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Conversion of an oil cavern to a coal storage facility
RESEARCH
Conversion of an Oil Cavern to a Coal Storage Facility P~ir Olsson and H&kan Stille
Abstract--A new, more environmentally safe coal storage facility is being built in central Stockholm. The coal-fired heat- and powergenerating plant incorporates a technique developed by Skanska liB for coal-firing, called PFBC (Pressurized Fluidized Bed Cornbastion), which permits the coal to be burnt more cleanly than is now possible in any existing plant. Construction of the plant involves converting an one of two oil caverns, excavated in rock, that formerly served an oil-fired heating and power-generating plant situated on the site. The authore describe the design and construction process, as well as the monitoring efforts that have been carried out to verify the design solutions.
Introduction s a result of increased public consciousness of environmentad problems, high demands are made on handling and storage of fuel and waste products in connection with new power plants. A new coal-fired heat and power generating plant is under construction at V~irtan in central Stockholm, Sweden (see Fig. 1). The plant will be taken into operation at the end of 1990. When fully commls~ioned, it will supply the central and northern part of the city with an additional 130 MW of electric power and 210 MW of thermal energy. To meet today's environmental demands, completely new technique for coal-firing, called PFBC (Pressurized Fluidized Bed Combustion), has been developed by ABB Asea Brown Boveri. Using this technique, the coal can be burned more cleanly t h a n is now possible in any existing plant. For environmental reasons, any handling of coal must be done in a closed systen~
A
R~um~---Un nouveau centre de stockage de charbon, plus salubre pour l 'environnement, a dtd construit en plein Stockholm. La centrale dlectrique et de chauffage au charbon incorpore une technique, d~velopp~e par Skanska AB, de combustion du charbon appelde CBFP (Combustion d'un lit de charbon rendu fluide et pressuris~), qui permet au charbon d'etre braid plus proprement qu'il n'est maintenant possible dans n 'importe quelle centrale existante. La construction de la centrale implique la transformation d'une des deux cavernes d pdtrole, excav~e clans la roche, qui servait prdc~demment duns centrale ~loctrique et de chauffage au p~trole situ~e sur le site. Les auteurs ddcrivent la conception du profit et le procddd de construction, et auesi les efforts de contr6le qui ont ~td entrepris pour vdrifier les solutions mises au point.
The site of the new plant is in an old industrial p a r k that is surrounded by a p a r t m e n t blocks. An old oil-fired heating and power-generating plant is situated on the site. The preliminary design of the new plant included a surface storage facility for coal. The necessary storage
\ \\
F-
~-COALSTORAGEIN ROCK CAVERN
volume of approTimAtely 150,000 m 8 implied buildings of a size that would considerably alter the appearance of the city skyline. Apprevalofthe project was therefore refused at an early stage by the Environmental Authority. A new preposal was put forward. The solution finally adopted involved
LL
HABOUR
.~
i
FOR COAL ASHES
COAL-FtREO HEAT AND POWER PLANT
Present address: Piir Olsson and H~tkan Stille, SkAnRka AB, S-182 25 Danderyd, Sweden.
Figure 1. Layout of the Vdrtan coal power plant, located in central Stockholm.
Tunnelling a~ul Undergroutul Space TecllnologT, Vol. 5, No. 4, pp. 376~384, 1990. Printed in Great Britain.
0886-7798/90 $3.00 + .00 Pergamon Press pie
379
using one of the two oil caverns that had been excavated in rock to serve the oil-firedplant at the beginning of the 1970s. In 1988, Stockholm's local energy authority, Stockholm Energy, contracted Skanska (through A B B ) to convert the oil cavern into a coal storage facility and to excavate some new transportation tunnels. Only very limited information on converting oil caverns is available. Furthermore, the demands on safety in a coal storage facility differ considerably from those required in a traditional oil storage cavern. Therefore, during the project a number of new questions have had to be answered, and the feedback from them will surely prove useful in the future when the development and testing of new forms of energy will be under constant review.
Excavation of the Original Oil Caverns Figure 3. A minor part of the roof had not been in contact with the oil. (Photo: S. Triidg~rdh)
Geology The rock in the area is of gneissic origin, intercepted by granite and pegmatite. The foliation of the gneiss is mainly horizontally oriented; fractures are mainly oriented parallel to the foliation. Steeply dipping fractures, almost perpendicular to the cavern, are also present. A crush zone with partially clayfilledjointsintersects the cavern almost perpendicularly. However, the fracture intensity is moderate or low and the rock conditions may be classified as good to very good. The two existing caverns were excavatedin 1974. They are located parallel to each other, approximately 20 m apart. The caverns were excavated to a height of about 30 m by means of a gallery and two benches (see Fig. 2). Detailed records were maintained ofthe excavation of the caverns and the reinforcement installed. Appro~mately 25% of the roof was supported by shotcrete. Spot bolting (approximately 1 bolt/m) was performed
in the remaining part. This support was adequate, taking into account the good quality of the rock and the purpose for which the cavern was built. Normally there is no need to enter an oil cavern after it has been commissioned. The existing demands on safety stipulate first and foremost that installations shall not be damaged by falling blocks. After construction was completed, crude oil was stored at a temperature of 50°C-60°C in both caverns. It is intended that one of the caverns shall continue to be utilized for storing heated crude oil. It should be noted that during construction, cracking was observed in the rock, which could indicate the presence of rockburst.
