Design/Build Replacement. Beaufort & Morehead Railroad Trestle. Over. Newport River, Morehead City, N.C.

Transcription

1 Design/Build Replacement Of Beaufort & Morehead Railroad Trestle Over Newport River, Morehead City, N.C. Neil T. Greenlee, P.E. The HNTB Companies 343 E. Six Forks Rd. Suite 200 Raleigh, NC Ph. (919) Fax (919)

2 ABSTRACT Design/Build Replacement of Beaufort & Morehead Railroad Trestle over Newport River, Morehead City, N.C. Neil T. Greenlee, P.E. In May 1999, the HNTB Companies and McLean Contracting Company completed the first design/build railroad bridge replacement project for the North Carolina Department of Transportation (NCDOT). Approximately 2,300 feet of open timber trestle approaches on the Beaufort & Morehead (B&M) Railroad over the Newport River in Morehead City, N.C. were replaced with concrete approaches meeting AREA and NCDOT requirements. The ballasted, on-line replacement trestle consists of prestressed concrete T-beams supported on precast concrete pile caps with composite steel and concrete pile foundations. The project was completed in 11 months at a cost of $8.45 million. The replacement structure was constructed on-line during four day weekly windows for maintenance of rail traffic. The profile of the 1,770 ft. west approach was raised nearly four feet to keep the new superstructure completely above the 100 year flood plain. Significant shimming was required to tie the new and existing trestles together in order to run trains in between the four day work windows. The HNTB/McLean team developed a unique, composite pile system consisting of 24 steel pipe piles encased above the scour line with grouted, 36 prestressed cylinder pile sleeves. Pipe pile embedment was 70 feet, and the concrete sleeves satisfied NCDOT s corrosion protection criteria for coastal zones. Global structural modeling and elastic second order analysis allowed the replacement structure to be designed using two composite piles per pier without transverse or longitudinal bracing.

3 INTRODUCTION In August 1996, the North Carolina Department of Transportation (NCDOT/Owner) requested proposals for engineering and construction services for the replacement of timber railroad trestle approaches on the Beaufort & Morehead (B&M) Railroad in Morehead City, N.C. This would become the first design/build bridge replacement project either highway or railroad ever undertaken by the NCDOT. The HNTB Companies (Consultant) and its subconsultants teamed with McLean Contracting Company (Contractor) to win this project with a replacement structure which received the Owner s highest possible technical rating in addition to offering the lowest replacement cost. The subject project will first be discussed from the perspective of the overall design/build process utilized for its performance. Detailed descriptions of the salient technical features have been omitted from this discussion since they are not necessary to understand the design/build process as it relates to the project. The technical features of the project will then be described, with special emphasis on those particular features which make this project unique from a railroad engineering perspective. The above information is preceded by a brief background description so that it may be understood within the context of the overall project. PROJECT BACKGROUND The B&M Railroad crosses the 2,400 foot wide Newport River to link Radio Island to the State Port at Morehead City, N.C. The previous bridge originally constructed in 1908 consisted of a 100 foot single-leaf, heel trunnion bascule span located approximately 600 feet from the west shoreline (State Port side) with open deck timber trestle approaches (Fig. 1). The bascule span across the Atlantic Intracoastal Waterway provides 80 feet of horizontal clearance in the navigation channel. Prior to advertising this project, the Owner inspected the timber approaches and assigned them a sufficiency rating of 9 points out of a possible 100. The B&M Railroad is the only rail link between the North Carolina mainland and Radio Island, and for nearly 50 years its primary industrial client was an aviation fuel terminal located on the island. Rail access to Radio Island is a critical

