橋梁尖兵 (A Soldier of the Bridges

A Study of PSM Construction Method (1/2)

A type of modern viaduct construction method proposed

for

The Taiwan High Speed Rail Project

ABSTRACT

This paper describes a type of modern viaduct construction method, the PSM (Precast Span Method), from the background of the Taiwan High Speed Rail Project, one Contract C215 of the Project which the writer is joining now, to the introduction of PSM construction method and its analysis and comparison with other construction methods, such as MSS (Movable Scaffolding System), on the same basis. A further comparison between two different proposed locations of precasting yard with their individual production lines in C215 will also be carried out. In the mean time a professional report of PSM from the original inventor, an Italian construction company, FERROCEMENTO – RECCHI, is introduced and its assessment is given by the writer. Finally the proposal comes to a significant conclusion, the PSM is the best selection for achieving the goals of high quality, shortest time, and lowest cost in construction of the long viaducts, according to the results from the analysis and comparison with others.

1. BACKGROUND AND INTRODUCTION

1.1  PROJECT BACKGROUND

Taiwan is bustling and vibrant with 23 million educated and energetic residents. Taiwan is an economic power in the Asia-Pacific Region, the world’s 15^th largest trading country, with 3^rd highest foreign exchange reserve. In order to continue supporting the island’s economy, the Government has embarked on an extensive US$ 300 billion infrastructure construction program that includes both mass transit networks for major cities and inter-city transportation systems. The High Speed Rail Project is an integral part of the Government’s overall transportation plan to address the growing demand in medium and long distance inter-city travel.

The Taiwan High Speed Rail Project is a key transportation component of the economic future of Taiwan. The Government of the Republic of China on Taiwan decided that the implementation of the Taiwan High Speed Rail Project would be on a Build Operate Transfer (BOT) basis. This represents a major departure from the traditional methods of contracting for infrastructure projects in Taiwan. The Project that cost US$15 billion approximately is the largest current private railway project in the world.

Investment proposals were invited from the private sector and two groups participated and submitted proposals. The Taiwan High Speed Rail Consortium was selected as the Best Applicant. The Government concluded the negotiation of the Construction & Operation Agreement (C&OA) with the Consortium to secure financing for design, construct, operate, maintain and ultimately transfer the complete system back to Government.

The Taiwan High Speed Rail Corporation (THSRC) was formed and registered in May 1998 and the C&OA was signed in July the same year. The concession calls for THSRC to build and operate the Taiwan High Speed Rail for a period of 35 years. It also grants THSRC the right to undertake property development around station areas for a period of 50 years. THSRC is currently in the process of implementing the Taiwan High Speed Rail Project, with its main focus on construction of the Project.

The Government of the Republic of China on Taiwan has and will continue to facilitate the development of the Project. The Government has taken the lead in advance planning of the High Speed Rail Project including enactment of the “Statute for Encouragement of Private Participation in Major Transportation Infrastructure Projects” and the integration of other transportation modes that will feed passengers to the Project. The Government has also taken the lead in land acquisition and in designing and constructing of underground portions of the Project in the Greater Taipei Metropolitan area.

The West Taiwan High Speed Rail Corridor runs approximately 345 Km from Taipei in the North to Kaohsiung in the South. The Project will incorporate proven High Speed Rail technology. General features of the Project include :

l        Two tracks (north bound and south bound) ; multiple tracks at stations.

l        A 350 km/hr design speed and maximum operating speeds of 300km/hr.

l        Trains operating at minimum headways of 4 minutes in the peaks.

l        Double deck trainsets seating 1,100 to 1,400 people allowing 420,000 seats per day.

l        Eight intermediate stations in addition to the terminal stations in Taipei and Kaohsiung.

l        Three main depots and stabling yards.

l        Automatic train control.

l        Computerized reservation/ticketing.

l        Safety monitoring devices for earthquake, wind, rain, rockfalls, derailment, etc.

The Civil Works on the Taiwan High Speed Rail system primarily consists of route structure, which in turn consists of embankments, cuttings, bridges/viaducts, and bored tunnels/cut & cover tunnels. The total length of the civil works is approximately 330 kilometers, which does not include approximately 15 kilometers of underground track in the urban Taipei area. There will be roughly 247 kilometers of bridges/viaducts, 50 kilometers of tunnels/covered-cuts, and roughly 33 kilometers of embankments/cuttings. The distribution percentage of these engineering features is shown in the Figure 1 as follows.

