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Spiling bolts and reinforced ribs of sprayed concrete replace concrete lining
Tunnelling and Underground Space Technology 17 (2002) 403–413
Spiling bolts and reinforced ribs of sprayed concrete replace concrete lining Kristin H. Holmøya,*, Bent Aagaardb a
Norwegian University of Science and Technology, Department of Geology and Mineral Resources Engineering, N–7491 Trondheim, Norway b O.T. Blindheim AS, Kjøpmannsgt, 61, N–7011 Trondheim, Norway Received 30 April 2002; received in revised form 1 August 2002; accepted 12 August 2002
Abstract This article discusses how and when spiling bolts and reinforced ribs of sprayed concrete should be used to achieve safe progress in poor rock mass conditions. It is discussed when bolting and reinforced ribs of sprayed concrete can replace cast in place concrete as permanent rock support. Examples from the challenging Frøyatunnel illustrate how this was achieved in poor to extremely poor rock mass conditions. Numerical models comparing sprayed concrete and cast in place concrete are briefly reported. 䊚 2002 Elsevier Science Ltd. All rights reserved. Keywords: Tunnelling; Rock support; Spiling bolts; Reinforced ribs of sprayed concrete; Concrete lining
1. Introduction The use of spiling bolts, sprayed concrete and reinforced ribs of sprayed concrete close to the tunnel face has increased in Norwegian tunnels over the last 10 years. It is a consequence of projects with demanding excavation conditions, where tunnelling in weak rock and passing of weakness zones have been substantial. In the same period, the development of sprayed concrete in terms of quality has been fast. Among other things, an alkalifree accelerator which gives the sprayed concrete higher early strength has been developed. Alkali-free sprayed concrete has opened the possibility for spraying thicker layers of sprayed concrete in one round, and is therefore, suitable for support in connection with weakness zones. In Norway there exists a tradition for flexible decision on primary support as the tunnel is being excavated. Therefore, many different support alternatives for excavating through weakness zones have been developed. The development has been fast, and it is now important to try to sum up the experience gained about the use of *Corresponding author. Tel.: q47-73-59-48-94.
spiling bolts and reinforced ribs of sprayed concrete in connection with the passing of weakness zones. The fact that spiling bolts and reinforced ribs of sprayed concrete are about to replace concrete lining will be discussed in this paper based on experience from the 5.3 km long Frøyatunnel. Some points of view on how to make use of spiling bolts and reinforced ribs of sprayed concrete when excavating through weakness zones are given. Also some examples are included that show how the use of numerical modelling can shed light on possible differences between the mode of operation for sprayed concrete in combination with boltsyribs vs. concrete lining. 2. The Frøya sub-sea tunnel In this paper the Frøya sub-sea tunnel is used as main example. It is therefore, adequate to present some facts about the Frøya tunnel. The Frøya sub-sea Tunnel is 5.3 km long and connects the two islands Hitra and Frøya on the north-west coast of Norway, West of Trondheim, see Fig. 1. Deepest
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Fig. 1. The Frøya tunnel, connecting the two islands Hitra and Frøya, and the Hitra tunnel connecting the islands to the mainland Norway.
point is 155 m below sea level, and it has a major part (3.6 km) below the sea, where the rock overburden varies between 37 and 155 m. The two-lane tunnel has a cross sectional area of 50 m2. The maximum gradient is 10%. The Frøya tunnel is the second sub-sea tunnel of the Hitra-Frøya Mainland Fixed Link. The 5.7 km long and 264 m deep Hitra tunnel was completed in 1994. The total tunnel cost for the Frøya tunnel (including preinvestigations and contract administration) was 417 million NOK (52 million Euro, 8 NOKs1 Euro) which equals 78700 NOKym tunnel. The tunnelling works started in February 1998, with hole-through in September 1999, and opening of the tunnel for traffic in June 2000. Compared to other, similar projects, rather comprehensive pre-investigations were carried out. The refraction seismic measurements have shown more of low velocity (weakness) zones than for any of the other sub-sea tunnels constructed in Norway, see Fig. 2. A main geological feature is the Tarva fault which can be followed more than 150 km towards NW on the Norwegian mainland. The material in many zones consists of soil-like material (clay, silt, sand and gravel). Often, the clay material shows a high degree of swelling with low strength and low friction properties. The metamorphic rocks in the area are of Precambrian age with gradual transitions between various gneisses, such as granitic gneiss, micagneiss, and migmatite. The strike of the bedrocks is mainly ENE-WSW with steep dip towards NW. 3. Spiling bolts In this paper, the term spiling bolts refers to the use of rock bolts ahead of the tunnel face. The bolts are
installed outside the theoretical blasting profile in a fan shaped pattern, often oriented approximately 158 relatively to the tunnel axis, see Fig. 3. Spiling bolts are used to secure the stability before next blast round. When spiling bolts are used, the danger of any fall-outs is reduced so that it is much easier to maintain the correct profile until rock support after blasting is established. One should therefore use spiling bolts whenever poor rock mass conditions are expected on the next blasting round. Because the rock quality on the next round is so difficult to predict, spiling bolts are often omitted and are not used until poor rock mass has already resulted in fall-outs and instability. The use of spiling bolts is an important tool to secure proper control of the stability in zones with poor rock mass conditions. In most cases where a weakness zone is expected, probe drilling is carried out. If the drilling reveals poor rock mass conditions in front of the tunnel face, spiling bolts should be considered. The bolt spacing and the extent of bolting in the profile is decided based on the rock mass conditions. Poorer rock conditions demands denser installation of the spiling bolts. In rock mass with Q-values, (Nilsen and Palmstrøm, 2000, pp. 146–149), lower than 0.1, the bolt spacing should not exceed 0.4–0.5 m. In extremely poor rock mass conditions it may be necessary to reduce the spacing to 25–30 cm. The bolt spacing has to be adjusted to the local conditions. If a spacing of 0.7 m does not lead to a noticeable fall-out, it should be considered whether the spiling bolts have any purpose compared to the cost. How much of the profile that needs spiling bolts, should be considered in connection with how the weakness zone is expected to appear after the next blasting round.
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Fig. 2. Assumed main weakness zones in the tunnel area, as interpreted from geological maps, aerial photos and field investigations.
