- Email: [email protected]
Investigating the effect of nailed sleepers on increasing the lateral resistance of ballasted track
Computers and Geotechnics 71 (2016) 1–11
Contents lists available at ScienceDirect
Computers and Geotechnics journal homepage: www.elsevier.com/locate/compgeo
Research Paper
Investigating the effect of nailed sleepers on increasing the lateral resistance of ballasted track Morteza Esmaeili ⇑, Alireza Khodaverdian, Hossein Kalantar Neyestanaki, Saharnaz Nazari School of Railway Engineering, Iran University of Science and Technology, Narmak, Tehran 16846, Iran
a r t i c l e
i n f o
Article history: Received 28 April 2015 Received in revised form 20 June 2015 Accepted 29 August 2015
Keywords: Lateral resistance of ballasted track Single tie (sleeper) push test (STPT) Nailed sleeper Numerical study Structural requirements
a b s t r a c t Up to now, various techniques such as using dual-block sleeper, frictional sleeper, safety cap installation, Xitrack utilization and so forth have been proposed in order to improve the lateral resistance of ballasted railway tracks. In all existing methods, no engagement has been considered for track subgrade in this regard. In this paper, benefiting from the steel-driven nails, a new technique called ‘‘nailed sleeper” is introduced for enhancing the lateral resistance of concrete sleepers. In this regard, a 3D numerical model of ballasted track with single tie (sleeper) was developed using ABAQUS software and it was validated using laboratory single tie (sleeper) push test (STPT) results. Thereafter, by inserting two steel nails in the concrete tie of the model, some sensitivity analyses were conducted on the effective parameters of nails such as length, diameter, location through the sleeper and the elasticity modulus of the subgrade. In the next stage, from a structural point of view, the effect of the nail presence on the flexural and shear behavior of B70 concrete sleeper was controlled based on Australian design code AS 1085.14 requirements. As a result of the numerical and analytical analyses, the most appropriate dimensions and locations of nails were defined. Finally, after constructing a nailed B70 sleeper and installing it in a test track, its lateral resistance was evaluated under cyclic loading, and the obtained results were compared with common B70 sleeper at the same track. Consequently, it was observed that using a pair of nails of 40 mm in diameter and 1500 mm in length can increase the lateral resistance more than 200% compared to the normal condition. This technique can be efficiently used for horizontal anchoring the curved ballasted railway tracks. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Various sources of lateral loads can be introduced for an under operation ballasted track and in this regard the induced forces by the wheel/rail contact, the wind lateral load on rolling stock, track buckling due to rail heating and the centrifugal forces on the curves are worth mentioning. Providing sufficient lateral resistance in under operation ballasted tracks are of main concern in high speed railways and in small radius curves, particularly in continuous welded rail (CWR) lines [1]. Generally, the super structure components of the ballasted tracks including the rails, fastenings, sleepers and ballast layer are the main sources of providing the lateral resistance; in this matter the role of sleeper-ballast interaction in horizontal direction is significant since the ballast layer creates lateral resistance in track through the contact between sleepers ⇑ Corresponding author. E-mail addresses: [email protected] (M. Esmaeili), ali.khodaverdian@ aut.ac.ir (A. Khodaverdian), [email protected] (H.K. Neyestanaki), [email protected] (S. Nazari). http://dx.doi.org/10.1016/j.compgeo.2015.08.006 0266-352X/Ó 2015 Elsevier Ltd. All rights reserved.
and ballast at the bottom, on the sides and in shoulders [2]. In the numerical investigation that was carried out by Kabo [3], the effect of these values was analyzed and a sensitivity analysis was conducted. In another research, based on the experimental results presented by Le Pen and Powrie [4], it was concluded that 26–35% of the lateral resistance is provided by interaction between the sleeper bottom with the ballast and the proportion of side and shoulder ballasts are 37–50% and 15–37% respectively. Moreover, other factors such as the sleeper type, its shape, the sleepers spacing, ballast characteristics, ballast depth, and shoulder ballast width and height can affect the lateral resistance [5]. The lack of lateral resistance in railway tracks can result in lateral displacement of tracks which leads to train or wagon derailment. This phenomenon usually happens in newly constructed or maintained (or tamped) curved tracks and it is followed by serious financial and physical damages. In this respect, for improving the lateral resistance of the tracks, many techniques have been proposed by various researchers and companies. For instance, changing the shape and sleeper geometry for better interaction with ballast layer in horizontal direction can be pointed out and in this regard, the
2
M. Esmaeili et al. / Computers and Geotechnics 71 (2016) 1–11
‘winged sleepers’ suggested by Montalban et al. [6] is worth mentioning. As the results of the numerical analysis depicted, creating two wings at the bottom of the sleeper can increase the lateral resistance up to 53%. In another research, Zakeri et al. [7] proposed the utilization of the ‘frictional sleepers’, including the jags in the sleeper base. Their lab experiments with this type of sleeper confirmed 68% enhancement in the lateral resistance due to more interaction of ballast particle with sleeper base. In other studies Ciotla˘uș and Kölló [8] and Beck et al. [9] suggested employing ‘frame’ and ‘Y shape sleepers’ respectively for enhancing the lateral stability and resistance of ballasted tracks. In contrast to above-mentioned techniques, which have focused on implementing a modification on sleepers, Xitrack technology uses a liquid polyurethane which is poured into the ballast to improve the lateral stiffness of shoulder ballast [10]. Scrutinizing all presented methods for enhancing the lateral resistance, it is obvious that none of them rely on subgrade stiffness and bearing capacity. So, the current study presents an innovative idea called ‘‘nailed sleeper”, which is based on the lateral bearing of driven steel nails in conjunction with the lateral resistance of common concrete sleepers. In this method, the possibility of restricting sleepers horizontally using subgrade without limiting the vertical displacement is investigated. For this purpose, by getting the idea from driven micropiles, two vertical steel nails are inserted through B70 concrete sleeper in subgrade to restrict its lateral displacement. In order to implement this idea, firstly a 3D finite element model of single tie (sleeper) push test was developed in ABAQUS software and its results were compared with the laboratory STPT results. Afterwards, due to a good agreement between the numerical and experimental results, two steel nails were included in the model and an extensive sensitivity analysis was performed on the important parameters of nailed sleeper such as: nail diameter, nail length, nail location through sleeper and subgrade stiffness. Consequently, the best dimensions of the nails were obtained. Knowing the optimum nails geometry, the effect of nail presence on the sleeper’s structural behavior was controlled based on Australian design code AS 1085.14 [11]. Finally, utilizing the achieved results in this stage, the nailed B70 sleeper was manufactured and installed in an under-operation railway route and its efficiency in increasing the lateral resistance of ballasted railway track was confirmed through many single tie (sleeper) push tests in comparison to common B70 sleepers.
2. Introducing the idea of nailed sleeper The idea of nailed sleepers is originated from utilizing steel nails (or steel micropiles) in railroads aiming at connecting super and substructure of ballasted railways together in order to engage the lateral bearing capacity of nail/subgrade system in the lateral resistance of the ballasted tracks. In this system, the interaction between the nails and subgrade in lateral direction in conjunction with concreter sleeper as the capping system acts similar to a group of micropiles which has an adequate potential to enhance the lateral stiffness of the whole track system. Although this idea does not have any background in the field of railway engineering, many researchers in the field of geotechnical engineering have been engaged in it. For instance, the results of a series of experiments conducted by Richards and Rothbauer [12], indicate that the behavior of micropiles under lateral loads depends on the type and the resistance of the first two to five-meter soil layers at the top of the micropiles. In another field experiment and the consequent numerical analysis conducted by Abd Elaziz and El Naggar [13], the behavior of a single hollow micropile under the
monotonic and cyclic lateral loading in clay soils was investigated. Through their experiments it was proved that the lateral behavior of micropiles is affected by soil properties in a depth equal to 10 times of micropiles diameters from the top, while the soil layers in lower level does not have any effect in this regard. Yet, in another experiment done by Kershaw and Luna [14], the effect of both vertical and lateral loading on micropiles was studied. Based on their experiments, the presence of vertical static load on micropiles does not have any effect on its lateral resistance. In this research, according to what is shown in Fig. 1, driving two vertical steel nails (which work as micropiles) in track subgrade through the holes made in concrete sleeper (which acts as pile cap), leads to the interaction of the track superstructure (sleeper and ballast) and track substructure (subgrade) which resulted in an increase in overall lateral resistance in ballasted tracks against the applied lateral forces. Obviously, the nails diameters, lengths and locations along the sleeper have considerable effects on the proposed system performance. On the other side, the lateral stiffness and bearing capacity of the subgrade play important roles in mobilizing the lateral stiffness of the nailed sleeper system. Another important issue in real performance of the nailed sleeper system is the effect of the nails on structural performance of prestressed concrete sleepers due to insertion of two holes for nail deriving. Considering all the factors mentioned is essential in developing the idea of using nailed sleeper system. In response to this essence, utilizing the numerical and analytical approaches as analysis and design tools should be considered. For this reason, the next section is devoted to detailed numerical and analytical calculations to support the nailed sleeper idea. Meanwhile, it should be noted that in the proposed system no restrictions have been considered in vertical direction for the nails in the sleepers, so two low friction plastic sheaths were inserted in the concrete sleeper prior to concrete casting during the sleeper production. This issue facilitates the tamping operation on ballasted tracks and it has no negative effects on the vertical stiffness of the tracks as well. 3. Numerical analysis of lateral resistance of the nailed sleeper The behavior of nailed sleepers under lateral loads is not only affected by nails diameter, length and location along the sleeper, but it considerably depends on geotechnical parameters of ballast layer and subgrade soil. Therefore, in the first place, using ABAQUS 6.13 [15] as finite element software, the interaction of nails and sleeper was investigated considering all issues mentioned before, and a comprehensive sensitivity analysis was done on the effective parameters. In the following section, the 3D numerical model of a single nailed sleeper and the analysis done under lateral loading are discussed in detail. 3.1. The model geometry, mesh and boundary conditions Since the main purpose of this research is to study the behavior of B70 concrete nailed sleeper in standard single-track railway, in the process of modeling, the dimensions and the geometry of the B70 sleeper and also the cross-section of single track railway [16] are used according to Fig. 2. In Fig. 3, the discretized finite element model is shown. It should be mentioned that in the direction perpendicular to the page, the model’s thickness was considered equal to 1 m. In the numerical analysis with ABAQUS, various elements were utilized for track super and substructure meshing. In this regard, for sleeper and ballast and subballast layer, the C3D20 type of hexahedral element with 20 nodes was used while for the subgrade soil modeling, the C3D8 type of hexahedral element with 8 nodes was utilized. On the other hand, the wedge element type C3D15
M. Esmaeili et al. / Computers and Geotechnics 71 (2016) 1–11
3
Fig. 1. Schematic of (a) longitudinal section of ballasted track with nailed sleepers and (b) cross section of ballasted track with nailed sleeper.
