مطالعه‌ي موردي بر روي تاثير شكل ترانشه‌هاي باز و پر در كاهش ارتعاشات ناشي از عبور قطار

Study on the shape effect of open and in-filled trenches in reducing vibrations due to passing trains

به علت توسعه‌ي احداث خطوط مترو و راه‌آهن در مناطق شهري كشورهاي جهان، بيشتر شهرها با مشكل نزديكي خطوط راه‌آهن به برخي از مناطق حساس به ارتعاش مواجه‌اند. در اين بين، از عمده روش‌هاي كاهش ارتعاش در منبع و مسير مي‌توان به ترانشه‌هاي باز و پر اشاره كرد. با عنايت به آنكه در خصوص تاثير شكل ترانشه‌هاي كناري، پژوهش‌هاي مشخصي صورت نگرفته است؛ لذا در اين پژوهش با تمركز بر ترانشه‌هاي باز و پر به بررسي اثر سه شكل ترانشه‌ي مستطيلي، پلكاني و مثلثي پرداخته شده و براي اين منظور از روش عددي اجزا محدود/نامحدود (المان‌هاي ميراگر و جاذب انرژي) استفاده شده است. نتايج حاصل از تحليل عددي، نشان‌دهنده‌ي آن است كه ترانشه‌ي مثلثي و پلكاني در هر دو حالت باز و پر نسبت به ترانشه‌ي مستطيلي با توجه به مساحت برابر و در نظرگرفتن بار متناوب قطار، كارايي بهتري دارند.

Ground-borne vibrations due to passing trains are a complex dynamic problem. Different types of vibrations can be produced due to passing trains on the track and existing irregularity of wheel and rail surface. Due to the construction of subway and railway lines in the most urban areas in the world, they have been encountered with adjacency problem of railroads lines to some sensitive areas such as buildings, laboratories, hospitals, research centers with the precise instruments or monuments. Four major steps exist for transferring train induced vibrations to infrastructures that follow: (A) Generation of vibrations due to repeated train loads (B) Transmission of vibrations in the surrounding medium (C) the effects of vibrations on adjacent reception structures (D) Interception of vibrations. Waves transfer through structure of track including rail, traverse, ballast, sub ballast and soil so with arriving to the buildings, their effects cause discomfort of residents. Studies of Researchers indicate that the various methods are presented for controlling the train induced vibrations in the path of wave propagation (e.g. trench and wave obstacle), source of vibration (e.g. Rail grinding, resilient wheels and Floating slabs) and reception structure of the vibrations (e.g. elastic support systems). Among proposed methods for reduction of train induced vibrations, the use of open and in-filled trenches can be pointed out. In all presented researches for reduction of train induced vibrations with using trenches, the rectangular shape of trenches is considered. Because of the shape effect of trenches hasn’t been investigated in reduction of train induced vibrations, therefore firstly the numerical model of present study has been verified with accomplished work by Ni et al (1994) with using two techniques for simulating boundary elements(infinite elements and dashpot elements) for common rectangular shaped trench. Finally the three shape effects of rectangular, triangular and step shaped trenches for both open and in-filled concrete ones have been investigated. In this regard, the effect of train induced vibration has been investigated with finite/infinite element method (or energy-absorbing damper elements). Amplitude reduction ratio (Ar) has been used for estimating and comparing the efficiency of the rectangular, triangular and step shaped trenches. The obtained results for three shapes of rectangular, triangular and step trenches indicate that Triangular and step shaped trenches have better efficiency than the common rectangular trench with considering equal areas and harmonic train load applied for both open and in-filled trenches.