Over the years in the development of tunnelling many methods have been tried to achieve the optimum in efficiency of excavation, particularly of hard rock. In addition, the need to tackle ‘mixed ground’ more efficiently has to deal with hard ground, whether stratified in the form of inclusions like boulders. As outlined in the article beginning p.21, the types of mixed ground have caused great problems when using existing cutting tools, causing excessive wear or damage, and resulting delays in tunnelling progress.

Various forms of tools have been tried for mechanical cutting of rock as part of the general development of tunnelling equipment. Most of these have involved the localised crushing of hard rock and simultaneously releasing chips of rock from the tunnel face between the cutting lines of the tools. The force may be applied perpendicularly, as with disc cutters or point-attack roadheader picks, or at an angle nearer to zero such as by a drag bit.

Scraper tools are generally reserved to soft rock or to helping collect previously broken material. While such tools dominate the ‘business end’ of TBMs, other approaches have been considered.

It is worth keeping such methods in mind to avoid ‘reinventing the wheel’, to avoid repeating past mistakes or to re-examine that may assume new relevance under different circumstances.

Bizarre

The attempted development of alternative methods to today’s range of cutters took place mainly in the 1970s, but as if to show that competitive development is rarely as lost cause, this period also resulted in the development by Robbins of disc cutters, of which similar designs are now used worldwide.

As ‘necessity is the mother of invention’, some of the driving forces for intense development included the requirement for rapid excavation of tunnels to serve the US’s intercontinental ballistic missile silos, and drive for more efficient mining of narrow hard-rock ore-bodies, particularly in South Africa. However, the consideration of such methods as nuclear blasts leads one to think that a desperate drive for development sometimes causes a divorce from reality.

Boro Lukajic of the Tunnelling Association of Canada says, "Nuclear cratering charges were once considered, but the environmental impacts would be prohibitive.

"Even artillery rounds were used to study the impact on rock breakage."

"I remember very well the enthusiasm and rhetoric at the conferences in the late 1970s," he adds. "One speaker showed us a video of artillery round and water-jetting. It was exciting to watch."

Water jet

It is the use of high-pressure water jets, whether on their own or in assistance with more conventional cutters, that has established rather more credibility than more potentially destructive methods. It is now a well-established technique in industrial situations for cutting rock, precisely if necessary, and also many other materials.

In the construction sector it is often used in directional drilling and guided boring for ‘trenchless’ pipe installation, in some demolition techniques and in pipe cutting. More recently air and water jets have been used in conjunction with vacuum spoil removal for excavation in utility applications in which, with careful selection of pressure and jet type, sub-soil can be removed without damaging buried pipes and cables. In the first, however, the pressures employed are insufficient to cut hard rock as the applications usually involve installations in soil, sand and clay. Installations have also been carried out in gravels with the assistance of picks and other hard material elements on boring or reaming heads.

In the context of rock excavation it is important to distinguish between the capabilities of water jets based on the water pressure developed, with or without other cutting elements. Here we are concerned mainly with hard rock applications that are much more demanding than the more widespread use of water-jet cutting in soft and medium-strength ‘rocks’.

The use of jets of water to excavate ground has been around for many years, such as for hydraulic mining. But early applications relied as much on water flow as pressure to dislodge material. Water-jet cutting should not be confused with ‘water pressure jetting’ for cleaning surfaces in which the pressure employed is around 1,000-2,750bar.

Histories of water jet cutting refer to its use in hydraulic mining of coal in the USSR and New Zealand, but to move previously blasted material, and so it was already loose. Other areas employing this technique were South Africa, Prussia and in California in the gold rush of 1853-1886. However, it was not until the 1930s that, in Russia, attempts were made to cut massive rock with water jets, using a water cannon with a developed pressure of around 7000bar. This is also at the upper end of the pressure range employed today in most industrial applications of water-jet cutting known as ‘UHP cutting’. A major limitation on pressures used has been found to be metal fatigue in all parts of the piping system.

Industrial applications of water-jets, limited in pressure, often employ an additional abrasive cutting medium such as garnet grit, introduced to a mixing tube just behind the jet nozzle to accelerate cutting. Alternatively, the abrasive can be included in a slurry formed earlier in the system.

