The APCO Chetak Expressway JV executed work on this package. Innovative planning, design and construction methods marked the construction of the viaduct of this project for reducing construction time by the selection of materials, technology and sequence of operations.
The construction scheme was evolved looking at the traffic congestion, safety, traffic management through the construction zone and safety of commuters and mitigation of environment parameters. Hence, highlights Sanjay Shrivastava, Vice President, APCO Chetak Expressway, it was decided that maximum off-site activities would be promoted, such as:
Precast pre-stressed I-girders (28.1-m x 1.9-m-high) in casting yard.
Precast pier caps in two parts 7.6 m each (cantilever length of 8.4 m). Section sizes were selected for ease of construction, handling and erection (mostly weight governed).
A module of continuous integrated span totaling to 90 m (3 x 30 m) has intermediate piers, are monolithic with the deck and the deck is made to rest on free bearing only on expansion joints. The foundation pile cap required to support the elevated viaduct structure of 7.3 m x 7.3 m, accommodating eight piles of 1,000 mm dia, at every 30 m centre to centre. The size of pier is 1.6 m x 6 m, with a pier cap of 22.8-m-long. The top deck slab of 25.65 m has a base of seven I-shaped PSC girders, stationed on the pier cap.
The viaduct part of the project involved construction of 1,097 precast pre-stressed I-girders of 1.9 m height, 310 precast post-stressed pier caps 7.6 m x 3.0 m, 1,148 piles (cast in-situ) of 1,000 mm x 24-m-deep, 157 pile caps (cast in-situ) 7.3 m x 7.3 m, 157 piers (cast in-situ) 1.6 m x 6 m, deck slab (cast in-situ) 156 nos x 30 m x 25.65 m, 120,042 sq m, 416 pot bearing and 104 guided bearing, single strip seal expansion joint – 53 nos.
The structural scheme developed mainly considered maximum off site activities, and assembling it in-place, says Shrivastava. “It was decided to construct girders in ‘precast casting yard’ of 12,000 sq m, having 12 beds, for three girders casting at a time at one bed. The expected production is 108 girders per month and staking capacity of 220 nos. It was arrived that in case of reduction of cycle time, the girder production can be increased to 180 nos per month by using early setting admixture or application of steam curing. Accordingly, four boilers (600 kg) were also commissioned for steam or hot water curing. The precast pier cap casting unit was also developed with 25 beds, for a production cycle of 25 nos per month. A steel cutting bending and binding automatic yard was conceived (50 m x 18 m), accommodating a shear line, a bend line, a straightening machine, three automatic stirrup benders, spiral, etc.”
The key construction activities and parameters that helped complete the project in a record time are:
Adds Shrivastava, “All the above mentioned activities are in a series, but by selecting two major activities as precast activities – I-girders and pier caps – they are operational on parallel basis. By these two precast activities the working width on the actual road reduced to 8.5 m in the road centre, with free flowing traffic on either side. The erection of precast I-girders and pier caps are being done with the help of gantries (tracks at 30 m centre to centre) instead of crane launching, thereby reducing the stoppage of traffic.”
When asked what is the USP of the innovation and what were the improvised construction techniques to obtain ‘the aim’, Shrivastava elaborates:
a. Pre-cast girders at casting yard three numbers at a time, resulting in excellent and precision quality factory like output of I-girders.
b. Using of two gantries in casting yard to achieve higher quality and efficient production output.
c. High grade RCC, M55 grade with micro silica and admixture, to obtain early strength of 45 N per sq mm, within four to five days, required for de-stressing the HT cables, and early de-moulding.
d. Use of steam boilers for gaining early strength is an improvement technique lifted from working in cold country techniques.
e. Completion of pier cap in seven stages of construction work is a new innovation in the field of road construction. The detailed stages are as: (1) Pre-casting of left pier cap segment (2) Pre-casting of right pier cap segment (3) Stitching and casting of flange porting over pier (4) Stitching and casting of web portion (5) Stressing of two cables of the pier cap after completion of casting, thereby removing of bottom supports (6) Stressing of next two cables of the pier cap after erection of seven I-girders (7) Stressing of the last two cables of the pier cap after completion of deck slab casting (8) Pre-stressing the pier cap and removal of bottom supports.
f. Erection of I-Girders (seven nos in one span) over the pier cap.
g. Use of deck sheet for deck slab casting.
The above methodology resulted in ease of construction at site by breaking down of activities and has largely resulted in decreased traffic congestion, increased safety, better traffic management through the construction zone, safety of commuters and better mitigation of environment parameters.
Most importantly, the pioneered technologies used in this project are easy to adopt and do not attract any upfront working capital. “This innovation will leave a lasting impression on every forthcoming project in the nation on state and national highways. It will be an extraordinary practice, if adapted in road construction,” prides Shrivastava.
- SERAPHINA D’SOUZA