Emergency Bridge Stabilization at Mile Watrous Subdivision

Transcription

1 Emergency Bridge Stabilization at Mile Watrous Subdivision Christophe Deniaud, Ph.D., P.Eng. Bridge Engineer, CN Avenue Edmonton, Alberta, Canada T5J 0K2 Tel: (780) Fax: (780) Tim Saunders, P.Eng. Structural Engineer, CN PO Box Lagimodiere Bvd Winnipeg, Manitoba, Canada R3C 2Z6 Tel: (204) Fax: (204) /

2 ABSTRACT In the spring of 2006, an ongoing monitoring program indicated that the east abutment of the bridge over the South Saskatchewan River at Mile Watrous Subdivision was settling. A geotechnical consultant confirmed that the east embankment of the South Saskatchewan River was experiencing a landslide in the vicinity of the east abutment of the bridge. This landslide was causing the failure of a soil mass that encompassed the abutment of the bridge and unattended to, the slope failure would have been catastrophic with eventual loss of the entire embankment as well as the abutment and the 175 through-truss span resting on it. CN crews as well as multiple contractors were immediately dispatched to the site to begin construction on the preventative and restorative measures. Over a period of one month, work on site continued seven days per week in a race against the accelerating settlement occurring at the east end of the structure. At its peak, settlement of the abutment was occurring at a rate of approximately 3/8 per day, with additional shims added on numerous occasions to keep the track in operation. Furthermore, throughout the entire duration of the emergency, Edmonton to Winnipeg mainline corridor traffic was never stopped. The work on site involved the successful strengthening of the existing abutment and effectively isolating the bridge abutment from the embankment should embankment settlement continue. The response proved to be very successful since a major track and service outage was avoided as well as the avoidance of costs for potential replacement of the entire abutment and span. 2/

3 INTRODUCTION Bridge Description The bridge at Mile Watrous Subdivision is an eight-span steel structure consisting of six decktruss spans, one deck plate girder span and a single 175 foot long through-truss span on the east end. The bridge crosses the South Saskatchewan River on the south side of Saskatoon, Saskatchewan with an overall length of 1501 feet and maximum height of 78 feet (see Figure 1). The single-track structure was built in 1908 and is on the Winnipeg-Edmonton corridor of CN s Western Region. The track profile consists of a 0.40% grade decreasing to the west over the entire length of the bridge. The east abutment of the bridge, where the settlement occurred, is unreinforced concrete founded on five rows of 12 timber piles that were driven to a depth of 17 feet below the base of the abutment. Initial Assessment Vertical misalignment of the east end of the bridge was first reported to the Bridges and Structures department in late October 2005 (see Figure 2). Upon receiving this report a survey is immediately conducted to determine the current track profile and pick up elevation data from other components of the structure including the bearing seat of the east abutment and the bearings, floor beams and stringers of the through-truss (Span 1) resting on this abutment. Although this survey indicates a definite vertical alignment change when compared with the design elevations, there is not any recent data available to compare with in order to establish the rate and acceleration of the potential settlement. As such, this initial survey served as a baseline for a weekly monitoring program consisting of subsequent surveys to confirm any change. This data was also used to establish a vertical curve of the track profile and to determine shim height requirements for the east end of span 1. On November 25, 2005 the east end of Span 1 is shimmed three inches bringing the alignment to an acceptable condition. During the subsequent monitoring surveys, there is no indication of any settlement when compared against the baseline data and as such the monitoring program is put on hold until the spring of Furthermore, there is also no further settlement of the east abutment during the winter. 3/

4 In late February 2006 the East abutment begins to settle again and the monitoring program is reinstated. Weekly surveys are completed to monitor the rate of settlement of the east embankment and its influence on the bridge abutment. Now that new settlement of the east embankment is reported and confirmed, P. Machibroda Engineering, a geotechnical consultant, was commissioned in March to complete a slope stability analysis of the east embankment of the South Saskatchewan River, in the vicinity of the bridge. During the next month, while the geotechnical analysis was being completed, monitoring continued and it is decided to excavate a test pit adjacent the abutment to confirm the condition of the timber piles under the abutment. The excavation is completed and the timber piles are confirmed to be in good condition, free of any deterioration. Results of the geotechnical investigation are received in mid April and confirm that an active landslide is occurring at the east end of the bridge. The soil stratigraphy of the area creates a perched water table behind the embankment, increasing pressure on the back of the abutment as well as increasing the overall weight of the material at the top of the embankment and reducing the factor of safety against a slope failure as indicated in Figure 3. The retrogressive landslide has a failure plane where embankment material is lost at the toe of the slope followed by a vertical settlement of a wedge of material that encompasses the bridge abutment, which explains the elevation changes of the bridge abutment. DETERMINATION OF SOLUTIONS Based on the geotechnical investigation, Machibroda recommends a series of remedial works listed below. 1) Slope stabilization with large ground water wells installed on the river bank 2) Placement of slope indicators throughout the area with weekly monitoring and ongoing survey 3) Placement of fill at the toe of the slope or slope reconstruction using soil reinforcing geogrid 4/

