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.

Tech Talk - fracking tight sand and shales

The recent news that Saudi Arabia has not found natural gas to be as available as it had thought from its shale deposits, and is shifting to exploring for gas in their tight sand formations has not caught a lot of attention. But it is worth considering some of the aspects of this – and hence this post.

The Oil and Gas Journal define tight sands in this way:

The term tight gas sands refers to low-permeability sandstone reservoirs that produce primarily dry natural gas. A tight gas reservoir is one that cannot be produced at economic flow rates or recover economic volumes of gas unless the well is stimulated by a large hydraulic fracture treatment and/or produced using horizontal wellbores. . . . . . . . . Tight sands produce about 6 tcf of gas per year in the United States, about 25% of the total gas produced.

The production of natural gas from tight sands is different to that from shale sources, for, as the Canadian Center for Energy notes:

Initial production rates are low; however, productivity is long lived.

To understand why this is, and why this differentiates from shale production requires a little bit of an explanation.

Back in the time that the plants were growing that went on, over time, to produce the oil and natural gas we now extract, the surrounding ground was typically soil, with intrusions of sand as the tides and rivers shifted. As succeeding layers of ground covered these layers, so the soil turned into shale and the sands into sandstone. Sand particles tend to be more round, and so as the particles packed together the gaps between the grains provided space for the oil and gas to collect. Clay particles, that went on to form the shales, on the other hand are flatter and pack together more tightly. Further, while the sand formed continuous layers of material, with relatively few cracks, the shale, on the other hand cracked as it was compressed, dried and was squeezed – simplistically in the same way as the lake beds crack when they are exposed to the sun after a drought has driven off all the water.

The result of this is that the fine sands that formed into the tight sands have relatively low permeability – fluids within them find it difficult to get through the very narrow passages and driving pressures have to be high to overcome the frictional forces along these passages. At the same time, because there are relatively few natural fractures in the rock, artificial cracks have to be driven through the rocks, in order to provide enough large passage ways for the gas to escape. It still flows relatively slowly through the rock around these cracks before it reaches them, and this is why the production is relatively low, but continues over time.

On the other hand the shale structure holds much of the oil/natural gas in a different way. Firstly there are the natural fractures within the rock, which tend to run into one another, and which hold much of the fluid that is released early in the production of these formations. Secondly the rock immediately around the fractures has been itself fractured somewhat during the formation of the layer, and has a higher porosity (i.e. holds more fluid) and permeability than the central segments of each of the larger pieces.

To simplify the situation therefore, prior to hydro-fracking the natural gas in a tight sand is permeated relatively evenly throughout the rock (given the variety of the conditions that control rock structure over even relatively short distances). On the other hand the natural gas in a shale is preferentially concentrated in the natural fractures and the rock adjacent thereto.

So now a horizontal well is drilled along the rock, and, at intervals pressurized to drive fractures into the surrounding formation. The rock structural differences mean that the sandstone and the shale will respond differently. And a slight qualification at the beginning of the explanation, because of the length of the feed lines to the injection point where the fracture will start, the pressure rises relatively slowly on the rock, rather than with the sharp pressure spike from an explosive blast, which produces multiple fractures. (The use of a small shaped charge to induce the fracture by driving a small hole into the rock, will similarly only produce a single fracture extending out from the well bore for each charge).

Contrary to the pictures that are shown by oil and natural gas companies on how fracking cracks grow out into the rock, the cracks will not branch out under their own normal development. Crack splitting only occurs at relatively high energy transfer rates, when the crack speed reaches the maximum, and the crack has to bifurcate to absorb the input energy, those conditions normally will not exist at any distance from the wellbore.

What happens in the case of the shale formations is that the pressure opens the natural fractures in the rock and as these link together the pressure can open them over a significant volume of the rock, giving the equivalent spread of fractures away from the wellbore as the initial pressurized joints intersect others as the sand and fracking fluid migrate along the joints into the rock. Opening up these passages and giving access to the relatively higher porosity rock along their edges allows a relatively high rate of flow for the initial volume of natural gas which is found in these zones. But as they deplete, and the natural gas has to travel from the more central part of the shale blocks, so there is the dramatic drop in production which has been remarked by many (around 65% drop within the first year in some cases). Production continues to fall as the more easily produced volumes drain through the intersected fractures, and the residual production falls to a slight fraction of its initial value.

In contrast, the tight sand formation will flow in a different manner. Because of the low number of natural fractures and the lack of permeability changes along the joints, much of the channeling through the rock has to be put in place by the fracking process itself. Because there is much less collection of fluids into natural fractures (though there is still some) the initial production rates are not as high as they are from the shale formations, since the overall induced fracture density and dispersed penetration is not as pervasive. On the other hand once the fractures are in place, although permeability is low, if the formation does initially produce at an acceptable rate, it is more likely that the well will continue to flow with a lower, but steadier flow, than the shale well, and for a considerably longer time.

Thus the news that Saudi Arabia is now moving to try and develop their tight sand fields suggests that they will need to carry out much more extensive development in order to get the initial volumes of natural gas that they will need, but that once these wells are producing, that they will last longer than they would in the shale fields.

