Settlement Minimisation above Shallow Cover Driven Tunnels using a Shotcrete Lining

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1 Settlement Minimisation above Shallow Cover Driven Tunnels using a Shotcrete Lining E. J. Nye Senior Project Manager and Tunnel Specialist Mott MacDonald Australia Pty Ltd Level 2, 60 Pacific Highway St Leonards, New South Wales, 2065, Australia edward.nye@mottmac.com.au ABSTRACT The design of large diameter driven tunnels with shallow ground cover requires a thorough understanding of both design and construction. The ground conditions are of course an important parameter, however, it appears quite common to assume that such a tunnel is predominately a geotechnical problem rather than a structural one. When the structural interaction with the ground and structural performance are both understood it is possible to provide an efficient structural solution which will cause minimal surface settlement. There are various structural options and construction methodologies to choose from when designing a shallow cover tunnel, this paper demonstrates that a tunnel fully supported initially by canopy tubes and shotcrete (with shotcrete replacing heavy steel sets), offers the most efficient and most likely method to achieve the desired minimum and predicted surface settlement values and consequently not damage surface structures or disrupt surface traffic. KEYWORDS Shotcrete, Support, Canopy, Driven, Tunnel, Settlement, Prediction, Mitigation, Stability, Construction, Building, Damage, Face, Loss, Analysis, Monitoring, Shallow, Cover. INTRODUCTION The excavation of a shallow tunnel in weak ground has the potential to cause significant surface settlement that if not mitigated, may lead to the selection of an alternative more disruptive and potentially slower method of construction such as a cut and cover tunnel. Over the past fifteen years there has been a significant shift from the use of heavy steel sets to shotcrete as the initial ground support in shallow cover driven tunnels. The shift has not, however, been universal, especially from the author s experience in Australia. Even when the method has been demonstrated through project examples there still appears to be a lack of understanding or perhaps a lack of confidence to try something which appears to be new. This paper therefore tries to increase the industry s understanding of a tunnelling method that generally has more acceptance overseas than is apparent in Australia with a brief theoretical background together with reference to two completed tunnel case studies. BACKGROUND Design and Construction There are two important elements to providing a serviceable structure like a driven tunnel. There is firstly its design and secondly its construction. The design is very much dependent upon the designer having a detailed knowledge of the construction process. There are two basic functions to be performed by a tunnel lining, firstly and most obviously is load bearing of both dead loads (ground and surface structures) and live loads (vehicle traffic, cars and

2 trucks, trains etc) and secondly for the minimisation of ground movements generally for the purpose of mitigating excessive settlement (and also to inherently provide increased ground stability). Settlement Estimation Theory Ground movements can be related to face loss which in a driven tunnel excavation is the result of ground convergence into the tunnel opening away from the tunnel excavation face, down from the tunnel crown and convergence inwards of the tunnel walls. The face loss is in the theoretical tunnel face area reduction per unit length (or volume loss) of the tunnel caused by these movements. These combined movements result in the development of a surface settlement trough parallel to the tunnel alignment of varying width and depth. The area under a normal probability curve at the surface defining the settlement trough profile is assumed to equal the face loss area in the tunnel itself. Computer analyses including both 2D and 3D finite element methods, are becoming more universally adopted and in many cases provide a more accurate method of settlement prediction when combined with sound experience of tunnel practice and a good geological model. The empirical method referred to above uses the following equation which describes a normal distribution curve to define the profile of the settlement trough. Y = Y max.exp(-x 2 /2i 2 ) Where Y is the settlement at a distance X from the tunnel centreline, Y max is the maximum settlement (at X = 0), and i is the distance to the point of inflection of the normal probability curve. If an initial estimate of Y max and i are determined it is possible to plot the theoretical predicted settlement curve over the tunnel. Back calculation of the of the settlement trough volume, V s, under the probability curves is given by the equation: V s = 2.5iY max Example plots of empirical settlement troughs compared to actual field data are given in: (Nye, 2001 and Rankine, 1987 who explains fully the formulae defining the empirically derived settlement trough). Note that the construction methods in the Rankine paper have largely been superseded by further refining the tunnelling methods or by more recent tunnel machine technology developed over the past 25 years. Quoted V s values in Table 2 (Rankine) have been significantly reduced (quoted figures in Table 2 are in the range of 0.5% to 10 %). The canopy tube method used in conjunction with shotcrete as used in the tunnels referred to in this paper have V s values in the range of 0.2% to 0.5%. While there are differences in the geology between tunnel sites (which may or may not be significant), many in this industry still have an expectation that higher values of V s are the norm, particularly quoting V s in the range of 1% to 1.5% even when canopy tubes have been specified (canopy tubes support the ground above the initial excavation opening prior to applying shotcrete, refer to Figures 3, 4 and 5). Load - Deformation A hypothetical Load Deformation curve with two support reaction lines has been plotted in Figure 1 below and is based on a similar diagram that is given in Hoek and Brown (1980). However, this reference diagram was developed particularly for deep tunnels and the introduction to Chapter 8 (Hoek) states The principal objective in the design of underground excavation support is to help the rock mass support itself. While this is a basic premise of deep tunnels to optimise the support design the opposite is true of very shallow tunnels where The principal objective of the tunnel support and construction methodology is to attract as much ground load to the tunnel support as possible. The premise here is that the tunnel lining will be much stiffer than the surrounding ground and not allow the ground to relax and arch with a consequential outcome of relaxation being ground movements including surface settlement.

