100-Year Design Life of Rock Bolts and Shotcrete

Transcription

1 100-Year Design Life of Rock Bolts and Shotcrete R. Bertuzzi Pells Sullivan Meynink Pty Ltd ABSTRACT: One of the main technical challenges of underground construction for public space is ensuring the long-term design life of support components. It is common for owners to specify a 100-year design life. Can designers, suppliers and constructors guarantee this? A few papers discussing this topic have been published over the past decade but as more underground public spaces are being built in Australia, the terms permanent rock bolt and permanent shotcrete have come under greater scrutiny. This paper presents the authors recent experience in relation to providing a permanent rock bolt and shotcrete support. 1 INTRODUCTION The trend in tunnel design in Australia is to specify a 100 year design life for permanent support. Often that support is provided by rock bolts and shotcrete. Experience with concrete in compression extends back to Roman times but experience with the longevity of elements in tension is limited. For example, the British Standard BS8081 (Code of Practice for Ground Anchorages) has been available to guide the design of permanent rock bolts since the early 1990s. This paper looks at the projects in Sydney, Australia undertaken since 1990 and the details of the installed rock bolts which are supposed to have equivalent design lives. The term rock bolt is used here in a generic sense and covers reinforcing elements comprising bars and strands. 2 DESIGN LIFE 2.1 Rock Bolts The life expectancy of rock bolts can be addressed from two viewpoints. The first is to attempt to assess the probable functional life of a given type of rock bolt in a given hydrochemical environment. For example one may attempt to assess how long a Split- Set bolt may last in a particular tunnel given knowledge of the groundwater chemistry. This is the approach typically adopted by the soil anchor industry by incorporating an allowance for corrosion rate and including sacrificial steel (example NSW RTA s specification for the Design of Reinforced Soil Walls). The second approach is to attempt to eliminate uncertainty by developing corrosion protection measures to provide a substantial level of safety. This is the approach presented in BS8081, which suggests permanent rock anchors require double corrosion protection of all components. The intent is that in the event of perforation of one of the two barriers during installation or loading, the remaining barrier must remain intact (Barley 1997). In following this path it becomes readily apparent that attention to detail is critical (Pells & Bertuzzi, 1999). It is worth noting that BS8081 discounts the use of sacrificial steel stating it gives no effective protection, as corrosion is rarely uniform and extends most rapidly and preferentially at localised pits or surface irregularities. Table 1 provides a list of recent tunnelling projects in Sydney and the type and accepted design life of rock bolt support. A good engineering description of the rock mass conditions in Sydney is given in Pells, 2003 and Bertuzzi & Pells, All that can be said of the actual durability of the rock bolts used in these projects is that there have been no failures to date. Limited over-coring of rock bolts has been carried out and to the author s knowledge this has been done on temporary bolts. Nevertheless, there appears to be an acceptance by the industry that carbon steel bolts cement grouted in an open ended plastic sheath are acceptable for 100 year design life in Sydney tunnels.

2 Table 1 Permanent Rock Bolts Used in Sydney. Project Year Permanent Rock Bolt Opera House Carpark 1990 Epoxy coated steel bolts fully cement grouted in 45mm diameter holes M2 tunnel 1993 Black steel 24mm diameter bolts fully cement grouted in 44mm diameter holes Soil nail structures at 1995 Open ended sheathed black steel M20 bolts fully Olympic Park & Devlin cement grouted in 45mm diameter holes (CT-Bolts) St Wombarra drainage tunnel West Ryde drainage tunnel Eastern Distributor tunnel 1997 Black steel 24mm diameter deformed bolts fully chemical resin encapsulation in 27mm diameter holes 1998 Black steel 24mm diameter deformed bolts fully chemical resin encapsulation in 27mm diameter 1998 Epoxy coated steel bolts fully cement grouted in 45mm diameter holes Design Life Reference 50 Pells et al, Braybrook, Project design 80 to 100 Project design 80 to 100 Project design 50 Pells and Bertuzzi, 1999 Closed ended sheathed multi-strand cable bolts fully 50 cement grouted in 45mm diameter holes (Freyssibolts) Stainless steel bolts fully cement grouted in 45mm 75 diameter holes 1998 Stainless steel bolts fully cement grouted in 45mm 75 Project design diameter holes 1999 Fibreglass bolts fully resin encapsulated 100 Asche & Quigley, 1999 Bondi pumping chamber repair Northside Storage tunnel M5 East tunnel 2000 Open ended sheathed black steel M20 bolts fully cement grouted in 45mm diameter holes (CT- Bolts), and open sheathed black steel cable bolts fully cement grouted in 50mm diameter holes (MegaBolts) Cross City tunnel 2003 Partially closed ended sheathed black steel bolts with stainless head assembly fully cement grouted in 45mm diameter holes (BBB-Bar) Epping to Chatswood Rail Link station caverns 2003 Specially designed open ended sheathed multistrand cable bolts (MegaBolts) and single strand cable bolts (CT-Strand) Open ended sheathed black steel, coarse threaded steel bar bolts with stainless head assembly fully cement grouted in 45mm diameter holes (DCP & CT-Bolt) 100 Adams et al, Asche & Lechner, Project design 2.2 Shotcrete Concrete technology is applicable to shotcrete. In the case of the Eastern Distributor a sacrificial thickness of shotcrete was required because of the local high acidity of the groundwater chemistry. No special treatment was required for the other projects in Sydney. However, a complete water barrier may be required in groundwater environments more aggressive than Sydney, which gets us away from a rock reinforcement design to one of a passive lining. Shotcrete is not discussed further in this paper. 3 WHAT ARE THE PROBLEMS NOW Some of the aspects currently being considered by tunnel designers are the details relating to the rock bolt head assemblies, temporary anchorage during grouting, rupture of plastic sheathing due to ground movement and final shotcrete cover. Of these the main issue in the author s recent experience is the potential for the plastic sheath to rupture when subjected to tension and shear loading. Design solutions typically offered are based on defining a maximum value for acceptable movement above which something must be done, including re-

