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Rail. Eng. Science (2020) 28(2):113–128 https://doi.org/10.1007/s40534-020-00210-1 The mechanism and effects of subgrade fluidisation under ballasted railway tracks 1,2 1 2 • • Buddhima Indraratna Mandeep Singh Thanh Trung Nguyen Received: 21 January 2020 / Revised: 24 March 2020 / Accepted: 4 April 2020 / Published online: 28 May 2020 The Author(s) 2020 Abstract The rapid growth in railway infrastructure and help to prevent mud pumping by alleviating the build-up of the construction of high-speed heavy-haul rail network, excess pore pressures under moving train loads. especially on ground that is basically unsuitable, poses challenges for geotechnical engineers because a large part Keywords Mud pumping Ballasted track Subgrade of the money invested in the development of railway lines fluidisation Fouled ballast Prefabricated vertical drains is often spent on track maintenance. In fact around the world, the mud pumping of subgrade fines is one of the common reasons why track performance deteriorates and track stability is hindered. This article presents a series of laboratory tests to examine following aspects of mud 1 Introduction pumping: (1) the mechanisms of subgrade fluidisation under undrained condition, (2) the effects of mud pumping Over the last few decades the growth of railway infras- on the engineering characteristics of ballast, and (3) the use tructure across the world has been exponential, with mas- of vertical drains to stabilize subgrade under cyclic loads. sive strides being taken towards the development of high- The undrained cyclic triaxial testing on vulnerable soft speed heavy-haul rail networks. However, this rapid subgrade was performed by varying the cyclic stress ratio advancement in the railway industry often presents (CSR) from 0.2 to 1.0 and the loading frequency f from 1.0 geotechnical engineers with challenging ground site con- to 5.0 Hz. It is seen from the test results that for a specimen ditions. In Australia alone, a major chunk of the money compacted at an initial dry density of 1790 kg/m , the top invested is spent on annual track maintenance [1] that portion of the specimen fluidises at CSR = 0.5, irrespective stems from the common, and worldwide, problem of mud of the applied loading frequency. Under cyclic railway pumping that has adverse effects on the track substructure loading, the internal redistribution of water at the top of the and also increases the overall cost of track maintenance subgrade layer softens the soil and also reduces its stiff- [2–4]. To illustrate, at present, along the east coast of New ness. In response to these problems, this paper explains South Wales, Australia there are almost 300 active how the inclusion of vertical drains in soft subgrade will pumping locations where wet cohesive fines have been pumped to the surface [5]. Put simply, mud pumping is the upward migration of & Buddhima Indraratna subgrade soil fines into the coarser ballast layer. Where the indra@uow.edu.au ground-water table is high, subgrade slurry forms at the ballast-subgrade interface, and when this slurry is subjected Centre for Geomechanics and Railway Engineering, University of Wollongong, Wollongong City, NSW 2522, to repetitive rail loading, it is then pumped up to the surface Australia of the ballast [6]. The extent of mud pumping in ballasted tracks around the world can be seen in the examples shown Australian Research Council (ARC) Industrial Transformation Training Centre, ITTC-Rail, University of in Fig. 1. The pumped slurry not only reduces the bearing Wollongong, Wollongong City, NSW 2522, Australia 123 114 B. Indraratna et al. Fig. 1 Sightings of mud pumping on ballasted rail track in Australia capacity of the track foundation, it also forms mini-soil permeability tests to assess the hindered drainage capacity volcanoes along the sides of the track. Furthermore, the and (2) large-scale triaxial tests to evaluate the reduced intrusion of fines into the ballast layer hinders its free shear strength and degraded resilient modulus. Addition- draining capacity and also reduces interlocking between ally, the effectiveness of prefabricated vertical drains the angular ballast particles [7]. (PVDs) in reducing the pore pressure build-up in soft The term ‘‘ballast fouling’’ is widely used to indicate the subgrade through field and numerical simulations is deterioration of ballast which has degraded its engineering presented. characteristics. Ballast fouling generally has three sources: (1) ballast abrasion and breakage, (2) external sources such as coal waste and dust, and (3) subgrade fluidisation and 2 Subgrade fluidisation under undrained cyclic infiltration through the subballast and ballast layers. While