Waterjetting 34e - Hole completions and core removal

When I began writing about hole cutting and drilling, a month ago, I was intending to talk just about the relative efficiencies of cutting the core into larger pieces, rather than designing a cutting pattern that would completely cover the surface of the excavation, milling and removing the core in fine particles. Other topics intruded, however, and it is only now that I am going to conclude this theme by discussing that particular point.

Earlier posts have discussed how, by inclining two jet paths so that they come into close proximity within the target (or intersect in some cases) a much larger volume can be removed with no increase in input energy to the process. The gain in production comes from working out which are the best angles to set the cutting jets at, relative to the overall work piece.

Figure 1. Intersection of two jets in cutting clay. The cuts were made with the sample lying horizontally – in the actual operation the jets cut up and down vertically and the included wedge would normally fall out.

There is no universal rule for selecting the best angle for this, or for selecting the best relative depth for the intersection. It depends on the material being removed, and on the logistics and relative sizes of the hole being driven, and the components that fit into it. The material responses depend very much on the strength and structure of the material. The paths of both jets will have to intersect to remove materials such as steel, whereas with clays, and many rocks the two paths need only be relatively close at their lower end for the intervening rib to separate.

The problems are not just constrained to the removal of that core. One of the significant problems that exists even when using conventional techniques to drive tunnels and other large holes is that of making sure that the diameter that is being cut remains the same size as the tunnel advances. Because it is easier to break the rock within the tunnel wall, because of the release of the surrounding rock pressure, it requires additional effort to cut out beyond the projected perimeter in order to give enough space for the tunnel. With explosive blasting of the tunnel this means that the perimeter holes are drilled out beyond the projected tunnel line, and in cutting with a waterjet a similar strategy is required.

This problem is not normally that severe, since the size of the nozzles and support equipment are not that much larger than the jet that does the cutting, but, when the jet is cutting at the edge of the excavation the jet will need to be inclined outward by an angle of somewhere around 20 - 25 degrees in order to cut clearance.

An additional problem arises with the need to break the pieces being removed from the solid into small enough fragments so that these can be moved out of the way and into a transport line, so that the cutting head can continue to advance. In the case of the Soil Saw (for which Figure 1 showed one of the earlier test cuts) the nature of the clay was such that, once it was broken from the solid it disintegrated relatively easily and could be moved. Had this not been the case the cutting tool could not have passed without cutting the piece into smaller chunks. And in this regard, once a piece has been broken from the solid and is floating in a suspension within the cut, it becomes much more difficult to cut, since it can be deflected away from the jet before the full force of the jet can cut into it.

Further most materials are not as friable as clay – particularly those that are manufactured , and the pattern of cuts has thus to be designed so that the fragments are positively cut to the right size, to make sure they fit through the various gaps and feeds. For most of our work this meant that the pieces should be smaller than walnuts, and usually of around half-an-inch in size.

Ensuring that these cuts intersect in materials of varying properties will often require that the jets be designed to overcut in more favorable conditions, which wastes considerable energy. The alternative is to use the jets to cut relieving slots into the target, but to ensure that all the material is removed to the required depth on a pass by also including a mechanical component to the cut surface.

A cutting head designed by Rogaland Research shows the type of design required to achieve this, illustrating the angles of the two jets that will cut into the target as the head moves around the hole. In this case the design is to fit into a large diameter drill pipe to create a larger overall hole size.

Figure 2. The cutting head design developed by Rogaland Research (Vestavik, O.M., Abrasive Water-Jet Drilling Experiments, Progress Report, Rogaland Research, Stavanger, Norway, May, 1991.)

In this case the ribs of rock that are isolated by the jet cuts are removed by the action of the mechanical cutters in the second part of the bit.

Figure 3. Location of the jetted slots in the face of the drill-hole using the Rogaland tool. (After Vestavik)

In many cases the combination of a waterjet action to provide a free surface for the material to break to, and to relieve some of the confining stress on the material within the hole can significantly lower the mechanical forces required to break out the material. In such cases it makes much more sense to combine a waterjet action with that of a second removal device (which can be mechanical or thermal in some cases) to obtain a much more efficient combined system than that which would otherwise be the case. Where such systems have been used they have been shown to be more efficient in a number of cases than either of the component systems alone.

Unfortunately combining two systems to achieve optimal performance is not as easy as just merging the two sets of components, since there are additional benefits that come where the combination is further optimized so that the two parts work synergistically together. (One of the factors not included in the Rogaland design). I have written of this in the past, and will again soon.

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.

Tech Talk - of grouting, ground freezing and answers at Fukushima (and a gentle cough to PBS)

One of the reasons that I started to write blogs was to help folks to understand some of the technical background that fed into corporate decisions. I was watching the PBS Newshour this past week, and they were discussing the change in philosophy at the Fukushima nuclear plant. After trying to seal the flow of groundwater using a grout wall, the new plan is, instead, to spend some $470 million and build an ice wall.