The length of the spiling bolts has to be adjusted to the rest of the excavation process. It is required that the bolts reach approximately 2 m beyond the length of the blasting round. Most commonly, spiling bolts with a length of 6 m, and 2–4 m length of the blasting round are used. With reduced length of the blasting round, the overlap increases. The diameter of the spiling bolts should be 32 mm, since it is often necessary for the bolts to carry high weights on long spans. It is important to ensure that the bolts are fixed close to the face of the next blasting round. The typical way to proceed is to use both rock bolts and steel straps, and cover them with sprayed concrete, Fig. 4 shows an example. If a heavier support is desirable as initial support, one can install a reinforced rib of sprayed concrete as suspension for the spiling bolts. Spiling bolts have so far only been used as initial support, and have not been considered in the permanent
support program. Therefore, the spiling bolts have usually not been protected against corrosion. It is, however, very conservative not to take into account the stabilising effect the spiling bolts will contribute to the permanent support. This should be considered on the basis of how the spiling bolts work together with other rock support methods. Also, guidelines should be defined on how the installation of spiling bolts should be carried out, and how the protection against corrosion should be. Today, no such guidelines exist. 4. Reinforced ribs of sprayed concrete The commonly used method in Norway for reinforced ribs of sprayed concrete, consists of layers of sprayed concrete which is reinforced and anchored to 6 pieces of 16 mm rebars that are bent around the profile. The spacing between the rebars is 5–10 cm. The rebars are
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Fig. 3. Sketch showing spiling bolts anchored at the rear edge with bolts and steel straps, the sketch also shows invert cast concrete.
attached with radial bolts welded to a crossbeam. Typical spacing between the bolts is 1.5 m, and grouted bolts are used. Sprayed concrete is first used to create a smooth foundation where the arch is going to be mounted, and afterwards to cover the reinforcement with minimum 5 cm. Often a combination of reinforced and unreinforced concrete is used. Several ready-made arches exist which may be installed directly inside the tunnel profile with radial bolts. An example of this kind of reinforced ribs of sprayed concrete is Pantex-arches, see Fig. 5. The Pantex-arches are constructed of three ribbed bars
mounted together in a framework. When using these arches to create a reinforced rib, it is extremely important that the profile is smooth and close to the arch. The ready-made arches have the disadvantage that they can not be adjusted to the profile. Therefore, they are less suitable if the profile has great out-falls. Fig. 6 shows a sketch of both type of arches. The condition of the rock mass determines the spacing between the reinforced ribs of sprayed concrete, and normal centre distance is 1.5–2.5 m. Centre distances more than 2.5 m are unsuitable if a continuous zone is to be supported.
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● reduced blast length (3 m); ● before blasting 6 m long spiling bolts, 32–37 pieces 32 mm diameter (0.4 m centre distance), steel straps and 12–13 anchoring bolts (CT-bolts) for attaching the bolts in the rear edge; ● sprayed concrete to cover the steel straps and bolt heads; ● immediately after mucking, sprayed concrete, one layer (5–6 cm); ● bolting, a total of 30 CT-bolts per blast round (including the anchoring bolts), i.e. 1.3–1.8 m spacing; ● supplementation with 1–2 layers of sprayed concrete when the next blast round was sprayed, a total of 10–15 cm thickness; ● a total of 3–4 m3 sprayed concrete per tunnel metre. 5.2. Profile No. 3985–4025 In the Frøya tunnel there are several weakness zones with Q-values lower than 0.1 (extremely poor). Cast in place concrete lining at the tunnel face was necessary
Fig. 4. Picture of exposed spiling bolts after one short blasting round, (from the Frøya tunnel).
5. Examples from the Frøya tunnel on the use of spiling bolts, reinforced ribs of sprayed concrete and cast in place concrete In the Frøya tunnel, spiling bolts were used for 750 m of the tunnel length when passing weakness zones. A total of 10 439 spiling bolts were used, in average 37 spiling bolts per round, (Lien et al., 2000). This was mostly 6 m long spiling bolts, but in a few occasions 8 m long bolts were used. All spiling bolts had a diameter of 32 mm. In the following text CT-bolts are mentioned several times, CT-bolts are point anchored (immediate support) and grouted later to be included in the permanent support. 5.1. Profile No. 3380–3415 A description of the excavation and support method used when excavating through a weakness zone in the Frøya tunnel with Q-value around 0.1 (very poor to extremely poor), follows below:
Fig. 5. Pantex arches installed as suspension for the spiling bolts, ready for applying sprayed concrete, (from the Frøya tunnel).
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Fig. 6. Sketch showing how reinforced ribs of sprayed concrete and pantex-arches are constructed.
in several of the weakness zones. In a 40 m continuous weakness zone with extremely poor to exceptionally poor rockmass conditions, (qs0.013 and with some parts where the Q-value was as low as 0.004), the following supportyactions were carried out: ● reduced blast length (3 m); ● before blasting 6 m long spiling bolts, 36–64 pieces 32 mm diameter (0.25–0.4 m centre distance), steel straps and 14–16 anchoring bolts (CT-bolts) for attaching the bolts in the rear edge; ● sprayed concrete to cover the steel straps and bolt heads; ● immediately after mucking sprayed concrete, one or two layers (6–12 cm); ● bolting, a total of 23–31 CT-bolts per blast round (including the anchoring bolts), i.e. 1.5 m spacing; ● supplementation with 2–3 layers of sprayed concrete, a total of 19–31 cm thickness; ● risk of the walls caving in (cracking was discovered in the sprayed concrete), led to a necessity for invert cast concrete on a total of 35.5 m. ● one section (5 m) concrete lining at the end of the zone, where the rock conditions were so extremely bad that it was impossible to use bolts (fall-in and water behind the sprayed concrete).
In the poorest parts, the excavation was carried out with a front loader. Convergence measurements were carried out to control if the performed support was adequate. The results showed that the deformations had come to a stop. The deformation measurements were actively used as part of the decision basis for the permanent support. Convergence was recorded at 3 profiles, profile No. 3992, 4003 and 4012. The convergence stopped after approximately one month at profile No. 3992 and 4003. The total deformation was 2 and 4.5 mm. At profile No. 4012 the deformation developed over a long period. The deformation came to a stop after approximately 8 months, and the total deformation was approximately 3 mm. Except for the 5 m concrete lining executed at the tunnel face, it was concluded that cast in place concrete was not necessary. As permanent support 5 reinforced ribs of sprayed concrete with centre distance 2 m were installed on the section close to the concrete lining (profile No. 4004–4014). In the remaining parts of the zone, approximately 10 cm extra sprayed concrete was used, with even thicker layers in depressions. The crossing of this zone is an example of active follow-up at the tunnel face, with mapping of the rock
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mass conditions and convergence measurements, optimizing the rock support. 5.3. Profile No. 6840–6940 In a 100 m long weakness zone with Q-values between 0.01 and 0.1, extremely poor, reinforced ribs of sprayed concrete were systematically mounted at the tunnel face in combination with spiling bolts. For excavation through this zone, the following excavation method and rock support was carried out: ● reduced blasting length (3 m), partly digging, no blasting necessary. ● before blasting 6 m long spiling bolts, 26–66 pieces diameter f32 mm every short blast round. ● approximately 15 anchoring bolts (CT-bolts) in combination with steel straps to ensure that the spiling bolts were properly attached in the rear edge. Where the rock mass conditions were extremely poor the spiling bolts where secured in the rear edge with reinforced ribs of sprayed concrete. ● immediately after mucking, sprayed concrete, one or two layers (6–12 cm) ● bolting, a total of 25–31 CT-bolts per blast round (including the anchoring bolts), i.e. 1.3 m spacing ● supplementation with 2–3 layers of sprayed concrete, a total of approximately 20 cm thickness ● reinforced ribs of sprayed concrete, 2–3 m spacing, one pantex arch, alkali-free sprayed concrete was used for covering the ribs. ● invert cast concrete on a total of 65 m. The concrete work where done in 20–30 m long sections towards the tunnelface, i.e. max. 10 days after excavation. Convergence was recorded at 3 profiles, profile No. 6880, 6920 and 6930. At profile No. 6880 and 6920 the deformations stopped after approximately 4 months. The total deformation was 1 and 3 mm. In profile No. 6938 the deformation developed over a long period. The deformation continued to develop, approximately 0.5 mm per month. After 9 months the total deformation was 10.2 mm. Cracking was observed in the sprayed concrete in the wall at profile No. 6890. Based on the observation of cracks and the convergence measurements, the permanent support was decided. It was concluded that it was necessary to strengthen the contact between invert cast concrete and reinforced ribs with extra sprayed concrete, and 4 extra reinforced ribs of sprayed concrete were installed. An additional 5 cm of sprayed concrete was used to cover some reinforcement ribs and between some ribs.