Waterjetting 20c - Critical Distances intro

I remember going down the New Orleans just after Hurricane Katrina, and being a member of one of the first inspection teams that drove down the Delta inspecting the damage. One of the things that has remained in my memory was that of driving past one of the large coal piles being stored outside one of the local power stations, and being surprised that – despite the wind and rain – it was still there. Some years previously I had visited several places that had build ultra-ultra-high pressure jet generating devices. (Not sure what else you call a system that produces jet impact pressure that begin to approach 1 million psi). The one that has struck in my memory was visiting the underground test for Dr. Bill Cooley’s water cannon. Built to advance the concept of using high pressure gas to drive a pulse of water at a rock target (this was during the days examining new ways to drive underground tunnels) the device fired a small slug of water through a very carefully designed nozzle to generate a jet measured to produce 500,000 psi on impact.

We stood around and watched as the device was loaded, charged and fired and were, I suppose, a little disappointed at the damage induced around the impact point. Only a small amount of fragments were produced from the shot, and as you can see from the figure above, there was not a lot of evident surface damage from the shots. Actually, for the amount of energy involved, the amount of material removed was quite significant and some of the pieces were several inches in size:

Figure 2. Fragments recovered after a single shot of the cannon.

Later analysis explained why the damage had not been greater. The problem arises because of the way that the pressure is generated to drive the jet. Most of the devices of the time, whether our own, others in the US, the UK or in Russia, used a gas driver to generate the driving pressure, either using smokeless powder (as we did) or storing gas volumes under pressure and then suddenly releasing the accumulated volume to provide the driver.

The problem with this approach is that the driving pressure is not sustained as the water moves down the relatively long nozzle, and the driving pressure on the surface is thus very transient. One recording of such from the UK showed a typical pulse for their unit:

Figure 3. Pressure pulse for the water cannon device developed in the UK.

Dr. Cooley’s cannon had a much shorter and more rapidly decaying pulse, and this was largely the reason that the damage that it induced was not greater. The high impact energy was able to generate large cracks into the surface as the jet penetrated, some of which coalesced with the surface and allowed fragments to release, but the energy pulse was not long enough at the high pressure and with inadequate volume to fill the cracks produced and pressurize them to cause rapid extension and result in larger volume material removal.

The relatively low pressure impact of the rain on the coal in the Gulf, and the high pressure impact on the rock in the test mine both saw the ability of the water to penetrate into the surface layers of the material. But in both cases the impact pressure on the water that permeated into the cracks within that surface was not enough in the secondary phase of the failure process, to sustain internal pressures within the cracks that would lead to large volume material removal.

Now, as I mentioned last time, there are ways to enhance the effect of an impact, by creating secondary surfaces for the rock to break to. The best illustration of this, perhaps is to consider two small (six-inch side) blocks of plexiglas. We drilled a small hole through to the center of each and placed a detonating cap in that hole. We then fired the detonator.

Figure 4. Block of Plexiglas after a detonating cap has been fired in the middle of the block.

Note that the block was big enough to contain the crack damage. But note also that there is very little material removed, because the cracks that were formed by the explosion were contained within the block and did not interact very much. Now consider what happened when we first drilled a circular relief slot around a similar hole and charge. (We based the radius of the cut on the results of the first shot, shown above). After the detonation, this is what we achieved:

Figure 5. Similar bock to figure 4 except that a relief slot was first cut into the block

Note that there are (for those of us interested in driving tunnels) several interesting improvements. Firstly the cracks that radiated out to the walls of the first block now stopped within the central isolated core, and the wall of the excavation is now smooth and undamaged. Secondly the interaction of the radiating cracks within the core all reached the relief slot, and broke the core into fragments that were liberated. And that this also broke to the back of the slot, which was left relatively flat and at the end of the relief slot, so that it would be easy to start a new drilling and breaking operation to deepen the tunnel without having to redrill any damaged zone at the end of the previous excavated section.

But, for the purpose of today’s exercise, note that in the first place – even though there were free surfaces at the edge of the block, they were too far away for the cracks to reach them, and effectively break out the material. It was only when the relief slot was moved closer to the exploding detonator that the central volume was broken out and the effective result that was desired was achieved.

The critical parameter is the correct assessment of the distance over which the interaction between the event and the free surface (or in the case of last time’s discussion between two concurrent jet cuts) will take place.

As I will discuss in later posts, sometimes the distance that is critical to effective jet use is only on the order of fractions of an inch, and if done correctly the result (as above) is impressive, if the distance is too great, then nothing happens. But we’ll talk more about this, in many applications, in the future.

Waterjetting 14e - Cavitation and Comminution

In the last post on this subject I discussed how, by adjusting the back pressure in the relatively stationary fluid surrounding a high-speed jet of water, it is possible to intensify cavitation damage. The simple way to find the optimal value for the back pressure for a given jet pressure and size was, we found, to listen to the sound of the cavitation collapse, and by adjusting the back pressure tweek both the sound and range of the cavitation cloud surrounding the jet.

The damage that the cavitation would induce on samples of rock is a function of the time that the cloud plays on the surface. By slowly moving the sample under the nozzle, in a confined cell, different levels of damage could be achieved, based on the speed at which the sample moved.

Figure 1. Traversing specimen cell, with the front cover removed to show the sample in the holder.

