Waterjetting 20a - Making holes in soil

There are two problems that often arise when applying waterjets to a soil-like material. The first of these is that the water can spread into the surrounding soil around the hole being excavated so that it loses its strength and can collapse into the hole. (This can be used to advantage in some cases.) This is a particular problem when excavating trenches, where the hole has to be as small as possible, yet the sides have to be stable so that work can be done at the bottom. The other is that, once the soil is loosened it has to be picked up and moved, and a way has to be found to be sure that the particles don’t settle out before they should.

Figure 1. Three consecutive frames from a video record of a jet firing into glass beads behind a glass wall. The framing rate was 30 fps.

In the sequence of frames shown above the jet is seen to first penetrate down through the beads (which were simulating soil, being easier to handle and see through), and then when it reaches about a nine inch depth it stops penetrating and starts to widen and fill the hole, which until the is relatively open. Note that at this stage there is no ejected material from the hole and very little penetration of the water outside the line of the hole. The penetration stops when the water no longer has the energy to push the particles aside, and continue to penetrate. (The test was at relatively low pressure to keep the penetration small enough to remain inside the box). Note also that the hole is largely hollow at this time.

If the jet is allowed to continue to play on the surface, the water will now penetrate into the material on either side. This can be better seen if the fluid color is changed to black by adding fine carbon particles to the water. The pressure was further lowered (to 100 psi) to keep the jet penetration down to below three inches, and in this case the jet was a fan shape to encourage the spread, rather than being projected through a round orifice.

Figure 2. Video frame taken as a waterjet laced with fine carbon penetrates into glass beads, note that the carbon starts to be carried into the surrounding material, and that again, in this short time interval there is very little material being ejected from around the hole.

In passing it might be noted that this is a relatively effective and simple way to inject remedial chemicals into layers of clay and soil that could contain undesirable chemicals (such as PCBs) but where going in to remove the contaminant might be difficult and cause other problems. This could occur if the contaminated layer is now covered with more material, and trying to dig the contaminated material out would cause it to disperse into a stream, river or bay where the problem has been found to lie.

But if we want to remove the soil, then the process, as it stands with using a single jet, is fairly inefficient. The water, at this pressure, is cutting into the soil, making a hole, penetrating into the soil around the hole, but not much is being moved. Again that could be an advantage.

Those who play golf know that good maintenance requires that golf greens need to be aerated at regular intervals to keep the grass healthy. At the same time, pulling plugs of material out of the green is disruptive, and conventional mechanical tools will still make a bit of a mess, and take some time. On the other hand Toro has developed and improved a tool – the Toro Hydroject – which has a series of jets that are spaced at adjustable intervals (but typically around 3 inches) along a distribution manifold, so that when the jets pulse they drive holes down into the soil, with no surface spillage of soil. And this can be done at walking speed – between foursomes, and without disrupting play.

Figure 3. Toro aerator at work. (After Toro )

The tool is also effective in poking holes under pools of water to speed drainage. The most effective pressures vary for different soil types and conditions, but are typically in the low thousands of psi, with penetration depths of up to eight inches. (They are also a potential tool for finding land mines, but that is another story).

Figure 4. Cut through a hole jetted into soil (turned on its side for convenience) (Toro)

Yet these applications again illustrate that the tool might be more difficult to use, where the main purpose is to remove the soil, and where a sequence of passes of a jet over the surface won’t potentially move much material.

The answer to this problem is to use more than one jet at once, and to place them at some distance apart, depending on the soil and jet parameters this might be more than an inch or two. What now happens is that the resistance of the soil is removed when it the jets pass along either side of the intervening rib at the same time. This very rapidly liquefies the rib in the middle, and it is removed as the jets cut past.

This can be seen where, for example, two jets simultaneously traverse over a clay bed. When only one jet was used it cut only a thin slice into the clay, with little material removed.

Figure 5. Single consecutive cuts into clay with a water jet that also contains kaolin so as to show where the cuts were made. (Note that even where the cuts are close together there is no removal of the ribs between cuts.)

Figure 6. Material removed when two jets cut side-by-side into clay. Note that all the intervening clay between the jets has been removed, to a depth of four inches.

The contrast between the two figures shows that by changing the way in which the jets cut into the material (concurrently rather than consecutively) up to ten times or more material can be removed from the surface for the same amount of input energy.

Figure 7. Slot cut in the ground by a combination of jets acting together as a head was moved through the ground (Halliburton - the Soil Saw)

This also works with some rock types, and I will discuss how we used it to design a machine for mining coal in a later post, but it is not the end of the story in developing the soil cutting aspect. The problem that can then arise comes from the type of soil that is being cut, and the distance between the jets. As the soil becomes more coherent (clay laden) the jets need to be brought a little further together (and the more sandy the soil, the further apart they can be.) But if the jets don’t have enough time to totally break up the rib of material into particles (controlled by how fast the jets are being moved over the surface and the relative depth of cut) then the pieces may come out in lumps of varying size and shape. These are more difficult to break up, once they break away from the solid.