Fig. 2. (a) B70 concrete sleeper longitudinal section, (b) B70 concrete sleeper cross-section and (c) cross section of single track railway (all in mm).
coefficient in interface of the sleeper and the ballast was assumed as 0.9 [6], while it was considered as 0.1 for the contact surface of nail with ballast and subballast layer and it was assigned as 0.4 for nail and subgrade soil interface [17]. As free vertical sliding of the nails into concrete sleepers is one of the key features of the proposed system, the frictionless contact was imposed for nail/sleeper interface. In the modeling process, concrete sleeper and steel nails (ST37 type) were considered as linear elastic materials. This is justifiable due to minor value of lateral displacement in concrete sleeper, during single tie push test. On the other hand, an elastoperfectly plastic model with Mohr–Coulomb failure criterion was assigned to ballast, subballast and subgrade material. The detailed material properties of the model components including density, elasticity modulus, Poisson’s ratio and shear strength parameters are summarized in Table 1 [3,18]. 3.2. Lateral loading of a single sleeper Fig. 3. 3D FE mesh employed in modeling of nailed sleeper.
with 15 nodes was employed for nails. Overall, 7428 elements and 20219 nodes were used in the whole model. In order to impose the boundary conditions, the lateral surfaces were fixed in X direction, the longitudinal surfaces were fixed in Y direction and the bottom surface was fixed in all 3 directions, X, Y and Z. It should be noted that the model is free to settle in vertical direction. For interaction modeling between all different parts of the model, the tangential behavior of the contacts was assumed to allow sliding in existing interfaces. Regarding this, the friction
STPT, which is the abbreviation of Single Tie Push Test, is one of the experiments for defining lateral resistance. In this test, in order to determine the lateral resistance of a single sleeper, at first, the sleeper fastenings should be unlocked from the rails and then the lateral displacement of the single sleeper, due to the applied lateral load, should be recorded. The lateral load resulted in 2 mm displacement is usually known as the lateral resistance in the test [16]. In the developed numerical model of the present study, similar to STPT condition, a gradual concentrated load was applied on the sleeper lateral side and the loading was terminated while a 2mm lateral displacement took place.
4
M. Esmaeili et al. / Computers and Geotechnics 71 (2016) 1–11
Table 1 Mechanical characteristics for elements [3,18]. Model component
Material model
Density (kN/m3)
Elasticity module (MPa)
Friction angle (°)
Cohesion (kPa)
Poisson’s ratio
Concrete sleeper Steel nail Ballast Subballast Subgrade
Linear elastic Linear elastic Mohr–coulomb Mohr–coulomb Mohr–coulomb
24 78.5 15.6 16.7 18
37,500 210,000 140 80 45
– – 45 35 30
– – 2 2 50
0.15 0.3 0.4 0.35 0.33
3.3. Numerical model validation and results In this section, the results of the conducted numerical analyses are presented in two stages. In the first stage, the results of the common STPT, which exclude the subgrade material in numerical model, are presented and compared with the existing experimental STPT results. In the next stage, using this validated model, a comprehensive sensitivity analysis is performed on the proposed ‘nailed sleeper’ system to reveal the most important parameters affecting the new system. As shown in Fig. 4, the results of the common STPT test are in good agreement with the experimental results reported by Zakeri et al. [7]. In this diagram, two separate phases can be observed. In the first phase, a linear relation between force and displacement confirms the frictional behavior of sleeper in contact with ballast material, while in the second phase, the constant force with incremental displacement displays the sleeper sliding. Regarding AREMA [16] recommendations, the force resulted in 2 mm displacement, which is equal to 12.5 kN, can be considered as lateral resistance of single sleeper in both experimental and numerical approaches. Based on the obtained results in pervious stage and the adequate validity of developed FEM model in ABAQUS as a suitable analysis tool, in the next stage after adding the subgrade and nail elements to the model, an extensive sensitivity analysis is conducted to show the effect of various engaging parameters on real performance of nailed sleeper system. For this reason, a sensible range of nail and subgrade specifications are considered to be implemented in the numerical model as shown in Table 2. 3.3.1. The effect of the nails length In Fig. 5, the active lateral load–displacement diagram is plotted for a conventional sleeper and also for a nailed sleeper with a specific nail diameter of 40 mm, for various lengths of 1, 1.5 and 2 m. In these results the subgrade soil elasticity modulus is considered 45 MPa, which meets the minimum required value recommended by UIC719 R [19]. As can be seen, increasing the length of the nails from 1 m to 2 m does not affect the lateral resistance of the whole system. On the other hand, focusing on distribution pattern of internal forces along the nails, as is shown in Fig. 6, it can be observed that
Fig. 4. Comparing load–displacement curves of numerical model results and experimental test.
Table 2 Used parameters in sensitivity analysis of ‘‘nailed sleeper” behavior numerical model. Parameter
Symbol
Unit
Value
Nail diameter Nail length Nail distance to sleeper center
D L B
mm m mm
Subgrade elasticity modulus
E
MPa
16, 24, 32, 40 1, 1.5, 2 100, 200, 300, 400, 500, 1100 45, 80, 100, 120
Fig. 5. Load–displacement diagram of conventional sleeper and nailed sleeper system for different lengths of the nail.
the shear and moment in the nails reach to a negligible value in depths more than 1 m. Although this result confirms the suitability of nail length of 1 m for ‘‘nailed sleeper” system, considering the possibility of increase in ballast layer depth during the maintenance operations and the length of nails embedded in concrete sleeper (as micropile cap), the length of 1.5 m was adopted. This greater length guaranties the more safety factor of the proposed system against lateral loads. As illustrated in Fig. 5, the nailed sleeper with 1.5 m length results in lateral resistance of 20.5 kN that is approximately 150% more than normal sleeper without the nails which is 8.3 kN and it proves that the proposed nailed sleeper can meet the requirements of the tracks against all kinds of imposed lateral loads mentioned before such as the wind lateral load on rolling stock, the centrifugal forces in curves and so forth. It should be mentioned that the loading direction isn’t an important issue in the proposed system because the pair of the nails on the both side of the sleeper withstands against all lateral loads in all directions. From structural design point of view, supposing the yielding stress (F y ) of 240 MPa for steel nails, the ratio of plastic moment (Mp = 2.56 kN m) to maximum existing moment (M d = 0.62 kN m) provides a minimum safety factor of 3.6. Moreover, comparing the allowable shear stress of 0.4F y [20] with maximum existing shear stress of 21 MPa shows a safety factor of 4.5 in the nail element. Therefore, it can be stated that the selected dimensions for the nails guarantee the suitable performance of the nailed sleeper system. 3.3.2. The effect of the nails diameter Adopting the suitable length of 1.5 m from the results in the previous section, and considering the same specifications for sub-
M. Esmaeili et al. / Computers and Geotechnics 71 (2016) 1–11
5
Fig. 6. Shear force, bending moment and lateral displacement variations in depth for various nail lengths.
Fig. 7. Load–displacement diagram for various nail diameters.
grade soil, the results of numerical model for various diameters of the nail are illustrated in Fig. 7. As shown in this figure, by increasing the nail diameter, the ratio of lateral resistance of nailed sleeper system to the common system increases with a linear pattern. As shown, for nails with 16, 24, 32 and 40 mm in diameters, the lateral resistance of nailed sleeper have raised up to 55%, 90%, 120% and 150% respectively in comparison to conventional sleepers. Although a bigger nail diameter results in more lateral resistance, it can also reduce the efficient sleeper width which may causes some inefficiency for sleeper from a structural point of view. Overall, a maximum diameter of 40 mm was selected for nails in the proposed system. As shown in Fig. 8 for various nail diameters, after depth around 1 m, all shear, bending and lateral displacements converge to zero which means the minor effect of the nail with lengths more than this value in mobilizing the more lateral resistance. 3.3.3. The effect of nails location along sleeper To study the effect of nails location along the sleeper, a series of sensitivity analyses were performed and the nails position were modified. In these analyses, the nail diameter and length were selected as 40 mm and 1.5 m respectively, while the subgrade soil module of elasticity was chosen as 45 MPa. According to Fig. 9, when the nail location is far from the centerline up to 400 mm, the lateral resistance of nailed sleeper arises but thereafter it remains constant. Consequently, based on the achieved results, the nails location was fixed in 400 mm in order to provide the maximum lateral stiffness and avoid any inference
between nails and tamping picks during the maintenance operations. 3.3.4. The effect of subgrade stiffness To investigate the effect of this parameter on lateral resistance of the proposed system, four various elasticity modulus – 45, 80, 100 and 120 MPa – were allocated to the subgrade soil in FEM model. These values were selected based on UIC719 [19] recommendation for subgrade elasticity modulus (EV2 > 45 MPa). Considering 1.5 m for nails length and 40 mm for their diameter, Fig. 10 depicts the lateral resistance curve against the subgrade soil elasticity modulus. As it is shown, increasing the modulus from 45 to 120 MPa results in an increase of around 10% in the lateral resistance. Fig. 11 shows the shear force, bending moment and displacement diagrams along the nails length. It should be emphasized that both Figs. 10 and 11 are drawn based on the results achieved from 2 mm lateral displacement in nailed sleeper system. 4. Structural control of nailed sleeper The main purpose of this section is to discuss the structural effect of holes inserted in concrete sleeper on its structural design especially on flexural reinforcements. Based on the presented results in previous section, although nails with bigger diameters evidently result in higher lateral resistance, they can cause some insufficiencies in structural performance of concrete sleeper under
6
M. Esmaeili et al. / Computers and Geotechnics 71 (2016) 1–11
Fig. 8. Shear force, bending moment and lateral displacement for nails with various diameters.
