Sunday, April 6, 2014

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.

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