4 aspect of the Ports Authority s long-range industrial development plans, thus continued operation of the B&M Railroad is crucial to the success of current and future operations of the Port of Morehead City. Accordingly, the N.C. Ports Railway Commission which owns and operates the B&M Railroad attempted to replace the timber approaches in 1990 with a similar structure but could not generate the estimated $4.5 million construction fee. Several major industrial clients declined to locate their operations on Radio Island primarily because of the severely deteriorated condition of the timber approaches (Fig. 2), so the Ports Authority petitioned the State Legislature for an adequate replacement structure. The State Legislature responded by deeding the entire structure to NCDOT and amending the North Carolina General Statutes to allow immediate replacement of the timber approaches through design/build procurement procedures. NCDOT advertised the project in August 1996, requiring a comprehensive scope of design/build services consisting of environmental permitting, structure design and construction, and construction engineering and inspection services. The replacement approaches were completed nearly 3 years later in May The bascule span was rated but not replaced or rehabilitated as part of this project. DESIGN/BUILD PROCESS OVERVIEW The design/build process for the subject project is best described in three chronological phases. Phase 1, Pre-Contract, progressed from project advertisement to formal execution of the design/build contract, and consisted primarily of the Owner prequalification and selection process and the preparation of technical concepts and bids. Phase 2, Pre- Construction, progressed from contract execution to the beginning of construction, and consisted of environmental permitting, final design and plan production, and prefabrication of precast structural units. Phase 3, Construction, progressed from permit acquisition to project completion, and included continuation of final design and plan production, trestle construction, and construction management activities. Phase 1 Pre-Contract The prequalification and selection process officially began with the August 1996 request for statements of qualifications. From a pool of seven respondents, the Owner shortlisted three teams to prepare technical proposals and bids. Each shortlisted team was required to make a technical presentation to the Owner, who would evaluated and

5 rank each proposal prior to the bid opening. The maximum possible score was 100 points, with technical merit weighted at 35% and cost at 65%. To compute a team s final score, the ratio of the low bid to that team s bid was multiplied by 65 (maximum cost score = 65 points) and the resulting number was added to the number of points awarded for technical merit (maximum technical score = 35 points). This scoring methodology gave the Owner a greater measure of control over the outcome of the selection process than if the project had been awarded based on cost alone. Preparation of technical proposals and bids began in early October A scoping meeting was held in late September 1996 to explain the scoring methodology, distribute design criteria and appropriate background information, and answer relevant questions. Each team received a lump sum fee of $50,000 and approximately seven weeks for preliminary design and preparation of proposals and bids. Technical interviews and the subsequent bid opening occurred in late November 1996, but due to insufficient funding all bids were rejected, the project was rescoped with the original shortlisted teams, and a second interview and bid opening was conducted in late February No additional design fees were paid to the teams for this second round of preliminary design and bidding. Upon completion of the second round of interviews and bids, the HNTB/McLean team was awarded the project by virtue of receiving the maximum possible score of 100 points based on technical merit and cost. The lowest cost of approximately $7 million submitted by HNTB/McLean, combined with precontract design fees of $150,000 and $1.3 million of extra work requested by the Owner, resulted in a total project cost of approximately $8.45 million, or $3,700 per linear foot of replacement structure. The $7 million bid was nearly $2 million less than the low bid submitted in the first round, but funding was still insufficient so the contract was not executed until July 1997 (the beginning of the next fiscal year). No work was performed on the project between the award date and the contract execution date. Phase 2 Pre-Construction The pre-construction phase began in July 1997 with a meeting to review the technical concept and define the roles of the Owner and the design/build team in the performance of the project. During this meeting, the Owner requested that the design/build team evaluate the cost of several additions and/or revisions to the original technical concept. The

