Development of the Countermeasure against Roadbed Degradation under Ballastless Tracks for Existing Lines

Transcription

1 Challenge H: For an even safer and more secure railway Development of the Countermeasure against Roadbed Degradation under Ballastless Tracks for Existing Lines Katsumi MURAMOTO, Takahisa NAKAMURA Track Structures and Geotechnology Laboratory, Railway Technical Research Institute, Tokyo, Japan Recently, ballastless tracks with cement grouted into ballast have been constructed on a full-scale construction operation in Japan Metropolitan Area. If the ballastless tracks are laid on clayey roadbeds, the soils are likely to be fluidized and flow out by trainloads. Consequently, track degradations typified by a large track depression sometimes occur. In this study, the authors carried out the test with a full-scale ballastless track model laid on a saturated clayey roadbed. From the results of the test, it was confirmed that degradation of the ballastless track on a clayey roadbed is caused by local progressive failure; specifically, the outflow of the fine-grain fraction in roadbed surface. In addition, the authors confirmed that the BLITS (Bentonite Liner for Track-bed Surface) method is one of the reasonable countermeasures for the roadbed degradation under the ballastless tracks. Keywords: roadbed, ballastless track, roadbed degradation, mud pumping, bentonite 1. Introduction When ballastless tracks (Fig. 1) are constructed on sound roadbeds, the maintenance work and the maintenance costs become generally less than conventional ballast tracks. However, if the ballastless tracks are laid on clayey roadbeds, the soils are likely to be fluidized and flow out by the trainload. Consequently, track degradations typified by a large track depression sometimes occur. The authors, therefore, have carried out moving-load tests with small-scale ballastless track models on clayey roadbeds and reappearance tests of the roadbed degradation using small-scale cylindrical molds, so that the basic defect mechanism of clayey roadbeds has been clarified (Fig. 2). 1) Accordingly, three fundamental policies of countermeasures against clayey roadbed defects were clarified as follows 2) : 1) Decrease a dynamic water pressure on the roadbed surface, which is excited by trainload 2) Decrease roadbed pore water level 3) Increase roadbed soil cohesion The results of small-scale model tests, however, might not be applicable to actual conditions because the phenomenon involving pore water movement differs significantly with the effect of scale. Furthermore, the progressive failures involving roadbed soil outflow are hardly predictable using numerical simulation at 1

2 this time. In this study, we performed tests with full-scale track models to clarify the details of roadbed degradation and countermeasure effectiveness. Rail PC-Sleeper Cement Grouted Layer Ballast Ballast Penetration Ground Fig. 1 Ballastless Track for Existing Lines Side Ditch Train Load Outflow of mud Inflow of rain water Cave Formation Water Channel Fig. 2 Basic Defect Mechanism of Clayey Roadbeds 2. Simulation of the roadbed degradation process by full-scale model tests 2.1 Setup of full-scale clayey roadbed model Where a full-scale roadbed model is made with clayey soil, the roadbed is, generally, constructed by two methods; compaction with the soil, which is controlled by optimal water content; consolidation with slurry, which is controlled by higher water content than the liquid limit. Because roadbed saturation and loading history were more important in these tests, the models were made by consolidation. Fig. 3 shows an outline of the construction process of the full-scale roadbed model. The clay slurry was pumped into an earth tank (Fig. 4), which was then depressurized after the roadbed surface had been covered with a polyethylene film. The differential stress between the internal pressure and atmospheric pressure acts on the roadbed surface and squeezes pore water from the slurry. As a result, a saturated clayey roadbed with a controlled stress history is constructed, namely by the vacuum consolidation method. 2

