Time Dependent Movements on the Billy Bishop Toronto City Airport Pedestrian Tunnel, Ontario, Canada

Jon Hurt, Sean Lee, and Amirreza Ghasemi Arup, New York, NY Seth Pollak Arup, Houston, TX Andrew Cushing Arup, Toronto, ON Abstract: Billy Bishop Toronto City Airport Pedestrian Tunnel is a passenger access tunnel under the Western Channel of Lake Ontario. Southern Ontario is known for very high horizontal in-situ stresses and time dependent behavior of shale formations, with continued rock deformation and final linings experiencing distress in several projects, sometimes years after installation. This paper discusses construction sequencing, methods used for predicting time dependent deformation, instrumentation and monitoring, and comparison of the predicted against the actual movements during construction. Results from a custom-made FLAC sub-routine along with a closed form solution are provided and the differences are discussed. INTRODUCTION

The Billy Bishop Pedestrian Tunnel is an underground pedestrian tunnel under construction for Billy Bishop Toronto City Airport (BBTCA), which is located on the Toronto Islands in Lake Ontario. The location of the BBTCA relative to downtown Toronto and the other major Toronto airports is shown in Figure 1.

Figure 1. Location of the Billy Bishop Toronto City Airport

PROJECT HISTORY

In early 2010, the Toronto Port Authority (TPA) announced that it was seeking a private partner to construct the pedestrian tunnel. In July 2011, an agreement involving an exchange of land between the TPA and the City of Toronto permitted the tunnel project to go forward. Three consortia were invited to respond to the Request for Proposal (RFP) for the project, with bids being submitted in October 2011. In January 2012, a Public Private Partnership (P3) was formed between the TPA and Forum Infrastructure Partners, a consortium consisting of Forum Equity Partners (Developer and Equity Partner), PCL Constructors, Inc. (General Contractor), Technicore Underground (Shaft and Tunneling Contractor), Johnson Controls (Facilities Manager), Arup Canada, Inc. (Lead Designer Structures and Tunneling), ZAS (Architect), and EXP (Geotechnical Engineer of Record), with groundbreaking for the project taking place in March 2012. PROJECT OVERVIEW

The island airport is separated from the mainland by the 120m wide Western Gap Channel, and passengers currently access the airport by Ferry. The project consists of the construction of shafts on the mainland and island sides of the channel and a tunnel between. On the mainland side, the shaft will accommodate six elevators. The island shaft contains two elevators and two banks of three escalators running up from the tunnel to the airport lobby, as shown in Figure 2. The internal dimensions of the tunnel are 9.3m wide and 6m high, to provide a spacious environment and accommodate two moving walkways. In addition, three utility conduits water and sanitary force mains - will be run through the temporary construction works above the tunnel crown permanent lining. Each of the conduits consists of a thermally welded HDPE pipe installed within a temporary steel sleeve pipe. Including the mains within the tunnel project saved the City of Toronto approximately $10m.

Figure 2. Proposed Billy Bishop Airport Pedestrian Tunnel (BBAPT) [Image by ZAS Architects] The tunnel will be constructed within the horizontally bedded Georgian Bay Shale. This shale unit consists of typically moderately weathered to fresh, grey to dark grey, fine to very fine grained fissile shale interbedded with slightly weathered to fresh grey, fine grained calcareous siltstone and limestone

Interbeds [Project GBR]. There are two distinctive features of the shale in the Toronto region. One is a high horizontal stress regime, and the second is long-term time dependent swelling behavior which occurs when the following factors occur:

Stress relief of the rock mass Availability of fresh water

The swelling is a consequence of the reduction in confined stress in the rock which occurs upon excavation in combination with a differential gradient in salinity between the saline rock porewater and freshwater from Lake Ontario or even humid air. Osmotic and diffusive processes result in a decrease in the salinity of the rock porewater achieved by an overall increase in the water content, resulting in volumetric expansion of the shale rock over time. The development of this time dependent deformation (TDD) relative to the time of installation of the permanent lining has a direct impact on the long-term moments and forces induced on the lining. Following the methodology developed by Lo et al. (1978), results from free swell tests, semi-confined well tests and no-swell tests are used to identify the Swelling Potential of the rock in different directions. These tests are performed as follows:

Free swell test: Sample is exposed to water; and vertical and horizontal deformation of sample in time is recorded. Eight of these tests were performed for the Billy Bishop project.

