Dr. Sanjeev Kumar Garg, F.I.E, L.M.I.R.C., I.R.S.E., Chief Project Manager, Delhi Metro Rail Corporation, New Delhi, India
Metro projects are capital intensive public oriented projects and their timely completion is of paramount importance both from economic and convenience to public aspects. Due to cost economics, elevated lines are the first choice for metro corridors, which from land availability point of view are generally planned along the major city roads. The construction of Metro puts constraints on traffic movement in the construction reach and therefore to reduce the construction time and space, choice of suitable construction methodology becomes more relevant and important in such conditions.
As metro projects are generally linear in nature, in-situ construction requires barricading of site by traffic diversion or restrictions over long stretches. Cast-in-situ methodology requires more time & space and hence adds inconvenience to public. From environmental consideration also, cast-in-situ methodology is not a preferred option. Therefore use of precast elements to the maximum extent becomes the natural choice for the elevated metro projects in cities.
Advantages of Precast Concrete: Precast concrete has distinct advantages over cast in situ construction. As the elements are already cast and cured requiring only installation, site activities are reduced to minimal, saving significant site activity time. Being a casting yard made product using same moulds in same environment, the elements are nearly uniform adding to aesthetics. Further, quality control and curing of elements is much better and easier in casting yard in comparison to cast-in-situ construction resulting in more durable end product. With proper planning, precast construction is more economical in overall scenario. In addition, as casting is done in yard where environment pollution control can be done more effectively and due to minimum site activities resulting in less noise and pollution at actual site of work, precast construction is more environmental friendly option. Further, this also results in more convenience to public during the construction phase.
Challenges in use of Precast Concrete in Metro Projects: However, for Metro Projects, precast construction has some associated challenges. Selection of size and location for casting yard is a critical activity for precast construction in metro projects. As heavy and large size members are to be dealt with and transported, casting yard should not be located far away from the corridor. Ideally, it should be near the middle of the project length. Besides that, it should have good and wide approach roads which are capable to ply the heavy transport vehicles carrying the members. Turning radius at turns is another factor to be considered as longer vehicles require larger turning radius. Loading of large precast elements weighing hundreds of tons requires gantries of matching capacity. Transportation of heavy precast elements from casting yard to the site requires special vehicles which can not only carry the required load but also negotiate the city roads, bridges, flyovers and turns etc safely. Due to high traffic in day, such transportation is normally possible only at night. Safe erection/ installation of precast elements at site is another challenge. High capacity precision movement hydraulic cranes, equipped with modern safety devices, are required to lift and install precast elements. After erection, the element has to be secured in place before detachment of cranes from the element. Higher accuracy of site work is required while using precast elements as any error beyond tolerance of element dimensions may require demolition or alteration in site work as modification of precast element is not possible.
Use of Precast Concrete in Delhi Metro: Delhi Metro has been the pioneer in metro construction in India and methodologies and technologies developed/adopted by DMRC are generally considered by other Metro Systems in the country also. Though DMRC has used precast elements for superstructures (segmental box girders, I girders etc) since inception, precast technology has been used in a big way for construction of viaduct in Phase-III of Delhi Metro Elevated Projects and in Phase-IV corridors now under construction are by and large being constructed using precast elements including stations also.
Viaduct: In viaduct, there are four main elements, viz, foundation, pier, pier cap and superstructure. Due to heavy supported loads, space availability and soil conditions, normally pile foundation are used in metro viaduct. Piles are bored cast-in-situ leaving no scope for pre-casting. Pier can also be cast over the footprint of pile cap thus requiring no extra space for construction. However, pier cap requires some extra and superstructure require significant extra space for construction and hence are fit elements for precast construction. In superstructure, application of precast elements has extensive application in bridges/viaducts.
Segmental Precast PSC Box: Segmental precast PSC box girder has been used in Delhi Metro since beginning. The segments of a girder need to be match-cast in yard and then transported to site. Due to smaller size of segments (in comparison to complete superstructure), it is easier to load and transport segments from casting yard to site using available trailers and cranes/ gantries. These are assembled using launching girders supported on piers and then the segments are pre-stressed together. No special arrangement is required for lifting at site as this work is performed using the launcher itself. The methodology of construction of segmental precast pre-stressed box girders is fairly simple & standardised and thus widely used in metro viaducts.
