بررسی انتشار موج در شمع‌های پیش‌ساخته از بتن سبک و مشخصات رفتاری آنها در حین کوبش

با توجه به استفاده‌ی گسترده از شمع‌ها در سازه‌های مختلف، دانش فنی و روش‌های اجرایی و نیز طراحی این نوع پی‌های عمیق، نیازمند مطالعه‌ی بسیار و نوآوری می‌باشد. در این مقاله، به بررسی و تحلیل رفتار کوبشی شمع‌های پیش‌ساخته از انواع بتن سبک با مقاطع استوانه‌ای و مخروطی پرداخته شده که این مسئله موجب شناخت بیشتر این فرآیند و در نتیجه افزایش راندمان عملیات کوبش شمع‌ها می‌گردد. از جمله روش‌های بررسی مسائل کوبش، استفاده از روش عددی تفاضل محدود می‌باشد، به‌گونه‌ای که بتوان به شکلی منطقی رفتار شمع و خاک را در هنگام کوبش شمع مدل‌سازی نمود. بدین‌منظور، ابتدا انتشار موج در اثر اعمال بار ضربه به رأس یک میله‌ی الاستیک بررسی شده و سپس با مدل‌سازی شمع استوانه‌ای و مخروطی و تخصیص مشخصات مکانیکی انواع بتن سبک به آنها، رفتار این‌گونه شمع‌ها نیز تحت کوبش مورد تجزیه‌وتحلیل قرار گرفت. با مقایسه‌ی نتایج به‌دست‌آمده از تحلیل‌ها، استنباط می‌شود که سرعت انتشار موج در میله‌های ساخته شده از بتن سبک تیپ L3 و S2 بیشتر از میله‌های با بتن معمولی و سایر طرح اختلاط‌ها می‌باشند. همچنین شمع‌های بتنی سبک دارای رفتار مشابهی مانند بتن‌های معمولی در حین کوبش بوده و دارای عملکرد بهتری نیز نسبت به بتن معمولی هستند. این مسئله علاوه بر کاهش وزن شمع‌های بتنی پیش‌ساخته، می‌تواند موجب کاهش هزینه‌های اقتصادی شمع‌کوبی شده و همچنین با انتخاب طرح اختلاط مناسب و بیشتر شدن میزان نفوذ نهایی، افزایش راندمان شمع‌کوبی حاصل می‌گردد.Wave Propagation Analysis and Behaviour of Precast Light-Weight Concrete Piles during Driving Omid Tavasoli 1* and Mahmoud Ghazavi21. Assistant Professor, Department of Civil Engineering, East Tehran Branch, Islamic Azad University, Tehran, Iran, * Corresponding Author, email: o.tavasoli@iauet.ac.ir2. Professor, Civil Engineering Department, K. N. Toosi University of Technology, Tehran, Iran  The pile driving process can be easily modeled prior to installation to determine adequate and appropriate equipment selection. Pre-casted lightweight concrete (LWC) provide an attractive alternative to conventional pile materials such as steel and common concrete by improving the durability of deep foundations. In this paper, the drivability of cylindrical and tapered piles with lightweight concrete was investigated and compared with traditional pile materials using the finite difference analysis. The three-dimensional model was considered to simulate pile-soil system in drivability by FLAC3D. The vertical LWC pile was assumed to behave linear elastic, and the soil was acted in elasto-plastic material obeyed the Mohr-Coulomb failure criterion. Interface elements were also used at the soil-pile contact surfaces along the pile shaft and toe to allow the slip occur during the driving procedure. Quiet boundaries were considered to prevent waves traveling in the lateral and vertical directions for the soil. The different concrete mixtures with Leca and Scoria were assumed to compare the size of the physical properties of light aggregate, which letters S and L represent Leca and the Scoria in each concrete mix design.The analysis of wave propagation in LWC rods without and with damping effects were performed with fixed and free end boundary conditions. The rod gravity was neglected with no soil or other supports around the rod shaft. A half-sine stress wave was applied on the rod head. To represent the dynamic responses, the force and velocity records were monitored below the rod head to prevent the mixing up the upward stress wave and downward reflection from the rod head. The obtained results were exactly in accordance with one dimensional wave propagation theory in rods. The immediate F and Z.v waves shifted down after tip reflections are the reflections from the rod free head boundary condition. In fact, the downward initial compressive wave is reflected as compression type at rod fixed-end and reflected tension type at the rod free-top boundary conditions. The F wave and Z.v wave amplitudes were attenuated with time as expected due to the damping presence in the rod.The pile drivability with light weight concrete and cylindrical and tapered geometry was also investigated in clayey soil and the results were compared. Based on signal matching for LWC piles, as expected, residual displacements of pile S2 and L3 are 8 to 20% greater than common concrete piles, and pile S3 has approximately the same behaviour as pile CC.The analyses results indicate that LWS piles with selecting the appropriate mixture design and ratio between elastic modulus and specific weight have a better performance compared with common used concrete and therefore, it can affect the pile optimum penetration and economic saving of pile driving procedure. Keywords: Lightweight Concrete; Pile Driving; Wave Propagation; Tapered Pile; Finite Difference Method.