When the sample was moved under the nozzle at 2-inches a minute, the cavitation cloud attacked the surface relatively uniformly, with only localized increases in damage. In Figure 2 the red lines mark the width of the cavitation cloud on impact, it then spreads and collapses over the surface to give the wider erosion path.

Figure 2. Traverse over the surface of a dolomite sample at 2-inches a minute. The red lines define the width of the jet. Note the additional depth of removal under the sample ID (15) where the ink chemical had slightly weakened the rock making it more susceptible to erosion.

As the speed of the sample movement is slowed, however, the cavitation attack starts to find weakness planes in the rock and preferentially begins to erode these. As these channels are formed so the jet will flow into them to escape from the following flow of water in the consequent jet flow. As the cloud moves into these narrower spaces, so the pressure increases, inducing more of the bubbles to collapse and thus intensifying the erosion attack along that weakness plane (Figure 3).

Figure 3. Traverse of a cavitating jet over dolomite at a speed of 0.5-inches per minute. Note how the jet is now eating into zones of weakness which are beginning to define pieces of rock that are then liberated as cracks grow all around them.

As the traverse speed is further reduced to 0.4-inches per minute the erosion pattern which is developing in Figure 2 becomes consistent under the full width of the cavitation cloud, and the intersection of developing cracks means that the rock is now being removed in larger pieces and the erosion rate suddenly increases significantly.

Figure 4. Effects of moving the cavitating jet over the rock at 0.4-inches per minute. The cavitation is now developing cracks in the rock that join and break out larger pieces of rock, to a depth of around 0.5 inches over the cloud width.

This ability to focus the jet attack on weaknesses in the rock structure can be useful if, for example, the rock under attack is a mineral ore. Because the ore is defined with weakness planes around the individual constituent grains of the minerals and host rock, at a slow traverse speed the cavitation cloud will preferentially attack those boundaries, in the process liberating the individual grains, and separating the rock into its constituent materials. This liberation can be achieved as the rock is being mined (we have demonstrated this in the lab) so that the valuable mineral can be separated from the waste rock at the mining machine. This means that the waste can be left, in a larger size range than is conventionally left after separation, at the mining site, and does not have to be transported to the surface and ground to powder in order to separate out the valuable minerals. The energy savings that this achieves can be potentially as high as 75% of the total energy currently used at the mine.

Where the mining breaks out the rock without achieving complete liberation a secondary process can be used where the particles of material are fed into a secondary tube, where the particles pass through a second cavitation cloud. The attack of the very small bubbles on the mineral particles is such that the fragments of ore are rapidly broken (comminuted) into much smaller sizes in a process which, because there are so many events occurring sequentially , can appear almost instantaneous.

Tests at Missouri University of Science and Technology, for example, have shown that 0.5-inch sized pieces of coal can be reduced to 5-micron size in a single step. There is a video attached to this post of one of these tests. Figure 5 shows the equipment, with the coal in the inner metal tube, while the surrounding space is filled with water under slight pressure. 

Figure 5. Equipment to comminute coal to 5-microns (The size of the feed coal can be seen in the plastic box on the right).

Water to the cell does not have to be at any great pressure. The test has been successfully run with the water fed from a pressure washer obtained from the local hardware store for less than $100.

Figure 5a. Early in the test the water flowing out of the inner tube is filled with fine particles of coal as the cavitation breaks the pieces down to the required size.

B) A short while later and the outer tube begins to fill with the fine material.

One of the advantages of coal at 5-microns is that it can be mixed with water in about a 50% slurry and fed into a diesel engine, which will then run. GE has tested a locomotive and shown that it is possible to run the engine on the mixture, should conventional diesel no longer be economically available.

The process also works when a harder rock, such as dolomite (a host for galena and other minerals) is placed in the inner tube. The cloud color in this case is white.

Figure 6. Using cavitation to crush dolomite. The original particle sizes are in the box on the left, the cloud of particles is at around 5-microns.

Waterjetting 9d - Deepening a hole and cautions with glass

The last three posts have described what happens when a jet of water first arrives on a surface, and then starts to penetrate into the material. At a close stand-off distance the erosion starts around the edge of the jet, and continues to widen the hole as it gets deeper, until a point where the pressure at the bottom of the hole falls, and the jet stops going deeper. The lateral flow away from the bottom of the jet continues to erode material, however, and so the hole gets a little wider at the bottom. This creates a small chamber under the entrance hole and this can build up enough pressure that it can cause the material around the hole to break.


Figure 1. Progress in the high-pressure waterjet drilling of a hole in rock.

In the last post I showed where this happened with a 1-ft cube of rock that had been broken with a single pulse, but this fracture of the target can occur when piercing glass or other brittle materials. So the question becomes how to stop the fracture if one is trying to cut glass. This applies when the job calls for making an internal cut in the glass, and not when cutting in from the side, although that also has some problems that I will address in a later post.

When starting an internal cut, obviously, if it is possible, it means piercing a starter hole through the glass in a region that is going to be part of the scrap, if this is possible, as it would be, for example when cutting a sculpture. A secondary reason for that location, apart from confining any small cracks that might happen during the pierce, is that these starter holes are larger in diameter (for the reason given above) than the cut line once the jet starts to move, and that hole section would appear as a flaw on a final cut line.

