Rehabilitation of Masonry Piers at Nipigon, Ontario

Transcription

1 Rehabilitation of Masonry Piers at Nipigon, Ontario Daniel E. J. Adamson, P. Eng. Canadian Pacific Railway 2881 Alyth Road SE Calgary, AB Canada, T2G 5S3 Phone (403) Fax (403)

2 ABSTRACT 62.4 Nipigon is multi-span bridge carrying the CPR mainline across the Nipigon river in Ontario, Canada. Originally completed in 1885, the 65 foot high masonry piers have gradually deteriorated in the form of cracks in the masonry and lateral displacement of the piers. In 2001, a project to stabilize the piers was undertaken. Significant construction issues included; minimizing pier settlement during construction due to sensitive soils, working in constrained site conditions, reduction of interference with train traffic, and minimizing cost. A reinforced concrete jacket on drilled micropiles selected as the best method of rehabilitation, due to micropiles low disturbance to soil during installation, the ability to be installed in constrained locations, and their large amount of resistance to loads. This paper discusses the history of the bridge, the movement and deterioration observed in the piers, the various rehabilitation options considered, and the design and construction of the method of rehabilitation. Keywords: micropile, bridge, pier, railway, CPR

3 INTRODUCTION The bridge at 62.4 Nipigon is a single track, multi-span bridge carrying the CPR mainline across the Nipigon river in Ontario, Canada. The Nipigon subdivision is part of CPR s transcontinental mainline along the north shore of Lake Superior and carries about 38 MGT per year and 22 trains per day. The bridge was originally completed in 1885 with iron superstructures on masonry piers and abutments. The masonry piers were supported on timber mats, which in turn were supported on a combination of timber piles and bearing on the soil. In 1905 and 1910, the iron superstructures were replaced with steel spans on the existing foundations. Figure 1 General Arrangement, 62.4 Nipigon Over time, pier deterioration has been observed in the form of cracks in the masonry and lateral movement of the piers. The deterioration was mainly associated with increasing live loads, some scour events under the timber mats supporting the piers, and soft ground conditions. The pier movement was first noted in 1954 and had continued consistently up to the present. Since the pier movement was showing no indications of stopping, and spans were now in tight contact, a project to stabilize the piers and reposition the spans was undertaken. Due to the current tilted position of the piers, the sensitivity of the underlying soils, constrained site conditions, and the

4 need to minimize interference with train traffic, the rehabilitation used a concrete jacket supported on micropiles. HISTORY 62.4 Nipigon was originally constructed between 1883 and 1885, during the original construction of the Canadian Pacific Railway. It was designed by C.C. Schneider of St. Louis, Missouri. The 745 structure over the Nipigon River originally consisted of two 155 iron Deck Trusses (DT), a 115 DT, and a 320 viaduct with DPG spans (Figure 2). On the viaduct, the track enters a spiral and then curves off the bridge at 4.5 degrees. Figure 2 Original Structure of 62.4 Nipigon According to the original foundation drawings, the foundations at Piers 1 and 2 were constructed with masonry stone on a double layer mat of 12 by 12 timbers. The timbers were supported on 12 x12 timber caps, which in turn were supported on timber piles (Figure 3). Between the timber caps, wet concrete was poured just prior to the Timber mat being installed, to allow some load to be transferred directly to the soil as a footing, in addition to the support provided by the

5 timber piles. At Pier 3, there was a similar arrangement, except that there were no timber piles used. The masonry was supported on a timber mat approximately 20 below the ground surface. The reason for this was possibly because there were better soils under pier 3 (sand and gravel), and because Pier 3 was not located in the river channel. Figure 3 - Foundation Arrangement at Piers 1 and 2 In 1905, the 115 DT was replaced with a 115 DPG (Coopers E50 design), with a rocker bent on Pier 2 for expansion, and a knee braced bent on Pier 3 (Figure 4).