Figure 1  The percentage of engineering classification in the THSR Project

Based on the condition of private railway project the lowest cost and the earliest completion are two priority considerations before construction. It will be a challenge how to get a better plan especially in the viaducts and bridges that make up nearly 75% of the route. This is also the real reason why the writer likes to study and research the viaduct superstructure construction methods on this Project.

The major Civil Works components will be procured through a number of separate fixed price lump sum design/build contracts. Eleven Civil Works contracts from C210 to C295 will be let for the construction of the guideway. Contracts are evaluated using "best value" (price and others factors such as time and quality) as the method of selection. An extensive pre-qualification process had been completed amongst local and international contractors. Civil Works Contracts have been awarded. The Key Plan of the Taiwan High Speed Rail Project and the section of Contract C215 are shown in Figure 2 on the next page.

1.2  INTRODUCTION OF THE CONTRACT C215

The Contract C215 includes design and construction of the section of the Taiwan High Speed Rail Civil Works between Chainage TK 28+080 and Chainage TK 68+540. The contract interfaces with, but excludes the construction of, Taoyuan Station, which is located between Chainages TK 42+064 and TK 42+506.

The Works includes the construction of approximately 29,000 meters of viaduct and bridge construction, approximately 6,300 meters of tunnel, which includes the Hukou Tunnel 4,290 meters long, cut and cover approach tunnels to the only underground Taoyuan station in the THSR project and a 1,100 meters at grade sections.

At the northern end the alignment passes through a heavily industrialized area and bisects a number of operating factories. At Taoyuan Station the alignment will be underground to allow for future development and the interfacing with the proposed railway connecting Taipei to CKS International Airport. The general topography of the area includes a number of lakes and ponds, including one that lies over the cut and cover approach tunnels to Taoyuan Station. The railway also crosses a small residential development that is to be retained. This requires the construction of foundations within the basement of the development while maintaining vehicular access to the basement for the residents at all times.

At the southern end of the Contract the alignment passes through a hilly section that includes a military camp and training area. The HSR alignment is in mined tunnel and coordination with Military Authorities is required for gaining access to land to be used as a works area for constructing an access adit to be used for constructing the southern part of this tunnel.

For the reason of avoiding verbose writing, the PSM construction method, which will be used in the viaduct section 2 of Contract C215, that the writer is now responsible for its construction monitoring, will only be discussed in this paper.

1.3  INTRODUCTION OF THE VIADUCT SECTION 2 OF CONTRACT C215

The viaduct section 2 of Contract C215 includes about 20 Km of viaducts and bridges with 120 meters earthwork (cut & fill) in between the chainages TK 44+775 and TK 64+114 from Jungli to Hukou. Following is a detailed structure layout table for the viaduct section 2 which including 17.5 Km of superstructure with PSM construction method.

Table 1  Detailed structure layout table for the viaduct section 2

Because the Project is on a BOT basis, therefore, the investors do not like to extend the 5 year duration time of the Project construction and to reduce the operation time of 30 years. In the mean time for escaping interface problems between design and construction the THSRC delivers the contract in Design/Build method. Hence the Contractor has the right to decide the types of structures and their construction method. PSM (Precast Span Method) is selected to be the most fast and the cheapest construction method for long distance viaduct superstructure under such tight contract constrain conditions.

1.4  PSM CONSTRUCTION METHOD

1.4.1 INTRODUCTION OF PSM CONSTRUCTION METHOD

PSM (Precast Span Method) is a type of modern viaduct construction method proposed for use on the Taiwan High Speed Rail Project.

PSM has adopted the solution to prefabricate single full span elements in purpose-made sheds, situated nearby the viaduct to be constructed, using special hydraulically operated formworks. This procedure allows for the placement of reinforcement-bars and prestressing tendons as well as the uninterrupted pouring of the concrete to take place in near-ideal and easily controlled conditions, without any subjection to weather conditions. (Refer to Picture 1. Prefabricating Yard on the next page)

The prestressing of the element is achieved by strengthening the strands before the pouring of the concrete and subsequently cutting them after the curing. Owing to its cyclical nature, this method also allows for increased productivity as the work progresses.

The curing of the concrete is ensured by controlled steam heating. Once the concrete has achieved the required strength, the hydraulically operated formwork is withdrawn and flat-jacks are utilized to lift the finished prefabricated element and place it on a specially designed transporter vehicle which has been positioned underneath the element. (Refer to Picture 2. Transporter vehicle on the next page)

During its transportation along the completed viaduct deck, the element is kept in a horizontal position by means of a hydraulic system able to absorb the transverse slope along the alignment.

