In this section (part 2) of the series on Waterjetting, the focus is on the way in which high-pressure waterjets grow cracks in their target. As John Field showed, even the presence of microscopic cracks on a glass surface are enough to initiate the larger cracks that lead to failure. In many cases, however, the most useful growth can be achieved if the cracks only extend to the point that they remove a desired amount of material. This becomes important where there are weaknesses and flaws in the material – such as the layers between plies of wood, or even Kevlar – which should not be grown as the jet cuts down through the material. And in a later article this topic will be a part of a discussion as exactly what happens as a jet drills a hole into a target. But, for today, I would like to talk about crack growths in rock and soil, both because it is one of the oldest ways in which water can penetrate into material, and also because it holds the potential to be one of the newest areas into which waterjetting is growing, and will likely further advance into a more significant business.
And to begin consider that, as water penetrates into the cracks in a rock, and grows those cracks slowly, under natural forces, rocks with minerals in them, will see those mineral particles separately broken out. The classic example of this is with gold. One of the ways in which the Forty-Niners found the gold in California was by panning for the gold particles in the rivers, and tracking the gold deposits back up-stream until they reached the original gold deposits of the Sierra Mountains. Not that this was the first time that water transport had helped in gold mining. One of my favorite stories to begin classes is to remind them of Jason and the Argonauts.
It is a theme that has been made into a movie several times, (see, for example, here) and tells the story of how the Greek Prince Jason and a band of companions go in search of the Golden Fleece, and the adventures that he has along the way. Despite the mythical creatures the story is thought to be likely based on some measure of truth, with the voyage taking place some time before 1300 B.C. But our focus is on the fleece, rather than the voyagers.
Within the Caususus mountains of Georgia lies the modern town of Mestia, which was thought in Roman times, to be the site of Colchis, where Jason found the Golden Fleece. The reality is not quite as dramatic as the legend since, as the Roman historian Strabo noted
“It is said that in the country of Colchis, gold is carried down by mountain torrents, and that the barbarians obtain it by means of perforated troughs and fleecy skins, and that this is the origin of the myth of the Golden Fleece”
It is thought that the miners of the time directed the streams so that they flowed over the veins of gold and eroded out the particles so that the gold was carried down to the valley. Here it was fed through the troughs that Strabo described, and the heavy gold particles were captured as they tangled in the wool of the fleece. To recover the gold the miners would then hang the fleeces in trees, so that they would dry, and the gold could be shaken loose. Unfortunately as the fleeces hung in the trees they provided a tempting target for Greek thieves. (In a later version that I will write about in the next post the sheep fleece was replaced with brush that could be dried and burned to release the gold).
Water was thus, in one of the earliest “automated” mining processes, used to both dislodge and then carry the valuable mineral from the mining site The overall power of water to move soil has been used to wash away material for over a hundred years. In the 1973 War between Egypt and Israel the Egyptian Army gained a significant advantage in the early hours of the war by using waterjet monitors to wash away the defensive barrier along the edges of the Suez Canal, rather than using conventional mechanical excavators.
To deal with the massive earthen ramparts, the Egyptians used water cannons fashioned from hoses attached to dredging pumps in the canal. Other methods involving explosives, artillery, and bulldozers were too costly in time and required nearly ideal working conditions. For example, sixty men, 600 pounds of explosives, and one bulldozer required five to six hours, uninterrupted by Israeli fire, to clear 1,500 cubic meters of sand.
The quoted Sunday Times report of the time suggested that the Israeli Army had anticipated that it would take 24-hours to remove the barriers giving time for their Army to mobilize and arrive. However, using a set of five pumps per breech site the Egyptian Army was able to make an opening in as short as a 2-hour time, with the mobilized water cannon opening 81 breeches, and removing 106 million cubic feet of material in that first day of the war. They were thus able to initially advance into the Sinai with relatively little resistance.
The pressure of the water does not have to be high to disaggregate the soil, but large volumes were needed in that application both to break the soil loose and to move it out of the way. Moving the debris out of the way is an important part of the operation, and while, in the above case it could be just pushed to one side, in many more localized jobs, particularly in cities, that is not an answer. However if the soil can be collected with the water, then the fluid can help to move the soil down a pipe away from the working area. And, more importantly, if the soil can be captured as it is being broken loose, then both can be collected before the water has had a chance to penetrate into the soil around the hole, and so the walls of the hole will not get wet, and will remain stable and not fall in.
One way that we have achieved this is to rotate a pair of waterjets relatively rapidly (depending on the material the jet pressure can range from 2,000 psi to 10,000 psi) so that the surface layer is removed, and to immediately take this away by combining the jet action with a vacuum for removal. (In the initial trials we used a Shop Vac to remove both water and debris). This combination has become known as hydro-excavation, and will be the topic of a couple of posts in the future.
Similarly the use of high pressure to break an ore down into its different parts, so that the valuable mineral can be separated from the host rock at the mining machine, is become a new way to reduce the costs of transporting and processing the ore, and make mining more efficient. As yet this latter is still more of a laboratory development, though it will develop for greater use in the future, and there will be additional posts on this too in the future. But, in both cases, the use of waterjets to effectively rely on extending pre-existing cracks makes the systems work. In the next post I’ll write about a couple of other ways of getting enough cracks into the rock as ways of making it easier to separate and remove valuable materials from underground.