Conditions for Conversion A first inspection of the cavern after the oil had been pumped out revealed,
ROCKCAVERN FOR COALSTORABE
I
ROCKCAVERNFOR OIL STORA6E
290m
c'"
L"
18m
;
as expected, that the walls and the major part of the roof were covered with oil (see Fig. 3). Only a small portion of the roof had not come into contact with the oil. This condition was explained by a certain reserve volume and also by the formation of air pockets. It could also be assumed that oil had penetrated the joints in the rock and reduced the friction in the joints, (see Fig. 4). Extensive rockfalls were observed in some parts of the cavern. Some of the blocks had fallen after the cavern had been emptied, as evidenced by the fresh, non-oil-covered breakage surfaces on the walls, the roof, and on the fallen blocks themselves. The fallen blocks were generally slab-shaped and up to a few cubic meters in size. The rockfalls, which were considerable in some places, can be explained by the sparseness of the support installed during excavation ofthe cavern. The parts of the cavern where most of the falls had occurred corresponded to the areas in which the poorest rock quality was observed during excavation. It was in this part of the cavern that indications of rockburst were observed during excavation. The additional temperature-induced stresses caused by the rise in temperature in the cavern, in combination with the reduced strength caused by oil penetration in the joints, have also had a negative effect on the stability of the cavern. The cavern temperature, which has become lower as a result of the conversion work, will remain at this lower level when the cavern is used for storing coal. The reduction in temperature
Figure 2. Plan and section of the caverns. 380 TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY
Volume 5, N u m b e r 4, 1990
will also cause changes in the stresses in the rock mass. There is an obvious risk that further falls of loose rock will occur. However, based on experience from the excavation of the cavern and inspection of the empty cavern, the overall stability is not judged to be at risk. In the future, when the cavern is being used for storing coal, staff and machines will be at work continuously and, hence, major demands will be made on insuring the safety of the caverns. Therefore, the support and cleaning works in the future coal storage facility were directed towards creating a safe working place, from a rock engineering point of view. One condition was that these works could be performed with safe working methods. To meet the high safety demands in the coal storage facility, it was judged necessary to apply bolting and shotcrete in the entire roof. On the other hand, the walls could be secured by spot bolting, in combination with more frequent inspections of the rock during the operating life of the cavern. All rock surfaces in the cavern had to be cleaned, partly to permit inspection, scaling and support work to be done under satisfactory conditions and partly to obtain a good working environment for its future operation as a coal storage facility.
Working Method Considering the height and condition of the cavern, it was judged to be impossible to carry out the cleaning, scaling and support of the roof and the upper parts of the walls safely from
Figure 5. The work was performed from a floating platform. (Photo: S. Trddg&rdh)
equipment placed on the floor of the cavern. By filling the cavern to about twothirds of its height with water and working from floating platforms, two useful purposes would be achieved simultaneously: (1) cooling of the rock and (2) a suitable working height of about 10 m (see Fig. 5). Aiter the work on the roof and upper parts of the wall was completed, the water level could be lowered in stages, thus providing a suitable working height at all times. To permit the safe performance of the works, it was necessary at an early
stage to install temporary support to take immediate effect. The film ofoil covering the rock surface made it difficult to judge the quality of the rock, and sometimes impossible to detect joints and loose rock. A possible alternative was the systematic installation of Swellex bolts in connection with scaling. The support measures and cleaning could then be done beneath a secured area. However, it was difficult to justify this solution from an economic standpoint. From the point of view of safety, however, such a solution was fully satisfactory.
Cleaning Test
Figure 4. The rock surface behind loose rock was covered with oil that had penetrated open joints and reduced the friction. (Photo: P. Olsson)
Volume 5, Number 4, 1990
Before presenting a program for support measures, a cleaning test had to be done on the oil-covered rock to d e t e r m i n e the expected adhesion properties of the cleaned rock surface. The tests were performed using various cleaning techniques. Table 1 shows the results of cleaning tests using different techniques. The use ofliquid solvent in the waterfilled cavern would cont-mlnate the water to such an extent that it would be necessary to treat all water pumped out of the cavern. Because the costs of s u c h an a r r a n g e m e n t would be considerable, the method was judged to be impractical. The adhesion obtained by wet blasting with sand and water was not sufficient to meet the design criteria of the support required in the cavern. Because it would not be possible to make the shotcrete stick to the cleaned rock, the shotcrete had to be applied in some other way. However, the effect of the wet-blasting was sufficient to permit the required support measures to be judged.