     Picture 1.  Prefabricating Yard

     Picture 2.  Transporter Vehicle

     Picture 3. Transporter vehicle and Launching Gantry

The prefabricated element is then placed into position by a sophisticated launching equipment consisting of a steel truss with travelling gantry crane, which once the element has reached the launching position, hoists up the box girder, shifts it forward and lowers it into place on the viaduct piers. (Refer to Picture 3. Transporter vehicle and Launching Gantry on this page)

1.4.2 WHY IS THE PSM CONSTRUCTION METHOD SELECTED ?

Among the advantages encountered in the production of factory made structures, virtually free of the possible defects that could be found in structures constructed in open-air sites, this method also allows for the possibility of placing one full span per day and, most importantly, it ensures that viaducts be completed in a fraction of the traditional time. (Refer to Table 2. PSM construction cycle time schedule on the next page)

Taking into account that this method allows for span lengths ranging between 25 m and 45 m and an average production of 250 spans per year, 6 ~ 10 Km viaducts can be easily realized each year by means of only one launching equipment and transporter unit, making the method extremely competitive even in terms of unit cost per square meter of viaduct.

Finally, this method allows for all work operations to be carried out on the viaduct section, which has already been completed, thus eliminating interference with traffic below as well as any of the problems which typically arise when crossing rivers or valleys. In addition, the width of most ROW in viaduct section is only 18 m given by BOTHSR, which makes no sufficient space for construction on the ground.

Therefore, the PSM is selected in the construction of viaduct superstructure of the Taiwan High Speed Rail Project.

1.5   DEFINITIONS AND ABBREVIATIONS

1.5.1 PSM : Precast (Prefabricated) Span Method.

1.5.2 MSS : Movable Scaffolding System.

1.5.3 BOT : Build, Operate and Transfer.

1.5.4 C&OA : Construction and Operation Agreement between MOTC & THSRC.

1.5.5 MOTC : Ministry of Transportation and Communications.

1.5.6 BOTHSR : Bureau of Taiwan High Speed Rail, MOTC.

1.5.7 THSRC : Taiwan High Speed Rail Corporation.

2. LITERATURE REVIEW

A professional report that the topic is Prestressed Concrete “Speedy Giant” Prefabrication occurred in the Proceedings of International Bridge Conference of 14^th Annual Meeting held in Pittsburgh, Pennsylvania on June 1997 was selected to be reviewed and then the assessment will be given in this section. The report was presented by FERROCEMENTO – RECCHI, a very famous Italian Construction Company.

2.1  A SUMMARY OF THE REPORT

2.1.1 BACKGROUND

The Precast Span Method (PSM) was invented and first used about 25 years ago to build the deck of a 3 Km motorway viaduct with simply supported spans. Each pre-stressed box girder span, 32 m long, 13 m wide and weighing 5000 kN, was precasted, cured, transported and launched in 24 hours, using the hours of darkness for the steam curing phase.

Subsequently, this method was applied in the construction of the decks of a number of viaducts, totally 10 Km, on a section of the Rome – Florence high speed rail line, which were designed with 25 m simply supported spans. In this case, however, some modifications to the equipment were made in order to speed up transportation, because the viaducts were distributed along a 35 Km stretch.

2.1.2 THE POGGIO IBERNA VIADUCT

l         INTRODUCTION

The most advanced and recent application of the PSM is represented by the Poggio Iberna Viaduct, a big step ahead for this construction method. The transversal section of this viaduct and its cast-in-situ joint are shown in Figure 3. as follows.

Figure 3. Poggio Iberna viaduct – transversal section of one carriageway  and cast-in-situ joint.

This viaduct is a part of the Cecina – Leghom section of the Rome – Civitavecchia – Leghom turnpike. It crosses, in a seismic zone, the Poggio Iberna valley, a vast and environmentally important uncontaminated area of Mediterranean bushes and scrubs.

Apart from the usual requirements imposed on these kinds of works, such as construction cost-effectiveness, speed of execution, durability, tailoring the structure to the geomorphology of the surrounding area, respect for the environment and ease of inspection and maintenance. In this particular case the design specifications also included the requirement that the deck should consist of 300 M long structurally continuous stretches and should have a box-shaped cross section. The design was furthermore conditioned by innumerable obstacles such as the presence of a gas pipeline, an aqueduct and State Road #206.

