Ballast sinking is a commonly occuring problem in railway track structures founded on wet fine-grained solis like clays and silts,especially where an adequate and functioning blancket layer is absent.This necessitates frequent and costly mainteinance 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 trackbad stabilization using geosynthetics as a lasting solution to this problem, NF 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 an appropriate solution,and executing the work
TECHGRID STABILIZATION USING TECHGRID BIAXIAL GEOGRID AND NONWOVEN GEOTEXTILE
A 100m long stretch between km 323/0 to 323/2 between jamguri and Oating stations,in Golaghat District of Assam,experiencing a servere 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 3 m height constructed largely of silty clays(Photo 1).Because of very high ballast sinking rate,tamping and packing operations had to be undertaken very frequently.
A combination of reasons could have contributed to the servere and recurring ballast sinking problem at this location-subgrade comprising fine-grained plastic soils (Photo 2);excessive moisture content of the subgrade 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 properly functioning blanket course
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
Providing a nonwoven geotextile as a separator and filter between the subgrade and the ballast:
The geotextile acts as a separator preventing the penetration of the ballast particles into the fine-grained subgrade.The geotextile also acts as a filter which prevents the pumping of subgrade 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.
Techgrid TGB-40,a knitted and PVC coated biaxial geogrid with a tensile strength of 40kN/m in both machine directions,aperature diemensions of 25x25 mm and roll width of 5 m,manufactured by Techfab India at its state-of-the-art plant in Silvassa, Union Territory Of Dadra & Nagar Haveli was proposed for this purpose.The geogrid is manufactured from superior grades of high tenacity, high molecular weight and low carboxyl and group polyester yarns which are formed into a grid structure using a highly sophisticated warp-knitting process and is then precision coated with a specially formulated PVC plastisol to produce a strong,flexible,tough,dimensionally stable and durable geogrid. A layer of Techgrid biaxial geogrids installed below the ballast constraints and confines the ballast particles reducing the lateral and vertical movement of ballast and enhances the strength and stiffness of the ballast.Reinforcement of the ballast results in a marked improvement of the track performance through several mechanisms: Substantial reduction in the lateral spreading of ballast and penetration of the ballast into the subgrade and associated track settlements.
Reduction in subgrade attrition and wear and tear of ballast
Reduction in vertical stresses on the subgrade because of better load distribution by the reinforced ballast with enhanced strength and stiffness.
Lower shear stresses on the subgrade with consequent increase in bearing capacity.
Placing cleaned ballast over the geogrid and restore the track to the desired geomatery.
The work was executed by Techfab India under the overall supervision of the permanent way Inspector.Because of the remoteness of the location and the small quantum of work,the work was carried out manually.The blocks allocated allowed completion of about 10 to 12 m of track each day.The rehabilitation work involved the following steps:
Removal of the ballast and preparation of the exposed subgrade subgrade by dressing to ensure a cross-fall of of 1 in 30 and ramming with wooden tampers(photo 4). Spreading of a thin layer of sand(about 50 mm thick)on the prepared subgrade to cover any ballast particle from projecting from the subgrade(photo 5). Laying of the geotextile over the sand layer without any folds or wrinkles(photo 6). Spreading of thin layer of sand(approximately 50 mm thick) over the geotextile(photo 7). Installing the geogrid over the second layer of sand(photo 8). Placing and compacting cleaned ballast over the geogrid(photo 9).
Approximately two years has passed since the completion of the rehabilitation.As per the feed back from NF Railways, any significant ballast sinking problem has not been observed in the locationtreated with geogrids and geotextiles and there is an appreciable improvement in the track performance.Taking note of the success of this trail,NF Railways rehabilitated another stretch using the same materials and technique with satisfactory results.An inspection of the site in the first week of June 2008 showed that the condition of the treated portion of the track is quite satisfactory.