پديد آورندگان :
عساكره, عادل دانشگاه هرمزگان - دانشكده فني مهندسي - گروه عمران , شعباني, مهديه دانشگاه هرمزگان - دانشكده فني مهندسي - گروه عمران
كليدواژه :
روان گرايي خاك , اضافه فشار آب حفره اي , آزمايش سانتريفوژ , روش عددي و مدل رفتاري خاك
چكيده فارسي :
تخمين روان گرايي يكي از اهداف مهم در مهندسي ژئوتكنيك است. بدين منظور روش هاي آزمايشگاهي و عددي مختلفي ارائه شده اند. يكي از گام هاي مهم در پيش بيني روان گرايي، پيش بيني اضافه فشار آب حفره اي ايجاد شده است. وقتي كه بارگذاري لرزه اي به خاك ماسه اي اشباع اعمال مي شود، فشار آب حفره اي تا زماني كه به فشار محصوركننده اوليه برسد، افزايش مي يابد و موجب وقوع روان گرايي مي شود. به طوركلي دو روش مهم براي تحليل هاي ديناميك خاك وجود دارد: روش هاي غيروابسته تنش كل و روش وابسته تنش موثر. هدف مهم اين پژوهش ارزيابي ظرفيت مدل موجود در نرم افزار تفاضل محدود فلك بر پايه روش هاي تجزيه و تحليل تنش موثر براي پيش بيني اضافه فشار آب حفره اي در طول بارگذاري لرزه اي است. براي كاليبره كردن مدل عددي، آزمايش سانتريفوژ شماره يك پروژه ولاكس كه روي ماسه نوادا با تراكم 40% انجام شده، مدل سازي شده است. پس از صحت سنجي مدل عددي، خاك محل احداث پروژه مسجد جامع شهر بندرعباس، براي بررسي روان گرايي با استفاده از روش عددي مدل سازي شده است. براي مدل سازي، از نتايج آزمايش هاي عمومي مكانيك خاك انجام شده روي نمونه هاي خاك محل و آزمايش نفوذ استاندارد انجام شده در سايت، براي كاليبره كردن پارامترها و انتخاب ثابت هاي مدل بهره گرفته شده است. نتايج حاصل از مدل عددي با نتايج حاصل از آزمايشگاه تقريبا مطابقت دارد. بنابراين با انتخاب دقيق پارامترهاي مدل عددي مي توان به طوركلي با استفاده از نتايج مدل سازي ديد كلي از تشكيل فشار آب حفره اي و تخمين عمق روان گرايي داشت.
چكيده لاتين :
Estimation of Liquefaction is one of the main objectives in geotechnical engineering. For this purpose, several numerical and experimental methods have been proposed. An important stage to predict the liquefaction is the prediction of excess pore water pressure at a given point. In general, there are two important methods for soil dynamics analyses, fully coupled effective stress and uncoupled total stress analysis. The main purpose of this study is to evaluate the model capacity of the finite difference software, FLAC, based on effective stress analysis methods to predict the excess pore water pressure during seismic loading. A level ground centrifuge test conducted during the VELACS project on the Nevada sand with a density of 40%, was utilized to calibrate the numerical model. After the validation of the numerical model, a model was conducted to predict excess pore pressure and consequently the liquefaction for the site of Bandar Abbas Mosque.
Theoretical bases
A fully coupled u–P formulation, where pore pressures and displacements are computed simultaneously and interactively at each time step, is used in FLAC software. This feature is used to simulate the excess pore water pressure time histories during cyclic loading. The finite difference based software, FLAC, used the Finn model that incorporates two equations correlating the volumetric strain induced by the cyclic shear strain and excess pore water pressure produced during cyclic loading. As mentioned above, the pore water pressure generation can be computed from two sets of equations: Martin et al. (1975) and the Byrne (1991) formulations in which the volumetric strain that was produced in any cycle of loading is depended on the shear strain that was formed during that cycle as well as the previously accumulated volumetric strain.
Modeling and Results
The VELACS model # 1 centrifuge test representing a level ground site constituted of the Nevada sand at 40% relative density has been numerically simulated in the current study to validate the numerical model. The centrifuge model contains a laminar box with slipping “rings” that allows differential horizontal displacements. This was simulated in the FLAC model by free-field boundary conditions which prevent reflection of the waves in the side walls. Figure 1 shows comparison of EPWP time histories ratio of numerical modeling and centrifuge test. Static analysis was carried out before dynamic analysis in order to find initial stress and strain state. At the next stage, the dynamic loads were applied at the base of the model and dynamic analysis was performed.The Bandar Abbas mosque project is located approximately 500 meters from the coast. In the project, due to the groundwater level and the existence of loose layers of silt, investigating the potential of liquefaction is necessary. For numerical modeling the results of the general soil mechanics test on soil samples and standard penetration test performed on the site were used to calibrate the parameters and select the model constants.
Conclusion
The results of numerical modeling have been matched to experimental results of the centrifuge test using both Martin and Byrne formulations, except for the case of 5 m the numerical model has predicted lower excess pore water pressure values than the experimental values. This may be originated from the fundamental assumption of the Martin et al. (1975) EPWP theory, in which excess pore water pressure is directly related to the relevant volume changes. On the other hand, the Martin et al. (1975) model was adopted for one-dimensional measures of shear strain, while, in a 2D analysis under both horizontal and vertical shakings, there are three strain rate measures. FLAC uses some assumptions to solve this problem and it can affect the results.The results of the numerical model showed liquefaction to a depth of about 5 meters that is almost compatible with the results from the lab, which has declared that the depth 2 to 5 m is liquefiable. With careful selection of numerical model parameters one can generally use the simulation results to have a general sense on the pore water pressure generation and liquefaction prediction.