6 Figure 4 Span 5 (new in 1905) In 1910, the remainder of the bridge superstructure was replaced on the original masonry foundations (Figure 5). One 155 DT was replaced with a 127 DT and a 30 Deck Plate Girder (DPG), and a small pedestal foundation was added. The viaduct on the west end was also replaced.

7 Figure 5 - Steel Superstructure Replacement (1910) In 1955, a large flood occurred on the Nipigon River. This flood caused significant scouring under the timber mat supporting Pier 2. An underwater diver was engaged to inspect the underside of the timber mat, and it was discovered that about 10 of material had been scoured from underneath the NE corner of the mat and pier. To remediate the scour, a steel sheetpile wall was driven around the timber mat, and the void underneath the timber mat was tremie filled with lean concrete. At Pier 1, a sheetpile cofferdam was also installed as a preventative measure.

8 Figure 6 - Pier 2 with Sheetpile Protection Figure 7 Bearing Arrangement at top of Pier 2

9 In 1962, there was a report of lateral movement of the track to the south over Pier 2. Pier 2 supports the expansion bearings of the 155 DT, and the rockerbent of Span 4 (Figure 7). A detailed inspection of the bearings on pier #2 revealed that the rocker bent was beginning to show significant leaning to the west. The gap between the span 3 and 4 bearings was measured at 1.5", compared to the original shop drawing dimension of 3.75", and the span 3 roller bearings were at fully expanded. As a result of the inspection, it was felt that the expansion rollers were seized, thus causing the expansion/contraction of both spans to be taken by the rocker bent (which caused the excessive tilt), and by bending of the pier. The roller bearings were replaced with lubricated bronze bearings, and it was reported in subsequent years that the rocker bent moved back to the proper position. At the same time, Pier 3 was inspected and found to be heavily cracked. A waffle type reinforced concrete jacket was installed to contain the cracked stones. From 1962 until 2001, the gap between the bearing plates of spans 3 and 4 and the angle of the rockerbent with respect to vertical was periodically measured. In 1976 the gap between the bearing plates closed. In 1993 and 2001, the base of the rockerbent was observed to be pushed off the bedplate, and the angle of the rockerbent continually increased (Figure 8). Also in 2001, vertical splitting in the top stones of the pier, in line with the rockerbent anchor bolts was observed, suggesting the anchor bolts were being subjected to considerable horizontal forces.

10 Figure 8 Span 3 Bearings Pushing Rockerbent off Bedplate, 2001 REHABILITATION OPTIONS In 2001, due to the continuing movement of Pier 2, and the condition of the masonry in all the piers, a project was initiated to deal with the moving pier and shifted bearings. Two options were considered: 1) Replacement of the Piers, and 2) Rehabilitation of the Piers. The Replacement option consisted of the construction of 3 new piers and an abutment about 25 beside the existing substructures, in line with the existing track. The proposal was to construct the new piers, slide the existing spans over to the new piers, and install a new longer span between Pier 3 and the first viaduct tower. Preliminary designs were done to determine material quantities and costs. The Rehabilitation option involved installing a new piled foundation around the base of the existing piers and the timber mat. A concrete jacket would then be poured around the new piles and up the existing masonry pier. Due to the sensitivity of the masonry pier to additional

11 movement, a low impact method of pile installation was required. Drilled micropiles were investigated and, due to their small diameter, they would have very little impact on the surrounding soil during installation. They were also capable of resisting large vertical forces. Micropiles were considered an appropriate method of pile support for the concrete jacket. The cost of both options were estimated. The replacement option was estimated to cost about $2,000,000 higher than the rehabilitation option. However, in addition to the lower cost, the biggest benefit of rehabilitation over replacement was in the reduced amount of track blocks required. For the replacement option, it was expected that track blocks from hours would be required to slide the spans over to the new piers. For the rehabilitation option, the track blocks would be limited to 4-6 hours, mainly for bearing work. DESIGN CONSIDERATIONS The detailed design of the rehabilitation option was commenced in October of Most of the design and drafting was performed internally by the CPR Structures Planning and Design department. The design of the micropiles was performed by Golder Associates, based on load combinations provided by CPR. All designs were in accordance with the AREMA Manual of Railway Engineering, 2003 Edition. The design load was Cooper s E80. The soil conditions varied under each of the 3 piers. Under the Pier 1 timber mat was a very thick layer of compact to very dense silt. Under the Pier 2 timber mat, there was a 17 ft thick layer of Compact to very dense sand, followed by a 9 ft layer of very dense silt, 17.4 ft of dense sand, and the remainder dense to very dense silt. Pier 3 had about 23.6 ft of compact sand fill, underlain by dense to very dense sand.

