Waterjetting 30a - Why cut slots in rock

Looking back over the period where we first started coming together to discuss high-pressure water jets, some 40-odd years ago I was reminded of the work of one of my then graduate students (and subsequent faculty member in Egypt) Dr. Ahmed El-Saie. He obtained his doctorate in 1977 and looking back on that work it is interesting to see how long it took for some of the ideas he worked on to come to fruition.

His dissertation focused on using a waterjet system (which I will discuss in a later post) as a way of cutting a slot around the edge of a tunnel before excavating the contained volume. Early in his dissertation, for example, he discussed the use of impact breakers as a method for improving the economics of tunnel driving over conventional drill and blast techniques. He felt that this would be particularly useful where the volume of overbreak around the tunnel beyond the desired size could be controlled by cutting a perimeter slot.

Apart from the benefits that come from mechanical excavation over blasting (workers don’t have to leave the working area during a blast, for example) other benefits can be shown by contrasting the damage to a block of Plexiglas when a detonator is fired in a small central hole in Plexiglas, with and without that perimeter cut.

Figure 1. Damage to a block of Plexiglas from a detonator fired in a central hole.

If, however, a relieving slot is first cut around the perimeter of the anticipated damage zone (we used the distance to some of the longest cracks) then a different result is obtained.

Figure 2. Effect when the experiment is repeated with a pre-cut slot around the perimeter.

As the photos clearly showed with the free outer surface the central core of material is broken out in pieces, there is a nice relatively flat front surface to the excavated hole, which lies at the back of the drilled hole. (This is a relatively important point in driving tunnels, since often the last third of the blast-hole length is not effectively broken out of the solid, and has to be re-drilled).

It is also important to notice that the cracks from the detonator explosion did not grow out beyond the edge of the slot, so that the tunnel wall would be stable and, because there would be no overbreak, the cost of tunnel support would be reduced considerably.

However, in the larger scale the depth that this slot would have to be cut is around 7-ft. This would require that the jets cut a slot wide enough for the nozzles to advance into the slot, and this required a considerably higher volume of rock to be removed by the waterjets.

Tests of such a device in a German coal mine used two different methods for cutting the slot. Initial trials at Rossenray Colliery in Germany used a waterjet assisted mechanical set of tools to to cut the slot to the desired width. The head, seen moving along the slot at the edge of the tunnel, had to make a number of passes to reach the depth needed.

Figure 3. Tunnel profiling in Germany using a combination of waterjets and metal tools to cut to the perimeter of a tunnel (after Bauman and Koppers)

Subsequent trials replaced the mechanical cutter with a set of waterjet nozzles alone, and this reduced even further the cutting forces required to make the slot (and would make the machine smaller and lighter as a result). Although the trial was successfully concluded the tool did not move into production, perhaps in part because of the change in the mining economy at that time.

To prevent the cracks from growing into the wall, however, a wide slot is not needed, and even a continuous crack around the edge of the hole can be effective. But how to control crack growth to a single direction from the borehole?

The answer came as part of the Master’s degree of another student, Steve McGroarty. If one drills a hole into a block of Plexiglas, and then notches the side, fills the hole with water, and fires an air rifle pellet into the hole, then the pressurized water will flow into the notches and cause the cracks to grow. These are a little difficult to see in the following picture, but the cracks grow at the bottom of the hole and from the edges of the v-cuts (made at the time with a saw).

Figure 4. Individual cracks growing out from 3 bored holes in plexiglas

The above test showed that energy could be focused into cracks if they could be properly aligned. (We could break off a corner of the block in a single piece, using a single notched hole). This work was then in the field by Steve in comparing results when he used explosives to drive a short tunnel underground.

In Steve’s case he drilled holes around the edge of the tunnel, and then notched some of these with a waterjet system. (Others were left un-notched to provide a comparison). The lance used had two jet nozzles and was fairly simple to insert, and the lance was run to the back of the hole, and the two opposing jets aligned to the proposed tunnel wall, raised to pressure and pulled back out of the hole, notching the walls. This is a fairly fast process, and used relatively little water.

The holes were then charged with a small amount of powder and fired just before the rest of the blasting round, which was distributed around the rest of the core rock, in order to break it into fragments.

Figure 5. Tunnel wall after the round had been cleared showing the clear break at the back of the holes, (the next round has been drilled along the edge) and parts of the drilled hole still evident in the wall of the tunnel. (after McGroarty)

The role of the waterjets was much smaller than if a complete slot had been cut, and this significantly reduced the cost financially, in energy and in time, and produced much the same desired result.

In later work we used the same notched borehole idea to break out large pieces of rock as we excavated the Omnimax Theater under the Gateway Arch in St. Louis, but that is a story for another day.

Next time I will discuss some of the ways that Dr. El-Saie used to cut the slot.

A.A. El-Saie “Investigation of Rock Slotting by High Pressure Water Jet for use in Tunneling”, Doctoral Dissertation, Mining Engineering, University of Missouri-Rolla, 1977.
Bauman L. and Koppers M. “State of Investigation on High Pressure Waterjet Assisted Road Profile Cutting Technology,” BHRA 6th ISJCT, paper G2, pp. 283 – 300, 1982).
S.J. McGroarty “An Evaluation of the Fracture Control Blasting Technique for Drift Round Blasts in Dolomitic Rock”, M.S. Thesis, Mining Engineering, University of Missouri-Rolla, 1984.

