Contractors like to reduce risk as well as make a profit, and frequently this comes to the same thing. When selecting an excavation method for mixed or medium strength ground where water is not a great problem, there is much to be said for a method that is reliable, well known and readily available at modest costs.

The versatility of roadheaders in terms of both excavation dimensions and ground types is well known. If the ground is relatively dry, or water can be excluded, roadheaders allow a full view of the face so that the location of any change in ground conditions can be observed. This can be a major advantage over TBMs, where the only indications of changing ground will be the nature of the spoil or machine performance. Therefore, roadheaders have particular applications for tunnels in which geological faults are expected but the size or locations are not known.

Backhoes have similar benefits in softer ground, especially in materials like boulder clay, from which hard inclusions have to be removed. In harder ground, hydraulic impact breakers have increasing applications where other methods cannot be justified.

True roadheaders are mounted on crawler chassis and are equipped with a gathering arm or spinner loaders and a discharge conveyor to load trains or tyred dumpers. Shield machines can be equipped with roadheader-type boom cutters, usually employing the lip of the shield to lift spoil to a conveyor. In the case of the Dosco SC series, the conveyor runs from the tow of the shield. A recent development takes the form of cutterheads which, without booms, can be mounted on conventional hydraulic excavators.

Mucking out

Backhoes in specialist tunnelling equipment are similarly integrated into the excavation system but also assist in mucking out by dragging back spoil from well in front or to the sides of the equipment towards a gathering arm loader or chain conveyor. On the Dosco Mk 2B machine, a backhoe can be interchanged with roadheader boom. Prime movers can be diesel-electric or electro-hydraulic.

In addition to the advantages of simplicity, economy and robustness for excavating medium-to-soft ground, shielded backhoes can bring mechanisation to small diameter manned operations, including pipejacking. One specialist in this sector is Akkerman, which offers backhoes in a steerable shield with sand shelves if required. The rotating backhoe can pass through the shelves to dig and can be extended to penetrate the face. Other features are the in-shield control module, a loading out chain conveyor and optional flood prevention doors. Another option is a bracket for mounting hydraulic breakers in case hard inclusions are encountered.

Northwest Boring employed an Akkerman articulated shield with excavator in pipejacking operations for the Columbia Slough Consolidation Project in Portland, Oregon, in the US. Ground conditions ranged from silty sand to gravel with cobbles. Northwest carried out pipejacking at diameters of 1524mm i.d. and 3658mm o.d. and drive lengths up to 70.1m. Pacific Boring has also used an Akkerman excavator shield for pipejacking in San Jose, California.

Hydraulic impact breakers can be mounted on specialist tunnelling equipment or conventional hydraulic excavators as well, but need additional means of mucking out in the latter case.

Multiple machines

ITC Schaeff‘s tunnel excavators can carry hydraulic impact breakers (typically Montabert or Rammer) as well as backhoe excavator buckets and a loader apron. Two such units, a Model 312 and a 412, were used to excavate the crown heading on a 600m long tunnel forming part of the Sierre by-pass on the N9 motorway between Lausanne and Brig in Switzerland. The predominantly shale ground included hard blocks, which were tackled successfully by the breakers. A single lever retracts the breaker and extends the backhoe bucket for action. Production rates of up to 600m³/machine are claimed.

Dimensional capabilities of the ITC Schaeff units range from the 112 (max. height 4.9m; min. height 2.5m; unit width 1.9m) to the 420 (max. height 8m; min. height 4m; unit width 2.9m).

Rammer, which recently introduced an addition with tunnelling applications to the top end of its range, has reported that three large hammers were used on construction of the Rome-Naples section of Italy’s high-speed rail network. A Rammer E 68 CITY was used to break up the bench section of the Briccele Tunnel near Capua, while larger S 86 and S 84 units mounted on Fiat-Hitachi excavators removed ground in the upper section of the tunnel.

Roadheaders, backhoes of all types and, to some extent, hydraulic breakers, have a natural place in excavation of tunnels carried out according to NATM and SCL principles. This particularly applies to tunnels of large section such as those for motorways, rail way stations and crossovers, where staged excavation may be practised.

The extent to which roadheaders are accepted for main civil tunnel excavation may be gathered from their recommendation for the pending Girsberg Tunnel in Switzerland. They have been chosen because they can form an exact profile with a plane surface for better sealing, with consequent reduction in support requirements as a result of low vibration excavation. As an added bonus, roadheader excavation in the project’s ground conditions creates spoil of suitable size for transportation and disposal.

The successful contracting JV of Walo; Rothpletz Lienhard; Schlitter; and Brunner Erben employed a 65 tonne Eickhoff ET 250-Q to cut the molasse (sandstone and marl) ground in one of two tunnels of 85m² section each. The ET-250 has a transverse cutting head designed for the hard sandstone. A 110 tonne Eickhoff ET 410-Q roadheader with a 300kW cutterhead drive was scheduled to start work on the second tube last month (September).

