انتخاب مقیاس بهینه مدل سازی فیزیکی شیب ماسه ای خشک توسط مطالعه آزمایشگاهی

در این تحقیق به کمک اجرای مدل های فیزیکی ، تحلیل های ارتعاش آزاد و فرکانس های طبیعی انجام شده است مدلسازی های آزمایشگاهی در مهندسی ژئوتکنیک می تواند میدان شتاب 1g انجام شود در مدلسازی های ژئوتکنیکی، ارتباط بین فرکانس های مدل و نمونه واقعی از اهمیت کلیدی برخوردار است. در این مقاله با انجام آزمایش های دینامیکی پالس ضربه چکش، محدوده فرکانس های بهینه تحریک مدل بر مبنای ابعاد و مقیاس هندسی مدل های فیزیکی شیب انتخاب شده است. شیب های ماسه ای مدل با زوایای مختلف 25 تا 60 درجه ای داخل جعبه مدلسازی ایجاد شده اند. دانسیته نسبی مدل های شیب ماسه ای متوسط و در حدود 50 تا 52 درصد است. محدوده فرکانسی مورد بررسی با توجه به مطالعه پیش لرزه ای مدل های میز لرزان بین 0.001 تا 150 هرتز درنظرگرفته شده است. مطابق یافته های پژوهش حاضر، در یک مدل شیب ثابت، فرکانس هایی که در آن ها حداکثر انرژی لرزه ای نهفته است، در مقایسه با فرکانس های با حداکثر دامنه پاسخ بزرگنمایی شده شتاب، کاملا متفاوت هستند. نتایج مطالعه حاضر وجود رابطه ای مابین زاویه شیب ماسه ای، فرکانس های مدل فیزیکی و فرکانس بزرگنمایی را اثبات می کند. به طوری که با افزایش زاویه شیب های مدل در یک ارتفاع ثابت، مقادیر بزرگنمایی پاسخ شتاب ضربه کاهش می یابد.

selection of optimal scale for physical modeling of dry sandy slope by laboratory study

in this research, the free vibration or natural frequency analyzes have been performed with the help of small-scale physical models. laboratory modeling in the geotechnical engineering can be performed in the acceleration field of 1g.  in each of the physical modeling modes, the relationship between the model and prototype frequencies is very essential. in this paper, with the help of hammer impact pulse tests (hipts) -dynamic experiments- the optimal frequency ranges and the best geometric scales for physical modeling are investigated by a strongbox. the frequency range studied has been selected according to the study of shaking table models between 0.001hz and 150hz. to perform impact pulse tests, the physical models of dry sandy slope with different inclination angles from 25 to 60 degrees (and a constatnt slope height) have been instrumented by the piezoelectric acceleration sensors.  the relative density of the sandy slope models is medium dense and about 50% to 52%. in addition to 8 physical models of sandy slopes, two models of level-ground and empty box have also been investigated. the time-history of the acceleration function of the input excitation shock at the slope floor (base point) and the response acceleration at the slope crest are recorded by the acceleration sensors.  these acceleration time responses last for a short stroke (short impact) of less than 1.0 second duration. after extracting temporal responses, the frequency analyzes including transfer function (tf), fourier response spectrum ratio (rfrs), and spectral energy density function (psd-function) are derived from the temporal results. using the transfer function or rfrs, quantitative values of natural frequencies of the physical model of the sandy slope and the storngbox are extracted in different vibration modes. according to the findings of the present study, for a constant slope model the frequencies at which the maximum seismic or dynamic energy is emitted are quite different from the frequencies with the maximum magnified response amplitude. the results of the present study prove the existence of a logic relationship between the sandy slope inclination angle (physical model natural frequencies) and the model response amplification frequency. so that by increasing the angle of inclination of the model slopes at a constant height, the magnification values of the impact acceleration response decrease. because in general, the amount of sandy materials magnifies or weakens the amplitude of frequency responses. the presence of low sandy materials (on steep slope models) reduces the magnification range of the acceleration response and high sandy materials (on gentle slopes) increase the response range. optimal frequencies in strong box modeling in the 1g acceleration field are frequencies that do not interfere with acceleration magnifications before or during seismic excitation (pre-seismic mode). acceleration magnification causes resonance and premature failure in the physical model, which is generally undesirable and unmeasurable in laboratory studies. in this research, the optimal frequency range according to the measurements is proposed for the physical modeling of the 1g acceleration field. these ranges and frequency values are presented according to the various constraints such as the type of strong box, slope angle, relative density of sand, the actual frequency effect of the horizontal components of earthquakes, and so on.