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 lon-gevity of elements in tension is limited. For exam-ple, 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, Austra-lia undertaken since 1990 and the details of the in-stalled rock bolts which are supposed to have equivalent design lives. The term rock bolt is used here in a generic sense and covers reinforcing ele-ments 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 ex-ample one may attempt to assess how long a Split-Set bolt may last in a particular tunnel given knowl-edge of the groundwater chemistry. This is the ap-proach 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 cor-rosion protection of all components. The intent is that “in the event of perforation of one of the two barriers during installation or loading, the remain-ing 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 protec-tion, 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 pro-jects 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, 2002. 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 car-ried 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 accept-able for 100 year design life in Sydney tunnels.

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 pub-lished 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.

Table 1 Permanent Rock Bolts Used in Sydney.

Project Year Permanent Rock Bolt Design

Life Reference

Opera House Carpark 1990 Epoxy coated steel bolts fully cement grouted in

45mm diameter holes

50 Pells et al, 1991

M2 tunnel 1993 Black steel 24mm diameter bolts fully cement

grouted in 44mm diameter holes

100 Braybrook,

1993

Soil nail structures at

Olympic Park & Dev-

lin St

1995 Open ended sheathed black steel M20 bolts fully

cement grouted in 45mm diameter holes (CT-Bolts)

100 Project design

report

Wombarra drainage

tunnel

1997 Black steel 24mm diameter deformed bolts fully

chemical resin encapsulation in 27mm diameter

holes

80 to 100 Project design

report

West Ryde drainage

tunnel

1998 Black steel 24mm diameter deformed bolts fully

chemical resin encapsulation in 27mm diameter

80 to 100 Project design

report

Epoxy coated steel bolts fully cement grouted in

45mm diameter holes

50

Closed ended sheathed multi-strand cable bolts fully

cement grouted in 45mm diameter holes (Freyssi-

bolts)

50

Eastern Distributor

tunnel

1998

Stainless steel bolts fully cement grouted in 45mm

diameter holes

75

Pells and

Bertuzzi, 1999

Bondi pumping cham-

ber repair

1998 Stainless steel bolts fully cement grouted in 45mm

diameter holes

75 Project design

report

Northside Storage tun-

nel

1999 Fibreglass bolts fully resin encapsulated 100 Asche & Quig-

ley, 1999

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)

100 Adams et al,

2001

Cross City tunnel 2003 Partially closed ended sheathed black steel bolts

with stainless head assembly fully cement grouted

in 45mm diameter holes (BBB-Bar)

100 Asche &

Lechner, 2003

Specially designed open ended sheathed multi-

strand cable bolts (MegaBolts) and single strand

cable bolts (CT-Strand)

Epping to Chatswood

Rail Link station cav-

erns

2003

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 Project design

report

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 environ-ments 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 sub-jected to tension and shear loading. Design solu-tions typically offered are based on defining a maximum value for acceptable movement above which something must be done, including re-

bolting, multiple stage grouting and the inclusion of a frangible or compressible grout. It goes with-out 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, neck-ing and ultimately tensile failure. The failure in-volves composite paths: failure along the outer duct face over a proximal length translating to group strand failure and thence to multiple individ-ual 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 axi-ally and shear. A cable bolt typically can accom-modate even more. However, if the object is to maintain the corrosion protection, then the amount of deformation that a bolt can be designed to with-stand is that which ruptures the plastic sheathing. In other words, in many civil applications high ca-pacity steel rock bolts are now being designed on the tolerance of the plastic sheath.

5 EXISITNG EXPERIENCE

5.1 Exhumed Support

Weerasinghe & Anson (1997) investigated the con-dition of multiple strand cable anchorages after 22 years in a marine environment. The cables com-prised greased and sheathed free lengths and ce-ment grouted unsheathed fixed lengths. Interest-ingly, while there was evidence of general corrosion there was negligible loss of strand sec-tion 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

connects to the stressing head and locking wedges. This case study suggests that that perhaps BS8081 is too restrictive in dismissing cement grout encap-sulation 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 corro-sion protection.

During 1997, excavation of a basement at No.2 Bond St Sydney Steel intersected several steel strand cables that had been installed in 1972. 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 re-places BS8081, does allow cement grout to be con-sidered 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 plas-tic sheaths, made after approximately 37mm of shear, were that “the sharp edged grout fragments had severed and torn it (the sheath).” Barley fur-ther 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 Common-wealth Games Stadium in Manchester, UK, they were replaced with straight anchors during con-struction. In underground excavation, it is not pos-sible to restrict permanent rock bolts to axial loads.

6 CURRENT EXPERIMENTS

6.1 Procedure

This author with his colleagues has carried out lim-ited 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 to-gether 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).

The second series of tests was substantially more sophisticated and involved combined shear and axial loading of the corrugated plastic duct (re-fer 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 lo-cated on top of the bottom block to ensure the top block slide smoothly when pushed. A 65mm di-ameter 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 in-spected for damage. In one of the tests the hori-zontal movement was incrementally advanced; the test taking a week to reach the 18mm of horizontal movement.

Figure 2. General layout of the apparatus in the first series of tests.

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 epoxy-coated 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 shortcom-ings 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 pre-tension.

Figure 3. General layout of the apparatus in the second series of tests.

Figure 4. Close-up of damaged plastic duct after 15mm of di-rect shear movement

Figure 5. Close-up of damaged plastic duct subjected to com-bined shear and axial movement (after 18mm of shear)

Figure 6. DSI's rig for direct shear test

Figure 7. DSI's epoxy coated strand subjected to 23mm of di-rect shear movement

7 CONCLUSIONS

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 Syd-ney tunnels. However, in order to maintain the cor-rosion protection, the amount of deformation that a bolt can be designed to withstand is that which rup-tures the plastic sheathing. In many civil applica-tions 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 encap-sulated 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 influ-ence 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.

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