Background

Ballast sinking is a commonly occurring problem in railway track structures founded on wet fine-grained soils like clays and silts, especially where an adequate and functioning blanket layer is absent. This necessitates frequent and costly maintenance resulting in sub-optimal utilization of track capacity. Large stretches of the tracks of Northeast Frontier Railways are prone to this problem. When Techfab India Industries Ltd put forward a proposal for track bed stabilization using gesynthetics as a lasting solution to this problem, Northeast Frontier Railways agreed to go for a trial installation on an experimental basis to evaluate the efficacy of this technology. Techfab India Industries Ltd. was assigned the task of assessing the site conditions, designing on appropriate solution, and executing the work.

The Site

A 100 m long stretch between km 323/0 to 323/2 between Jamguri and Oating stations, in Golaghat District of Assam, experiencing a severe and recurring ballast sinking problem was selected for the trial. Here the track alignment crosses paddy fields and the track structure is founded on embankment of approximately 3m height constructed largely of silty clays (Photo 1). Because of very high ballast sinking role, tamping and packing operations had to be undertaken very frequently.

The Problem

A combination of reasons could have contributed to the severe and recurring ballast sinking problem at this location - sub grade comprising fine-grained plastic soils (photo 2); excessive moisture content of the sub grade due to heavy rainfall in the area, ponding of water on the formation because of lack of proper grading and cross-fall of formation and capillary rise due to standing water in the paddy fields; and absence of a properly functioning blanket course. Because of the above three factors ballast sinking could have taken place both through lateral displacements due to the poor restraint offered by the soft and saturated sub grade and penetration of the ballast particles into the sub grade (photo 3). Packing of additional ballast provides relief only for a short time since none of the above factors are addressed and hence sinking would continue with time.

The Solution

The solution proposed comprised the following measures:

1. Building up/dressing the embankment to the correct level at the required locations and dressing the formation with a cross-fall of 1 in 30 to facilitate drainage.

2. Providing a nonwoven geotextile as a separator and fiber between the sub grade and the ballast:

The geo textile acts as a separator preventing the penetration of the ballast particles into the fine-grained sub grade. The geo textile also acts a filter which prevents the pumping of sub grade fines into the ballast. A needle-punched nonwoven geotextile with a mass per unit area of 250 g/m2 was used for this purpose. To protect the geotextile from puncture and abrasion by the ballast particles it was decided to sandwich the geotextile between two layers of sand, each 50 mm thick.