|January 17, 1948||Railway Age||Vol. 124 No. 3|
This view, looking north, or downstream, at the bridge across Twelve-Mile river, was taken after the piers had been constructed but before the spans had been shifted.
Combination of steel piles and Prepakt concrete "streamlines" the reconstruction of important deck plate-girder structure on existing alinement [sic] without falsework or traffic delays
By Charles P. Disney—M.E.I.C.
Consulting Engineer, Toronto, Ont.
In the recent reconstruction of a double-track deck plate-girder main-line bridge the Canadian National adopted a highly unusual design for the substructure, which proved to have important advantages. In this design each pier consists essentially of steel piles grouped in accordance with design requirements and extending nearly the full height of the pier. By integral bonding of the steel piles within Prepakt concrete a monolithic pier shaft is developed which has such strength and lateral rigidity as to permit of a relatively slender pier of uniform section. By using this design for the piers the construction time and the cost of the work were substantially less than would have been the case had conventional methods been used.
Adding further to the interest attached to this project is the fact than an important additional saving in both cost and time was realized by reconstructing the bridge on its original alinement, with the substructure work being done entirely beneath the track level and without interfering with train movements over the bridge. In this connection the pier design chosen was the essential factor in expediting the work and enabling it to be executed on the existing alinement The superstructure work involved only the shifting of the existing spans longitudinally into position on the new substructures, with the addition of only one short new span to increase the length.
The structure involved in this project is located on the double-track main-tine of the Canadian National between Hamilton, Ont., and Niagara Falls, where it crosses the 100 ft. deep valley of Twelve-Mile river which serves as the tailrace for a large hydro-electric power plant near St. Catharines, Ont. The old bridge was composed of six spans, each consisting of four lines of deck plate girders (two under each track), having a total length of 416 ft. Three of the spans were 100 ft long, two were tower spans 25 ft. in length, and the sixth was a span 65 ft. long. The substructure consisted of masonry abutments, two steel towers and a steel bent, all carried on masonry pedestals.
The river which is spanned by this bridge serves as the tailrace for the $7,700,000 DeCew hydro-electric generating plant where power is developed under a head of 254 ft. The output of 70,000 hp. of electrical energy was found insufficient to meet present industrial requirements and a new unit was therefore added to double the power output. This involved the removal of 3,000,000 cu. yd. of material to widen, straighten and deepen the river to provide for three times as much tailrace capacity, with the resultant imperative requirement for a new bridge of greater length and much deeper foundations.
To meet the new conditions at the bridge site without interrupting traffic, conventional methods would have necessitated the building of an entirely new structure either upstream or downstream of the existing bridge, which would also have involved the relocation of the approach tracks and their high embankments. This procedure would not only have been extremely costly but would also have seriously delayed the urgently required output of additional hydro-electric power.
A study of the unusual constructional and economic problems involved in this project led to the conclusion that a pier design, primarily originated and employed by the writer to expedite and simplify the construction of bridges, particularly those involving difficult foundation and underwater problems, was even more advantageous in this particular instance as it not only provided the constructional advantages in cost and time inherent in the design but also eliminated the necessity for track realinement and the reconstruction of the bridge on a new site, as well as obviating interruption of traffic.
The existing superstructure, which was originally erected and put into service in 1902, had been well maintained and was fully satisfactory in condition and load-carrying capacity to meet the present-day requirements of high-speed heavy traffic on the line in question. Consequently, it was economically desirable that the steel superstructure be continued in service, and the design adopted was therefore predicated on the continued employment of the existing spans. Specifically the re-use of the three 100-ft. spans and the 65-ft span gave the new bridge its required total length of 430 ft. 8 in. between the faces of backwalls, with the addition only of a new 57-ft. span.
The substructural layout was planned in such a way that it was possible to provide for the required increase in the total length of the bridge by relocating only one of the abutments, the other being retained unchanged for the new bridge. A new west abutment was built and four intermediate piers were constructed, designated from east to west as Piers 1 to 4, so spaced as to be constructed closely adjacent to the existing substructures.
The essential feature of the scheme adopted for the reconstruction of this bridge lay in the design of the new piers, which permitted them to be built to their final elevation from underneath the bridge without using the deck for construction purposes and in close proximity to the old piers without endangering the existing structure even though founded at depths much below the old footings. This type of pier consists essentially of H-section steel piles, grouped in accordance with design requirements, and monolithically encased within a pier shaft of Prepakt concrete, making the whole into a relatively slender column of uniform section having great bearing capacity. Briefly Prepakt concrete is formed by first filling the forms with compacted coarse aggregate and then consolidating the whole mass with a special cement-sand-water-admix grout which is forced to fill all the voids in the aggregate mass under pressure by pumped intrusion.