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6. Convergence measurements In the Frøya Tunnel the convergence measurements were performed in all weakness zones where the rock engineer considered that there could be a risk of harmful deformations. The results were used as a basic input for design of the permanent support. Control of the stability during excavation was another important purpose. The bolts could not be installed too close to the tunnel face, because the work close to the tunnel face could damage the bolts. Therefore, it was not possible to measure the whole deformation development. But the important part of the measurements is to evaluate the deformation over time. A stable situation is obtained when total displacement converges towards a constant value Readings were made approximately once a week just after measuring bolts had been installed in a profile. The frequency dropped gradually, and could at the end be several months. Maximum convergence measured at profile 7370 was 23 mm. The first reading was taken 13 days after the tunnel face was at that profile, and based on the shape of the graph showing deformation over time one can presume that there has been 10–15 mm unrecorded deformation (the first 13 days after blasting). Convergence was recorded at 27 profiles all together. In case the convergence measurements revealed that the deformations did not stop, additional support was considered. Dependent on already performed rock support and the progress of deformation, it was decided which actions were needed and when to implement them. Additional support in the form of invert cast concrete and extra bolting in the walls were carried out in the zone between profile 7350–7390 before breakthrough (Nov. 1998) to get acceptable deformation. This solution was working well, since the deformations almost came to an end, only minor deformations were recorded after the invert cast concrete was mounted, see the graph showing convergence measurements at profile No. 7370, Fig. 7. 7. Experience from the Frøya tunnel The practise of using spiling bolts worked very well for the purpose of stabilising the next blasting round when excavating through weakness zones. The spiling bolts limited the rock falls and the shape of the profile was kept. With Q-values below 0.4, spiling bolts were generally used. The typical centre distance (spacing) was approximately 0.4 m, but both smaller and larger spacing was used. Anchoring at the rear edge of the spiling bolts with bolting, steel straps and sprayed concrete before blasting
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Fig. 7. Convergence measurement at profile No. 7370, (from the Frøya tunnel).
was successful. The combination of reduced blast round down to 3 m and the use of 6 m long spiling bolts worked well. In extremely poor rock mass conditions (q-0.1), reinforced ribs of sprayed concrete as anchor for the spiling bolts proved necessary. In some instances where cast in place concrete lining at the tunnel face had been performed, the spiling bolts were mounted to the concrete lining (boring through), see Fig. 8. A total of 17 pantex arches were installed, and experience shows that the pantex arches do not necessarily give any saving of time compared with mounting traditional reinforced ribs of sprayed concrete (as long as the workers are experienced). The pantex arches are stiffer and demands a smooth contour to avoid using too much sprayed concrete. Rock support by the use of pantex arches therefore became a little more expensive compared with traditional reinforced ribs of sprayed concrete. The convergence measurements together with the engineering geological investigations and inspection from a working platform, was used as basis for making decisions concerning the permanent support. The experience from the Frøya tunnel demonstrates the importance of having an experienced rock engineer at the working site so that engineering geological investigations can be carried out before using sprayed concrete. Any possible weakness zone that may alter the total stability and in the worst case lead to failure, may then be recorded. Making the right decision at the working site is critical to achieve the best solutions. In weakness zones with exceptionally poor rock mass conditions, the tunnel floor became crushed due to the
loads from the quarrying trucks. Invert cast concrete was therefore placed 15–20 m behind the tunnel face. Invert cast concrete was also placed in those weakness zones where the convergence measurements revealed that the deformations did not culminate. The invert cast concrete proved to be very effective stopping further deformation. It created an arch effect that gave considerably better stability. In some parts of the tunnel problems with the bonding between the sprayed concrete layers were observed. The cause of the bonding capacity problem was that it was
Fig. 8. Picture showing spiling bolts anchored in the concrete lining, (from the Frøya tunnel).
