Water jets offer precise cutting10 November 1999
Water jet cutting allows precise cuts to be made, even in damaged rock, but development work has been slow. David A Summers looks at current developments in this still-promising technique for underground work
Excavating underground space is becoming much more controlled, and blasting is giving way to using machines to cut the rock. Machines are effective in cutting an opening, but it is the edges and the corners which frequently require much more power and which cause the greatest wear to the tools. An alternative technology, in which high pressure water jets are used to cut the rock, can give considerable depths of cut without damaging the adjacent wall rock.
The technique has taken some time to develop: it was 27 years ago that the then British Hydromechanics Research Association hosted the First International Symposium on Water Jet Cutting Technology. A British paper concluded that the technology would be unlikely to benefit the construction industry, since it cost more than £100,000 (US$150,000) to develop a practical tool.
At that time the equipment available for generating water jets at high pressure was still being developed, and the necessary pumps, couplings, fittings and hoses were not available for long term reliable operation.
Since then the picture has changed. High pressure water jet pumps and associated equipment have become increasingly reliable. Equipment now costs more than the £100,000 which was the original barrier to progress, but its productivity routinely justifies the investment. It is increasingly being used for concrete demolition, and particularly for the removal of damaged concrete. The water jet industry is now a multi-billion dollar growth industry beginning to make a significant impact across a diverse range of applications.
The potential for underground work at hydro stations is undoubted — indeed one of the early areas where water jets were expected to have a great impact was in underground construction — but this application is now lagging. In recent years there has been more interest in developing applications above ground, without the restrictions that hamper underground construction.
Two recent projects illustrate some of the benefits of water jet cutting. The first describes work in an underground room, while the second, currently in progress, involves precise cutting of granite.
The Jefferson National Expansion Memorial in St Louis, US, has an 189m tall arch of stainless steel under which the National Park Service created an underground museum. Lack of funds meant the museum was not completed: one room measuring 18x24m was left with bare concrete walls and a dirt floor for future development. Recently the University of Missouri-Rolla (UMR) agreed to help the Park Service lower the floor of that room in a stepped manner to make it 5m deeper, so it could be used as a large screen theatre.
It was necessary to remove 2000m3 of dolomite, chert and clay from the room without visitors to the site being disturbed. The rock had to be removed through a corridor whose entrance was 5m above the floor of the room, and which was 2.4m wide and with a minimum height of 2.4m. Limited access precluded heavy equipment, and blasting so near the foundations of the Arch was not to be even discussed.
To extract the central volume of rock a free face was established at each level 30-50cm deep. At about 50cm from that face, holes were drilled to the bottom of the level at about 75cm intervals. A Darda rock splitter was placed in the holes and activated, cracking the rock parallel to the face. A small impact hammer on a 700 series Bobcat was then used to grow the cracks out to the free surface, using the cracked hole to locate the moil. The individual blocks, weighing 500-1000kg, were loaded into a hopper suspended from the roof trusses and swung out to the corridor, where they were hauled away using a fork lift.
The room was left with irregular walls which the impact hammer could only slowly straighten, particularly where chert was present in layers over 10cm thick. To cut the walls straight an abrasive slurry jet system was used. High pressure water (at 350bar) was pumped through a hose. Between the pump and the cutting nozzle approximately 8% abrasive (a local blasting sand) was added to the water. The resulting stream cut through the dolomite and abrasive to around 30-40cm deep on each pass. To carry the cutting nozzle and support lance, a small section of radio antenna mast was slung from four come-alongs (two at each end) from small concrete anchors set into the wall of the opening. Two supports were required at each end so that the lance position could be adjusted, and the jets cut inward at an angle of 1:12 to the wall. This was because the nozzle assembly had a finite thickness and cutting in this way maintained the wall alignment.
