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Precast Concrete Tunnel Segments – Go beyond the limit!

As published in the Ontario Concrete Pipe Association’s Concrete Pipe Journal Spring 2019 Issue.

Written By: Lui Sammy Wong, P.Eng., M.E.Sc. – VP Engineering & Quality – Con Cast Pipe

Introduction

Precast concrete has been widely used in constructing infrastructure for decades. It’s ability to accelerate the construction schedule and improve the quality of the concrete are considerably superior to cast-in-place applications. The use of precast though, is limited by the product transportation, lifting and handling capacities. The Eglington Crosstown LRT Mount Dennis Station was a recent project that went well beyond the limits of previous precast infrastructure applications. This article presents a successful project that has gone beyond the typical limits of precast concrete structures by overcoming numerous technical challenges.

Project Information

The Eglinton Crosstown LRT Project, a 19-km long 25-station/stop rapid light rail transit project, will serve mid-town Toronto along Eglington Avenue between Weston Road and Kennedy Road. It is a $5.2 billion investment funded by the Province of Ontario and is a part of Metrolinx’s regional transit strategy expected to be completed in 2021. Mount Dennis Station, a future station located at the west terminus of the LRT line, will provide an interchange station between the GO Kitchener Line and the Union Pearson Express Line. This project is managed by Crosslinx Transit Solutions, a design and construction consortium that is comprised of four global and local leaders in transportation infrastructure. Con Cast Pipe was awarded the contract to supply all precast tunnel segments to Mount Dennis Station in 2017.

At Mount Dennis Station, two train tunnels and a pedestrian tunnel connect the LRT station platform and the future GO transit station. Because the LRT line crosses underneath the busy GO transit west corridor, constructing these tunnels cannot shut down the train line on any given day. Precast tunnels were selected to shorten the construction time line. The three tunnels consist of 38 precast box units with sizes of 8.1m X 6.1m, 7.5m X 5.7m, and 5.5m X 6.9m for the eastbound, westbound LRT tunnels and the pedestrian tunnel respectively. The unit masses of the tunnel sections are 76.8 tonnes, 71.2 tonnes and 66.8 tonnes respectively. The lay length of each unit is 1.5m. With given weights and sizes that are beyond the typical limits in precast concrete products, the manufacturing, delivery and installation phases of the project faced various degrees of difficulty.

Reinforcement

Each reinforced steel cage weighs close to 7000 kg. The principle steel consisted mostly 35M bars at 125 mm spacing with 15M stirrups distributed along the perimeter of the wall providing rigidity against torsion during the movement between the fabrication shop and the mold without any distortion; and handling of the precast unit during installation. The cage required embedded lifting inserts, weld plates, post-tensioning reinforcements and tubes, as well as 1-inch flexible grout tubes. A special jig system was used for accuracy and precision. Concrete Challenges With the given geometry, a 1.5m thick section located at the two upper haunches places the unit in the mass concrete category where concrete temperatures during the initial hydration become critical. Exceeding the limit specified by CSA A23.1 entails the risk of delaying ettringite formation which may lead to durability problems. Prior to official production, concrete raw materials and eligible mix designs were carefully examined. Software named Concrete Works was used to model the cement hydration and estimate the temperature profile of the precast units. Three trial mockups of miniature units made of different concrete mixes and their size equivalents to the maximum thickness of the full-size units were used to develop low heat of hydration self-compacting concrete.

Forty per cent slag blended with GU cement was used to mitigate the risk of overheating the concrete. During execution, real time concrete temperature monitoring provided by EXACT Technology was adopted to control the critical phase of curing.

Due to the dimensions, the precast units could only be produced outdoors. The concrete temperature reached its peak between 55 degrees Celsius and 60 degrees Celsius and exceeded the stripping strength of 20 MPa between 28 and 30 hours of age. During winter day production, cold weather protection was extremely important to avoid rapid loss of surface heat. The stripping process was completed quickly followed by winter protection immediately after. The surface temperature dropped extremely rapidly during this process. Failing to complete this protection would result in thermo-cracking.

Delivery Challenges

All the tunnel units exceeded the typical oversize overweight annual permit shipping limits. The tunnel sections were delivered using 13 axle special tractor trailers to distribute the weight evenly during transportation. In advance of the permit application, a mock-up delivery was conducted to ensure the selected route would provide the moving road block a navigational path free of any potential obstructions to Mount Dennis Station. Once the Traffic Plan was established, approved and permits granted, a coordinated convoy of both police and private escorts traveled down Hwy 401 to Black Creek Drive in Toronto, maneuvering through all the local narrow streets and sharp turns.

Due to the permit restrictions and rush hour curfews the delivery window was limited between 10 a.m. and 3 p.m. A limited area for staging was used to accommodate four 40m (133’) long tractor trailers near the installation site as a buffer. With the assistance of a fully loaded dump truck, the final move was to back the 40m long tractor trailer with the 77 tonne tunnel section down a 12% sloped ramp to within 3m of the crane.

Installation Challenge

The precast units were produced in the “flower-pot” (laid down) position for handling and transportation. The tilt-up process was engineered utilizing two groups of four 37mm diameter hooked cast-in lifting inserts. The box unit was carefully set on a pile of sand. A spreader beam connected between the hook of the crane and an eight-pulley system to the swivel lifting plates was required to ensure each sling handled equal weight. The tilt-up process, pivoting along the bottom edge of the unit, took around three minutes to rotate the unit to a stand-up position. This process was engineered by Con Cast Pipe with flawless execution by Western Mechanical. Expanded form strips were used to seal both the external and internal perimeters of the joint face and to cushion the joint space between the units at about 19mm. Upon homing the unit, an immediate post tensioning process took place after the alignment of the box unit was confirmed. Four 46mm diameter threaded bars, complete with structural assemblies, were engaged with 100 tonne hydraulic jacks and were pulled simultaneously to a minimum of 500 kN. A 300mm by 300mm by 35mm bearing plate was used at each rod to transfer the tension load into the concrete. This is an important design requirement in precast tunnel elements. The remaining spaces were pressure grouted using the cast-in grout tubes. This ties all precast units together as one integral structure.

With limited space and the amount of equipment required the installation became more challenging when approaching the last unit of each tunnel. The clear distance between the last unit and the adjacent concrete caisson wall barely allowed one person to pass through. Water proofing of the exterior face and backfilling took place after all inspections of the as-installed structure were completed.

The successful installation depended on adequate engineering design as well as sound product quality, planning and execution. It relied on professionals and skilled trades from each of the stakeholders in order to make this happen. The complete installation can be reviewed on YouTube.

Conclusion

In conclusion, the precast application at the Mount Dennis Station has gone beyond the typical limits of precast box units. The manufacturing, delivery and installation challenges made this project a one of a kind for future reference. A construction project that fully exploits the use of precast components can enjoy an accelerated schedule and superior concrete quality. Understanding and implementing the project specific tolerances in both product and process are key success factors. This project also strived for technological advancement in using real time monitoring of concrete temperature and strength during the initial critical curing phase by EXACT Technology Company. The information provided allowed for critical decision making in the manufacturing process. This in turn achieved the best result in precast concrete manufacturing, setting a new benchmark for the precast industry.

1 Metrolinx (2013) The Big Move Baseline Monitoring Report