Applying a two-tiered reinforced soil system for a rail overbridge
LOCATION
Darah-Jhalawar-Teendhar, NH-52, India
Client
National Highway Authority of India (NHAI)
Product used
StrataGrid™ (uniaxial geogrids) and 1S Type StrataBlock™
Application
Approaches to a railway overbridge (ROB)
About the project
The client specified the need for a rapid solution for approaches to a rail overbridge (ROB). The existing railway track is laid on an embankment that is 9m-10m above the natural ground level. Hence, the construction of a bridge to cross these tracks required approaches of a maximum height of 19m-20m.
While the bearing capacity requirements for such structures would be high, fortunately, this was not an issue since soft rock was available at a depth of 1m-2m. The daunting task was to analyse and design the ≈20m high reinforced soil walls for the approaches. IRC guidelines recommend that the full height of a reinforced soil structure should not exceed 15m. Strata proposed a design that considered a two-tiered reinforced soil system to cover the required height of ≈20m. The bottom tier, beyond the approach height of 15m, is maintained at 12.20m. There are berms on either side at the top of the bottom tier, each of 1.5m width. The balance height is made up by the upper tier reinforced soil system atop the first tier, above the berms on either side.
The top tier has been embedded into the lower tier appropriately. The berms have also been suitably sloped outward and with a flexible but impervious topping to prevent the ingress of rainwater into the lower tier through the berm surface. The berm is also required along the closing cross-wall of the lower tier. This increased the span between the reinforced soil system of the upper tier and the first pier of the main bridge structure, which would necessitate a longer span of the approach slab and heavier bearing requirements. To reduce the span of the approach slab, the first set of bridge piers was taken through the cross-wall berm. The reinforcement geogrids for the lower-tier cross-wall were appropriately located, and the cross-wall was designed with the adjusted geogrids.
Availability of appropriate non-plastic soil in adequate quantities for both approaches was a major problem. Non-plastic material is an essential component of any reinforced soil system. In several cases earlier, Strata had successfully used pond ash (a non-plastic material deemed as a waste from coal-fired thermal power plants) as the non-plastic component. Hence, while non-plastic soil was used for one approach, pond ash was used for the other.
CONSULTATION From know-how to manufacture, installation |