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difficult to wash out the clay dust before spraying the next layer with sprayed concrete. Especially, a red clay in some of the weakness zones gave an adhesive clay dust when blasting. It was therefore focused on performing a good cleaning before spraying the next layer. 8. When should concrete lining replace reinforced ribs of sprayed concrete? Examples on conditions where spiling bolts and use of reinforced ribs of sprayed concrete are sufficient also as permanent rock support, have been shown. There are, however, situations when concrete lining is the proper and necessary choice. 8.1. Block falls When block falls have occurred and a risk for further development exists, concrete lining is often the best solution. This is because there will be a need for an inappropriate amount of sprayed concrete to fill the cavities before it is possible to install reinforced ribs of sprayed concrete. This will be a time-consuming and an expensive solution. Concrete lining is a proper alternative. 8.2. Inadequate hold for radial bolts When the rock mass conditions were so poor that it was difficult to anchor the radial bolts at the Frøya tunnel, concrete lining was used as initial support at the tunnel face. In such conditions it was also impossible to anchor the spiling bolts at the rear edge in a satisfactory manner, and excavating further without concrete lining was unjustifiable. It is important in the stability evaluation to achieve a good dialogue with those performing the rock support. They are working close to the rock mass, and can give valuable details about rock mass quality and can, for example, give reports about problems with anchoring radial bolts. 8.3. Clay in combination with water The presence of clay is by many interpreted as a crucial factor for choosing concrete lining as permanent support. But the clay content in a weakness zone is not alone conclusive. In the Frøya tunnel several weakness zones with high clay content were supported permanently with spiling bolts and reinforced ribs of sprayed concrete. It is of great importance to study how the clay behaves in the weakness zone. Experience from the Frøya tunnel reveals that zones with high content of solid clay often had no water leakage, and this led to satisfactory stability achieved by the use of spiling bolts, radial bolts, sprayed concrete and reinforced ribs of sprayed concrete. In weakness zones where the clay appeared in gouges, mixed with crushed rock mass and usually with minerals
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like mica and calcite, water was frequently present. Less clay and more mica combined with water led to poorer stability. It was therefore, necessary to use cast in place concrete in a 35 m long weakness zone. The most important thing when deciding if cast in place concrete lining at the tunnel face is necessary, is to be present at the tunnel face for mapping of the rock mass conditions. 8.4. Continued deformations For decision about type of permanent rock support, convergence measurements gives important input, in addition to visual observation. If no cracks are observed in the sprayed concrete, and the convergence measurements indicate that the deformations have stopped, there is no need for cast in place concrete. 9. Numerical modelling of variable rock support in weakness zones at the Frøya tunnel For evaluating the effect of different types of rock support of weakness zones in the Frøya tunnel, numerical modelling (UDEC) was performed, (Løset, 1998). In the model, the zone at profile No. 4000 was chosen as a representative zone, and input parameters were selected based on mapping, evaluation of cores covering all of the actual zone together with on-site inspection in the tunnel. The analysis was performed with variable rock support and in two stages. Stage 1: A. 25 cm sprayed concreteqbolts 1.5=1.5 mqinvert cast concrete placed some time after the other rock support. B. 25 cm sprayed concreteqbolts 1.5=1.5 mqinvert cast concrete laid at the same time as the other rock support. C. 25 cm sprayed concreteqbolts 1.5=1.5 m (no invert cast concrete). Stage 2: D. 25 cm sprayed concreteqbolts 1.5=1.5 mqreinforced ribs of sprayed concrete (cyc 2 m)qinvert cast concrete laid some time after the other support. E. Cast in place concreteqinvert cast concrete laid some time after the other support. The model assumed that all support, except from invert cast concrete, was installed after a 10–12 mm deformation had been developed. In this rock mass, that means approximately 1–2 h after blasting. The results revealed that the time for invert cast concrete in relation to the potential deformation development was important. The invert cast concrete led to less deformation and thereby less load on the remaining
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support. When the invert cast concrete was placed at the same time as the other rock support, both deformation and bolt load were at minimum. However, the most practical was to place the invert cast concrete some time after the other rock support. The relatively strong reinforced ribs of sprayed concrete that were modelled, gave a rock support as good as cast in place concrete.
● Do differences exist in the concrete quality when using several layers of sprayed concrete compared with massive casting? Would it be wise to reduce the number of layers with sprayed concrete? ● Which requirements have to be made concerning the concrete quality? Is there a difference in durability between sprayed concrete and conventional constructional concrete?
10. Further research on the effect of spiling bolts
12. The durability of sprayed concrete with alkalifree accelerators
Further research is necessary to answer when and how spiling bolts should best be used. Aspects that ought to be elaborated more: ● Prepare guidelines for when and how spiling bolts should be used in relation to rock mass quality, water, fall outs on last blast round, results from probe drilling and core drilling. ● Type of results from spiling bolts, dimension and length, anchoring length within the blast round, steel qualityycorrosion shield. ● Anchoring at the rear end, spacing between bolts and how much of the profile needs spiling bolts. ● The effect as permanent rock support, or as integrated part of the permanent support. To find the answers to these questions, it is necessary to learn and put together the experience from projects both in Norway and abroad. 11. Practical effect of cast in place concrete and reinforced ribs of sprayed concrete In an environment, like the Norwegian tunnelling industry, where cast in place concrete is normally not a requirement, it is often discussed when it is right to use cast in place concrete lining, and when sprayed concrete and possibly reinforced ribs of sprayed concrete is the right choice. Although some practical guidelines have been established, more research is needed. There are several questions that have to be answered. ● Gather experience from projects in Norway and abroad, especially from cases where the stability has been unsatisfactory. ● Is it possible to document load transfer with the aid of stress measurements in cast in place concrete and reinforced ribs of sprayed concrete? Is there a difference between the two supporting methods concerning what practical effect they have? ● Further studies with the aid of numerical modelling of cast in place concrete and reinforced ribs of sprayed concrete. Is it correct that these supporting methods work equally? What are the limits for use of reinforced sprayed concrete compared with cast in place concrete?
When excavating through weakness zones and using reinforced ribs of sprayed concrete, concrete with alkalifree accelerators will be used extensively in near the future. It is therefore a natural question to ask if concrete with alkali-free accelerators is as durable as traditional sprayed concrete with alkali silicate. Norwegian Public Roads Administration, Directorate of Public Roads has, in connection with the Project ‘Health Environment and Safety—Sprayed Concrete’, examined the accelerators impact on the concrete quality, (Storas ˚ et al., 1999). The results revealed that sprayed concrete with alkali-free accelerator does not give any poorer durability compared to traditional alkali silicate accelerator. This is despite the fact that one can achieve a considerably higher early strength under otherwise equal conditions. Generally, it seems like the use of alkali-free accelerator leads to a more uniform material compared with the use of alkali silicate. 13. Conclusion In many occasions use of spiling bolts and reinforced ribs of sprayed concrete may replace cast in place concrete. It is difficult to set limits for when spiling bolts should be used with the aim of Q-values. As a guideline in very poor to extremely poor rock mass conditions, centre distances from 50 to 25 cm could be a starting point. Reinforced ribs of sprayed concrete is a flexible supporting alternative, where it is simple to adjust the distance between the arches and to vary the thickness of the sprayed concrete depending on the rock mass conditions. The traditional reinforced ribs of sprayed concrete are more flexible concerning installation, and works better than ready-made arches when large block falls have occurred. In order to optimise the choice of rock support in poor zones, careful excavation is important, where probe drilling and possibly pregrouting are part of the excavation method. Besides that, it is of great importance to have a well qualified crew at the tunnel face representing both contractor and owner. An experienced rock engineer should be present at the tunnel face to carry out
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the geological mapping of the rock mass before it is covered with sprayed concrete. Convergence measurements should be a natural part of the decision basis for determination of permanent support in weakness zones. This gives good documentation on how the initial support works, and if it is sufficient. Further research is necessary to find answers to when and how spiling bolts should be used. In addition, research is needed to shed light on the practical effect of cast in place concrete compared to reinforced ribs of sprayed concrete.
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References Nilsen, B., Palmstrøm, A., 2000. Handbook No 2, Norwegian Group for Rock Mechanics. The Q-system, pp. 146–149. Lien, J.E., Mehlum, A., Moe, L.E., Lillevik, S. Soknes, S. (2000): The Frøya tunnel and adjoining road network—Final report, Norwegian Public Roads Administration (in Norwegian). Løset, F. 1998, The Frøya tunnel, numerical modelling of a typical weakness zone (in Norwegian), Norwegian Geotechnical Institute (NGI) report 981040-01. Storas, ˚ I., Bakke, B., Hauck, C., Ulvestad, B., Davik, K.I., Moen, A.B. (1999), Norwegian Public Roads Administration, Planning and Construction Dpt., Project Health Environment and Safety— Sprayed Concrete, publication nr. 94 (in Norwegian).