6 environmental documentation and permitting process was deemed independent of the outcome of these evaluations, so these activities commenced immediately upon execution of the design/build contract. However, final design and plan production could not commence until the Owner rendered a final decision regarding a particular item related to the foundation type. This decision was made in October 1997, and final design and plan production commenced at that time. The coastal area in which the project is located is regulated by the Coastal Area Management Authority (CAMA) of the State Department of Health and Natural Resources. The Newport River is a navigable waterway which is regulated by the Corps of Engineers (COE) and the United States Coast Guard (USCG). Therefore, permits were required from all three of these agencies before construction could begin. (Actually, no Coast Guard permit was required, but the agency required a formal application to be submitted and reviewed before officially rendering this decision.) In addition to these agencies, a Federal Aviation Administration (FAA) permit was required for crane booms which could potentially penetrate the runway clear zone of a small airport located in the vicinity of the project. The CAMA/COE permit application required the submission of a State Environmental Assessment/Finding of No Significant Impact (SEA/FONSI) document, which required approximately two months to prepare and four months for the State Department of Administration to review and approve. Though the replacement structure is located on the same alignment as the old structure thus requiring no additional right-of-way acquisition it still required approximately one year to obtain construction permits because of the number of regulatory agencies involved and the amount of environmental documentation required. Final design and construction documents were substantially complete by the end of the pre-construction phase. The Owner agreed to an intermittent review process in which construction documents were submitted, reviewed and approved for structural elements requiring prefabrication and storage at the construction site. This allowed construction to begin as soon as all permits were obtained. Structural optimization occurred throughout this process, but the overall technical concept was never substantially changed. Prior to submitting construction documents to the Owner, all structural details were subjected to rigorous, internal constructability reviews to ensure their compatibility with the Contractor s preferred construction techniques. This process of internal constructability reviews and intermittent Owner reviews ultimately streamlined the construction process and allowed substantial amounts of

7 construction material to be fabricated and stored at the project site well in advance of their required use by the Contractor. As a result, design and plan production efforts were consistently ahead of construction, and no claims were filed by the Contractor during or after construction. Phase 3 Construction Construction began in June 1998 when all permits had been acquired, and was substantially complete by early May Construction management and inspection activities during this phase were performed by the design/build team s Consultant. The Consultant assigned one, full-time field inspector to the project who was responsible for all inspection activities and documentation typically required by the Owner. Materials testing when required was performed by the Owner using field test specimens prepared by the Consultant s field inspector. In addition to the Consultant s inspector, the Owner sent a representative from its local Resident Engineer s office to check on the project several times each week during construction. The operating railroad though not contractually involved in the project was also consulted by the Owner in matters having a direct impact on immediate and long-term railroad operations. Throughout this process, the Contractor and the Consultant acted as one contractual entity from the standpoints of work performance and coordination with the Owner and the operating railroad. This contractual bond resulted in an amicable union between the Contractor and the Consultant, which further resulted in swift and effective resolution of occasional problems which arose during construction. TECHNICAL FEATURES The description of the project technical features begins with an explanation of the design and construction criteria established by the Owner, followed immediately by a general overview of the replacement structure. As explained in the design/build process overview, two rounds of interviews and bids were conducted by the Owner in an attempt to reduce the cost of the replacement structure as much as possible. To that end, the Owner revised the original project criteria prior to the second round of bidding. Since the actual replacement structure was designed and constructed under the revised criteria, the structural concept associated with the original criteria is not discussed. Detailed

8 descriptions of primary structure and trackwork features follow the general structural overview, and the section concludes with a brief description of the weekly sequence of construction. Design/Construction Criteria Principle design and construction criteria for the replacement structure were as follows: Use AREA Manual for Railway Engineering, 1996 edition. Use Cooper E-80 live load w/diesel impact & 25 mph design speed. Use 120 mph design wind speed. Use design stream flow of 4 ft./sec. Use NCDOT corrosion protection guidelines for structures in highly corrosive areas. (These guidelines disallowed the use of exposed structural steel above the 100-yr. scour line, which was located an average of six feet below the existing soil line.) Perform on-line replacement w/o interruption to scheduled rail and marine traffic. (Scheduled rail traffic occurred three consecutive days per week, leaving four consecutive days per week for construction access to the trestle.) Remove and dispose of the existing timber approach trestles. Re-use existing 100 lb. bolted rail and 6 x 8 crossties. General Structure Overview To satisfy the project design and construction criteria, the design/build team developed a ballasted replacement structure (Fig. 3) consisting of the following primary features: Dual, prestressed T-beams supported on transverse 33-0 centers. Precast, reinforced concrete pier caps with rotational pile fixity. Composite steel and concrete piles (two plumb 6-0 centers each pier).

