Issue 30

A. De Iorio et alii, Frattura ed Integrità Strutturale, 30 (2014) 478-485; DOI: 10.3221/IGF-ESIS.30.58 479 I NTRODUCTION rom the analysis of the technical literature concerning the continuous welded rail (CWR) problems, it can be inferred that together with the noteworthy advantages that it offers when compared to the traditional solution, in terms of both maintenance, comfort and performance, some essential precautions are needed. Among these, the most important from the structural design point of view is related to the track buckling (usually in the horizontal plane, although the phenomenon can occur also in the vertical plane [1]) caused by the rail temperature increases due to sun irradiation or train induction brakes [2]. The phenomenon takes place especially in presence of eccentricity due to track misalignments, in the neighbourhood of bends, bridges and other track singularities or when the neutral temperature is decreased due to creep [3, 4]. This latter is an effect particularly important near tunnels, in correspondence of strong gradients or in bents [2]. Also if the track neutral temperature can be restored to a safe value by periodic maintenance, very often the effects of local variability cannot be eliminated [4]. When rail temperature exceeds the maximum allowable value it is mandatory to reduce the rolling stock speed. Furthermore, several authors have pointed out that loads (included the longitudinal ones) exerted on the track by trains can contribute to exceed the lateral strength limit [6], also due to the up-lift: design data for high speed trains and examples of horizontal track loads evaluation are also reported in [6]. In [6-8] it is demonstrated that track lateral strength offers a safety margin against lateral buckling and that most of the track strength is due to the friction between the sleepers base and the ballast; other important lateral strength contributions are given by the presence of ballast near the heads of the sleepers, the ballast parts included between successive sleepers and the ballast shoulders. The relative importance of these strength contributions depends on the track weight [6]. However, it is very difficult to trace a curve representative of the weight effect also because of the experimental data lack: until now direct measurements of track lateral strength have been carried out only with no more than two different weights of the track. Other parameters that can affect the lateral strength are time, since the ballast mechanical properties degrade with time and, finally, the sleeper shape that affects the resultant friction force against the ballast. Also the kind of fastening system between sleepers and rails can affect the track lateral strength [9]. When the track moves sideways, sleepers can remain parallel to each other or show a relative rotation [10, 11]. Which of these behaviours predominates depends on the ballast strength and the primary and secondary twisting stiffness of the fasteners. However, it has not been possible to the authors finding in literature test data parameterized with fastener stiffness. In consequence of ballast maintenance operations, the lateral strength can be reduced up to the 40% of its full value. Therefore, some operations cannot be carried out when the rail temperature is high, while some others are done imposing a reduced train speed until the ballast is consolidated [12]. A reliable solution to improve the track lateral stability is the adoption of sleeper anchors; unfortunately they involve additional costs. A related problem also very often examined is the track displacement in curves due to the thermal deformations of the CWR. It lays down precise limits to the CWR construction: curves having radius less than a given threshold value cannot be made by the continuous rail. Several studies have been carried out on the contributions brought to the lateral strength by the geometrical details of the track [3, 6, 12]. Most of them do not cover all the aspects involved: very often the scheduled experimental activities were not finished [6]. The tests usually were not repeated, when they were, the number of repetitions was quite low or however aprioristically fixed. Sometimes, the reported experimental results are partially censored (See, f.e., [3]); in addition, data are referred to an uniform ballast also if generally the presence of water pockets [6] or other singularities can alter the ballast behaviour. On the basis of these data, that have limited and different reliability, several theoretical models and design software’s have been developed in order to make deterministic and probabilistic previsions about the CWR buckling behaviour (See, f.e., [3, 5, 7, 8, 11, 13, 14, 15]) and to establish an allowable buckling risk [1, 2, 5, 16], if the risk based approach is adopted. In addition to the buckling phenomenon probability, this latter approach takes account of the seriousness of its consequences [5]. In order to improve the reliability of the aforementioned tools adopted for the prediction of the ballast behaviour, an experimental activity wider and more realistic than those usually carried out till now by the Institutions interested in this topic and in funding such research programs is needed. Starting from this latter observation, RFI and DII have shown their interest in sharing resources and competences to develop a demanding research program. Together with Italcertifer, Italian Institute of Railway Research and Certification, F