Phase-I: after 2nd flooding (with sand blanketing)
The first Phase of Experiment-2 was conducted with a 150mm sand-blanket (Network Rail standard, RT/CE/S/033) overlying the subgrade soil, in order to investigate the influence of sand blanketing on track behaviour during and after flooding. In this Phase, subgrade soil was not modified except for making the surface layer level. It was then allowed for two weeks for air-drying. The track was flooded for second time for a week. After the draining stage completed, the tank was placed immediately under the LOS to investigate the influence of sand blanketing on track performance. The drainage period
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was one week; it was ensured that no water coming out from the drainage holes. At this Phase, only 1400 cycles were applied at 2Hz. The test was repeated after two (2000cycles), four (10,000cycles) and six (10,000cycles) weeks.
Phase-II: after 3rd flooding (with sand blanketing and water inside the tank)
The test was conducted with a 150mm sand blanket (Network Rail standard, RT/CE/S/033) and with water inside the tank. It was found in Phase-I that the sand blanketing protected the subgrade from subgrade erosion, slurry formation and ballast movement into the soil. However, the sand blanket caused additional problems such as water becoming entrapped between ballast and the sand blanket layer, as well as sand migration etc. The track was marked rectangular to investigate the ballast movement during cyclic loading under water. After one week of flooding, the track was placed under LOS without any water being drained away. The track submerged rapidly after only 1500 cycles. The initial loading frequency was 2Hz, which was reduced to 1Hz. The test was repeated after four weeks (at 2,000 cycles) and six (at 10,000 cycles) weeks.
Phase-III: New surface layer (dry)
The test was conducted with a new surface layer considered as a dry phase to compare the track performance with the initial dry Phase. The surface layer became saturated due to repeated flooding; therefore, it was decided to replace the surface layer. In addition, the bottom layer of the tank also became saturated water passed through the layer via the side wall of the tank. The problems due to a raised of water table was studied in this Phase. After removing the ballast, the surface layer (100mm) was also removed and replaced with a new soil layer. The soil was mixed with approximately 12% moisture content in a big mixture machine. The reason for mixing in low moisture was as the remaining subgrades moisture content was considerably higher. When the soil was placed on top of the subgrade it would be easy to compact. The soil was placed in four layers and each layer was compacted by both manually and an electric compactor. The tank was then placed under the LOS to compact the new surface layer. The compacting procedure under the LOS followed the same procedure as discussed in section 3.4.2. After compaction, the track was allowed to air dry for 8 weeks, as the test was required to run in dry conditions. Before placing ballast, soil samples were collected to measure the soil properties (moisture content, void ratio and suction). The moisture content was 12% and matric suction was approximately 700kPa. The applied cycle was 2.3×105 at 3Hz. The behaviour of the track is discussed in section 5.4.
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3.5 Summary
In this chapter, the experimental techniques and procedures have been presented. The cyclic load was applied in the GRAFT in unsaturated and saturated conditions and the subgrade soil behaviour was investigated separately. Soil suction was measured by the filter paper method and WRC determined by both filter paper and pressure plate techniques. Double and single point methods were used to investigate the collapse behaviour of subgrade soil. Subgrade stiffness was determined by the PLT and the relationship between suction and stiffness was studied. A range of tests of ballast characteristics tests was conducted including a particle size distribution test and the large shear box test. The main objective of the experimental programme was to obtain reliable and realistic data that can be used in track design and maintenance recommendations. Table 3.4 presents a summary of all the performed experiments and conditions of the test in this research. The following chapters explain the results from the experimental programme that was designed to fulfil the stated objectives of this research.
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Table 3.5 List of all experiments performed in this experiment
Tests Condition of test Performed test Applied cycles
Experiment-1 LDSB, PSD and PP
Phase-I Dry: Initial PLT, WC, FP, OM and
GRAFT
500,000
Phase-II Wet: 1st flooding WC, FP and GRAFT 230,000
Phase-III Dry: Recovery period
WC, FP, GRAFT, OM and PLT
50,000
Experiment-2 LDSB, PSD and PP
Phase-I Wet: 2nd flooding (with sand blanket)
PLT, WC, FP, and GRAFT
13,400
Phase-II Wet:3rd flooding (with water inside and sand blanket)
PLT, WC, FP, OM and GRAFT
11,500
Phase-III Dry: New surface layer
PLT, WC, FP and GRAFT
300,000
LDSB = Large Direct Shear Box, PSD = Particle Size Distribution, WC = Water Content, PLT = Plate load test, FP = Filter Paper, PP = Pressure Plate and OM = Oedometer test
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CHAPTER FOUR IMPACT OF FLOODING ON TRACK
PERFORMANCE
4.1 General overview
This chapter presents details of an experimental study of railway track behaviour both before and after immediately flooding and subsequently, after a period of drying. The railway structures are constructed on compacted soils (unsaturated soil). The strength and stiffness of unsaturated soil are greatly influenced by soil suction; therefore, design and construction measures should be implemented to maintain the unsaturated conditions throughout the service life (Siekmeier, 2011). It should be emphasised that the saturated condition is the critical situation that reduces long-term performance. Unsaturated soil behaviour being highly dependent on soil suction and so varying with the changes of water content (Fredlund and Rahardjo, 1993). In unsaturated soil mechanics, matric suction is a key parameter which controls the state of stress (Fredlund and Morgenstern, 1977; Fredlund and Rahardjo, 1993; Houlsby, 1997; Gupta et al., 2007; Sawangsuriya et al., 2008; Ng and Xu, 2012). Unsaturated soil behaviour has been highlighted in the literature review. In this chapter, the subgrade behaviour is explained in terms of changes in moisture content associated with matric suction.
To understand the influence of flooding on track performance, this part of the experimental programme was divided into three phases.