Fig. 10. Lateral resistance diagram of nailed sleeper system for various subgrade elastic modulus. Fig. 9. Lateral resistance diagram of nailed sleeper system for various locations of nails along sleeper.
R¼JQ service load. Considering the possible limitation regarding nail diameter, a detailed calculation is performed in the present section to control all relevant structural issues in nailed sleeper system. It should be noted that all calculations and controls are conducted based on Australian standard, AS1085.14 [11] regarding concrete sleepers. The structural controls are especially conducted in critical sections of sleeper where the nails are inserted. As shown in Fig. 12, based on AS1085.14 [11], three types of loading models have to be applied under sleeper for defining the critical moments, as follows: 1. LM1: for calculating the positive flexural moment at rail seat by assuming the uniform pressure distribution beneath the sleeper on both sides at the width of l = L g (Fig. 12a). 2. LM2: for calculating the positive flexural moment in the center of the sleeper assuming the uniform pressure distribution beneath the sleeper on both sides at the width of l = 0.9(L g) (Fig. 12b). 3. LM3: for calculating the negative flexural moment in the center of sleeper by assuming the uniform pressure distribution over the whole sleeper length at the bottom (Fig. 12c). According to the proposed formula by Australian standards, the vertical applied load on the rails seats is equal to:
D:F 100
ð1Þ
where R is the rail seat load, J is the combined vertical design load factor, Q is the load carried by sleeper and D.F is the axle load distribution factor. As recommended in this standard, the minimum impact factor (J), which is equal to 2.5, was used in calculations. Moreover, the remaining parameters such as sleepers’ center to center spacing and vehicle axle load were considered as 600 mm, 250 kN respectively. In addition, in this case the D.F is considered equal to 51. Substituting the assumed values in the formula above, the vertical load of rail seat equaled 318.75 kN. In Fig. 13, variation diagrams of positive and negative moments along the sleeper are plotted against loading models of LM1, LM2 and LM3. The process of controlling the critical sections of the sleeper from the structural stand point is done according to the flowchart in Fig. 14, using the value shown for the moments in the diagram above. Flexural tension stress has to be controlled in 3 sleeper constructional steps as follows: Step1: the concrete stresses immediately after transfer, and before deferred losses. Step2: the concrete compressive stress after all losses without any applied load. Step3: the concrete stresses under the working conditions and after allowing for all losses of pre-stress.
M. Esmaeili et al. / Computers and Geotechnics 71 (2016) 1–11
Fig. 11. Shear, bending moment and displacement for nails with various elasticity modulus of subgrade.
Fig. 12. Various loading patterns for calculating critical flexural moments in sleeper.
Fig. 13. Moment diagram respect to three loading pattern LM1, LM2 and LM3.
It is worth mentioning that based on Australian standard; there is no need to control the shear stresses in all sleeper sections. Flexural stresses were calculated at the highest (Ftop) and the lowest (Fbot) axes of the sleeper sections at the 400 mm distance
Fig. 14. Flowchart of structural controls in concrete sleeper.
7
8
M. Esmaeili et al. / Computers and Geotechnics 71 (2016) 1–11
from the center of the sleeper. All calculated tension stresses should be compared with allowable values proposed by AS 1085.14 [11]. Moreover, the bearing stress, which is created in side contact area of the nails with concrete sleeper, should be controlled in the walls of the hole. The lateral load on a single nailed sleeper is considered approximately 22.5 kN based on the maximum lateral load calculated in numerical analysis chapter. Subtracting the share of base, crib and shoulder ballast in lateral resistance, the pure lateral load imposed to one of the walls of the hole was calculated. By dividing this load on the side contact area of nail and sleeper the bearing pressure can be obtained. The amount of the applied stress was compared with the proposed values of ACI 318 [21]. Considering the values calculated for tension and bearing stresses and comparing them with allowable stresses in each part, a summary of results is presented in Table 3. As it is shown, all of the calculated stresses are considerably less than the allowable stresses recommended by the standard, so it can be concluded that no negative effects are expected from the steel nail presence on structural performance of concrete sleeper in nailed sleeper system; neither in the construction phase nor under the service load.
5. Field test on a B70 nailed sleeper After completing the numerical and analytical investigations on the nailed sleeper system and selecting appropriate dimensions for length, diameter and location of the nails in the sleepers, in this section, many field tests are conducted on both common and nailed sleeper to compare their real performance on site in order to prove the superiority of the new system in improving the lateral resistance of ballasted track. In this regard, some details concerning fabrication, field installation and single tie push test are presented. 5.1. Nailed sleeper production Concerning the production of nailed sleepers, some changes should be applied on common B70 sleeper. To this end, the sheath, made up of compressed plastic with outer and inner diameters of 50 and 40 mm respectively, was placed 400 mm away from sleeper center in the mold in the sleeper production factory. The other steps such as tendon reinforcement installation, fabricating and curing are the same as the conventional B70 sleeper. Finally a sample of B70 sleeper containing two holes for inserting the steel nails was manufactured as shown in Fig. 15.
Table 3 Structural control for both flexural and bearing stresses in critical section of nailed sleeper B70. Control type
Bending stressesa Step 1
Created stresses (MPa) Allowable compression stresses (MPa) Allowable tension stresses (MPa) a
Bearing stressesa Step 2
Step 3
F top
F bot
F top
F bot
F top
F bot
1.67 24 –
2.6 24 –
1.42 <1 –
2.21 <1 –
1.43 27 3.098
2.2 27 3.098
0
All calculations were performed considering the compression strength of sleeper concrete equal f c ¼ 60 MPa.
Fig. 15. Produced B70 sleeper including two holes.
Fig. 16. Schematic of instrumentation for STPT on nailed sleepers.
1.3 21 –
9
M. Esmaeili et al. / Computers and Geotechnics 71 (2016) 1–11
5.2. Installation, instrumentation and lateral resistance testing on sleeper In this research, the field lateral resistance tests were carried out in a part of Iran railway network in the main route of Tehran–Karaj near Karaj station, which is located 30 km away from Tehran station. This site is equipped with ballasted tracks with superstructure, including 25 cm ballast and conventional B70 sleeper. The shoulder width in the site is about 40 cm. For conducting the STPT on the common and nailed sleeper, firstly the fabricated B70 sleeper including two holes was substituted with a B70 sleeper in the track. Moreover, on one side of the track and outside the shoulder a concrete support was established to provide enough reaction for lateral loading of the nailed sleeper by hydraulic jack. The common instrumentation of sleeper was performed by installing a LVDT at the end using a supporting frame to record the lateral displacement of the pushed sleeper while the load of hydraulic jack was recorded by a load cell installed between the jack and lateral side of sleeper as shown in Fig. 16. The utilized LVDT and the load cell specifications are presented in Tables 4 and 5. It should be noted that all recorded data during the loading procedure were recorded via installed data acquisition system on a laptop. The data logger system used in this project was the TMR211 model, made by the Tokyo Sokki Kenkyujo Company [22]. After instrumentation, many single tie push tests were carried out on a B70 sleeper before and after nails installation. It should be noted that in the case of nailed sleeper testing, the nails were driven into the track substructure – through provided holes in B70 sleeper – using a pneumatic hammer (model D25980K of DeWALT Company) [23] and a special coupling as shown in Fig. 17. The installed nailed sleeper is illustrated in Fig. 18.
Table 5 Load cell specifications. Rated capacity (R.C) Rated output (R.O) Zero balance Combined error Repeatability Creep (30 min)
50, 100, 200, 500kgf, 1, 2, 5, 10, 20tf 2.0 ± 0.005 mV/V 0 ± 0.02 mV/V 0.05% 0.02% 0.03%
Temperature effect on Zero value Output value
0.03%/10 °C 0.03%/10 °C
Excitation Recommended Maximum
10 V 15 V
Resistance Input Output Insulation Compensated temp range Operating temp range
400 ± 30 Ω 350 ± 3.5 Ω >2000 MΩ 10 to +40 °C 30 to +80 °C
5.3. STPT results and discussion After installation of special B70 sleeper (including two holes) in the selected track and before the steel-nails installation, three STPT tests were carried out. The results of these tests are illustrated in Fig. 19. In the case of nailed sleeper, STPT tests were carried out with some modifications. These tests were performed under cyclic loading so that the loading and unloading were done consecutively in a manner that the loading step was continued up to 2 mm lateral displacement occurrence in the sleeper. Load and displacement were recorded during these cycles, the results of which are shown in Fig. 20. It is worth considering that in total, four series of tests were conducted which the first one (Fig. 20a) was completed
Fig. 17. Nails driving using pneumatic hammer and the attached coupling.
