The Toronto-York Spadina Subway Extension (TYSSE) is an 8.6km extension that will connect the York Region north of the city to downtown Toronto as shown in figure 1 This subway extension will be built entirely underground within twin 5.4m-diameter bored tunnels. The tunneling will be performed by four EPB machines and will utilize a precast segmental lining for permanent support. Distributed along the 8.6km extension are six stations spaced to serve the communities.
During the planning of the extension it was determined by the Toronto Transit Commission (TTC), York University and the community that providing a new station within York University would be a major goal of the project. The station will provide an efficient way for the students to commute to and from the academic center and reduce public bus traffic to the campus. The most desirable location for the station was located directly beneath the campus’s central green space. However, this station location would require the twin bored tunnels to pass beneath several campus buildings, including the newly constructed Seymour Schulich School of Business. In addition to the tunnels passing beneath it, a 20m deep excavation for the York University Station would need to be located directly adjacent to the Schulich Building.
The Schulich Building is a 3-story reinforced concrete structure supported by continuous foundation walls and interior spread footings approximately 8m below grade. The exterior façade is comprised of both glass and marble cladding while the interior spaces are finished concrete. The crowns of the proposed TYSSE running tunnels are approximately 14m below grade or 6m from the base of the building foundation. The support of excavation for the proposed York University Station is approximately 8m from the exterior face of the school. Apart from the exterior finishes of the building the most challenging aspect of the Schulich Building to the compensation grouting team was the foundation system. The substructure is comprised of 5m-tall continuous exterior walls with interior columns founded on independent spread footings. The basement mostly consists of storage spaces, utility rooms and maintenance areas. The area of the building closest to the station excavation is founded on independent caissons connected by grade beams that support a partially enclosed colonnade and upper floors. This variation in the foundation system makes the compensation grouting program significantly more challenging.
Risk analysis
Early on in the design process the project team completed a comprehensive risk register of more than 150 major concerns that could affect the schedule, cost or safety of the entire extension. Each one of the risks was categorized and rated, based on likelihood of occurrence and severity. Based on this risk analysis, the potential to damage the Schulich Building by construction-induced settlement, if no mitigation measures were implemented, was found to be significant. The potential cost of repair/disruption could also be high, which meant that, unmitigated, this would be a large project risk. The relocation of the station to avoid the tunnels passing beneath the Schulich building was considered and although desirable from a building protection perspective, it was not feasible due to tunnel alignment constraints.
A technical review of the likely ground losses and potential mitigation measures was then carried out, and an extensive compensation grouting program was recommended. The follow up risk review session confirmed that this recommendation would significantly reduce the risks, and was therefore adopted to protect the Schulich School of Business. Throughout the design development process, consideration was consistently given to construction cost versus potential risk.
Project constraints
The project schedule is very tight and requires trains to be running in 2015. In order to achieve this, a number of schedule-saving measures are required. These include the need to have four TBMs operating simultaneously, and for the Client to pre-purchase all four TBMs rather than leaving the selected contractor to do it after contract award. The station construction and TBM mining also have to overlap as the schedule does not allow one to be completed before the other starts. In order to facilitate a very quick start up an enabling works contract was also let to construct the TBM launch shafts, thus on the day of Notice to Proceed (NTP), the launch shafts and TBMs would be ready.
These schedule savings do, however, introduce several challenges to the compensation grouting program;
•The compensation grouting system installation, testing and pre-condition grouting have to be completed within a very short time between NTP and the first TBM reaching the building (approximately seven months).
•The whole compensation grouting system needed to be fully operational before the first TBM arrived because of the short interval between the first and second TBM drives.
•The grouting system would have to remain operational for the duration of the station excavation.
•The pre-installed York University Station headwall would effectively block any settlement advancing ahead of the TBM.
•Tunneling through this pre-installed headwall could cause additional ground loss.
In addition to the schedule-based constraints we also had to consider other constraints related to the station excavation and compensation grouting program being in the middle of an active university campus (figure 2). These included:
•All roads would need to remain open or be effectively detoured.
•All pedestrian traffic would need to be able to easily negotiate routes around the site.
•Noise controls would have to be implemented.
•The Schulich Building would need to remain open and function as usual.
•Limited laydown and working space.
As the design progressed all of these constrains were carefully integrated with the technical challenges of the compensation grouting system.