TUNNELLINGANDUNDERGROUNDSPACETECHNOLOGY381
Table 1. Results of cleaning tests using different techniques.
Method
Tensile Strength
Wet blasting with sand + water
0.2 MPa
Wet blasting with sand + water plus liquid
0.9 MPa
solvent
Blasting with rotor nozzle-type "Water Jet"
By providing the Swellex bolts with some form of support for the shotcrete, it would be possible to utilize them both for the temporary securing of the roof, as well as to compensate for the poor adhesion properties of the rock. The Swellex bolts had to be treated with a non-corrosive coating to enable them to be used as part of the permanent support. Coated Swellex bolts are treated with a dual:component protective coating. One component is elastic and expands with the Swellex bolt without rupturing;
0.4 MPa
the other is a corrosion-resistant material contulning zinc oxide.
Support of the Roof As noted above, the overall stability of the cavern was not judged to be at risk. The support measures were designed to secure the stability of the cavern against the falling of large and smaller blocks. Because smaller blocks would be difficult to detect when scaling, it was judged impossible to carry out the spot bolting r e q u i r e d to secure them.
Therefore, shotcrete had to be applied. The shotcrete was designed to support blocks of 0.4 m thickness. Coated Swellex bolts were used to fasten the shotcrete to the rock (see Fig. 6). The bolts were equipped with two plates, one rectangular and one triangular (see Fig. 7). The rectangular plate was placed against the rock to prevent the fall of the smaller stones that came loose in connection with expansion of the bolt. The triangular plate was placed 5 cm beyond the rectangular plate and functioned as an abutment for the shotcrete. The triangular shape of the plate made it easier to apply shotcrete behind it. Theoretical calculations showed that the bolts needed to be placed systematically 2.0 m on centers to be able to support 0.4 m thick slabs of rock. The Swellex bolts were judged to be sufficient to secure the rock surface temporarily during the reinforcement work. The adhesion between the shotcrete and the rock has not been considered in these calculations and can be regarded as an extra margin of safety. In areas of loosened rock and loose blocks of a thickness greater than 0.4 m, spot belting was performed with grouted rock bolts. It was judged possible to make highly accurate observations of the areas requiring spot bolting. The grouted rock bolts which were installed selectively were equipped with transverse iron bars 5 cm from the surface of the rock to provide additional support for the shotcrete. Figure 8 shows a diagram of the support system for the roof. On completion of the systematic and spot bolting the rock surface was cleaned by high-pressure blasting with sand and water. A supplementary inspection was then carried out to determineif further bolting was required. After supplementary bolting, if required, fiber-reinforced shotcrete was applied on the roof. All bolt-heads were thoroughly covered with shotcrete to assure the corrosion resistance of the plates. Drains were installed in areas where water had been observed.
S u m m a r y of Working Method
Figure 6. Safety work with Swellex bolts. (Photo: S. Tr'ddgdrdh) 382 TUNN~,I,INGANDUNDERGROUNDSPACETECHNOLOGY
The working method used when supporting the roof can be s,i m m~rized as follows: • ScAling of loosened rock. • Systematic bolting with non-corrosive treated Swellex bolts, 1 = 1.8 m, c/c 2.0 m, equipped with two plates. • Spot bolting with grouted rock bolts, 1 = 3.0 m. • Cleaning by sand- and waterblasting at high pressure.
Volume 5, Number 4, 1990
instances of corrosion penetration are judged to h a v e little effect on the strength of the bolt. According to the report of the Corrosion Research Institute, it was possible to use the Swellex bolts as part of the permanent support system.
Drilling Out of Swellex Bolts After the facility has been in operation for 10-15 years, the i n ~ n t / o n is to drill out a number of bolts especially installed for this purpose and to perform pullout tests on t h e m At the same I~me, information on the non-corrosive coating and bearing capacity of the belts will be obtained. Some bolts have already been drilled out in order to study how the bolts are interacting with the rock in the drilled hole. Figure 7. The Swellex bolts were provided with one rectangular and one triangular plate. (Photo: P. Olsson)
* I n s p e c t i o n of t h e r o c k a n d supplementary bolting. * Application of fiber-reinforced shotcrete.
Support of the Walls (Fig. 9) From the peint of view of safety, it was not necessary to apply shotcrete on the entire area of the walls. Suppert of the walls consisted of sc~llng and spot bolting with grouted rock bolts. In places where the rock was poor and could not be scaled to a stable surface, and also around openings and installations, the walls were secured by means of systematic bolting using Swellex bolts and shotcrete. To create an acceptable working environment in the coal storage plant, the walls had to be cleaned. From a rock mechanical point of view cleaning was needed only to permit an accurate assessment of the bolting requirement. In the future, however, it will be necessary to carry out inspections and preventive scaling of the rock more frequently t h a n is normally the case, particularly in consideration of the changes t h a t are expected to occur in the cavern climate.