5 4) Regrading (excavation) at the crest of the slope 5) Underpinning of the existing abutment with deep foundations down into the stable glacial till stratum with HP piles all around the perimeter of the abutment. The Network Operation Bridge and Structure Design Group based in Edmonton got involved in April 2006 to design the remedial repairs as the abutment was continuously moving down and started to affect train operations. The preliminary design concludes that it is not possible to drive HP piles all around the perimeter of the existing abutment without affecting train operation. Furthermore, this option would require a large amount of HP piles to carry the vertical loads. The earth pressure behind the abutment would also require additional piles and walls installed further up the crest of the slope with dead man anchor systems. The pile driving operations close to the track with a slope failure situation are also a serious concern to maintain the integrity of the track bed. This solution would require a construction period in the order of a few months with a constant danger to the train operation. The second option consists of large diameter caissons installed on either side of the abutment (Figure 4). This design allows for the installation of the caissons outside the track, which enables continuity of the service. The existing abutment is then to be tied to the caisson concrete cap with Dywidag bars, which are cored into the existing abutment and post-tensioned. In addition, the piling contractor is able to mobilize its equipment within days and to start accessing the underside elevation of the existing abutment. Instead of installing soldier piles and lagging walls perpendicular and away from the track, it is decided to build these retaining walls parallel to the track. This design feature provides two fold benefits: 1) Access for the contractor to both sides of the abutment and 2) Decrease rate of slope movement at the bridge approach. 5/

6 Thus, the construction period is significantly reduced to 6 weeks with an increase confidence in the stability of the construction site. REPAIR DESIGN In cooperation with the piling contractor, readily available 30 diameter pipe piles are sourced out of British Colombia and shipped to the site immediately. While the contractor and CN forces are installing the shoring walls along the track at the back of the East abutment, the design of the repairs is optimized with the available pipe pile size and the available piling equipment. A total of 4 caissons per side are required to support the vertical loads from the abutment and the earth pressure from the moving embankment slope failure. The salvation of the existing abutment requires a change in the design philosophy. The existing abutment was supported on timber piles closely spaced and was cast with unreinforced concrete. Now, the new design ignores the timber piles, which are too short to be effective, and assumes the abutment seat behaves as a beam between the two rows of caissons. The design concludes that steel reinforcement is required and cores are drilled transversally through the abutment to insert high strength Dywidag bars. In addition, a total of 6 out of 20 bars installed are post-tensioned to tie the pile caps and the existing structure together. High strength concrete is used for the pile caps to provide an early strength suitable for post-tensioning operation after 3 days only. CONSTRUCTION AND EXECUTION Work begins onsite with an initial on-site meeting and site orientation with the contractors. Contractors and suppliers retained for the work include: 1. Agra Foundations Ltd. Responsible for pile-driving, caisson pipe installation, caisson concrete placement 2. Western Redi Mix Inc. Concrete Supply 3. P. Machibroda Engineering Ltd. Slope installation, concrete coring 4. F. Peters Excavating Ltd. Site excavation, equipment rentals 6/