Semi-confined swell test: Sample is exposed to water and a constant load is applied to the specimen. The deformation in the direction of the applied load is recorded in time. Twelve SCST tests were performed for the project.

Null swell test: Sample is exposed to water and variable load is applied. No deformation is allowed in the direction of the applied load. Change of load in time is recorded. Three samples were tested for the project.

Swelling potential is defined as the average slope of the swelling strain versus the logarithm of time and is defined for a specific direction, since behavior in the vertical and horizontal directions is typically noticeably different. The swelling potential decreases as the applied pressure is increased. The pressure where swelling potential is zero and no swell occurs, is called the Critical Stress and is defined with the result of the no-swell test. Hawlader, Lee, and Lo (2003) studied the impact of applied load on the swelling potential of different samples. They concluded that the applied stress in one principal stress direction reduces swelling strain not only in that direction but also in the perpendicular directions.

Figure 3 shows the relation between applied pressure and swelling potential in horizontal and vertical directions for samples of Billy Bishop project. The point of zero swelling potential (Critical stress) is also clear at the end of the lines. Similar to previous experience of other projects in the area, the swelling potential in the vertical direction is two to three times higher than the horizontal value.

Figure 3. Swelling potential vs. stress in vertical and horizontal directions

The swelling potential of shales tends to increase with decreasing calcite content, and an increasing outward salt concentration gradient from the pore fluid of the rock to the ambient fluid (Lee and Lo, 1993). Therefore, calcite content and salt concentrations (salinity) of pore water in the rock samples were also considered in the tests. TUNNEL CONSTRUCTION

The construction sequence is shown in Figure 4. Once the seven TBM drift tunnels had been constructed and filled with concrete, two further TBM tunnels were driven (#8 and #9). These provided an opening that simplified the main excavation, Cut 1, which was excavated by breaking the rock around these pilot tunnels. With the arched roof and inclined side walls, Cut 1 was performed with the need for any additional rock support. Figure 4 also shows the location of the three force main utility conduits, one installed in each of three of the western drift bores (numbers 2, 4, and 7). The steel pipe sleeves were hung from the crown of each drift bore with steel cables, with a final HDPE pipe being installed incrementally within each steel sleeve. The next stage in the sequence, Cut 2, involved excavating the final section of either side of the first cut. The sidewalls were bolted, with particular attention paid to ensuring the rock under the arch was fully supported. The final excavation stage, Cut 3, was the removal of the invert, and this was carried out in two stages a rough cut for the bulk of the excavation following by trimming to the required profile. Bulk excavation was typically performed by hoe-ramming and Dosco roadheader, and trimming by roadheader mounted on an excavator.

Figure 4. Excavation Sequence of proposed TBM Drifts Drilling and Backfilling

Figure 5. Tunnel Excavation Cut 1 The presence of design engineers on the site allowed several modifications to the excavation sequence to allow optimization of the schedule, based on observations of the rock and monitoring results. This included allowing the Cut 1 excavation to proceed for the whole length of the tunnel before the Cut 2 commenced, and delaying the application of shotcrete on the sidewalls, which removed this operation from the critical path. Following the excavation, the lining was placed. A fully tanked PVC compartmentalized membrane system was used. The reinforcement for the lining was prefabricated to allow for rapid assembly in the tunnel. After the invert concrete was poured, the arch concrete was placed using a 12m long arch form. The arch lining was designed with steel fiber reinforced concrete, with reinforcement only provided up to the shoulders, to maximize the speed of construction. SHORT TERM TUNNEL SUPPORT DESIGN

The tunnel has a relatively shallow rock cover of eight meters for the 10.5m-wide excavation span. Given that the excavation is under the lake and there was not a comprehensive knowledge of the condition of the rock, a rather novel temporary lining (pre-support) system was selected that involved a series of interlocking, horizontal TBM-driven secant drift bores which are backfilled with 15 MPa strength concrete to form an integral arched roof under which mass excavation can occur (see Figures 3 and 4). This mitigated the risk from encountering a water bearing feature under the channel, as the small 1.8m diameter TBMs would provide a much greater means of face stability than an open face SEM excavation.