triaxial tests the mechanism of types (1) and (2) has already been addressed to some degree [8–10], existing studies on the To further improve our understanding of mud pumping fouling mechanism induced by subgrade fluidisation are under ballasted rail tracks, a detailed geotechnical charac- limited. The following paper addresses the mechanism of terisation of vulnerable subgrade soil was needed. This subgrade fluidisation induced by high cyclic stresses section describes the various factors that affect subgrade induced by the railway loading through a series of fluidisation, followed by a pertinent discussion on the undrained cyclic triaxial tests. As a consequence of sub- cyclic response of pumping prone subgrade. grade fluidisation, the performance of the clay-fouled bal- last is further investigated through: (1) large-scale Rail. Eng. Science (2020) 28(2):113–128 The mechanism and effects of subgrade fluidisation under ballasted railway tracks 115 2.1 Factors affecting subgrade fluidisation • Atterberg limits The upper and lower bounds of water content where soil exhibits plastic behaviour are defined as the liquid limit (w ) and the plastic limit (w ) respectively [11]. LL PL These limits are collectively known as the Atterberg limits. The subgrade soils that were reported to have already pumped are mainly in the low-to-medium plasticity region on the soil plasticity chart [5, 12]. However, several of the mud pumping locations in New South Wales, Australia, are along the east coast, so they consist of low compressible estuarine clays with water levels that are often close to the liquid limit [5]. Therefore, this subgrade soil is in a fully saturated state and the continual passage of trains results in serious deformation followed by the subgrade becoming softer. • Presence of fines Indraratna et al. [13] reported that this problematic subgrade soil has an overall fines fraction (\ 75 lm) of Fig. 2 Pore pressure dissipation at different time intervals under the about 30%. As well as fines, this soil also has a filter subbase particle (re-analysed after [20]) ratio (D /D , where D is the diameter 15coarse 85fine 15coarse that corresponds to 15% finer by weight of coarse particles and D is the diameter at 85% finer by 85fine actuator (± 5 kN up to 5.0 Hz), (2) a rigid loading frame, weight of fine particles, respectively) of 5.8, this indicates there is a high susceptibility towards internal (3) a trixial cell, (4) an inlet for filling the cell, (5) a pore instability [14–16]. Since these clayey fines have a pressure transducer, (6) a pneumatic cell pressure con- larger specific surface area, they adsorb more water, troller (up to 650 kPa), (7) a back pressure controller to whereas other studies reported a large amount of fines saturate the soil specimens, (8) a data logger (10 data in the subgrade [4, 17–19]. points per cycle were recorded), and (9) a computer. One- • Hydraulic gradient way stress-controlled tests were carried out in accordance Subgrade fines cannot migrate unless there is a certain with the ASTM D5311-92 standards. The soil sample was level of hydraulic gradient over the track foundation, oven-dried and mixed with 15% water by weight and compacted in ten layers using the non-linear under com- but the repeated passage of trains causes a large increase in excess cyclic pore pressure in the saturated paction criterion [21]. The specimen was 50 mm in diameter by 100 mm high. Further details of sample subgrade layers. Alobaidi and Hoare [20] carried out finite element simulations to study the pore pressures preparation can be sought elsewhere [13]. generated near the top of the subgrade under static loading. As Fig. 2 shows, the pore pressure at the end 2.3 Undrained cyclic triaxial testing of the simulated subbase particle dissipated from 10.0 to 0.6 kPa within 0.25 s. This rapid dissipation of pore To investigate the mechanism of mud pumping, a series of pressure between the centre and the end of the subbase undrained cyclic triaxial tests were carried out on samples particles amounts to an equivalent hydraulic gradient of of remoulded subgrade soil collected from a track site near 147; therefore, a high hydraulic gradient is considered Wollongong City, Australia. This particular track had shown frequent signs of previous mud pumping and was to be one of the main forces which drive the migration of subgrade slurry. subjected to frequent maintenance. The soil was collected from the top of the subgrade layer after the rails, the sleepers, and the ballast and sub-ballast layer had been 2.2 Experimental setup removed. The soil was then transported to the lab facilities at the University of Wollongong for further classification. An image of the GDS ELDYN apparatus at the University The basic