Precast I Girders for viaduct: Precast I girders is another choice for bridges and these too have also been used in superstructure extensively. Unlike segments of box girders, I girders are heavier and longer. Usually, a 25 m I girder weighs about 60 MT. Hence, these are either cast on ground at site and then erected by local shifting or if casted in yard, these require special transportation arrangement owing to the length and weight of the member. Their erection also requires special lifting beams or tandem working of cranes. These need to be secured soon after erection against movement and toppling to avoid accidental fall.
Precast U-Girders and Pier caps: With the availability of larger capacity cranes and special transport vehicles, it became possible to transport and erect heavier members. In phase-III of the Delhi Metro, U-girders along with precast pier cap have been extensively used which has resulted in faster construction and less obstruction at site and that too for short duration. A typical precast pier cap has (approx) footprint of 3x10m and weighs 72 T. Pier Caps are cast in yard - post-tensioned elements and transported to site on trailers. These are fixed over cast-in-situ piers with stitch concrete pour. However, use of precast pier caps requires tight construction tolerances for pier construction else cap may not fit properly on the pier head.
U-girders are precast, pre-tensioned- pre-stressed concrete members which have revolutionised the construction of metro viaducts. These have many advantages over other forms of superstructure, especially in rail structure interaction analysis as rail is placed near the neutral axis of the girder. These weigh about 5.5 T/m, 5 m wide and vary from 17 to 28 m in span. These form complete superstructure supporting one track and spanning from pier to pier thus requiring only erection. Uniform cross-section for all spans is not only aesthetically pleasing but also allows easy and uniform fixing of track. However, due to their size, weight and thin sections, these require careful handling in storage, transportation and erection.
Parapets of Viaduct: The parapets, besides being a safety feature of the bridge, provide aesthetics to the viaduct. Hence, it is important that parapet is of good surface finish and of uniform size for better aesthetic appearance of the viaduct. Further, these are lighter elements which can easily be lifted/ transported and fixed with girder using stitch pour. Hence, precast concrete becomes natural choice for parapets.
Precast concrete in Station Construction: Due to easy availability of heavy duty cranes and high capacity longer transport vehicles, it has been possible to construct the metro station floor plates also with precast elements. However, foundations, piers, staircases, lift wells etc are cast in situ. Figure-1 presents details of a typical station showing the extent of precast elements.
The top pier cap supporting track and platform is constructed in three parts. Middle part is fixed on pier with stitch pour. Then side parts are connected which are stitched together by pre-stressing. Similarly, concourse cross arm is also in three parts and fixed in similar way. Platform and concourse floor are constructed using p girders. Two adjoining p girders are joined by stitch concrete pour. This method of construction provides three distinct advantages, namely faster construction, better finish and least obstruction at site. After completion of floor plates; staircase, lift well and other partitions are provided in a conventional way.
Conclusion: Use of precast concrete has revolutionised the construction of metro projects. Use of precast elements is environment and user friendly. For successful implementation, use of precast elements require due consideration since planning stage along with incorporation of relevant provisions in tender. Choice of location and size of casting yard plays an important role in speed of construction. Use of precast technology requires relatively less construction space at site making it more suitable for construction along crowded city roads. The form and size of elements should be chosen considering the available construction facilities. To ensure safety during transportation and erection of precast elements, handling equipment of suitable capacity and good conditions should be used. DMRC has successfully used precast technology since inception and has increased its use over the course of time. Presently, most of the elements of both viaduct and stations are precast leading to much faster construction with better quality control.
Acknowledgement: Sh D K Saini/Director Project, Sh C.P. Singh/ CPM, Sh C S Mudgal/PM, Sh Viraj Gupta/DGM Design, Sh Ankit Mittal/DGM Design, Internal reports and colleagues.