Vanessa Cutler, in New Technologies in Glass discusses the process of cutting in more detail, but suggests that the starter hole be pierced at a lower pressure than that to be used in the cut. This is so that the pressure within the cavity will remain lower during the pierce, and insufficient to cause the glass to break. She suggests (and she has a vastly greater experience than I in this) that the piercing pressure be around 11,000 to 18,000 psi – this varies a bit with abrasive grit size, machine size and glass type.

Figure 2. Detail of the glass sculpture "p1", by Vanessa Cutler. (Note that these holes do not pierce all the way through the glass but all end at the same depth.)

She also recommends, when there are multiple cuts to be made on a sheet, that all the piercing holes be completed before any cutting begins. One of the reasons for this is to avoid constantly resetting the cutting pressure, which could be a problem, if you forget to lower the pressure back down before starting the next pierce. (Would I as an Emeritus Professor ever be that absent-minded? Why else bring it up?)

You will notice, with abrasive cutting into glass, that there is not the belling at the bottom of the cut that there is with plain waterjet cutting, and that the hole tapers with depth, as the cutting effectiveness reduces with the fall in pressure with depth, and the jet is less able to cut into the side walls of the opening at these lower pressures.

Stepping back from the cutting of glass to the more general condition where the jet runs out of power at the bottom of the hole, the main reason for this is the conflict between the water in the fresh jet coming into the hole, and the spent water trying to make it out of the hole at the same time.

One way of overcoming the problem is to interrupt the flow of water into the hole. Back in my grad student days we tried doing this by breaking the jet into slugs, so that one slug would have enough time to travel to the bottom of the hole, cut a little, and then rebound out of the hole, before the next slug of water arrived. There was relatively little sophistication in the tool we designed to do this. Simply it was a disk, with holes drilled in it at an angle.


Figure 3. Interrupter disk placed in the path of a continuous jet. (My PhD Dissertation)

The reason for the angled holes was to make the disk self-propelling as it rotated under the jet, since the angled edges of the hole forced the disk to continue rotating once started. (On a minor note the disk would rotate at several thousand rpm, and the noise that it made was loud enough that I was instructed to only carry out the tests after the staff had left for the evening).


Figure 4. The penetration of a waterjet into sandstone with the jet running continuously (black), with the jet interrupted (red) and with the jet rotated slightly off-axis (green). (Brook, N. and Summers, D.A., "The Penetration of Rock by High Speed Waterjets", Intl. Journal Rock Mechanics and Mining Science, May, 1969)

As can be seen in figure 4, with the pulsating jet more energy was getting to the bottom of the hole, without interference, and the hole continued to deepen over time. However the interruption tool had a number of disadvantages, apart from the noise and that the disk would be very rapidly destroyed under an abrasive jet. It was wasting a significant portion of the energy, in a more optimized design, that I won’t discuss further, the energy loss was about 50%.

But if the jet was moved slightly over the surface, and in these early tests the easy way to do this was to have the target rotate with the jet hitting the rock just offset from the axis of rotation. (At the time high-pressure swivels weren’t yet available). This gave the upper curve in figure 4, and a much more rapid penetration of the target.

In more modern times the nozzle is moved, either by causing it to move slightly around the hole axis, or by causing a slight oscillation or “dither” in the nozzle while the pierce is taking place. This is generally a feature of the control software that drives the cutting table. But the reason for the movement is to get the water flowing in such a way that the water going out of the hole does not interfere with that going in, and so there is a reduced risk of pressure build-up in the hole, with the consequent cracking that this would cause.

Waterjetting 9c - Starting to make hole

This small sequence of posts describes the initial milliseconds during which a high-pressure waterjet penetrates into a target material. Because this work was largely developed using rock targets, most of the illustrations will be with that material, but the concept apples, to a degree, also with abrasive laden jets penetrating into materials such as glass.

For this post I am going to discuss just what happens with the jet being fired down onto the target surface, without either the nozzle or the target moving. Much of this work was carried out in the 1960’s in the UK, though I will begin with some tests that Dr. Bill Cooley carried out using his modification to a Russian hydraulic cannon that he redesigned so that it was capable of firing at 500,000 psi – and yes I have seen it fired at that pressure (I took the photo).


Figure 1. The Cooley Cannon ready to fire at 500,000 psi in an underground mine.

In order to see how effective different processes were in cutting into different materials the international scientific community that was developing waterjet technology at the time needed a method to compare the different approaches. The metric that was used was to define the Specific Energy as the amount of energy that it took to remove a unit volume of the target material. (And in time that will be subject of some more specific posts).

Bill’s cannon used stored gas that was suddenly released as a way of driving the water at the desired pressure, and measured the pressure indirectly by timing the break of two pencil leads in front of the nozzle. This gave the jet velocity, and pressure could be back-calculated from that value.

Dr. Cooley took results from his work and from other scientists working with similar devices, to produce the following graph.


Figure 2. Specific energy as a function of the impacting jet length, measured in nozzle diameters. (Cooley, W.C., "Correlation of Data on Erosion and Breakage of Rock by High Pressure Water Jets," Chapter 33, Dynamic Rock Mechanics, ed., G.B. Clark, 12th Symposium on Rock Mechanics, University of Missouri-Rolla, November, 1970, pp. 653 - 665.)