3 Supernatant Water Polyethylene Film Air-Water Separation Layer (Plastic Pallet) Clay Slurry Sand Layer Filter (Nonwoven Cloth) 1) Cast in slurry and settle out 2) Set plastic pallet and polyethylene film to Vacuum Pump Atmosphere Pressure Vacuuming Atmosphere Pressure Ballastless Track Model Clay Slurry Water Trap Clayey Roadbed Drain Pipe Vacuuming 3) Vacuum and consolidate 4) Set track model Fig. 3 Making Procedures of Roadbed Table 1 Properties of Arakida clay Particle Density 2.712g/cm 3 Liquid Limit 49.4% Plastic Limit 27.7% Plasticity Index 21.7 Rate of Sand Content 2.7% Rate of Silt Content 50.6% Rate of Clay Content 46.7% Fig. 4 Slurry Casting 2.2 Model outline The properties of Arakida clay, which was used in this experiment, were as shown in Table 1. This clay is derived from volcanic cohesive soil and distributed around the Arakawa (a river in Japan s Kanto region). Although Arakida clay is well known as a good compaction soil and used for playing fields, we had already clarified that this clay easily causes degradation under trainloads 3). Fig. 5 and Fig. 6 show the outlines of the model. The slurry was formed into a clayey roadbed approximately 700 mm thick by the vacuum consolidation method. Ballast penetration and concrete plates that simulate irregularities at the bottom of the grouted layer were buried in the roadbed surface. In addition, pore pressure meters were set near these buried items. In substitution for the ballastless track for existing lines, an A-type Shinkansen concrete slab was used 3

4 for the model in this experiment because the slab width is practically equal to that of a cement grouted layer of the ballastless track. Besides, it had been confirmed by FE analysis that bending rigidity of the slab is also almost equal to the ballastless track (Fig.7) 4). A-Type Concrete Slab for Shinkansen Concrete Plate Pore Water Pressure Meter Ballast Penetration Layer Depth=700mm 3000mm(5 fastenings) Pore Water Pressure Meter 2340mm Ballast Penetration Concrete Plate Fig. 5 Cross Section of the Model Fig. 6 Roadbed Surface 0.0 Wheel Load = 50kN Track Structure Model Loading Actuator Vertical Displacement (mm) Ballastless Track for Exisiting Lines A-Type Slab Track for Shinkansen Roadbed Displacement Rail Displacement Distance from Center of the Model (mm) Fig. 7 Result of FE Analysis Slab Fig. 8 Situation of the Cyclic Loading 2.3 Test specifications Fig. 8 shows a situation of the cyclic loading. The loading points were the center of the rails with five rail fasteners on the slab. The cyclic load, which was a 0 to 100kN sine wave, was acted on the loading points with a 1-Hz frequency. This 1-Hz frequency was determined from the predominant frequency of the dynamic water pressure that was measured under an actual ballastless track on a conventional (non-shinkansen) line (Fig.9). This frequency depends on car length; therefore, eighty thousand times correspond to the monthly operation of a busy line in Japan. The roadbed settlements under ballastless tracks on operation lines must be approximately finished because sufficient loading history was applicable under the operation with ballast tracks. Therefore, the maximum consolidation pressure was set at 80kPa because the model roadbed has to be over 4

5 consolidation condition. In addition, static preloading, in which a drainage layer is put on the roadbed, was imposed with a 300kN axle load for 24 hours before a cyclic loading test with a 100kN axle load. Fig. 10 shows an outline of the loading history. 2 Water Pressure (kpa) Water Pressure of the Roadbed Surface Car Length Bogie to Bogie Roadbed Displacement Time (sec) Displacement (mm) Mean Roadbed Pressure (kpa) Consolidation Pressure (80kPa) Self Weight of The Slab Approximately 3 to n Preloading (Axle Load = 300kN) Cyclic Loading Test (Axle Load = 100kN) Fig. 9 Water Pressure under Ballastless Track Fig. 10 Outline of Loading History 2.4 Observation of the degradation Fig. 11 shows the primitive phase of the degradation, when muddy water with a high water content moves in and out through water channels that have developed around the ballast penetration layers or the concrete plates. Then, as shown in Fig. 12, silty soil, which has lower water content than the primitive phase muddy water, is pushed out from the water channels and accumulates on the roadbed at the terminal phase of degradation. Fig. 13 shows the roadbed surface when the concrete slab was removed after the loading test. Emanating from the ballast penetration layers, ramal channels filled with soft mud arose on the roadbed surface. The entire roadbed surface was thinly covered with soft mud. The relationship between number of times under load and track settlement, in other words rail settlement, is shown in Fig. 14. The track settlement increased by 4 mm to 5 mm at a stretch until primal thousands times under loading. It was assumed that this primitive settlement occurred because the roadbed surface became softer and weakened by the cyclic water pressure. The primitive settlement corresponded to the primitive outflow of muddy water shown in Fig. 11. The settlement rate of the track was reduced immediately after the primitive settlement, however that eventually accelerated by degrees. It was assumed that this acceleration occurred due to spreading of the water channels over the roadbed surface (Fig. 13) and flowing out of silty soil which has less viscosity with little clay content (Fig. 12). Fig. 15 shows the relationship between number of times under load and amplitude of the track displacement. The amplitude increases at a stretch until primal thousands times under loading and then increases at a constant rate. It is conceivable that a primitive rising of the amplitude occurred in a softening process of the roadbed surface, and that the latter constant-rate rising of the amplitude occurred while a spreading of the roadbed degradation decreases a bearing rigidity of the slab. 5