&#8220The extent to which road-headers are accepted for main civil tunnel excavation may be gathered from their recommendation for the Girsberg Tunnel in Switzerland”

Hard rock

Although roadheaders and backhoes are now items of equipment with which most tunnellers are familiar, development progresses steadily on components and ancillary equipment, and the momentum for increased use in hard rock has been continuous over many years. While roadheaders cannot compete with drill+blast on full faces of hard rock, it is important that they should withstand encounters with hard bands in mixed faces. The main focus of development to this end has been in the cutting elements and in the durability of bearings for cutter and slewing drives. There has also been a major trend towards increased power and weight, provided the excavated floor can withstand the machine weight and the churning up from crawler tracks.

One example is the Dosco TTM 100, a 150 tonne machine that can cover a face section of 30m³ from a single position. The cutting head, having a 150 kW drive, can excavate rock with a compressive strength of around 300MPa. For larger sections, Dosco has produced twin-boom miners such as the TB3000 which can cover a section of 6m x 9m from one position (approximately 54m³ excavated volume).

Voest-Alpine was one of the first developers of roadheaders with the Alpine Miner for medium-strength rock cutting in mining. Later models – the AM 85 and AM 105 – are designed for the reliable and economic excavation of harder rocks up to 160MPa. Cutterhead drive power ranges from 40kW in the smallest unit through 400kW in the largest to power Twinheader cutting units which comprise two cutterheads on the single boom, each rotating into the face and slewed across it. Voest-Alpine AM105 roadheaders have been employed widely on road and rail tunnels in Sydney for the 2000 Olympics, as well in the Madrid and Lisbon metros and in Germany.

Roadheader and continuous miner manufacturer IBS says that it is its early diversification into equipment repair and modernisation that have given its engineers extensive experience in the design shortcomings of existing models. This was used as the base of the development of IBS’s own range, which ranges from the SM 200 V 20 tonne compact Boom Miner to the SM 760 VP 65 tonne machine for hard rock work. All have transverse cutting heads. Available options include on-board probe drill and roof bolting rigs, automatic lubrications system, load-out conveyors and hydraulic power take-off for auxiliary equipment.

Accuracy

As with drill+blast, there is considerable potential for wasteful overcutting of the designed tunnel profile when roadheaders, backhoes and breakers are used. It is important, therefore, that there is some means of guiding or limiting cutting actions to the required section dimensions. Early development of guidance systems was not very successful commercially but it incorporated the control of set transverse or vertical patterns for the most economical face excavation. The main problem has been allowing for the moving base of the roadheader in automatic control. Most roadheader use still relies on tracing the required profile on the face using a laser beam as reference.

Roadheaders and backhoes mounted securely within shields should have no reference problems for automatic control. Dosco’s SB series has an optional profile guidance system, which is said to reduce driver fatigue as well as eliminate overbreak. It is also offered as an option on the Mk 2B.

W L Hollingsworth of Precision Centerline has described a guidance system which has been adapted for use on roadheaders (SME Annual Meeting 1999 and RETC 1997). The basic system is computer controlled using a motorised theodolite with integral target recognition, two prisms for electronic distance measurement (EDM) and a dual-axis precision electronic inclinometer. When used for a roadheader, the system needs sensors to measure the boom position.

Data are stored and checked by an independent computer with an operator’s display. The exact position of the cutterhead is determined by combining the results from the basic system and the boom sensors; a colour coded display provides an indication of the accuracy of cutting operations.

Dust control

Responsible contractors and safety authority regimes have long promoted the use of ventilation and water sprays for dust control adjacent to roadheaders. Many roadheaders have their own extractor fans with dust scrubber units. High-pressure water flushing sprays at the cutterhead may also be a precaution against ignitions of any methane, as well as acting as dust suppressors. Whatever special arrangements are made, dust control in the face area is aided by the use of extraction ventilation at the face rather than forced ventilation, which only drives any dust into the faces of the tunnel workers.

Telescopic boom mounts for roadheaders and backhoes offer improved safety when the machines are cutting near an unsupported roof, as well as the potential for deeper ‘pulls’ and the limitation of carrier movements. While the boom provides a possible means of access for support installation (concrete spraying, mesh and rockbolt installation) and surveying, it should be equipped with a properly designed access platform with secure, fail safe operation.

Compact units

In parallel with larger machines, roadheaders or similar equipment with cutterheads have been developed at the other end of the scale for tasks such as renovation and shaft sinking. The recent introduction of hydraulic drive cutting heads for mounting on other equipment has promoted versatility and economy of milling excavation. Such units provide an alternative to hydraulic impact breakers mounted on conventional excavators, mainly for trimming profiles and removing hard inclusions in mixed faces.

Eickhoff has introduced the ETH series of milling cutters in which a gearbox and a pair of cutting heads replace the bucket of a hydraulic excavator using the same power supply. Of the ten units supplied by Eickhoff so far, two have been put to use on the CERN project in France/Switzerland and at least two will begin work soon. A 100kW ETH30 and a 150kW ETH50 are being used for shaft sinking for profiling and removing remaining spoil. The Zschokke/Locher/Baresel/Porr/Asdag JV will use a similar procedure for large cavern excavation, and the Seli–Dragados JV will use a TH30 unit for excavation in frozen ground.