Geofabrics
Synthetic Replacement for Sand-Blankets to Prevent Pumping Failure
Abstract
This paper describes the development process undertaken to produce a lightweight, geosynthetic filter to replace a sand-blanket used within trackbed as a method of preventing and solving the subgrade erosion problem known as ‘pumping’. The issue of ‘pumping failure’ is examined, with an explanation of the cause and effect, as well as the various methods that have been tried as a means of tackling the problem.
The development program is then explained, including the design and construction of a full scale trackbed test facility that simulates real conditions in the harshest of environments. There is some discussion relating to the project deliverables and methodology used to test and prove the functionality of proposed materials. The materials selection process is discussed, with some explanation of the way in which each component within the geocomposite was selected and evaluated.
The movement from laboratory trials to full scale use is explained, with some evaluation of the field use and the scope for further development.
Introduction
Geosynthetics have been employed to perform a number of functions in track construction and rehabilitation for almost half a century with varying degrees of success. When properly specified and installed, the use of geosynthetics has been proven to significantly enhance the performance of the trackbed in a number of ways, often reducing maintenance costs and increasing the lifetime of the design.
There are four principle functions (see fig.1) that geosynthetics fulfil when they are used within and beneath and around ballast and sub-ballast layers: separation, filtration, drainage and reinforcement/stabilisation (Pimentel, Bathurst, & Palmeira). The functions of separation and filtration are often considered as singular and for the purposes of this paper we will focus on this.
The introduction of geosynthetics for use below track precipitated as an indirect consequence of a transformation in maintenance techniques from a labour intensive technique to the employment of automated mechanical equipment and the requirement for the installation of deep ballast layers. Faults in track geometry were previously corrected by the manual removal of ballast which did not necessitate the disturbance of deep, well consolidated lower ballast layers. Over time, lower ballast layers would break down considerably, retaining granular characteristics and enabling natural filtration between the ballast and the clay subgrade. The use of automated equipment and this installation of deep ballast layers resulted in the removal of this well graded granular layer and a consequential lack of effective filtration.
The removal of this well graded subgrade precipitated rapid subgrade erosion pumping failure (EPF), this being the migration of slurried clay into the overlaying ballast. This migration occurs when cyclic loading on ballast in contact with a clay subgrade which is abraded and when mixed with water is pumped upwards. This results in a failure in ballast performance and track modulus and consequential reduction in bearing capacity. Figure 2 shows a severely failed subgrade.
The way in which this issue was tackled in the UK was to apply a layer of compacted sand of a thickness of between 2-300mm. An assortment of sands and gravels were employed, with varying success, until ultimately a grading envelope was established with the correct particle size distribution necessary to prevent the upward migration of fines. (RT/CE/S/03, 1998).
The commercial availability of geotextiles in the mid 1970’s presented the promise of replacing the granular layer as an effective means of preventing erosion pumping failure. However, it quickly became evident that there was no geotextile that could effectively function as a slurry filter. Testing on a number of geotextiles by the British Rail Board (BRB) soil mechanics section found that slurried London clay could pass easily through available fabrics (Ayres, 1986).
The installation of a sand blanket having the correct proportions of fines was proven to either treat an existing slurry problem or prevent one from developing. It achieves this by filling in all of the depressions in the surface of the excavated formation, i.e. it conforms to the shape of the ground (see fig.3). This was recognised as being fundamentally important because it had been identified that the development of a slurry can be triggered in small voids which become filled with water, causing localised softening and the subsequent formation of slurry, which is then pumped out under pressure of the passing of a train.
Although a compacted sand layer established itself as a very effective as a means of preventing slurry migration, it was not, conversely, popular with renewals engineers. The use of a sand blanket necessitated the need for large volumes of excavation which required additional trains to remove spoil and deliver new materials. The process is very time consuming and consequentially very expensive. It was therefore desirable to find an effective replacement.
