Waterjetting 34d - Drilling small holes through steel and concrete

In the earlier posts in this section I have concentrated more on cutting the profiles of a hole, and the different ways in which this can be done efficiently. However there are many cases where the hole has to travel into the work piece to a depth greater than can be easily achieved without the nozzle entering the hole, or where the hole is being drilled into something with no realistic “other side.”

In these cases, choice of the best strategies to remove the central core of material from the hole varies, depending in part on how large the hole is, and in what sort of material. For relatively small diameter holes, then the jet itself can drill a hole large enough for the head to enter the target. (In part this is because the diameter of the nozzle and supporting pipe can either be made quite small, or the jet can be made more diffuse.) In other cases the cutting head has to be designed to remove that central volume in a way that allows the fragments to pass the nozzle assembly as it feeds down the hole.

To illustrate the first approach consider some work that took place at the University of Alabama-Huntsville. The tool that was needed had to drill down through the reinforced concrete of bridge abutments and foundations to give access to tools that would evaluate their condition, and that of the surrounding ground. However putting large holes in such structures is, in itself, a potentially weakening process and can provide a path for subsequent corrosion. The tool developed therefore was reduced in size to around 1/4th inch in diameter, capable of drilling holes of around 3/8th of an inch in size through the reinforced concrete, using a Direct Injection of Abrasion or suspension jet, operated at 5,000 psi.

Figure 1. Portable abrasive suspension injection system. Operates at 5,000 psi (pump is on the left of the platform). This unit fits in the bed of a pickup truck and cost less than $10,000 for the MS&T HPWL lab to build. The device built in Alabama cost roughly $14,000). The tank at the front mixes the abrasive with polymer and water before feeding it into the two supply tanks that alternately supply the feed line to the nozzle. The polymer is used to keep the abrasive in suspension and not to settle back out of the suspension.

The University team coined the name Multi-Intrusive Testing, (MIT) for the method and showed that it was capable of driving holes more than 3-ft deep, and of the required small size. In this case the use of a suspension jet, where the abrasive is added to the high-pressure water upstream of the nozzle assembly means that there is only a single feed line going down-hole. This is one of the features that allows the operational size of the tool to be reduced.

As existing infrastructure (bridges, roads etc) exists through the changes of typical yearly weather cycles the concrete and surrounding materials will slowly deteriorate, and erosion over time can remove the support for a bridge, corrode the internal components and wear away critical parts of the overall assembly. The, fortunately relatively infrequent, collapse of major bridges shows the risks of leaving this damage without repair, yet the very massive nature of many of the structures makes it difficult to detect this damage before it reaches critical size. Hence the need for the tool.

Laboratory tests showed that the rig could drill holes up to 3-ft deep, with the drill, which was rotated at speeds between 60 and 120 rpm, being capable of drilling through not only the concrete, but also the rebar embedded within it.

Figure 2. Hole drilled through reinforced concrete showing the ability to penetrate the steel reinforcing within the concrete. (after Graettinger et al ibid).

Within the geometry of the very small holes of this type, which are required to minimize damage to the structure, it is difficult to offset the nozzle to a sufficient angle that the hole is large enough for the nozzle to pass. In most cases this needs to be at an angle greater than 12 degrees, since shallower angles tend to have the abrasive and jet rebound into the center of the core without fully cutting into the wall to a depth to ensure that the hole diameter is maintained.

One way of overcoming this problem is to force the jet to diverge as it leaves the nozzle, since the induced spreading (typically 15 degrees or more) not only ensures that all the material ahead of the jet is removed, but also allows the wall diameter to be maintained.

The problem with the design, as with many similar drilling applications for waterjet and abrasive waterjet systems, is that the jet cuts to the required diameter at some distance ahead of the nozzle body itself. This can be illustrated with a picture of a diffused waterjet nozzle, that had been used, at MS&T, to drill through a sheet of steel (representing a borehole cased section), a layer of concrete (which would act as the sealing element in wells drilled to recover oil and natural gas for example) and then out into the sandstone rock beyond.

Figure 3. Dispersed abrasive jet (not rotated) used to drill through the simulated borehole wall. Note the two black lines that define the edges of the hole as it cuts into the steel plate.

Although the hole diameter was acceptable for passage of the tool once the steel was penetrated, as with the rebar in the earlier example, the hole diameter through the steel was too small to allow the head to progress. The answer, as has been discussed in earlier posts, is to advance the shroud (the outer cover over the nozzle) forward until it touches the edge of the cutting jet diameter at the required size. At this point, should the jet not initially cut the hole to the required size, then the drill will stop advancing, but the jet will be in contact with the obstructing material – and in a short interval will penetrate through it allowing the jet to drill further forward.

Difffusing the jet and removing the material will work for hole diameters up to about four inches, and we have used such a technique to drill through gravel and similar beds of loose material (with care, since while the jets have little disturbing force on the material around the hole much vibration can cause that material to destabilize and continually collapse into the hole being drilled – which ends up defeating the object of the exercise). I’ll talk about approaches for larger holes next time.