The temporary support of the TBM bores was provided by steel ring beams at a nominal 1.2m spacing supplemented with plywood sheet lagging. The design was based on short term convergence confinement calculation (which included the effect of swelling for the time the bores were expected to remain open) and validated by the discrete element program UDEC (v.4.0) by Itasca Consulting Group, Inc. The UDEC model was used to model the entire construction sequence of excavating and backfilling each individual drift bore followed by the main tunnel excavation. Of particular interest was the horizontal shear displacement of the bedded shale rock and how this affected stress redistribution around and through the backfilled concrete arch. Additionally, the joints around and within the arch were assigned properties consistent with a concrete-concrete or concrete-rock interface created by the backfill pours. Two cases of horizontal in situ stress were considered to bound the expected short term behavior. Key results are summarized in Table 1. Table 1. Summary of key results for short term (temporary) stability of backfilled arch and tunnel

H [MPa]

Max. dowel axial force

[kN]

Vertical displacement

under arch [mm]

Max. displacement

under arch abutment

[mm]

Max. horizontal

tunnel convergence

[mm]

Depth of plastic zone under invert

[m]

Max. Arch Compressive Stress [MPa]

(without swell)

2.5 176 11 13 5 0 4

6.9 395 27 40 7 4 8.5

A small percentage of boundary joint length along the arch perimeter indicated debonding (Figure 6). However, the remaining bonded length was more than enough to prevent a downward translation of the structure. The internal concrete-concrete joints did not show any loss of contact or debonding (i.e. both joint normal and shear stiffnesses > 0) which indicated that no tensile forces exist within the arch, even in the critical case of a potential wedge failure in one of the side abutments. The results confirmed the assumption of the arch acting as a single, contiguous member in response to the stress distribution resulting from the main tunnel excavation (Figure 7).

Figure 6. Backfilled concrete arch stability modeling in UDEC showing zones of joint debonding upon excavation of main tunnel.

Figure 7. Backfilled concrete arch stability modeling in UDEC showing maximum principal stress contour upon excavation of main tunnel. The temporary main tunnel support design consisted of installing 32mm diameter rock dowels in the sidewall abutments under the backfilled arch once the final excavation profile was cut. The aim of the dowels was to stabilize any overbreak or wedge failures resulting from the occasional sub vertical joint dipping into the tunnel.

The final lining of the tunnel was design to maximize productivity during construction, and utilizes a conventional reinforced concrete invert slab with prefabricated reinforcement panels, and a concrete arch with a minimum 400mm thick steel fiber reinforced concrete (SFRC) lining with partial steel rebar reinforcement in the sidewalls to counter long term swelling pressures. DESIGN PROCEDURE FOR TIME DEPENDENT DEFORMATION

