DESIGN AND INSTALLATION OF THE SHORING AND DECKING FOR TTC YONGE STATION INTRODUCTION By Dawn Tattle, P. Eng. Anchor Shoring & Caissons Ltd. TTC Yonge Station is the largest and most complicated of the new five-station Sheppard Subway Line under construction in Toronto, Ontario. The new station is being built on top of an operating station and will connect the existing north-south TTC Yonge Subway Line with the new east-west TTC Sheppard Subway Line. The work site is located at a major intersection in Toronto. All construction is being completed while keeping the existing subway in operation, maintaining traffic flow as well as access to existing businesses and residences. The tender for construction of TTC Yonge station closed in July 1998. The contract was awarded to the joint venture team of Ellis Don Construction Ltd. and Belor Construction Ltd. The subcontract for the design and construction of the shoring and decking systems was awarded to Anchor Shoring & Caissons Ltd. The schedule for the work was extremely aggressive and there were substantial financial penalties if the completion of the work was late. Design of the shoring and fabrication of materials began immediately after award and installation of the shoring began on September 14, 1998. GENERAL DESCRIPTION OF THE WORK The new Sheppard Subway line crosses above the existing operational Yonge line at the main traffic intersection of Yonge St. and Sheppard Ave., in Toronto, Ontario (figure 1). The actual design of the shoring and decking was not provided in the tender documents, only the design parameters and physical limits of the shoring and decking work were specified. During the bid stage Anchor Shoring worked together with RWB Engineering Ltd. to develop preliminary designs of the various areas of the project to be used in tender pricing. The scope of work included design and installation of 1,000 soldier piles, 900 struts, 285,000 ft2 of timber lagging, 35,000 ft2 of contiguous caisson walls, 100,000 ft2 of traffic decking, utility supports, and temporary support of existing stair structures and vent shafts. The limits of the site work are as shaded on the attached Key Plan (figure 2). The shoring design was required to incorporate numerous traffic detours, existing utilities and demolition of the substantial portions of the existing structures to allow connection of new subway line and station. In the majority of roadway areas 12 x12 timbers were installed on W36 deck beams to allow traffic to drive on a temporary traffic deck surface while the shoring, excavation and concreting continued beneath the deck. Intermediate support piles ( king piles ) were installed to support the deck at locations where the width exceeded 75 or traffic staging was required. 1
At the actual intersection of Yonge St. and Sheppard Ave. there was not sufficient clearance available between the top of the new station and the road surface to allow construction of a traffic deck. In this location it was also necessary to demolish the roof of the existing Yonge subway station to allow construction of the new Sheppard line. It would not be possible to close this extremely busy intersection for the duration which would be required for the construction of the new line and station works. This situation was resolved by relocating the existing Yonge/Sheppard intersection to the west to allow construction at the intersection (Figure 3) to occur while still maintaining traffic. Prior to implementation of this traffic detour it was necessary to complete a substantial portion of the Sheppard Ave West decking, all decking on Yonge St. and a section of the Sheppard Ave. East decking to allow the construction work below to continue after the traffic detour was in place. Figure 4: View looking east on Sheppard Ave. East prior to detour of Yonge St. intersection. Figure 5: Relocation of intersection complete. Demolition of roof of existing Yonge Station and installation of struts and lagging along Sheppard is in progress. 4
In order to allow transfer of future trains between the two subway lines 2 curved sections of tracks were required (figure 6). These two curved sections are called the S/East Wye and the S/West Wye. It was necessary to demolish a length of approximately 200 feet along the east and west sides of the existing Yonge line structure at the south limits of the project to allow construction of the Wye connector lines. The depth of excavation at the south limits of the Wyes was approximately 70 feet. A backhoe will not reach to this depth. In order to excavate this section the material was pushed to a bail out point with a loader. The stockpiled material was then removed by clamming from the surface through an opening in the deck. This process was time consuming and difficult. Figure 6: View looking north along Yonge St. Work in progress on S/West and S/East Wyes and decking on Sheppard West at north west corner of photo. The presence of a maze of utilities within the road allowance and intersection complicated the shoring and excavation work considerably. Many of the utilities were relocated in advance of the construction to expedite the work. Utilities within the limits of the excavation were supported off the shoring and decking. 6
