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.