There are two common design methods available to assess the impacts of TDD on the tunnel lining. Lo and Yuen (1981) developed a closed form solution method to predict the long term loads and displacement at any point in time in lining and rock. However, the closed-form solution method does not consider the effect of time-dependent swelling-induced rock stress on the swelling potential of the shale rock. As a result, the closed-form solutions for the final unlined rock swelling displacement and lining moments and forces are conservatively over-estimated. To obtain a more realistic estimate of the lining loads, a numerical model to account for swelling was developed by Itasca on behalf of Arup and implemented in the FLAC 2D finite difference program. The swelling rock constitutive model in Hawlader et al. (2003; 2005) was developed based on the Mohr-Coulomb elastic/perfectly plastic material model. This is based on the observations in the laboratory experiments that the swelling strains in the principal swelling directions of a Shale rock specimen increase linearly with the logarithm of time, and the swelling strains are reduced in both parallel and perpendicular directions by the application of stress on the rock specimen. In this project, the model formulation was implemented for use with the two-dimensional code FLAC in plane strain mode. A virtual critical stress concept was introduced for vertical swelling in the zone between the rock surface and the depth where each of in-situ stress is equal to the critical stress (c). This zone would have swelled before any construction activity started, and was assumed to be in a stable condition with the in-situ stress level. To avoid modeling vertical swelling in this zone, the critical stress zone in the vertical direction was set equal to the in-situ stress (a virtual critical stress, which is reduced from the original critical stress). The critical stress below this zone was maintained at the actual level. Derivation of Swelling Parameters The time-dependent model requires eight input parameters/properties, i.e., the slope of the bedding plane (), the time (t0) for initiation of swelling, Youngs modulus (E), Poissons ratio (v), the three free swell potentials in principal swelling directions (mx(0), my(0) and mz(0)), one pseudo-Poissons ratio (), a threshold stress (th) (below which no swelling strain reduction occurs) and the critical stress (c) (above which the swelling is suppressed completely). A series of laboratory swelling tests on shale samples, along with in-situ rock stress measurements, were performed, obtained and used in the engineering analysis of the tunnel lining, including; Table 2 lists the values that were interpreted from the tests. Values of other projects and the back analysis described below are also presented for comparison.

Table 2. TDD parameters of the Georgian Bay shale at different projects in Southern Ontario

Project In-situ Stress (MPa)

Swell potential Critical Stress (MPa)

Elastic Modulus

(Mpa)

Time Dependent Elastic modulus

(MPa)

Viscosity (MPa-day) Vertical Horizontal

Billy Bishop Toronto City Airport Pedestrian Tunnel

GBR 2.5-6.9 0.04 to 0.26% 0.04 to 0.26% 2 to 5 5,875 320 to 1100 38000 to

73800 Project Specific Testing (Exp)

8.2* 0.7 to 1.1% 0.15 to 0.3% 0.64 4,000 440 29500

Design 2.5-8.2

0.7% (Base) 0.24 to 1.2% (Sensitivity Analysis)

0.4% (Base) 0.05 to 0.45 (Sensitivity Analysis)

3 (Base) 0.64 to 3

(Sensitivity Analysis)

N/A N/A

Shaft Backanalysis

4.7 (N-S), 5.1 (E-W) 0.7 to 1.1% 0.3 to 0.4% 3 6,800 N/A N/A

Heart Lake Tunnel, Mississauga (Lo et al 1979)

Vert. 0.435 Hor. 2 to 8 (Sensitivity Analysis)

0.42 % 0.13 to 0.17% 1.9 to 2.6 8,300 to 12,000

250 490 140

41200 52600 20800

Deep Lake Water Cooling Centre Island Tunnels, Toronto (Lo and Micic 2010)

- - 0.12% - 1,500 to 4,700 1100 73600

Skydome and John Street Tunnel, Toronto (Lo et al 1987)

Vert. 0.2 Hor. 10 0.2 to 0.62% 0.04 to 0.26% -

2,200 to 2,300 380 38000

*Single test, not considered representative

Numerical Analysis Continuum modeling using the finite difference code FLAC (v.7.0 with the Swello module) was used to design the final tunnel lining including the effect of the TDD behavior of the shale rock mass. As the temporary condition (prior to permanent lining installation) is expected persist for approximately 75 days, the effects of TDD deformation must be considered. The numerical modeling included all stages of the construction sequence, including the boring and backfilling the seven interlocking TBM drift bores (as well as two lower pilot tunnels to facilitate conventional mass excavation underneath the resulting arch), the three cuts for the main tunnel excavation and the installation of the lining. Figure 8 shows the FLAC 2D model