The alternative is to move the jets relatively rapidly over the surface. This shortens the depth of the soil that is being moved at one time, but if, for example, the jets are being spun around an axis inside a shroud connected to a vacuum system, then the particle sizes can be controlled to fit within the vacuum line, and the depth of cut is small enough to hold the partial vacuum around the edge of the shroud to make sure that all the particles and water are removed and the walls are kept dry enough to remain stable. But, again that brings us into hydro-demolition and I’ll cover more of this in a later post.

Waterjetting 18c - Abrasive choices

Picking the right abrasive can make a big difference in the profit that a waterjetting operation makes. But the question, of course, is which abrasive is the best? And, as I have done in the past, I am going to hedge a little in my answer. The reason for this is that there are different factors that control the price of the abrasive – how far has it to be transported, how it was prepared and what it is made of – for example. And while some abrasives generally cut better than others, if the unit is only going to be cutting a narrow range of material, then the abrasive that is best for cutting a wide range of materials (garnet) may not necessarily be the best choice in that particular case.

Figure 1. Different types and sizes of abrasive particles.

And further, just to make life a little more complicated, not all garnet abrasive (or other types of abrasive product) are created to give equal performance. As I have mentioned in a previous post the cutting performance of the abrasive can be rapidly reduced as the particle size falls below 100 micron. Thus, if the particles have not been well graded, so that there is a significant amount of abrasive below this size (even though the vendor tells you that it is a 250 micron size) then the cutting performance will not be as good as it would be if the mix contained no fine fraction that far below the stated particle size.

Figure 2. The effect of particle size on cutting performance

On the other hand if particle sizes are too large, then for a given abrasive feed rate there will be a smaller number of particles hitting the surface, and this can also reduce cutting performance if carried too far.

Figure 3. The effect of increasing particle size on the cutting of cast iron at a constant abrasive feed rate (0.88 lb/min) (after Hashish*)

There are other factors that has also to be considered in selecting the best abrasive, and, while I am going to discuss this below, in most cases it is going to be something that you will have to test in your own shop, comparing the results that you get with the cost of the abrasive (often worked out in dollars per square foot of cut, or similar unit) to decide which gives you – in your particular circumstance – the best performance.

But the testing of different abrasives can be reduced to a manageable range of tests (and we prefer the triangle test that I have described in an earlier piece) if some basic thoughts are born in mind.

Decide what it is that you will be cutting – is it mainly going to be a metal that is going to respond in a ductile way when cut by the abrasive. In which case the abrasive should have enough sharp edges to cut into the material, and then to plow up some of the surface so that as more particles hit the surface, pieces are broken off.

Figure 4. Mechanisms for cutting into a metal or other ductile material.

On the other hand, if the material that you are going to cut is a brittle one, say for example rock or glass, then the material is going to be removed by crack growth. Here relatively spherical particles can be more effective because the energy of the particle is concentrated at the point of impact, and more easily causes cracks to grow. On the other hand relatively flat particles with multiple impact points reduce the pressure under any one and reduce the effectiveness of the particles in getting the cracks to grow as quickly and as long as possible.

Thus, for example, we can compare the effect of using the same amount of steel shot (round) and garnet in cutting granite (a brittle material) and tool steel.

Figure 5. Cuts in granite and tool steel using the same abrasive feed rate, but the cut on the right is with garnet and that on the left is with steel shot. Note that the steel cuts the granite to a deeper distance within the cut, while in the tool steel it bounces off without leaving much impression. (However harder steel grit on softer steels can be an effective choice).

The difference that particle shape makes in cutting ductile materials can be shown when the same AFR is used but in one case the abrasive is broken glass particles and in the other it is glass beads.

Figure 6. Effect of particle shape (broken glass and glass beads) in cutting composite material at the same abrasive feed rate (after Faber***)

Further some particles, for example steel shot or grit, can be recycled through the system a number of times without much degradation, so that if they can be simply collected below the cut, they can be profitably reclaimed, providing that the particles below 100 micron are separated out of the flow after each cycle.

Figure 6. Effect of recycling abrasive through a slurry abrasive jet system on depth of cut achieved (after Kiyoshige et al**)

Others however are more friable, this is perhaps more true of alluvial garnet, which often has a much higher density of internal cracks than mined garnet, and so is more liable to fracture either in the mixing chamber or on first contact with the target, so that the amount that remains above 100 microns is substantially smaller than that with the mined alternative.

Yet cost is a factor, and so it is best, for your material to test the different alternatives available, before making a final decision.

* Hashish, M., "Abrasive Jets," Section 4, in Fluid Jet Technology, Fundamentals and Applications, Waterjet Technology Association, St. Louis, MO, 1991.
**Kiyoshige, M., Matsamura ,H., Ikemoto, Y., Okada, T., "A Study of Abrasive Waterjet Cutting using Slurried Abrasives," paper B2, 9th International Symposium on Jet Cutting Technology, Sendai, Japan, October, 1988, pp. 61 - 73.
*** Faber, K., Oweinah, H., "Influence of Process Parameters on Blasting Performance with the Abrasive Jet," paper 25, 10th International Symposium on Jet Cutting Technology, Amsterdam, October, 1990, pp. 365 - 384.