geotechnical properties of the soil are listed in of Wollongong used for this study is shown in Fig. 3. The Table 1. The soil is classified as low plastic clay; the CL major components of the equipment include (1) a dynamic has a plasticity index of 11 and a liquid limit of 26% [22]. Rail. Eng. Science (2020) 28(2):113–128 116 B. Indraratna et al. Fig. 3 a GDS ELDYN triaxial equipment at University of Wollongong. b Schematic layout of the cyclic triaxial equipment Cyclic tests were carried out on remoulded soil specimens present experimental study was carried out to represent the compacted at various initial dry densities (q ) to study the worst case scenario wherein the ballast layer is excessively cyclic stress ratio (CSR) and the loading frequency (f) af- fouled, either through excessive ballast breakage or by coal fected the development of cyclic axial strains (e ) and the particles [5], rendering the subgrade layer below in a near ac normalised mean excess pore pressure (EPP). The experi- undrained state. mental procedure is summarised in the flowchart, refer to Fig. 4, while the test details are tabulated in Table 2. The Rail. Eng. Science (2020) 28(2):113–128 The mechanism and effects of subgrade fluidisation under ballasted railway tracks 117 Table 1 Basic geotechnical characterisation of subgrade soil (data between 0.4 and 0.5, but when the specimen was com- from [13]) pacted at a lower density (q = 1680 kg/m ) the CSR d c dropped to between 0.3 and 0.4. The situation where the Soil property Value specimen fluidises when subjected to the critical cyclic Liquid limit, w 26% LL stress ratio, including the mechanism, is discussed in detail Plasticity index, PI 11% in further sections. In situ moisture content 14.6% Specific gravity, G 2.63 2.4.2 Loading frequency, f Maximum dry density (kg/m ) 1814 The loading frequency has a pronounced effect on the It is the natural water content of the subgrade after frequent occur- rence of mud pumping cyclic axial strains (Fig. 6), so when the CSR \ CSR and the specimens do not fluidise under cyclic loading, the resulting higher frequency causes a higher cyclic axial strain. For example, Fig. 6 shows that at 1.0 Hz the cyclic axial strain slowly accumulated to 0.25% after 50,000 The CSR is defined as the ratio between half of the cycles, but at 5.0 Hz it was as high as 0.9% after the same applied cyclic stress (r ) and the effective confining pres- number of cycles. When the CSR = 0.5 ([ CSR ), how- sure (r ): ever, the higher frequency delayed the fluidisation (failure) 3c of the specimen. Similar observations of frequency CSR ¼ : ð1Þ dependence were made by earlier researchers [26, 27] 3c albeit with different soil and loading characteristics. This Since mud pumping is a shallow surface phenomenon, means that in the time domain, asmaller frequency imparts an effective confining of 15 kPa was used to a load on the soil specimen for a longer period of time, and anisotropically consolidate the specimens (the ratio of this results in higher cyclic axial strains and excess pore horizontal to vertical stress k = 0.6). These specimens pressure. were subjected to a wide range of CSR (0.2–1.0) and The critical number of cycles N can be estimated by the loading frequency (f = 1.0–5.0 Hz) to represent varying inflection point on the concave plot of strains that were magnitudes of axle load and train speeds [13]. subjected to CSR [ CSR [13]. Figure 7 shows that when CSR [ CSR the relationship between the critical number 2.4 Results and discussion of cycles N and the loading frequency is almost linear, irrespective of the initial dry densities. Moreover, the The pore pressure transducer and linear variable differen- loading frequency is more prominent at a higher dry den- tial transformer (LVDT) were calibrated before each test to sity because the critical number of cycles N is higher. ensure the data was measured accurately. The development of excess pore pressure and the accumulated cyclic axial 2.4.3 Relative compaction (RC) strain (e ) was influenced by the soil properties [e.g. rel- ac ative compaction (RC)] and the loading conditions (e.g. RC (or the degree of compaction) is defined as the ratio CSR and f). The following section presents the key results between the initial dry density of a specimen and the from the undrained cyclic triaxial testing of remoulded maximum dry density obtained from the Standard Proctor subgrade soil specimens. test [28]. The samples compacted at higher initial dry density, i.e. having a higher RC, tended to resist the applied 2.4.1 Critical CSR cyclic stresses better than the loosely compacted specimens. Figure 8 shows that the samples with q = 1790 kg/m As Fig. 5 shows, when the CSR increases from 0.2 to 1.0, fluidised at a CSR C 