7 5. UMA Engineering Ltd. Site Surveying 6. United Rentals Ltd. Miscellaneous rentals 7. Vector Construction Ltd. Grout placement 8. Harris Rebar Reinforcing Steel Supply Excavation The first order of business is the excavation and removal of excess embankment material from the top of the slope to reduce the overall load on the embankment. Concurrent with this excavation, a hydrovacuum truck is employed to locate the underground cables east of the abutment in preparation for driving piles for the retaining wall parallel to the track. Once all cables have been exposed and protected, the pile driving starts. The piles are driven on the south side first followed by the north. Four days are required to install all of the piles required for the retaining wall (see Figure 5 and 6). During this period, the east abutment had settled approximately ¾. Additional shims are placed in order to raise the east end of the bridge span 2-1/2. At this stage an additional measure is taken to assist with monitoring of the settlement through the installation of a cable running between the bridge abutment and the recently driven piles for the retaining wall. This cable is incorporated into the CTC system with the intent that any major movement of the bridge abutment would disconnect the cable shunting the circuit and inducing a stop signal on all approaching signals to the bridge location. Retaining Walls Once all piles are driven for the retaining wall, timber lagging, steel channel walers and tieback bars are installed to complete the retaining wall construction. The wall is constructed progressively lower; therefore at the completion of the retaining wall, the base elevation is 15 feet below the base of rail. 7/

8 Steel Caisson The drill rig is mobilized to the south side of the bridge abutment and installation of the steel caissons commences. The drill rig augurs to a depth of 75 feet and then concrete is placed in the bottom of the hole followed by the installation of the steel pipe (See Figure 7 and 8). Additional concrete is placed to the top of the pipe. This process is repeated for the four caissons on the south side. During the installation of the caissons on the south side, holes are cored in the existing abutment to permit the installation of post-tensioning bars (see Figure 9). During the coring, it becomes evident that the interior of the existing abutment had previously cracked and may become a problem during the post-tensioning phase. Because of this, six Dywidag bars are immediately installed to aid in keeping the existing concrete intact. Once all caissons are installed on the south side, the coring contractor moves to the south side and the north side was prepared for the drill rig to install caissons there. There is concern of overloading the embankment with the large equipment, therefore a small retaining wall is built and piles are driven to transfer the equipment loads directly to the till layer rather than onto the embankment slope. Installation of the caissons proceeds on the north side. The north side proves to be more of a challenge on account of large boulders encountered below grade in the location of the piles. Additional equipment including rock-breakers are employed to assist in the removal of the rock. Formworks and Dywidag Bar Installation After all of the caissons are installed, a concrete mud slab is placed. The remaining Dywidag bars are installed through the post-tensioning ducts and the steel reinforcement for the pile caps is installed (see Figure 10). The construction of the formwork concludes preparations for the placement of concrete for the new pile caps. 28 cubic metres of concrete are required for each pile cap. 8/

9 Once the required strength has been achieved in the concrete pile caps, post-tensioning of the Dywidag bars is required to complete the process of transferring abutment loads to the new substructure. High-capacity jacks are used to post-tension the bars (see Figure 11). Following the post-tensioning, all holes are pressure grouted, as are all internal cracks of the existing abutment. POST-CONSTRUCTION ACTIVITIES The slope indicators are still in place within the vicinity of the abutment and are monitored on a regular basis. So far, these indicators have shown little movement in the embankment and no movement of the abutment. The through-truss bearings had been raised several times with numerous shim plates (see Figure 12) that need to be replaced with steel grillages. This work is scheduled as part of the regular maintenance program. CONCLUSION This effort contributed to the salvation not only of the existing abutment but also of the through-truss span. The cost of replacing both the steel span and the abutment would have been approximately $6,000,000. More importantly the train operation was never completely interrupted and track speed was re-established as soon as the repairs were completed. This achievement averted a major service disruption on the core route between Winnipeg and Edmonton, which prevented the re-routing and delays of trains over an extended period. 9/

10 Figure 1: CN Bridge at Mile Watrous Subdivision Facing East Figure 2: Mile Watrous Subdivision Initial Signs of Settlement 10/

11 Figure 3: Stratigraphic Section through the East embankment Figure 4: Strengthened Abutment with Caissons down to the Till Layer 11/

12 Figure 5: Pile Installation for Retaining Wall Construction Figure 6: Completed Retaining Wall on North Side 12/

13 Figure 7: Caisson Installation on South Side of Abutment Figure 8: Caisson Installation on South Side of Abutment 13/

14 Figure 9: Concrete Coring for Installation of Post-tensioning Reinforcement Figure 10: Placement of Reinforcing Steel and Formwork Construction 14/

15 Figure 11: Post-tensioning of Dywidag Bars Figure 12: East Bearings of Through-Truss Span 1 With Shim Plates Added Throughout the Project 15/