While it is not within the purpose or scope of this article to discuss the qualities or technique of Prepakt it is considered necessary, in order to ensure a full understanding of this important pier design, to mention at least briefly some of the characteristics of this kind of concrete for the benefit of those who may not already' be familiar with it. In the course of some ten years of experience in the uses and applications of Prepakt in connection with a great many important bridge jobs, the author has found that this type of concrete has definite advantages as compared with ordinary concrete, such as substantially higher strength, particularly at later ages, assuming equal cement content; greater resistance to freezing and thawing; greatly-reduced drying shrinkage; absolute bond; greater resistance to cracking; and a higher degree of impermeability.
The steel H-piles in place for Pier 3. Note their proximity to the pedestals of existing steel tower
Pier 3, having been built in "reverse", before the concrete for the lower portion of the shaft and the footing was placed
This drawing shows essential details of the Twelve-Mile river bridge before and after it was reconstructed.
In addition to these and other advantageous factors its method of production lends itself particularly favorably to the type of pier construction used on this job. due to the fact that Prepakt concrete can be as readily made under water as in the air. It is my opinion that the fundamental fact that one cubic yard of Prepakt concrete actually contains one cubic yard of coarse aggregate is an advantageous factor which favorably influences all its properties as compared with ordinary concrete.
The two highest of the four piers (2 and 3) of the reconstructed bridge are both situated within the confines of the enlarged river as required by the expansion of the hydro-electric plant. They are of the same height, each being approximately 640 ft. high. Measured from the new water level to the top of the bridge seat. The location of Pier No. 2 actually positioned it in the extremely fast tailrace water which existed in the original more confined course of the river; and Pier No. 3 is located directly adjacent to one of the old steel towers at a point where the existing ground was to be excavated to a considerable depth in deepening the channel of the river. In fact. at this location the new channel is considerably below the level of the footings of the masonry pedestals supporting the old steel tower.
While construction of both of these piers as located would have been out of the question for both structural and economic reasons by any other method except as designed, the type of pier adopted and the construction procedure employed enabled the work to be carried out expeditiously and relatively simply as will be described later.
Piers 2 and 3 are identical in design. Each of them embodies thirty-six 12-in. 65-lb. steel H-piles, arranged in three rows of 12 each. These piles were driven to rock and extended upward to the cut-off elevation which was approximately 6 ft. below the finished bridge seat. That portion of each pier extending through the water and into firm material below the channel bed was enclosed in steel sheet-piling and filled with Prepaid concrete. From the top of the sheet-piling to the cut-off level of the steel piles, these piles were encased in Prepakt concrete in such a manner as to form a slender shaft of uniform section 9 ft. wide and .40 ft. 6 in. long, with rounded ends constructed on a 4-f1. 6-in. radius. Each shaft is topped by a heavily-reinforced cap, also of Prepakt concrete, which forms the bridge seat.
In constructing Pier No.2, which, as already indicated, was located within the fast water of the existing river, the work involved six separate steps. The first of these consisted of driving steel sheet-piling in conformity with the designed perimeter of the underwater portion of the pier. This steel sheet-piling pier-base "form" was 11 ft. wide and 41½ ft. long and was driven about 15ft. below the final grade line of the new channel. The sheet-piling was driven by a Northwest crawler crane equipped with swinging leads and a steam hammer. The second step involved the excavation of river-bed material from within the steel sheet-piling by means of an orange-peel bucket operated by the crawler crane.
The third step in the construction of the pier was the driving of the H-piles by means of the crane and steam hammer. This work was done from underneath the bridge and the piles were spliced as necessary until driven to refusal with their upper ends above the normal water level. The average penetration below water level was 70 ft. In the fourth step additional lengths of H-piles were spliced by welding to those already driven, these lengths being such as to bring the piles up to the cut-off level. The piling forming the shaft of the pier was braced at intervals by welded steel angles.
In the fifth operation the steel sheet-piling pier-base "form", or cofferdam, was filled with coarse aggregate, thus expelling about 60 per cent of the water without pumping, assuming the compacted coarse aggregate to have a normal void content- of approximately 40 per cent. The aggregate mass was then intruded with the special cement-sand-admix grout which, on account of its inherent immiscibility, forced the remaining water out as it rose within the "form", thus forming Prepakt concrete. Step number six was to construct a conventional form around the steel H-piles up to the bridge-seat elevation, place the reinforcing rods for the pier-cap, fill the whole with coarse aggregate and intrude the mass to complete the Prepakt shaft of the pier.
