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The test results confirmed a significant difference in track performance (in terms of settlement) before and after flooding. Track is not safe to be operational immediately after the drained water from the track, without an investigation of soil properties including moisture content, suction and the degree of saturation. In the event of flooding, the upper layer of the subgrade is most affected and sensitive to changes of water content and soil suction. However, over time, the entire subgrade can be affected if the water stays in the track for a long period. An effective drainage system is required to direct water away from the track as quickly as possible. The research suggests that, without checking the subgrade properties, the track should not be open for operation and train services should be halted.

For the periods of scheduled maintenance work, all the attention is given to ballast properties. However, subgrade properties should be checked as well, especially after an extreme event, for example rainfall sufficiently heavy to result in flooding. The results showed that if tamping of the track does not work, then it is a fault of the subgrade. Therefore, it cannot bring the track up to level due to permanent settlement; in addition, frequent tamping can damage ballast properties.

During the recovery period over the first two weeks, the track showed little improvement. The subgrade soil properties indicated that the subgrade soil had reached an equilibrium

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condition. Consequently, track settlement increased significantly, particularly after two weeks of the recovery period. However, four and six weeks later the track settlement decreased, but not significantly. The research indicates the wetting process is considerably faster than the drying process. In this situation, extra care was required to protect the track from further damage; for instance, load restriction can be applied.

The performance varied with changes of soil suction; higher suction values specify a stiffer track, whereas lower suction indicates poor performance. The settlement of the track is much higher in the wet condition due to the loss of soil suction. The results also confirmed that the subgrade properties did not change significantly, as the subgrade is covered by the ballast. However, the subgrade soil dried out quickly after removal of the ballast. Finally, this research suggests more attention should be given to subgrade behaviour; particularly checking subgrade properties to avoid further track damage and reduce maintenance costs.

CHAPTER FIVE INFLUENCE OF SANDBLANKETING ON

TRACK BEHAVIOUR DURING AND AFTER FLOODING

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5.1 Introduction

This chapter reports and discusses the influence of traditional sand blanketing on track behaviour. Traditionally, sand blanketing is one of the common techniques that has been used as a drainage material as well as to protect the subgrade from erosion. A sand blanket is generally a permeable layer of fine granular material which is placed as a drainage layer on subgrade soil to allow water to drain from the subgrade surface (Bonnet, 2005). The sand blanket also prevents ballast penetration into the subgrade soil, together with the pumping action of clay (Selig and Waters, 1994; Li et al., 2007b).

Progressive shear failure occurs due to over stressing on the clay subgrade soil; an event which can be avoided by placing granular material to enhance drainage. Wenty (2005) reported attrition results in the development of a slurry of the ballast-subgrade interface due to the presence of water and heavy dynamic loading. Overloading the subgrade creates water pockets which, as a result, cause attrition and can be avoided by placing a granular blanket. Sharp and Caddick (2006) reported that the sand blanket prevents upward movement of the slurry by filling the voids in the subgrade. If slurry is formed under the clay it is retained in the clay, hence it dries out in time, sand blanket therefore increases granular layer stiffness. The track becomes vulnerable in the wet condition; a situation which impacts on each component of the track, particularly the subgrade soil. To protect the subgrade soil, it is important to know that the soil’s behaviour, specifically its drainage system. Sand blanketing is a common and useful method of protecting subgrade soil but it cannot overcome all the water related problems. Sometimes, it creates additional problems to the track if the drainage system is not efficient.

Experiement-2 is divided into three phases which are described as below:

Phase-I (after flooding with a sand blanket): After removing all the ballast the soil samples were collected at different depths under the three sleepers to measure the soil properties. The aim of this phase was to investigate the influence of a sand blanket at saturated conditions. The moisture content of the subgrade surface was approximately 27% (see Figure 5.2a). The subgrade matric and total suction was between 150-570kPa respectively (see Figures 5.2c & d). A 150mm sand blanket was placed on the subgrade according to Network Rail standard RT/CE/S/033. The optimum moisture content of sand

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was 12.8%. The track was flooded for a week as in the first experiment. After this time, the water was then allowed to drain for a week, after which it was placed under the LOS. It was ensured that there was no water coming out from the drainage holes. The test was repeated after two, four and six weeks to record any improvement of the track. Track performance and behaviour are discussed in section 5.3.

Phase-II (with water inside in the GRAFT and a sand blanket): In this Phase, the flooded track was placed under load without the water being drained. After the first Phase was completed, the moisture content increased over the entire subgrade, particularly at the surface and bottom sections. The water passed through at the side wall of the tank. The surface moisture content was above 35%. The surface layer became fully saturated (degree of saturation = 100%). The matric suction was between approximately 50-80kPa at the depth between 200-400mm. The total suction varied between 100-300kPa (at the depth between 200-400mm). The test was repeated after four and six weeks. A discussion about the track performance during this phase is presented section 5.4.

Phase-III (Dry phase with a new surface layer): The surface layer of the track became fully saturated and the test was discontinued to allow further maintenance. The subgrade surface layer (100mm) was replaced by a new surface layer. This Phase investigated the track performance in dry condition. At the bottom depth, (500-700mm) the subgrade soil was almost saturated. The moisture content was approximately 28% and the degree of saturation was 98%. The track often experiences seepage problems in real life in the wet period or in wet conditions. The water can get into the track from the bottom layer of soil if the water table rises (Figure 1.1). The track behaviour of this phase is discussed in section 5.5.