In rock excavation applications this would appear to be impractical and uneconomic, at least in the view of major tunnelling machinery manufacturers.

However, a member of the KMT Group, Waterjet Corp, offers a frame-based high-pressure water-jetting machine developed for tunnelling and underground car park applications called Ediljet powered by Mitsubishi Electric. The design is based on Waterjet’s Quarryjet system. KMT’s history goes back to 1947 when the McCartney Manufacturing was established in Baxter Springs, Kansas, US. The company was sold to Ingersoll Rand in 1971 that began work on waterjet cutting to replace commercial saws in manufacturing applications. Around the same time Flow Industries began to market industrial waterjet cutting equipment and introduced sand abrasive for cleaning.

In 2003, Karolin Machine Tool (KMT) of Sweden purchased the waterjet division of Ingersoll Rand and based it back in Baxter Springs. Various companies in the KMT group supply equipment for cutting all types of materials using waterjet cutting, with or without other cutting methods.

Pros and cons

The major potential advantage of water jet cutting in tunnelling is that it is a continuous process. Although TBMs with disc, roller and pick cutters, plus roadheaders for that matter, are also described as continuous, the need to replace work or damaged cutting elements can cause considerable disruption of a continuous tunnelling process.

However it may be found that waterjetting can have similar problems with the need to work on jet nozzles and other plumbing due to wear and fatigue caused by the high pressures involved. This can eliminate one of the major advantages of water jet cutting.

Water jets can be particularly effective against a face of fractured, highly jointed or porous rock since the disruptive effect of the high-pressure water can be introduced relatively deep into the rock to be excavated. More massive rock with a uniform face will be more difficult to remove.

Other potential pitfalls include:

  • Extra water in the working space can be an additional nuisance unless the water can be recycled or integrated into a slurry shield handling system.
  • Extra water could also make spoil more difficult to handle.
  • The presence of high-pressure equipment will be an additional safety concern requiring precautions and special training to site personnel.
  • Extra space required for tanks and pressure vessels such as an attenuator, although not a serous problem with large TBMs.

Waterjet assistance

As part of the drive for alternative rock cutting methods in the 1970s, water jet interaction with more conventional cutting methods was considered. At first this was to improve the life of drag bits by cooling to prevent failure, but it was also found that necessary penetrating force could be reduced by using adjacent water jets, albeit at relatively low pressure of 83-550bar. This and other related work was undertaken by the South African Chamber of Mines Research Organisation.

Around the same time both Robbins and Wirth were involved in trial work on the effect of waterjet assistance with TBM cutters.

In Germany the work with Wirth equipment was conducted by the mining research institute Bergbau-Forschung to try and design more compact TBMs for use in collieries. In the US the Colorado School of Mines (CSM) worked for the Bureau of Mines to try and increase the rate of TBM progress.

In 1975 CSM applied high-pressure (over 4,100bar) water jets to a 7ft (2.13m) diameter Robbins hard-rock TBM with disc cutters. The cutterhead carried 16 disc cutters and 31 water jet nozzles to excavate massive granite. In a recent presentation, founder and developer Dick Robbins explained that the intention was for the water jets to cut groves in the face, the ‘ridges’ of which could then be broken by the disc cutters. The main reason for not following this route of development was that the disc cutters actually worked faster than the water jets.

The water-jet equipment on the Wirth and Robbins machines, of different designs, was supplied by US-based Flow Industries, which was also involved in the early development of industrial high-pressure waterjet cutters, and also the FlowMole guided boring system for underground utility pipe installation. The results of applying water jets to the cutterhead gauge were disappointing, but jets applied more generally over the cutterhead produced substantial cuts in the forward thrust required. Interest in the technology largely fizzled out, perhaps due to the problems listed earlier, excepting use in roadheaders in conjunction with dust suppression by Sandvik, Dosco and others.

Rock saws

Chiefly employed in the ‘dimension stone’ and masonry industry, rock saws could, in theory, be used for cutting from solid rock using edges usually impregnated with diamonds or carbide insert teeth depending on the rock hardness. However, in tunnelling this would still leave the need to dislodge the cut material for removal, or to have a second free face.

There is also a likelihood of high levels of dust generation and a need for safety precautions to prevent accidental operator interaction with the cutting element (as is the case with roadheaders).