12 A basic premise in the design of the concrete jacket and micropiles was avoid excessive settlement of the existing piers. In their current state, the spans on Pier 2 were in tight contact and under very high internal stress. Since the amount of the stress could not be measured, the effect of any additional settlement of the pier was to some extent unknown.

13 Figure 9 Proposed Concrete Jackets and Micropile Foundation

14 The general arrangement of the micropiles and concrete jackets is shown in Figure 9. A detailed view of the micropile layout at Pier 2 is shown in Figure 10. For Piers 1 and 2, the micropiles were located around the perimeter of the existing timber mat, in the space between the existing steel sheetpiling and the edge of the timber mat. This was done to reduce the drilling effort required by avoiding the mat, and reduce any potential settlement incurred by vibration during installation of the piles. Figure 10 Layout of Micropiles at Pier 2 The micropiles consisted of a #20 central reinforcing bar grouted for the full length of the hole (about 65 long), and a 10 diameter casing pipe for the top 25, to provide lateral resistance. At Pier 1, due to large lateral loads from train longitudinal forces, 2 casings of 10 and 6 diameter

15 per pile were required. The longitudinal forces at Pier 1 were large because it supported the fixed bearing for both span 2 (127 DT) and 3 (155 DT). Pier 2 supported the expansion bearings for spans 3 and 4 (115 DPG). The vertical working loads of the piles ranged from 225 kips for the micropiles used at Pier 2 to 270 kips for the micropiles used at Pier 1. The lateral deflection during a maximum longitudinal force event was limited to ½. Also included in the project were bearing replacements on top of Pier 2, 3. The replacement bearings at Pier 2 involved installing grillages, lubricated bronze plates under span 3, and new bearing plates under the rockerbent. At pier 3, new fixed bearing plates were installed to replace the heavily corroded existing plates. The details of the reinforced concrete jacket are shown in Figure 11. At Pier 2, there was concern about the additional weight that would be placed on the timber mat when the concrete jacket was poured. For the first concrete pour, and a large portion the second pour, much of the weight of the concrete would be transferred directly to the timber mat (approximately 550 kips). The effect of this additional weight on the timber mat and pier was unknown. Therefore, it was required that the formwork for these pours would be supported such that no load was applied to the timber mat. At piers 1 and 3, the weight of the concrete was not an issue because the weight of existing overburden was at least as heavy as the first 2 pours of concrete. By having the micropiles 14 from the centerline of the pier, there was a very large moment in the jacket that needed to be resisted. The moment was resisted by coring holes through the base of the masonry and grouting in #20 steel reinforcement ties. These ties were anchored by plates. The remainder of the jacket was attached to the pier using hooked #9 dowels.

16 CONSTRUCTION Figure 11 - Typical Concrete Jacket for Piers 1 and 2 Construction began in April of The general contractor was Leo Alarie and Sons, of Timmins, Ontario, and Geo-Foundations Contractors Inc. of Toronto, Ontario, was the micropiling subcontractor. Construction Management and site supervision was done by Hatch Mott MacDonald (HMM), of Mississauga, Ontario. Golder Associates was subcontracted to HMM for the inspection of micropile installation.

