ارزيابي ميزان تاثير عوامل مختلف در نشست شالوده‌هاي سطحي واقع بر خاك‌هاي دو لايه‌ي روان‌گرا تحت بار سيكليك

Effect of Different Parameters on Shallow Foundation Settlement Rested on Two-Layered Soil Subjected to Earthquake Loading

اغلب ساختمان‌هاي معمول بر روي شالوده‌ي سطحي ساخته مي‌شوند. بررسي نشست شالوده‌هاي سطحي خصوصاً وقتي بارهاي لرزه‌يي به روان‌گراشدن خاك زير شالوده منتهي مي‌شوند، موضوع تازه‌يي است. در اين نوشتار سعي شده است با مدل‌سازي عددي، تاثير عوامل مختلف در نشست شالوده‌ي سطحي بر روي خاك دو لايه‌ي روان‌گرا در دو حالت مختلف مورد بررسي قرار گيرد. در همين راستا، پارامترهاي عرض شالوده، بسامد تحريك ورودي، بيشينه‌ي دامنه‌ي تحريك ورودي، و نسبت عمق لايه‌ي بالايي به كل ضخامت خاك مورد بررسي قرار گرفته است. نتايج حاصل از مدل‌سازي عددي صورت‌گرفته نشان مي‌دهد كه با افزايش عرض شالوده، كاهش بسامد، و افزايش دامنه‌ي تحريك ورودي، ميزان نشست سازه افزايش مي‌يابد. همچنين هر چه نسبت عمق لايه‌ي بالايي به كل دامنه بيشتر باشد، در حالت خاك متراكم بر روي خاك سُست، شاهد كاهش و در حالت خاك سُست بر روي خاك متراكم، شاهد افزايش نشست سازه خواهيم بود.

Settlement and tilt of structures due to the liquefaction of subsoil layers is a major cause of damage during earthquake. Among different soil profile types, two-layered sub soil is very common. The numerical study presented in this research addresses the effects of different parameters that are influential on liquefaction in saturated sand deposits underlying the foundation of structures. In this regard, a 3D finite element model, with a fully coupled dynamic analysis of saturated porous media has been utilized. For the constitutive model, a well-calibrated bounding-surface plasticity model, capable of accounting for the monotonic and cyclic response of saturated sand, in a wide range of densities and confining pressures, has been used. Another main feature of the proposed numerical model is taking the variations of permeability into account during liquefaction. The numerical simulations of this study have been performed using OpenSEES, which is an Open-source software framework. In this research, two 3D u-p elements with novel variable permeability functions have been used, which were implemented in OpenSEES. The numerical model has been verified by simulation of a series of centrifuge experiments performed on models of footing and the analysis results are compared with experimental measurements. After verification of the numerical model, comprehensive parametric studies were conducted and, based on the obtained results the effects of the parameters were reported. The results showed that existance of a liquefiable layer plays a significant role in shallow foundation settlement during earthquake. In two- layered subsoil, if the footing rests on the liquefiable layer, the underlying dense sand can reduce the amount of settlement more effectively.