For those running a conventional cutting table the water orifice is around 10 thousandths of an inch in diameter. So what this graph is saying is that once the first thousand diameters of length (1000 x 0.001= 10 inches) has hit the surface, then the process starts to become significantly less efficient. If the jet is moving at 2,000 ft/sec that length arrives in around 0.0005 seconds. Why this rising inefficiency after that time, and how do we get around it?

Earlier in this series I mentioned that one of the tests to find the pressure at which a waterjet penetrates a target is to note the point at which, instead of the water hitting the surface, and flowing along it, it changed direction to flow back towards the nozzle. This is because, as the jet penetrates it makes a hole, and the only way out of that hole is back along the way the jet came. Unfortunately there is more water still coming down into the hole, and so the water leaving the hole (at the same volume flow rate) meets the water coming into the hole. The rapidly moving water going out is moving about as fast as that coming in, and so, as the hole gets deeper, so the pressure at the bottom of the hole gets less. This has been measured by a number of folk, but Dr. Stan Leach was the first, and produced this plot:


Figure 3. Depth at the bottom of a hole, as a function of the incoming jet pressure. (Leach, S.J., and Walker, G.L., "The Application of High Speed Liquid Jets to Cutting," Philosophical Transactions, Royal Society (London), Vol. 260 A, 1966,pp. 295 - 308.)

Because the holes were preformed of metal (to hold the transducer) and were sized to the nozzle diameter, this is not, as it turns out totally accurate, although it illustrates the problem.

It isn’t totally accurate because, as the illustration from the last two posts showed, the erosion occurs initially around the edge of the jet, rather than under it, and thus the hole created is about twice to three times the jet diameter, rather than being of the same size.


Figure 4. Damage pattern around the impact point of a 10,000 psi pressure, 0.04 inch diameter jet on aluminum, target close to the nozzle.

Nevertheless as the hole deepens the pressure at the bottom of the hole gets less, and after a while the jet penetration slows to almost a halt.


Figure 5. Penetration as a function of time (My Dissertation)

The sides of the hole, however, continue to erode, but from the bottom upwards so that, after a short while, the narrower entry hole starts to constrict the flow out, and pressure begins to build-up in the hole.

Remember that a waterjet works by growing existing cracks in the material. So that if there is a natural crack in the rock, which may be as small as a grain boundary, or the scratch made by an abrasive particle as it moves back out of a hole in glass, then the water entering that small crevice will pressurize the walls and cause the crack to grow. Often there is more than one, and the result can be, in rock:


Figure 6. Rock breakage around the jet impact point on a 1-ft block of sandstone (after Moodie and Artingstall Moodie, K., Artingstall, G., "Some Experiments in the Application of High Pressure Water Jets for Mineral Excavation," paper E3, 1st International Symp on Jet Cutting Technology, Coventry U.K., April, 1972, pp. E3 25 - E3 44.)

In rock that might not be such a bad thing, since in many cases the intent is just to break the rock out of the way, so that a tunnel can be created that folk can walk or drive through. But in the case of glass and other such brittle materials, where the object is just to make a very fine cut, with no side cracks, cracking the sheet is disastrous. This can be illustrated by the results when a jet was fired along the central axis of a 2-inch diameter core of granite. The escape of water into the cracks allowed the cycle to repeat several times, and the hole was, as a result, much deeper than it would have been if the cracking had not occurred.


Figure 7. 2-inch diameter granite core that split when a short jet pulse was fired into the core, along the axis.

And so, next time, I’ll write about some of the ways in which we can get around this problem.

Waterjetting 8b - Repairing concrete

Some years ago we were on a bridge in Michigan, working on a demonstration of the ability of high-pressure jets to remove damaged concrete from the surface of the bridge. Before the demonstration began the state bridge inspector walked over the bridge armed with a length of chain. He would drop the lower links of the chain against the concrete at regular intervals, and depending on the sound made by the contact, would decide if the concrete was good, or not. He then marked out the damaged zones on the concrete, and suggested that we get to work and remove those patches.


Figure 1. Automated removal of damaged concrete from a bridge in Michigan

The change in the sound that he heard, and used to find the bad patches in the 1concrete, was caused by the growth of cracks in that concrete. It was these longer cracks, and delaminations in the concrete that made it sound “drummy” and which identified it as bad concrete.

Now here is the initial advantage that a high-pressure waterjet has in such a case. The water will penetrate into these cracks. As I mentioned in an earlier post, water removes material by growing existing cracks until they intersect, and pieces of the surface are removed. The bigger the cracks in the surface, the lower the pressure that is needed to cause them to grow. This is because the water fills the crack, and pressurizes the water, the longer the crack, the greater the resulting force, and thus the greater the ease in removing material.

At an operating waterjet pressure of between 11,000 and 12,500 psi, for a normal bridge-deck concrete, the cracks that are long enough for an inspector to call the bridge “damaged” will grow and cause the damaged material to break off. The pressure is low enough, however, that it will not grow the smaller cracks in “good” concrete, which is therefore left in place.


Figure 2. Damaged area of bridge after jet passes.

In order to cover the bridge effectively and at a reasonable speed, six jets were directed down from the ends of a set of rotating crossheads, within a protective cover. The diameter of the path was around 2 feet, and the head was traversed over the bridge so that it took about a minute for the head to sweep the width of a traffic lane.