Fig. 18. Nailed sleeper system after the nails installation. Table 4 LVDT properties. Type Capacity Related output Sensitivity Non-linearity Spring force Frequency response Temperature effect on zero Compensated temperature range Temperature range Input/output resistance Recommended exciting voltage Allowable exciting voltage Weight
CDP-100 100 mm 5 mV/V + or – 0.1% (10,000–106 strain + or – 0.1%) 100–106 strain/mm 0.1%RO 4.9 N 3 Hz 0.01%RO/-C 0 to +40-C (no condensation) 10 to +60-C (no condensation) 350 X Less than 2 V 10 V 580 g
instantly after nail driving and other tests were performed five months later. During this period the track had been under service loads so comparing, the results of the first test with other three tests can indicate the short-term and long-term behavior of the proposed nailed sleeper system. As shown in Fig. 19, the averages of the lateral resistance corresponding 2 mm lateral displacement is around 10.4 kN for three STPT tests on common sleeper. While in the case of nailed sleeper, as it is illustrated in Fig. 20, when the number of loading and unloading cycles grows, the lateral resistance increases gradually and according to this figure, in the first loading cycle, the lateral resistance resulted in 2 mm displacement are recorded as 18.7, 24.4, 20.2 and 21 kN corresponding to tests 1–4. The existing difference between the obtained results in test 1 and other tests can be justified by altering the soil of the subgrade phase from undrained to drained condition. Tracing the presented results in
10
M. Esmaeili et al. / Computers and Geotechnics 71 (2016) 1–11
Fig. 19. STPT test results for conventional sleeper.
Fig. 20 reveals that due to the continuing load cycles, a better interaction between the nails and surrounding subgrade soil can be established which results in enhancing the lateral resistance up to 30 and 35 kN. Comparing the mobilized lateral resistance in this condition with corresponding value in the case of common B70 sleeper confirms a more than 200% enhancement in the lateral resistance. As it stated before, during the cyclic tests, the loading phase in each cycle is continued till 2 mm lateral displacement takes place in the sleeper. But during the unloading phase, the sleeper moves in reverse direction and a part of imposed displacement is recovered as the elastic deformation of the system. This phenomenon can be justified because of the nails presence in the system as influential elastic elements. On the other hand, after unloading, another part of the displacement still remains in sleeper which represents the plastic or permanent deformation of the system and is justifiable because of nonlinear behavior of ballast and subgrade materials. By increasing the loading and unloading cycles, the proportion of the elastic displacement from the total displacement increases. Scrutinizing the presented results reveals that in all conducted cyclic tests after 10 cycles, almost all displacements remain elastic and a steady state could be observed in the nailed sleeper system which confirms no plastic deformation occurrence. This matter can be described by evaluating the values of load– displacement slopes G1 and G2. As indicated in Fig. 21. The G1
Fig. 21. The nailed sleeper lateral stuffiness during the loading and unloading (G1 and G2).
Fig. 22. G1/G2 diagram versus number of cycles.
which is loading slope is calculated as the slope of the line connects the start and peak (maximum load) points together and in the same manner the unloading slope, G2, is calculated as the slope of the line connects the end and peak points together in each cycle for all tests. As it is illustrated in Fig. 21, comparing cycles 1 and 12 in test 3 (Fig. 20c) as a sample, it is found out that due to an increase in the number of cycles both G1 and G2 are going to converge to the same value and based on Fig. 22, generally in all tests mentioned before, the proportion of G1 to G2 tends to 1, which
Fig. 20. Results of lateral cyclic loading test on the nailed sleeper.
M. Esmaeili et al. / Computers and Geotechnics 71 (2016) 1–11
Fig. 23. Maximum active lateral load versus elastic displacement in each cycle.
confirms the steady state happening in the system. The speed of convergence is considerably depends on high elasticity of steel nails regarding to compacted soil. From another point of view the high elasticity of the nails helps to recover the imposed lateral displacement in lateral direction and in overall the nails behavior controls the whole system elasticity. This finding guarantees the stable performance of the proposed system in practical cases when the train wheel-loads apply on the nailed sleeper for long terms. From another point of view, in Fig. 23, the maximum load in each cycle is plotted against the elastic displacement in the same cycle for all 4 tests. According to this figure, the result of test that was carried out instantly after nail driving has lower slope than the later ones. This problem may be justifiable due to the disturbance caused by the nail driving and the short term behavior of the subgrade soil. On the other hand, in the first test which was done instantly after nail driving the system stiffness is 10.8 kN/mm while in other three tests the results for system stiffness is 16.9, 18.8 and 27.9 kN/mm respectively. 6. Conclusion In this research, after introducing the nailed sleeper as an innovative approach for increasing the lateral resistance of conventional ballasted track – by engaging the lateral bearing capacity of subgrade – in the first place, the technical feasibility of the method was investigated using FEM method, and the suitable diameter, length and appropriate location of nails along the sleeper were determined. Moreover, the effects of nails on flexural performance of sleeper were discussed. Next by installing a nailed sleeper in real track and conducting the STPT, lateral resistance of single sleeper with and without nails were measured and the good performance of this sleeper was confirmed. The main findings of the present study can be summarized as follows: 1. As a result of numerical modeling, the lateral resistance of nailed sleepers using nails with 1, 1.5 and 2 m in length are identical. In other words, increasing the length of the nails to more than 1 m does not have any effect on lateral resistance. 2. FEM numerical results reveal that nails with diameters of 16, 24, 32, and 40 mm can increase the lateral resistance to about 55, 90,120 and 150 percent correspondingly compared to conventional B70 sleepers. 3. According to the numerical analyses results, it was concluded that when the nails are located away from the sleeper centerline – up to 400 mm – the lateral resistance of nailed sleeper arises but thereafter it remains constant.
11
4. The FEM model results show that the lateral resistance is increased about 10% by increasing the subgrade elasticity modulus from 45 to 120 MPa. 5. Based on the field test results, by increasing the loading and unloading cycles, the amount of lateral resistance increases. Lateral resistance of nailed sleeper using two nails with the diameter of 40 mm and the length of 1.5 m, that have installed 400 mm away from centerline of the sleeper has increased by more than 200% in comparison to conventional B70 sleepers. 6. By increasing the loading/unloading cycles, slopes of loading cycles (G1) have increased and slopes of unloading cycles (G2) have decreased. After 10 cycles, the proportion of G1/G2 tended to 1 which means long term stability of the nailed sleepers. To ensure the suitable performance of the nailed sleeper in increasing the lateral resistance of ballasted track, the subsequent stage of the present research will be devoted to monitoring the nailed sleeper performance in horizontal fixation of the curved track by complementary field tests. References [1] Zakeri JA. Lateral resistance of railway track. In: Perpinya X, editor. Reliability and safety in railway. InTech; 2012. p. 430. [2] Selig ET, Waters JM. Track geotechnology and substructure management. T. Telford; 1994. [3] Kabo E. A numerical study of the lateral ballast resistance in railway tracks. Proc Inst Mech Eng, F: J Rail Rapid Trans 2006;220(4):425–33. [4] Le Pen LM, Powrie W. Contribution of base, crib, and shoulder ballast to the lateral sliding resistance of railway track: a geotechnical perspective. Proc Inst Mech Eng, F: J Rail Rapid Trans 2011;225(2):113–28. [5] Lichtberger B. Track compendium: formation, permanent way, maintenance, economics. Eurailpress; 2005. [6] Montalbán Domingo L, Real Herraiz JI, Zamorano C, Real Herraiz T. Design of a new high lateral resistance sleeper and performance comparison with conventional sleepers in a curved railway track by means of finite element models. Latin Am J Solids Struct 2014;11:1238–50. [7] Zakeri J-A, Mirfattahi B, Fakhari M. Lateral resistance of railway track with frictional sleepers. Proc ICE-Trans 2012;165(2):151–5. [8] Ciotla˘uș M, Kölló G. Increasing railway stability with support elements. Special sleepers; 2012. [9] Beck A, Hempe T, Schlender T. Experience gained with Y-steel sleeper track on german rail (DB AG). Rail Eng Int 2008;37(2). [10] Woodward PK, Kennedy J, Medero GM, Banimahd M. Application of in situ polyurethane geocomposite beams to improve the passive shoulder resistance of railway track. Proc Inst Mech Eng, F: J Rail Rapid Trans 2012;226 (3):294–304. [11] AS-1085.14. Railway Track Material Part 14. Prestressed Concrete Sleepers. Standard Australia; 2012. [12] Richards JT, Rothbauer M. Lateral loads on pin piles (Micropiles). GeoSupport 2004:158–74. [13] Abd Elaziz AY, El Naggar MH. Performance of hollow bar micropiles under monotonic and cyclic lateral loads. J Geotech Geoenviron Eng 2015:04015010. [14] Kershaw K, Luna R. Full-scale field testing of micropiles in stiff clay subjected to combined axial and lateral loads. J Geotech Geoenviron Eng 2014;140 (1):255–61. [15] ABAQUS 6.13. Providence (RI, USA): Dassault Systèmes Simulia Corp. [16] AREMA. Manual for Railway Engineering, Volume 1, Chapter 1, Roadway and Ballast: American Railway Engineering and Maintenance of way Association; 2012. [17] Engineers ASoC, Engineers USACo. Design of Sheet Pile Walls. ASCE Press; 1996. [18] Indraratna B, Nimbalkar S. Implications of ballast breakage on ballasted railway track based on numerical modelling; 2011. [19] Railways IUo. Earthworks and track-bed layers for railway lines: UIC code. 2nd ed. Paris: International Union of Railways; 1994. [20] AISC. Specification for structural steel buildings. Chicago; 2010. [21] ACI. Building code requirements for structural concrete: (ACI 318-95); and commentary (ACI 318R–95). American Concrete Institute; 1995. [22] http://www.tml.jp/. [23] http://www.dewalt.com/.