Testing of Non-Corrosive Coating on Swellex Bolts To permit the Swellex bolts to be included as p a r t of the p e r m a n e n t support, they had to be given a noncorrosive coating. The Corrosion Research Institute was asked to assess the risk of corrosion in coated bolts. T h e following c o n c l u s i o n s w e r e reached: 1. Bolts with u n d a m a g e d noncorrosive coating will not suffer any appreciable attack by corrosion for m a n y years. 2. In places where the coating is damaged, the risk of severe general corrosion is slight, but spots of corrosion may occur in this connection. 3. It is judged that any spots of corrosion that occur will take a long time--probably over 20 years--to penetrate right through the steel. Isolated
Pullout Tests on Non-Corrosive Coated Swellex Bolts Because our experience of non-corrosive coated Swellex bolts is very limited, some pullout tests were performed on the belts (see Fig. 10). The results corresponded with the technical data received from the supplier. The bolts ruptured when subjected to a load of about 11 tons.
Adhesion Test After cleaning and shotcreting, cores were drilled to determine the adhesion between rock surface and shotcrete. The adhesion obtained varied between Almost nil and a mswlmuln of 0.2 MPa.
Pressure Test on ShotcreteCoated Swellex Plates The triangular plates on the Swellex belts functioned as abutments for the shotcrete. Tests showed t h a t the suppert structure could withstand a load of about 5 tons before rupturing.
DoweLs
Testing and Verification of Solutions The support philosophy used in connection with the conversion of the oil storage plant in V~rtan combines old and new techniques. Therefore, it has been necessary to carry out a number of tests to verify and document the solutions. The tests also have provided an excellent oppertunity to increase our knowledge with reference to similar projects in the future.
Volume 5, Number 4, 1990
Cooled Sweltex SfeeLfibr, reinforced shofcreh 0
Figure 8.
1
2m
Outline of the support system for the roof.
TuN~.Lr~e ANDUNDF_J~ROUNDSPACETECHNOLOGY383
Figure 9. Support work on the walls with the working level adjusted by pumping out water from the cavern. (Photo: S. Trddg~rdh)
Drilling Out Bolt Heads The plate functioning as an abutment for the shotcrete was given a triangular shape in order to obtain a maximum shotcrete cover. Drilled-out bolt heads showed a shotcrete cover of at least 80% behind the plates.
On first entering the cavern after the oil had been pumped out, an extensive rockfall was observed in the roof. This was explained by a combination of the sparse support during construc-
tion, the additional temperature-induced stresses, and the reduction of the friction in the oil-filled joints. Acleaning test was performed at an early stage to estimate what degree of adhesion could be expected from different methods ofcleanlng the sticky rock surface. The tests showed that it was impossible to achieve the desired adhesion by practical methods. The support system had to be designed to give both instant temporary support for safe working conditions and some kind of abutment for the shotcrete. Swellex bolts were used to obtain instant temporary support. The bolts were coated for protection against corrosion and were provided with plates. The long-term stability of the cavern was secured by grouted rock bolts selectively installed. Aiter bolting had been performed, the rock surface was cleaned by means of high-pressure sand- and waterblasting. This was necessary to permit the final assessment of the adequacy of the reinforcement. Following supplementary bolting, a final layer of shotcrete reinforced with steel fibers was applied on the roof. The plates on the Swellex bolts were designed to carry the shotcrete, and compensated for the lack of adhesion. To verify the technical solutions, it was necessary to perform tests while the conversion work was in progress. In particular, the adhesion achieved between the rock and shotcrete, the bearing capacity of the plates on the Swellex bolts, and the protection against corrosion were documented.
[]
Summary A new coal power plant is under construction in central Stockholm. The coal will be stored in an old oil cavern constructed 15 years ago in good, mainly gneissic rock. The conversion of oil cavern to coal storage cavern has involved some technical innovations and has resulted in the development of a partially new technical solution. During its 15 years of operation the oil cavern has been used to store crude oil at a temperature ofup to 60°C. When excavated, the cavern was sparsely supported by rock bolts and shotcrete. This degree of reinforcement was sufficient for its function as an oil storage facility. However, the demands for safety in a coal cavern with people and machines constantly at work are much higher. To achieve the required degree of safety, it was clear that the roof would have to be covered with shotcrete.