Figure 3. Scarifying jets, with the head raised above the deck so that their location can be seen. Normally the nozzles are positioned just above the deck, so that the rebounding material is caught in the shroud.

Unfortunately, while this means that the rotating waterjet head could distinguish between good and bad, and remove the latter while leaving the former, it could not read marks on concrete. So where the bridge inspector was not totally accurate, the jet removal did not follow his recommendations. It was, however, quite good at removing damaged concrete from reinforcing bar in the concrete, where the water migration along the rebar had also caused the metal to rust. And, since the pressure was low enough to remove the cement bonding, without digging out or breaking the small pebbles in the concrete, they remained partially anchored in the residual concrete. As a result when the new pour was made over the cleaned surface, the new cement could bond to the original pebbles, and this gave a rough non-laminar surface, which provided a much better bond than that left had the damaged material been removed mechanically with a grinding tool.


Figure 4. Rebar cleaned by the action of the jet as it removes the surrounding damaged concrete.

Waterjets had an additional advantage at this point in that, in contrast with the jackhammer that had previously been used to dig out the damaged region, but which vibrated the rebar when it was hit, so that damage spread along the bar outside the zone being repaired, with the jet action there was no similar force, so that the delamination was largely eliminated.

Now this ability to sense and remove all the damaged concrete is not an unmixed blessing. Consider that a bridge deck is typically several inches thick, and it is usually sufficient to remove damaged concrete to a point just below the top layer of the reinforcing rods. Once the damaged material is removed, then the new pour bonds to the underlying cement and the cleaned rebar. But the waterjets cannot read rulers either. So in early cases where the deck was more thoroughly damaged than the contractor knew at the time that the job began, the jet might remove all the damaged concrete, and this might mean the entire thickness of the bridge deck. And OOPS this could be very expensive in time and material to replace.

What was therefore needed was a tool that still retained some of the advantages of the existing waterjet system, that it cut through weakened concrete, and cleaned the rebar without vibration, but that it did so with a more limited range, so that the depth of material removal could be controlled.

There was an additional problem that also developed with the original concept. For though the jets removed damaged concrete well in this pressure range, the jets were characteristically quite large (about 0.04 inches or so). The damaged concrete is contaminated with grease and other deposits from the vehicles that passed over it. Thus any large volumes of cleaning water would also become contaminated, and, as a result will have to be collected and treated. That can be expensive, and so any way of reducing the water volume would be helpful.

The answer to both problems was to use smaller jets at higher pressures. Because of the smaller size, their range is limited, and at the same time the amount of water involved can be dramatically reduced. It does mean that the jet is no longer as discriminatory between “good” concrete and “bad.” This is not, however, a totally bad thing, since when working to clean around the reinforcing rods, there has to be a large enough passage for the new fill to be able to easily spread into all the gaps and establish a good bond.

Thus the vast majority of concrete removal tools that are currently in use are operated at higher pressures, and lower flow rates. This allows the floor to be relatively evenly removed down to a designated depth, and this makes the quantification of the amount of material to be used in repair to be better estimated, and the costs of disposal of the spent fluid and material to be minimized.


Figure 5. Scarified garage floor showing the rough underlying surface. This will give a good bond to the repair material, as will the cleaned rebar.

The higher pressure system has the incidental advantage of reducing the back thrust on the cutting heads, so that the overall size of the equipment can be reduced, allowing repair in more confined conditions.

Waterjetting 2b - crack growth and granite sculpture

The last post in this series showed that the main way in which waterjets penetrate into materials is by growing cracks that already exist within the material, and I used glass as an example to show that this was true.

It is this process during which water penetrates into cracks, and then comes under pressure, either by the impact of more falling water (say under a waterfall in nature) or because the water freezes and then thaws, that causes the cracks in the rock to grow under natural attack, and the rock to slowly erode. As this happens the cracks slowly grow and extend to the point that they meet one another, separating small pieces of rock from the solid.

Within the body of a piece of rock the largest cracks that exist are normally at the boundaries of the grains of different minerals that make up the bulk of the rock. (Back in 1961 Bill Brace showed that the strength of a rock reduced as the square root of the increase in the grain size of that rock ).( Brace, W. F. (1961): Dependence of fracture strength of rocks on grain size. Bulletin of the Mineral Industries Experiment Station, Mining Engineering Series. Rock Mech. 76, 99± 103.) More recently, though still back in 1970, my second grad student, John Corwine, showed that it was possible to predict the strength of a block of granite, knowing the size of its crystals.

Which makes a good time to tell a little anecdote. Back when I was doing my own doctorate at the University of Leeds (UK) we were looking at how waterjets drilled through rock, and how that might be used to make a drill. We had already run some tests of different rocks that we placed under a nozzle, and gradually raised the pressure of the jet to see what pressure it took to make a hole in the rock. Tests on granite had shown that the jet (with a maximum pressure of just under 10,000 psi) would not drill a hole into those rock samples, and so the granite had been set aside. But, with the equipment just finished and yet having to go to lunch, I asked Dennis Flaxington, the lab technician helping me, to put a new sample into the rig so that we could run a test in the afternoon. When I came back I found that he had used a piece of granite. I made several disparaging remarks, at which point he noted that, having spent some significant time putting the rock in the apparatus, I should just go ahead and run the test (which normally took about 5 minutes) rather than being an unmentionable. And so we did, and as I posted earlier, this is the resulting hole in the rock, which we were now able to drill right through in a process that took about half-an-hour.