3 Figure 1. Load/Deformation curves for a shallow tunnel with a stiff (AD) and less stiff support type (ABC). The small diameter canopy tubes over the arch at the tunnel face provide initial deformation restraint of the excavation, a stiff shotcrete lining applied at the tunnel face will maintain the line AD after the tunnel face advances well past this point in the tunnel. This concept is further discussed later in this paper. Note that the canopy tubes function is transient and they do not contribute to the long term support of the tunnel. SHOTCRETE PROPERTIES The wet mix shotcrete is pneumatically applied concrete using compressed air. The shotcrete mix design is specifically optimised for compaction and workability (densities of 2300 kg/m 3 are achievable) and early strength gain (using a low water cement ratio and accelerator). An accelerator is added at the shotcrete nozzle to ensure early high strength gain in the first few hours after application. To ensure pump-ability and to provide a smooth shotcrete surface the maximum aggregate sizes are in the range of 7mm and 10mm. For the tunnel construction as described in this paper early strength gain of the shotcrete is paramount for the confinement of the surrounding ground and to allow the construction progress of the tunnel to proceed at an acceptable rate. Early strength gain also allows thicker shotcrete to be installed in one pass, especially overhead. For this reason early strength tests are carried out in the field. Table 1 gives typical early strengths specified against time and the appropriate test applicable at each time interval.

Compression (ASTM C116) 24 hours 18 Core Compressive Strength (AS1012) 3 days 26 Core Compressive Strength (AS1012) 7 days 35 Core Compressive Strength (AS1012) 28 days 40 Core Compressive Strength

4 Table 1. Early and minimum strengths, test applied and calculated E-shotcrete Age Minimum strength (MPa) Test applied 3 hours 1 Initial in-situ strength Meyco Needle Penetrometer 12 hours 6 Sprayed Beam Compression (ASTM C116) 24 hours 18 Core Compressive Strength (AS1012) 3 days 26 Core Compressive Strength (AS1012) 7 days 35 Core Compressive Strength (AS1012) 28 days 40 Core Compressive Strength (AS1012) Elastic Modulus (MPa) 1,000 12,000 20,000 24,000 28,000 30,000 Where in Table 1 the elastic modulus has been calculated using the following AS3100 formula at Clause 6.1.2, E c = 0.043ρ 1.5 f c. In which ρ is the density of the shotcrete in kg/m 3 and f c is the compressive strength. (a) (b) Figure 2. Early strength testing in the field using a Meyco Penetrometers (a) and Sprayed Beam Compression test (ASTM C116) (b). Field testing equipment, Figure 2(a) is a photograph of the Meyco Penetrometers which is required at the 1 hour time point and 2(b) similar for the Beam Test at 12 hours. Table 2 is a summary of actual field data and also includes standard cylinder strength results. The f c of the shotcrete was specified as 40MPa. Table 2. Actual minimum and maximum strength range of shotcrete for various age ranges and tests (Boggo Road Busway driven tunnel) Age Specification Core Strength* Cyl. Strength* Hours Days (MPa) Min Max Min Max (MPa) 1.2 (MPa) 0.6 (MPa) 1.2 (MPa) (B) 7.3 (B) 5.2 (B) 7.3 (B) days days days days * Unless otherwise noted, MP = Meyco Needle Penetrometer, and B = Sprayed Beam Compression

CASE STUDY EXAMPLES Two case studies are described here (refer to Figures 3, 4 and 5 following).

The tunnel profiles are horse shoe shaped with the curved arch over the tunnel crown.

support of canopy tubes and shotcrete). So we are talking here about very large openings and relatively very small movements.

