Geofabrics
The Development of a Geocomposite to Prevent Mud Pumping
Abstract
In 2010 GEOfabrics launched a new anti pumping geocomposite in the UK, which could be placed directly on the sub-grade or on a failed sub-ballast and prevent the development of mud pumping.
Subgrade erosion (mud pumping) is a serious problem in the UK. The upwards migration of fine soil from the subgrade into the ballast can significantly reduce the life of the ballast, typically from an expected 1000MGT sometime to less than 100MGT.
Although the percentage of track renewal sites affected by mud pumping annually is small, typically less than 10%, the renewal costs are proportionally much higher, as they require deep excavation and installation of additional granular layers. The implications of not treating the source of the problems are a potential risk to safety of the line; trackbed failure may occur sometimes a little as 1 year after reballasting.
While the use of an anti-pumping geocomposite does not offer significant savings on material costs, it has been found that the simplified logistics and reduction in construction time typically allow trackbed installation time to be reduced by up to 50%. Alternatively, the length of track that could be treated in a given track possession could be doubled.
This paper summarises the main causes of subgrade erosion. It gives a historical perspective on the identification and treatment of mud pumping problems in the UK. It then outlines the rigorous product development/test program undertaken by GEOfabrics over several years using a full scale track bed load test facility simulating the harshest conditions encountered under a main line railroad. The paper concludes with a case history summarizing performance on one of the first commercial installations.
Introduction
In the UK the traditionally solution to a severe mud pumping problem would be to place a layer of well graded sand, typically up to 200mm thick, directly on the subgrade before placing new ballast. This was expensive and time consuming, but it provided a permanent solution to the problem.
When geosynthetics were first introduced some 40 years ago it was initially hoped that these could replace the thick layer of sand, but none were found that could successfully prevent the passage of clay particles and at the same time survive in the abrasive environment encountered at the base of a ballast layer. The geotextiles available at the time did however act as a separator between sand and ballast, which allowed the thickness of sand to be considerably reduced. For over 20 years this remained the standard solution for a subgrade pumping problem.