0.5, whereas at lower densities (1600 there is a critical cyclic stress ratio (CSR ) beyond which and 1680 kg/m ) the specimens fluidised at CSR = 0.3 and the cyclic axial strain (e ) and mean excess pore pressure 0.4, respectively. Furthermore, the residual axial strain e ac ar ratio increase rapidly; this result is similar to those found increased from 0.08% to 0.4% when the density decreased by previous researchers investigating the threshold cyclic from 1790 to 1600 kg/m . Therefore, increasing the dry stress ratio [23–25]. For example, when the specimen was density or RC will reduce the void ratio and may help to compacted at an initial dry density of q = 1790 kg/m and control the cyclic axial strains and increase the cyclic shear then subjected to a frequency f = 1.0 Hz, the CSR is strength of the subgrade soil [13]. Rail. Eng. Science (2020) 28(2):113–128 118 B. Indraratna et al. Fig. 4 Flowchart of the test scheme adopted for the undrained cyclic triaxial tests 2.5 Mechanism of fluidisation portion of the specimen has formed a thick suspension-like slurry. Chawla and Shahu [18] simulated a large-scale 2.5.1 Based on the physical changes within the specimen physical track model and observed an increment in the water content near the top of the subgrade when mud Figure 9a shows a typical specimen which fluidised under pumping took place. Similarly, the water content measured cyclic loading (f = 1.0 Hz and CSR = 0.5) where the top at the top of one selected specimen (q = 1790 kg/m , Rail. Eng. Science (2020) 28(2):113–128 The mechanism and effects of subgrade fluidisation under ballasted railway tracks 119 Table 2 Testing scheme for undrained cyclic triaxial tests (data sourced from [13]) 3 a Initial dry density q (kg/m ) Relative compaction (RC) (%) Initial void ratio e Loading properties d 0 Frequency f (Hz) Applied CSR 1600 88 0.644 1, 2 and 5 0.1–0.4 1680 93 0.556 1, 2 and 5 0.2–0.5 1790 99 0.469 1, 2 and 5 0.2–1.0 The specimens were compacted to an initial water content of 15% Fig. 6 Effect of loading frequency on subgrade fluidisation Fig. 5 Variation in the cyclic axial strains and mean excess pore pressure at 1.0 Hz frequency CSR = 0.5, and f = 1.0 Hz) revealed that post-fluidisation was 23.1%, which is close to the liquid limit of the soil. Furthermore, when the particle size distribution curves of specimens that fluidised at high cyclic stress ratios were examined, a significant amount of finer fractions (\ 75 lm) had migrated from the central region to the top of the specimen (Fig. 9b). Therefore, under repeated loading, an upward migration of fines and an internal redistribution of the water content fluidised the test speci- mens. Indraratna et al. [29] measured the liquidity index (LI, which is the ratio of the difference between the current water content and the plastic limit to the plasticity index) of the specimen that have fluidised. It was observed that the fluidised specimens have the LI close to 1.0 at the top part of the specimen (Fig. 10). In other words, the top portion of the fluidised specimen has a water content close to the Fig. 7 Influence of loading frequency on the critical number of liquid limit. Thereby, the top portion changes from a solid cycles and residual axial strain state to a fluid-like state. Rail. Eng. Science (2020) 28(2):113–128 120 B. Indraratna et al. 3 Influence of mud pumping on the performance of ballast Mud pumping associated with ballast fouled by a fluidised subgrade has adverse effects on the performance of bal- lasted tracks, which is why it has been comprehensively investigated at the University of Wollongong over the past few years. This adverse influence can be categorised as (1) a reduced drainage capacity, (2) a reduced shear strength and friction angle, (3) a degraded resilient modulus, and (4) substantial track deformation under cyclic loads. The fol- lowing sections will describe these aspects further by considering the laboratory test data. 3.1 Reduced drainage capacity of ballasted tracks The permeability associated with the drainage capacity of ballast is one of the primary parameters that are directly and severely affected by mud pumping. The infiltration of Fig. 8 Influence of initial dry density on the critical CSR and N fluidised subgrade soil into the ballast reduces its porosity, c c permeability and drainage capacity, so a series of perme- ability tests were carried out on fresh ballast mixed with different percentages of clay to investigate the drainage of 2.5.2 Stiffness degradation fouled ballast [30]. Typical Kaolin clay with a liquid limit (w ) of 52% and a plasticity index (PI) of 26% was used LL As discussed in earlier sections, when a specimen was as fouling material. To carry out these tests, a large subjected to a CSR