5 CASE STUDY EXAMPLES Two case studies are described here (refer to Figures 3, 4 and 5 following). The case studies are for tunnels that are 14m wide with 5m (Boggo) cover and 19m wide at the portal and with 3m ground cover (Buranda). The tunnel profiles are horse shoe shaped with the curved arch over the tunnel crown. The magnitude of ground movements are in the order of millimetres, perhaps 2mm to 5mm in the tunnel walls and tunnel face and in the tunnel crown in the order of 5mm to 20mm (with combined initial support of canopy tubes and shotcrete). So we are talking here about very large openings and relatively very small movements. The geology is weak rock or a mixed face including stiff clays (Nye, Kitson and Cinniah, 2009). For an example of a soft ground tunnel which also includes a fully shotcreted invert refer to Gibbs et al (2002). Larger movements may cause building damage, or for speed restrictions to be placed on suburban trains running overhead. So to achieve the minimum of settlement a high degree of ground control is required and canopy tubes in combination with a shotcrete lining placed close to the tunnel face has the ability to consistently provide a high degree of ground control and settlement predictability. At Buranda the maximum settlement was 20mm (allowable 25mm) with a 200mm thick shotcrete lining and at Boggo Road 10mm (predicted 10mm) shotcrete with 350mm shotcrete applied near the tunnel face. Both tunnel used canopy tubes. Boggo Road had stiff clay above the tunnel crown. (a) (b) Figure 3. Photographs of Buranda (a) and Boggo (b) , tunnels (Brisbane). Figure 4. Boggo Road Jail tunnel vertical alignment and geological model.

6 (a) (b) Figure 5. Tunnel cross section (a) with 12m long canopy tubes over the tunnel crown and long section with canopy tube profile (b). The question then becomes how sensitive is the surface/near surface structure to settlement that could cause damage or surface disruption and then was it the most appropriate support method and construction methodology appropriate to a particular situation. In the past small diameter (around 100mm to 140mm dia.) canopy tubes in combination with heavy steel sets have been used, however, with all human endeavour the boundaries of knowledge are always moving forward and in recent years light steel lattice girders with shotcrete and even more lately shotcrete alone is becoming more common. Table 3. Comparing the axial stiffness of a shotcrete lining with steel sets/lattice girder. Material/Section /age Thickness or section size (mm) X-area mm 2 /m length of tunnel Modular Ratio* Equivalent X-Area (mm 2 ) to 28 day old shotcrete Axial Stiffness Ratio to Shotcrete at 24 Hours Shotcrete1 (12 hrs) , , Shotcrete2 (24 hrs) , ,000 1 Shotcrete3 (3 days) , , Shotcrete 4 (28 days) , , Steel Section UC37 4, , Steel Section UC89 11, , Lattice Girder 3** 2x32,1x25 2, , * the modular ratio of steel to 28 day old shotcrete is 7. **Lattice girder - 3 is the girder used on Boggo Road.

7 The author is not aware that any tunnels have been constructed with canopy tubes and shotcrete alone in Australia. A final qualification here is that the tunnels in question have an arched profile with little, if any, tension on the inside face of the shotcrete. The arched profile provides a compressive thrust throughout the arch which should be confirmed by finite element analysis. Steel or GRP (Glass Reinforced Plastic) fibres can also be included in the shotcrete mix design to provide a reserve of flexural capacity (as can light lattice girders). The section of tunnel under the heritage buildings with canopy tubes and lattice girders (Figures 3(b), 4 and 5 was excavated at a rate of approximately 9m per week (11 shifts, two 10 hours working and 4 hours maintenance over five and half days per 24 hour working) with each excavation cycle 1m in length. In practice the full 350mm thickness shotcrete was kept very close to the tunnel face. In the worst ground conditions from the east portal the surface settlement was 10mm above the centreline of the tunnel which was equal to the predicted value. Steel sets have their place but not in this particular situation. Apart from their technical disadvantages the cost of supply and steel fabrication alone today is around $10,000/tonne and the void between the steel sets spaced say at 1m intervals along the tunnel has to be filled anyway with either shotcrete or in-situ concrete. The use of shotcrete alone, without steel sets, also raises the justification of the shotcrete being used as both the initial tunnel support and the permanent support. There is a durability advantage where it is possible to not have steel embedded in shotcrete. With steel reinforcement are numerous examples of corrosion and spalling in permanent tunnel linings even when the steel is embedded in in-situ concrete. Lattice girders are also more preferable to steel sets (typically UC sections) because the likelihood of shotcrete voids caused by shadowing around the individual steel bars is significantly less. Referring back to the Load Deformation plot Figure 1, line ABC is what we would might expect during tunnelling using steel sets (even with canopy tubes). The initial deformation AB could be the result of poor timber blocking between the back of the sets and the surrounding ground and settlement of the isolated steel set footing. While past practice has been to wedge the footings of the sets or timber block around the excavation perimeter behind the steel set to preload the set this environment is equivalent to working under unsupported ground which is now considered bad practice. In contrast shotcrete is applied robotically and more quickly (with outside a 3m to 5m exclusion zone near the tunnel face) and the shotcrete is in direct contact with the ground with no voids. Also slope of line BC is less than line AD because the steel sets are not as stiff as an uncracked shotcrete lining (refer Table 3). For this reason line AD is more likely to be achieved. The footing of the shotcrete arch is also a strip footing, which can also be locally widened if necessary (if poorer ground is encountered to decrease the load bearing pressure). This can be compared to the isolated footing under steel sets spaced at 1m intervals. To explain further as the tunnel advances the load imposed by the ground increases to eventually reach near equal to the full overburden of the ground above. This process is complete at about one tunnel diameter from the tunnel face. At this point the surface settlement should have asymptotically reached its maximum value. Also compared to two previous tunnels one in Sydney (New Southern Railway at Cleveland Street, Ashe. H, 1996) and later used in Brisbane (Vulture Street) it has been possible to demonstrate with using the Boggo Road as an example that pre-excavation of large side drifts and back filling these with mass concrete to form side wall drifts is not necessary. There are two other items worth mentioning here as well. Firstly, at Boggo Road it was decided to a have a constant width tunnel (to eliminate the tunnel width variable on settlement). The saw tooth profile resulting from the method required to install the canopy tubes was in the crown alone, the canopy tubes did not fan outward along the side walls of the tunnel. Secondly, as the canopy tubes were 12m long with a 3m overlap (refer to Figure 5, (b)) each cycle of canopy tube installation was spaced at 9m (i.e. less than a tunnel diameter). The space created by the step in the