6 Fig. 16 shows the relationship between number of times under load and water pressure of the roadbed surface around the buried items. The water pressure increases at a stretch until primal thousands times under loading and then decreases by degrees. Because the clay content does not flow out at the primary phase, the roadbed surface maintains a low permeability and thus the large water pressure occurs. Then, after the clay content flows out, the permeability of the roadbed surface increases slightly; besides, water channels on the roadbed grow to water pressure meter. Consequently, the water pressure seems to abate. Fig. 11 Primitive Phase of the Degradation Fig. 12 Terminal Phase of the Degradation Installation Location of The Slab Concrete Plate 0 Track Settlement (mm) Ballast Penetration Water Channels 12 80,000 times is correspond to a monthly train operation The Number of Loading Times (1,000 times) Fig. 13 Roadbed Surface after Loading Fig. 14 Track Settlement 6

7 Amplitude of Track Displacement Water Pressure of Roadbed Surface Concrete Plate Ballast Penetration Max Min The Number of Loading Times (1,000 times) The Number of Loading Times (1,000 times) Fig. 15 Amplitude of Track Displacement Fig. 16 Water Pressure of Roadbed Surface 2.5 Roadbed degradation process According to the above results, roadbed degradation under ballastless tracks is assumed to be due to the following process; 1) Dynamic water pressure between the grouted layer or a slab and the roadbed occurs due to trainloads; therefore the roadbed surface, on which effective stress hardly acts, is fluidized. 2) The clay content becomes mud water and flows out at the primary phase; therefore, density of the roadbed surface decreases. Consequently, the water in the boundary layer becomes free to move; therefore, water channels arise on the roadbed. 3) The roadbed surface, from which clay content has flown out, has reduced cohesion; therefore, the surface soil becomes prone to move with water. The surface soil consequently flows out through the water channels. 4) The water channels extend all over the roadbed; therefore roadbed degradation is accelerated. In addition, bearing rigidity of the track is reduced; as a result, the grouted layer finally collapses. To conclude, degradation of a clayey roadbed under ballastless tracks is not caused by shear deformation or by consolidation, which are due to a lack of roadbed strength, but rather by a local progressive failure, which is due to the outflow of small-particle content from the roadbed surface. 3. Countermeasures against roadbed degradation 3.1 Basic countermeasure policies From the reappearance tests it was evident that the clayey roadbed degradation under ballastless tracks is a local progressive failure. In consequence, countermeasures to increase the strength of the entire roadbed can be dispensed with. In fact, it is assumable that the degradation is preventable by some appropriate treatments only for the roadbed surface. However, even if the surface is simply reinforced with likes of cement, a new border is formed between the cemented layer and the uncemented layer; therefore, the result must be similar to an untreated case. The improvement method should satisfy either 7

8 of the condition to show below at least: (1) Prevention of decrease in effective stress (2) Ejection or confinement of free water between the roadbed and the grouted layer 3.2 BLITS method To prevent a decrease in effective confining stress on the roadbed surface, a permeable layer set between the grouted layer and the roadbed is assumed to be effective, because that layer diffuses the water pressure that acts directly on the roadbed. But actually, this method might not be able to continue the effect for a long term, because the permeable layer has to use a filter which will be clogged by small particles sooner or later. For the practical application, therefore, the confinement of free water between the roadbed and the grouted layer has been adopted. As one of the method to confine free water movement, we regarded Bentonite-clay that has become often used as the material of impermeable layer at waste disposal sites. As a result, the Bentonite liner for track-bed surface (BLITS) method, in which the Bentonite liner is used as a protection layer of the roadbed surface, has been developed. Fig. 17 shows an example of the BLITS method. If water is present between the roadbed and the grouted layer, the Bentonite liner hydrates and swells; as a result, it forms an impermeable layer to protect against an inrush of the free water. In the BLITS method, a special granular Bentonite (Fig. 18) which is colored with red food coloring for visibility against roadbed soil is used. The thickness of the Bentonite liner is basically from 5 mm to 10 mm. Fig. 19 shows a concept of the Bentonite liner. Because the Bentonite has little bearing strength, train roads from the grouted layer are directly borne by the penetrated ballast on the roadbed. Bentonite Liner (approximately 5-10mm) Fig. 17 Example of BLITS Method Bentonite Grouted Layer Ballast Penetration Fig. 18 Granular Bentonite Fig. 19 Concept of the Bentonite Liner for BLITS Method 8

