بررسي تأثير شكل موج بارگذاري بر روي پارامترهاي ديناميكي مخلوط ماسه- پودر لاستيك با استفاده از آزمايش‌هاي ميز لرزه

امروزه استفاده از مشتقات تايرهاي لاستيكي فرسوده در پروژه هاي مختلف ژئوتكنيكي جهت جذب و كاهش ارتعاش ناشي از بارهاي لرزه‌اي و ديناميكي گسترش يافته و بررسي تاثير پارامترهاي مختلف بر رفتار ديناميكي آن‌ها در تركيب با خاك ازاهميت فراواني برخوردار مي‌باشد. با توجه به اينكه بارهاي ديناميكي از منابع متعدد و با شكل موجهاي متفاوت ممكن است بر خاك وارد شوند، لذا در اين مقاله تاثير شكل موج بارگذاري بر روي پارامترهاي ديناميكي مخلوط ماسه – پودر لاستيك از قبيل مدول برشي (G) و ضريب ميرايي (D) مورد بررسي قرار گرفته‌است. يك سري آزمايش ميز لرزه g1 بر روي مخلوط ماسه – پودر لاستيك انجام گرفت. نمونه‌هاي خاك دردو حالت غير‌مسلح و مسلح به پودر لاستيك با درصدهاي وزني 5%، 10%، 15% و 20% پودرلاستيك، و در تراكم نسبي صفر درصد تحت سه نوع بارگذاري مستطيلي، سينوسي و مثلثي در فركانس ثابت 2 هرتز و تحت شتاب ورودي g1/0و g3/0 قرار گرفتند. نتايج نشان داد كه در همه حالات، نمونه‌هاي خاك تحت بارگذاري مستطيلي بيشترين مقدار و تحت بارگذاري مثلثي كمترين مقدار مدول برشي و نسبت ميرايي را از خود نشان مي‌دهند. پارامترهاي ديناميكي نمونه‌ها تحت شكل موج‌هاي سينوسي و مثلثي نسبتا نزديك به‌هم مي‌باشند. تاثير شكل موج بر روي ضريب ميرايي در سطوح كرنش‌ برشي پايين، ناچيز بوده ولي با افزايش سطوح كرنش، افزايش مي‌يابد. ضمن اينكه با افزايش كرنش برشي مدول برشي خاك كاهش يافته ولي نسبت ميرايي افزايش مي‌يايد. از طرف ديگر با افزايش درصد پودر لاستيك از مقدار مدول برشي كاسته شده ولي بر ميزان ضريب ميرايي افزوده مي‌شود.

Today, the use of waste tires mixed with soil has been expanded in various geotechnical projects to absorb and reduce the vibration caused by seismic and dynamic loads. Therefore, the objective of this work was to evaluate the dynamic properties of such mixtures prior to practical applications. Given that the different kinds of exterior cyclic loading affect the natural soil, such as earthquakes, high buildings, high speed rails, wave loads, oil tanks, reservoirs and so on, and they demonstrate different wave patterns. So far independent research on the effect of loading frequency on the dynamic properties of the sand-tire mixture has not been carried out. Therefore, in this paper, 1-g shaking table tests were employed to investigate the effect of loading waveform on dynamic properties of sand-tire mixture. A hydraulic shaking table with a single degree of freedom, designed and constructed at the Crisis Management Center of Urmia University, was used to conduct the experiments. Firoozkuh No. 161 sand was used in all the experiments and tire powders were used as a soil reinforcement material. Tire powders are made from discarded tires that have been broken into pieces and sieved by an industrial tire-shredder system. Also, accelerometers were used to measure the acceleration of the input to the sample as well as to record the acceleration caused by the input excitation at different depths of the soil sample. The displacement transducers (LVDT sensors) were also used to measure linear displacement. To record information, all sensors were plugged into a 16- channel dynamic data logger ART-DL16D. Samples were constructed in both unreinforced (pure sand) and reinforced form and with a relative density of zero. In reinforced samples, tire powders were added to the sand with 5%, 10%, 15% and 20% in gravimetric basis. To prepare the sample, a wet tamping method was utilized in both the unreinforced (pure sand) and the reinforced (sand mixed with tire powders) specimens. In this method, first, the sand was mixed with 5% water. Samples were subjected to rectangle, sinusoidal and triangle waveform at constant frequency of 2 Hz and input acceleration of 0.1g and 0.3g. The results showed that in all cases, soil samples exhibit the highest shear modulus and damping ratio under rectangle loading. Therefore, the values of G and D for the rectangular waveforms are greater than those of the sinusoidal and triangle waveforms. The shear modulus and damping ratio for the sinusoidal waveforms are marginally greater than those of triangle waveforms. The effect of loading waveform on the damping ratio of the soil at low levels of strain is negligible, but it increases with increasing strain levels. The shear modulus reduced by increasing the tire powder and the highest reduction is observed in the mixture with 10% to 15% of tire powder. By increasing the tire powder, the damping ratio values of samples increased so that the mixture with 20% of the tire powder has the highest damping ratio. In all cases, the shear strain increased by increasing the amplitude of the input acceleration, and as a result, the shear modulus decreased and the damping ratio increased. In addition, with increasing acceleration, the difference between the values of the shear modulus and the damping ratio increases between different loading waveforms