Contents lists available at ScienceDirect
Computers and Geotechnics journal homepage: www.elsevier.com/locate/compgeo
Research Paper
Investigating the effect of nailed sleepers on increasing the lateral resistance of ballasted track Morteza Esmaeili ⇑, Alireza Khodaverdian, Hossein Kalantar Neyestanaki, Saharnaz Nazari School of Railway Engineering, Iran University of Science and Technology, Narmak, Tehran 16846, Iran
a r t i c l e
i n f o
Article history: Received 28 April 2015 Received in revised form 20 June 2015 Accepted 29 August 2015
Keywords: Lateral resistance of ballasted track Single tie (sleeper) push test (STPT) Nailed sleeper Numerical study Structural requirements
a b s t r a c t Up to now, various techniques such as using dual-block sleeper, frictional sleeper, safety cap installation, Xitrack utilization and so forth have been proposed in order to improve the lateral resistance of ballasted railway tracks. In all existing methods, no engagement has been considered for track subgrade in this regard. In this paper, benefiting from the steel-driven nails, a new technique called ‘‘nailed sleeper” is introduced for enhancing the lateral resistance of concrete sleepers. In this regard, a 3D numerical model of ballasted track with single tie (sleeper) was developed using ABAQUS software and it was validated using laboratory single tie (sleeper) push test (STPT) results. Thereafter, by inserting two steel nails in the concrete tie of the model, some sensitivity analyses were conducted on the effective parameters of nails such as length, diameter, location through the sleeper and the elasticity modulus of the subgrade. In the next stage, from a structural point of view, the effect of the nail presence on the flexural and shear behavior of B70 concrete sleeper was controlled based on Australian design code AS 1085.14 requirements. As a result of the numerical and analytical analyses, the most appropriate dimensions and locations of nails were defined. Finally, after constructing a nailed B70 sleeper and installing it in a test track, its lateral resistance was evaluated under cyclic loading, and the obtained results were compared with common B70 sleeper at the same track. Consequently, it was observed that using a pair of nails of 40 mm in diameter and 1500 mm in length can increase the lateral resistance more than 200% compared to the normal condition. This technique can be efficiently used for horizontal anchoring the curved ballasted railway tracks. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Various sources of lateral loads can be introduced for an under operation ballasted track and in this regard the induced forces by the wheel/rail contact, the wind lateral load on rolling stock, track buckling due to rail heating and the centrifugal forces on the curves are worth mentioning. Providing sufficient lateral resistance in under operation ballasted tracks are of main concern in high speed railways and in small radius curves, particularly in continuous welded rail (CWR) lines [1]. Generally, the super structure components of the ballasted tracks including the rails, fastenings, sleepers and ballast layer are the main sources of providing the lateral resistance; in this matter the role of sleeper-ballast interaction in horizontal direction is significant since the ballast layer creates lateral resistance in track through the contact between sleepers ⇑ Corresponding author. E-mail addresses: [email protected] (M. Esmaeili), ali.khodaverdian@ aut.ac.ir (A. Khodaverdian), [email protected] (H.K. Neyestanaki), [email protected] (S. Nazari). http://dx.doi.org/10.1016/j.compgeo.2015.08.006 0266-352X/Ó 2015 Elsevier Ltd. All rights reserved.
and ballast at the bottom, on the sides and in shoulders [2]. In the numerical investigation that was carried out by Kabo [3], the effect of these values was analyzed and a sensitivity analysis was conducted. In another research, based on the experimental results presented by Le Pen and Powrie [4], it was concluded that 26–35% of the lateral resistance is provided by interaction between the sleeper bottom with the ballast and the proportion of side and shoulder ballasts are 37–50% and 15–37% respectively. Moreover, other factors such as the sleeper type, its shape, the sleepers spacing, ballast characteristics, ballast depth, and shoulder ballast width and height can affect the lateral resistance [5]. The lack of lateral resistance in railway tracks can result in lateral displacement of tracks which leads to train or wagon derailment. This phenomenon usually happens in newly constructed or maintained (or tamped) curved tracks and it is followed by serious financial and physical damages. In this respect, for improving the lateral resistance of the tracks, many techniques have been proposed by various researchers and companies. For instance, changing the shape and sleeper geometry for better interaction with ballast layer in horizontal direction can be pointed out and in this regard, the
2
M. Esmaeili et al. / Computers and Geotechnics 71 (2016) 1–11
‘winged sleepers’ suggested by Montalban et al. [6] is worth mentioning. As the results of the numerical analysis depicted, creating two wings at the bottom of the sleeper can increase the lateral resistance up to 53%. In another research, Zakeri et al. [7] proposed the utilization of the ‘frictional sleepers’, including the jags in the sleeper base. Their lab experiments with this type of sleeper confirmed 68% enhancement in the lateral resistance due to more interaction of ballast particle with sleeper base. In other studies Ciotla˘uș and Kölló [8] and Beck et al. [9] suggested employing ‘frame’ and ‘Y shape sleepers’ respectively for enhancing the lateral stability and resistance of ballasted tracks. In contrast to above-mentioned techniques, which have focused on implementing a modification on sleepers, Xitrack technology uses a liquid polyurethane which is poured into the ballast to improve the lateral stiffness of shoulder ballast [10]. Scrutinizing all presented methods for enhancing the lateral resistance, it is obvious that none of them rely on subgrade stiffness and bearing capacity. So, the current study presents an innovative idea called ‘‘nailed sleeper”, which is based on the lateral bearing of driven steel nails in conjunction with the lateral resistance of common concrete sleepers. In this method, the possibility of restricting sleepers horizontally using subgrade without limiting the vertical displacement is investigated. For this purpose, by getting the idea from driven micropiles, two vertical steel nails are inserted through B70 concrete sleeper in subgrade to restrict its lateral displacement. In order to implement this idea, firstly a 3D finite element model of single tie (sleeper) push test was developed in ABAQUS software and its results were compared with the laboratory STPT results. Afterwards, due to a good agreement between the numerical and experimental results, two steel nails were included in the model and an extensive sensitivity analysis was performed on the important parameters of nailed sleeper such as: nail diameter, nail length, nail location through sleeper and subgrade stiffness. Consequently, the best dimensions of the nails were obtained. Knowing the optimum nails geometry, the effect of nail presence on the sleeper’s structural behavior was controlled based on Australian design code AS 1085.14 [11]. Finally, utilizing the achieved results in this stage, the nailed B70 sleeper was manufactured and installed in an under-operation railway route and its efficiency in increasing the lateral resistance of ballasted railway track was confirmed through many single tie (sleeper) push tests in comparison to common B70 sleepers.
2. Introducing the idea of nailed sleeper The idea of nailed sleepers is originated from utilizing steel nails (or steel micropiles) in railroads aiming at connecting super and substructure of ballasted railways together in order to engage the lateral bearing capacity of nail/subgrade system in the lateral resistance of the ballasted tracks. In this system, the interaction between the nails and subgrade in lateral direction in conjunction with concreter sleeper as the capping system acts similar to a group of micropiles which has an adequate potential to enhance the lateral stiffness of the whole track system. Although this idea does not have any background in the field of railway engineering, many researchers in the field of geotechnical engineering have been engaged in it. For instance, the results of a series of experiments conducted by Richards and Rothbauer [12], indicate that the behavior of micropiles under lateral loads depends on the type and the resistance of the first two to five-meter soil layers at the top of the micropiles. In another field experiment and the consequent numerical analysis conducted by Abd Elaziz and El Naggar [13], the behavior of a single hollow micropile under the
monotonic and cyclic lateral loading in clay soils was investigated. Through their experiments it was proved that the lateral behavior of micropiles is affected by soil properties in a depth equal to 10 times of micropiles diameters from the top, while the soil layers in lower level does not have any effect in this regard. Yet, in another experiment done by Kershaw and Luna [14], the effect of both vertical and lateral loading on micropiles was studied. Based on their experiments, the presence of vertical static load on micropiles does not have any effect on its lateral resistance. In this research, according to what is shown in Fig. 1, driving two vertical steel nails (which work as micropiles) in track subgrade through the holes made in concrete sleeper (which acts as pile cap), leads to the interaction of the track superstructure (sleeper and ballast) and track substructure (subgrade) which resulted in an increase in overall lateral resistance in ballasted tracks against the applied lateral forces. Obviously, the nails diameters, lengths and locations along the sleeper have considerable effects on the proposed system performance. On the other side, the lateral stiffness and bearing capacity of the subgrade play important roles in mobilizing the lateral stiffness of the nailed sleeper system. Another important issue in real performance of the nailed sleeper system is the effect of the nails on structural performance of prestressed concrete sleepers due to insertion of two holes for nail deriving. Considering all the factors mentioned is essential in developing the idea of using nailed sleeper system. In response to this essence, utilizing the numerical and analytical approaches as analysis and design tools should be considered. For this reason, the next section is devoted to detailed numerical and analytical calculations to support the nailed sleeper idea. Meanwhile, it should be noted that in the proposed system no restrictions have been considered in vertical direction for the nails in the sleepers, so two low friction plastic sheaths were inserted in the concrete sleeper prior to concrete casting during the sleeper production. This issue facilitates the tamping operation on ballasted tracks and it has no negative effects on the vertical stiffness of the tracks as well. 3. Numerical analysis of lateral resistance of the nailed sleeper The behavior of nailed sleepers under lateral loads is not only affected by nails diameter, length and location along the sleeper, but it considerably depends on geotechnical parameters of ballast layer and subgrade soil. Therefore, in the first place, using ABAQUS 6.13 [15] as finite element software, the interaction of nails and sleeper was investigated considering all issues mentioned before, and a comprehensive sensitivity analysis was done on the effective parameters. In the following section, the 3D numerical model of a single nailed sleeper and the analysis done under lateral loading are discussed in detail. 3.1. The model geometry, mesh and boundary conditions Since the main purpose of this research is to study the behavior of B70 concrete nailed sleeper in standard single-track railway, in the process of modeling, the dimensions and the geometry of the B70 sleeper and also the cross-section of single track railway [16] are used according to Fig. 2. In Fig. 3, the discretized finite element model is shown. It should be mentioned that in the direction perpendicular to the page, the model’s thickness was considered equal to 1 m. In the numerical analysis with ABAQUS, various elements were utilized for track super and substructure meshing. In this regard, for sleeper and ballast and subballast layer, the C3D20 type of hexahedral element with 20 nodes was used while for the subgrade soil modeling, the C3D8 type of hexahedral element with 8 nodes was utilized. On the other hand, the wedge element type C3D15
M. Esmaeili et al. / Computers and Geotechnics 71 (2016) 1–11
3
Fig. 1. Schematic of (a) longitudinal section of ballasted track with nailed sleepers and (b) cross section of ballasted track with nailed sleeper.