9 3.3 Confirmation of the improvement effect The full-scale model test with improved roadbeds using BLITS methods, as referred to above, was performed under the same condition as the unimproved case. Fig. 20 shows the relationship between number of times under load and track settlement. It is clear that the BLITS method shows sufficient improvement because the track settlement was much less than the unimproved case. Fig. 21 shows the relationship between number of times under load and amplitude of the track displacement. However the BLITS method showed larger displacements than the unimproved case at the primary phase, those displacements became small according to number of times under load. This phenomenon shows that because the Bentonite liner is compacted by cyclic load, the bearing strength of the track increases. With regard to the BLITS method, more term loading tests were performed with other frequencies, as shown in Fig. 22. The frequency dependence of the settlement is hardly shown; in the end, the final track settlement was estimated in 2 mm up to one million loadings. Because there was not enough consolidation time in these tests, the roadbed consolidation has not been finished yet. Therefore, almost all of the settlement is thought to have been caused by roadbed consolidation. Generally, the existing roadbed was already consolidated enough; hence, if only the degradation of the roadbed surface can be prevented, the settlement of the ballastless track on an existing roadbed can be kept to a very small level Track Settlement (mm) BLITS Method 80,000 times is correspond to a monthly train operation Unimproved Amplitude of The Track Displacement unimproved BLITS Method The Number of Loading Times (1,000 times) The Number of Loading Times (1,000 times) Fig. 20 Track Settlement Fig. 21 Amplitude of Track Displacement 9

10 0 1Hz 2Hz 5Hz Track Settlement (mm) BLITS Method 12 Unimproved 4. Conclusions The Number of Loading Times (1000 times) Fig. 22 Track Settlement (Long-Term Tests) From the results of the full-scale model tests, it can be confirmed that degradation of the ballastless tracks on a clayey roadbed is caused by local progressive failure; specifically, the outflow of the fine-grain fraction in the roadbed surface. Even if only the roadbed surface is saturated with water, the roadbed is likely to be degraded. The degradation, therefore, is caused not only by groundwater, but also by temporal surface water from rainfall. In addition, the degradation mechanism of the ballastless track is assumed to be different from the mud pumping of the ballast track; therefore, the ballastless track has the potential to degrade after replacement, though mud pumping had not occurred when the ballast track was used in service. The ultimate countermeasure is improvement of whole roadbed, for example, replacement of the poor roadbed soil or cement stabilization. However, if consolidation of the existing roadbed has been sufficiently finished, additionally, if the ballast track has been used without any train running problems, it is assumed that the reasonable treatment of the roadbed surface as discussed in this report can prevent roadbed degradation after replacement of ballast track by ballastless track. References [1] Muramoto, K., Sekine, E. et al., Dominant Factors of Degradation of Cohesive-Soil Roadbed under Ballastless Tracks RTRI Report, Vol. 18, No. 3, pp , 2004 (in Japanese) [2] Muramoto, K. and Sekine, E., Fundamental Tests of Anti-degradation Methods of Soft Roadbed under Train Load, JGS, 39th Japan National Conference of Geotechnical Engineering, pp , 2004 (in Japanese) [3] Muramoto, K. and Sekine, E., A Study on Degradation under Ballastless Tracks, JSCE, 59th JSCE Annual Meeting, Vol. 3, pp , 2004 (in Japanese) [4]Muramoto, K. Nakamura, T. and Sekine, E., "An Effective Repairing Method of Ballastless-track for Existing Lines", JSCE Journal F, Vol. 63, No.3, pp , 2007 (in Japanese) 10