The jet cut in past the thickness of the nozzle on each cut. The lance was then lowered, and the process repeated, until the full depth had been achieved. The requirement was that the wall should be straight within 5cm over the 5m maximum depth cut. Water and sand collected in the bottom of the excavation and were pumped from a sump. It had been intended to recirculate both but the site contained a layer of clay which ran horizontally through the room, about 3m below the original floor. Vertical seams were also present, and the clay was difficult to remove. Instead, the waste was pumped out of the room. Because the clay was a long term threat to the stability of the room, it was removed using low pressure water and a hand-held lance to a depth of roughly 1m.
The Park Service also asked that the walls be reinforced with 4.5m Dywidag bolts. The bolts were installed in patterns below the existing concrete walls. Initially the holes were to be drilled with a pneumatic drill but the clay seams within the wall were up to 50cm thick and it was difficult to collar the hole beyond them. Close to the surface, the broken and blasted rock from the original excavation had been left in place under the concrete and it was difficult to drill conventionally.
The abrasive slurry system was modified so that it would feed a lance with a rotary water jet drilling head on the front. This was a relatively light assembly and could be set up to drill into the wall at all locations. The drill penetrated all the rock and cut a hole large enough to insert the bolts over their full length. The bolts were inserted through resin cartridges, with a fast setting cartridge located at the back of the hole, and could be tensioned to full design load before the locking nuts were tightened in place. None of the water jet drilled holes carried less than the design load, even those installed through the broken and blasted ground.
Since this work was completed there have been changes in the tools available. Abrasive slurry jetting systems are now available operating at 690bar, and the current state-of-the-art unit is operating in Germany at a pressure of 1400bar. With increased pressure, cutting speed increases and much less abrasion is required.
The Arch project showed that it is possible to cut through quite hard rock (dolomite and chert) with commercially available pressures and to leave a surface cut to the right size without damaging the underlying rock surface. One perceived problem is with water: no-one likes to work wet, and though it is much less a real concern than a perceived one, it does have an impact.
The ability of water jets to cut rock without damage, to an increasing depth, is illustrated by another UMR project. Under commission from artist Edwina Sandys, the High Pressure Water, Jet Laboratory is carving a statue to commemorate the parents and wife of an alumnus.
The construction is a granite arch that stands 5m high and 0.75m deep. Two vertical rocks 1.5m wide and 1.2m apart support an overhanging rock. A figure 3.45m tall, 1m wide and cut though the 75cm thickness, has been removed from each of the two verticals. The figures are polished and offset 15m from the arch.
The two vertical members were very irregular and roughly 30cm oversize in each dimension. To remove the oversize a slot 2.1m deep and 6cm wide was cut around the blocks using water alone at a pressure of 1000bar. Two jets were used in a rotating head that entered the slot and cut it straight over the full depth. Once the rocks had been shaped they were turned on their side and the figures cut out. Because the artist wanted close agreement between the shape of the figure and the hole, the slot width to release the figures was kept under 2cm. This slot was cut using plain water at 1000bar, and rotating two 1mm diameter nozzles over the rock surface at 200rpm as the lance moved over the required path. Speed of cutting was controlled by marked variations in granite strength as the rock was penetrated; typically 8mm cutting depths were achieved on a single pass. High-pressure water was used, rather than abrasive-laden water jets, because of the 2cm slot width required. Abrasive water jets will normally only cut a slot width of several millimetres.
These two projects were carried out as much to demonstrate the capabilities of the tool as to accomplish the task. They confirm that these tools have potentially significant benefits for construction. Since the initial work was carried out the growth of the water jet industry in other areas has helped to develop reliable tools. Most of these are still in industrial sites, where less robust design is adequate. As its presence and capabilities become more well known it is likely to find increasing application. It probably took 15-20 years to use this technique to remove damaged concrete and now it is becoming the universal method of removal, and also of removing unsatisfactory shotcrete during construction.
Some uses of the technique will be of benefit in almost all rock excavation work. The ability to cut straight or curved edges without any damage to the walls even in weakened rock, and with accurate dimension control, can be applied in almost every case. There is still some way to go in developing the best way of applying this tool, but its impact is very likely to be significant within the next decade.