Fig. 2. (a) B70 concrete sleeper longitudinal section, (b) B70 concrete sleeper cross-section and (c) cross section of single track railway (all in mm).
coefficient in interface of the sleeper and the ballast was assumed as 0.9 [6], while it was considered as 0.1 for the contact surface of nail with ballast and subballast layer and it was assigned as 0.4 for nail and subgrade soil interface [17]. As free vertical sliding of the nails into concrete sleepers is one of the key features of the proposed system, the frictionless contact was imposed for nail/sleeper interface. In the modeling process, concrete sleeper and steel nails (ST37 type) were considered as linear elastic materials. This is justifiable due to minor value of lateral displacement in concrete sleeper, during single tie push test. On the other hand, an elastoperfectly plastic model with Mohr–Coulomb failure criterion was assigned to ballast, subballast and subgrade material. The detailed material properties of the model components including density, elasticity modulus, Poisson’s ratio and shear strength parameters are summarized in Table 1 [3,18]. 3.2. Lateral loading of a single sleeper Fig. 3. 3D FE mesh employed in modeling of nailed sleeper.
with 15 nodes was employed for nails. Overall, 7428 elements and 20219 nodes were used in the whole model. In order to impose the boundary conditions, the lateral surfaces were fixed in X direction, the longitudinal surfaces were fixed in Y direction and the bottom surface was fixed in all 3 directions, X, Y and Z. It should be noted that the model is free to settle in vertical direction. For interaction modeling between all different parts of the model, the tangential behavior of the contacts was assumed to allow sliding in existing interfaces. Regarding this, the friction
STPT, which is the abbreviation of Single Tie Push Test, is one of the experiments for defining lateral resistance. In this test, in order to determine the lateral resistance of a single sleeper, at first, the sleeper fastenings should be unlocked from the rails and then the lateral displacement of the single sleeper, due to the applied lateral load, should be recorded. The lateral load resulted in 2 mm displacement is usually known as the lateral resistance in the test [16]. In the developed numerical model of the present study, similar to STPT condition, a gradual concentrated load was applied on the sleeper lateral side and the loading was terminated while a 2mm lateral displacement took place.
4
M. Esmaeili et al. / Computers and Geotechnics 71 (2016) 1–11
Table 1 Mechanical characteristics for elements [3,18]. Model component
Material model
Density (kN/m3)
Elasticity module (MPa)
Friction angle (°)
Cohesion (kPa)
Poisson’s ratio
Concrete sleeper Steel nail Ballast Subballast Subgrade
Linear elastic Linear elastic Mohr–coulomb Mohr–coulomb Mohr–coulomb
24 78.5 15.6 16.7 18
37,500 210,000 140 80 45
– – 45 35 30
– – 2 2 50
0.15 0.3 0.4 0.35 0.33
3.3. Numerical model validation and results In this section, the results of the conducted numerical analyses are presented in two stages. In the first stage, the results of the common STPT, which exclude the subgrade material in numerical model, are presented and compared with the existing experimental STPT results. In the next stage, using this validated model, a comprehensive sensitivity analysis is performed on the proposed ‘nailed sleeper’ system to reveal the most important parameters affecting the new system. As shown in Fig. 4, the results of the common STPT test are in good agreement with the experimental results reported by Zakeri et al. [7]. In this diagram, two separate phases can be observed. In the first phase, a linear relation between force and displacement confirms the frictional behavior of sleeper in contact with ballast material, while in the second phase, the constant force with incremental displacement displays the sleeper sliding. Regarding AREMA [16] recommendations, the force resulted in 2 mm displacement, which is equal to 12.5 kN, can be considered as lateral resistance of single sleeper in both experimental and numerical approaches. Based on the obtained results in pervious stage and the adequate validity of developed FEM model in ABAQUS as a suitable analysis tool, in the next stage after adding the subgrade and nail elements to the model, an extensive sensitivity analysis is conducted to show the effect of various engaging parameters on real performance of nailed sleeper system. For this reason, a sensible range of nail and subgrade specifications are considered to be implemented in the numerical model as shown in Table 2. 3.3.1. The effect of the nails length In Fig. 5, the active lateral load–displacement diagram is plotted for a conventional sleeper and also for a nailed sleeper with a specific nail diameter of 40 mm, for various lengths of 1, 1.5 and 2 m. In these results the subgrade soil elasticity modulus is considered 45 MPa, which meets the minimum required value recommended by UIC719 R [19]. As can be seen, increasing the length of the nails from 1 m to 2 m does not affect the lateral resistance of the whole system. On the other hand, focusing on distribution pattern of internal forces along the nails, as is shown in Fig. 6, it can be observed that
Fig. 4. Comparing load–displacement curves of numerical model results and experimental test.
Table 2 Used parameters in sensitivity analysis of ‘‘nailed sleeper” behavior numerical model. Parameter
Symbol
Unit
Value
Nail diameter Nail length Nail distance to sleeper center
D L B
mm m mm
Subgrade elasticity modulus
E
MPa
16, 24, 32, 40 1, 1.5, 2 100, 200, 300, 400, 500, 1100 45, 80, 100, 120
Fig. 5. Load–displacement diagram of conventional sleeper and nailed sleeper system for different lengths of the nail.
the shear and moment in the nails reach to a negligible value in depths more than 1 m. Although this result confirms the suitability of nail length of 1 m for ‘‘nailed sleeper” system, considering the possibility of increase in ballast layer depth during the maintenance operations and the length of nails embedded in concrete sleeper (as micropile cap), the length of 1.5 m was adopted. This greater length guaranties the more safety factor of the proposed system against lateral loads. As illustrated in Fig. 5, the nailed sleeper with 1.5 m length results in lateral resistance of 20.5 kN that is approximately 150% more than normal sleeper without the nails which is 8.3 kN and it proves that the proposed nailed sleeper can meet the requirements of the tracks against all kinds of imposed lateral loads mentioned before such as the wind lateral load on rolling stock, the centrifugal forces in curves and so forth. It should be mentioned that the loading direction isn’t an important issue in the proposed system because the pair of the nails on the both side of the sleeper withstands against all lateral loads in all directions. From structural design point of view, supposing the yielding stress (F y ) of 240 MPa for steel nails, the ratio of plastic moment (Mp = 2.56 kN m) to maximum existing moment (M d = 0.62 kN m) provides a minimum safety factor of 3.6. Moreover, comparing the allowable shear stress of 0.4F y [20] with maximum existing shear stress of 21 MPa shows a safety factor of 4.5 in the nail element. Therefore, it can be stated that the selected dimensions for the nails guarantee the suitable performance of the nailed sleeper system. 3.3.2. The effect of the nails diameter Adopting the suitable length of 1.5 m from the results in the previous section, and considering the same specifications for sub-
M. Esmaeili et al. / Computers and Geotechnics 71 (2016) 1–11
5
Fig. 6. Shear force, bending moment and lateral displacement variations in depth for various nail lengths.
Fig. 7. Load–displacement diagram for various nail diameters.
grade soil, the results of numerical model for various diameters of the nail are illustrated in Fig. 7. As shown in this figure, by increasing the nail diameter, the ratio of lateral resistance of nailed sleeper system to the common system increases with a linear pattern. As shown, for nails with 16, 24, 32 and 40 mm in diameters, the lateral resistance of nailed sleeper have raised up to 55%, 90%, 120% and 150% respectively in comparison to conventional sleepers. Although a bigger nail diameter results in more lateral resistance, it can also reduce the efficient sleeper width which may causes some inefficiency for sleeper from a structural point of view. Overall, a maximum diameter of 40 mm was selected for nails in the proposed system. As shown in Fig. 8 for various nail diameters, after depth around 1 m, all shear, bending and lateral displacements converge to zero which means the minor effect of the nail with lengths more than this value in mobilizing the more lateral resistance. 3.3.3. The effect of nails location along sleeper To study the effect of nails location along the sleeper, a series of sensitivity analyses were performed and the nails position were modified. In these analyses, the nail diameter and length were selected as 40 mm and 1.5 m respectively, while the subgrade soil module of elasticity was chosen as 45 MPa. According to Fig. 9, when the nail location is far from the centerline up to 400 mm, the lateral resistance of nailed sleeper arises but thereafter it remains constant. Consequently, based on the achieved results, the nails location was fixed in 400 mm in order to provide the maximum lateral stiffness and avoid any inference
between nails and tamping picks during the maintenance operations. 3.3.4. The effect of subgrade stiffness To investigate the effect of this parameter on lateral resistance of the proposed system, four various elasticity modulus – 45, 80, 100 and 120 MPa – were allocated to the subgrade soil in FEM model. These values were selected based on UIC719 [19] recommendation for subgrade elasticity modulus (EV2 > 45 MPa). Considering 1.5 m for nails length and 40 mm for their diameter, Fig. 10 depicts the lateral resistance curve against the subgrade soil elasticity modulus. As it is shown, increasing the modulus from 45 to 120 MPa results in an increase of around 10% in the lateral resistance. Fig. 11 shows the shear force, bending moment and displacement diagrams along the nails length. It should be emphasized that both Figs. 10 and 11 are drawn based on the results achieved from 2 mm lateral displacement in nailed sleeper system. 4. Structural control of nailed sleeper The main purpose of this section is to discuss the structural effect of holes inserted in concrete sleeper on its structural design especially on flexural reinforcements. Based on the presented results in previous section, although nails with bigger diameters evidently result in higher lateral resistance, they can cause some insufficiencies in structural performance of concrete sleeper under
6
M. Esmaeili et al. / Computers and Geotechnics 71 (2016) 1–11
Fig. 8. Shear force, bending moment and lateral displacement for nails with various diameters.