9 The west approach trestle consists of one end 11-1 ½, 15 interior 33-0, and one end 10-0 (moving west to east) for a total length of approximately 516 feet. The east approach trestle consists of one end 10-0, 53 interior 33-0, and one end 11-1 ½ (moving west to east) for a total length of approximately 1,770 feet. Most of the features which make this structure unique are hidden underwater or underneath the surfaces of the various structural elements. These features are examined in detail in the sections immediately following. The most notable external feature of these approach structures is the absence of transversely battered support piles and periodic, longitudinal brace piles and/or brace piers. Both approaches were modeled and analyzed independently, and each was designed such that a large percentage of its total longitudinal force would be directed to its ends. End supports and individual piers were thus designed to resist their share of the total longitudinal force based on their relative stiffnesses in the longitudinal direction. For transverse stability, frame action was achieved in the piers through cast-in-place, reinforced concrete pile plugs which created rotational fixity at the pile to pier cap connections. When warranted due to the unbraced heights of the piers, an elastic second order analysis was performed in both directions on the piers to account for P- effects. The amount of additional analysis performed was justified by the overall nature of the project as well as constructability issues. The Owner desired an economical replacement structure which could be quickly constructed and would satisfy an uncommonly strenuous set of corrosion protection criteria for a railroad trestle. Furthermore, the existence of physical access barriers high voltage power lines on the north side of the trestle and a privately owned fishing pier on the south side, for instance created construction access issues which required a minimal amount of materials and uniformity of details to be competitive. These and other factors required a willingness on the part of the design/build team to employ non-traditional engineering and construction techniques to successfully complete the project.

10 Prestressed T-beams The original concept submitted to the Owner involved the use of single void, 36 T-beams of the type commonly used on the BN and UP railroads. Span lengths were initially set at the maximum length for these beams of During the internal constructability review process, the precaster requested that the beams be cast solid (i.e. that the void be removed) for ease of fabrication. At this point, the beams had already been lengthened to 33-0, and the additional weight of the solid beam would tax the Contractor s 40 ton crane limit. Therefore, the beams were redesigned as solid with external pockets on the sides (Fig. 4) to accommodate the fabricator and minimize the pick weight. Full depth blockouts in the ends of the beams accommodated anchoring assemblies needed for lateral shear transfer and vertical restraint against potential wave uplift. Fiberglass tees were used as ballast stops in all horizontal and vertical open joints. Precast Pile Caps Precast, non-prestressed pile caps were designed with moment connections at the piles. Pile fixity was accomplished in the actual structure by fabricating the pile caps with protruding rebar cages and tapered, preformed holes centered above the cages (Fig. s 5 & 6). After placing the pile caps, fresh concrete was poured through the preformed holes to form a reinforced plug in the tops of the piles. These pile plugs transfer the design moments from the piles to the precast caps. T-beam anchoring assemblies were grouted into additional preformed holes on top of the caps after placement of the pile plug and T-beams (Fig. s 5 & 6). Composite Piles The composite piles developed specifically for this structure consist of 24 diameter, ½ wall, open ended steel pipe piles (A252 Grade 3) encased with 36 diameter, 5 wall prestressed concrete cylinder pile sleeves. The sleeves extend from the tops of the pipe piles to approximately 3 feet below the existing scour line and were used primarily to prevent corrosion of the unembedded portions of the steel pipe piles. The system was made composite by filling the 1 annular