Figure 8. FLAC 2D model of TBM drifts and main tunnel showing strain due to TDD in the rock and displacement of the sidewalls 150 days after excavation. The initial designs were based on the permanent invert slab for the main tunnel being placed between 50 days after the bench excavation, with the permanent lining for the tunnel crown and sidewalls cast between 125 days after the main tunnel excavation. However, as excavation proceeded and monitoring data was collected from the shaft, the model was repeated with 30 days to invert placement and 75 days before placing the arch. A series of seven design cases were considered using a selected range of parameters for in-situ stresses, swelling potential values, critical stresses and boundary conditions. The FLAC 2D runs were carried out to determine the long-term axial thrust and bending moment developed within the lining caused by the swelling rock mass. These analyses were carried out to a design life of 100 years, and were considered both with and without the additional loads caused by a full hydrostatic water pressure build up around the tunnel. Figure 9 shows a comparison between the predictions of the FLAC model with the movements predicted by the closed form solution from Lo and Yuen. It can be seen that for this size excavation, the movements predicted by numerical analysis are considerably lower.

Figure 9. Comparison of results from Closed Form Solution and Numerical Analysis. MONITORING DURING CONSTRUCTION TO VALIDATE DESIGN APPROACH

To verify the design assumptions for the long-term swelling behavior of the shale rock during construction of the BBTCA Pedestrian Tunnel, a number of instrumentation and monitoring programs were implemented. Mainland Shaft The mainland shaft provided valuable information since it was the first excavation of the project in the Geotgian Bay Shale and remained open the longest. The initial support requirements consisted of bolts and mesh on the north and south walls to protect against loose blocks formed due to clusters of vertical joints. Three inclinometers were installed two on the north side (0.5m and 3m from the excavation face) and one on the west side (0.5m from the face). The results showed elastic movements during each successive shaft excavation was performed, followed by very small TDD movements. It should be noted that the shaft walls were very wet, providing the ideal conditions for TDD to occur. Back-analysis was complicated by the degree of restraint provided by the shaft invert. To model this, the elastic response to the shaft excavation was first modeled in 3D using Midas GTS. This allowed the appropriate movement at each inclinometer location to be obtained for each stage of the shaft excavation, as the shaft invert was gradually lowered and the restraining effect on the shaft movement reduced. This data was then used in a 2D FLAC model of a horizontal slice of the shaft, which used a support pressure on the inside of the excavation boundary to model the staged excavation of the shaft. The TDD routine was used to assess the TDD that occurred at each stage. The back-analysis provided results that gave relatively good agreement with the recorded movements in the inclinometers, as shown in Figure 10. The movement of the shaft walls during excavation was used to investigate in-situ stress and elastic modulus. The ongoing TDD was used to investigate parameters for critical stress and the swell potentials. While there are a number of inter-related parameters for the swelling, meaning it was not possible with the data available to isolate each individual effect, the data indicated that the critical swell potential was higher than predicted by testing. Within the limits of the accuracy of the recorded data and the analysis, the back-analysis provided justification for the following key parameters:

Horizontal Stress: ~4.7 MPa (N-S), ~5.1 MPa (E-W) Horizontal Swell potential: In the range between 0.03 to 0.04% Critical Stress: ~3 MPa

Figure 10. Shaft Back-analysis (Elev. +57.4 m) TBM Driven Secant Bore The radial deformation of the first 1.8m diameter TBM-driven secant drift bore were measured with time using tape extensometers at two separate locations. The installed support was light steel ribs, and these were not observed as being under load at the monitoirng locations. The drift tunnels were generally dry with the occassional water seep, particularly above a 200mm thick band of limestone that was located at tunnel springline. Within the limits of accuracy of the tape extensometeer (+/- 0.5mm) a general convergance trend was detected which was open for approximately 10 weeks. The monitoring showed convergence on both horizontal and vertical chords, with the horizontal convergence of around 1mm,

