impeller on the side of the snowblower will more efficiently feed debris into the airstream and increase the production rate. Offsetting the weight of the ejector tube directing the debris over the tunneller will lower the stress on the extension frame and increase the overall reliability of the assembly. A separate debris feed from the machine is also under consideration.

Related Files
Operations diagram of the South Pole Tunnelling System