Figure lO.Pullout tests were performed to verify the load capacity of the plates on the Swellex bolts. (Photo: P. Olsson)
384 TUNNELLINGANDUNDERGROUNDSPACETECHNOLOGY
Volume 5, Number 4, 1990
Conversion of an Oil Cavern to a Coal Storage Facility P~ir Olsson and H&kan Stille
Abstract--A new, more environmentally safe coal storage facility is being built in central Stockholm. The coal-fired heat- and powergenerating plant incorporates a technique developed by Skanska liB for coal-firing, called PFBC (Pressurized Fluidized Bed Cornbastion), which permits the coal to be burnt more cleanly than is now possible in any existing plant. Construction of the plant involves converting an one of two oil caverns, excavated in rock, that formerly served an oil-fired heating and power-generating plant situated on the site. The authore describe the design and construction process, as well as the monitoring efforts that have been carried out to verify the design solutions.
Introduction s a result of increased public consciousness of environmentad problems, high demands are made on handling and storage of fuel and waste products in connection with new power plants. A new coal-fired heat and power generating plant is under construction at V~irtan in central Stockholm, Sweden (see Fig. 1). The plant will be taken into operation at the end of 1990. When fully commls~ioned, it will supply the central and northern part of the city with an additional 130 MW of electric power and 210 MW of thermal energy. To meet today's environmental demands, completely new technique for coal-firing, called PFBC (Pressurized Fluidized Bed Combustion), has been developed by ABB Asea Brown Boveri. Using this technique, the coal can be burned more cleanly t h a n is now possible in any existing plant. For environmental reasons, any handling of coal must be done in a closed systen~
A
R~um~---Un nouveau centre de stockage de charbon, plus salubre pour l 'environnement, a dtd construit en plein Stockholm. La centrale dlectrique et de chauffage au charbon incorpore une technique, d~velopp~e par Skanska AB, de combustion du charbon appelde CBFP (Combustion d'un lit de charbon rendu fluide et pressuris~), qui permet au charbon d'etre braid plus proprement qu'il n'est maintenant possible dans n 'importe quelle centrale existante. La construction de la centrale implique la transformation d'une des deux cavernes d pdtrole, excav~e clans la roche, qui servait prdc~demment duns centrale ~loctrique et de chauffage au p~trole situ~e sur le site. Les auteurs ddcrivent la conception du profit et le procddd de construction, et auesi les efforts de contr6le qui ont ~td entrepris pour vdrifier les solutions mises au point.
The site of the new plant is in an old industrial p a r k that is surrounded by a p a r t m e n t blocks. An old oil-fired heating and power-generating plant is situated on the site. The preliminary design of the new plant included a surface storage facility for coal. The necessary storage
\ \\
F-
~-COALSTORAGEIN ROCK CAVERN
volume of approTimAtely 150,000 m 8 implied buildings of a size that would considerably alter the appearance of the city skyline. Apprevalofthe project was therefore refused at an early stage by the Environmental Authority. A new preposal was put forward. The solution finally adopted involved
LL
HABOUR
.~
i
FOR COAL ASHES
COAL-FtREO HEAT AND POWER PLANT
Present address: Piir Olsson and H~tkan Stille, SkAnRka AB, S-182 25 Danderyd, Sweden.
Figure 1. Layout of the Vdrtan coal power plant, located in central Stockholm.
Tunnelling a~ul Undergroutul Space TecllnologT, Vol. 5, No. 4, pp. 376~384, 1990. Printed in Great Britain.
0886-7798/90 $3.00 + .00 Pergamon Press pie
379
using one of the two oil caverns that had been excavated in rock to serve the oil-firedplant at the beginning of the 1970s. In 1988, Stockholm's local energy authority, Stockholm Energy, contracted Skanska (through A B B ) to convert the oil cavern into a coal storage facility and to excavate some new transportation tunnels. Only very limited information on converting oil caverns is available. Furthermore, the demands on safety in a coal storage facility differ considerably from those required in a traditional oil storage cavern. Therefore, during the project a number of new questions have had to be answered, and the feedback from them will surely prove useful in the future when the development and testing of new forms of energy will be under constant review.
Excavation of the Original Oil Caverns Figure 3. A minor part of the roof had not been in contact with the oil. (Photo: S. Triidg~rdh)
Geology The rock in the area is of gneissic origin, intercepted by granite and pegmatite. The foliation of the gneiss is mainly horizontally oriented; fractures are mainly oriented parallel to the foliation. Steeply dipping fractures, almost perpendicular to the cavern, are also present. A crush zone with partially clayfilledjointsintersects the cavern almost perpendicularly. However, the fracture intensity is moderate or low and the rock conditions may be classified as good to very good. The two existing caverns were excavatedin 1974. They are located parallel to each other, approximately 20 m apart. The caverns were excavated to a height of about 30 m by means of a gallery and two benches (see Fig. 2). Detailed records were maintained ofthe excavation of the caverns and the reinforcement installed. Appro~mately 25% of the roof was supported by shotcrete. Spot bolting (approximately 1 bolt/m) was performed
in the remaining part. This support was adequate, taking into account the good quality of the rock and the purpose for which the cavern was built. Normally there is no need to enter an oil cavern after it has been commissioned. The existing demands on safety stipulate first and foremost that installations shall not be damaged by falling blocks. After construction was completed, crude oil was stored at a temperature of 50°C-60°C in both caverns. It is intended that one of the caverns shall continue to be utilized for storing heated crude oil. It should be noted that during construction, cracking was observed in the rock, which could indicate the presence of rockburst.