Spiling bolts and reinforced ribs of sprayed concrete replace concrete lining Kristin H. Holmøya,*, Bent Aagaardb a
Norwegian University of Science and Technology, Department of Geology and Mineral Resources Engineering, N–7491 Trondheim, Norway b O.T. Blindheim AS, Kjøpmannsgt, 61, N–7011 Trondheim, Norway Received 30 April 2002; received in revised form 1 August 2002; accepted 12 August 2002
Abstract This article discusses how and when spiling bolts and reinforced ribs of sprayed concrete should be used to achieve safe progress in poor rock mass conditions. It is discussed when bolting and reinforced ribs of sprayed concrete can replace cast in place concrete as permanent rock support. Examples from the challenging Frøyatunnel illustrate how this was achieved in poor to extremely poor rock mass conditions. Numerical models comparing sprayed concrete and cast in place concrete are briefly reported. 䊚 2002 Elsevier Science Ltd. All rights reserved. Keywords: Tunnelling; Rock support; Spiling bolts; Reinforced ribs of sprayed concrete; Concrete lining
1. Introduction The use of spiling bolts, sprayed concrete and reinforced ribs of sprayed concrete close to the tunnel face has increased in Norwegian tunnels over the last 10 years. It is a consequence of projects with demanding excavation conditions, where tunnelling in weak rock and passing of weakness zones have been substantial. In the same period, the development of sprayed concrete in terms of quality has been fast. Among other things, an alkalifree accelerator which gives the sprayed concrete higher early strength has been developed. Alkali-free sprayed concrete has opened the possibility for spraying thicker layers of sprayed concrete in one round, and is therefore, suitable for support in connection with weakness zones. In Norway there exists a tradition for flexible decision on primary support as the tunnel is being excavated. Therefore, many different support alternatives for excavating through weakness zones have been developed. The development has been fast, and it is now important to try to sum up the experience gained about the use of *Corresponding author. Tel.: q47-73-59-48-94.
spiling bolts and reinforced ribs of sprayed concrete in connection with the passing of weakness zones. The fact that spiling bolts and reinforced ribs of sprayed concrete are about to replace concrete lining will be discussed in this paper based on experience from the 5.3 km long Frøyatunnel. Some points of view on how to make use of spiling bolts and reinforced ribs of sprayed concrete when excavating through weakness zones are given. Also some examples are included that show how the use of numerical modelling can shed light on possible differences between the mode of operation for sprayed concrete in combination with boltsyribs vs. concrete lining. 2. The Frøya sub-sea tunnel In this paper the Frøya sub-sea tunnel is used as main example. It is therefore, adequate to present some facts about the Frøya tunnel. The Frøya sub-sea Tunnel is 5.3 km long and connects the two islands Hitra and Frøya on the north-west coast of Norway, West of Trondheim, see Fig. 1. Deepest
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Fig. 1. The Frøya tunnel, connecting the two islands Hitra and Frøya, and the Hitra tunnel connecting the islands to the mainland Norway.
point is 155 m below sea level, and it has a major part (3.6 km) below the sea, where the rock overburden varies between 37 and 155 m. The two-lane tunnel has a cross sectional area of 50 m2. The maximum gradient is 10%. The Frøya tunnel is the second sub-sea tunnel of the Hitra-Frøya Mainland Fixed Link. The 5.7 km long and 264 m deep Hitra tunnel was completed in 1994. The total tunnel cost for the Frøya tunnel (including preinvestigations and contract administration) was 417 million NOK (52 million Euro, 8 NOKs1 Euro) which equals 78700 NOKym tunnel. The tunnelling works started in February 1998, with hole-through in September 1999, and opening of the tunnel for traffic in June 2000. Compared to other, similar projects, rather comprehensive pre-investigations were carried out. The refraction seismic measurements have shown more of low velocity (weakness) zones than for any of the other sub-sea tunnels constructed in Norway, see Fig. 2. A main geological feature is the Tarva fault which can be followed more than 150 km towards NW on the Norwegian mainland. The material in many zones consists of soil-like material (clay, silt, sand and gravel). Often, the clay material shows a high degree of swelling with low strength and low friction properties. The metamorphic rocks in the area are of Precambrian age with gradual transitions between various gneisses, such as granitic gneiss, micagneiss, and migmatite. The strike of the bedrocks is mainly ENE-WSW with steep dip towards NW. 3. Spiling bolts In this paper, the term spiling bolts refers to the use of rock bolts ahead of the tunnel face. The bolts are
installed outside the theoretical blasting profile in a fan shaped pattern, often oriented approximately 158 relatively to the tunnel axis, see Fig. 3. Spiling bolts are used to secure the stability before next blast round. When spiling bolts are used, the danger of any fall-outs is reduced so that it is much easier to maintain the correct profile until rock support after blasting is established. One should therefore use spiling bolts whenever poor rock mass conditions are expected on the next blasting round. Because the rock quality on the next round is so difficult to predict, spiling bolts are often omitted and are not used until poor rock mass has already resulted in fall-outs and instability. The use of spiling bolts is an important tool to secure proper control of the stability in zones with poor rock mass conditions. In most cases where a weakness zone is expected, probe drilling is carried out. If the drilling reveals poor rock mass conditions in front of the tunnel face, spiling bolts should be considered. The bolt spacing and the extent of bolting in the profile is decided based on the rock mass conditions. Poorer rock conditions demands denser installation of the spiling bolts. In rock mass with Q-values, (Nilsen and Palmstrøm, 2000, pp. 146–149), lower than 0.1, the bolt spacing should not exceed 0.4–0.5 m. In extremely poor rock mass conditions it may be necessary to reduce the spacing to 25–30 cm. The bolt spacing has to be adjusted to the local conditions. If a spacing of 0.7 m does not lead to a noticeable fall-out, it should be considered whether the spiling bolts have any purpose compared to the cost. How much of the profile that needs spiling bolts, should be considered in connection with how the weakness zone is expected to appear after the next blasting round.
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Fig. 2. Assumed main weakness zones in the tunnel area, as interpreted from geological maps, aerial photos and field investigations.