11 space between the pipe piles and the cylinder pile sleeves with non-metallic, high strength grout. To the author s knowledge, this project marks the first use of this particular type of composite pile system. Composite piles afforded the dual advantage of fulfilling the Owner s corrosion protection criteria while providing increased lateral resistance when designing the piers for load cases involving secondary forces (wind, stream flow, longitudinal forces, etc.). Using exposed structural steel below the scour line was permitted by the Owner, but the pipe piles represented a technical merit risk for the design/build team due to the Owner s traditional hesitancy to use structural steel in highly corrosive environments. Using cylinder pile sleeves to protect the pipe piles above the scour line was an unorthodox solution, but pipe piles were easier to install and significantly less expensive than using full length cylinder piles at all of the piers. Once the issue of placing grout in the annular space was resolved in theory, at least the design/build team decided that the potential cost savings of this system was too great to ignore and elected to use it in the overall technical concept. To construct this system, a pipe pile was first vibrated to approximately 30 feet of embedment and then driven an additional 40 feet to grade (total embedment = 70 feet). A cylinder pile sleeve was then slipped over the pipe pile and driven into the soil until its top elevation matched the top elevation of the pipe pile. Grout was then pumped into the annular space through a small, flexible tube inserted into the space by securing it to a piece of #3 rebar which was long enough to reach the bottom of the sleeve. Pumping continued until all water in the annular space was displaced and the consistency of the grout at the top of the pile matched the consistency of the grout in the mixer. After installing both composite piles in this manner, the system generally sat overnight before the rest of the pier was constructed. Standing water inside the pipe piles was pumped out prior to placing the pile caps, and a plywood bulkhead was suspended from the pile cap rebar cage in order to facilitate placement of the cast-in-place plug. This procedure was tested and refined during the pre-construction phase to confirm its achievability on the actual structure within the 4 day construction window imposed by the Owner (Fig. 7).

12 Geotechnical/Foundation Issues The most challenging aspect of this project from a geotechnical perspective was obtaining enough skin friction capacity in open ended pipe piles to support E-80 live loads without sacrificing economy of construction. Since skin friction capacity and economy are both direct functions of pile length, this was a particularly difficult obstacle to overcome in preliminary design. Spiral welded pipe piles are typically furnished in maximum lengths of 45 to 55 feet, so design pipe piles in excess of 100 to 110 feet would require field assembly from three separate pieces. Considering the total number of pipe piles needed for the project, adding just one additional groove welded pile splice per design pile represented a significant amount of additional labor and expense for the Contractor. In addition, the Contractor wanted to keep design pile lengths under 100 feet, if possible, for ease of handling in the field. The 100 foot length limit was achieved by assuming that a nominal amount of end bearing would be provided by soil plugging at the design tip elevations. Nominal geotechnical data was provided at the original scoping meeting for use by the design/build teams in preliminary design. Since foundation type critically influences the strength and economy of any structure, the design/build team conducted pre-construction and construction phase test pile programs to verify the preliminary foundation design assumptions. As a result, the preliminary assumption that end bearing would be provided by soil plugging in the pipe piles was invalidated. Design pile lengths were increased accordingly, resulting in 110 foot piles being required for 7 or 8 piers on the west approach structure. In order to keep all design pile lengths under 100 feet, the cylinder pile sleeves at these piers were extended an additional 10 to 15 feet into the soil. The resultant skin friction and end bearing from these extended sleeves allowed the pipe piles at these locations to be reduced to their original design length of 100 feet. Another objective of the test pile programs was to provide the field inspector with a method of verifying production capacity during pile driving operations. To accomplish this, a pile driving analyzer (PDA) was used to monitor test piles driven at various locations along the trestle alignment. Restrikes were performed and monitored at 24 and 72 hour intervals to determine the amount of soil freeze occurring after initial installation. PDA data from initial driving tests and restrikes was analyzed to correlate the blow counts with pile capacity at a given depth of installation. For

13 field verification purposes, the inspector recorded the number of blows per foot of penetration during pile driving operations. Field blow counts had to exceed specified minimums in the final 10 feet of embedment for the piles to be accepted. Trackwork The project was scoped and bid assuming that the 100 lb. bolted rail and 6 x 8 timber crossties from the old trestle would be salvaged and reused on the new trestle. This initial directive was made by the Owner solely on the basis of reducing project costs. The existing rail and ties were in such poor condition, however, that the Owner decided to replace them after construction began. The operating railroad laid 131 lb. bolted rail and 13 double-shoulder tie plates on 7 x 9 timber crossties in the port area off the west end of the old trestle, so identical track materials were ultimately used on the new trestle. The belated decision to install new trackwork on the replacement trestle created several dilemmas for the design/build team. First, a great deal of consideration went into determining the most appropriate type of lift joint to be used at the bascule span. The need to minimize costs coupled with the limited amount of rail traffic and slow (10 mph) operating speed did not warrant the use of proprietary lift joints, which are expensive and often difficult to maintain. Furthermore, the simple easer rail lift joint on the old trestle though somewhat deformed after nearly fifty years of service was still performing adequately (Fig. 8). In light of these facts, the design/build team and the operating railroad elected to fabricate a lift joint (Fig. 9) similar to the easer rail joint already in use. Though similar in appearance to the original joint, the rail ends of the new joint are beveled at 45 degrees and the guide plate on the field side of the rails is for lateral joint stability only and does not support the wheels of passing trains. Rudimentary by today s standards, this joint has been in service for almost two years and the operating railroad has experienced no problems with its performance. The problem of thermal rail expansion was the second issue raised by trackwork replacement. On the old trestle, slip rail expansion joints were located for this purpose within 50 feet of each end of the bascule span. New joints of this type were investigated, but fabrication time and long-term maintenance concerns led the design/build team to consider

