9.2.2 Civil Engineering Structures
Four basic types of structure of LRT Guideway can be generally considered: elevated, embankment surface and underground structures. Structures in Phase 1 of this are, however, all elevated structures. Regarding the design of structures, there are conditions specific to India as detailed in the following sections. One such significant condition is that seismic loads are not considered at all.
(1)Geology
First, we explain the geological conditions at the construction site. According to available data, hard rock (granite) occurs under the surface soil at a maximum depth of 20 m. Therefore it is planned to adopt a support pile system in which piles extend to the bedrock, which is more reliable than a friction pile system that relies on fiction between pile and soil.
One geographical feature of the Deccan plateau is that hills of exposed hard rock occur at some places.
New tunnels on the Konkan Railway west of Decan plateau site are all hard rock tunnels without concrete linings or other earth retention measures.
There is relatively little precipitation. The groundwater level is low and groundwater infiltration rarely occurs during cast-in-place pile excavation work.
For the purpose of foundation construction work therefore, the geological conditions are comparatively good.
The following design is based on such geological conditions. The detailed design will be carried out after a detailed geological survey has been conducted at the site.
(2) Supports (Appendix 6.1-1,2 )
The standard support structure is composed of single round T-shaped columns erected at the street center. Where single T-shaped columns cannot be used due to spatial or geometrical constraints, portal rigid fame type structure covering the streets will be used.
In principle, single columns type will be used in streets wider than 12 m and portal rigid frame type will be used in narrower streets. Depending on street conditions, however, portal rigid fame type may be used in place of single columns.
There are two types of foundation: the pile foundation (with precast piles or cast-in-place piles) and the direct foundation. This LRT project will use cast-in-place piles up to a maximum depth of 20 m for the entire route. The single column system will be composed of four cast-in-place piles with a diameter of 1.0 m at a footing of 4.5 m (length) x 4.5 m (width) x 1.5 m (height), and a portal rigid fame type system of four cast-in-place piles with a diameter of 0.8 m at a footing of 3.7 m x 3.7 m x 1.4 m at both ends.
Due to geographical conditions at the site, special T-shaped supports will be used at eight places.
(3) Girders (Appendix 6.1-1,2)
The basic structure is a continuous trapezoidal box girder composed of 25-m prestressed concrete (PC) beams, for two parallel single-track Lines. Beam length will be expanded to 30 m at six