The length of the spiling bolts has to be adjusted to the rest of the excavation process. It is required that the bolts reach approximately 2 m beyond the length of the blasting round. Most commonly, spiling bolts with a length of 6 m, and 2–4 m length of the blasting round are used. With reduced length of the blasting round, the overlap increases. The diameter of the spiling bolts should be 32 mm, since it is often necessary for the bolts to carry high weights on long spans. It is important to ensure that the bolts are fixed close to the face of the next blasting round. The typical way to proceed is to use both rock bolts and steel straps, and cover them with sprayed concrete, Fig. 4 shows an example. If a heavier support is desirable as initial support, one can install a reinforced rib of sprayed concrete as suspension for the spiling bolts. Spiling bolts have so far only been used as initial support, and have not been considered in the permanent
support program. Therefore, the spiling bolts have usually not been protected against corrosion. It is, however, very conservative not to take into account the stabilising effect the spiling bolts will contribute to the permanent support. This should be considered on the basis of how the spiling bolts work together with other rock support methods. Also, guidelines should be defined on how the installation of spiling bolts should be carried out, and how the protection against corrosion should be. Today, no such guidelines exist. 4. Reinforced ribs of sprayed concrete The commonly used method in Norway for reinforced ribs of sprayed concrete, consists of layers of sprayed concrete which is reinforced and anchored to 6 pieces of 16 mm rebars that are bent around the profile. The spacing between the rebars is 5–10 cm. The rebars are
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Fig. 3. Sketch showing spiling bolts anchored at the rear edge with bolts and steel straps, the sketch also shows invert cast concrete.
attached with radial bolts welded to a crossbeam. Typical spacing between the bolts is 1.5 m, and grouted bolts are used. Sprayed concrete is first used to create a smooth foundation where the arch is going to be mounted, and afterwards to cover the reinforcement with minimum 5 cm. Often a combination of reinforced and unreinforced concrete is used. Several ready-made arches exist which may be installed directly inside the tunnel profile with radial bolts. An example of this kind of reinforced ribs of sprayed concrete is Pantex-arches, see Fig. 5. The Pantex-arches are constructed of three ribbed bars
mounted together in a framework. When using these arches to create a reinforced rib, it is extremely important that the profile is smooth and close to the arch. The ready-made arches have the disadvantage that they can not be adjusted to the profile. Therefore, they are less suitable if the profile has great out-falls. Fig. 6 shows a sketch of both type of arches. The condition of the rock mass determines the spacing between the reinforced ribs of sprayed concrete, and normal centre distance is 1.5–2.5 m. Centre distances more than 2.5 m are unsuitable if a continuous zone is to be supported.
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● reduced blast length (3 m); ● before blasting 6 m long spiling bolts, 32–37 pieces 32 mm diameter (0.4 m centre distance), steel straps and 12–13 anchoring bolts (CT-bolts) for attaching the bolts in the rear edge; ● sprayed concrete to cover the steel straps and bolt heads; ● immediately after mucking, sprayed concrete, one layer (5–6 cm); ● bolting, a total of 30 CT-bolts per blast round (including the anchoring bolts), i.e. 1.3–1.8 m spacing; ● supplementation with 1–2 layers of sprayed concrete when the next blast round was sprayed, a total of 10–15 cm thickness; ● a total of 3–4 m3 sprayed concrete per tunnel metre. 5.2. Profile No. 3985–4025 In the Frøya tunnel there are several weakness zones with Q-values lower than 0.1 (extremely poor). Cast in place concrete lining at the tunnel face was necessary
Fig. 4. Picture of exposed spiling bolts after one short blasting round, (from the Frøya tunnel).
5. Examples from the Frøya tunnel on the use of spiling bolts, reinforced ribs of sprayed concrete and cast in place concrete In the Frøya tunnel, spiling bolts were used for 750 m of the tunnel length when passing weakness zones. A total of 10 439 spiling bolts were used, in average 37 spiling bolts per round, (Lien et al., 2000). This was mostly 6 m long spiling bolts, but in a few occasions 8 m long bolts were used. All spiling bolts had a diameter of 32 mm. In the following text CT-bolts are mentioned several times, CT-bolts are point anchored (immediate support) and grouted later to be included in the permanent support. 5.1. Profile No. 3380–3415 A description of the excavation and support method used when excavating through a weakness zone in the Frøya tunnel with Q-value around 0.1 (very poor to extremely poor), follows below:
Fig. 5. Pantex arches installed as suspension for the spiling bolts, ready for applying sprayed concrete, (from the Frøya tunnel).
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Fig. 6. Sketch showing how reinforced ribs of sprayed concrete and pantex-arches are constructed.
in several of the weakness zones. In a 40 m continuous weakness zone with extremely poor to exceptionally poor rockmass conditions, (qs0.013 and with some parts where the Q-value was as low as 0.004), the following supportyactions were carried out: ● reduced blast length (3 m); ● before blasting 6 m long spiling bolts, 36–64 pieces 32 mm diameter (0.25–0.4 m centre distance), steel straps and 14–16 anchoring bolts (CT-bolts) for attaching the bolts in the rear edge; ● sprayed concrete to cover the steel straps and bolt heads; ● immediately after mucking sprayed concrete, one or two layers (6–12 cm); ● bolting, a total of 23–31 CT-bolts per blast round (including the anchoring bolts), i.e. 1.5 m spacing; ● supplementation with 2–3 layers of sprayed concrete, a total of 19–31 cm thickness; ● risk of the walls caving in (cracking was discovered in the sprayed concrete), led to a necessity for invert cast concrete on a total of 35.5 m. ● one section (5 m) concrete lining at the end of the zone, where the rock conditions were so extremely bad that it was impossible to use bolts (fall-in and water behind the sprayed concrete).
In the poorest parts, the excavation was carried out with a front loader. Convergence measurements were carried out to control if the performed support was adequate. The results showed that the deformations had come to a stop. The deformation measurements were actively used as part of the decision basis for the permanent support. Convergence was recorded at 3 profiles, profile No. 3992, 4003 and 4012. The convergence stopped after approximately one month at profile No. 3992 and 4003. The total deformation was 2 and 4.5 mm. At profile No. 4012 the deformation developed over a long period. The deformation came to a stop after approximately 8 months, and the total deformation was approximately 3 mm. Except for the 5 m concrete lining executed at the tunnel face, it was concluded that cast in place concrete was not necessary. As permanent support 5 reinforced ribs of sprayed concrete with centre distance 2 m were installed on the section close to the concrete lining (profile No. 4004–4014). In the remaining parts of the zone, approximately 10 cm extra sprayed concrete was used, with even thicker layers in depressions. The crossing of this zone is an example of active follow-up at the tunnel face, with mapping of the rock
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mass conditions and convergence measurements, optimizing the rock support. 5.3. Profile No. 6840–6940 In a 100 m long weakness zone with Q-values between 0.01 and 0.1, extremely poor, reinforced ribs of sprayed concrete were systematically mounted at the tunnel face in combination with spiling bolts. For excavation through this zone, the following excavation method and rock support was carried out: ● reduced blasting length (3 m), partly digging, no blasting necessary. ● before blasting 6 m long spiling bolts, 26–66 pieces diameter f32 mm every short blast round. ● approximately 15 anchoring bolts (CT-bolts) in combination with steel straps to ensure that the spiling bolts were properly attached in the rear edge. Where the rock mass conditions were extremely poor the spiling bolts where secured in the rear edge with reinforced ribs of sprayed concrete. ● immediately after mucking, sprayed concrete, one or two layers (6–12 cm) ● bolting, a total of 25–31 CT-bolts per blast round (including the anchoring bolts), i.e. 1.3 m spacing ● supplementation with 2–3 layers of sprayed concrete, a total of approximately 20 cm thickness ● reinforced ribs of sprayed concrete, 2–3 m spacing, one pantex arch, alkali-free sprayed concrete was used for covering the ribs. ● invert cast concrete on a total of 65 m. The concrete work where done in 20–30 m long sections towards the tunnelface, i.e. max. 10 days after excavation. Convergence was recorded at 3 profiles, profile No. 6880, 6920 and 6930. At profile No. 6880 and 6920 the deformations stopped after approximately 4 months. The total deformation was 1 and 3 mm. In profile No. 6938 the deformation developed over a long period. The deformation continued to develop, approximately 0.5 mm per month. After 9 months the total deformation was 10.2 mm. Cracking was observed in the sprayed concrete in the wall at profile No. 6890. Based on the observation of cracks and the convergence measurements, the permanent support was decided. It was concluded that it was necessary to strengthen the contact between invert cast concrete and reinforced ribs with extra sprayed concrete, and 4 extra reinforced ribs of sprayed concrete were installed. An additional 5 cm of sprayed concrete was used to cover some reinforcement ribs and between some ribs.