Conditions for Conversion A first inspection of the cavern after the oil had been pumped out revealed,
ROCKCAVERN FOR COALSTORABE
I
ROCKCAVERNFOR OIL STORA6E
290m
c'"
L"
18m
;
as expected, that the walls and the major part of the roof were covered with oil (see Fig. 3). Only a small portion of the roof had not come into contact with the oil. This condition was explained by a certain reserve volume and also by the formation of air pockets. It could also be assumed that oil had penetrated the joints in the rock and reduced the friction in the joints, (see Fig. 4). Extensive rockfalls were observed in some parts of the cavern. Some of the blocks had fallen after the cavern had been emptied, as evidenced by the fresh, non-oil-covered breakage surfaces on the walls, the roof, and on the fallen blocks themselves. The fallen blocks were generally slab-shaped and up to a few cubic meters in size. The rockfalls, which were considerable in some places, can be explained by the sparseness of the support installed during excavation ofthe cavern. The parts of the cavern where most of the falls had occurred corresponded to the areas in which the poorest rock quality was observed during excavation. It was in this part of the cavern that indications of rockburst were observed during excavation. The additional temperature-induced stresses caused by the rise in temperature in the cavern, in combination with the reduced strength caused by oil penetration in the joints, have also had a negative effect on the stability of the cavern. The cavern temperature, which has become lower as a result of the conversion work, will remain at this lower level when the cavern is used for storing coal. The reduction in temperature
Figure 2. Plan and section of the caverns. 380 TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY
Volume 5, N u m b e r 4, 1990
will also cause changes in the stresses in the rock mass. There is an obvious risk that further falls of loose rock will occur. However, based on experience from the excavation of the cavern and inspection of the empty cavern, the overall stability is not judged to be at risk. In the future, when the cavern is being used for storing coal, staff and machines will be at work continuously and, hence, major demands will be made on insuring the safety of the caverns. Therefore, the support and cleaning works in the future coal storage facility were directed towards creating a safe working place, from a rock engineering point of view. One condition was that these works could be performed with safe working methods. To meet the high safety demands in the coal storage facility, it was judged necessary to apply bolting and shotcrete in the entire roof. On the other hand, the walls could be secured by spot bolting, in combination with more frequent inspections of the rock during the operating life of the cavern. All rock surfaces in the cavern had to be cleaned, partly to permit inspection, scaling and support work to be done under satisfactory conditions and partly to obtain a good working environment for its future operation as a coal storage facility.
Working Method Considering the height and condition of the cavern, it was judged to be impossible to carry out the cleaning, scaling and support of the roof and the upper parts of the walls safely from
Figure 5. The work was performed from a floating platform. (Photo: S. Trddg&rdh)
equipment placed on the floor of the cavern. By filling the cavern to about twothirds of its height with water and working from floating platforms, two useful purposes would be achieved simultaneously: (1) cooling of the rock and (2) a suitable working height of about 10 m (see Fig. 5). Aiter the work on the roof and upper parts of the wall was completed, the water level could be lowered in stages, thus providing a suitable working height at all times. To permit the safe performance of the works, it was necessary at an early
stage to install temporary support to take immediate effect. The film ofoil covering the rock surface made it difficult to judge the quality of the rock, and sometimes impossible to detect joints and loose rock. A possible alternative was the systematic installation of Swellex bolts in connection with scaling. The support measures and cleaning could then be done beneath a secured area. However, it was difficult to justify this solution from an economic standpoint. From the point of view of safety, however, such a solution was fully satisfactory.
Cleaning Test
Figure 4. The rock surface behind loose rock was covered with oil that had penetrated open joints and reduced the friction. (Photo: P. Olsson)
Volume 5, Number 4, 1990
Before presenting a program for support measures, a cleaning test had to be done on the oil-covered rock to d e t e r m i n e the expected adhesion properties of the cleaned rock surface. The tests were performed using various cleaning techniques. Table 1 shows the results of cleaning tests using different techniques. The use ofliquid solvent in the waterfilled cavern would cont-mlnate the water to such an extent that it would be necessary to treat all water pumped out of the cavern. Because the costs of s u c h an a r r a n g e m e n t would be considerable, the method was judged to be impractical. The adhesion obtained by wet blasting with sand and water was not sufficient to meet the design criteria of the support required in the cavern. Because it would not be possible to make the shotcrete stick to the cleaned rock, the shotcrete had to be applied in some other way. However, the effect of the wet-blasting was sufficient to permit the required support measures to be judged.