Contract packaging
Selecting the most efficient way for the compensation grouting contract to be tendered was important to the overall success of the grouting system. It was not immediately obvious if the contract should be included with the tunnel drives, the station excavation or be tendered independently. However it was clear that if it was tendered independently it may become a source of conflict between the tunnel and station contractor and the grouting contractor. Similarly, if tendered in conjunction with either the tunnel or station excavation conflict could arise from the opposite contractor. Several options were considered including novation options before the current arrangement of a single contract for the tunnel construction, station excavation and compensation grouting was selected. This single contract option also included a requirement to utilize a compensation grouting contractor that had been pre-qualified by TTC.
Design requirements
In addition to meeting the general project requirements, the design of the compensation grouting system had to satisfy several parameters. A significant aspect was quantifying these parameters to design an effective settlement mitigation system. Design parameters that had to be established included:
•Determining probable amounts of ground loss for each of the construction activities.
•Calculating the possible ground movement based on the probable ground losses,
•Calculating settlement based on dewatering induced consolidation,
•Analyzing the structural response of the Schulich Building to different amounts of ground movement, and
•Establishing the maximum amounts of vertical and horizontal displacement each part of the building could resist both structurally and cosmetically.
Ground loss
Establishing the probable amount of ground loss for the area of the Schulich Building was a joint effort between the program manager, tunnel designer, station designer, compensation grouting designer and the TTC. In general, the contract documents state that any ground loss greater than the specified amount is considered to be the liability of the tunnel contractor or the station contractor, depending on the construction phase. Movements less than the limits are TTC’s responsibility. In the case of the Schulich building, TTC wanted the compensation grouting system to be able to protect the building in both cases (i.e., both above and below the contract limits applicable elsewhere).
To resolve the ground loss parameter, settlement predictions of the tunnel and station designers were first verified then subjected to a sensitivity analysis. Sensitivity analysis varied ground loss parameters, including volume and soil type, to give the compensation grouting design team a range of values (see Table 1).
Ground movement
The calculation of ground movements was an iterative process used to determine a range of potential settlements. Both vertical and lateral movements were explored as both could have a negative impact on the structure. Largely empirical methods were used to determine ground movements from both tunneling and station excavation. These methods were supplemented by historical data and computer analysis to provide a thorough baseline. Each of the empirical methods employed required specific input based on ground conditions as well as construction parameters. Ground conditions were obtained from geotechnical data reports developed by Golder Associates.
The ground movement due to tunneling was analyzed using Hatch Mott MacDonald’s in-house software Ansettle and Ground Response Program, which are both based on a Gaussian Curve analysis presented by O’Reilly and New (1982). In addition to ground loss, this method requires a trough width parameter (k) that is based on the type of soils expected to be encountered. A ‘k’ of both 0.4 and 0.5 were used to determine maximum probable trough width and depth. By entering ground losses between 0.5 and 2 per cent settlement profiles were determined for both ground surface and the elevation of the base of the foundation footings. It was also acknowledged that, as the TBM advances, it will develop a ‘bow wave’ or curve beginning at ground surface and extending along the tunnel centerline to the maximum settlement depth. This ‘wave’ was considered as its Slope has the potential to be steeper than the traditional transverse Gaussian curve. To determine the ‘bow wave’ the designers drew from previous similar projects.
Settlement propagating from the York University Station excavation was also calculated based on the anticipated secant pile support of excavation. The secant piles created a relatively stiff soil support structure. However due to the depth of the station box and proximity to the Schulich Building, it is anticipated that soil settlement will extend under the structure.
The settlement due to tunneling and settlement due to station excavation were combined to determine a maximum unmitigated profile as shown in figure 3. While this profile is useful to determine the areas requiring the most mitigation it does not accurately represent the transient stages of construction. Similarly, the unmitigated profile does not account for the advance rate of the TBM, which could significantly impact the compensation grouting. If the tunneling maintains consistent ground loss and increases its advance rate the compensation grouting contractor will be required to mitigate more volume in a fixed duration of time. Each of these issues was considered as the grouting program was designed.
Dewatering
Soil consolidation due to construction dewatering was considered for both the station excavation as well as for the compensation grouting shafts. Due to the schedule of construction any dewatering-induced settlement from the station excavation would need to be mitigated through the use of compensation grouting. However, if soil consolidation occurred during excavation of the shafts there would be no way to mitigate it. In conjunction with the project geotechnical consultant, it was determined that construction dewatering of the compensation grouting shafts would not cause significant soil consolidation as long as the completed shafts were sealed and the dewatering was abbreviated.