Fig. 10. Lateral resistance diagram of nailed sleeper system for various subgrade elastic modulus. Fig. 9. Lateral resistance diagram of nailed sleeper system for various locations of nails along sleeper.
R¼JQ service load. Considering the possible limitation regarding nail diameter, a detailed calculation is performed in the present section to control all relevant structural issues in nailed sleeper system. It should be noted that all calculations and controls are conducted based on Australian standard, AS1085.14 [11] regarding concrete sleepers. The structural controls are especially conducted in critical sections of sleeper where the nails are inserted. As shown in Fig. 12, based on AS1085.14 [11], three types of loading models have to be applied under sleeper for defining the critical moments, as follows: 1. LM1: for calculating the positive flexural moment at rail seat by assuming the uniform pressure distribution beneath the sleeper on both sides at the width of l = L g (Fig. 12a). 2. LM2: for calculating the positive flexural moment in the center of the sleeper assuming the uniform pressure distribution beneath the sleeper on both sides at the width of l = 0.9(L g) (Fig. 12b). 3. LM3: for calculating the negative flexural moment in the center of sleeper by assuming the uniform pressure distribution over the whole sleeper length at the bottom (Fig. 12c). According to the proposed formula by Australian standards, the vertical applied load on the rails seats is equal to:
D:F 100
ð1Þ
where R is the rail seat load, J is the combined vertical design load factor, Q is the load carried by sleeper and D.F is the axle load distribution factor. As recommended in this standard, the minimum impact factor (J), which is equal to 2.5, was used in calculations. Moreover, the remaining parameters such as sleepers’ center to center spacing and vehicle axle load were considered as 600 mm, 250 kN respectively. In addition, in this case the D.F is considered equal to 51. Substituting the assumed values in the formula above, the vertical load of rail seat equaled 318.75 kN. In Fig. 13, variation diagrams of positive and negative moments along the sleeper are plotted against loading models of LM1, LM2 and LM3. The process of controlling the critical sections of the sleeper from the structural stand point is done according to the flowchart in Fig. 14, using the value shown for the moments in the diagram above. Flexural tension stress has to be controlled in 3 sleeper constructional steps as follows: Step1: the concrete stresses immediately after transfer, and before deferred losses. Step2: the concrete compressive stress after all losses without any applied load. Step3: the concrete stresses under the working conditions and after allowing for all losses of pre-stress.
M. Esmaeili et al. / Computers and Geotechnics 71 (2016) 1–11
Fig. 11. Shear, bending moment and displacement for nails with various elasticity modulus of subgrade.
Fig. 12. Various loading patterns for calculating critical flexural moments in sleeper.
Fig. 13. Moment diagram respect to three loading pattern LM1, LM2 and LM3.
It is worth mentioning that based on Australian standard; there is no need to control the shear stresses in all sleeper sections. Flexural stresses were calculated at the highest (Ftop) and the lowest (Fbot) axes of the sleeper sections at the 400 mm distance
Fig. 14. Flowchart of structural controls in concrete sleeper.
7
8
M. Esmaeili et al. / Computers and Geotechnics 71 (2016) 1–11
from the center of the sleeper. All calculated tension stresses should be compared with allowable values proposed by AS 1085.14 [11]. Moreover, the bearing stress, which is created in side contact area of the nails with concrete sleeper, should be controlled in the walls of the hole. The lateral load on a single nailed sleeper is considered approximately 22.5 kN based on the maximum lateral load calculated in numerical analysis chapter. Subtracting the share of base, crib and shoulder ballast in lateral resistance, the pure lateral load imposed to one of the walls of the hole was calculated. By dividing this load on the side contact area of nail and sleeper the bearing pressure can be obtained. The amount of the applied stress was compared with the proposed values of ACI 318 [21]. Considering the values calculated for tension and bearing stresses and comparing them with allowable stresses in each part, a summary of results is presented in Table 3. As it is shown, all of the calculated stresses are considerably less than the allowable stresses recommended by the standard, so it can be concluded that no negative effects are expected from the steel nail presence on structural performance of concrete sleeper in nailed sleeper system; neither in the construction phase nor under the service load.
5. Field test on a B70 nailed sleeper After completing the numerical and analytical investigations on the nailed sleeper system and selecting appropriate dimensions for length, diameter and location of the nails in the sleepers, in this section, many field tests are conducted on both common and nailed sleeper to compare their real performance on site in order to prove the superiority of the new system in improving the lateral resistance of ballasted track. In this regard, some details concerning fabrication, field installation and single tie push test are presented. 5.1. Nailed sleeper production Concerning the production of nailed sleepers, some changes should be applied on common B70 sleeper. To this end, the sheath, made up of compressed plastic with outer and inner diameters of 50 and 40 mm respectively, was placed 400 mm away from sleeper center in the mold in the sleeper production factory. The other steps such as tendon reinforcement installation, fabricating and curing are the same as the conventional B70 sleeper. Finally a sample of B70 sleeper containing two holes for inserting the steel nails was manufactured as shown in Fig. 15.
Table 3 Structural control for both flexural and bearing stresses in critical section of nailed sleeper B70. Control type
Bending stressesa Step 1
Created stresses (MPa) Allowable compression stresses (MPa) Allowable tension stresses (MPa) a
Bearing stressesa Step 2
Step 3
F top
F bot
F top
F bot
F top
F bot
1.67 24 –
2.6 24 –
1.42 <1 –
2.21 <1 –
1.43 27 3.098
2.2 27 3.098
0
All calculations were performed considering the compression strength of sleeper concrete equal f c ¼ 60 MPa.
Fig. 15. Produced B70 sleeper including two holes.
Fig. 16. Schematic of instrumentation for STPT on nailed sleepers.
1.3 21 –
9
M. Esmaeili et al. / Computers and Geotechnics 71 (2016) 1–11
5.2. Installation, instrumentation and lateral resistance testing on sleeper In this research, the field lateral resistance tests were carried out in a part of Iran railway network in the main route of Tehran–Karaj near Karaj station, which is located 30 km away from Tehran station. This site is equipped with ballasted tracks with superstructure, including 25 cm ballast and conventional B70 sleeper. The shoulder width in the site is about 40 cm. For conducting the STPT on the common and nailed sleeper, firstly the fabricated B70 sleeper including two holes was substituted with a B70 sleeper in the track. Moreover, on one side of the track and outside the shoulder a concrete support was established to provide enough reaction for lateral loading of the nailed sleeper by hydraulic jack. The common instrumentation of sleeper was performed by installing a LVDT at the end using a supporting frame to record the lateral displacement of the pushed sleeper while the load of hydraulic jack was recorded by a load cell installed between the jack and lateral side of sleeper as shown in Fig. 16. The utilized LVDT and the load cell specifications are presented in Tables 4 and 5. It should be noted that all recorded data during the loading procedure were recorded via installed data acquisition system on a laptop. The data logger system used in this project was the TMR211 model, made by the Tokyo Sokki Kenkyujo Company [22]. After instrumentation, many single tie push tests were carried out on a B70 sleeper before and after nails installation. It should be noted that in the case of nailed sleeper testing, the nails were driven into the track substructure – through provided holes in B70 sleeper – using a pneumatic hammer (model D25980K of DeWALT Company) [23] and a special coupling as shown in Fig. 17. The installed nailed sleeper is illustrated in Fig. 18.
Table 5 Load cell specifications. Rated capacity (R.C) Rated output (R.O) Zero balance Combined error Repeatability Creep (30 min)
50, 100, 200, 500kgf, 1, 2, 5, 10, 20tf 2.0 ± 0.005 mV/V 0 ± 0.02 mV/V 0.05% 0.02% 0.03%
Temperature effect on Zero value Output value
0.03%/10 °C 0.03%/10 °C
Excitation Recommended Maximum
10 V 15 V
Resistance Input Output Insulation Compensated temp range Operating temp range
400 ± 30 Ω 350 ± 3.5 Ω >2000 MΩ 10 to +40 °C 30 to +80 °C
5.3. STPT results and discussion After installation of special B70 sleeper (including two holes) in the selected track and before the steel-nails installation, three STPT tests were carried out. The results of these tests are illustrated in Fig. 19. In the case of nailed sleeper, STPT tests were carried out with some modifications. These tests were performed under cyclic loading so that the loading and unloading were done consecutively in a manner that the loading step was continued up to 2 mm lateral displacement occurrence in the sleeper. Load and displacement were recorded during these cycles, the results of which are shown in Fig. 20. It is worth considering that in total, four series of tests were conducted which the first one (Fig. 20a) was completed
Fig. 17. Nails driving using pneumatic hammer and the attached coupling.