C CSR , there was a rapid accumula- chamber of 500 mm diameter by 500 mm high was tion of strains. The degradation of these soil specimens was designed to ensure there would be a minimal boundary evaluated using the axial dynamic stiffness at a given effect on the hydraulic results (Fig. 12). The specimen had loading cycle N (i.e., E ) as follows: d,N a constant water head flow through it so that the discharge r r velocity at the outlet could be measured. To evaluate the d;max d;min E ¼ ; ð2Þ d;N e e performance of fouled ballast, a novel void contamination ac;max ac;min index (VCI) was proposed, through which the volumetric where r and r are the maximum and minimum d,max d,min ratio between the fouled material filling the ballast and the deviatoric stress experienced by the specimen, respec- initial void of fresh ballast was estimated. tively; and e and e are the maximum and mini- ac,max ac,min Figure 13 shows how ballast fouled by a clay subgrade mum cyclic axial strains for the given loading cycle N, will hinder its hydraulic conductivity k; apparently k de- respectively. The stiffness degradation index is computed creases significantly when the VCI increases. For example, as follows: k dropped from about 0.1 m/s when the VCI was less than -6 d;N 10% to only 1 9 10 m/s at VCI = 75%, and k reduced to d ¼ ; ð3Þ almost the same value as clay’s when the VCI was more d;1 than 95%. In fact when the hydraulic conductivity falls where E is the axial dynamic stiffness of the first loading d;1 -4 below 1 9 10 m/s (0.36 m/h at VCI = 50%), which is cycle. almost the same as that of the nominal silty subgrade soil, it As Fig. 11 shows, when the specimen was subjected to can seriously affect the discharge capacity of the ballast CSR = 0.5 it experienced a sudden reduction in its axial and the track foundations. For example, if we consider a dynamic stiffness, so when the specimen had fluidised typical rainfall in Australia of 150 mm/h [31], this would there was a rapid accumulation of pore pressure and axial -4 3 result in a critical flow rate Q of 2 9 10 m /s for a unit strain, and a large reduction in its stiffness. length of the rail track, so if the actual drainage capacity of the track Q is less than Q , the track can be considered as a c -4 having poor drainage. Where k =1 9 10 m/s at VCI = 50% and the average ballast thickness is about 0.3 m, Q Rail. Eng. Science (2020) 28(2):113–128 The mechanism and effects of subgrade fluidisation under ballasted railway tracks 121 Fig. 9 Post-fluidisation analysis of the specimen (modified after [13]) -4 3.2 Reduced shear strength and friction angle is about 0.3 9 10 m/s which is much less than the estimated Q , and this would lead to very poor drainage of ballast and track that requires constant maintenance. Obviously, this is a rough estimate that only considers vertical drai- A series of large-scale triaxial tests using the unique in- nage, and actual field conditions can be far more complex house equipment at University of Wollongong [32] were and involve horizontal flows and non-uniform ballast carried out on fouled ballast (Fig. 14). In this investigation fouling. the VCI ranged from 10% to 80% and the confining Rail. Eng. Science (2020) 28(2):113–128 122 B. Indraratna et al. Fig. 10 Variation of the liquidity index (LI) of the specimens along the specimen height (modified after [29]) Fig. 12 Large scale permeability test for mud pumping fouled ballast [30] Fig. 11 Variation of the degradation index with number of cycles as a strain rate of 5.5 mm/min was applied onto the (q = 1790 kg/m , f = 1.0 Hz) specimens. Figure 15 shows that the shear strength of ballast pressures varied from 10 to 60 kPa. Kaolin was mixed with decreases as the fouling level VCI increases. It is apparent fresh ballast inside a concrete mixer and then placed into that the deviator stress q has dropped significantly, for the chamber in layers. The specimens were then compacted example, from 475 kPa at a confining pressure where with a vibrating plate until they attained a final height of p = 60 kPa to about 350 kPa when the VCI increases to 600 mm. The test took place under fully drained conditions 50%. Note that a VCI of 50% is a critical fouling level, as Rail. Eng. Science (2020) 28(2):113–128 The mechanism and effects of subgrade fluidisation under ballasted railway tracks 123 was accompanied by a reduction in the friction angle of ballast such that the friction angle decreased to less than 40 when the VCI reached 80% at a confining pressure of 60 kPa. Interestingly, the larger the confining pressure, the lower the friction angle when the same level of ballast fouling is considered. An empirical equation was proposed to estimate how the peak of q (i.e., q ) decreases with the VCI: peak