جستجوی مقالات مرتبط با کلیدواژه « بالاست » در نشریات گروه « عمران »

The growth of railway lines has caused engineers to seek to increase the efficiency of the ballasted railway track, and for this reason, they are looking for an increase in the maintenance intervals and a lower life cycle cost. In high-speed railways and lines with high traffic volume, especially railways with stiff and soft subgrade, the issue of line settlement is one of the challenging factors. According to the methods introduced to strengthen the subgrade so far, the use of geogrid is one of the conventional methods to reduce settlement and maintenance operations, which today requires more investigation in its optimal selection. In this article, an attempt has been made to investigate the effectiveness of geogrid to reduce stresses on the subgrade, sub-ballast layer, and ballast layer, and to explain how to choose it based on the technical parameters. In this study, the types of geogrid and the effect of mesh size, applied stress, and its effect on soil resistance have been investigated and the method of achieving the appropriate selection of geogrid has been suggested. Studies show that by using geogrid, the vertical settlement and horizontal movement of the railway panel can be reduced by 70%.

Ballast materials play a very important role in rail tracks, the most important of which is the distribution of stresses to the bottom layers and the provision of proper elasticity and drainage. At the same time, one of the main problems of these materials is particles breakage due to dynamic transit loads as well as tamping operations, which over time, especially for weaker rocks, lead to high fouling and the need for frequent maintenance and repair. One of the proven solutions to increase the service life of ballast materials and reduce the maintenance problems caused by it, is to add an optimal percentage of TDA to the ballast, which helps to improve the abrasion behavior and durability of the material and causes less particles breakage. In this paper, the behavior of ballast materials mixed with TDA is numerically investigated using two-dimensional simulation of ballast box test with discrete element approach. Using this numerical modeling in PFC software, which has been validated by actual laboratory study of ballast box test, the effects of increasing axial load (different stress levels) and operating speed (different loading frequencies) are evaluated by performing a parametric study within the optimal percentage range. Based on this, the optimal mixing percentage between ballast and TDA materials in different stress and frequency conditions has been determined. The results showed that the stress level has a very high effect on the results, while the effect of frequency is much lower. According to the results, also 5 to a maximum of 10% by weight of TDA can be proposed as the optimal mixing percentage both in terms of settlement and particles breakage.

Regarding to increasing demand for axle-load and fleet operation speed in the ballasted railway lines, provision of lateral-resistance is raised among the owners of railway industries. So, different solutions are presented to increase lateral resistance of the track in technical literature. Several factors affect the track lateral resistance but the most important one is the interaction between ballast layer and sleeper. Literature review shows that the interaction between ballast particles and sleepers has not widely been definitely studied by consideration of particle behavior of ballast material in lateral resistance problem. Therefore in this paper a numerical model has been developed to simulate single sleeper push test by discrete element numerical software PFC 2D. The results was compared with laboratory measurement results. Ballast grading used in this paper was according to the Group 2 of Iran Code No. 301 and the modeled sleeper was B70 concrete sleeper. To perform geometric modeling of the particles, clustering method was used as well as photography. Finally, regarding the results of the valid numerical model, sensitivity analysis were performed on the parameters affecting the lateral resistance including initial upright load, ballast material properties, ballast layer thickness and ballast shoulder width.

In the present study utilizing large scale direct shear test, the effect of ballast encasing with geogrid has been investigated. In this mater two ballast grading of 1 and 4 have been considered in conjunction with three types of geogrids GP35/35, GP40/20 and GP60/20. All direct shear test have been carried out under the vertical surcharges of50, 100 and 150 kPa with shear deformation rate of 1mm/min. The outcomes of the results reveals that in absence of geogrid encasing, the shear strength of ballast depends on the maximum particle size, uniformity coefficient and the vertical surcharge value. In the case of geogrid encasing of the ballast has promoted the shear strength behavior whereas for the grade 1 gradation of the ballast, the maximum friction angle corresponding to 50 and 150 kPa surcharges have been 74.5 and 68.96 degrees while the maximum reduction percentage in dilation angle has been 32.22. On the other hand for ballast grading 4, the mentioned variations have been correspondingly obtained as 30.12, 14.67 and 27.86 percentages.