Figure 1. 9-inch thick block of granite drilled through by a 10,000 psi waterjet at Leeds University. It took over 30 minutes. (Summers, D.A., Disintegration of Rock by High Pressure Jets, Ph.D. Thesis, Mining Engineering, University of Leeds, U.K., 1968.)

How could this now work, when a single jet clearly did not penetrate into the granite in the earlier tests? The answer is that as we moved the rock under the nozzle (we were slowly spinning the rock under the nozzle, and then raising the rock, since at the time there were no high-pressure swivels available for us to use) the jet passed successively over the edges of the different crystals in the granite. As it entered and pressurized these small fractures, the pressure in the crack was enough to grow the crack and remove individual crystals along the jet path. By starting at the center, and taking successive passes around the axis a large depression was cut into the surface, and the rock could then be raised, and a second smaller layer removed. Repeating this slowly removed the rock in front of the nozzle, and at the end of the test we had drilled through 9 inches of granite.

From this experience, over time we went on to cut, for a University, a lot of granite. Obviously, to cut at a competitive rate we had to cut at a higher pressure that just 10,000 psi. But, after showing that we could cut Georgia granite at a competitive rate in tests run at 15,000 psi down in Elberton, Georgia, Dr. Marian Mazurkiewicz and I led a group of our students in cutting 53 blacks of that granite to form the MS&T Stonehenge that now sits on the University campus.

Figure 2. View of the Stonehenge at Missouri University of Science and Technology, the vertical blocks are some 11 ft tall. The entire sculpture was cut by high pressure water jets operating at between 12,500 and 15,000 psi. (MS&T RMERC ).

Cutting commercially is not quite as simple as it might appear, since larger blocks such as those shown in Figure 2 will contain rock that varies quite significantly in properties as the cuts progress. In the Stonehenge case the rock came from close to the top of the quarry, and the cracks in the rock were quite well defined. Some fifteen years later we were fortunate enough to be asked to cut a second sculpture, but this time working with the internationally acclaimed artist, Edwina Sandys. Edwina had designed a sculpture for the campus, the Millennium Arch, which required that we cut two figures from blocks of Missouri granite, and polish them to create one group, while using the original pieces as part of an Arch that would stand some 50 ft away.

Figure 3. The Millennium Arch at Missouri University of Science and Technology. (Each vertical leg of the Arch is some 15 ft long, and the figures removed and in the background, are 11 ft tall). Better images can be found here.

The vertical legs were first cut to shape, and then the figures cut out from them. In order to contain the crack growth to limit the amount of material removed the cutting lance had two jets inclined outwards and the lance was rotated at around 90 rpm, as the lance made repeated passes over the surface, removing between a quarter and half-an-inch of rock on each pass, until it had penetrated through the rock. It took 22 hours of cutting to isolate the female figure from the host block. The slot width was around an inch, and there was some significant difficulty in cutting this slot as the quality of the rock changed within the blocks being cut. (The problem was solved by raising the cutting pressure).

Figure 4. Partial cut for one of the figures of the Millennium Arch, checking the depth.

This second sculpture illustrates both an advantage and a problem for the use of waterjets in cutting rock pieces. Use of the water gives a relatively natural look to the rock, although the vertical surfaces of the arch and the capstone were all actually “textured” to look natural using a hand-held lance at 15,000 psi. (The rock is a little harder than that from Georgia and most of the cutting took place at around 18,000 to 20,000 psi). But when the polished surfaces for the inside of the verticals and the isolated figures were prepared the rough initial surface required much more time to grind and polish flat, than a smoother initial cut would have needed.

Because water alone penetrates along crystal and grain boundaries in the rock the surface left is relatively rough. This gets to be even more of a problem if waterjets are used to cut wood. Here the “grain” boundaries are the fibers in the wood structure. Thus when a relatively low pressure jet (10,000 pai) cuts into the wood, it penetrates between the fibers and the cut quality is very poor. One of the first things I have asked students to do, when given the use of a high pressure lance for the first time, was to write their name on a piece of plywood. Here is an example:

Figure 5. Student name written with a high-pressure jet into plywood. Note that areas of the wood around the jet path are lifted by water getting into the ply beneath the surface layer, and that part of the top ply between cuts is removed in places.

I thought about having you guess the student name, Steve, but this is one of the more legible ones. (Female students generally cut the letters one at a time and were more legible, male students tried to write the whole name at once).

There are many similar examples that I could use to illustrate that, while there are tasks where waterjets alone work well, when it comes to precision cutting, then adding a form of sand to the jet stream to provide a much more limited range to the cutting zone can give a considerable advantage, and so the field of abrasive waterjet cutting was born, and discussion of that topic will lead, in time, to a whole series of posts.

Waterjetting 2a - growing cracks and glass cutting

High pressure abrasive waterjets (AWJ) are able to cut glass with considerable precision, and maintain the accuracy of cut through thick material.