Structural response
The greatest single element of the compensation grouting design was the structural response of the building, not only to settlement but also to potential grouting-induced heave. Based on the probable settlement and heave scenarios a number of potential ground displacement profiles were generated. These were then modeled within the analysis to determine the effect of these ground movements on the structure. To determine building behavior two different approaches were used which generated the following information:
3-D analysis
•Overall understanding of the building Behavior
•Predicted Demand-to-Capacity Ratios
•Predicted crack widths
Building response to excavationinduced Settlement
•Tensile strain,
•Angular distortion.
The entire building was modeled in a 3-D structural analysis program to determine how each foundation element would react to movement as shown figure 4. In addition to creating the building within the software, service loads were introduced into the model to create a realistic simulation of building response. The model confirmed the designer’s assumption that the exterior walls were significantly stiffer than the interior columns and would tend to span isolated areas of settlement. Correspondingly, the exterior walls would be relatively resistant to heaving from grouting. The interior columns were more susceptible to movement from ground settlement as were the exterior caissons. The difference in behavior had to be considered because it would intensify the probability of differential settlement between the foundation walls and columns.
In addition to confirming the behavior of the foundation elements, the software also analyzed the structure to determine the induced load in each structural member. These loads were then compared to their capacities to establish demand-capacity ratios.
The third use of the model was to provide the information necessary to calculate the predicted crack widths that could be induced in structural members under the different analyzed ground movements.
In addition to the 3-D computer model, an assessment of the effects of tensile strain and angular distortion was also conducted to relate the building to data collected from movements of other structures. Hatch Mott MacDonald’s in-house software Building Response to Excavation Induced Settlement was utilized to determine the maximum tensile strains and angular distortions.
The software simplifies the building into a single span beam and collects strains based on imported settlement profiles. These results were utilized in two ways:
1) They were tabulated and classified utilizing Burland’s (1974) maximum tensile strain classification.
2) They were plotted versus angular distortion on Boscardin and Cording’s (1989) relationship of damage to angular distortion and horizontal strain classification as in figure 5.
Compensation grouting program
Due to the geometric constrains of the Schulich Building and proposed construction, as well as cost of installation, grouting shafts were chosen instead of directional drilling for the grout injection system. The project lent itself well to the use of traditional Sleeve Port Grout Pipes (SPGP), also referred to as Tube-a-Manchette (TAM). These consisted of rigid steel pipes ported every 0.5m with rubber sleeve ‘valves’. The SPGP will be installed from shafts excavated around the perimeter of the building. The (three) shaft locations were chosen to minimize disturbance to the campus, optimize the SPGP coverage, reduce SPGP length and minimize damage to the building from their construction. The process for grouting includes inserting a packer or injection device into the SPGP. When each injection is complete the hole is flushed and the packer is reset.
The movement limits and design requirements of the Schulich Building were carefully reflected within the design of the compensation grouting program.
Array placement
The grouting arrays were located to efficiently distribute grout to all areas beneath the building anticipated to experience ground movement. This meant that a total of approximately 120 SPGP would be extended radially from each shaft. As figure 6 illustrates shafts 1 and 2 will be primarily for grouting of the tunnel-induced settlement, and shafts 2 and 3 will be for the station-induced settlement. The SPGP were placed with a maximum spacing of 2m to ensure a cohesive grout unit. Additionally, the SPGP are specified to be schedule 80 steel piping to provide increased reinforcement of the soil. Vertically, the SPGP were located between the lowest foundation structure and the crown of the tunnel. This was done to minimize the chances of grout communicating to either structure. If the grout were to communicate to either structure, it has the potential of filling utilities or causing localized over-stressing.
Grouting procedure
Grouting operation will occur in four steps. The first step is a trial grouting program that is set up separate from the working system to test grout flow, grout type and soil response. This is designed to ensure that the contractor can prove his approach before tunneling begins. The second will be pre-conditioning or pre-heaving the area. This step takes place prior to tunnel or station excavation and is important because it consolidates the soil to be treated, begins creating a soil/grout mass, ensures the ground will be responsive to heave from subsequent grouting and introduces a small amount of heave to the structure. Step three is the concurrent grouting that occurs during TBM mining and is a predefined program of injection and observation. This step occurs very quickly so it is critical to have a specific knowledge of how to control the building based on the pre-consolidation and trial grouting tests. The final step is controlled through observation of the structure and injecting additional grout if deemed necessary. This step will occur during the station excavation and be elongated if settlement issues persist.