3 bolting, multiple stage grouting and the inclusion of a frangible or compressible grout. It goes without saying that none of these remedial measures are particularly attractive to the client or the contractor. 4 FAILURE MECHANISM OF ROCK BOLTS Rock bolts typically fail in tension. It may well be that the start of the failure was shear movement but that typically leads to the rock bolt bending, necking and ultimately tensile failure. The failure involves composite paths: failure along the outer duct face over a proximal length translating to group strand failure and thence to multiple individual strand pull-out of the distal component (Figure 1). This occurrence of progressive debonding is commonly accepted in the industry. Figure 1 Encapsulation, group strand and individual strand failure interfaces (Barley, 2003) While the rock bolt itself fails in tension, its corrosion protection may fail much earlier in shear or actually puncture. A rock bolt can accommodate a relatively large amount of displacement, both axially and shear. A cable bolt typically can accommodate even more. However, if the object is to maintain the corrosion protection, then the amount of deformation that a bolt can be designed to withstand is that which ruptures the plastic sheathing. In other words, in many civil applications high capacity steel rock bolts are now being designed on the tolerance of the plastic sheath. connects to the stressing head and locking wedges. This case study suggests that that perhaps BS8081 is too restrictive in dismissing cement grout encapsulation as part of a corrosion protection system. The industry in Australia appears to be of the view that the cement grout does provide a layer of corrosion protection. During 1997, excavation of a basement at No.2 Bond St Sydney Steel intersected several steel strand cables that had been installed in The cables had been cement grouted in holes drilled through sandstone. These cables all showed a sign of corrosion and one was corroded. This case study suggests that cement grout alone does not provide long-term corrosion protection. The European Code EN1537, which partly replaces BS8081, does allow cement grout to be considered to be part of the corrosion protection if it is within a plastic sheath and under working loads the cracks of the cement grout are less than 0.1mm width. 5.2 Previous Experiments Barley (2003) describes results of the relatively limited testing of sheathed anchors subjected to shear that have been carried out carried out since the 1970s in the UK. His observations of the plastic sheaths, made after approximately 37mm of shear, were that the sharp edged grout fragments had severed and torn it (the sheath). Barley further states that as a result of these tests, compliance with BS8081 has been restricted to axially loaded anchors, to wit while the concept of axial loading of curved stands and their corrosion protection components was recently tried for the Commonwealth Games Stadium in Manchester, UK, they were replaced with straight anchors during construction. In underground excavation, it is not possible to restrict permanent rock bolts to axial loads. 6 CURRENT EXPERIMENTS 5 EXISITNG EXPERIENCE 5.1 Exhumed Support Weerasinghe & Anson (1997) investigated the condition of multiple strand cable anchorages after 22 years in a marine environment. The cables comprised greased and sheathed free lengths and cement grouted unsheathed fixed lengths. Interestingly, while there was evidence of general corrosion there was negligible loss of strand section within the single corrosion protection anchor. The main area affected by corrosion was that around and immediately beneath the anchor head that is in the detail where the grease-filled sheath 6.1 Procedure This author with his colleagues has carried out limited shear tests on grout encapsulated plastic ducts. Two series of tests were carried out. The first series comprised grouting a 2mm thick walled corrugated plastic duct within two hollow steel tubes. The two steel tubes were bolted together while the duct was grouted. After 7 days, one of the steel tubes was anchored to the concrete pavement and a jack was used to push the second tube to simulated direct shear (refer to Figure 2). This author has also requested similar tests of bolt manufacturers. At the time of writing, some of the tests carried out by bolt manufacturer DSI were made available (Stevens, 2004).

