تحليل تنش موثر نيمه وابسته بر مبناي انرژي كرنشي براي مدل‌سازي روان‌گرايي ماسه‌ها

Effective Stress Loosely Coupled Analysis of Sand Liquefaction based on Strain Energy

در اين مطالعه يك مدل غيرخطي نيمه وابسته به‌منظور پيش‌بيني رفتار سيكلي زهكشي‌نشده و روان‌گرايي ماسه‌ها ارايه شده است. از مدل هايپربوليك كندر-زلاسكو اصلاح‌شده، كه به صورت نيمه وابسته با يك مدل پيش‌بيني كننده‌ي فشار آب حفره‌‌يي بر مبناي انرژي كرنشي كار مي‌كند- استفاده شده است، تا تحليل تنش موثر انجام شود. اگر چه روش‌هاي بسيار زيادي براي تحليل تنش موثر و مدل‌سازي روان‌گرايي در دسترس هستند، اما اين روش‌ها غالباً نيازمند پارامترهاي بسياري هستند، كه تعيين برخي از آن‌ها نياز به صرف وقت و هزينه‌ي بسيار دارد. مدل ارايه‌شده در اين نوشتار ساده است و پارامترهاي اندك و قابل تعيين دارد و در عين سادگي پيش‌بيني قابل قبولي را نيز ارايه مي‌كند. نتايج مدل‌سازي نشان مي‌دهد با وجود اينكه مدل ارايه‌شده تحرك سيكلي را در نظر نمي‌گيرد، حد بالاي اضافه فشار آب حفره‌يي و روان‌گرايي را به دقت پيش‌بيني مي‌كند و از آن مي‌توان در تحليل يك بُعدي غيرخطي پاسخ زمين به‌منظور ارزيابي رخداد روان‌گرايي استفاده كرد.

Nonlinear site response analysis gives a more detailed description of soil behavior than the linear elastic or equivalent linear methods, but application of nonlinear codes in practice has been limited because of its greater number of parameters. In addition, there are some difficulties in calibration and adjustment of parameters in nonlinear codes. There are two main approaches for studying the response of soil deposits subjected to earthquake loading, including total stress and effective stress methods. The major deficiency of the total stress method is that it is unable to take into account the progressive stiffness degradation caused by pore pressure buidup in soil. Only the effective stress method can model the gradual loss of soil strength and stiffness due to the build-up of pore water pressure. In this study, a loosely coupled nonlinear model is presented to predict the undrained behavior of sand subjected to cyclic loading. A modified Kondner – Zelasko (MKZ) model, which is coupled with a pore water pressure generation model based on strain energy, is used for effective stress analysis. There are a couple of methods for the effective stress analysis and modeling of liquefaction, but these methods often require many parameters, and their determination requires greater time and cost. The numerical method presented in this paper is simple and needs a few and determinable parameters. It also provides accurate and acceptable predictions in triaxial condition. Although the proposed model is unable to account for the cyclic mobility and fluctuation of pore water pressure during cyclic loading, it accurately predicts peak points of pore pressure history during cyclic loading. Modeling of cyclic undrained triaxial tests conducted on Toyoura and Frazer River sands confirms that the proposed approach predicts the onset of liquefaction very well, and it can be used in nonlinear site response analysis to evaluate liquefaction occurrence.