Grouting efficiency
Grouting efficiency refers to the relationship between the volume of grout injected versus the volume of heave realized at the surface. Several components are considered when determining grouting efficiency such as water bleed, existing voids, plastic/elastic soil deformation and grout advancing in a direction not resulting in heave. Grouting efficiency is anticipated to vary, based on the phase of grouting, at 15-25 per cent. Determining the grouting efficiency also allowed the design team to estimate a required grout volume for each tunnel drive and the station.
Pre-heave limits
As discussed earlier, the pre-conditioning consolidates the soil, makes it more responsive, and also allows for pre-heaving of discrete areas. By heaving the structure prior to tunneling it allows the contractor to have a ‘head start’ on the settlement making it easier to maintain a more consistent elevation. For the Schulich Building the design team chose to pre-heave several of the interior columns and exterior caissons to minimize settlements.
Injection pressures
Specifying injection pressures required the consideration of several parameters; geometric and mechanical. The injection pressures must be great enough to fracture the existing grout however the pressures must be controlled to ensure that grout is not communicated to the tunnel or foundation. If the grout extends to either structure, at high enough pressure, the liner could fail or the basement could fill with grout. The pressures were therefore limited to an initial fracture pressure and sustained grouting pressure. To decrease the risk of communication relatively small volumes were specified for each injection. Injection pressures also rely a great deal on viscosity and grout type. Each of these parameters have been included within the trial program for optimum mix design.
Compensation rates
One of the most significant challenges of the compensation grouting program is to determine the required rate of grout injection during the TBM drives. The design therefore uses a calculated rate that will be verified in the field with data from the trial grouting program, pre-consolidation and the building reactions. The compensation rate relied on several parameters that were developed during the design process and several assumptions including the maximum predicted settlement, settlement limit, pre-heave, grout efficiency, injection volume, TBM advance rate, pump flow rate, number of packers and the packer set-time. One of the major components is the TBM advance rate as limiting the advance rate will allow greater time to compensate for ground loss, however previous experience notes that, generally, the more quickly a TBM advances the lower the ground losses tend to be. To mitigate this issue the design team specified an advance rate no greater than 35m per 24-h shift unless the contractor could prove the system installed would be adequate to protect the structure. This advance rate limitation should not however affect the schedule as the normal advance rate is expected to be approximately 15m per day.
Building monitoring
One of the key elements to protecting the Schulich Building from damage is to be able to accurately measure the movement of the structure. To continuously monitor the building, the design team specified a robust automated instrumentation system. The system will provide real-time data to a web-based database, which will allow workers, engineers and the owner to follow the progress of the work. The system will integrate precise settlement monitoring points installed onto the key structural elements of the buildings as well as robotic total stations surveying the exterior of the building. These instruments, as well as additional in-ground instrumentation will provide specific knowledge of the status of the structure. This information will then be used in conjunction with the trigger levels in the specifications and a preapproved contractor plan of work to perform work in an efficient and definitive manner.
Conclusion
The TYSSE compensation grouting program for the York University Seymour Schulich School of Business is designed to be both a proactive and reactive system closely linked to the behavior of the structure through a comprehensive monitoring system. The program has been designed to provide controlled mitigation through a system of movement limit criteria and integrated contractor design. Tunnel drives and station construction have also been sequenced within the overall project to provide a limited amount of simultaneous construction and to limit ground loss. With active settlement control
construction costs and safety risks have been significantly reduced and specifically allocated to either the owner or contractor. By clearly defining and rating project risks early in the design, the project team could provide effective mitigation tools and minimize contingencies.
Figure 1, map of the Toronto-York Spadina Subway Extension with directions of tunneling and position of York University Station Figure 2, Aerial photo map of proposed subway alignment in relation to Schulich building and proposed York University Station Figure 3, maximum unmitigated settlement contours Figure 4, exaggerated structural response along gridline BA due to tunnel excavation Figure 5, horizontal strain versus angular distortion Figure 6, the layout of SPGP or Sleeve Port Grout Pipes Table 1 – Probable and Maximum analyzed Settlement Values