Fig. 18. Nailed sleeper system after the nails installation. Table 4 LVDT properties. Type Capacity Related output Sensitivity Non-linearity Spring force Frequency response Temperature effect on zero Compensated temperature range Temperature range Input/output resistance Recommended exciting voltage Allowable exciting voltage Weight
CDP-100 100 mm 5 mV/V + or – 0.1% (10,000–106 strain + or – 0.1%) 100–106 strain/mm 0.1%RO 4.9 N 3 Hz 0.01%RO/-C 0 to +40-C (no condensation) 10 to +60-C (no condensation) 350 X Less than 2 V 10 V 580 g
instantly after nail driving and other tests were performed five months later. During this period the track had been under service loads so comparing, the results of the first test with other three tests can indicate the short-term and long-term behavior of the proposed nailed sleeper system. As shown in Fig. 19, the averages of the lateral resistance corresponding 2 mm lateral displacement is around 10.4 kN for three STPT tests on common sleeper. While in the case of nailed sleeper, as it is illustrated in Fig. 20, when the number of loading and unloading cycles grows, the lateral resistance increases gradually and according to this figure, in the first loading cycle, the lateral resistance resulted in 2 mm displacement are recorded as 18.7, 24.4, 20.2 and 21 kN corresponding to tests 1–4. The existing difference between the obtained results in test 1 and other tests can be justified by altering the soil of the subgrade phase from undrained to drained condition. Tracing the presented results in
10
M. Esmaeili et al. / Computers and Geotechnics 71 (2016) 1–11
Fig. 19. STPT test results for conventional sleeper.
Fig. 20 reveals that due to the continuing load cycles, a better interaction between the nails and surrounding subgrade soil can be established which results in enhancing the lateral resistance up to 30 and 35 kN. Comparing the mobilized lateral resistance in this condition with corresponding value in the case of common B70 sleeper confirms a more than 200% enhancement in the lateral resistance. As it stated before, during the cyclic tests, the loading phase in each cycle is continued till 2 mm lateral displacement takes place in the sleeper. But during the unloading phase, the sleeper moves in reverse direction and a part of imposed displacement is recovered as the elastic deformation of the system. This phenomenon can be justified because of the nails presence in the system as influential elastic elements. On the other hand, after unloading, another part of the displacement still remains in sleeper which represents the plastic or permanent deformation of the system and is justifiable because of nonlinear behavior of ballast and subgrade materials. By increasing the loading and unloading cycles, the proportion of the elastic displacement from the total displacement increases. Scrutinizing the presented results reveals that in all conducted cyclic tests after 10 cycles, almost all displacements remain elastic and a steady state could be observed in the nailed sleeper system which confirms no plastic deformation occurrence. This matter can be described by evaluating the values of load– displacement slopes G1 and G2. As indicated in Fig. 21. The G1
Fig. 21. The nailed sleeper lateral stuffiness during the loading and unloading (G1 and G2).
Fig. 22. G1/G2 diagram versus number of cycles.
which is loading slope is calculated as the slope of the line connects the start and peak (maximum load) points together and in the same manner the unloading slope, G2, is calculated as the slope of the line connects the end and peak points together in each cycle for all tests. As it is illustrated in Fig. 21, comparing cycles 1 and 12 in test 3 (Fig. 20c) as a sample, it is found out that due to an increase in the number of cycles both G1 and G2 are going to converge to the same value and based on Fig. 22, generally in all tests mentioned before, the proportion of G1 to G2 tends to 1, which
Fig. 20. Results of lateral cyclic loading test on the nailed sleeper.
M. Esmaeili et al. / Computers and Geotechnics 71 (2016) 1–11
Fig. 23. Maximum active lateral load versus elastic displacement in each cycle.
confirms the steady state happening in the system. The speed of convergence is considerably depends on high elasticity of steel nails regarding to compacted soil. From another point of view the high elasticity of the nails helps to recover the imposed lateral displacement in lateral direction and in overall the nails behavior controls the whole system elasticity. This finding guarantees the stable performance of the proposed system in practical cases when the train wheel-loads apply on the nailed sleeper for long terms. From another point of view, in Fig. 23, the maximum load in each cycle is plotted against the elastic displacement in the same cycle for all 4 tests. According to this figure, the result of test that was carried out instantly after nail driving has lower slope than the later ones. This problem may be justifiable due to the disturbance caused by the nail driving and the short term behavior of the subgrade soil. On the other hand, in the first test which was done instantly after nail driving the system stiffness is 10.8 kN/mm while in other three tests the results for system stiffness is 16.9, 18.8 and 27.9 kN/mm respectively. 6. Conclusion In this research, after introducing the nailed sleeper as an innovative approach for increasing the lateral resistance of conventional ballasted track – by engaging the lateral bearing capacity of subgrade – in the first place, the technical feasibility of the method was investigated using FEM method, and the suitable diameter, length and appropriate location of nails along the sleeper were determined. Moreover, the effects of nails on flexural performance of sleeper were discussed. Next by installing a nailed sleeper in real track and conducting the STPT, lateral resistance of single sleeper with and without nails were measured and the good performance of this sleeper was confirmed. The main findings of the present study can be summarized as follows: 1. As a result of numerical modeling, the lateral resistance of nailed sleepers using nails with 1, 1.5 and 2 m in length are identical. In other words, increasing the length of the nails to more than 1 m does not have any effect on lateral resistance. 2. FEM numerical results reveal that nails with diameters of 16, 24, 32, and 40 mm can increase the lateral resistance to about 55, 90,120 and 150 percent correspondingly compared to conventional B70 sleepers. 3. According to the numerical analyses results, it was concluded that when the nails are located away from the sleeper centerline – up to 400 mm – the lateral resistance of nailed sleeper arises but thereafter it remains constant.
11
4. The FEM model results show that the lateral resistance is increased about 10% by increasing the subgrade elasticity modulus from 45 to 120 MPa. 5. Based on the field test results, by increasing the loading and unloading cycles, the amount of lateral resistance increases. Lateral resistance of nailed sleeper using two nails with the diameter of 40 mm and the length of 1.5 m, that have installed 400 mm away from centerline of the sleeper has increased by more than 200% in comparison to conventional B70 sleepers. 6. By increasing the loading/unloading cycles, slopes of loading cycles (G1) have increased and slopes of unloading cycles (G2) have decreased. After 10 cycles, the proportion of G1/G2 tended to 1 which means long term stability of the nailed sleepers. To ensure the suitable performance of the nailed sleeper in increasing the lateral resistance of ballasted track, the subsequent stage of the present research will be devoted to monitoring the nailed sleeper performance in horizontal fixation of the curved track by complementary field tests. References [1] Zakeri JA. Lateral resistance of railway track. In: Perpinya X, editor. Reliability and safety in railway. InTech; 2012. p. 430. [2] Selig ET, Waters JM. Track geotechnology and substructure management. T. Telford; 1994. [3] Kabo E. A numerical study of the lateral ballast resistance in railway tracks. Proc Inst Mech Eng, F: J Rail Rapid Trans 2006;220(4):425–33. [4] Le Pen LM, Powrie W. Contribution of base, crib, and shoulder ballast to the lateral sliding resistance of railway track: a geotechnical perspective. Proc Inst Mech Eng, F: J Rail Rapid Trans 2011;225(2):113–28. [5] Lichtberger B. Track compendium: formation, permanent way, maintenance, economics. Eurailpress; 2005. [6] Montalbán Domingo L, Real Herraiz JI, Zamorano C, Real Herraiz T. Design of a new high lateral resistance sleeper and performance comparison with conventional sleepers in a curved railway track by means of finite element models. Latin Am J Solids Struct 2014;11:1238–50. [7] Zakeri J-A, Mirfattahi B, Fakhari M. Lateral resistance of railway track with frictional sleepers. Proc ICE-Trans 2012;165(2):151–5. [8] Ciotla˘uș M, Kölló G. Increasing railway stability with support elements. Special sleepers; 2012. [9] Beck A, Hempe T, Schlender T. Experience gained with Y-steel sleeper track on german rail (DB AG). Rail Eng Int 2008;37(2). [10] Woodward PK, Kennedy J, Medero GM, Banimahd M. Application of in situ polyurethane geocomposite beams to improve the passive shoulder resistance of railway track. Proc Inst Mech Eng, F: J Rail Rapid Trans 2012;226 (3):294–304. [11] AS-1085.14. Railway Track Material Part 14. Prestressed Concrete Sleepers. Standard Australia; 2012. [12] Richards JT, Rothbauer M. Lateral loads on pin piles (Micropiles). GeoSupport 2004:158–74. [13] Abd Elaziz AY, El Naggar MH. Performance of hollow bar micropiles under monotonic and cyclic lateral loads. J Geotech Geoenviron Eng 2015:04015010. [14] Kershaw K, Luna R. Full-scale field testing of micropiles in stiff clay subjected to combined axial and lateral loads. J Geotech Geoenviron Eng 2014;140 (1):255–61. [15] ABAQUS 6.13. Providence (RI, USA): Dassault Systèmes Simulia Corp. [16] AREMA. Manual for Railway Engineering, Volume 1, Chapter 1, Roadway and Ballast: American Railway Engineering and Maintenance of way Association; 2012. [17] Engineers ASoC, Engineers USACo. Design of Sheet Pile Walls. ASCE Press; 1996. [18] Indraratna B, Nimbalkar S. Implications of ballast breakage on ballasted railway track based on numerical modelling; 2011. [19] Railways IUo. Earthworks and track-bed layers for railway lines: UIC code. 2nd ed. Paris: International Union of Railways; 1994. [20] AISC. Specification for structural steel buildings. Chicago; 2010. [21] ACI. Building code requirements for structural concrete: (ACI 318-95); and commentary (ACI 318R–95). American Concrete Institute; 1995. [22] http://www.tml.jp/. [23] http://www.dewalt.com/.