17 Prior to micropile installation, a survey system was set up to monitor the piers during the construction. Rotational movement of the piers in the North-South and East-West directions was monitored, as well as vertical movement. This monitoring was done at least daily, and more frequently when required. It is interesting to note that even before micropile installation began, the top of Pier 2 was noticed to be moving back and forth parallel to the track. As more frequent readings were taken, it was noticed that the movement of Pier 2 coincided very closely with the air temperature fluctuations. Since span 3 was at full expansion and tight against the base of the span 4 rockerbent, the only way the thermal expansion of span 3 could be accommodated was by bending of pier 2. Two full scale sacrificial micropile load tests were done to confirm the soil conditions and the design soil-grout bond assumptions. One test was done on the east side of the river and one was done on the west side. The test micropiles were installed within the same soils that the production piles would develop their resistance in. The tests consisted of vertical compression and tension tests, and lateral bending tests. The test piles were loaded to a minimum of twice the working design load. The tests generally confirmed the soil properties and assumptions used in the design. The tests also provided an opportunity to examine the work procedure for the installation of the micropiles proposed by the contractor. The installation of micropiles began at Pier 2. The general procedure for drilling a micropile began by drilling a cased hole, filled with drilling mud, down to the design tip elevation. The central #20 reinforcing bar is then inserted into the hole along with 2 secondary grouting tubes attached to the #20. A tremie pipe is then lowered and grout is poured into the pipe, displacing the drilling mud. As the grout is poured down the tremie pipe, the casing pipe is slowly withdrawn to it s design elevation. The grout that is below the casing pipe flows against and into

18 the soil. This is the primary grouting stage. The grout is then allowed to harden for a few hours. After this is done, one of the grout tubes is pressure grouted (2 nd stage grouting). This cracks the hardened primary grout, and forces it against the soil, causing densification of the soil and improved friction resistance. Depending on how much grout is used in the 2 nd stage, this step may be done a third time. At Pier 2, the timber mat was found to be wider than shown on the original drawings. Since there was not enough room to move the pile off the mat, because of the steel sheetpile cofferdam, the piles had to be drilled through the timber mat. For efficiency, a Tricone drill bit was used to core through the timber mat. Once this hole was complete, a cased pipe with a down-the-hole hammer was inserted in the hole, and drilling continued to the design pile tip elevation. During installation of the first couple of micropiles at pier 2, the top of the pier moved about 1 to the east. It was found that installation of the micropiles in the south east part of the pier caused more settlement of the pier. The drilling was stopped and the contractor reassessed the drilling methodology. It was decided to perform some soil conditioning by pre-grouting to try and improve the soil for the micropiles. This was done and the micropile installation continued. The Pier movement slowed down but did not stop. As more and more piles were installed, the pier moved to the south and east. The track speed was lowered and the track alignment was watched very carefully. At the end of the micropile installation, the final movement was about 2.5 to the south (movement perpendicular to track), 1 to the east (movement parallel to track), and a differential vertical settlement of 1 on the north side of the pier and 1.75 at the south side. This left the track over about 2.5, which had to be spike lined back to tangent at the end of the project.

19 Once the micropiling at Pier 2 was complete, work at Pier 3 was begun. Even though the soil conditions were different, the micropiling induced movement to this pier as well. Fortunately, the movement was quite predictable. When the micropiles were installed on the east side of Pier 3, the pier tilted to the east, and when the micropiles were installed on the west side of Pier 3, the pier moved west. By selectively choosing the piles to be drilled, the pier movement was mitigated. At the completion of the micropiling, the pier was approximately 5/8 away from the original position. At Pier 1, the micropiling was installed without any significant issues. Figure 12 Completed Micropiles at Pier 1 Concrete forming and pouring began at Pier 2. For Pour #1, the contractor designed a suspended formwork to meet the requirement of not adding additional weight to the timber mat (see Figure 13). A steel frame was installed above the construction joint of Pour #1, supported on top of some of the micropiles. Tierods were hung from the steel frame to support steel decking, which