TUNNELLINGANDUNDERGROUNDSPACETECHNOLOGY381
Table 1. Results of cleaning tests using different techniques.
Method
Tensile Strength
Wet blasting with sand + water
0.2 MPa
Wet blasting with sand + water plus liquid
0.9 MPa
solvent
Blasting with rotor nozzle-type "Water Jet"
By providing the Swellex bolts with some form of support for the shotcrete, it would be possible to utilize them both for the temporary securing of the roof, as well as to compensate for the poor adhesion properties of the rock. The Swellex bolts had to be treated with a non-corrosive coating to enable them to be used as part of the permanent support. Coated Swellex bolts are treated with a dual:component protective coating. One component is elastic and expands with the Swellex bolt without rupturing;
0.4 MPa
the other is a corrosion-resistant material contulning zinc oxide.
Support of the Roof As noted above, the overall stability of the cavern was not judged to be at risk. The support measures were designed to secure the stability of the cavern against the falling of large and smaller blocks. Because smaller blocks would be difficult to detect when scaling, it was judged impossible to carry out the spot bolting r e q u i r e d to secure them.
Therefore, shotcrete had to be applied. The shotcrete was designed to support blocks of 0.4 m thickness. Coated Swellex bolts were used to fasten the shotcrete to the rock (see Fig. 6). The bolts were equipped with two plates, one rectangular and one triangular (see Fig. 7). The rectangular plate was placed against the rock to prevent the fall of the smaller stones that came loose in connection with expansion of the bolt. The triangular plate was placed 5 cm beyond the rectangular plate and functioned as an abutment for the shotcrete. The triangular shape of the plate made it easier to apply shotcrete behind it. Theoretical calculations showed that the bolts needed to be placed systematically 2.0 m on centers to be able to support 0.4 m thick slabs of rock. The Swellex bolts were judged to be sufficient to secure the rock surface temporarily during the reinforcement work. The adhesion between the shotcrete and the rock has not been considered in these calculations and can be regarded as an extra margin of safety. In areas of loosened rock and loose blocks of a thickness greater than 0.4 m, spot belting was performed with grouted rock bolts. It was judged possible to make highly accurate observations of the areas requiring spot bolting. The grouted rock bolts which were installed selectively were equipped with transverse iron bars 5 cm from the surface of the rock to provide additional support for the shotcrete. Figure 8 shows a diagram of the support system for the roof. On completion of the systematic and spot bolting the rock surface was cleaned by high-pressure blasting with sand and water. A supplementary inspection was then carried out to determineif further bolting was required. After supplementary bolting, if required, fiber-reinforced shotcrete was applied on the roof. All bolt-heads were thoroughly covered with shotcrete to assure the corrosion resistance of the plates. Drains were installed in areas where water had been observed.
S u m m a r y of Working Method
Figure 6. Safety work with Swellex bolts. (Photo: S. Tr'ddgdrdh) 382 TUNN~,I,INGANDUNDERGROUNDSPACETECHNOLOGY
The working method used when supporting the roof can be s,i m m~rized as follows: • ScAling of loosened rock. • Systematic bolting with non-corrosive treated Swellex bolts, 1 = 1.8 m, c/c 2.0 m, equipped with two plates. • Spot bolting with grouted rock bolts, 1 = 3.0 m. • Cleaning by sand- and waterblasting at high pressure.
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instances of corrosion penetration are judged to h a v e little effect on the strength of the bolt. According to the report of the Corrosion Research Institute, it was possible to use the Swellex bolts as part of the permanent support system.
Drilling Out of Swellex Bolts After the facility has been in operation for 10-15 years, the i n ~ n t / o n is to drill out a number of bolts especially installed for this purpose and to perform pullout tests on t h e m At the same I~me, information on the non-corrosive coating and bearing capacity of the belts will be obtained. Some bolts have already been drilled out in order to study how the bolts are interacting with the rock in the drilled hole. Figure 7. The Swellex bolts were provided with one rectangular and one triangular plate. (Photo: P. Olsson)
* I n s p e c t i o n of t h e r o c k a n d supplementary bolting. * Application of fiber-reinforced shotcrete.
Support of the Walls (Fig. 9) From the peint of view of safety, it was not necessary to apply shotcrete on the entire area of the walls. Suppert of the walls consisted of sc~llng and spot bolting with grouted rock bolts. In places where the rock was poor and could not be scaled to a stable surface, and also around openings and installations, the walls were secured by means of systematic bolting using Swellex bolts and shotcrete. To create an acceptable working environment in the coal storage plant, the walls had to be cleaned. From a rock mechanical point of view cleaning was needed only to permit an accurate assessment of the bolting requirement. In the future, however, it will be necessary to carry out inspections and preventive scaling of the rock more frequently t h a n is normally the case, particularly in consideration of the changes t h a t are expected to occur in the cavern climate.