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6. Convergence measurements In the Frøya Tunnel the convergence measurements were performed in all weakness zones where the rock engineer considered that there could be a risk of harmful deformations. The results were used as a basic input for design of the permanent support. Control of the stability during excavation was another important purpose. The bolts could not be installed too close to the tunnel face, because the work close to the tunnel face could damage the bolts. Therefore, it was not possible to measure the whole deformation development. But the important part of the measurements is to evaluate the deformation over time. A stable situation is obtained when total displacement converges towards a constant value Readings were made approximately once a week just after measuring bolts had been installed in a profile. The frequency dropped gradually, and could at the end be several months. Maximum convergence measured at profile 7370 was 23 mm. The first reading was taken 13 days after the tunnel face was at that profile, and based on the shape of the graph showing deformation over time one can presume that there has been 10–15 mm unrecorded deformation (the first 13 days after blasting). Convergence was recorded at 27 profiles all together. In case the convergence measurements revealed that the deformations did not stop, additional support was considered. Dependent on already performed rock support and the progress of deformation, it was decided which actions were needed and when to implement them. Additional support in the form of invert cast concrete and extra bolting in the walls were carried out in the zone between profile 7350–7390 before breakthrough (Nov. 1998) to get acceptable deformation. This solution was working well, since the deformations almost came to an end, only minor deformations were recorded after the invert cast concrete was mounted, see the graph showing convergence measurements at profile No. 7370, Fig. 7. 7. Experience from the Frøya tunnel The practise of using spiling bolts worked very well for the purpose of stabilising the next blasting round when excavating through weakness zones. The spiling bolts limited the rock falls and the shape of the profile was kept. With Q-values below 0.4, spiling bolts were generally used. The typical centre distance (spacing) was approximately 0.4 m, but both smaller and larger spacing was used. Anchoring at the rear edge of the spiling bolts with bolting, steel straps and sprayed concrete before blasting
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Fig. 7. Convergence measurement at profile No. 7370, (from the Frøya tunnel).
was successful. The combination of reduced blast round down to 3 m and the use of 6 m long spiling bolts worked well. In extremely poor rock mass conditions (q-0.1), reinforced ribs of sprayed concrete as anchor for the spiling bolts proved necessary. In some instances where cast in place concrete lining at the tunnel face had been performed, the spiling bolts were mounted to the concrete lining (boring through), see Fig. 8. A total of 17 pantex arches were installed, and experience shows that the pantex arches do not necessarily give any saving of time compared with mounting traditional reinforced ribs of sprayed concrete (as long as the workers are experienced). The pantex arches are stiffer and demands a smooth contour to avoid using too much sprayed concrete. Rock support by the use of pantex arches therefore became a little more expensive compared with traditional reinforced ribs of sprayed concrete. The convergence measurements together with the engineering geological investigations and inspection from a working platform, was used as basis for making decisions concerning the permanent support. The experience from the Frøya tunnel demonstrates the importance of having an experienced rock engineer at the working site so that engineering geological investigations can be carried out before using sprayed concrete. Any possible weakness zone that may alter the total stability and in the worst case lead to failure, may then be recorded. Making the right decision at the working site is critical to achieve the best solutions. In weakness zones with exceptionally poor rock mass conditions, the tunnel floor became crushed due to the
loads from the quarrying trucks. Invert cast concrete was therefore placed 15–20 m behind the tunnel face. Invert cast concrete was also placed in those weakness zones where the convergence measurements revealed that the deformations did not culminate. The invert cast concrete proved to be very effective stopping further deformation. It created an arch effect that gave considerably better stability. In some parts of the tunnel problems with the bonding between the sprayed concrete layers were observed. The cause of the bonding capacity problem was that it was
Fig. 8. Picture showing spiling bolts anchored in the concrete lining, (from the Frøya tunnel).