20 was used as the bottom form (see Figure 14). The bottom tip of the steel decking was supported on angles that were welded to the micropiles. With this arrangement of formwork, the entire weight of the first pour of concrete was supported off the micropiles, not the timber mat. After Pour #1 was cured, the steel frame was removed. The weight of Pour #2 was mostly supported by Pour #1. Once Pour #2 was cured, all subsequent pours and train live load were now being supported by the micropiles. The void underneath Pour #1 was grouted with a low strength flowable fill. Figure 13 Steel Beam Frame Supporting the Formwork

21 Figure 14 Suspended Formwork Once Pours 1 and 2 were completed, the remaining pours up the pier were done (Figure 15). These were done using conventional form and pouring construction. The concrete work at Piers 1 and 3 was similar.

22 Figure 15 Pier 2 Concrete Jacket underway Once the jacket was completed to the top of Pier 2, the replacement of the existing bearings was undertaken. The top of the existing pier was severely cracked and deteriorated. Therefore, in addition to replacing the bearings, it was decided during the design phase to remove the top course of masonry stone (about 2 ft) and replace it with steel grillages. The grillages would then be encased in concrete. In order to minimize the track blocks required, the bearing replacement work at Pier 2 was broken down into 4 track blocks of approximately 4-6 hours long each. Prior to the track blocks, the masonry on either side of the bearings was removed (Figure 17). A rock splitter was used to break-up the masonry.

23 Figure 16 Masonry Removed from around Bearings The work during the track blocks consisted of: Block #1) Repositioning of the rockerbent to a vertical position, and replacement of the rocker bent bearing plates, #2) on the north side, removal of the masonry under the bearings, installation of the grillages, and installation of the bearing plates (Figure 17 and 18), #3) removal of the masonry, and installation of the grillages and bearings plates on the south side, and #4), grouting underneath all the grillages. Prior to the grouting, plywood shims were temporarily used between the grillages and the masonry.

24 Figure 17 Spans in Jacked Position. Masonry has been removed prior to Grillage and Bearing Installation Figure 18 Grillage and Bearing Plates installed. After the grillages and bearing plates were installed, the grillages were encased in concrete. This completed the work at Pier 2. The work at Pier 3 was similar.

25 CONCLUSIONS 62.4 Nipigon, built in 1885, experienced masonry pier deterioration and pier movement after a many years in service. A project to economically stabilize and strengthen the piers, with minimal disruption to the track, evolved into the use of micropiles and concrete jacketed construction. At the completion of the project, CPR will have a safe and reliable structure crossing the Nipigon river for many years. Figure 19 Piers 2 and 3 completed. Pier 1 concrete work underway in 2006.

26 ACKNOWLEDGEMENTS LIST OF FIGURES Figure 1 General Arrangement, 62.4 Nipigon Figure 2 Original Structure of 62.4 Nipigon Figure 3 Foundation Arrangement at Piers 1 and 2 Figure 4 Span 5 (new in 1905) Figure 5 - Steel Superstructure Replacement (1910) Figure 6 - Pier 2 with Sheetpile Protection Figure 7 Bearing Arrangement at top of Pier 2 Figure 8 Span 3 Bearings Pushing Rockerbent off Bedplate, 2001 Figure 9 Proposed Concrete Jackets and Micropile Foundation Figure 10 Layout of Micropiles at Pier 2 Figure 11 - Typical Concrete Jacket for Piers 1 and 2 Figure 12 Completed Micropiles at Pier 1 Figure 13 Steel Beam Frame Supporting the Formwork Figure 14 Suspended Formwork Figure 15 Pier 2 Concrete Jacket underway Figure 16 Masonry Removed from around Bearings Figure 17 Spans in Jacked Position. Masonry has been removed prior to Grillage and Bearing Installation Figure 18 Grillage and Bearing Plates installed. Figure 20 Piers 2 and 3 completed. Pier 1 concrete work underway in 2006.