The problem that I had with the broadcast was that the person appearing to explain the change in philosophy seemed to me to be more concerned with spreading fear and confusion, than in explaining the fairly logical engineering decision to change from one technique to another. And so, with some repetition from earlier posts I thought to explain why the change, and why it is logical, and not a highly dramatic concern.

The ground upon which the Fukushima plant sits is permeable, so that groundwater is continually flowing through it from rainfall in the surrounding countryside, which migrates down into the ground, and then flows down to the sea. At the time of the tsunami the basements in the reactor buildings were damaged, to the point that this groundwater can now enter the buildings. Unfortunately the reactors themselves were also damaged, so that water being used to cool the reactors can escape, and flow down into the basements. Here it can mix with the groundwater, contaminating it, and all the downstream regions to the sea.It has been suggested that there is about 100 tons of cooling water, and 300 tons of contaminated ground water currently flowing into the sea every day. It is not clear if this flow is into the immediately adjacent sea, or whether the contamination is getting into layers of bedrock, which don’t come to the seabed until some distance offshore.

The initial plan was just to rely on a chemical grout that would be injected into the ground to seal the passages in the rock/soil around the plant. I described, in an earlier post, how grouting can be used to seal off the water channels within rock, making it impermeable. At the same time, depending on the grout injected, it is also possible to add strength to the rock/soil so that it is better able to withstand loads. (Thus the rock over and around a tunnel might be grouted). To explain a little of that let me quote from the earlier post:

Figure 1. Drilling pattern used to inject grout around a tunnel line.

In a grouting operation, that is the goal. Normally a ring of holes are drilled into the wall of the tunnel so that they fan out around the planned tunnel path, and they are about 40 ft long. At this point the cement is brought to the site, ready to be injected. However, it is not just a case of bringing in say 8 bags of cement from the local hardware store. When dealing with the choice of cement, its physical and chemical contents and the pressure at which it will be pushed into the rock there are a number of factors that have to be established first.

The temperature and water chemistry of the surrounding rock are some of the initial critical factors. Changing either will change how fast the cement sets, or if it will. The object in this case is to get the cement to flow into the cracks around the drilled holes, so that the cement will flow to fill those spaces completely, before it sets. But it has to set in a reasonable time for work to continue on schedule. And since water chemistry in a tunnel, and temperature, change – so the mix has to be altered to accommodate those changes.


Figure 2. Types of grout used to meet different needs.

The next thing that has to be checked depends on the size of the cracks that the cement is being injected into. If the cracks are very thin, and the cement contains particles that are bigger than the crack, then the wall of the opening will act as a filter paper, stopping the cement particles from getting back into the crack and filling it. On the other hand if the particles are too small, then they will not bridge together to block the crack, and stop the fluid flow long enough for the cement to set up. If there are too many large particles then, when they lock together, they leave too large a gap between them, and fluid can still flow, and the rock will remain weak.

Over the years a rough correlation has been developed between how much fluid is flowing though the rock, and the type of rock, and the size of initial particles needed to provide an initial seal of it. But, as with cementing in an oil well, when the first cement injection has been finished, and allowed to set, then the rock is tested to see if the flows have stopped. Very often they have not, though hopefully they have diminished. This is because the first shot into the rock is more aimed at narrowing the flow passages and slowing the flow of fluid through the rock so that when finer particles are used, in secondary grouting, they won’t be carried away into the rock, before they can set up and block the remaining passages.

And so, typically, after the first grouting operation, there will be a second, to further fill the narrower passages in the rock, and those bits not properly sealed by the first injection. The cement grouts also act to give some strength to the rock, since they are filling the spaces within the rock structure with the set-up cement, that has some strength to it.


Figure 3. Rock after grouting (white lines)

However to fill the finer cracks, and to stop the flow cement may be too coarse a material in some of the rocks found. In such a case, then a chemical gel might be injected into the ground to fill those finer cracks. These tend to set up rather more like Jello, and while strong enough to resist water flow, do not usually give any additional strength to the rock.

There is one caution in injecting grouts into the rock that has to be borne in mind (and I know of cases where it wasn’t). Grouting operations force liquid into existing cracks within the rock surface. The liquid hopefully fills those cracks, before it sets, but if it is injected at too high a pressure, then the force on the walls of the crack can cause it/them to grow. At that point the rock will become weaker, instead of stronger, and the section in the tunnel roof/walls that is already open can fall in. Which is not good!

In the main it is not economic to keep injecting more and finer grouts into a rock until the flow is totally stopped. As the flows diminish the costs to stop them rise, and so it is usually the case that the operator accepts a certain small flow rate as the most economic alternative, and makes arrangements to deal with that water as it enters the tunnel. (If not I have seen tunnel floors lifted by the pressure that develops in the water trapped behind them).

Of course, if all else fails, then you can cut a slot into the wall and fill it with cement to completely seal off the excavation – though this is often done with a series of drilled holes, it can also be done with a variety of saw, known as a soil saw.

Figure 4. Grout wall exposed to show the 12-inch thickness and integrity.