peak;f ¼ pffiffiffiffiffiffiffiffiffi ; ð4Þ 1 þ b VCI peak;b where q and q are the peak of q in fresh and peak;f peak;b fouled ballast, respectively; b is an empirical parameter which varies with different confining pressures p . For Fig. 13 Reduced hydraulic conductivity of ballast with increasing example, b = 0.094, 0.047, and 0.05 when p changes from void contamination index [30] 10 to 30 and 60 kPa, respectively. The prediction using the above equation is shown in Fig. 15a as a comparison with the experimental data. The results show a good agreement indicated by the permeability testing shown in the previous between the proposed model and the experimental data. section. However, increasing the VCI from 50% did not reduce q very much, so there is a critical threshold of clay fouling where any change within this range will have an adverse influence on the shear behaviour. This degradation Fig. 14 Large-scale triaxial test on fouled ballast [32] Rail. Eng. Science (2020) 28(2):113–128 124 B. Indraratna et al. Fig. 15 Adverse effects of clay mud pumping on the shear strength of ballast: a reduced deviator stress, and b reduced friction angle [33] different levels of fouled ballast. Figure 16 shows that M increased linearly with VCI when N \ 4000; for example, M decreased from about 120 MPa to almost 100 MPa at N = 1000. This reduction of M became more severe when N exceeded 10,000 cycles. Specifically, M decreased from 190 to 120 MPa at N = 100,000, and moreover, when N is greater than 10,000 cycles, the increment of M becomes less significant while N continues to increase; in fact M is almost unchanged when N increases from 10,000 to 100,000 cycles when the VCI = 80%. 3.4 Substantial track deformation It was apparent that increasing the amount of slurry (i.e., mud pumping) inside the ballast reduced its friction and bearing capacity, as shown in the previous sections, and Fig. 16 Variation of resilient modulus with VCI over number of cycles N [10] this led to an increase in the deformation of ballast track. The experimental results based on the large-scale cyclic triaxial tests shown in Sect. 3.3 indicate that the axial strain 3.3 Degraded resilient modulus increased significantly when the VCI rises (Fig. 17). For example, the axial strain increased from 9% to 17% when To understand how mud pumping affects the resilient the VCI increased to 80%; therefore, the larger the number modulus (M ) of ballast [10], a series of cyclic tests were of cycles, the more the axial strain. Note also that the carried out using the large-scale model, as shown in excess pore pressure increased rapidly when the VCI Fig. 14. A similar methodology was used in this investi- increased from 50% to 80%, especially when N [ 100. gation and a cyclic load that mimicked real time loading This is because when the percentage of clay reached a behaviour induced by train passage was applied. Specifi- certain high degree, the permeability of the clay-ballast cally, a frequency f = 20 Hz and a confining pressure of mixture decreased dramatically, which caused a large 10 kPa were used. The change of the resilient modulus M build-up of excess pore pressure. This study clearly over the number of loading cycles (N) was estimated at Rail. Eng. Science (2020) 28(2):113–128 The mechanism and effects of subgrade fluidisation under ballasted railway tracks 125 Fig. 17 Effect of mud pumping on ballast deformation under cyclic load: a axial strain and b excess pore pressure indicates that when mud pumping develops to a certain discussed by Indraratna et al. [34]. A large-scale cyclic level, it can lead to severe deformation of rail tracks. triaxial apparatus fabricated at the University of Wollon- gong, Australia was used to evaluate the efficacy of vertical drain inclusions on specimens 300 mm in diameter by 4 Role of vertical drains in alleviating cyclic excess 600 mm high. The installation of PVD for the large-scale pore pressure triaxial testing was carried out by scaling down the size of the vertical drain to represent the unit cell. The ratio 4.1 Laboratory investigation between the radius of the unit cell and the PVD was kept the same as in the field, thus eliminating any boundary Prefabricated vertical drains (PVDs) are commonly used to effects. Further details of the scaling down of the vertical stabilise deposits of soft soil by applying a surcharge with drain can be found elsewhere [34]. The cyclic excess pore or without vacuum preloading. The ability of PVDs to pressure was measured through the bottom of the triaxial dissipate cyclic excess pore pressure has already been cell by miniature pore pressure transducers installed at prescribed locations in the subgrade sample. These tests took place at frequencies 5.0–10.0 Hz to simulate train speeds between 60 and 100 km/h and axle loads of 25–30 t. Figure 18 shows that the excess pore pressure ratio increased rapidly to 0.9 in about 100 cycles, which resulted in undrained shear failure. Without the inclusion of PVD, an anisotropically consolidated undrained testing (CK U) caused large strains and excess pore pressure to develop after about 400 cycles, but with PVDs installed, the axial strain stabilised after 1000 cycles and the cyclic excess pore pressure within the sample was regulated. These results indicated that PVDs can reduce the magnitude of excess pore pressure induced by cyclic loading and it can better resist the cyclic stresses arising from railway loading. 