An unique feature in connection with the substructural work is the fact that Pier 3 had to be constructed in "reverse" as compared with usual practice. In other words the procedure in constructing this pier was to place the concrete for the upper portion of the pier shaft first and that for the lower portion, including the footing, last.
The procedure employed was dictated by the considerable depth to which this pier had to be carried below the existing ground level, which level was much higher than at Pier 2. and the further fact that the proximity of the footings for one of the existing steel towers would have imposed severe complications in carrying out the excavating work. Steel sheet-piling was first driven to a depth of about 15 ft. below the final grade line of the channel, after which the steel H-piles for the pier were driven to refusal, being spliced as necessary to bring them to the cut-off elevation.
Derricks at work on the superstructure of the bridge. Note that, awaiting removal of the adjacent steel tower and its footings, the concrete for the lower part of Pier 3 (center) has not yet been placed
When this had been done and the angle bracing applied, a conventional form was constructed around the steel H-piling above the existing ground level and the Prepakt for the pier was placed above that level.
Further work on Pier 3 was then held in abeyance until the superstructure had been landed on the new substructure, removing the load from the steel tower adjacent to this pier. When this had been done the excavation of the existing ground around the lower portions of the steel H-piles was carried down to the proposed new water level. As this work proceeded, steel bracing was applied to the piles to develop lateral rigidity at this stage. When the material inside the cofferdam had been excavated to the desired depth the concreting of the footing and shaft of the pier was completed by bringing the concrete up from the foundation to meet that which had been previously placed in the upper half of the pier.
As steel H-piles were not readily available for Piers 1 and 4 which, being located high up on the side slopes of the channel, are only about 31 ft. high above the ground, it was decided to use steel tubular piles, which were on hand, encasing them above the ground line with Prepakt concrete. These two piers are identical and in each of them there were placed 26 steel tubular piles, arranged in two rows of 13 each. All of these piles are 14 in. in diameter except the two piles on the center line of each pier which are of the 12-in. size. These piers are 37 ft. 6 in. long and 6 ft. wide and have semi-circular ends constructed on radii of 3 ft. Like Piers 2 and 3 they are topped by heavily-reinforced concrete caps 6 ft. in depth.
The tubular piles were also driven by the crawler-mounted crane operating under the bridge, and were driven in sections which were welded to each other to obtain piles of the necessary length, about 80 ft. The procedure involved in constructing these piers was similar to that employed for Piers 2 and 3 except that it was not necessary to drive steel sheet-piling for the footings.
As stated the superstructure work consisted of shifting the existing 100-ft. spans and the 65-ft. span into position on the new substructures and installing the new 57-ft. span. In this work the superstructure for each track was handled by bridge derricks operating from the other track. Only about six actual working days would have been required to complete the superstructural work for each track had not a strike of derrick runners held up the work seriously.
Employing the methods described in this article the work was accomplished at a cost of approximately $350,000, which represented a remarkable saving as compared with the estimated cost of $1,300,000 which would have been required if a new bridge had been constructed on a different alinement. Furthermore, a very important aspect of the work was the time saved. Only eight months were required to do the substructure work, whereas it is estimated that 2½, years would otherwise have been needed. The relatively short time in which the job was completed permitted the enlarged hydro-electric power development to be completed and put into operation about two years earlier than would have been the case otherwise.
The quantities of materials used on this job included 2,932 cu. yd. of concrete, 27,323 lb. of reinforcing steel, 157,888 lb. of structural steel, 608,400 lb. of steel H-piling, 4,160 lin. ft. of steel tubular piles, and 112,630 lb. of steel sheet-piling.
The type of pier used was devised by the writer who also originated the scheme of operation and the bridge design for its application as the solution to the unusual circumstances and problems encountered. The work was carried out under the general supervision of E. R. Lgoie [sic], chief engineer, Central region, Canadian National, and under the direct supervision of K. Huffman, construction engineer of the Central region. The substructure work was performed under contract by C. A. Pitts & Co. in conjunction with the Prepakt Concrete Company, while all work involving the superstructure was carried out by the Hamilton Bridge Company. As the existing bridge was quite ample and satisfactory for the railway's present and future needs, the project was wholly necessitated by the DeCew power development, and the work was therefore executed at the expense of the Hydro-Electric Power Commission of Ontario.