Figure 1. Cutting an Eagle from glass using AWJ (courtesy of KMT)

Because of this precision, and because the glass can be cut to leave very delicate webs between adjacent cuts, AWJ glass-cutting has been used to create art objects for a number of years. It is not as easy as it might at first seem, and Dr. Vanessa Cutler an international leader in advanced cutting, has used the tool to create significant works of art, She has also written on the problems that can arise in cutting what often seems to be a simple, consistent material. (Noting, in passing that through the combination of computer control and memory it is easier at times to re-create works that break than would be the case with other tools for artistic creation). For the more mundane cutting world that comprises the rest of us, cutting glass is more likely restricted to simple activities such as cutting the parts for the windows of wood furnaces.

When the cuts are this simple, time can be saved by stacking two or more sheets of glass, one on top of the other, and cutting all of them at the same time. As one learns the parameters, thicker and greater numbers of plates can be stacked, and still successfully cut.

Figure 2. Cutting through four sheets of glass simultaneously (courtesy of KMT)

However, if one gets too ambitious, and stacks too many plates then the lower plates may start to crack, often after the cut has started into the plate. As Dr. Cutler has noted in her new book “New Technologies in Glass”, cracks can also create problems for the unwary in dealing with internal stresses in the structure of the glass.

There can be several reasons for this, but it primarily goes back to the point I made in the introduction, about water pressure causing existing cracks and weakness planes to grow, as a way of removing material. There are two sorts of cracks that exist in glass, those created by the impact of the abrasive particles themselves, and those that were already present in the glass.

Figure 3. Micro-photograph showing cracks growing out from the point where two abrasive particles struck a piece of glass. (This was adjacent to the main cutting path).

Figure 4. Micro-photograph of the edge of the main cut by an AWJ on glass, showing that it is made up of the intersection of adjacent cracks created by the abrasive impact.

I’ll write about the mechanics of cutting glass in a later post or two, but for the moment I would like to write about the basics of crack growth from the point of view of cracks that already exist in the material. In large part this won’t be using waterjets alone to cut glass. Rather there are lots of other materials, particularly soil and rock, which have much higher crack densities, and longer cracks which make it easier to cut and remove material.

But first a little demonstration you can carry out. Take a strip of paper, and cut a slit in it half way along the strip and half way through the paper. Now take both ends of the paper in your hands and pull them apart. This causes the cut (the crack) to grow through the paper and gives you two halves. If you do this a second time you should find that by stopping moving your hands you can stop the cut from growing all the way through the paper. Now repeat the process, but use a piece of paper that you have not cut a slot in. The amount of force you need to pull the paper apart is much higher, and I seriously doubt that one you get the tear (crack) to start that you can stop it before it goes all the way through the paper. (Remember this, and I'll come back to it a bit later in time).

The idea of putting cracks on the edge of packages to lower the force you need to tear them open can be found on the edge of lots of candy bars, packs of peanuts and other goodies in stores. The serrated edge acts as a series of cuts or cracks, that concentrate the force applied when you pull on the edges of the packet so that the package tears at a much lower force, and you can control the tear so that you don’t end up throwing all the contents around the room.

Figure 5. Serrations and tear at the top of a packet of honey

Now at this point you might say that there aren’t any cracks in glass, when we start to cut it. If the glass is very new this is true, but with all the chemicals in the air, and the dust that is carried in the wind, although glass can look clear, the surface actually contains a lot of very fine cracks.

John Field, one of the earlier investigators of high-pressure waterjet impact, showed this in one of those brilliant, yet simple demonstrations that, in this case, he carried out some forty-five-odd years ago. If waterjet impact grows surface cracks, and glass acquires surface cracks from damage through being out in the air, then if that surface layer is removed, then the underlying glass will have no cracks. So John took a glass slide, and etched off the surface of the lower half of the slide, by immersing it in acid. Then he fired a very high-speed droplet of water at the point on the slide where the acid etch stopped.

Figure 6. Impact of a high-speed droplet of water on glass. Above the dividing line the glass surface contains the micro-cracks and flaws that come with being exposed to the air over time. The lower section below the line has had these flaws removed. As can be seen the cracks only develop in the unetched part of the glass, where they grow pre-existing cracks, even into the side of the glass that was etched. (Field J.E. “Stress Waves, Deformation and Fracture Caused by Liquid Impact,” Phil. Trans. Royal Society, 260A, July 1966, pp. 86 – 93.)

In a single picture he captured the evidence that waterjets work by growing cracks (top half), and that without cracks there is no damage (bottom half). Understanding this opens up a whole vista of different applications, from the removal of soil from around pipelines underground (the new technology of hydro-excavation) to the removal of damaged concrete, while leaving healthy concrete in place (the developed field of hydro-demolition). And these, and other topics will be part of this series, as it moves forward.

But as John showed, not all the cracks a jet will grow can be seen, and as Vanessa found, they don’t have to be at the surface to create problems. One of her early pieces was entitled “p1.” Within it are an uncountable series of holes, drilled deep into the glass.

Figure 7. Detail of the glass sculpture "p1", by Vanessa Cutler.

One of the skills Vanessa has learned is in controlling the quality of the pierce, and its dimension, but initially there had to be a period of learning.

Figure 8. Single cracks growing out from partial piercings in a test piece during development (Vanessa Cutler).

And so, in the next sequence of posts the simple idea of growing existing cracks will be explored. Mainly, in the beginning, this will focus on cracks that are already there, and how to usefully make them grow. But in some cases we don’t want all those cracks to grow, and that will also come up, as this series continues.