14 other options. After much internal discussion and input from the operating railroad, the team elected to mitigate the problem by box anchoring all crossties on the new trestle within 160 feet of each end of the bascule span. Again, the operating railroad has experienced no problems associated with thermal rail expansion at the draw span after two seasonal temperature cycles. Sequence of Construction As stated earlier, the replacement structure was constructed on the existing alignment during weekly periods of four consecutive days. New structure was typically erected at a pace of three spans per week. The sequence of construction during these weekly periods was as follows: Day 1 - Remove existing track in panelized sections; demolish approximately 100 feet of existing timber structure; erect pile driving template and work platform. Day 2 - Day 3 - Day 4 - Install composite piles at each new pier location. Place pile cap and T-beams. Replace previously removed track panels on temporary timber blocking and perform tieins for resumption of train traffic. The sequence above does not include ballast and new trackwork because this was done in one continuous operation after the new structure was fully erected. New track was laid in 39 foot, prefabricated panels which were prefabricated and stored on-site until they were ready for use. Ballast was fully tamped and surfaced after installation and lining of track panels. The east approach of the replacement structure was constructed on a level profile. This created a top of rail grade differential of approximately four feet which had to be dealt with during construction. The solution to this problem involved shimming the old trestle to the new profile for several hundred feet ahead of the weekly demolition zone (Fig. 10). Salvaged timber from the west approach which was replaced first on the same vertical alignment was used to shim the track on the east approach.

15 Original Timber Trestle, Looking East to West, w/morehead City Port in Background Fig. 1

16 Sample Pile Deterioration, Original Timber Trestle Fig. 2

17 Concrete Replacement Trestle As Seen From Adjacent US Hwy. 70 Bridge Fig. 3

18 End View of Prestressed T-beams During Construction (note holes in pile cap for beam anchors) Fig. 4

19 Precast Pile Caps Fabricated Upside Down w/ Protruding Rebar Cages For Pile Fixity Fig. 5

20 Top View of Precast Pile Cap Being Righted For Installation (note large holes for pile plugs, small holes for anchoring assemblies) Fig. 6

21 Pre-Construction Phase Composite Pile Grout Pumping Test Fig. 7

22 Easer Rail Lift Joint on Original Trestle (Field Side) Fig. 8

23 Lift Joint Fabricated for Replacement Trestle Fig. 9

24 Shimmed Timber Trestle (Foreground) and New Concrete Trestle (Background) Fig. 10

25 LIST OF FIGURES Fig. 1 Fig. 2 Fig. 3 Fig. 4 Fig. 5 Fig. 6 Fig. 7 Fig. 8 Fig. 9 Fig. 10 Original Timber Trestle, Looking East to West, w/morehead City Port in Background Sample Pile Deterioration, Original Timber Trestle Concrete Replacement Trestle As Seen From Adjacent US Hwy. 70 Bridge End View of Prestressed T-beams During Construction Precast Pile Caps Fabricated Upside Down w/ Protruding Rebar Cages For Pile Fixity Top View of Precast Pile Cap Being Righted For Installation Pre-Construction Phase Composite Pile Grout Pumping Test Easer Rail Lift Joint on Original Trestle (Field Side) Lift Joint Fabricated for Replacement Trestle Shimmed Timber Trestle (Foreground) and New Concrete Trestle (Background)