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difficult to wash out the clay dust before spraying the next layer with sprayed concrete. Especially, a red clay in some of the weakness zones gave an adhesive clay dust when blasting. It was therefore focused on performing a good cleaning before spraying the next layer. 8. When should concrete lining replace reinforced ribs of sprayed concrete? Examples on conditions where spiling bolts and use of reinforced ribs of sprayed concrete are sufficient also as permanent rock support, have been shown. There are, however, situations when concrete lining is the proper and necessary choice. 8.1. Block falls When block falls have occurred and a risk for further development exists, concrete lining is often the best solution. This is because there will be a need for an inappropriate amount of sprayed concrete to fill the cavities before it is possible to install reinforced ribs of sprayed concrete. This will be a time-consuming and an expensive solution. Concrete lining is a proper alternative. 8.2. Inadequate hold for radial bolts When the rock mass conditions were so poor that it was difficult to anchor the radial bolts at the Frøya tunnel, concrete lining was used as initial support at the tunnel face. In such conditions it was also impossible to anchor the spiling bolts at the rear edge in a satisfactory manner, and excavating further without concrete lining was unjustifiable. It is important in the stability evaluation to achieve a good dialogue with those performing the rock support. They are working close to the rock mass, and can give valuable details about rock mass quality and can, for example, give reports about problems with anchoring radial bolts. 8.3. Clay in combination with water The presence of clay is by many interpreted as a crucial factor for choosing concrete lining as permanent support. But the clay content in a weakness zone is not alone conclusive. In the Frøya tunnel several weakness zones with high clay content were supported permanently with spiling bolts and reinforced ribs of sprayed concrete. It is of great importance to study how the clay behaves in the weakness zone. Experience from the Frøya tunnel reveals that zones with high content of solid clay often had no water leakage, and this led to satisfactory stability achieved by the use of spiling bolts, radial bolts, sprayed concrete and reinforced ribs of sprayed concrete. In weakness zones where the clay appeared in gouges, mixed with crushed rock mass and usually with minerals
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like mica and calcite, water was frequently present. Less clay and more mica combined with water led to poorer stability. It was therefore, necessary to use cast in place concrete in a 35 m long weakness zone. The most important thing when deciding if cast in place concrete lining at the tunnel face is necessary, is to be present at the tunnel face for mapping of the rock mass conditions. 8.4. Continued deformations For decision about type of permanent rock support, convergence measurements gives important input, in addition to visual observation. If no cracks are observed in the sprayed concrete, and the convergence measurements indicate that the deformations have stopped, there is no need for cast in place concrete. 9. Numerical modelling of variable rock support in weakness zones at the Frøya tunnel For evaluating the effect of different types of rock support of weakness zones in the Frøya tunnel, numerical modelling (UDEC) was performed, (Løset, 1998). In the model, the zone at profile No. 4000 was chosen as a representative zone, and input parameters were selected based on mapping, evaluation of cores covering all of the actual zone together with on-site inspection in the tunnel. The analysis was performed with variable rock support and in two stages. Stage 1: A. 25 cm sprayed concreteqbolts 1.5=1.5 mqinvert cast concrete placed some time after the other rock support. B. 25 cm sprayed concreteqbolts 1.5=1.5 mqinvert cast concrete laid at the same time as the other rock support. C. 25 cm sprayed concreteqbolts 1.5=1.5 m (no invert cast concrete). Stage 2: D. 25 cm sprayed concreteqbolts 1.5=1.5 mqreinforced ribs of sprayed concrete (cyc 2 m)qinvert cast concrete laid some time after the other support. E. Cast in place concreteqinvert cast concrete laid some time after the other support. The model assumed that all support, except from invert cast concrete, was installed after a 10–12 mm deformation had been developed. In this rock mass, that means approximately 1–2 h after blasting. The results revealed that the time for invert cast concrete in relation to the potential deformation development was important. The invert cast concrete led to less deformation and thereby less load on the remaining
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support. When the invert cast concrete was placed at the same time as the other rock support, both deformation and bolt load were at minimum. However, the most practical was to place the invert cast concrete some time after the other rock support. The relatively strong reinforced ribs of sprayed concrete that were modelled, gave a rock support as good as cast in place concrete.
● Do differences exist in the concrete quality when using several layers of sprayed concrete compared with massive casting? Would it be wise to reduce the number of layers with sprayed concrete? ● Which requirements have to be made concerning the concrete quality? Is there a difference in durability between sprayed concrete and conventional constructional concrete?
10. Further research on the effect of spiling bolts
12. The durability of sprayed concrete with alkalifree accelerators
Further research is necessary to answer when and how spiling bolts should best be used. Aspects that ought to be elaborated more: ● Prepare guidelines for when and how spiling bolts should be used in relation to rock mass quality, water, fall outs on last blast round, results from probe drilling and core drilling. ● Type of results from spiling bolts, dimension and length, anchoring length within the blast round, steel qualityycorrosion shield. ● Anchoring at the rear end, spacing between bolts and how much of the profile needs spiling bolts. ● The effect as permanent rock support, or as integrated part of the permanent support. To find the answers to these questions, it is necessary to learn and put together the experience from projects both in Norway and abroad. 11. Practical effect of cast in place concrete and reinforced ribs of sprayed concrete In an environment, like the Norwegian tunnelling industry, where cast in place concrete is normally not a requirement, it is often discussed when it is right to use cast in place concrete lining, and when sprayed concrete and possibly reinforced ribs of sprayed concrete is the right choice. Although some practical guidelines have been established, more research is needed. There are several questions that have to be answered. ● Gather experience from projects in Norway and abroad, especially from cases where the stability has been unsatisfactory. ● Is it possible to document load transfer with the aid of stress measurements in cast in place concrete and reinforced ribs of sprayed concrete? Is there a difference between the two supporting methods concerning what practical effect they have? ● Further studies with the aid of numerical modelling of cast in place concrete and reinforced ribs of sprayed concrete. Is it correct that these supporting methods work equally? What are the limits for use of reinforced sprayed concrete compared with cast in place concrete?
When excavating through weakness zones and using reinforced ribs of sprayed concrete, concrete with alkalifree accelerators will be used extensively in near the future. It is therefore a natural question to ask if concrete with alkali-free accelerators is as durable as traditional sprayed concrete with alkali silicate. Norwegian Public Roads Administration, Directorate of Public Roads has, in connection with the Project ‘Health Environment and Safety—Sprayed Concrete’, examined the accelerators impact on the concrete quality, (Storas ˚ et al., 1999). The results revealed that sprayed concrete with alkali-free accelerator does not give any poorer durability compared to traditional alkali silicate accelerator. This is despite the fact that one can achieve a considerably higher early strength under otherwise equal conditions. Generally, it seems like the use of alkali-free accelerator leads to a more uniform material compared with the use of alkali silicate. 13. Conclusion In many occasions use of spiling bolts and reinforced ribs of sprayed concrete may replace cast in place concrete. It is difficult to set limits for when spiling bolts should be used with the aim of Q-values. As a guideline in very poor to extremely poor rock mass conditions, centre distances from 50 to 25 cm could be a starting point. Reinforced ribs of sprayed concrete is a flexible supporting alternative, where it is simple to adjust the distance between the arches and to vary the thickness of the sprayed concrete depending on the rock mass conditions. The traditional reinforced ribs of sprayed concrete are more flexible concerning installation, and works better than ready-made arches when large block falls have occurred. In order to optimise the choice of rock support in poor zones, careful excavation is important, where probe drilling and possibly pregrouting are part of the excavation method. Besides that, it is of great importance to have a well qualified crew at the tunnel face representing both contractor and owner. An experienced rock engineer should be present at the tunnel face to carry out
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the geological mapping of the rock mass before it is covered with sprayed concrete. Convergence measurements should be a natural part of the decision basis for determination of permanent support in weakness zones. This gives good documentation on how the initial support works, and if it is sufficient. Further research is necessary to find answers to when and how spiling bolts should be used. In addition, research is needed to shed light on the practical effect of cast in place concrete compared to reinforced ribs of sprayed concrete.
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References Nilsen, B., Palmstrøm, A., 2000. Handbook No 2, Norwegian Group for Rock Mechanics. The Q-system, pp. 146–149. Lien, J.E., Mehlum, A., Moe, L.E., Lillevik, S. Soknes, S. (2000): The Frøya tunnel and adjoining road network—Final report, Norwegian Public Roads Administration (in Norwegian). Løset, F. 1998, The Frøya tunnel, numerical modelling of a typical weakness zone (in Norwegian), Norwegian Geotechnical Institute (NGI) report 981040-01. Storas, ˚ I., Bakke, B., Hauck, C., Ulvestad, B., Davik, K.I., Moen, A.B. (1999), Norwegian Public Roads Administration, Planning and Construction Dpt., Project Health Environment and Safety— Sprayed Concrete, publication nr. 94 (in Norwegian).