Saturday, March 29, 2008

Alternative Design for Underpass

Alternative Design for Underpass
Recently BBMP has adopted precast box underpasses on Bellary road corridor for faster construction. BBMP planned to cast the precast panels elsewhere and get transported to underpass construction location site. The entire construction was delayed due to underground utilities noticed during excavation.

An alternative design for precast Underpass construction had been adopted for Bangalore Mysore Expressway. The Segmental precast arch type structure was constructed for the Underpasses and other drainage culverts. This is first of its kind in the country adopted in this project.
Technically both the designs are safe and stable. The main advantage of Precast arch shape is that all the overburden pressure is transformed to compression in its entire cross section, which can effectively taken by concrete with minimum reinforcement. This makes the Arch section more economical. In case of box section all the four sides are subjected to bending requiring considerable reinforcement.
Minimum lane configuration in urban areas require two lanes with footpaths on both sides. In the case of 4mx 4m
Box it requires two boxes kept side by side to meet the requirement. But by adopting precast arch ample of space will be available for the traffic lanes and footpaths. There will be adequate reserved space for side drains and Utilities.
Foundation base preparation and segmental erection can be done in a very short time. After erection of the segments the joints need to be sealed which is followed by back filling and pavement construction. The entire construction shall be 6 weeks.
As this structure is proposed to be erected on the ground it will not disturb the most of the existing utilities. However the location of underpass may be planned to avoid the services being fall under the foundation. The foundation is a very simple concrete pad with a key type groove to rest the arch segment at base as shown in photograph below. Added to this if any utilities to be shifted for safety reasons where several water mains and sewer lines are running it may further require some additional construction time. Even though OFC and telephone lines are easily manageable in parallel with ground work, it definitely slows down the construction work.
Traffic diversion plan must be meticulously implemented for smooth traffic flow during construction and should create room for the hindrance free construction activity.
The precast arch Underpass gives wider view of the road for the driver which helps greatly in safer driving where as the box underpass narrows down the visibility and a clustrophodic feeling.
The other advantages of this precast arch underpasses are the ease of transportation as these elements can be kept side by side and are lighter. These elements are assembled at site with a joint at top and finally stitched with a longitudinal beam

The advantages of arch type of construction

* Simple, Rapid, predictable construction without scaffolding or formwork.
* Most reliable precast arch system ideally suiting for Indian condition.
* As concrete is good in compression the basic shape of the the arch segment transfers all the overburden stresses into compression
* It creates ample space for mixed use of traffic as well as footpath for Pedestrians.
* All the services and utilities can be taken underground and repaired/ relaid without disturbing the structure.
*Arch shape is amenable for 2 lane road with
adequate clearance without erecting 2 boxes, side by side.
* It is cost effective.


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