4.2 Field investigation PVDs approximately 8.0 m long were installed at Sand- gate, Australia in that part of the track that passes over 30 m deep soft estuarine clays. This track stabilisation Fig. 18 Efficiency of PVDs in regulating excess pore pressure and using relatively short vertical drains was carried out with axial strain development (redrawn after [34], with permission from no additional preloading, but by the passage of trains at low ASCE) speeds (40 km/h). This site investigation included six Rail. Eng. Science (2020) 28(2):113–128 126 B. Indraratna et al. Fig. 19 Soil profile at the Sandgate Rail grade separation project [35] (with permission from ASCE) boreholes, fourteen piezometric cone penetration tests (CPTU), two in situ vane shear tests, and two test pits. The soil profile is shown in Fig. 19 and its properties can be sought elsewhere [35]. The water content of these layers of soil was close to their liquid limits, and its unit weight varied between 14 and 16 kN/m . Field monitoring inclu- ded settlement plates, inclinometers, and vibrating wire piezometers to ensure that no adverse movements would hamper track stability. 4.3 Numerical predictions Indraratna et al. [34] performed finite element (FE) simu- lations that incorporated the properties of soil taken from Sandgate and then made Class A predictions for the track stabilised with 8.0 m long vertical drains. Class A predic- tions enable all the design considerations to be identified and discussed prior to construction [36]. The FE analysis is based on the soft soil model and the Mohr–Coulomb model incorporated in the finite-element package PLAXIS, while the 2D plane strain FE analysis considered triangular ele- ments with six displacement nodes and three pore pressure nodes. An equivalent plane strain converted from axisymmetric to 2D was adopted based on equivalent plane strain unit geometry [33]. As Fig. 20 shows, the inclusion of PVDs at 1.5 m spacing helped to dissipate excess pore- water pressure rapidly and stabilise the soft clay. In fact almost 90% of the excess pore pressure dissipated within 50 days when PVDs were installed, whereas without PVDs the excess pore pressure was still close to zero after more Fig. 20 a Prediction of excess pore pressure 2.0 m below the subgrade and along the centre line of the track. b Profile of lateral than 10 years (Fig. 20a). These PVDs also control lateral displacement at the rail toe embankment at 180 days [35] (with displacement at the toe of the embankment while the permission from ASCE) numerical predictions agree with the field data (Fig. 20b). This study indicates how efficiently the PVDs reduced the Rail. Eng. Science (2020) 28(2):113–128 The mechanism and effects of subgrade fluidisation under ballasted railway tracks 127 Open Access This article is licensed under a Creative Commons build-up of excess pore pressure in soft subgrade under Attribution 4.0 International License, which permits use, sharing, cyclic loading, and therefore proves that installing vertical adaptation, distribution and reproduction in any medium or format, as drains is a viable alternative for delaying the onset of long as you give appropriate credit to the original author(s) and the subgrade fluidisation. source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not 5 Conclusion included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright This paper discussed the results obtained from large-scale holder. To view a copy of this licence, visit http://creativecommons. laboratory testing as well as investigations carried out on org/licenses/by/4.0/. the subgrade fluidisation under undrained cyclic triaxial loading. The salient findings from this study are as follows: • The soil plasticity, fines content and the hydraulic References gradient generated by the cyclic excess pore pressure plays a crucial role in the pumping of subgrade soil. In 1. 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Railway Engineering Science – Springer Journals
Published: Jun 28, 2020
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