4 The second series of tests was substantially more sophisticated and involved combined shear and axial loading of the corrugated plastic duct (refer to Figure 3). This series attempted to simulate the rock bolt within the rock mass. Two sandstone blocks separated by 5mm of clay were placed into a loading frame. Smooth fiberglass strips were located on top of the bottom block to ensure the top block slide smoothly when pushed. A 65mm diameter borehole was drilled through the blocks at 45 angle and a complete rock bolt (steel cable in this case and corrugated plastic duct) grouted into place. After 7 days, a jack was used to horizontally push the top sandstone block 15 to 18mm whilst restraining vertical movement of the block. The relative movement of the two blocks was measured using crack monitor gauge installed on two sides. Following the test, the bolt was over-cored and inspected for damage. In one of the tests the horizontal movement was incrementally advanced; the test taking a week to reach the 18mm of horizontal movement. 6.2 Results The data suggests that the tested corrugated plastic ducts are damaged at approximately 15mm of shear movement. The damage was consistent in all tests being caused by sharp fragments of broken grout puncturing the plastic duct. In the tests, little or no local failure of the sandstone around the bolthole occurred. The tests carried out by DSI on their epoxycoated cable, which were similar to the first series, suggest that this product is not damaged until about 20mm of shear movement. It is acknowledged that the first series of tests are simplistic because they do not represent the crushing of the rock; the dilatancy of the joint plane or the local debonding of the rock bolt; and hence may be overly aggressive. These shortcomings were partly addressed in the second series of tests. It is expected that the second series closely resembles the real case, although the test frame was too light to assess the influence of bolt pretension. Figure 3. General layout of the apparatus in the second series of tests. Figure 2. General layout of the apparatus in the first series of tests. Figure 4. Close-up of damaged plastic duct after 15mm of direct shear movement

5 7 CONCLUSIONS Figure 5. Close-up of damaged plastic duct subjected to combined shear and axial movement (after 18mm of shear) There appears to have been an acceptance by the industry that (i) cement grouts alone do not provide long term corrosion protection for carbon steel; and (ii) carbon steel bolts cement grouted in a plastic sheath is acceptable for 100 year design life in Sydney tunnels. However, in order to maintain the corrosion protection, the amount of deformation that a bolt can be designed to withstand is that which ruptures the plastic sheathing. In many civil applications high capacity steel rock bolts are now being designed on the tolerance of the plastic sheath. This author has reviewed the available data and has carried out limited shear tests on grout encapsulated plastic ducts. The data from these tests suggests that plastic ducts are punctured by sharp fragments of broken grout at approximately 15mm of shear movement. DSI carried out basic tests on their epoxy coated cable which suggest that this product is not damaged until about 20mm. The testing frame used was too light to assess the influence of bolt pre-tension however, further test work is continuing. 8 ACKNOWLEDGEMENTS The assistance of Hasan Wijaya, Matt Lowing and Mark Fowler in carrying out the tests and of Philip Pells in reviewing this paper is appreciated. Figure 6. DSI's rig for direct shear test Figure 7. DSI's epoxy coated strand subjected to 23mm of direct shear movement

6 REFERENCES Adams, D. N. & Lechner, M. K. & Lamb, I. M5 East Tunnels: A Flat Roofed, Bolt and Shotcrete-lined Highway RETC Proceedings, 2001 Asche, H. R. & Quigley, A Tunnelling design on the Northside storage tunnel project. 10 th Aust. Tunnelling Conf, IEAust, Melbourne, 1999 Asche, H. R. & Lechner, M. K. Design For The Cross City Tunnel, Sydney, RETC Proceedings 2003 Barley, A.D. Ground Anchor Tendon protected against corrosion and damage by a double plastic layer. Int. Conf. Ground Anchorages and Anchored Structures, Inst. of Civil Engineers, London, Barley, A.D. Double shear test on a corrugated plastic duct strand tendon encapsulated. Personal communication, August Baxter, D. Do all rock bolts rust? Can Q.A. help? Does it matter? Proceedings of 9th Australian Tunnelling Conf., IE Aust, Sydney, Braybrooke, J. Personal communication, 1993 British Standards BS8081, Code of Practice for Ground Anchorages, European Standard ENV Extensions of Special Geotechnical Work- Ground Anchors. Mothesville, D.K.V. & Weerasinghe, R.B. Current Rock bolting Practice, Tunnels and Tunnelling International, October Stevens, N. R. Dywidag-Systems International Personal communication, 2004 Pells, P.J.N & Bertuzzi, R. Permanent rockbolts - the problems are in the detail. Proc 10th Australian Tunnelling Conf., IE Aust, Melbourne, Pells, P.J.N., Poulos, H.G. & Best, R.J. Rock reinforcement design for a shallow large-span cavern. Proc 7th Int. Cong. Rock Mech, Aachen Weerasinghe, R.B. & Anson, R.W.W. Investigation of the Long Term Performance and Future Behaviour of Existing Ground Anchorages. Int. Conf. Ground Anchorages and Anchored Structures. Inst. of Civil Engineers, London, 1997.