Waterjetting 19d - waterjets and material removal

When miners first began to use water as a means of moving soil and ore from valuable deposits at, or near the surface, stream flow, or the energy from stored volumes of water was the main source of the power used. However, at these low pressures it was necessary to use large volumes of water (often in flows of over a thousand gallons a minute) to move the material. At the same time, even at these large flow volumes, the flow has to be confined in order to ensure that there is enough water around the particles to keep them in suspension. There is also a certain amount of turbulence required in this flow to retain the suspension and to stop the particles from settling out before the riffles where the gold or other valuable minerals can be collected.

As pressure is increased these jets can be increasingly productive, and can be used for a variety of functions, including the rapid removal of soil. Raising the pressure allows the tool to be used in mining soft rock. As an example there is a layer of relatively soft sandstone that lies in a roughly horizontal layer and appears along the banks of the Mississippi river and can be found, for example, under Minneapolis. There is a sand mine that lies along the banks of the river, that has a high-grade sand that can be used for making glass, but which also has a vein within it that has larger grains that can be used in the fracking stage of increasing oilwell production. However, with conventional mining (using blasting) it was not economic to screen the sand after it had been mixed during mining. The Bureau of Mines of the time (since closed) tried some experiments to see if a waterjet could be used to “high-grade” the sand, mining the larger layer first, and then that surrounding it to let machines and men progress further (and also to produce the sand for glass making).

Figure 1. Bureau of Mines experiment washing out sandstone, with 4,000 psi water and cutting about 9 ft. deep. Note the yellow color of the water as the sand settles out but the clay contaminant is suspended in the water and washed away.

The experiment, carried out by Dr. George Savanick’s team, was successful, and had the unexpected advantage, since the grains were all separated, of stripping the small amount of clay contained in the sandstone and carrying it away with the water, while leaving the sand on the floor, where it had to be picked up mechanically.

The use of pressurized water is used both to mine sand, and also, in Cornwall, for example, in the mining of clay, although the pressure and volumes needed are a function of the quality and amount of weathering that the clay has seen.

Figure 2. Mining clay in Cornwall (Pathe video here)

Engineers have even accelerated the movement of landslides, using clay mine pumps, in order to move the soil away faster and allow the slide to be remediated. At Dawlish in the UK, for example, railway engineers have added water under pressure to remove the sliding soil as a slurry, making clean-up faster, safer and less costly.

Figure 3. Moving soil in a landslide that has covered the railway line at the bottom of the slope, Dawlish UK March 2014. (The Packet)

A jet was also used from the bottom of the slide to liquefy the soil, which then flowed through the railway path and into the sea.

Figure 4. Removing the soil from the landslide from above the railway. (From a video at The Packet)

As a comment, for those who watch the video that the above picture was taken from, the jet cuts much more effectively closer to the nozzle, and had they used it to undercut the bank they could slurry the soil lower down, and have removed the material a bit faster than dispersing most of the jet energy in the air as they tried to reach the back of the slope. (If they had undercut the bank then the soil would have slid down towards them shortening the reach and speeding the process).

During the Second World War engineers also used water jets to uncover land mines that had been planted on beaches along the coast. (video from Pathe here ).

However the control of the water, and debris, can quite quickly become a problem, and containing the water and keeping the soil/sand particles suspended in it requires more preparation. As an illustration the civil engineers who work under Minneapolis are aware of the benefits of using higher pressure waterjet streams in driving tunnels (and occasionally rooms) under the city. For example, in driving a sewer tunnel (the St Anthony Park Storm Sewer extension) the engineers set up an extensive train behind the tunnel face, so that the resulting slurry could be pumped out of the tunnel.

Figure 5. Train behind the tunnel face, required to supply the jets and to pump the slurry from the excavation (after Nelson*)

The tunnels can be driven by two main jet operators at speeds of up to 120 ft per day () with a third jet being used to break the larger pieces down to slurry so that it can be pumped down the tunnel to the river, where it is barged and sold as glass-making sand.

Figure 6. Driving the storm sewer tunnel under Minneapolis. Two operators are carving the face into small pieces and the third is slurrying the sand (after Nelson*)

By feeding the sand and water into a channel cut into the floor of the tunnel it was possible to confine it, so that the blocks could be broken up more easily, and this also then provided a catchment for the intake to the slurry pumps.

The tunnels were pre-cut along the profile, so that the arch girders that provided support could be slid into place with the central core of rock still there to support them. This made the support easier to install, and provided immediate support of the working area ahead of the place where the miners were working.

(That ability to use waterjets to penetrate ahead of the tunnel and allow a support to be installed before the main core of the tunnel rock is removed has since been improved, and in the Advanced Austrian Tunneling Method high pressure jets drill out cylindrical bores along the profile ahead of the tunnel, which are filled with a grout that can be generated partially using the surrounding material as the holes are drilled. But I will talk about the use of cement mixed with the jets in a later post).

*Nelson C. Tunneling under Minneapolis, Water jet Workshop, Rolla, MO 1975.