8 saw tooth profile allowed extra shotcrete over the arch to form an even stiffer arch section potentially halting the asymptotic settlement curve trend before the tunnel face had advanced the full tunnel width of 14m (additional details in Nye and Alt 2010). Finally, it is important to understand that the tunnel arch will not be as stiff if the base/side walls of the arch are not restrained laterally, in weak rock as is the case in these two examples this was the geological setting. Provided this is the case the quality of the ground above the arch is of very much less consequence. CONCLUSIONS Large diameter driven tunnels can be constructed under sensitive structures. There is a fine line between success and failure as measured by either damage or by the surface disruption caused. The settlement risks are mitigated by specifying in detail the construction methodology as well as good design. There is increasing evidence that because shotcrete is much stiffer support than say isolated steel sets that not only are settlement magnitudes minimised but the chances of a consistently successful result are much higher. Shotcrete applied with today s modern robotic equipment also means tunnel construction is much safer. The installation of large steel sets still requires some man handling near the tunnel face which may not necessarily be stable. Again precautions at the face are paramount, for example using fibre glass face nails. Quality control of the shotcrete is always important but more experience is being gained with each project and the field testing methods for low strength shotcrete in its initial stages of curing have proven to be reliable indicators. ACKNOWLEDGEMENT The Buranda Busway tunnel was constructed by Thiess Contractors with SMEC as designer, the Boggo Road Busway driven tunnel was delivered by the Boggo Road Busway Alliance which included the Department of Main Roads and Transport, Queensland, Thiess Contractors and Sinclair Knight Merz (SKM). The author was the lead tunnel designer when employed previously by SKM. Mott Macdonald Australia Pty Ltd has recently completed a reference design for a significant shallow cover driven tunnel railway crossing in NSW. The author was also a reviewer on behalf of the Roads and Traffic Authority of the M5 East exit ramp shotcrete tunnel (Gibbs et al 2002) and the New Southern Railway tunnel (Ashe, 1996) on behalf of the Rail Access Corporation. REFERENCES Hoek E. And Brown. E. T. (1980). Underground Excavations in Rock. The Institution of Mining Engineers, London. Figure 129, pp 245. Rankine W. J. (1987). Ground movements resulting from urban tunnelling: prediction and effects. Engineering geology underground movements, ed. F.G. Benn, Geological Society of London. pp Asche H. (1996). The Prediction of Ground Settlement and the Design of Support Systems in Shallow Tunnels in Weak Rock. IX Australian Tunnelling Conference, Sydney, pp Nye E. J. (1999). The Soft Ground Tunnel Under Sydney Airport. Tenth Australian Tunnelling Conference, Engineers Australia/AusIMM, pp Gibbs P. W., Lowrie J., Keiffer S. and McQueen L. (2002). M5 East The design of a shallow soft ground shotcrete motorway tunnel. AITES ITA Downunder 2002, 28 th ITA Genral Assembly and World Tunnel Congress. pp Nye E. J., Kitson M. and Cinniah R.(2009). Boggo Road Busway Project, Brisbane. Rapid Excavation and Tunneling Conference, Las Vegas, pp Nye E. J. and Alt D.(2010). Shotcrete Application on the Boggo Road Busway driven tunnel Shotcrete Elements of a System, Taylor & Francis, London, pp,

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