This paper presents post-cyclic-monotonic behavior of BOF and EAF steel slags as ballast and subballast layers with and without geogrid or tire chips, and comparing their behavior with limestone material. Tests were carried out by the large scale triaxial equipment. The confining pressures are chosen with respect to the stress levels in typical rail road track. The tested specimens were prepared by maximum dry density. The used square shape geogrid has aperture sizes of 25*30.5 mm and the depth of placement about 5cm above and middle of subballast. Size of tire chips was 12.7-25 mm and mixed 7% by weight, only in ballast layer. Maximum deviatoric stress, secant modulus, unloading-reloading modulus, friction angle and particle breakage are investigated. Results indicated that performing cyclic loading on ballast layer before the post cyclic-monotonic tests decreases the axial strain corresponding to maximum deviatoric stress. Post-cyclic-monotonic friction angle of ballast and subballast with and without geogrid ranges from 57-65°. The friction angles of the steel slag of ballast, subballast and ballast/subballast specimens are similar to traditional ballast materials. The values of and decrease as moisture of specimen increases. Average poison ratio for mention layers is about 0.3. Marsal breakage index (Bg) of the single layered steel slag ballast is almost two times of the limestone ballast. Presence of steel slag subballast increases Bg of the steel slag ballast to about 2 times. According to fouling and breakage indexes of steel slag ballast under post cyclic-monotonic loading is relative clean. Saturation of ballast specimen causes increase of particle breakage, therefore the drainage of ballast materials is necessary. Usage of tire chips is not recommended in ballast due to high rate of decreasing of modulus and friction angle. Results indicated that the steel slag particle may be used as natural aggregate in ballast and subballast layers.

Suggestion of fracture toughness for controlling the ballast materials quality. Abstract . . . . . . . . Among the existing transportation systems, rail transport systems have special privileges including high capacity, safety, economy and etc. Structure of railways is composed of two main sections of substructure and Superstructure. Ballast is a layer between sleeper and sub-ballast that is composed of broken stones. These stones have uniform gradation. Since ballast bears heavy loads, its high quality is important. If the ballast is gradually crushed and fines are generated, stone permeability and drainage property will be damaged and finally, instability will be resulted. If the above problems occur at ballast, its substitution is necessary that has high costs. Ballast layer quality depends on materials and their density. Ballast must be resistant against applied load. For Ballast quality control several tests has been suggested in 301 railway magazine that after the test on Ballast, the results are compared with allowable values and Ballast quality was controlled. Ballast grains subjected to train loads, contain numerous cracks. From the fracture mechanic point of view, when stress intensity factor at the crack tip reaches the mixed-mode fracture toughness, crack growth begins and Ballast grains crush. Ballast crushing and abrasion are the main reasons for ballast contamination which is the main source of various defects, particularly geometric defects in the railway. Since disc-type specimens are among favorite test samples for determining mode I and mixed mode fracture toughness in brittle materials like rocks, Therefore it is suggested to add the determination of the mixed-mode fracture toughness I and II with the semi-circular bend specimens subjected to three-point-bend loading to the ballast quality control in 301 railway magazine. In this research, discussed the importance of fracture toughness in extraction, crushing and operation of Ballast, then the finite element method and experimental is used to analyze a semi- circular disc specimens under bending load and the fracture toughness of two Ballast mine, Anjilavand and Gaduk was determined. Anjilavand and Gaduk mine is two Ballast mines in Iran that using them for Ballast layers in railway substructure. The crack parameters KI, KII and T are calculated for different mixed-mode from pure mode I to pure mode II. YI, YII, and T* are the non-dimensional forms of KI, KII, and T, respectively. These parameters are functions of the crack length ratio, the crack angle and also the location of loading supports in the semi-circular bend specimen. The curves of YI, YII, and T* extracted for various combinations of modes I and II. Since the results for mixed mode fracture resistance of brittle materials are usually presented in a normalized form as KII/KIc versus KI/KIc, where KIc is a material constant called the pure mode I fracture toughness, using the mixed-mode fracture criteria, capped failure modes I and II for both ballast rocks were extracted and compared. Fracture toughness of Anjilacvand specimen is greater than Gaduk specimen that shows the Anjiavand Ballast for operation in railway is better than Gaduk Ballast.

Track buckling is formation of large lateral misalignments due to high compressive thermal stresses in continuous welded rail (CWR) tracks and often results in catastrophic derailments. Investigating the actual behavior of this phenomenon is a complex interaction between the vertical, lateral and torsional modes. In the present study, the effects of different superstructure components such as rails, sleepers, fasteners and ballast materials on lateral stability of CWR tracks have determined utilizing a three-dimensional developed model. The parametric studies have been done for both straight and curvilinear tracks. The validity of the model is verified through comparison with theoretic results and other experimental works. The studies show that the rails and ballast materials have highly affect on track

Rail bed modulus is one of the main parameters in optimal track design which is determined based on components of ballasted railway tracks. Perception of track bed behavior in different conditionsand on substructure with complex characteristic has an important role in the rail system analyses. Technical and geometric characteristics of track may have a significant impact on rail bed modulus. Review of recent researches on railways in different countries indicates the importance of rail supporting system in rail analysis results. This paper studies the importance of change in rail bed components characteristics in determining track modulus as one of the parameters of rail system analysis and design. This study is divided into two parts including field tests and numerical analysis. To achieve this goal, ballasted railway tracks are modeled according to technical and geometrical characteristics. Then, model is calibrated using field test in order to confirm the validity of model, and it is analyzed by employing finite element method based on tracks components classification. Finally, some diagrams and tables are presented for different conditions of rail roads according to ballast layer thickness and the type of the subgrade in order to determine track modulus and control rail vertical deflection.