During the construction Anchor was required to continue to provide access to the riders of the TTC Yonge line at all times. As a result of this it was necessary to support three existing concrete stair structures throughout the duration of construction. Two existing vent structures were also supported by Anchor Shoring as shown below. Figure 7: Support of S/East Wye vent shaft using Dywidag Hangers hung from deck beams. Figure 8: Support of S/West Wye vent using steel WF brackets, prestressed Dywidag bars and HSS steel pipe props. SUBSURFACE CONDITIONS The original soils investigation work completed in 1967 for construction of the original Yonge Subway line was supplemented by several phases of additional borehole drilling and testing. The Geotechnical Baseline Report was prepared by Golder Associates Ltd. Their report described the anticipated subsurface conditions and discussed the geotechnical aspects of the design and construction. The site is typically underlain by a thin layer of fill. Beneath the fill are alternating deposits of cohesive clay and cohesionless sands and silts. From the surface to maximum depth of explorations the deposits were given the designations of: Recent Sand/Silt, Upper Till, Upper Sand/Silt, Upper Clay, Middle Sand/Silt, and Middle Clay Deposits. Refer to the attached borehole (figure 9) illustrating the typical soil conditions at the site. 7
DESCRIPTION OF SHORING Anchor Shoring retained RWB Engineering Ltd. to design and inspect the shoring system. The majority of areas were shored using a soldier pile and lagged system. Contiguous caisson walls were specified where excavations would be close to settlement-sensitive structures. Installation of Soldier Piles and Filler Caissons In order to meet the schedule Anchor Shoring provided up to 4 drill rigs on site plus associated service cranes, vibratory hammers and loaders. This was the maximum amount of equipment which could work efficiently considering all the traffic and utility restrictions. In areas where the depth of excavation was less then forty feet it was possible to drill the pile holes without the use of casing. In these locations a Watson 3000 or a Taurus XL track mounted drill rig was used. The majority of the areas at the site required drilling below the water table and casing of these holes was necessary. Liners were vibrated through the wet layers to seal in the cohesive clay material using either an ICE 4450 or an ICE 812 vibro mounted on an LS 318 Linkbelt service crane. Drilling inside the liner was then completed using either a Haine drill mounted on an LS 208 Linkbelt crane, or a 155 Caldwell drill mounted on a 318 LS Linkbelt crane. If a soldier pile was to be installed it would be placed into the hole and the concrete toe poured. Lean mix concrete was placed above the toe and the liner extracted. The contiguous caisson walls were composed of a series of interlocked 915 mm diameter drilled holes. Intermediate or filler holes were drilled between soldier pile holes. All holes were backfilled with 4 Mpa concrete. A specific sequence of drilling was followed to ensure that the adjacent holes were overlapped or interlocked by at least 150 mm to ensure a seal below the water table. Bracing of Shoring The excavation depths varied from 30 to 70 feet. Up to 4 support levels of bracing were installed. Typically the bracing consisted of horizontal pipe struts in addition to a top deck beam (figure 11). The struts were installed between opposing lines of shoring with the exception of the south end of the site where the new shoring was strutted to the existing Yonge Subway Structure (figure 10). Struts are horizontal steel members. They can be used to brace opposing lines of shoring or brace a line of shoring against an existing structure such as the existing Yonge Subway structure. They can be steel pipes or wide flange sections. They were preloaded to minimize potential movements of the shoring adjacent to structures which were sensitive to movement. Excavation below the deck was completed using loaders to push the soil to a backhoe or clam sitting at grade. A V of soil would be removed to allow a loader clearance below the struts. After the strut was installed the berms would be removed and the excavation lagged. 9
Figure 12: Drilling of second row of regroutable tiebacks at Sheppard West by Crown Drilling Ltd. (a sister company of Anchor Shoring) Figure 11: S/West Wye lagged shoring braced by HSS pipe struts and upper deck beams. Note V cut in soil. Regroutable cased soil anchors were used to brace an area on Sheppard Ave. West as the strut lengths required were prohibitive. The soil anchor drill diameter was 7. Each drill hole was cased full length to prevent loss of ground. Once drilling was completed the steel cables and grout hose assembly were lowered into the centre of the casing. Grout was pumped into the casing and the casing extracted. The following day the tiebacks were regrouted under pressure. This creates the formation of bulbs along the anchor length providing a substantial increase in anchor capacity. Anchor Shoring retained Canadian BBR to supply, grout and stress the tieback anchors. CONCLUSIONS Performance of the shoring, decking, structure supports, and building response were monitored by TTC representatives during construction. Inclinometers were installed to monitor lateral movements of the shoring. The earth retention systems selected for this project performed in a satisfactory manner to limit ground movements and satisfy the specified performance criteria. Planning of each phase of the work, and communication of these plans to the other parties involved in the construction project was critical to ensure that the required traffic detours were arranged and that the work of other trades would not be interrupted. Cooperation between TTC, contractors and consultants was key to the successful completion of this extremely complex project. The majority of the concrete structure is now complete and train rails are currently being placed in the tunnels. Backfilling above the structure is ongoing. The balance of the deck removals will be completed this summer ending the traffic headaches at the intersection. The Sheppard line is scheduled to open in the fall of 2002. 11