Moreover, the alignment crosses the Traversa – Leghom provincial road, the Pisa – Cecina railroad twice, the Fine stream four times and finally it also crosses an old stone arch bridge that is an historic landmark.

l         DESCRIPTION OF THE VIADUCT

The viaduct consists of two separate carriage-ways each of which is 2,520 M long and 12.4 M wide. It presents two horizontal curves, having a radius of 900 and 1,000 M, with maximum transversal slope of 6.0%, and a maximum longitudinal gradient of 1.5%. The each span length is 42 M, and hence each carriageway consists of 60 spans.

Each span of the pre-stressed reinforced concrete deck consists of a single giant box girder that is 2.6 M high. The deck is structurally continuous for stretches of 6 to 8 spans, each of which is connected to the next by expansion joints. The end elements of each stretch are entirely prefabricated like the other elements and they are 41.9 M long and weigh 8,500 kN. The structural continuity is accomplished only after all the spans of the carriageway have been positioned. At the ends of the elements, where the walls and the slabs of the box girder are thicker to provide stiffness and to create a sort of diaphragm, there is also a passage way to allow inspection of the pier top as well as a cavity designed to house the anti-seismic device.

The prefabricated element is pre-stressed by 216 pre-tensioned strands, 0.6² diameter each, embedded in the concrete, of which 104 are straight and 112 are deflected at 14.5 M from both extremities. The resulting total pre-stressing force of each element is around 45,000 kN. The structural continuity is achieved through 22 post-tensioned cables of 12 strands each, resulting in a total pre-stressing force of around 55,000kN.

Another important aspect that characterizes this viaduct is the system developed to resist earthquakes. As mentioned above, the static scheme is a continuous beam resting on multisliding bearings connected with anti-seismic devices to the piers. These devices, which work like hinges installed in the center of each pier, allow slow longitudinal relative movements caused by variations in temperature, creep and shrinkage while they impede relative dynamic movements, such as those caused by earthquakes.

The upper pier cap is goblet-shaped, 3.5 M high and it is hollow for inspection purposes. The piers, whose maximum height is 24 M, have a hollow circular cross-section with an external diameter of 3.1 M and a thickness of 45CM. The foundations are mostly spread footings resting on 120 CM diameter piles which either “float” or rest on the bedrock.

In addition to the advantages provided by monolithic casting, such as a fast construction rate and a high standard of structural reliability, for all the viaducts described so far, the Poggio Iberna, apart from its dimensions and doubled weight, is also characterized by the structural continuity of the deck that is a determining factor for maintenance and user comfort.

l         CONSTRUCTION METHOD

The choice of the type and number of equipment and machinery units to be used in the construction of the foundations, piers and pier caps was conditioned by the production rate of the precast elements as demanded in the contract. The piers were built in 12 months with the aid of three sets of sliding formwork each having a mean production rate of 3 M per day. The pier caps were built with four sets of formwork designed to allow the positioning of the pre-assembled reinforcement cage and concrete casting in a single phase. Even though the inner shape of the pier cap is complex, the formwork has been designed to make its assembly and dismantling easy.

The technology adopted to build the bridge box girder consists in : prefabricating the spans as single giant elements, transporting and launching them in place on provisional supports, accomplishing the structural continuity of the stretch, followed by positioning of the elements on the final supports and connecting the anti-seismic devices. The elements are constructed in a purpose-built facility located on the site.

After positioning the pre-assembled reinforcement cage, the 340 M³ of concrete required are poured uninterruptedly for 8 hours. In order to produce prefabricated elements that are suited to the tortuous geometry of the motorway alignment, and in order for each to have the variable transversal slope required, the external part of the formwork consists of a multi-form steel mould, adjustable by computer control. Therefore, the cross section of the element can be made to rigidly rotate along its longitudinal axis.

When the concrete pouring, the steam treatment and the pre-tensioning strand-cutting phases are over, the internal form folds inwards in order to be drawn out longitudinally, so that the element is ready for the lifting phase. Transport occurs by means of a vehicle consisting of a train of 24 motorized trolleys which, after having moved in underneath the deck casting platform, loads the element by raising it off the formwork with hydraulic jacks.

On rails, the train then carries its load along the completed bridge deck. The prefabricated element is placed into position by sophisticated launching equipment which basically consists of a gantry and an underbridge which are capable of advancing themselves. This equipment rests on the piers on which the element is to be placed. The position of the underbridge ensures continuity with the rails along which the trolleys move. The element is thus brought under the gantry moving along the extrados of the underbridge, where it is hooked and lifted off the trolleys so that they are free to return to the casting area for the next cycle.