Testing of Non-Corrosive Coating on Swellex Bolts To permit the Swellex bolts to be included as p a r t of the p e r m a n e n t support, they had to be given a noncorrosive coating. The Corrosion Research Institute was asked to assess the risk of corrosion in coated bolts. T h e following c o n c l u s i o n s w e r e reached: 1. Bolts with u n d a m a g e d noncorrosive coating will not suffer any appreciable attack by corrosion for m a n y years. 2. In places where the coating is damaged, the risk of severe general corrosion is slight, but spots of corrosion may occur in this connection. 3. It is judged that any spots of corrosion that occur will take a long time--probably over 20 years--to penetrate right through the steel. Isolated
Pullout Tests on Non-Corrosive Coated Swellex Bolts Because our experience of non-corrosive coated Swellex bolts is very limited, some pullout tests were performed on the belts (see Fig. 10). The results corresponded with the technical data received from the supplier. The bolts ruptured when subjected to a load of about 11 tons.
Adhesion Test After cleaning and shotcreting, cores were drilled to determine the adhesion between rock surface and shotcrete. The adhesion obtained varied between Almost nil and a mswlmuln of 0.2 MPa.
Pressure Test on ShotcreteCoated Swellex Plates The triangular plates on the Swellex belts functioned as abutments for the shotcrete. Tests showed t h a t the suppert structure could withstand a load of about 5 tons before rupturing.
DoweLs
Testing and Verification of Solutions The support philosophy used in connection with the conversion of the oil storage plant in V~rtan combines old and new techniques. Therefore, it has been necessary to carry out a number of tests to verify and document the solutions. The tests also have provided an excellent oppertunity to increase our knowledge with reference to similar projects in the future.
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Cooled Sweltex SfeeLfibr, reinforced shofcreh 0
Figure 8.
1
2m
Outline of the support system for the roof.
TuN~.Lr~e ANDUNDF_J~ROUNDSPACETECHNOLOGY383
Figure 9. Support work on the walls with the working level adjusted by pumping out water from the cavern. (Photo: S. Trddg~rdh)
Drilling Out Bolt Heads The plate functioning as an abutment for the shotcrete was given a triangular shape in order to obtain a maximum shotcrete cover. Drilled-out bolt heads showed a shotcrete cover of at least 80% behind the plates.
On first entering the cavern after the oil had been pumped out, an extensive rockfall was observed in the roof. This was explained by a combination of the sparse support during construc-
tion, the additional temperature-induced stresses, and the reduction of the friction in the oil-filled joints. Acleaning test was performed at an early stage to estimate what degree of adhesion could be expected from different methods ofcleanlng the sticky rock surface. The tests showed that it was impossible to achieve the desired adhesion by practical methods. The support system had to be designed to give both instant temporary support for safe working conditions and some kind of abutment for the shotcrete. Swellex bolts were used to obtain instant temporary support. The bolts were coated for protection against corrosion and were provided with plates. The long-term stability of the cavern was secured by grouted rock bolts selectively installed. Aiter bolting had been performed, the rock surface was cleaned by means of high-pressure sand- and waterblasting. This was necessary to permit the final assessment of the adequacy of the reinforcement. Following supplementary bolting, a final layer of shotcrete reinforced with steel fibers was applied on the roof. The plates on the Swellex bolts were designed to carry the shotcrete, and compensated for the lack of adhesion. To verify the technical solutions, it was necessary to perform tests while the conversion work was in progress. In particular, the adhesion achieved between the rock and shotcrete, the bearing capacity of the plates on the Swellex bolts, and the protection against corrosion were documented.
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Summary A new coal power plant is under construction in central Stockholm. The coal will be stored in an old oil cavern constructed 15 years ago in good, mainly gneissic rock. The conversion of oil cavern to coal storage cavern has involved some technical innovations and has resulted in the development of a partially new technical solution. During its 15 years of operation the oil cavern has been used to store crude oil at a temperature ofup to 60°C. When excavated, the cavern was sparsely supported by rock bolts and shotcrete. This degree of reinforcement was sufficient for its function as an oil storage facility. However, the demands for safety in a coal cavern with people and machines constantly at work are much higher. To achieve the required degree of safety, it was clear that the roof would have to be covered with shotcrete.
Figure lO.Pullout tests were performed to verify the load capacity of the plates on the Swellex bolts. (Photo: P. Olsson)
384 TUNNELLINGANDUNDERGROUNDSPACETECHNOLOGY
Volume 5, Number 4, 1990