In case the company of Fukushima the company chose to use a form of waterglass which forms a gel when exposed to an acid environment. This has the advantage that the fluid is quite mobile pre-gelling and can thus penetrate even the finer crack networks, and then, when it sets, it forms the seal.

Unfortunately there are a couple of problems with the grouting approach, there is no assurance that all the cracks will be intersected, and it is often necessary to re-inject successively finer grout materials into the ground in order to seal off systems of cracks missed in the earlier injections. The other is that the grout does not have a lot of strength, and if it is being injected into ground with a relatively high lateral flow rate, then the grout can be washed away before it can set.

In circumstances such as these, ground freezing has been an alternative that has been used for decades. It was, for example, used in the Dig Dig in Boston, being at the time the largest frozen earth retaining project in history. To describe that process let me quote from another post I wrote on the topic, back in 2010.

So how does it work? There are a number of different ways of going about the process, but I am only going to briefly describe a couple of them. The first is the more conventional approach, using a brine coolant, and the second is more commonly used when, for example, you’re refurbishing a road tunnel, and the roof suddenly collapses all the way to the surface. (The injection of liquid nitrogen).


Figure 5. Collapse of the Blackwall tunnel(Photos courtesy of Mott McDonald)

By completely freezing the gravel and other constituents of the roof and tunnel line, it was possible to restabilize and excavate through the area, putting in new supports at the same time.


Figure 6. RE-excavating the tunnel, showing the columns of frozen ground that stop water flow and form a wall to hold the ground in place.

Ground freezing can effectively form a temporary roof over an excavation, even if very close to the surface. It was used, for example, during the Big Dig, to create a bridge under the railway lines in Boston, while the new tunnel lining was pushed into place beneath it, using hydraulic jacks. It was also used in Vienna, where a subway had to pass relatively closely under existing buildings.

Figure 7. Ice wall as temporary support (image Joe Summers)

The conventional method of freezing involves inserting two sets of concentric pipes into the ground, inside pre-drilled holes. The outer pipe is sealed at the bottom end, so that as the freezing fluid (typically a chilled brine) is pumped down the inner pipe, and then flows back up the outside, it draws heat from the surrounding rock and soil, lowering the temperature until the water freezes. After circulation the brine returns to the refrigeration plant where it is re-cooled and re-circulated.

It is important to know the chemistry of the water in the ground, since with the wrong combination the water may not freeze at the expected temperature. It is also important that the outer pipe be sealed since if there is any leakage, then the brine may spill into the surrounding rock. At that point it can’t be frozen with the system, any longer, and an alternative method has to be used.

It is generally smart also, particularly when digging near the surface, to make sure that there aren’t any pipes (such as water and sewer) that can act as heat sources during the process. There can be embarrassing results if one of these (particularly the latter) is undetected, and the surrounding ground collapses as the shaft is then dug.


Figure 8. Hole in an ice wall (arrow) note that it depends on how the ground is being dug out, how fast this is detected.

Brine operations generally take a considerable time, and there is a project schedule so that you can get an idea (depending on the depth and size of the hole) of what might be involved in such an operation. Remember that the ice wall has to be kept cold during the excavation, but that keeping the central material unfrozen makes it easier to dig out. Cement poured against a frozen wall, if properly designed, will set as planned, since the heat of hydration overcomes the surrounding heat loss.


Figure 9. A ground freezing schedule.

This requires, obviously, a lot of preplanning. Where there is an emergency this is not possible, and thus the use of faster freezing methods, such as the injection of liquid carbon dioxide, or nitrogen, into the ground, in order to freeze and stabilize it more rapidly.

There are two ways of making the injection. There is the dual-pipe approach where the nitrogen is in a closed circuit, and then there is the simpler process where a lance is, simplistically, pushed into the ground and the resulting gas percolates upward from the end. That is a little less precise, given that the gas moves through the ground following the path of least resistance, but it does have the advantage of being quick, and generally effective in stopping an imminent disaster. Relative to the months of a brine installation the work takes a week or so (depending on size).

Copper freeze pipes with a standard diameter of 2” (54 mm) are installed, at an average distance of 2” (54 mm) On the inside, downpipes with diameters of ½” (10–12) mm are installed.

LIN is fed into the pipes through insulated supply lines. The LIN vaporizes, with 1 kg of LIN extracting about 200 kJ of energy from the surrounding soil, cooling and freezing it. The vaporized cold nitrogen (i.e., exhaust gas) extracts another 100 kJ from the ground. After about one week, this process forms a frozen wall with a diameter of about 1 m. This so-called “establishing phase” lasts four to seven days, and about 300 - 500 gal (1,500–2,500 l) of LIN is used to freeze 1 cubic yard (0.75 m³) of soil.

Notice from the photographs how the equipment can be fielded very quickly to provide emergence stability to the structure, and then other treatments can be used once the flow has stopped, and the structure is stable. One could, for example stop the flow using ground freezing, and then – using the Brown and Root Soil Saw (and a head design that vestigial traces of modesty suggests I don’t discuss) – a channel can be cut down through the permeable ground and filled (as cut) with bentonite, as shown in the figure above.