The underbridge can then move to the next span, hence leaving room for the element to be lowered into place. In about two hours the precast element is transported and launched on the piers and left resting on the provisional supports. These supports are removed later, when the structural continuity of a stretch has been achieved and the continuous deck now rests on the final multisliding bearings, while equilibrium is provided by the anti-seismic devices. Structural continuity is obtained by joining the adjacent ends of the spans by post-tensioning cables. This operation is performed once all the elements of a viaduct are in position.

However, it should be pointed out that the connection between one span and the next is achieved without recourse to supplementary castings, as the joint is obtained by pumping a layer of highly resistant mortar into the narrow space left in between. This kind of connection is nearly a “no-thickness joint”, so it can be said that the deck remains, even in its continuity, monolithic and entirely prefabricated.

2.1.3 ONGOING PROJECTS

Out of the ongoing projects using the PSM, mention can be made of :

l         The decks for the Seoul – Pusan section of the high speed railway line in Korea. The transversal section of this project and its cast-in-situ joint are shown in Figure 4. as follows.

Figure 4. High Speed Railway line in Korea – Transversal section and cast-in-situ joint.

They are designed with box girder elements that are 14.0 M wide, 2.4 M high, that weigh 6,000 kN and that are structurally continuous for two or three 25.0 M long spans. The elements are pre-stressed in a first phase with pre-tensioned strands and, in a second phase, with post-tensioned tendons to achieve structural continuity. This is ensured through spliced reinforcement cast-in-situ joints with variable thickness between 50 and 100 CM. As in the previously mentioned railway viaduct, the elements are transported with a faster machine on rubber wheels, while the launching equipment basically consists of a self-advancing gantry crane that lifts, moves and lowers the elements into position ;

l         Decks for the Italian high speed railway line under construction. They are designed with 33 M simply supported spans and box girders that are 13.6 M wide and 2.7 M high. The elements weigh 8,000 kN and are prestressed by pre-tensioned strands. The transport and launching equipment are the same as those described above ;

l         Alternative proposal for the deck of a dual carriageway toll road in Indonesia, which is 12 kM long. The longitudinal and transversal sections of this project are shown in Figure 5. on this page. This has been designed with 35 M simply supported spans having a continuous slab to avoid an expansion joint on each pier. Each element, weighing 7,000 kN, is 14.0 M wide and 2.1 M high. The substructures have been designed based on the almost simultaneous advancement of the twin precast elements.

Figure 5. Viaduct in Indonesia – Longitudinal and Transversal sections

2.1.4 CONCLUSIONS

Even though the PSM deals with giant prefabrication, it is very flexible and entire spans of different lengths can be produced using the same equipment. Very large box girders can also be designed as in the case of a motorway project in progress which envisages the use of a single precast element, weighing about 16,000 kN, which is 27.0 M wide and 50.0 M long.

As in all prefabrication-based construction processes, it is evident that site preparation and equipment costs are justified when a minimum of 80 ~ 100 elements, depending on the dimensions, are to be built. For larger numbers of elements, the PSM becomes a highly competitive construction method due to its durability, cost-effectiveness, and speed of construction.

2.2  ASSESSMENT OF THE QUALITY OF PRESENTATION

According to above description PSM is really a quick, reliable and economical solution with cost-effective and advanced concept in modern bridge/viaduct construction.

FERROCEMENTO – RECCHI is a very famous and successful Italian construction company who have, over many years, developed the viaduct superstructure execution with the Prefabricated Span Method, a specialized technique which uses very advanced technology for the execution of viaducts. Their continuous research and development have led to the realization of optimal structural solutions which have been adopted in the construction of long viaducts for expressways, high speed railways, and light rail transport. More particularly, Ferrocemento – Recchi was the first contractor to successfully achieve the difficult task of constructing an entire motorway span with a single prefabricated element. This method, referred to as the Precast Span Method (PSM), has been developed by Ferrocemento – Recchi using state of the art technology and readily adopted by the relevant Authorities in countries like Italy, France, Singapore and South Korea as the principal solution for the construction of viaduct superstructures for motorways and high speed railway lines.

Every civil engineer shall acknowledge that the lower cost and the shorter construction time are two very important factors in any successful project. The Construction Company can get his deserved profit if he will pay more attention to the research and development of construction method. Ferrocemento – Recchi performed this very well. (To Be Continued...)