approximately double the vertical. The prediction model showed that TDD movement with a similar horizontal convergence, but slight divergence on the vertical chord. Tunnel Monitoring Construction monitoring consisted of tunnel movement measurements obtained from optical survey of prism points, multi-point borehole extensometers (MPBXs) installed horizontally in the tunnel sidewalls, and measurements of horizontal convergence using a mechanical tape extensometer. The MPBXs were installed horizontally within the final tunnel sidewalls to measure the tunnel wall movements during and after the tunnel excavation. They were installed 20m from the mainland shaft to allow installation as early as possible without the presence of the shaft excavation impacting the results. On the east wall, the base point of the MPBX was 10m into the rock, and 8m on the west side. The MPBXs were installed during the cut 1 excavation, with localized niches cut in the tunnel sidewalls. This allowed them to be in place when the cut 2 excavation took place, recording both the elastic ground movement and the TDD that occurred. Figure 11 shows a typical plot of the MPBX data.

Figure 11. Measured east wall (MPBX) movements from August 14, 2013 to January 21, 2014 The tape extensometer readings for the tunnel were recorded on a regular basis at twenty meter intervals along the tunnel, and displayed a similar pattern of movement, although with a slightly higher magnitude than the MPBX data. The range of movements recorded with the tape extensometer is shown on Figure 12, along with the MPBX data, and the predicted movements from the FLAC models used in the design. The survey targets were primarily intended to monitor the overall stability of the tunnel and the accuracy range was not high enough to track sidewall TDD movement in detail, but overall the survey measurements showed general agreement with the tape extensometer readings.

Figure 12. Measured versus predicted TDD in the tunnel Figure 12 demonstrates that the measured TDD was less that the prediction. It was believed that this may have been due to a lack of water to support the swelling, since the tunnel was relatively dry. As a result, the design was reviewed to ensure that the design could accommodate a greater proportion of the TDD movements after the lining was installed. CONCLUSION

The project has used a customized routine within a FLAC2D model to design the tunnel lining to accommodate the loading from Time Dependent Deformation in the Georgian Bay Shale. Monitoring during construction has been performed to validate the parameters used and to justify the design approach. REFERENCES

Hawlader, B.C, Y.N. Lee, and K.Y. Lo. (2003) Three-Dimensional Stress Effects on Time-Dependent Swelling Behavior of Shaly Rock, Canadian Geotechnical Journal, Vol. 40, pp. 501-511.

Hawlader, B.C., K.Y. Lo, and I.D. Moore. (2005) Analysis of Tunnels in Shaly Rock Considering Three-Dimensional Stress Effects on Swelling, Canadian Geotechnical Journal, Vol. 42, pp. 1-12.

Lo, K.Y. and S. Micic. (2010) Evaluation of swelling properties of shales for the design of underground structures, ITA-AITES 2010 World Tunnel Congress, Vancouver, B.C. Canada.

Lo, K.Y. and C.M.K. Yuen. (1981) Design of Tunnel Lining in Rock for Long Term Time Effects, Canadian Geotechnical Journal, Vol. 18, pp. 24-39.

Lo, K.Y., R.S.C. Wai, J.H.L. Palmer and R.M. Quigley. (1978) Time-dependent deformation of shaley rocks in Southern Ontario, Canadian Geotechnical Journal, Volume 15, pp. 537-547.

Introductiontunnel CONSTRUCTIONDesign Procedure for TIME DEPENDENT DEFORMATIONDerivation of Swelling ParametersNumerical Analysis

MONITORING DURING CONSTRUCTION TO VALIDATE DESIGN APPROACHMainland ShaftThe mainland shaft provided valuable information since it was the first excavation of the project in the Geotgian Bay Shale and remained open the longest. The initial support requirements consisted of bolts and mesh on the north and south walls to pr...TBM Driven Secant BoreThe MPBXs were installed horizontally within the final tunnel sidewalls to measure the tunnel wall movements during and after the tunnel excavation. They were installed 20m from the mainland shaft to allow installation as early as possible without the...

CONCLUSIONREFERENCES