Waterjetting 19d - waterjets and material removal

When miners first began to use water as a means of moving soil and ore from valuable deposits at, or near the surface, stream flow, or the energy from stored volumes of water was the main source of the power used. However, at these low pressures it was necessary to use large volumes of water (often in flows of over a thousand gallons a minute) to move the material. At the same time, even at these large flow volumes, the flow has to be confined in order to ensure that there is enough water around the particles to keep them in suspension. There is also a certain amount of turbulence required in this flow to retain the suspension and to stop the particles from settling out before the riffles where the gold or other valuable minerals can be collected.

As pressure is increased these jets can be increasingly productive, and can be used for a variety of functions, including the rapid removal of soil. Raising the pressure allows the tool to be used in mining soft rock. As an example there is a layer of relatively soft sandstone that lies in a roughly horizontal layer and appears along the banks of the Mississippi river and can be found, for example, under Minneapolis. There is a sand mine that lies along the banks of the river, that has a high-grade sand that can be used for making glass, but which also has a vein within it that has larger grains that can be used in the fracking stage of increasing oilwell production. However, with conventional mining (using blasting) it was not economic to screen the sand after it had been mixed during mining. The Bureau of Mines of the time (since closed) tried some experiments to see if a waterjet could be used to “high-grade” the sand, mining the larger layer first, and then that surrounding it to let machines and men progress further (and also to produce the sand for glass making).

Figure 1. Bureau of Mines experiment washing out sandstone, with 4,000 psi water and cutting about 9 ft. deep. Note the yellow color of the water as the sand settles out but the clay contaminant is suspended in the water and washed away.

The experiment, carried out by Dr. George Savanick’s team, was successful, and had the unexpected advantage, since the grains were all separated, of stripping the small amount of clay contained in the sandstone and carrying it away with the water, while leaving the sand on the floor, where it had to be picked up mechanically.

The use of pressurized water is used both to mine sand, and also, in Cornwall, for example, in the mining of clay, although the pressure and volumes needed are a function of the quality and amount of weathering that the clay has seen.

Figure 2. Mining clay in Cornwall (Pathe video here)

Engineers have even accelerated the movement of landslides, using clay mine pumps, in order to move the soil away faster and allow the slide to be remediated. At Dawlish in the UK, for example, railway engineers have added water under pressure to remove the sliding soil as a slurry, making clean-up faster, safer and less costly.

Figure 3. Moving soil in a landslide that has covered the railway line at the bottom of the slope, Dawlish UK March 2014. (The Packet)

A jet was also used from the bottom of the slide to liquefy the soil, which then flowed through the railway path and into the sea.

Figure 4. Removing the soil from the landslide from above the railway. (From a video at The Packet)

As a comment, for those who watch the video that the above picture was taken from, the jet cuts much more effectively closer to the nozzle, and had they used it to undercut the bank they could slurry the soil lower down, and have removed the material a bit faster than dispersing most of the jet energy in the air as they tried to reach the back of the slope. (If they had undercut the bank then the soil would have slid down towards them shortening the reach and speeding the process).

During the Second World War engineers also used water jets to uncover land mines that had been planted on beaches along the coast. (video from Pathe here ).

However the control of the water, and debris, can quite quickly become a problem, and containing the water and keeping the soil/sand particles suspended in it requires more preparation. As an illustration the civil engineers who work under Minneapolis are aware of the benefits of using higher pressure waterjet streams in driving tunnels (and occasionally rooms) under the city. For example, in driving a sewer tunnel (the St Anthony Park Storm Sewer extension) the engineers set up an extensive train behind the tunnel face, so that the resulting slurry could be pumped out of the tunnel.

Figure 5. Train behind the tunnel face, required to supply the jets and to pump the slurry from the excavation (after Nelson*)

The tunnels can be driven by two main jet operators at speeds of up to 120 ft per day () with a third jet being used to break the larger pieces down to slurry so that it can be pumped down the tunnel to the river, where it is barged and sold as glass-making sand.

Figure 6. Driving the storm sewer tunnel under Minneapolis. Two operators are carving the face into small pieces and the third is slurrying the sand (after Nelson*)

By feeding the sand and water into a channel cut into the floor of the tunnel it was possible to confine it, so that the blocks could be broken up more easily, and this also then provided a catchment for the intake to the slurry pumps.

The tunnels were pre-cut along the profile, so that the arch girders that provided support could be slid into place with the central core of rock still there to support them. This made the support easier to install, and provided immediate support of the working area ahead of the place where the miners were working.

(That ability to use waterjets to penetrate ahead of the tunnel and allow a support to be installed before the main core of the tunnel rock is removed has since been improved, and in the Advanced Austrian Tunneling Method high pressure jets drill out cylindrical bores along the profile ahead of the tunnel, which are filled with a grout that can be generated partially using the surrounding material as the holes are drilled. But I will talk about the use of cement mixed with the jets in a later post).

*Nelson C. Tunneling under Minneapolis, Water jet Workshop, Rolla, MO 1975.